Sewage sludge ash--A promising secondary phosphorus source for fertilizer production

Absorbing oxygen carriers promotes phosphorus recovery from sludge via the microwave thermal conversion process

This study aims to apply the Absorbing oxygen carriers (AOCs) to induce the migration and transformation of phosphorus compounds during the microwave thermal conversion of sludge so the hard-to-extract organic phosphorus (OP) can be converted to easy-to-extract inorganic phosphorus (IP) and be enriched onto the sludge char. The AOCs were recycled by screen separation from the IP-rich sludge char, with the latter being a renewable phosphorus source from sludge. The AOCs in this novel process enhanced the conversion efficiency of OP into non-apatite inorganic phosphorus (NAlP), which was further converted to apatite inorganic phosphorus (AP). Most phosphorus in the sludge char is presented in the form of orthophosphate.

Migration of trace elements and radioisotopes to various fractions of solid wastes generated as a result of the sewage sludge incineration process

The research was aimed at providing new knowledge in the field of chemical characteristics of solid waste generated in the process of combustion of sewage sludge in fluidized bed furnaces. The research material consisted of disposed fluidized beds (DFB), sewage sludge ash (SSA) and air pollution control residues (APC) from three Polish installations for the thermal treatment of sewage sludge. Natural radionuclides as well as anthropogenic isotope 137Cs were determined in the tested materials and the migration of a wide spectrum of trace elements to various waste fractions generated in the process of sewage sludge combustion was examined. It was observed that both radioisotopes and most of the trace elements determined accumulate in SSA and DFB, while the APC fraction contains a much smaller amount of them. The exceptions are mercury and selenium, whose volatile compounds migrate to the exhaust gas dedusting system and accumulate in the APC fraction (up to 40 mg/kg and 13 mg/kg, respectively). A potential threat from the 226Ra isotope in SSA is identified in the context of the management of this waste in the production of building materials because the typical activity of 226Ra in SSA collected from areas with very low Ra content in natural environment exceeds 1.5-6 times the activity of this isotope in conventional cement mixtures. When managing SSA and DFB, special attention should be paid to the content of metalloids such as As, B and Se, due to the high content of mobile forms of these elements in the mentioned materials.

Bioavailability transition path of phosphorus species during the sewage sludge incineration process

Sewage sludge incineration ash (SSIA) is rich in phosphorus (P), thus being considered as a reliable source of phosphorus recovery. Different P species behaved significant bioavailability. Based on this, a comprehensive investigation into the bioavailability transition path of P species during sewage sludge (SS) incineration was conducted. P predominantly existed in the form of inorganic phosphorus (IP) in SS with a higher concentration of non-apatite inorganic phosphorus (NAIP) and less concentration of apatite inorganic phosphorus (AP). During the SS incineration process, OP existed in the flocs and cell structures of SS underwent destruction, the released P then combined with metal elements such as Ca, Mg, Fe, and Al to form AP species (Ca/Mg-P) and NAIP species (Fe/Al/Mn-P), and the NAIP decomposition to release into gas phase. This was the initial step for enhancing the bioavailability of P species. As temperature increased and the incineration process progressed, the low-temperature-resistant NAIP dissociated, and the metal-binding sites of Al, Fe and Mn in NAIP species were gradually replaced by the Ca and Mg thus forming thermal stability AP species (Ca/Mg-P, such as CaHPO4, Ca2PO4Cl, and Mg3(PO4)2 et al.). This step was crucial for the bioavailability improvement of P species during the incineration process. Therefore, the IP proportions in TP were extremely high (＞98%), and this value gradually increased as incineration temperature raised. The higher incineration temperature, the lower NAIP concentration and higher AP concentration. Besides, additives such as coal/rice husk/eggshell played a significant affect. Additives wither higher Ca content were inclined to react with P to form Ca/Mg-P (AP), while the presence of SO2 would react with Ca metals to form CaSO4 thus inhibiting the formation of AP species (such as CaHPO4 and CaPO4Cl). This results could provide theoretical support for the efficient and directional migration of P during sewage sludge incineration.

Morphology and phosphate distribution in bottom ash particles from fixed-bed co-combustion of sewage sludge and two agricultural residues

The purpose of this study was to provide detailed knowledge of the morphological properties of ash particles, including the volumetric fractions and 3D distributions of phosphates that lay within them. The ash particles came from digested sewage sludge co-combusted with K- and Si-rich wheat straw or K-rich sunflower husks. X-ray micro-tomography were combined with elemental composition and crystalline phase information to analyse the ash particles in 3D. Analyses of differences in the X-ray attenuation enabled calculation of 3D phosphate distributions that showed high heterogeneity in the slag particles. This is underscored by a distinct absence of phosphates in iron-rich and silicon-rich parts. The slag from silicate-based wheat straw mixtures had lower average attenuation than that from sunflower husks mixtures, which contained more calcium. Calculated shares of phosphates between 7 and 17 vol% were obtained, where the highest value for a single assigned phosphate was observed in hard slag from wheat straw with 10 % sewage sludge. The porosity was notably higher for particles from pure wheat straw combustion (62 vol%), compared to the other samples (15-35 vol%). A high open pore volume fraction (60-97 vol%) indicates that a large part of the pores can be accessed by the surroundings. For all samples, more than 60 % of the discrete (closed) pores had an equivalent diameter < 30 μm, while the largest volume fraction consisted of pores with an equivalent diameter > 75 μm. Slag from sunflower husk mixtures had larger pore volumes and a greater relative number of discrete pores >75 µm compared to wheat straw mixtures.

Recent advances in enhanced technology of Cr(VI) bioreduction in aqueous condition: A review

Due to the extensive application of chromate in industry, chromium-contaminated water has emerged as a significant hidden danger that threatens human health and the safety of the ecological environment. The reduction of Cr(VI) to Cr(III) through microbial processes has become one of the most notable methods for remediating water polluted by chromium due to its economic efficiency and environmentally friendly nature. However, several issues persist in its practical application, such as low reduction rates, the need for additional nutrients, and challenges in solid-liquid separation. Therefore, there is a growing focus on seeking enhanced methods for Cr(VI) microbial reduction, which has become a key area of research. This review represents the initial effort to systematically classify and summarize the means of enhancing Cr(VI) microbial reduction. It categorizes the enhancement methods into two main approaches: microbial-based and multi-method combined enhancement, offering detailed explanations for their mechanisms. This research provides both inspiration and theoretical support for the practical implementation of the Cr(VI) microbial reduction method.

Effecting mechanisms of iron-rich sludge on ash fusion characteristics of coal with high ash fusion temperature under reducing atmosphere

Co-gasification is crucial for large-scale clean conversion of coal and sludge. In this study, the effects of municipal sewage sludge (MSS, Fe2O3:48.11 %) and pharmaceutical sewage sludge (PSS, Fe2O3: 67.80 %) on ash fusion temperature (AFT) of high AFT Xiangyuan coal (XY) were explored using an AFT analysis, X-ray fluorescence spectrometry, X-ray diffraction, scanning electronic microscopy, and thermodynamics FactSage calculation. The results showed that when MSS or PSS ash mass ratios reached 20 % or 16 % (for XY mixtures, the mass ratio of MSS or PSS should be >5.81 wt% or 5.07 wt%), respectively, the AFT met the requirement of liquid-slag discharge for entrained-flow bed gasification. Under a reducing atmosphere (6:4, CO/CO2, volume ratio), Fe2+ destroyed the bridging-oxygen bonds in the network structure and generated low melting-point (MP) hercynite (FeAl2O4). This resulted in the AFT decreases in the XY mixtures with the additions of PSS or MSS. Meanwhile, the high calcium content (CaO: 13.40 %) easily reacted with Al2O3 and SiO2 and formed anorthite (CaAl2SiO8), which inhibited high-MP mullite formation and decreased the mixed XY AFT. With the increasing SS mass ratio, the surface of the ash sample and thermodynamic FactSage calculation were in good agreement with the experimental results.

Envisioning the innovative approaches to achieve circular economy in the water and wastewater sector

Given the challenges of urbanization and rapid resource depletion, policymakers have been compelled to abandon the old sequential paradigm of "take-make-use-dispose" to a circular approach that prioritizes preservation of natural resources. The circular economy represents a sustainable management concept that focuses on reducing, recovering, reusing, and recycling waste. While significant strides have been made in implementing circular economy principles in various industries such as automotive, electronics, and construction, particular attention has been given to the water and wastewater domains due to imbalances in water resources. Here we review the global progress of circular economy adoptability in the water and wastewater domains, considering technical, environmental, economic, and social perspectives. It assesses the current state of circular economy integration in the wastewater domain worldwide and presents approaches to promote and accelerate its adoption. The study critically examines the principles of waste management, known as the 6Rs (reclaim, restore, recycle, reduce, recover, reuse), in order to formulate effective strategies for integrating circular economy practices in the water and wastewater domains. Additionally, the study provides an overview of existing research conducted on different aspects of circular economy. Finally, the study analyzes the challenges and opportunities associated with implementing circular economy principles in the water sector.

Phosphorous extraction and heavy metal separation from sewage sludge ash by two-compartment electrodialysis in an upscaled tube reactor

Two-compartment electrodialytic extraction (2C-ED) is a one-step process for the simultaneous phosphorous extraction and separation of heavy metals from sewage sludge ash (SSA). The process is driven by an applied electric DC field, which can be supplied from renewable sources. The proof-of-concept of the method was conducted in small laboratory cells; however, upscaling to a continuous 2C-ED process, which additionally can treat SSA suspensions at a low liquid-to-solid (L:S) ratio, requires a new design. This paper presents such a new design. In principle, ED consists of two compartments separated by a cation exchange membrane. One compartment contains a suspension of SSA in water and the anode. A cathode is placed in the other compartment. Electrolysis at the anode acidifies the suspension causing the dissolution of phosphorous and heavy metals. The heavy metals are separated from the suspension by electromigration into the catholyte, whereas the dissolved phosphorous remains in the dispersion solution. In the new design, the SSA was suspended in a tube-shaped reactor with the cation exchange membrane covering the outside. The reactor was placed in a container with the catholyte. Periodically turning off the reactor kept SSA in suspension even at a low L:S ratio without corners and pockets where the SSA otherwise tends to settle. Five 2C-ED experiments were conducted with 1.5 to 3 kg SSA at varying currents and durations. Up to 89% P was extracted. The extracted P was concentrated in the dispersion solution of the SSA suspension, where the obtained P-related concentrations of heavy metals were far below the limiting values for spreading on agricultural land. The experiments underlined that treating the SSA in a suspension with a low L:S ratio is advantageous. A comparison to previous laboratory experiments in small cells treating 50 g SSA shows a significantly more efficient use of the applied current in the new reactor setup. Thus, the new reactor design for 2C-ED fulfilled the set criteria for the operation and did additionally result in a higher efficiency than the laboratory setups, i.e., the design can be the first step towards an upscaling.

Characteristic of phosphorus rich compounds in the incinerated sewage sludge ashes: a case for sustainable waste management

Growing concern over mineral resources supply forces us to search for alternative sources of Phosphorus. The possibility to recover phosphorus from incinerated sewage sludge ashes appears to be an important aspect in anthropogenic phosphorus cycle and sustainable economy. To make phosphorus recovery efficient it is important to learn the chemical and mineral composition of ash and phosphorus speciation. The phosphorus content in the ash was over 7%, what corresponds to medium rich phosphorus ores. The main phosphorus rich mineral phases were phosphate minerals. The most widespread was tri-calcium phosphate Whitlockite with various Fe, Mg and Ca proportions. In minority Fe-PO₄ and Mg-PO₄ were detected. Whitlockite commonly overgrown with hematite, influences negatively mineral solubility and thus recovery potential and indicates low bioavailability of phosphorus. Considerable amount of phosphorus was found in the low crystalline matrix where phosphorus content was around 10 wt% however low crystallinity and dispersed phosphorus also does not strengthen the potential to recover this element.

Phosphorus recovery from incinerated sewage sludge ash using electrodialysis coupled with plant extractant enhancement technology

Phosphorus (P) is an integral mineral nutrient for the growth of plants and animals. As the increasing population worldwide, the demand for P resources keeps increasing. Therefore, it is necessary to recover P from secondary resources. Unlike conventional P recovery processes, this work focused on the recovery of P from incinerated sewage sludge ash (ISSA) using electrodialysis as the main technology coupled with plant extractants. In this study, Amaranthus and hydrolyzed polymaleic anhydride (HPMA) were used as P extractants, investigating the effects of HPMA concentration and pH of the compound agent on the migration of P and heavy metals from ISSA. The results showed that the concentration of HPMA and pH of the compound agent had a significant influence on the mobility of P and heavy metals. Meanwhile, the impacts of eggshell additions and voltage on the recovery efficiency of P was also studied by using waste eggshells as calcium sources. We found that when eggshells were added at 10 g/L and the voltage was 10 V, the recovery efficiency of P reached 96.05%. Moreover, XRD patterns revealed that the mineral phase of recovered P-containing products was predominantly hydroxyapatite, which had good environmental benefits. Generally, the favorable results have been achieved in the recovery efficiency of P and has practical implications for P recovery from ISSA.

Urban mining from biomass, brine, sewage sludge, phosphogypsum and e-waste for reducing the environmental pollution: Current status of availability, potential, and technologies with a focus on LCA and TEA

Rapid industrialization, improved standards of living, growing economies and ever-increasing population has led to the unprecedented exploitation of the finite and non-renewable resources of minerals in past years. It was observed that out of 100 BMT of raw materials processed annually only 10% is recycled back. This has resulted in a strenuous burden on natural or primary resources of minerals (such as ores) having limited availability. Moreover, severe environmental concerns have been raised by the huge piles of waste generated at landfill sites. To resolve these issues, 'Urban Mining' from waste or secondary resources in a Circular Economy' concept is the only sustainable solution. The objective of this review is to critically examine the availability, elemental composition, and the market potential of the selected secondary resources such as lignocellulosic/algal biomass, desalination water, sewage sludge, phosphogypsum, and e-waste for minerals sequestration. This review showed that, secondary resources have potential to partially replace the minerals required in different sectors such as macro and microelements in agriculture, rare earth elements (REEs) in electrical and electronics industry, metals in manufacturing sector and precious elements such as gold and platinum in ornamental industry. Further, inputs from the selected life cycle analysis (LCA) & techno economic analysis (TEA) were discussed which showed that although, urban mining has a potential to reduce the greenhouse gaseous (GHG) emissions in a sustainable manner however, process improvements through innovative, novel and cost-effective pathways are essentially required for its large-scale deployment at industrial scale in future.

Acidification and alkalinization pretreatments of biowastes and their effect on P solubility and dynamics when placed in soil

BACKGROUND

Sustainability concerns as well as recent increases in fertilizer prices exacerbates the need to optimise the use of biowastes as fertilizers. For this reason, we investigated how different pretreatments affect the P dynamics when biofertilizers are placed in the soil.

METHODS

Sewage sludge (SS), sewage sludge ash (SS-ash), meat and bone meal (MBM), and the solid fraction of biogas digestate (BGF) were pretreated with H₂SO₄, NaOH, and Ca(OH)₂ and incubated for 2 and 12 days, respectively, in a one-dimensional reaction system for detailed studies of the interactions in the biomaterial-soil interface and the soil adjacent to the placement zone.

RESULTS

Our results showed that acidification and treatment with NaOH increased the P solubility of the biomaterials. The P loss from the biomaterial layer to the soil was correlated with water-extractable P in the biomaterials (0.659) and water-extractable P in the soil (0.809). Acidification significantly increased the total amount of P depleted from the biomaterial to the soil whereas NaOH pre-treatment did not. However, for NaOH-treated SS and SS-ash, the apparent recoveries were significantly higher compared to the acidification due to a decrease in soil P sorption capacity as the soil pH increased due to residual alkalinity in the biomaterials.

CONCLUSIONS

Acidification showed promising results by increasing the P solubility of all the biomaterials, and the alkalinization of SS and SS-ash with NaOH by increasing the apparent recovery in the soil. However, further studies are needed to assess the effects of these treatments on plant growth and P uptake.

Mining phosphorus from waste streams at wastewater treatment plants: a review of enrichment, extraction, and crystallization methods

Two interrelated problems exist: the non-renewability of phosphate rock as a resource and the excess phosphate in the water system lead to eutrophication. Removal and recovery of phosphorus (P) from waste streams at wastewater treatment plants (WWTPs) is one of the promising solutions. This paper reviews strategies for P recovery from waste streams in WWTPs are reviewed, and the main P recovery processes were broken down into three parts: enrichment, extraction, and crystallization. On this basis, the present P recovery technology was summarized and compared. The choice of P recovery technology depends on the process of sewage treatment and sludge treatment. Most P recovery processes can meet the financial requirements since the recent surge in phosphate rock prices. The safety requirements of P recovery products add a high cost to toxic substance removal, so it is necessary to control the discharge of toxic substances such as heavy metals and persistent organic pollutants from the source.

Investigating phosphorus loads removed by chemical and biological methods in municipal wastewater treatment plants in Poland

The article presents an analysis of the current methods for phosphorus removal applied in municipal wastewater treatment plants in Poland. Within the study, 131 wastewater treatment plants were investigated, constituting 17 630 500 population equivalent, which is about 1/3 of the overall population equivalent (designed) in Poland. The research was based on a detailed technical questionnaire analysis obtained from wastewater treatment plants operators and calculations of pure metal doses in the applied coagulants and their type per a treated wastewater volume, population equivalent and phosphorus load removed. Moreover, a basic statistical analysis based on Pearson's correlation coefficient was applied to validate the relationship between the consumption of coagulants per the removed P load and the treated wastewater volume in 3 categories of wastewater treatment plants in terms of their population equivalent. The analysis results show that a minimum of 1470 Mg of phosphorus removed by 35 wastewater treatment plants based entirely on biological treatment methods could be used for phosphorus recovery to produce struvite, calcium phosphate or other highly bioavailable alternative fertilizer products. Moreover, 1490 Mg of phosphorus removed by other 17 wastewater treatment plants with a minimal coagulant dose (<1 g of metal per m³ of wastewater), increases the base for phosphorus recovery to approx. 2960 Mg per year using sewage sludge or its dewatering liquors. These results suggest that the implementation of the means mentioned above would significantly increase the possibilities for obtaining phosphorus from secondary sources, especially in wastewater treatment plants without sewage sludge incineration plants.

Variation of the element composition of municipal sewage sludges in the context of new regulations on phosphorus recovery in Germany

Phosphorus (P) recovery is obligatory for all sewage sludges with more than 20 g P/kg dry matter (DM) from 2029 in Germany. Nine wastewater treatment plants (WWTPs) were chosen to investigate variations of phosphorus contents and other parameters in sewage sludge over the year. Monthly sewage sludge samples from each WWTP were analyzed for phosphorus and other matrix elements (C, N, H, Ca, Fe, Al, etc.), for several trace elements (As, Cr, Mo, Ni, Pb, Sn) and loss of ignition. Among the nine WWTPs, there are four which have phosphorus contents both above and below the recovery limit of 20 g/kg DM along the year. Considering the average phosphorus content over the year, only one of them is below the limit. Compared to other matrix elements and parameters, phosphorus fluctuations are low with an average of 7% over all nine WWTPs. In total, only hydrogen and carbon are more constant in the sludge. In several WWTPs with chemical phosphorus elimination, phosphorus fluctuations showed similar courses like iron and/or aluminum. WWTPs with chamber filter presses rather showed dilution effects of calcium dosage. As result of this study, monthly phosphorus measurement is highly recommended to determine whether a WWTP is below the 20 g/kg DM limit.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s12302-022-00658-4.

Highly efficient phosphorus recovery from sludge and manure biochars using potassium acetate pre-treatment

Pyrolysis converts nutrient-rich residues (e.g., sewage sludge and manures) into biochar with low levels of organic contaminants and high nutrient contents. However, the availability of phosphorus (P) as one of the key nutrients in such biochar tends to be low and new approaches are needed to enhance P-availability. In this work we tested and optimised one such method, doping biomass prior to pyrolysis with potassium (K) as potassium acetate. The treatment worked effectively in both pyrolysis units tested (microscale and lab-scale, continuous unit) and all three feedstocks (two types of sewage sludges and swine manure). The most dramatic effect was observed in the microscale pyrolysis unit at 400 °C where 5% K doping increased the water-extractable P content 700-fold to 43% of total P. Of the added K, on average 90% was retained in biochar after pyrolysis of which ∼50% was water-extractable. The proposed method enables conversion of low-value residues into valuable resources with agronomically relevant total and available P and K levels. This approach does not require specialised equipment or process modifications and is therefore easy to implement and relatively cheap (∼US$ 60-80 t^-1 treated feedstock). It can present an urgently required solution to fulfil regulatory requirements for P-recovery.

Extracting phosphorus and other elements from lake water: Chemical processes in a hypolimnetic withdrawal and treatment system

Hypolimnetic withdrawal provides a way to remove phosphorus (P) from eutrophic lakes, but the method is still rarely combined with water treatment for capturing this P. Thus, little is known about the chemical interactions of P and other elements upon the treatment of hypolimnetic lake water. We investigated these chemical processes in a hypolimnetic withdrawal and treatment system (HWTS) in which hypolimnetic water is first led into a treatment unit for dissolved P (dP) precipitation and subsequently filtered before being circulated back into the lake. We studied three different field-scale treatment unit setups and water treatments (aeration only, aeration + calcium hydroxide (Ca(OH)₂), aeration + biopolymer) to compare their effectiveness for dP removal and the geochemical properties of the resulting precipitate. In the aeration only treatment, most of the dissolved iron (dFe) (91-95%) and dP (71-91%) were removed when sand filters were used. The addition of Ca(OH)₂ and biopolymer enhanced Fe flocculation, leading to more effective removal of dFe (d99-100%) and dP (88-95%) from the water. Regardless of the water treatment method, dP was always precipitated by amorphous Fe oxides formed in the hypolimnetic water upon aeration. The P content of the resulting precipitates was somewhat lower than expected (2 439-4 145 mg kg^-1), which may be linked to chemical interactions between Fe and other components in the hypolimnetic water, such as organic matter. The precipitates also contained some heavy metals such as copper and zinc. We conclude that all the tested water treatments were effective in removing dP from hypolimnetic water, but the enhanced precipitation by the addition of treatment chemicals is beneficial when a mesh or other rapid filtration method is used, or when there is only negligible accumulation of dFe in the hypolimnion of the treated lake. Depending on the water treatment method and the water chemistry of the treated lake, the precipitate may have potential for nutrient recycling, although it may sometimes require preliminary processing to enhance bioavailability for plants and to reduce the concentration of heavy metals.

Co-digestion of sewage sludge and wood fly ash

This study uses a new approach for the recycling of plant nutrients by co-digesting sewage sludge with fly ash from a wood combustion. Sewage sludge and fly ash both are enriched with nutrients of the wastewater resp. wood, which makes these products an enhanced source for recycled fertilizers. The effects of the ash addition to the anaerobic digestion are studied in several lab scale experiments including effects on the gas production and microbial activity. Following that, the fertilizing qualities of the digestate are evaluated by plant growth experiments. The results show that the fertilizing qualities of the digested sludge were improved by the ash addition. Next to this, gas production results show that the methane production was not affected by the ash addition, while the total gas release was reduced. The sulphur addition by the ash stimulated sulphate reducing bacteria. The sulphate reducing bacteria did not markedly inhibit the methanogens.

Phosphorus recovery from a pilot-scale grate furnace: influencing factors beyond wet chemical leaching conditions

Phosphorus is a non-renewable resource going to be exhausted in the future. Sewage sludge ash is a promising secondary raw material due to its high phosphorus content. In this work, the distribution of 19 elements in bottom and cyclone ashes from pilot-scale grate furnace have been monitored to determine the suitability for the phosphorus acid extraction. Moreover, the influence of some parameters beyond wet chemical leaching conditions were investigated. Experimental results showed that bottom ash presented lower contamination in comparison to cyclone ash and low co-dissolution of heavy metals (especially Cr, Pb and Ni), while high phosphorus extraction efficiencies (76-86%) were achieved. High Al content in the bottom ash (9.4%) negatively affected the phosphorus extraction efficiency as well as loss on ignition, while the particle size reduction was necessary for ensuring a suitable contact surface. The typology of precipitating agents did not strongly affect the phosphorus precipitation, while pH was the key parameter. At pH 3.5-5, phosphorus precipitation efficiencies higher than 90% were achieved, with a mean phosphorus content in the recovered material equal to 16-17%, comparable to commercial fertilizers. Instead, the co-precipitation of Fe and Al had a detrimental effect on the recovered material, indicating the need for additional treatments.

Phosphorus Fertilizers from Sewage Sludge Ash and Animal Blood as an Example of Biobased Environment-Friendly Agrochemicals: Findings from Field Experiments

Wastes of biological origin from wastewater treatment systems and slaughterhouses contain substantial amounts of phosphorus (P) with high recovery potential and can contribute to alleviating the global P supply problem. This paper presents the performance of fertilizer (AF) and biofertilizer (BF) from sewage sludge ash and animal blood under field conditions. BF is AF incorporated with lyophilized cells of P-solubilizing bacteria, Bacillus megaterium. In the experiments with spring or winter wheat, the biobased fertilizers were compared to commercial P fertilizer, superphosphate (SP). No P fertilization provided an additional reference. Fertilizer effects on wheat productivity and on selected properties of soil were studied. BF showed the same yield-forming efficiency as SP, and under poorer habitat conditions, performed slightly better than AF in increasing yield and soil available P. Biobased fertilizers applied at the P rate up to 35.2 kg ha^-1 did not affect the soil pH, did not increase As, Cd, Cr, Ni, and Pb content, and did not alter the abundance of heterotrophic bacteria and fungi in the soil. The findings indicate that biobased fertilizers could at least partially replace conventional P fertilizers. Research into strain selection and the proportion of P-solubilizing microorganisms introduced into fertilizers should be continued.

Effect of Corn Straw Blending on Phosphorus Specification and Bioavailability of Incinerated Sludge Ash

Phosphorus is a depletable resource, and the consumption of phosphorus fertilizer increases with the growing population size. Phosphorus recycled from incinerated sludge ash can be a complement to phosphatic fertilizers in districts suffering from phosphorus resource shortages (e.g., Germany, Japan, and Sweden). The apatite inorganic phosphorus (AP) content in incinerated sludge ash is a key factor influencing the recoverability and bioavailability. Biomass straw is rich in calcium and magnesium minerals and can be used as an additive to be blended with sludge to increase the AP content. However, most of the current studies added excessive amounts of calcium-based or biomass additives, and the bioavailability of various Ca-Mg-P minerals generated after the addition of biomass has not been systematically discussed. In this study, the changes of the phosphorus form in the mixed sludge and biomass with Ca/P in the range of 1.0-2.5 are studied, and the influence of temperature and additives on the phosphorus form and the bioavailability of phosphorus in the ash samples are discussed by combining X-ray diffraction and citric acid (CA) leaching experiments. The AP content is very low in the residue of the sludge or corn straw (CS) that has been burned individually. The sludge and the blended sludge and CS were incinerated at various temperatures. As the incineration temperature increased, the conversion of non-apatite inorganic phosphorus (NAIP) to AP was promoted, but the bioavailability did not change until 1050 °C for samples with a Ca/P of 2.5. In the range from 750 to 950 °C, higher temperature promotes the formation of Ca₂P₂O₇ and CaP₂O₆. CaP₂O₆ is insoluble in CA; thus, the bioavailability changes little from 750 to 950 °C, although the AP content increases. With the increase of Ca/P, the conversion of NAIP to AP and the bioavailability of phosphorus were promoted. For the blended sludge and CS ash, Ca₇Mg₂P₆O₂₄ appears at 950 and 1050 °C and the bioavailability also increases.

Utilization and recycling of wood ashes from industrial heat and power plants regarding fertilizer use

To achieve a sustainable circular economy for wood ash, the reuse of wood ash in agriculture and forestry is important. To evaluate the usability of ash from the combustion of natural as well as waste wood for application as fertilizer, wood fuel and corresponding ash fraction samples (n = 86) of four industrial wood-fired heat and power plants (>20 MW) were investigated. In different ash fractions, the concentrations of heavy metals (As, Cd, Pb, Ni, Tl, Zn) and plant nutrients (N, P, K, Ca) were assessed with regard to relevant legislation from the European Union and from selected European countries (Germany, Austria and Finland). Depending on fuel composition and combustion kinetics within the respective plants, the investigated ash fractions showed a wide range of nutrient and heavy metal contents. Apart from N which is mainly emitted with the flue gas, plant nutrients were present in substantial concentrations with mean values corresponding to 20% P, 55 % K and >95% Ca content compared to nutrient compound fertilizer. A direct application of most ash fractions from both natural and waste wood combustion is however not possible due to high contents of Cd, Pb and Ni. In order to develop the ash for secondary use, investigation for a suitable treatment process is necessary. An estimation of the nutrient recovery potential from industrial wood energy plants in southwest Germany showed that approx. 3.1% (P), 7.5% (K) and 22.8% (Ca) of raw material for fertilizers could be substituted by ash.

The Impact of Bio-Based Fertilizer Integration Into Conventional Grassland Fertilization Programmes on Soil Bacterial, Fungal, and Nematode Communities

Phosphorus (P) is an essential plant macro-nutrient applied to soil in agriculture, mainly sourced from non-renewable mined phosphate-rock, of which readily accessible reserves are currently under pressure, while global food demand continues to grow. Meanwhile, an abundance of P is lost in waste-streams. Hence, bio-based fertilizers are increasingly produced using nutrient-recovery technologies and evaluated as a sustainable fertilizer alternative. However, there is little knowledge of how these products affect soil microorganisms. In this study, four new phosphate bio-based fertilizers (two struvite and two incinerator ashes) were assessed in permanent grassland-plots to understand their impact on soil bacterial, fungal, and nematode community responses. The experiment consisted of 40 plots (each 6 × 2 m2) of 8 treatments (2 struvite, 2 ash, cattle slurry, 100% mineral fertilizer, zero P fertilizer, and a control without fertilization) with 5 replications arranged in a randomized complete block design. Community data were obtained by amplicon sequencing of DNA extracted from soil samples and subsequent analysis of community composition, diversity, structure and influencing environmental variables. Diversity of the soil microorganisms was maintained by all bio-based fertilizer treatments. Results showed that soil bacterial, fungal, and nematode communities of the struvite-treatments were similar to those in 100% mineral treatment. Communities in ash-treatments were more disturbed in their compositions, abundances and structures, possibly due to their high pH and heavy metal content. From canonical correspondence analysis, available P, K, and Mg, as well as plant P uptake and biomass yield, were identified as factors significantly influencing bacterial and nematode communities across different treatment groups. In particular, the abundance of environmental disturbance sensitive nematodes (e.g., Dorylaimida) was significantly reduced by one of the ash products. Overall, results indicate that both struvites are benign to soil bacterial, fungal, and nematode communities and can be safely applied as a source of renewable P to meet crop nutrition requirement. The ash products require further investigations before recommending their regular application as fertilizer. As the application of novel bio-based fertilizers will increase in the foreseeable future, the findings of this study would be valuable to feed into developing environmental risk assessment protocols.

Techno-Economic Analysis of Scenarios on Energy and Phosphorus Recovery from Mono- and Co-Combustion of Municipal Sewage Sludge

This study evaluates the techno-economic feasibility of energy and phosphorus (P) fertilizer (PF) recovery from municipal sewage sludge (MSS) through incineration in new combustion plants. We evaluated the economic impact of five critical process design choices: (1) boiler type, (2) fuel (MSS mono-combustion/co-combustion with wheat straw), (3) production scale (10/100 MW), (4) products (heat, electricity, PF), and (5) ash destination. Aspen Plus modeling provided mass and energy balances of each technology scenario. The economic feasibility was evaluated by calculating the minimum selling price of the products, as well as the MSS gate fees required to reach profitability. The dependency on key boundary conditions (operating time, market prices, policy support) was also evaluated. The results showed a significant dependency on both energy and fertilizer market prices and on financial support in the form of an MSS gate fee. Heat was preferred over combined heat and power (CHP), which was feasible only on the largest scale (100 MW) at maximum annual operating time (8000 h/y). Co-combustion showed lower heat recovery cost (19–30 €/MWh) than mono-combustion (29–66 €/MWh) due to 25–35% lower energy demand and 17–25% higher fuel heating value. Co-combustion also showed promising performance for P recovery, as PF could be recovered without ash post-treatment and sold at a competitive price, and co-combustion could be applicable also in smaller cities. When implementing ash post-treatment, the final cost of ash-based PF was more than four times the price of commercial PF. In conclusion, investment in a new combustion plant for MSS treatment appears conditional to gate fees unless the boundary conditions would change significantly.

Indicators for resource recovery monitoring within the circular economy model implementation in the wastewater sector

The European Union is currently in the process of transformation toward a circular economy model in which different areas of activity should be integrated for more efficient management of raw materials and waste. The wastewater sector has a great potential in this regard and therefore is an important element of the transformation process to the circular economy model. The targets of the circular economy policy framework such as resource recovery are tightly connected with the wastewater treatment processes and sewage sludge management. With this in view, the present study aims to review existing indicators on resource recovery that can enable efficient monitoring of the sustainable and circular solutions implemented in the wastewater sector. Within the reviewed indicators, most of them were focused on technological aspects of resource recovery processes such as nutrient removal efficiency, sewage sludge processing methods and environmental aspects as the pollutant share in the sewage sludge or its ashes. Moreover, other wide-scope indicators such as the wastewater service coverage or the production of bio-based fertilizers and hydrochar within the wastewater sector were analyzed. The results were used for the development of recommendations for improving the resources recovery monitoring framework in the wastewater sector and a proposal of a circularity indicator for a wastewater treatment plant highlighting new challenges for further researches and wastewater professionals.

Trace metal elements vaporization and phosphorus recovery during sewage sludge thermochemical treatment - A review

Phosphorus (P) plays essential roles in crops growth. Natural mineral sources of phosphate are non-renewable, overexploited and unevenly distributed worldwide, making P a strategic resource for agricultural systems. The search for sustainable ways to secure P supply for fertilizer production has therefore become a critical issue worldwide. Sewage sludge (SS) is an organic waste material considered as a key alternative source of P. Switzerland and the European Union are about to make it mandatory to recover P from SS or its treatment residues. Among the many technical options to achieve this objective, SS thermochemical treatments spiked with Cl-donors appear as a promising approach to recover P from SS and separate it from mineral pollutants such as trace metal elements (TME). The purpose of Cl-donor additives is to fix P within the mineral residues, possibly in bioavailable P species forms, while promoting TME vaporization by chlorination mechanisms. This review paper compares the various thermochemical treatments investigated worldwide over the past two decades. The influence of process conditions and Cl-donor nature is discussed. The presented results show that, except for nickel and chromium, most TME can be significantly vaporized during a high temperature treatment (over 900 °C) with Cl addition. In addition, the fixation rate and solubility of P is increased when a Cl-donor such as MgCl₂ is added.

Bloom of tetracycline resistance genes in mudflats following fertilization is attributed to the increases in the shared potential hosts between soil and organic fertilizers

A field experiment was carried out in mudflats adjacent to the Yellow Sea, China, amended with sewage sludge and vermicompost by one-time input at different rates to reveal the fates of tetracycline resistance genes (TRGs) and their potential hosts in the soils. Quantitative PCR results showed that soils added with either sludge or vermicompost had more abundant TRGs compared with the non-fertilized soil. This situation was more obvious in sludge fertilized soils especially at high application rates. Vermicompost exhibited a promising outlook for improvement of the mudflats. The abundances of intI1 in the non-fertilized soils were significantly higher than those in fertilizers and fertilized soils. The potential hosts for intI1 were not shared with other TRGs-contained hosts, indicating that intI1 had little effects on the dissemination of TRGs in the mudflats. Moreover, the exclusive hosts for TRGs in fertilizers were not higher than those in the non-fertilized soils, illustrating little effects of fertilization on the introduction of exogenous TRGs into soil. The shared hosts between soil and fertilizers were highest among four possible sources, contributing vastly to the bloom of TRGs following fertilization. It was also shown that different organic fertilizers caused distinct categories of shared potential hosts for TRGs. RDA analysis further indicated that the abundances of the shared potential hosts were affected by soil nutrients. These results suggested that the development of TRGs in soil following fertilization depended on the shared potential hosts with similar ecological niches between soil and fertilizers.

Improving the prediction of fertilizer phosphorus availability to plants with simple, but non-standardized extraction techniques

In the framework of the circular economy, new P fertilizers produced from diverse secondary raw materials are being developed using various technologies. Standard extraction methods (neutral ammonium citrate (NAC) and H₂O) provide limited information about the agronomic efficiency of these often heterogenous new products. Here, we compared these extractions with two alternative methods: 0.5 mol L^-1 NaHCO₃ and a sink extraction driven by phosphate adsorption onto ferrihydrite ("Iron Bag") on 79 recycled and mineral reference fertilizers. We compared their capacity to predict shoot biomass and P content of rye (S. cereale L.) grown in a greenhouse on three soils of contrasting pH with a subset of 42 fertilizers. The median extracted P (% of total P) was H₂O (1%) < NaHCO₃ (25%) < Iron Bag (67%) < NAC (85%). The NaHCO₃ extraction stood out as a cost-effective and reliable method to predict plant shoot biomass and P content (R² ranging between 0.65 and 0.86 in the slightly acidic and alkaline soil). Notwithstanding, the other methods provide complementary information for a more detailed characterization of how P solubility may be impacted by e.g. soil pH, granulation, or time. The implications of this work are therefore significant for fertilizer production, regulation, and use.

An examination of maximum legal application rates of dairy processing and associated STRUBIAS fertilising products in agriculture

The dairy industry produces vast quantities of dairy processing sludge (DPS), which can be processed further to develop second generation products such as struvite, biochars and ashes (collectively known as STRUBIAS). These bio-based fertilizers have heterogeneous nutrient and metal contents, resulting in a range of possible application rates. To avoid nutrient losses to water or bioaccumulation of metals in soil or crops, it is important that rates applied to land are safe and adhere to the maximum legal application rates similar to inorganic fertilizers. This study collected and analysed nutrient and metal content of all major DPS (n = 84) and DPS-derived STRUBIAS products (n = 10), and created an application calculator in MS Excel™ to provide guidance on maximum legal application rates for ryegrass and spring wheat across plant available phosphorus (P) deficient soil to P-excess soil. The sample analysis showed that raw DPS and DPS-derived STRUBIAS have high P contents ranging from 10.1 to 122 g kg^-1. Nitrogen (N) in DPS was high, whereas N concentrations decreased in thermo-chemical STRUBIAS products (chars and ash) due to the high temperatures used in their formation. The heavy metal content of DPS and DPS-derived STRUBIAS was significantly lower than the EU imposed limits. Using the calculator, application rates of DPS and DPS-derived STRUBIAS materials (dry weight) ranged from 0 to 4.0 tonnes ha^-1 y^-1 for ryegrass and 0-4.5 tonnes ha^-1 y^-1 for spring wheat. The estimated heavy metal ingestion to soil annually by the application of the DPS and DPS-derived STRUBIAS products was lower than the EU guideline on soil metal accumulation. The calculator is adaptable for any bio-based fertilizer, soil and crop type, and future work should continue to characterise and incorporate new DPS and DPS-derived STRUBIAS products into the database presented in this paper. In addition, safe application rates pertaining to other regulated pollutants or emerging contaminants that may be identified in these products should be included. The fertilizer replacement value of these products, taken from long-term field studies, should be factored into application rates.

Toxicity effects of size fractions of incinerated sewage sludge bottom ash on human cell lines

Sewage sludge bottom ash (SSBA) from the incineration plant used for the production of construction materials possibly possess heavy metals which might cause a negative impact on human health. Considering biosafety, we investigated the toxicity effects of 0.5-2 mm (aggregate substitute) and < 0.075 mm (cement substitute) in its solid and leachate form on human lung fibroblast cells (MRC-5) and human skin epidermal cells (HaCaT) on exposure through contact. MTS assay revealed the cellular responses of lung and skin cell lines to the leachates showing that the skin cells, which often interact with the external environment displayed better tolerance than the lung cells, whereas solid ash showed a concentration and size-dependent toxicity. Solid ash was found to downregulate the intracellular glutathione/superoxide dismutase activities and upregulate lactate dehydrogenase/lipid peroxidation activities thus inducing oxidative stress to the cell and subsequently resulting in the cell membrane leakage, destructive mitochondrial membrane potential (Δψm), apoptosis, and DNA damage, which is nearly 7-fold higher than the negative control. At a high concentration, DNA damage index of 1.09 and 1.29 was observed for the 0.5-2 mm sized ash leachate on skin cells and lung cells respectively, whereas for ash (<0.075 mm size) leachate, this fraction was 1.29 and 2.96, respectively. Overall, the ash leachate is found to be safer/biocompatible if they come in contact with humans as compared to SSBA in its solid form.

Effect of acetic acid on struvite precipitation: An exploration of product purity, morphology and reaction kinetics using central composite design

Phosphorus recovery has attracted increasing interest due to the potential depletion of phosphorus resources. One promising solution is to recover phosphorus via struvite precipitation from wastewater or other waste that is in rich of phosphate. However, product quality control during such process is always challenging due to the variation and complexity of wastewater compositions. For example, subcritical wet oxidation (SCWO) effluent is rich in phosphorus and nitrogen but contains a large amount of acetic acid, while its effect on struvite recovery is hardly known. Therefore, central composite design (CCD), considering pH, acetic acid level, Mg level and Ca level, was used to evaluate the effect of acetic acid on struvite purity, phosphorus removal, morphology and reaction kinetics. The experimental data were statistically analysed by analysis of variance (ANOVA) and principal components analysis (PCA). The results indicate that pH and Mg level have a significant impact on phosphorus removal (pH: p-value < 0.0001, Mg: p-value < 0.0001) and struvite purity (pH: p-value = 0.0410, Mg: p-value < 0.0001), Ca level only affects the struvite purity (p-value = 0.0333). The presence of acetic acid, within the studied range (8.77-34.53 mM), has a negligible effect on struvite morphology, phosphorus removal and reaction kinetics, but a slightly positive effect on struvite purity. Findings of this research would be beneficial to determine the feasibility of acetic acid-rich wastewater as a phosphorus source for struvite recovery.

From wastewater to fertilizer products: Alternative paths to mitigate phosphorus demand in European countries

Phosphorus (P) is a non-renewable resource, irreplaceable for life and food production, and currently considered a Critical Raw Material to the European Union (EU). Due to concerns about the rate of consumption and limited reserves in countries with sensitive geopolitical contexts, it is urgent to recover P from urban and industrial flows. Indeed, the municipal wastewater treatment plants (WWTP) are considered relevant sources with several hot spots, especially sewage sludge with estimated recovery efficiencies of >80%. The most promising recovery strategies are based on thermal treatments (e.g., incineration of sludge) following by wet-chemical or thermo-chemical leaching, precipitation, and adsorption. The direct application of sludge on soil is no longer a primary route for P reintegration in the value-chain for countries as Switzerland, Germany, and The Netherlands. In fact, Switzerland and Austria paved the way for implementing P recovery legislation, focusing on recovery from raw sewage sludge or ashes. Indeed, industrial technologies with sludge ash as input show high recovery efficiencies (Ashdec® and Leachphos® with 98 and 79%) and lower environmental impacts, whereas Pearl® technology has about 12% recovery efficiency with wastewater as input. After all, struvite emerges as the most recovered product with recent access to the internal market of EU fertilisers and similar growth performance compared to triple-super-phosphate. However, several studies leave open the possibility of introducing loaded adsorbents with P as soil amendments as a new alternative to conventional desorption. Briefly, P recovery should be a compromise between efficiency, environmental impacts, and economic revenues from the final products.

Systematic Review of Dairy Processing Sludge and Secondary STRUBIAS Products Used in Agriculture

Worldwide dairy processing plants produce high volumes of dairy processing sludge (DPS), which can be converted into secondary derivatives such as struvite, biochar and ash (collectively termed STRUBIAS). All of these products have high fertilizer equivalent values (FEV), but future certification as phosphorus (P)-fertilizers in the European Union will mean they need to adhere to new technical regulations for fertilizing materials i.e., content limits pertaining to heavy metals (Cd, Cu, Hg, Ni, Pb, and Zn), synthetic organic compounds and pathogens. This systematic review presents the current state of knowledge about these bio-based fertilizers and identifies knowledge gaps. In addition, a review and calculation of greenhouse gas emissions from a range of concept dairy sludge management and production systems for STRUBIAS products [i.e., biochar from pyrolysis and hydrochar from hydrothermal carbonization (HTC)] is presented. Results from the initial review showed that DPS composition depends on product type and treatment processes at a given processing plant, which leads to varied nutrient, heavy metal and carbon contents. These products are all typically high in nutrients and carbon, but low in heavy metals. Further work needs to concentrate on examining their pathogenic microorganism and emerging contaminant contents, in addition to conducting an economic assessment of production and end-user costs related to chemical fertilizer equivalents. With respect to STRUBIAS products, contaminants not present in the raw DPS may need further treatment before being land applied in agriculture e.g., heated producing ashes, hydrochar, or biochar. An examination of these products from an environmental perspective shows that their water quality footprint could be minimized using application rates based on P incorporation of these products into nutrient management planning and application by incorporation into the soil. Results from the concept system showed that elimination of methane emissions was possible, along with a reduction in nitrous oxide. Less carbon (C) is transferred to agricultural fields where DPS is processed into biochar and hydrochar, but due to high recalcitrance, the C in this form is retained much longer in the soil, and therefore STRUBIAS products represent a more stable and long-term option to increase soil C stocks and sequestration.

Transition to Circular Economy in the Fertilizer Sector—Analysis of Recommended Directions and End-Users’ Perception of Waste-Based Products in Poland

Circular economy (CE) is an economic model, in which raw materials remain in circulation as long as possible and the generation of waste is minimized. In the fertilizer sector, waste rich in nutrients should be directed to agriculture purposes. This paper presents an analysis of recommended directions for the use of nutrient-rich waste in fertilizer sector and an evaluation of possible interest in this kind of fertilizer by a selected group of end-users (nurseries). The scope of research includes the state-of-the-art analysis on circular aspects and recommended directions in the CE implementation in the fertilizer sector (with focus on sewage-based waste), and survey analysis on the potential interest of nurseries in the use of waste-based fertilizers in Poland. There are more and more recommendations for the use of waste for agriculture purposes at European and national levels. The waste-based products have to meet certain requirements in order to put such products on the marker. Nurserymen are interested in contributing to the process of transformation towards the CE model in Poland; however, they are not fully convinced due to a lack of experience in the use of waste-based products and a lack of social acceptance and health risk in this regard. Further actions to build the social acceptance of waste-based fertilizers, and the education of end-users themselves in their application is required.

Helminth eggs inactivation efficiency by sludge co-composting under arid climates

Primary sludge (PS) from the wastewater treatment plants is constituted of organic matter and other nutrients that could be reused as soil fertilizers. However, primary sludge amendment is hampered due to the presence of pathogens that could be responsible for several infections in various ways depending on the prevalence of the human sources. This study investigates helminth eggs (HE) removal and inactivation efficiency by co-composting. So, PS was mixed with date palm waste as a bulking material at a 1:1 volume ratio. The C/N ratio decreased from 25:1 to 12:1 and temperatures above 50ºC were observed since 33 days. The initial concentration of HE eggs in the PS is 135.9 per 10 g DR. Microscopic examination of the PS, according to the Bailenger method, showed the presence of nematodes and Cestodes represented by: Ascaris lombricoïde, Ancylostomes duodenale, Trichuris trichiura, Capilaria spp, Hymenolepis nana, Taenia saginata, and Ascaris lombricoïde. After 60 days of co-composting, the charge of parasites was reduced to 18 eggs per 10 g per DR. The viability of ascaris eggs was examined using a light microscope and the percentage of the embryonated eggs was determined. No viable eggs were observed in the final compost. The result corroborates the WHO guidelines for the application of the composted sewage sludge safely for agriculture purposes.

Land application of sewage sludge incinerator ash for phosphorus recovery: A review

Phosphorus (P) is essential for all living things and an integral part of food production. However, significant amounts of P are functionally lost when wastewater byproducts, such as biosolids or sewage sludge incinerator ash (SSA), are not beneficially reused. Around 20% of sewage sludge produced in the US is incinerated and nearly 25% of sewage sludge is incinerated in European Union member countries. SSA contains significant amounts of P (up to 14% total P) and other beneficial elements but is typically sent to landfills for disposal. However, SSA has also been explored as one method of capturing and redirecting P back into the food system. Research investigating SSA characterization, P availability, and contaminant concentrations and behavior in soil is required to understand the effects of SSA land application on soil chemical properties and crop production. Several approaches for recovering P from SSA have been investigated that consider these factors. Ultimately, the opportunity for land application of SSA depends on the individual characteristics of a given SSA, ex. total P and contaminant concentrations, and the requirements and regulations of the region where it is produced and applied. In this review, we address the history of P recovery from SSA and discuss research regarding characterization, contaminants, P availability, and land application of SSA.

Electrochemical Recovery of Phosphorus from Acidic Cheese Wastewater: Feasibility, Quality of Products, and Comparison with Chemical Precipitation

The recovery of phosphorus (P) from high-strength acidic waste streams with high salinity and organic loads is challenging. Here, we addressed this challenge with a recently developed electrochemical approach and compared it with the chemical precipitation method via NaOH dosing. The electrochemical process recovers nearly 90% of P (∼820 mg/L) from cheese wastewater in 48 h at 300 mA with an energy consumption of 64.7 kWh/kg of P. With chemical precipitation, >86% of P was removed by NaOH dosing with a normalized cost of 1.34-1.80 euros/kg of P. The increase in wastewater pH caused by NaOH dosing triggered the formation of calcium phosphate sludge instead of condensed solids. However, by electrochemical precipitation, the formed calcium phosphate is attached to the electrode, allowing the subsequent collection of solids from the electrode after treatment. The collected solids are characterized as amorphous calcium phosphate (ACP) at 200 mA or a precipitation pH of ≥9. Otherwise, they are a mixture of ACP and hydroxyapatite. The products have sufficient P content (≤14%), of which up to 85% was released within 30 min in 2% citric acid and a tiny amount of heavy metals compared to phosphate rocks. This study paves the way for applying electrochemical removal and recovery of phosphorus from acidic P-rich wastewater and offers a sustainable substitute for mined phosphorus.

Critical analytical parameters for faecal sludge characterisation informing the application of thermal treatment processes

Thermal processes for the treatment of faecal sludge such as pyrolysis or combustion offer complete destruction of pathogens, whilst allowing for energy and nutrient recovery. The development of such processes is currently constrained by a lack of knowledge on thermally relevant faecal sludge characteristics. This study investigated thirty faecal sludge samples from three sanitation technologies (ventilated improved pit latrines (VIP), urine diverting dry toilets (UD), septic tanks (ST)) and compared these by non-parametric statistical analysis. A focus was placed on parameters necessary for thermal process development and recoverable nutrient concentrations. The relevant characteristics ranged widely within technology groups. Calorific values and ash concentrations of 2.1-25.7 MJ/kg and 9.5-88.4% were observed for STs, of 9.2-13.9 MJ/kg and 40.9-61.5% for VIPs and of 3.9-18.1 MJ/kg and 18.8-81.3% for UDs. These two parameters show a strong linear inverse correlation and determine the minimum dewatering requirements from which a net energy recovery may be possible. Results suggest that more than 90% of samples can meet these requirements following commonly used dewatering technologies. A comparison across technologies provided strong evidence that the faecal sludge source significantly influences sludge composition, emphasized by higher median ratios of fixed carbon to volatile matter in VIPs (0.23) and UDs (0.23) compared to STs (0.15). The sanitation technology also influenced recoverable nutrient concentrations, with phosphorus and potassium concentrations generally ranging between 5.8-49.2 g/kg and 1.4-26.1 g/kg respectively. Compared to STs, median concentrations of phosphorus and potassium in VIPs were 3.4 and 3.8 times higher respectively, and 3.0 and 8.8 times higher in UDs. The findings highlight the importance of considering the faecal sludge source in the development of thermal treatment processes. This study provides critical knowledge to further develop such processes through modelling, experimental and scaled approaches.

Incinerated Sewage Sludge Bottom Ash- Chemical processing, Leaching patterns and Toxicity testing

Sewage sludge bottom ash, which is the major fraction obtained from the incineration of sewage sludge was treated with various organic and inorganic acids for heavy metal removal, along with a comparative phosphate treatment for heavy metal fixation. Malonic acid, an organic acid, was found to remove heavy metals better as compared to nitric acid, a strong inorganic acid. The acid treated samples were further examined for heavy metal leaching, followed by marine toxicity/abnormality testing of the leachates, where acid treated and phosphate treated ash leachate displayed higher (with malonic acid proving to be most toxic) and similar toxicity profiles as compared to raw ash leachate respectively. Raw ash was tested for its leaching patterns at different liquid/solid ratios(L/S = 5 and 10), salinities and time points (24, 48 and 72 h), where the leaching was found to saturate at L/S = 5 and at 24 h with varied salinity effecting the leaching insignificantly. When raw ash was benchmarked against concrete sand for marine toxicity, a material commonly used for land reclamation, acute toxicity patterns were found to be mostly similar except in case of the sea urchin embryonic assay, where toxicity was detected, indicating the sensitivity of the assay to residual levels of heavy metals. The raw ash was also tested against human cell lines where it displayed size and dose-dependent toxicity. To enable the use of ash for environment applications such as coastal reclamation, appropriate treatments are required to minimize leaching of potential harmful contaminants and this study demonstrates the importance of post-treatment of ash on its subsequent toxicity to organisms.

Experimental Parameter Study on Synthesis Gas Production by Steam-Oxygen Fluidized Bed Gasification of Sewage Sludge

The conversion of biogenic residues to fuels and chemicals via gasification and synthesis processes is a promising pathway to replace fossil carbon. In this study, the focus is set on sewage sludge gasification for syngas production. Experiments were carried out in a 20 kW fuel input bubbling fluidized bed facility with steam and oxygen as gasification agent. In-situ produced sewage sludge ash was used as bed material. The sensitivity of the key operation parameters gasifier temperature, oxygen ratio, steam to carbon ratio, and the space velocity on the syngas composition (H2, CO, CO2, CH4, CxHy, H2S, COS, NH3, and tars) was determined. The results show that the produced syngas has high H2 and CO concentrations of up to 0.37 m3 m−3 and 0.18 m3 m−3, respectively, and is thus suitable for synthesis of fuels and chemicals. By adjusting the steam to carbon ratio, the syngas’ H2 to CO ratio can be purposely tailored by the water gas shift reaction for various synthesis products, e.g., synthetic natural gas (H2/CO = 3) or Fischer–Tropsch products (H2/CO = 2). Also, the composition and yields of fly ash and bed ash are presented. Through the gasification process, the cadmium and mercury contents of the bed ash were drastically reduced. The ash is suitable as secondary raw material for phosphorous or phosphate fertilizer production. Overall, a broad database was generated that can be used for process simulation and process design.

Phosphorus recovery and recycling – closing the loop

The natural phosphorus cycle has been disrupted by human activity, which necessitates the development of new methods for the sustainable production of phosphorus compounds, and efficient recovery and recycling schemes.

Agricultural use and pH correction of anaerobic sewage sludge with acid pH

Agricultural use is the main way of recycling sewage sludge. Besides providing nutrients and organic matter to crops and soils, it is an important alternative for recycling this residue. However, problems during the sewage treatment process may generate sludge batches with an acidic pH. Thus, it is essential to understand the consequences of using such sludge on soils and plants, and to explore ways to overcome this limitation. The objective of this study was to evaluate addition rates of anaerobic sewage sludge (ASS) with acidic compositions on the soil fertility and performance of lettuce plants. Additionally, a methodology for pH correction of ASS with acidic pH is proposed. An agronomic experiment was conducted in a greenhouse using seven addition rates of ASS (0.0, 0.25, 0.5, 1, 2, 4 and 8 g kg^-1 in dry basis), treated with an additional step of disinfection (solarization), and applied in an Albaqualf soil cultivated with lettuce (Lactuca sativa). Soil and leaf chemical composition, as well as chlorophyll index and the dry matter of lettuce leaves were evaluated. Failures during the acidogenesis phase of the anaerobic digestion process were probably the cause of ASS acidification. Although this ASS increased soil fertility indicators and plant dry matter, it significantly reduced soil pH, thereby requiring a complementary assay to correct its pH up to 6.0, which was achieved through liming. Anaerobic sewage sludges with an acidic pH can be effectively used in agriculture after being dried and disinfected through solarization, followed by pH correction, avoiding negative impacts on soil chemical attributes and plant response.

Thermochemical Treatment of Sewage Sludge Ash (SSA)—Potential and Perspective in Poland

Phosphorus (P) recovery from sewage sludge ash (SSA) is one of the most promising approaches of phosphate rock substitution in mineral fertilizers and might be a sustainable way to secure supply of this raw material in the future. In the current investigation, the process of thermochemical treatment of SSA was applied to SSA coming from selected mono-incineration plants of municipal sewage sludge in Poland (Cracow, Gdansk, Gdynia, Lodz, Kielce and Szczecin). The Polish SSA was thermochemically converted in the presence of sodium (Na) additives and a reducing agent (dried sewage sludge) to obtain secondary raw materials for the production of marketable P fertilizers. The process had a positive impact on the bioavailability of phosphorus and reduced the content of heavy metals in the obtained products. The P solubility in neutral ammonium citrate, an indicator of its bioavailability, was significantly raised from 19.7–45.7% in the raw ashes and 76.5–100% in the thermochemically treated SSA. The content of nutrients in the recyclates was in the range of 15.7–19.2% P2O5, 10.8–14.2% CaO, 3.5–5.4% Na2O, 2.6–3.6% MgO and 0.9–1.3% K2O. The produced fertilizer raw materials meet the Polish norms for trace elements covered by the legislation: the content of lead was in the range 10.2–73.1 mg/kg, arsenic 4.8–22.7 mg/kg, cadmium 0.9–2.8 mg/kg and mercury <0.05 mg/kg. Thus, these products could be potentially directly used for fertilizer production. This work also includes an analysis of the possibilities of using ashes for fertilizer purposes in Poland, based on the assumptions indicated in the adopted strategic and planning documents regarding waste management and fertilizer production.

Inventory of Polish municipal sewage sludge ash (SSA) - Mass flows, chemical composition, and phosphorus recovery potential

This paper presents an inventory of sewage sludge ashes (SSA) generated in the mono-incineration plants for municipal sewage sludge in Poland. This research focused on the detailed study of mass flows, chemical composition, and phosphorus recovery potential. There are currently 11 sludge mono-incineration plants operated with a total capacity of 160,300 Mg dry weight (d.w.) of sludge annually. Recently, a significant increase in the amount of SSA generated in these plants has been observed, reaching 26,756 Mg in 2018. Chemical composition of SSA showed significant amounts of the main nutrients: calcium (~14%), phosphorus (~13%), magnesium (~3%), and potassium (~1%). Additional main elements were iron (~14.5%), silicon (~13%), and aluminium (~6%). The main trace elements in the SSA were zinc (~3750 mg/kg) and copper (~899 mg/kg). Pollutants, according to fertilizer regulations of different countries, present in Polish SSA were chromium (~703 mg/kg), nickel (~260 mg/kg), lead (~94 mg/kg), and cadmium (~9 mg/kg). The radionuclides, uranium, and thorium often present in higher amounts in commercial phosphate rock-based fertilizers, were only detected in SSA at low levels of 4-9 mg/kg and 2-3 mg/kg, respectively. Theoretical phosphorus recovery potential from the SSA (from plants in Cracow, Lodz, Gdansk, Gdynia, Szczecin, and Kielce) was estimated at 1613.8 Mg, of which 33.9% is bioavailable. Currently, in Poland, the recommended approach is the production of fertilizers as a result of the extraction of phosphorus from the SSA with its use in the production of secondary mineral fertilizers. Further research in this area is required considering Polish conditions and legislation.

Study of Evolution of Microbiological Properties in Sewage Sludge-Amended Soils: A Pilot Experience

Large amounts of sewage sludge are generated in urban wastewater treatment plants and used as fertilizer in agriculture due to its characteristics. They can contain contaminants such as heavy metals and pathogenic microorganisms. The objective of this research work is to study, in real conditions, the evolution of microbial concentration in agricultural soils fertilized by biologically treated sewage sludge. The sludge (6.25 tons Ha⁻¹) was applied in two agricultural soils with different textures and crops. A microbiological (total coliforms, Escherichia coli, Staphylococcus aureus, Enterococcus sp., Pseudomonas sp., Salmonella sp. and total mesophylls) and physical-chemical characterization of the sludge, soils and irrigation water were carried out. The evolution of these parameters during sowing, growth and harvesting of crops was studied. Initially, sewage sludge had a higher concentration of microorganisms than soils. Irrigation water also contained microorganisms, fewer than sewage sludge amendment but not negligible. After amendment, there were no differences in the microbiological evolution in the two types of soil. In general, bacterial concentrations after crop harvest were lower than bacterial concentrations detected before sewage sludge amendment. Consequently, the application of sludge from water treatment processes did not worsen the microbiological quality of agricultural soil in this study at real conditions.

Using Food Waste in Organic Fertilizer: Modelling Biogenic Carbon Sequestration with Associated Nutrient and Micropollutant Loads

What are the effects, measured as flows of biogenic carbon, plant nutrients, and pollutants, of moving organic waste up the waste hierarchy? We present a case study of Denmark, where most of the organic fraction of household waste (OFHW) is incinerated, with ongoing efforts to increase bio-waste recycling. In this study, one-third of the OFHW produced in North Zealand, Denmark, is diverted away from incineration, according to the Danish Waste Resource Plan 2013–2018. Co-digestion of OFHW, and digestate application on agricultural soil, utilizes biogenic carbon, first for energy conversion, and the remainder for long-term soil sequestration, with additional benefits for plant nutrient composition by increasing the N:P ratio in the digestate. We show a dynamic model of the biogenic carbon flows in a mix of OFHW co-digested with livestock manure and sewage sludge, addressing the contribution of OFHW to long-term carbon sequestration compared to other agricultural residues and bio-wastes over a time span of 100 years. In addition, we trace the associated annual nutrient and cadmium loads to the topsoil. At constant annual input rates and management practices, a diversion of 33% of OFHW would result in an increased organic carbon build-up of approximately 4% over the current amounts applied. The addition of OFHW, moreover, beneficially adjusts the N:P ratio of the digestate mix upwards, albeit without reaching an ideally high ratio by that measure alone. Cd loads from OFHW remain well below regulatory limits.

The Ecological Risk Assessment and the Chemical Speciation of Heavy Metals in Ash after the Incineration of Municipal Sewage Sludge

Popular incineration of sewage sludge results in the increase in heavy metals content in ash. The knowledge of the total content of heavy metals in sewage sludge ash does not demonstrate a potential hazard. The toxicity of heavy metals in great measure depends on the form of their occurrence. The prevailing norms do not require the ecological risk assessment of the environmental burden with heavy metals for the choice of the method of the utilization of sewage sludge ash. The paper presents the research results on the mobility of heavy metals in sewage sludge ash after its incineration. The geo-accumulation index (IGAI), the potential ecological risk index (PERI) and the risk assessment code (RAC) were used for the evaluation of the potential soil contamination with heavy metals. The authors also suggested a new formula, which took into consideration more factors influencing the risk of the contamination of a water-soil environment with heavy metals—the water and soil environment risk index (WSERI). The calculated indices for sewage sludge ash indicate the risk of soil contamination with heavy metals.