Post-treatment of biogas digestate – An evaluation of ammonium recovery, energy use and sanitation

Biogas digestate as a sustainable phytosterol source for biotechnological cascade valorization

Every year, several million tonnes of anaerobic digestate are produced worldwide as a by-product of the biogas industry, most of which is applied as agricultural fertilizer. However, in the context of a circular bioeconomy, more sustainable uses of residual digestate biomass would be desirable. This study investigates the fate of the sterol lipids β-sitosterol and cholesterol from the feedstocks to the final digestates of three agricultural and one biowaste biogas plants to assess if sterols are degraded during anaerobic digestion or if they remain in the digestate, which could provide a novel opportunity for digestate cascade valorization. Gas chromatographic analyses showed that feedstock sterols were not degraded during anaerobic digestion, resulting in their accumulation in the digestates to up to 0.15% of the dry weight. The highest concentrations of around 1440 mg β-sitosterol and 185 mg cholesterol per kg dry weight were found in liquid digestate fractions, suggesting partial sterol solubilization. Methanogenic batch cultures spiked with β-sitosterol, cholesterol, testosterone and β-oestradiol confirmed that steroids persist during anaerobic digestion. Mycobacterium neoaurum was able to transform digestate sterols quantitatively into androstadienedione, a platform chemical for steroid hormones, without prior sterol extraction or purification. These results suggest that digestate from agricultural and municipal biowaste is an untapped resource for natural sterols for biotechnological applications, providing a new strategy for digestate cascade valorization beyond land application.

Design of a Centralized Bioenergy Unit at Comarca Lagunera, Mexico: Modeling Strategy to Optimize Bioenergy Production and Reduce Methane Emissions

A centralized bioenergy unit was simulated, focusing on optimizing the manure transport chain, installing a centralized biogas plant, operation costs of the process, biogas upgrading, organic fertilizer production, and economic analyses. Comarca Lagunera from northeast Mexico was chosen as a study zone due to the existing number of dairy farms and livestock population (64,000 cattle heads). Two scenarios were analyzed: The first centralized scenario consisted of selecting one unique location for the anaerobic digesters for the 16 farms; the second decentralized scenario consisted of distributing the anaerobic digesters in three locations. Optimal locations were determined using mathematical modeling. The bioenergy unit was designed to process 1600 t/day of dairy manure. Results indicated that biomethane production was a more profitable option than generating electricity with non-purified methane. The amount of biomethane production was 58,756 m3/day. Economic analysis for centralized bioenergy unit scenario showed a net production cost of USD $0.80 per kg of biomethane with a profit margin of 14.4% within 10.7 years. The decentralized bioenergy unit scenario showed a net production cost of USD $0.80 per kg of biomethane with a profit of 12.9% within 11.4 years. This study demonstrated the techno-economical and environmental feasibility for centralized and decentralized bioenergy units.

Investigation of nitrogen loss during laboratory scale fixed-bed drying of digestate

In addition to the drying kinetics of digestate, a comprehensive knowledge is necessary on nitrogen loss and ammonia emissions in order to appropriately develop and design a dryer. Otherwise, these phenomena would result in environmental concerns and reduce the nutrient value of digestate. Expectedly, drying parameters considerably affect drying time. In this study, drying experiments were conducted at a laboratory scale fixed-bed dryer. The drying experiments were performed on dewatered digestate that mainly consisted of grass and maize silage (initial moisture content 76.4% w.b., 0.026 kg N kg^-1 DM). The drying air temperatures were adjusted to 60, 70 and 80 °C, the airflow rate varied between 150 and 250 m³ h^-1, respectively. As the results have shown, the total nitrogen was reduced by 29 to 42% during drying. Almost all ammonium nitrogen was lost (92%). No relationship between nitrogen loss and drying air temperature or airflow rate could be proved. These results will be used as a basis for further investigation of continuous digestate drying at a semi-technical scale.

Optimized pre-treatment of high strength food waste digestate by high content aluminum-nanocluster based magnetic coagulation

Coagulation-based pre-treatment efficiency of high strength digestate of food waste (HSDFW) anaerobic digestion is negated by organic ligand-catalyzed decomposition of coagulants. In this study, an efficient HSDFW pre-treatment method, magnetic seeds (MS) coagulation, was employed by using highly stable Keggin Al₃₀ nanocluster (PAC₃₀), MS and polyacrylamide (PAM), and its operation was optimized by evaluating the performance of removing turbidity, total suspended solids (TSS), chemical oxygen demand (COD), and total phosphorous (TP) phosphate. Results showed that at the optimum dosage of 4.82 g/L, PAC₃₀ demonstrated excellent removals as high as 98.93% ± 0.1% of turbidity, 98.04% ± 0.1% of TSS, 58.28% ± 0.3% of total COD, 99.98% ± 0.01% of TP and 99.50% ± 0.01% of dissolved phosphate, respectively. Apparent molecular weight (AMW) and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy analyses demonstrated more efficient removal of dissolved organic matter (DOM), particularly non-biodegradable and hydrophobic components by PAC₃₀ than commercial coagulant. The sedimentation was much improved from 40 min by coagulation/flocculation to about 5 min settling by MS coagulation. The PAC₃₀ based magnetic coagulation (MC) presents theoretical guidance on a cost-effective and much less footprint pre-treatment alternative for high strength wastewater.

Biogas Plant Exploitation in a Middle-Sized Dairy Farm in Poland: Energetic and Economic Aspects

Although cow manure is a valuable natural fertilizer, it is also a source of extreme greenhouse gas emissions, mainly methane. For this reason, this study aims to determine the impact of investments in a biogas plant on the energy and economic aspects of the operation of a dairy farm. A farm with a breeding size of 600 livestock units (LSU) was adopted for the analysis. In order to reach the paper’s aim, the analysis of two different scenarios of dairy farm functioning (conventional–only milk production, and modern–with biogas plant exploitation) was conducted. The analysis showed that the investment in biogas plant operations at a dairy farm and in using cow manure as one of the main substrates is a more profitable scenario compared to traditional dairy farming. Taking into account the actual Polish subsidies for electricity produced by small biogas plants, the scenario with a functioning biogas plant with a capacity of 500 kW brings €332,000/a more profit compared to the conventional scenario, even when taking into account additional costs, including the purchase of straw to ensure a continuous operation of the installation. Besides, in the traditional scenario, building a biogas plant allows for an almost complete reduction of greenhouse gas emissions during manure storage.

Towards a bio-based circular economy in organic waste management and wastewater treatment - The Polish perspective

Bio-based solutions are expected to ensure technological circularity in priority areas such as agriculture, biotechnology, ecology, green industry or energy. Although Poland, unlike the other EU member states, has not yet adopted a precise political strategy to promote bioeconomy, it has taken several actions to enable smart, sustainable and inclusive growth. This goal can be achieved by developing selected bioeconomy-related areas such as the biogas industry together with novel technologies implemented to optimize treatment of municipal sewage and management of organic solid waste. Here, the relatively strong status of the Polish biogas sector is presented. The widely used practice of sewage sludge biomethanation has led to construction of numerous complex installations combining biological wastewater treatment plants with anaerobic digesters. Based on physico-chemical processing of biostabilized sludges, a novel method for efficient granulated soil fertilizer production is elaborated, in line with the concept of circular economy and the notion of "waste-to-product". It is also shown that anaerobic fermentation of sewage sludges can be optimized by co-digestion with properly selected co-substrates to increase bioprocess yield and improve the resultant digestate fertilizer quality. The problem of post-fermentation eutrophic sludge liquors, environmentally hazardous waste effluents requiring proper treatment prior to discharge or field application, is addressed. Attempts to optimize biological treatment of digestate liquors with complex microbial consortia are presented. The Polish innovations described show that the "zero waste" path in circular bioeconomy may bring advantageous results in terms of transformation of waste materials into commercial, added-value products together with recovery of water resources.

Self-sustaining treatment as a novel alternative for the stabilization of anaerobic digestate

Self-sustaining smouldering combustion (SSS) is a technology based on the flameless oxidation of an organic substrate and limited by the rate at which oxygen is diffused to the surface of the substrate. This work aims to evaluate the SSS combustion as a treatment process for the stabilization of anaerobic digestate, determining the limits of operational conditions, (moisture content (MC), air flux) that allow for a self-sustaining process. Maximum possible MC was found at 82 wt% with Darcy air flux of 50 cm/s. The digestate destruction rate (kg/(h·m2), and the addition of sand as an inert solid, to enhance the oxygen diffusion, were also investigated. A sand/substrate mass ratio of 1 allowed for SSS at 85 wt% MC, but decreased the digestate destruction rate. The average composition of the emitted gases showed ca. 25% CO and 10% H₂, whereas the analysis of the ashes showed almost complete digestate inertization.

Closing the food waste loop: Food waste anaerobic digestate as fertilizer for the cultivation of the leafy vegetable, xiao bai cai (Brassica rapa)

The increasing world population necessitates the production of larger amounts of food in a safe and environmentally sustainable manner, while concomitantly managing an increasing amount of food waste similarly. These needs can theoretically be met by the recycling of the nutrients in food waste via anaerobic digestion, which also produces renewable energy. This hypothesis is proven by the growing of a commonly consumed leafy vegetable, xiao bai cai (Brassica rapa), by the addition of food waste anaerobic digestate in place of commercial fertilizer. Different concentrations of the digestate were tested, as well as different heat treatments to simulate hygienization, and the results for most part (aerial fresh weight, dry weight, chlorophyll content) are not significantly different from growth utilizing commercial inorganic 15:15:15 NPK fertilizer. Microbial analysis of the growth media was also carried out to explicate digestate effects and to show that some common foodborne disease pathogens were not detected.

Using Digestate and Biochar as Fertilizers to Improve Processing Tomato Production Sustainability

The principal goal of the organic farming system (OFS) is to develop enterprises that are sustainable and harmonious with the environment. Unfortunately, the OFS yields fewer products per land than the non-organic farming system in many agricultural products. The objective of our study was to assess the effects of digestate and biochar fertilizers on yield and fruit quality of processing tomato produced under the OFS. The experiment was carried out in Po Valley, during the 2017 and 2018 growing seasons. Liquid digestate (LD), LD + biochar (LD + BC) and pelleted digestate (PD) were evaluated and compared to biochar (BC) application and unfertilized control. The results showed that plants fertilized with LD + BC recorded the maximum marketable yield (72 t ha−1), followed by BC (67 t ha−1), PD (64 t ha−1) and LD (59 t ha−1); while the lowest production (47 t ha−1) was recorded in unfertilized plants. Over the two cropping seasons, LD + BC, BC, PD, and LD, increased fruit number per plant (+15%), fruit weight (+24%), Brix t ha−1 (+41%) and reduced Bostwick index (−16%), if compared to the untreated control. Considering the overall agronomic performances, digestate and biochar can be useful options for increasing yield and quality of processing tomato production in the OFS. Hence, these fertilizers can be assessed in future research both on other crops and farming systems.

Ammonium Sulphate from a Bio-Refinery System as a Fertilizer—Agronomic and Economic Effectiveness on the Farm Scale

This paper presents the results of a pot experiment aimed at the assessment of the agronomic and economic effectiveness of ammonium sulphate from an agro bio-refinery (Bio-AS). The Bio-AS was obtained by means of the ammonia stripping process from effluent after struvite precipitation from a liquid fraction of digestate. The agronomic effectiveness of Bio-AS in a pot experiment with maize and grass in two different soils, silty loam (SL) and loamy sand (LS), was investigated. The fertilising effect of Bio-AS was compared to commercial ammonium sulphate fertilizer (Com-AS) and control treatment (without fertilisation). The crop yields were found to depend on both soil type and nitrogen treatment. Crop yields produced under Bio-AS and Com-AS exceeded those under control treatments, respectively for SL and LS soils, by 88% and 125% for maize and 73% and 94% for grass. Crop yields under Bio-AS were similar to those under the Com-AS treatment. The fertilizer use of Bio-AS affected the chemical composition of plants and soil properties similarly as Com-AS. This suggests that Bio-AS from a bio-refinery can replace industrial ammonium sulphate, resulting in both economic and environmental benefits.

Treatment of the liquid phase of digestate from a biogas plant for water reuse

Biogas plants struggle with managing nitrogen-rich digestate from manure co-digestion. In this study, the biologically treated liquid phase of digestate from an aerobic granular sludge batch reactor (GSBR) containing oxidized nitrogen forms (NO_x), phosphorus, COD and total suspended solids was post-denitrified (P-D), and then ultrafiltered. In P-D, various hydraulic retention times (from 10 to 60 h) and biomass concentrations (from 6 to 14 g MLSS/L) were tested. Then, waste glycerin (GL) was added to the P-D reactor at a COD_GL/NO_x ratio of 1.1, causing a large number of phosphate-accumulating and denitrifying Janibacter sp., and PHB-accumulating and denitrifying Paracoccus sp. and Thauera sp. to be present in granules, which improved nutrient removal. The effluent was ultrafiltered at 0.3 and 0.5 MPa. After biological treatment supported with GL and followed by ultrafiltration, the purified liquid phase of the digestate met FAO standards for water reuse for irrigation.

A Bio-Refinery Concept for N and P Recovery—A Chance for Biogas Plant Development

Biogas is an alternative source of energy for fossil fuels. In the process of transforming organic materials into biogas significant amounts of valuable digestate are produced. In order to make the whole process sustainable digestate should be utilized this is a constraining factor in the development of the biogas industry. Consequently, there is an on-going search for new technologies to process digestate, allowing to broaden the range of possible ways of digestate utilization. One of such possibilities is technology of nitrogen (N) and phosphorus (P) recovery from the anaerobic digestate. In this study results of physicochemical analysis of materials flowing through the farm-scale bio-refinery producing struvite (STR) and ammonium sulphate (AS) are presented. Struvite was precipitated from the liquid fraction of digestate (LFDS). Ammonia was bound by sulphuric acid resulting in obtaining ammonium sulphate. The STR obtained was of medium purity and contained other macronutrients and micronutrients that further enhanced its agronomic value. The P recovery effectiveness, counted as the difference between the Ptot content in the material before and after STR precipitation was 43.8%. The AS was characterized by relatively low Ntot and Stot content. The Ntot recovery efficiency reached 43.2%. The study showed that struvite precipitation and ammonia stripping technologies can be used for processing digestate however, the processes efficiency should be improved.