Potassium ferrate followed by alkali-stripping treatment to achieve short-chain fatty acids and nitrogen recovery from waste activated sludge

Precise determination of the total nitrogen content in activated sludge by ultrasonic pre-treatment assisted wet method

The study proposed a method for determining total nitrogen (TN) content in activated sludge by ultrasound pre-treatment assisted wet method. Based on the single-factor experiment, with the TN content as the response value, the response surface methodology was employed to examine the individual and interactive effects of three factors: the dilution multiple of the sludge mixture, ultrasonic time, and ultrasonic power. At the same time, the physico-chemical parameters and the digestion-oxidation parameters were optimised. The results indicated that the optimal parameters were as follows; sludge dilution multiple of 225 times, stirring rate of 400 r/min, ultrasonic time of 22 minutes, ultrasonic power of 720 W, and optimal added volume of potassium persulfate at 8 mL with a digestion time of 40 minutes. The relative standard deviation (RSD) for the parallel determination of TN in sludge samples using ultrasonic pre-treatment assisted wet method was ≤2.77%, with a spike recovery rate of 98.49-101.43%. The method, ultrasonic pre-treatment assisted wet method to determine TN concentration in activated sludge, was simpler to operate, more accurate.

Roles of quorum-sensing molecules in methane production from anaerobic digestion aided by biochar

Biochar has been used to enhance methane generation from anaerobic digestion through establishing direct interspecific electron transfer between microorganisms. However, the microbial communication is still inadequate, thereby limiting further methane production improvement contributed by biochar. This study investigated the roles of quorum-sensing molecules, acylated homoserine lactone (AHL), in anaerobic digestion of waste activated sludge aided by biochar. Results showed that the co-addition of separated biochar and AHL achieved best methane production performance, with the maximal methane yield of 154.7 mL/g volatile suspended solids, which increased by 51.9%, 47.2%, 17.9%, and 39.4% respectively compared to that of control, AHL-loaded biochar, sole AHL, and sole biochar groups. The reason was that the co-addition of separated biochar and AHL promoted the stages of hydrolysis and acidification, promoting the conversion of organic matters and short-chain fatty acids, and optimizing the accumulation of acetate acid. Moreover, the methanogenesis stage also performed best among experimental groups. Correspondingly, the highest activities of electron transfer and coenzyme F420 were obtained, with increase ratios of 33.2% and 27.2% respectively compared to that of control. Furthermore, biochar did more significant effects on the evolution of microbial communities than AHL, and the direct interspecific electron transfer between fermentative bacteria and methanogens were possibly promoted.

Biochar derived from alkali-treated sludge residue regulates anaerobic digestion: Enhancement performance and potential mechanisms

Biochar produced from bio-wastes has been widely used to promote the performance of anaerobic digestion. Waste activated sludge (WAS) is considered as a kind of popular precursor for biochar preparation, but the abundant resources in WAS were neglected previously. In this study, the roles of biochar prepared from raw, pretreated, and fermented sludge on anaerobic digestion were investigated. That is, parts of carbon sources and nutrients like polysaccharides, proteins, and phosphorus were firstly recovered after sludge pretreatment or fermentation, and then the sludge residuals were used as raw material to prepare biochar. The methane yield improved by 22.1% with adding the biochar (AK-BC) prepared by sludge residual obtained from alkaline pretreatment. Mechanism study suggested that the characteristics of AK-BC like specific surface area and defect levels were updated. Then, the conversion performance of intermediate metabolites and electro-activities of extracellular polymeric substances were up-regulated. As a result, the activity of electron transfer was increased with the presence of AK-BC, with increase ratio of 21.4%. In addition, the electroactive microorganisms like Anaerolineaceae and Methanosaeta were enriched with the presence of AK-BC, and the potential direct interspecies electron transfer was possibly established. Moreover, both aceticlastic and CO2-reducing methanogenesis pathways were improved by up-regulating related enzymes. Therefore, the proposed strategy can not only obtain preferred biochar but also recover abundant resources like carbon source, nutrients, and bioenergy.

Contribution identification of hydrolyzed products of potassium ferrate on promoting short-chain fatty acids production from waste activated sludge

Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

Synergistic effects and mechanisms of plasma coupled with peracetic acid in enhancing short-chain fatty acid production from sludge: Motivation of reactive species and metabolic tuning of microbial communities

Suitable waste activated sludge (WAS) pretreatments that boost short-chain fatty acid (SCFA) production from anaerobic fermentation are essential for carbon emission reduction and sludge resource utilization. This study established an efficient WAS pretreatment process combining atmospheric pressure plasma jet (APPJ) with peracetic acid (PAA). The maximum SCFA production (6.5-fold that of the control) largely increased under the optimal conditions (PAA dosage = 25 mg/g VSS (volatile suspended solids), energy consumption = 20.9 kWh/m3). APPJ/PAA pretreatment enhanced the production of multiple reactive species (including OH, CH3C(O)O, 1O2, ONOO-, O2-, and eaq-) and strengthened the effects of H2O2, heat, and light. This synergistically solubilized WAS and released organic substrates for SCFA-producing microbes. In addition, the enrichment of SCFA-producing bacteria and the decrease in SCFA-consuming bacteria favored SCFA accumulation. The key genes encoding for the main substrate metabolism and SCFA production in the metabolic pathway of fermentation were also enhanced.

Insights into response mechanism of anaerobic digestion of waste activated sludge to particle sizes of zeolite

Anaerobic digestion has been proved as one promising strategy to simultaneously achieve resource recovery and environmental pollution control for biosolid treatment, and adding exogenous materials is a potential alternative to promote the above process. This study investigated response mechanisms of anaerobic digestion of waste activated sludge (WAS) to particle sizes of zeolite. Results showed that the methane production reached 186.75 ± 7.62 mL/g volatile suspended solids (VSS) with zeolite of the particle size of 0.2-0.5 mm and the additive dosage of 0.1 g/g VSS, which increased by 22% compared to that in control. Mechanism study revealed that zeolite could improve hydrolysis, acidification, and methanogenesis stages. Rapid consumption rates of soluble polysaccharides and proteins were observed, correspondingly, the accumulation of SCFAs were enhanced, and the compositions of SCFAs were optimized. Moreover, the activities of F420 increased by 28% with zeolite, and the syntrophic metabolism between bacteria and methanogens were promoted.

Magnetite enhancing sludge anaerobic fermentation to improve wastewater biological nitrogen removal: Pilot-scale verification

Alkaline anaerobic fermentation for acids production has been considered as an effective method to recover resources from waste activated sludge, and magnetite could improve the quality of fermentation liquid. Here we have constructed a pilot-scale sludge alkaline anaerobic fermentation process enhanced by magnetite to produce short chain fatty acids (SCFAs), and used them as external carbon sources to improve the biological nitrogen removal of municipal sewage. Results showed that the addition of magnetite could significantly increase the production of SCFAs. The average concentration of SCFAs in fermentation liquid reached 3718.6 ± 101.5 mg COD/L and the average concentration of acetic acid reached 2368.8 ± 132.1 mg COD/L. The fermentation liquid enhanced by magnetite were used in the mainstream A²O process, and the TN removal efficiency increased from 48.0% ± 5.4%-62.2% ± 6.6%. The main reason is that the fermentation liquid is conducive to the succession of microbial community in the denitrification process, increasing the abundance of denitrification functional bacteria and realizing the enhancement of denitrification process. Besides, magnetite can promote the activity of enzyme to enhance biological nitrogen removal. Finally, the economic analysis showed that magnetite enhancing sludge anaerobic fermentation was economically and technically feasible to promote biological nitrogen removal of municipal sewage.

Role of potassium ferrate in anaerobic digestion of waste activated sludge: Phenotypes and genotypes

The specific effects of potassium ferrate (PF) on acid and methane production in anaerobic digestion need further exploration. This study comprehensively investigated the role of PF in organic matter conversion in waste activated sludge (WAS) digestion. Due to the high pH produced by PF self-decomposition, the hydrolysis of organic matter was promoted, whereas the methanogenesis was inhibited. PF could further directly oxidize protein and polysaccharides released by hydrolysis to produce volatile fatty acids (VFAs) and involve in the transformation of ammonia nitrogen. PF could induce the enrichment of functional genes related to fermentation pathways and lessen those related to methanogenesis, and the phylum resistant to PF oxidation and the strains capable of producing VFAs were enriched, resulting in VFAs accumulation. This study analyzed the participation way of PF in anaerobic digestion and provided a theoretical basis for the application of PF in promoting VFAs recovery from sludge digestion.

Application and improvement methods of sludge alkaline fermentation liquid as a carbon source for biological nutrient removal: A review

Alkaline fermentation can reduce the amount of waste activated sludge and prepare sludge alkaline fermentation liquid (SAFL) rich in short-chain fatty acids (SCFAs), which can be used as a high-quality carbon source for the biological nutrient removal (BNR) process. This review compiles the production method of SAFL and the progress of its application as a BNR carbon source. Compared with traditional carbon sources, SAFL has the advantages of higher efficiency and economy, and different operating conditions can influence the yield and structure of SCFAs in SAFL. SAFL can significantly improve the nutrient removal efficiency of the BNR process. Taking SAFL as the internal carbon source of BNR can simultaneously solve the problem of carbon source shortage and sludge treatment difficulties in wastewater treatment plants, and further reduce the operating cost. However, the alkaline fermentation process results in many refractory organics, ammonia and phosphate in SAFL, which reduces the availability of SAFL as a carbon source. Purifying SCFAs by removing nitrogen and phosphorus, directly extracting SCFAs, or increasing the amount of SCFAs in SAFL by co-fermentation or combining with other pretreatment methods, etc., are effective measures to improve the availability of SAFL.

Steel slag assists potassium ferrate to improve SCFAs production from anaerobic sludge fermentation

Potassium ferrate (PF) pretreatment in anaerobic sludge and its potential influence mechanisms have received widely attention. This study investigated the coupling effect of PF loading on steel slag (SS) on excess sludge anaerobic fermentation. Results showed that SS loading increase the treatment performance of PF on short chain volatile fatty acids (SCFAs) production from anaerobic fermented sludge. It was showed that the modified PF loaded SS (MPF-SS) promoted the dissolution and release of organic substrates from intracellular to extracellular. Further exploration showed the promotion of PF and MPF-SS exposure to acid production microorganisms was much more than that to acid consumption microorganisms. MPF-SS addition can also effectively reinforce the carbohydrate transport, amino acid metabolism and the key enhanced genes associated with fatty acid biosynthesis pathways. This study fills the knowledge gap about modified PF on sludge treatment and also expands a new perspective for its application for sludge resource recovery.

Calcium peroxide and freezing co-pretreatment enhancing short-chain fatty acids production from waste activated sludge towards carbon-neutral sludge treatment

Short-chain fatty acids (SCFAs) recovery through anaerobic fermentation is a promising technology to achieve carbon-neutral in waste activated sludge (WAS) management. After 0.15 g CaO₂/g volatile suspended solids (VSS) addition and three-cycle freezing co-pretreatments, the maximal SCFAs production of 438.5 mg COD/g VSS was achieved within 4 days fermentation, and more than 70 % of SCFAs was composed of acetate and propionate, which achieved a higher level than reported in previous studies. Mechanism explorations elucidated that co-pretreatment triggered sludge solubilization, promoting the release of biodegradable organics, providing more biodegradable substrates for SCFAs generation. Further microbial community analysis indicated that the abundances of hydrolytic microorganisms and acidogens were enriched, whereas methanogens were inhibited. Besides, environmental analysis suggested that co-pretreatment could achieve carbon reduction benefits of 0.116-0.291 ton CO₂/ton WAS, demonstrating its huge carbon-neutral potential benefits.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Sewage sludge treatment technology under the requirement of carbon neutrality: Recent progress and perspectives

In the context of climate policies that advocate carbon neutrality, carbon emission reduction provides a new restriction in evaluating the waste activated sludge (WAS) treatment technologies and procedures. This review provides an overview of current researches and development efforts in WAS treatment, focusing on the dual attributes of WAS as contaminants and resources. Firstly, the improved technical requirements posed by heavy metals, micro(nano) plastics, or other emerging plastics in WAS are studied. Furthermore, in terms of carbon emission reduction, the applications and limitations of widely deployed WAS treatment technologies are discussed. Based on carbon neutrality requirements, the anaerobic co-digestion and co-pyrolysis technologies are comprehensively discussed from the views of pollutants removing efficiencies, enhancement methods, carbon emissions, and resource recovery. Finally, a workable new route for WAS treatment is proposed for future technological advancement and engineering innovation.