Co-digestion Of Waste Activated Sludge Research Articles

Environmental impact of sewage sludge co-digestion with food waste and fat-oil-grease: Integrating plant-wide modeling with life cycle assessment approach

Anaerobic co-digestion of fat-oil-grease (FOG) and food waste (FW) with sewage sludge (SS) in wastewater treatment plants is a method used to increase biogas production. In this study, digestion scenarios were compared using plant-wide modeling and life cycle assessment: Scenario-0 (mono-digestion of waste-activated sludge (WAS)), Scenario-1 (co-digestion of WAS with FOG), and Scenario-2 (co-digestion of WAS with FW). Scenario-0, with the highest energy use and landfilling of FOG/FW, has the worst environmental impact. Scenario-1 and Scenario-2 minimize the environmental load by energy recovery and avoiding landfilling of organic waste. Scenario-wise, the change in greenhouse gas (GHG) emissions from treatment was negligible. However, due to the impact of landfilling, GHG emissions in Scenario-0 were 21% and 30% higher than in Scenario-1 and 2, respectively. The environmental benefit of anaerobic co-digestion of FOG/FW with SS is not only in the contribution to energy production but also in the recycling of organic waste.

Calcium peroxide pre-treatment improved the anaerobic digestion of primary sludge and its co-digestion with waste activated sludge

Primary sludge (PS) and Waste activated sludge (WAS) as two main sludge streams in wastewater treatment plants are commonly anaerobically co-digested, which though may be differently affected by pretreatment. Previous work has found that calcium peroxide (CaO2) pretreatment effectively enhanced anaerobic digestion of WAS. However, the feasibilities of this strategy on PS anaerobic digestion and co-digestion of WAS and PS are still unclear. Herein, this work provided new insights into these systems. Biomethane potential test demonstrated that CaO2 pretreatment at 0.02–0.26 g/g-volatile suspended solids (VSS) promoted anaerobic digestion of PS. Then the feasibility of CaO2 pretreatment for improving anaerobic co-digestion of PS and WAS mixture was confirmed, with the highest improvement in methane production, VSS destruction and sludge reduction being approximately 37.4%, 38.9% and 19.9%, achieved at 0.14 g/g-VSS of CaO2. Process modelling analysis revealed that CaO2 pretreatment increased both degradable faction and actually degraded fraction in sludge mixture. The changes of sludge characteristics via pretreatment and key enzyme activity in sludge anaerobic co-digestion system demonstrated that increased CaO2 concentration resulted in increased soluble organics release from sludge mixture in the pretreatment stage and inhibited activity of coenzyme F420 responsible for methanogenesis. Further mechanism investigation disclosed that OH−, O2− and OH were main contribution factors, and the order of their contributions were OH− >O2− >OH. This work laid the theoretical foundation and provided guidance for the practical application of CaO2 pre-treatment technology.

Filamentous foam disintegration with free nitrous acid: Effect on anaerobic digestion

Sludge foaming is a common problem in wastewater treatment plants negatively affecting operation of anaerobic digestion reactors. Therefore, in common practice, foam is removed from reactors without being fermented, leading to increase in sludge mass for disposal. However, foam is rich in lipids and can be a good source of methane if operational problems can be overcome. In this paper, in a two-stage experiment, we show that foam disintegration with free nitrous acid (FNA) can boost methane production and decrease foaming potential. In the first stage, the biochemical methane potential (BMP) of foam was evaluated to be higher by 19–63% (191-263NmL/gVS) than the BMP of waste activated sludge (WAS) (161 ± 1NmL/gVS) confirming previous assumptions. The main findings of the second stage (continuous experiments) are: (1) foam and WAS co-digestion leads to sludge stratification and thickened biomass accumulation in the upper part of the reactor, (2) FNA disintegration destroyed foam structure, resulting in lower biomass stratification and 14% higher methane production (134 mL/gVS) than observed in the reference reactor, (3) FNA disintegration of both substrates (foam and WAS) does not provide noticeable benefits in terms of biomass stratification. However, it does enhance methane production to 140 mL/gVS and sludge mineralization efficiency. A significantly higher impact of FNA on methane yield from foam than WAS was attributed to the high content of M.parvicella and the ability of these bacteria to adsorb and accumulate lipids. Anaerobic digestion of FNA disintegrated foam leads to substantial benefits in terms of methane production, reactor volume, and reagents consumption.

Taking Advantage of Waste Heat Resource from Vinasses for Anaerobic Co-digestion of Waste Activated Sludge under the Thermophilic Condition: Energy Balance and Kinetic Analysis.

Vinasses are not only an easily biodegradable substrate but also a heat energy resource. In this study, the energy balance and kinetic model of anaerobic co-digestion of waste activated sludge (WAS) with vinasses have been investigated in semicontinuous reactor experiments at 55 °C. Herein, the maximum energy balance value, the ratio of energy to mass, and the kinetic constants μmax and K of anaerobic digestion of WAS were −33.44 kJ·day–1, −5.72 kJ·VS–1·day–1, and 0.0894 day–1 and 0.7294, respectively, at an organic loading rate (OLR) of 1.17 VS·L–3·day–1; when the mixture ratio of WAS to vinasses was 2:1 (dry VS) for co-digestion, the maximum energy balance value, the maximum ratio of energy to mass, and the kinetic constants μmax and K of anaerobic co-digestion of WAS and vinasses were +39.73 kJ·day–1, 8.1 kJ·VS–1·day–1, and 0.2619 day–1 and 1.9583, respectively, at an OLR of 1.73 VS·L–3·day–1. The positive energy balance was obtained for two reasons: one is for making the best use of the high-temperature heat energy resource of vinasses and the other is for enhancing the amount of biogas yield. The bottleneck of the negative energy balance of thermophilic digestion of WAS can be broken by anaerobic co-digestion of WAS and vinasses. The results indicate a promising future in the application of anaerobic thermophilic co-digestion of WAS and vinasses. Methane production from digestion and co-digestion was also predicted by the Chen–Hashimoto kinetic model.

Fat, oil, and grease (FOG) deposits yield higher methane than FOG in anaerobic co-digestion with waste activated sludge

The formation of fat, oil, and grease (FOG) deposits in sewers is a global challenge for the maintenance of sewer collection systems. Tons of FOG deposits (FDs) are removed from sewer systems every year and present an opportunity for increased methane production via anaerobic co-digestion with waste activated sludge (WAS) at water resource recovery facilities with existing anaerobic digesters. We hypothesized that FDs have higher biomethane potential than that of FOG (e.g., FOG collected in grease interceptors), because of the reduction of inhibition of long chain fatty acids due to saponification. In this study, substantially enhanced methane production was found in anaerobic co-digestion of WAS with FDs within the substrate to inoculum (S/I) ratio range of 0.25–1.2, and the maximum ultimate methane production (685.7 ± 24.1 mL/gVSadded, at S/I = 0.5) was 4.0 times higher than in the control (with WAS only) after 42 days of incubation. Although the lag phase period was longer in FD co-digestion (S/I = 0.5) than in FOG co-digestion (S/I = 0.5) under the same organic loading (gVS) and two times the COD loading, the daily methane production rate became higher after Day 15 in FD co-digestion. Significantly higher cumulative methane production (10.2%, p < 0.05) was obtained in FD co-digestion than in FOG co-digestion after 42-days. Microbial community analysis revealed higher levels of Geobacter in FD co-digestion, possibly suggesting a role for direct interspecies electron transfer (DIET) between Methanosaeta and Geobacter. This work provides fundamental insights supporting anaerobic co-digestion of FDs with WAS, demonstrating the advantages of FDs compared to FOG as co-substrate for enhanced biomethane recovery.

OPTIMIZATION OF BIOGAS PRODUCTION FROM CO-DIGESTION OF WASTE ACTIVATED SLUDGE AND MODIFIED TAPIOCA STARCH SLUDGE

Waste activated sludge (WAS) obtained from a modified tapioca starch wastewater treatment plant has potential for use as feedstock for biogas production. However, it has a very low C/N ratio of 4.3, leading to a low methane yield. Co-digestion of WAS with starch sludge (SS) can potentially provide balanced nutrients and can enhance biogas production. The results from a biochemical methane potential (BMP) test showed that using WAS as a single feed stock provided a low biogas production at 80 L/kg TVSadded. In co-digestion, the addition of 30% and 50% SS increase the biogas yields by as much as 150 and 209 L/kg TVSadded, respectively. However, the addition of 70% SS reduces the biogas yield to as low as 117 L/kg TVSadded. The result also showed that the optimum condition for biogas production is achieved at 2% TVSadded of mixed WAS and SS (1:1) with a desired C/N ratio of 23.56. The increase in % TVSadded of the mixed feedstock resulted in a reduction of the biogas yield. The co-digestion of WAS and SS (1:1) also improved the rate of digestion and enhanced the system stability. TVS removal efficiency of the mixed WAS and SS was 41.44% higher than the efficiency when using WAS alone. Moreover, the result of continuous stirred tank reactor (CSTR) confirmed that the co-digestion ratio of WAS:SS (1:1) can enhance biogas yield and methane yield at 301 and 142 L/kg TVSadded, respectively at retention time of 11 days.

Effects of green waste addition on waste activated sludge and fat, oil and grease co-digestion in mesophilic batch digester

ABSTRACT Anaerobic digestion (AD) is regarded as an effective method to treat waste activated sludge (WAS) and fat, oil and grease (FOG). Co-digestion of WAS/FOG could promote the methane yield but it will cause acid and salinity inhibition. Green waste (GW) was added into the digesters, and its effects on co-digestion of WAS and FOG in the mesophilic batch digester were investigated. Digestive performances (such as hydrolysis, acidogenesis and methanogenesis) were studied emphatically. The results showed that digester L6 (WAS:FOG:GW = 1:2:1, VS basis) presented the highest specific methane yield (SMY, 341.5 mL/g VS). The results of kinetics study verified that there was a slower hydrolysis rate when GW was applied as a co-substrate, which could reduce the potential of acid inhibition. Volatile fatty acid (VFA) and electrical conductivity analysis showed that GW addition could keep moderate VFA concentrations and alleviate the negative effects of high-salinity substrates on the digestive systems. The microbial community and diversity analysis proved that GW addition was beneficial to keep the balance of hydrolytic bacteria, acidogens and acetogens. The results of this study indicated that GW addition could enhance the energy recovery and system stability in the WAS/FOG co-digestive system.

Biogas production from anaerobic co-digestion of waste activated sludge: co-substrates and influencing parameters

Anaerobic digestion is a versatile biotechnology to treat waste activated sludge (WAS), the main by-products of biological wastewater treatment, because it can achieve simultaneously energy recovery (biogas) and pollutant reduction (organic matter, pathogens). However, the potential of biogas production from mono-digestion of WAS is usually limited by the imbalance carbon to nitrogen (C/N) ratio of WAS and ammonia accumulation. Anaerobic co-digestion, simultaneous digestion of two or more substrates, should be a feasible option to resolve these disadvantages. The abundant organic wastes from municipal, industrial, and agricultural field have been the ideal co-substrates because they not only can balance the substrate nutrient to obtain the optimal C/N ratio, but also can adjust pH and dilute the toxic materials to mitigate the inhibition to methanogens, consequently improving the yield of biogas, especially methane. This paper classified the main organic co-substrates according to their source and reviewed their application in anaerobic co-digestion of WAS. Then the influence of temperature, pH, organic loading rate, hydraulic retention time, C/N ratio, digester type and pretreatment method on biogas production was extensively discussed. Finally, this review brought forward the challenges and outlooks of anaerobic co-digestion in the future.

A critical review of volatile fatty acids produced from waste activated sludge: enhanced strategies and its applications.

This paper reviews the recent achievements in the enhanced production of volatile fatty acids (VFAs) from waste activated sludge (WAS). The enhanced strategies are divided into two approaches. The first strategy focuses on the regulation of carbon-to-nitrogen (C/N) ratio by co-digestion of WAS with carbon-rich substrates, including municipal solid wastes (MSW), marine algae, agricultural residues, and animal manures. The other strategy is to enhance the solubilization and hydrolysis of WAS or inhibit the methanogenesis by applying various pretreatments, such as mechanical, chemical, enzymatic, and thermal pretreatment. Finally, the applications of WAS-derived VFAs are discussed. The future researches in enhancing VFAs production and wide application of the VFAs from both technical and economic perspectives are proposed.

Effect of enhancing nutrient balance in anaerobic digester feedstock by co-substrate addition on the microbial diversity and energy production from municipal sewage sludge

Enhancement of methane production during anaerobic digestion of waste activated sludge (WAS) could improve the energy self sufficiency of the municipal wastewater treatment plants (WWTPs). Therefore, mixing WAS with organic wastes improved process performance and stability. In this work, the anaerobic co-digestion of WAS combined with the olive processing wastewater (OPW) was investigated and associated with the energetic benefits and microbial populations shifts. The bio-methane potential (BMP) of various WAS and OPW mixtures corresponding to increased phenols concentrations were tested. The anaerobic digestion of better proportions (90%/10% and 80%/20%) was performed in anaerobic sequencing batch reactors (ASBRs). The biodegradation of phenols at concentrations up to 0.76g/L was confirmed by Sephadex gel filtration showing that ASBR, which is suspended growth reactor, can handle much higher concentration of toxic compounds. Microbial analysis showed that phenols induced significantly the archaea community dynamic, which showed highly richness and diversity in the well performed reactor. The dominant bacteria and archaea phylotypes were affiliated to Proteobacteria and Methanosarcinales, respectively. Therefore, OPW addition increased total energy production from 24.6kWh/ton to 64.7kWh/ton, which would provide 0.43M€/year net benefits only from the electric power. In addition it brings a payback time on investment of 2 years for WWTPs modification, which was considered interesting.

An efficient method to improve the production of methane from anaerobic digestion of waste activated sludge.

Methane production from waste activated sludge (WAS) anaerobic digestion is always low due to slow hydrolysis rate and inappropriate ratio of carbon to nitrogen (C/N). In this work, a novel approach, i.e., co-digestion of WAS and tobacco waste (TW) using ozone pretreatment, to greatly enhance the production of methane is reported. Experimental results showed the optimal C/N and ozone dosage for methane production was 24:1 and 90 mg/g suspended solids, and the corresponding methane production was 203.6 mL/g volatile suspended solids, which was 1.3-fold that in mono-WAS digestion. Further investigation showed the co-digestion of WAS and TW was beneficial to the consumptions of protein and cellulose; also, the presence of ozone enhanced the disruption of organic substrates and production of short chain fatty acids, which provided sufficient digestion substrates for methane generation. Analysis of microbial community structure suggested that members of the phyla Bacteroidetes and Firmicutes were the dominant species when ozone pretreatment was applied. The findings obtained in this work might be of great importance for the treatment of WAS and TW.

Biomethanation of Sewage Sludge with Food Waste Leachate Via Co-Digestion.

Anaerobic mono- and co-digestion of sewage sludge and food waste leachate (FWL) were performed by assessing methane production and characterizing microbial communities. Anaerobic digestion (AD) of waste activated sludge (WAS) alone produced the lowest methane (281 ml CH4), but an approximately 80% increase in methane production was achieved via co-digestion of WAS and FWL (506 ml CH4). There were less differences in the diversity of bacterial communities in anaerobic digesters, while archaeal (ARC) and bacterial (BAC) amounts reflected AD performance. Compared with the total ARC and BAC amounts in the mono-digestion of WAS, the ARC and BAC amounts increased two and three times, respectively, during co-digestion of FWL and WAS. In characterized archaeal communities, the dominant ratio of hydrogenotrophic methanogens in the mono-digestion of WAS approached nearly a 1:1 ratio of the two acetoclastic and hydrogenotrophic methanogens in the co-digestion of FWL and WAS. The ARC/BAC ratio in the digesters varied in the range of 5.9% to 9.1%, indicating a positive correlation with the methane production of AD.

Anaerobic co-digestion of municipal biomass wastes and waste activated sludge: Dynamic model and material balances

The organic matter degradation process during anaerobic co-digestion of municipal biomass waste (MBW) and waste-activated sludge (WAS) under different organic loading rates (OLRs) was investigated in bench-scale and pilot-scale semi-continuous stirred tank reactors. To better understand the degradation process of MBW and WAS co-digestion and provide theoretical guidance for engineering application, anaerobic digestion model No.1 was revised for the co-digestion of MBW and WAS. The results showed that the degradation of organic matter could be characterized into three different fractions, including readily hydrolyzable organics, easily degradable particulate organics, and recalcitrant particle organics. Hydrolysis was the rate-limiting step under lower OLRs, and methanogenesisis was the rate-limiting step for an OLR of 8.0 kg volatile solid (VS)/(m3·day). The hydrolytic parameters of carbohydrate, protein, and lipids were 0.104, 0.083, and 0.084 kg chemical oxygen demand (COD)/(kg COD·hr), respectively, and the reaction rate parameters of lipid fermentation were 1 and 1.25 kg COD/(kg COD·hr) for OLRs of 4.0 and 6.0 kg VS/(m3·day). A revised model was used to simulate methane yield, and the results fit well with the experimental data. Material balance data were acquired based on the revised model, which showed that 58.50% of total COD was converted to methane.

Improving methane production during the codigestion of waste-activated sludge and fatty wastewater: Impact of thermo-alkaline pretreatment on batch and semi-continuous processes

With the objective of maximising methane production from waste-activated sludge (WAS), codigestion with fatty wastewater (FW) collected from restaurants was studied and the application of both a 170°C thermal pretreatment and thermo-alkaline/saponification to mixed WAS/FW waste was evaluated. Batch anaerobic digestion tests showed that recalcitrant compounds were formed during the 170°C pretreatment of fatty wastewater and thermo-alkaline pretreatments (80°C or 120°C, pH 8, 9 or 10) led to an increased initial methane production rate and to a very slight impact on the methane potential of the mixed waste (+4–7%). Semi-continuous anaerobic digesters were fed over 4months with WAS, two mixed substrates WAS/FW 90/10 and WAS/FW 60/40, and with WAS/FW 90/10 pretreated at softer conditions (80°C and pH=8, 0.14gKOHgVS-1). This pretreatment led to a significant increase in the methane production of semi-continuous reactors (+58%). Finally, this study showed the feasibility of the codigestion of waste activated sludge with highly concentrated fatty wastewater (40% in volume, 49% in VS and 73% in COD, equivalent to 7gL−1 of lipids). It resulted in specific methane production of 362mLCH4g−1 VS whereas WAS alone produced only 116mLCH4g−1 VS.

Evaluating and modeling biogas production from municipal fat, oil, and grease and synthetic kitchen waste in anaerobic co-digestions

The feasibility of using synthetic kitchen waste (KW) and fat, oil, and grease (FOG) as co-substrates in the anaerobic digestion of waste activated sludge (WAS) was investigated using two series of biochemical methane potential (BMP) tests. Ranges of ideal substrate to inoculum (S/I) ratio were determined for the FOG (0.25–0.75) and KW (0.80–1.26) as single substrates in the first experiment. The second experiment, which estimated the methane production performances of FOG and KW as co-substrates for WAS co-digestion, was conducted based on the optimal parameters selected from the results of the first experiment. Results indicated that co-digestions with FOG and KW enhanced methane production from 117±2.02mL/gTVS (with only WAS) to 418±13.7mL/gTVS and 324±4.11mL/gTVS, respectively. FOG exhibited more biogas production than KW as co-substrate. Non-linear regression results showed that co-substrate addition shortened the lag phases of organic biodegradation from 81.8 (with only WAS) to 28.3h with FOG and 3.90h with KW.

Methane Potential of Waste Activated Sludge and Fatty Residues: Impact of Codigestion and Alkaline Pretreatments

The aim of this study was to maximise methane production from waste activated sludge (WAS) originating from extended aeration process and presenting a low methane potential (190 mL CH4.g -1 OM). WAS co-digestion with fatty residues (FR, 560 mL CH4.g -1 OM) produced during pretreatments of the effluents from wastewater treatment plants in the Lille area and fatty wastewaters (FW, around 700 mL CH4.g -1 OM) collected from restaurants was assessed by batch experiments. Moreover saponification/alkali pretreatments improved kinetics of anaerobic digestion but had a low impact on methane potential (+ 6-7%) of the mixed waste composed of 66.6% of FW, 33.3% of WAS and 0.1% of FR. As results did not depend on pH ranging from 8 to 10 (addition of 0.12 to 0.21 gKOH gOM -1 ) nor temperature ranging from 80 to 120 °C, the least severe studied pretreatment conditions (80°C and pH=8) may be selected for further studies on con- tinuous anaerobic reactors.