Relative kinetics of anaerobic digestion under thermophilic and mesophilic conditions

Synergistic effect of hydrothermal sludge and food waste in the anaerobic co-digestion process: microbial shift and dewaterability

While thermal hydrolysis technology is commonly employed for sewage sludge treatment in extensive wastewater treatment facilities, persistent challenges remain, including issues such as ammonia-induced digestive inhibition and reduced productivity stemming from nutrient deficiency within the hydrothermal sludge. In this study, the effects of hydrothermal sludge-to-food waste mixing ratios and fermentation temperatures on anaerobic co-digestion were systematically investigated through a semi-continuous experiment lasting approximately 100 days. The results indicated that anaerobic co-digestion of hydrothermal sludge and food waste proceeded synergistically at any mixing ratio, and the synergistic effect is mainly attributed to the improvement of carbohydrate removal and digestive system stability. However, thermophilic digestion did not improve the anaerobic performance and methane yield. On the contrary, mesophilic digestion performed better in terms of organic matter removal, especially in the utilization of soluble carbohydrates, soluble proteins, and VFAs. Microbial community analysis revealed that the transition from mesophilic to thermophilic anaerobic co-digestion prompts changes in the methane-producing pathways. Specifically, the transition entails a gradual shift from pathways involving acetoclastic and hydrogenotrophic methanogenesis to a singular hydrogenotrophic methanogenesis pathway. This shift is driven by thermodynamic tendencies, as reflected in Gibbs free energy, as well as environmental factors like ammonia nitrogen and volatile fatty acids. Lastly, it is worth noting that the introduction of food waste did lead to a reduction in cake solids following dewatering. Nevertheless, it was observed that thermophilic digestion had a positive impact on dewatering performance.

Unblocking the rate-limiting step of the municipal sludge anaerobic digestion

Anaerobic digestion stabilizes municipal sludge through total solids reduction and biogas production. It is generally accepted that hydrolysis accounts for the rate-limiting step of municipal sludge anaerobic digestion, impacting the overall rates of solids reduction and methane production. Technically, the sludge hydrolysis rate can be enhanced by the application of thermal hydrolysis pretreatment (THP) and is also affected by the total solids concentration, temperature, and solids retention time used in the anaerobic digestion. This study systematically analyzed and compared ways to take these four factors into the consideration of modern anaerobic digestion system for achieving the maximum solid reduction. Results showed that thermophilic anaerobic digestion was superior to mesophilic anaerobic digestion in terms of solids reduction but vice versa in terms of the methane production when integrated with THP. This difference has to do with the intermediate product accumulation and inhibition when hydrolysis outpaced methanogenesis in THP-enhanced thermophilic anaerobic digestion, which can be mitigated by adjusting the solids retention time. PRACTITIONER POINTS: THP followed by TAD offers the greatest solids reduction rate. THP followed by MAD offered the greatest methane production rate. FAN inhibition appears to be an ultimate limiting factor constraining the methane production rate. In situ ammonia removal technique should be developed to further unblock the rate-limiting step.

A perspective on the combination of alkali pre-treatment with bioaugmentation to improve biogas production from lignocellulose biomass

Anaerobic digestion (AD) is a bioprocess technology that integrates into circular economy systems, which produce renewable energy and biofertilizer whilst reducing greenhouse gas emissions. However, improvements in biogas production efficiency are needed in dealing with lignocellulosic biomass. The state-of-the-art of AD technology is discussed, with emphasis on feedstock digestibility and operational difficulty. Solutions to these challenges including for pre-treatment and bioaugmentation are reviewed. This article proposes an innovative integrated system combining alkali pre-treatment, temperature-phased AD and bioaugmentation techniques. The integrated system as modelled has a targeted potential to achieve a biodegradability index of 90% while increasing methane production by 47% compared to conventional AD. The methane productivity may also be improved by a target reduction in retention time from 30 to 20 days. This, if realized has the potential to lower energy production cost and the levelized cost of abatement to facilitate an increased resource of sustainable commercially viable biomethane.

Kinetics of aerobic cellulose degradation in raw municipal wastewater

Cellulose contributes approximately one third of the influent suspended solids to wastewater treatment plants and is a key target for resource recovery. This study investigated the temperature impact on biological aerobic degradation of cellulose in laboratory-scale sequencing batch reactors (SBR) at four different temperatures (10-33 °C) and two different solids retention times (SRT) of 15 days and 3 days. The degradation efficiency of cellulose was observed to increase with temperature and was slightly dependent on SRT (80%-90% at an SRT of 15 days, and 78%-85% at an SRT of 3 days). Hydrolysis followed 1st order kinetics, rather than the biomass dependent Contois kinetics (default in the activated sludge models), with a hydrolysis coefficient at 20 °C of 1.14 ± 0.01 day^-1.

Investigation of the anaerobic digestion of cosmetic industrial wastes: Feasibility and perspectives

This study assessed the anaerobic digestion (AD) of wastes deriving from cosmetics production: sludge from onsite wastewater treatment plant (sWWTP), residues of shampoo/conditioner (RSC) and sludge from mascara production (MS), considered as single substrates and as mixture according to the produced amounts (54 %-wt sWWTP, 31 %-wt RSC, 13 %-wt MS, plus 2 %-wt food waste from the canteen, FW). Total COD (COD_T) was 624-1436 g O₂/kg _VS, while soluble COD was 5-23 %-wt of COD_T. AD tests at 35 °C achieved the following biogas yields: 0.10 Nm³/kg_vs (70 %-v/v methane) for sWWTP; 0.07 Nm³/kg_vs (62 %-v/v methane) for RSC; 0.04 Nm³/kg_vs (67 %-v/v methane) for MS. The mixed substrates underwent physico-chemical pre-treatments (thermo-alkaline, TA: 120 min at 50 °C; thermo-alkaline-sonication, TAS: 15 min at 40 kHz and 80 °C, both based on the addition of 0.08 g NaOH per each g of total solid in the substrate), reaching 64-66% disintegration rate, and AD tests (5 %-wt dry substance) at 35 and 52 °C. Biogas yields were (for TA and TAS respectively): 0.22 and 0.20 Nm³/kg_VS (62-70% methane); 0.21 and 0.19 Nm³/kg_VS (66-66% methane) at 52 °C. At both temperatures, methane yields considerably improved (+71-100%), compared to mixed untreated substrates, and 5-8 %-wt total solids reductions were observed. A technical-economic scale-up assessment completed the research. The energy analysis highlighted the crucial role of TA pre-treatment in achieving the process energetic sustainability. The economic analysis showed that the AD of the considered cosmetic waste could be sustainable anyway, thanks to the savings related to the disposal of the digestate compared to current waste management costs.

Solid state anaerobic digestion of food waste and sewage sludge: Impact of mixing ratios and temperature on microbial diversity, reactor stability and methane yield

Food waste (FW) and sewage sludge (SS) were anaerobically co digested under solid state conditions (Total solids >15%) and observed that mixing ratio of 3:1 and 2:1 is optimum for mesophilic and thermophilic conditions respectively. The VS reduction and methane yield at optimized ratio was 76% and 0.35 L CH₄/(g VS reduced) respectively at mesophilic temperature whereas it was 88% and 0.42 L CH₄/(g VS reduced) at thermophilic temperature. The metagenomic analysis for these cases were done and high throughput DNA sequencing revealed that diversified bacterial groups that participate in the different metabolisms (hydrolysis, acidogenesis and acetogenesis) were mainly dominated by the phylum Firmicutes and Bacteriodetes. Genus Methanothrix is found to be dominant which is capable of generating methane by any methanogenic pathway among all the archaeal communities in the reactors followed by Methanolinea and Methanoculleus. However, it was understood through metagenomic studies that acetotrophic pathway is observed to be the major metabolic pathway in the reactors.

Reconsidering hydrolysis kinetics for anaerobic digestion of waste activated sludge applying cascade reactors with ultra-short residence times

Hydrolysis is considered to be the rate-limiting step in anaerobic digestion of waste activated sludge (WAS). In this study, an innovative 4 stages cascade anaerobic digestion system was researched to (1) comprehensively clarify whether cascading configuration enhances WAS hydrolysis, and to (2) better understand the governing hydrolysis kinetics in this system. The cascade system consisted of three 2.2 L ultra-short solids retention times (SRT) continuous stirred tank reactors (CSTRs) and one 15.4 L CSTR. The cascade system was compared with a reference conventional CSTR digester (22 L) in terms of process performance, hydrolytic enzyme activities and microbial community dynamics under mesophilic conditions (35 °C). The results showed that the cascade system achieved a high and stable total chemical oxygen demand (tCOD) reduction efficiency of 40-42%, even at 12 days total SRT that corresponded to only 1.2 days SRT each in the first three reactors of the cascade. The reference-CSTR converted only 31% tCOD into biogas and suffered process deterioration at the applied low SRTs. Calculated specific hydrolysis rates in the first reactors of the cascade system were significantly higher compared to the reference-CSTR, especially at the lowest applied SRTs. The activities of several hydrolytic enzymes produced in the different stages revealed that protease, cellulase, amino peptidases, and most of the tested glycosyl-hydrolases had significantly higher activities in the first three small digesters of the cascade system, compared to the reference-CSTR. This increase in hydrolytic enzyme production by far exceeded the increase in specific hydrolysis rate, indicating that hydrolysis was limited by solids-surface availability for enzymatic attack. Correspondingly, high relative abundances of hydrolytic-fermentative bacteria and hydrogenotrophic methanogens as well as the presence of syntrophic bacteria were found in the first three digesters of the cascade system. However, in the fourth reactor, acetoclastic methanogens dominated, similarly as in the reference-CSTR. Overall, the results concluded that using multiple CSTRs that are operated at low SRTs in a cascade mode of operation significantly improved the enzymatic hydrolysis rate and extend in anaerobic WAS digestion. Moreover, the governing hydrolysis kinetics in the cascading reactors were far more complex than the generally assumed simplified first-order kinetics.

High-Solid Anaerobic Digestion: Reviewing Strategies for Increasing Reactor Performance

High-solid and solid-state anaerobic digestion are technologies capable of achieving high reactor productivity. The high organic load admissible for this type of configuration makes these technologies an ideal ally in the conversion of waste into bioenergy. However, there are still several factors associated with these technologies that result in low performance. The economic model based on a linear approach is unsustainable, and changes leading to the development of a low-carbon model with a high degree of circularity are necessary. Digestion technology may represent a key driver leading these changes but it is undeniable that the profitability of these plants needs to be increased. In the present review, the digestion process under high-solid-content configurations is analyzed and the different strategies for increasing reactor productivity that have been studied in recent years are described. Percolating reactor configurations and the use of low-cost adsorbents, nanoparticles and micro-aeration seem the most suitable approaches to increase volumetric production and reduce initial capital investment costs.

Arrested methanogenesis digestion of high-strength cheese whey and brewery wastewater with carboxylic acid production

A new anaerobic digestion process based on arrested methanogenesis (AM) was developed to treat high-strength cheese whey and brewery wastewater with simultaneous carboxylic acid production. This study specifically determined the links between wastewater characteristics, microbial community structure, and the operation of AM digesters at the bench scale. The highest total carboxylic acids concentration (78 g/L) was achieved after 15 days under batch condition at 40 °C and near-neutral pH. Lactate conversion to chain-elongated volatile fatty acid was observed. Under fed-batch conditions, the highest total acid productivity was 16 g/(L_liq·d) with substrate conversion of 0.66 g COD_digested/g COD_fed at hydraulic residence time (HRT) of 4 days. Fed-batch digestion with biomass recycling resulted in a 2-fold increase in VFAs concentration (30 g/L) and a higher diversity in the microbial consortia. Experimental results show that highly efficient, robust, and productive community structure was established for sustainable carboxylate production from widely varying high-strength wastewaters.

Anaerobic Mineralization of Recirculating Aquaculture Drum Screen Effluent for Use as a Naturally-Derived Nutrient Solution in Hydroponic Cropping Systems

Operation and effluent treatment costs are limiting factors for the success of recirculating aquaculture systems (RAS) in meeting seafood demand in the United States. Adopting a capture-and-reuse waste management model similar to terrestrial agriculture farmers would allow RAS farmers to monetize effluent and offset production costs. The moisture content and nutrient profile of RAS effluent makes it a potential option for use as a hydroponic fertilizer. Treatment of RAS waste is needed to mineralize particulate-bound nutrients before becoming a viable hydroponic nutrient solution. Anaerobic treatment (AT), a method used by municipal and agricultural waste treatment facilities to reduce total solids, has been shown to successfully mineralize particulate-bound nutrients from RAS effluent. Continuously mixed anaerobic batch bioreactors were used to evaluate the degree to which AT may mineralize particulate-bound nutrients in solid RAS waste. Concentrations of twelve different macro- and micro-nutrients were analyzed in the waste before and after treatment. Effluent samples were analyzed to determine the fraction of each nutrient in the solid and aqueous forms. This study showed that AT is an effective method to mineralize particulate-bound nutrients in RAS effluent and the mineralization rate data may be used to design a pilot-scaled flow-through RAS effluent treatment system.

Biogas production from anaerobic co-digestion using kitchen waste and poultry manure as substrate-part 1: substrate ratio and effect of temperature

The rapidly declining fossil fuels are no longer able to meet the ever-increasing energy demand. Moreover, they are considered responsible for greenhouse gas (GHG) emission, contributing to the global warming. On the other hand, organic wastes, such as kitchen waste (KW) and poultry manure (PM), represent considerable pollution threat to the environment, if not properly managed. Therefore, anaerobic co-digestion of KW and PM could be a sustainable way of producing clean and renewable energy in the form of biogas while minimizing environmental impact. In this study, the anaerobic co-digestion of KW with PM was studied to assess the rate of cumulative biogas (CBG) production and methane percentage in four digester setups (D1, D2, D3, and D4) operated in batch mode. Each digester setup consisted of five parallelly connected laboratory-scale digesters having a capacity of 1 L each. The digester setups were fed with KW and PM at ratios of 1:0 (D1), 1:1 (D2), 2:1 (D3), and 3:1 (D4) at a constant loading rate of 300 mg/L with 50 gm cow manure (CM) as inoculum and were studied at both room temperature (28 °C) and mesophilic temperature (37 °C) over 24 days. The co-digestion of KW with PM demonstrated a synergistic effect which was evidenced by a 16% and 74% increase in CBG production and methane content, respectively, in D2 over D1. The D3 with 66.7% KW and 33.3% PM produced the highest CBG and methane percentage (396 ± 8 mL and 36%) at room temperature. At mesophilic condition, all the digesters showed better performance, and the highest CBG (920 ± 11 mL) and methane content (48%) were observed in D3. The study suggests that co-digestion of KW and PM at mesophilic condition might be a promising way to increase the production of biogas with better methane composition by ensuring nutrient balance, buffering capacity, and stability of the digester.

Cellulolytic and Xylanolytic Microbial Communities Associated With Lignocellulose-Rich Wheat Straw Degradation in Anaerobic Digestion

The enzymatic hydrolysis of lignocellulosic polymers is generally considered the rate-limiting step to methane production in anaerobic digestion of lignocellulosic biomass. The present study aimed to investigate how the hydrolytic microbial communities of three different types of anaerobic digesters adapted to lignocellulose-rich wheat straw in continuous stirred tank reactors operated for 134 days. Cellulase and xylanase activities were monitored weekly using fluorescently-labeled model substrates and the enzymatic profiles were correlated with changes in microbial community compositions based on 16S rRNA gene amplicon sequencing to identify key species involved in lignocellulose degradation. The enzymatic activity profiles and microbial community changes revealed reactor-specific adaption of phylogenetically different hydrolytic communities. The enzymatic activities correlated significantly with changes in specific taxonomic groups, including representatives of Ruminiclostridium, Caldicoprobacter, Ruminofilibacter, Ruminococcaceae, Treponema, and Clostridia order MBA03, all of which have been linked to cellulolytic and xylanolytic activity in the literature. By identifying microorganisms with similar development as the cellulase and xylanase activities, the proposed correlation method constitutes a promising approach for deciphering essential cellulolytic and xylanolytic microbial groups for anaerobic digestion of lignocellulosic biomass.

Review on current approach for treatment of palm oil mill effluent: Integrated system

Malaysia is one of the countries that is well known for its palm oil based products and exports all over the world. Over the years, palm oil mill has been rising at alarming rate in Malaysia, causing palm oil-based wastes to increase especially palm oil mill effluent (POME). POME in Malaysia are channelled into water bodies such as rivers after treated mostly with conventional biological method. However, with current technologies and knowledge, conventional POME treatments are seen to be outdated and require major improvements as greenhouse gaseous are emitted to the environment as well as being less cost effective. Integrated systems that combine two or more conventional methods are introduced and reviewed to provide insights on the advantages and disadvantages of the system if it is to be implemented in real life plant. Integrated systems that focus on combining conventional methods are compiled and reviewed specifically for POME treatment. Among the integrated methods that are reviewed includes biological with membrane, adsorption with magnetic field exposure, adsorption with membrane and electrocoagulation with membrane. The systems are seen to give excellent color, chemical oxygen demand (COD) and total suspended solids (TSS) removal with average of higher than 90%. Reduction in space utilization, improved treatment time as well as simplified operating system were reported when integrated systems are applied as compared to conventional treatment of POME.

In-situ alkaline enhanced two-stage anaerobic digestion system for waste cooking oil and sewage sludge co-digestion

Anaerobic digestion is a promising way for resource recovery from waste cooking oil (WCO) due to its high bio-methanation potential. In-situ mild alkaline (pH 8) enhanced two-stage continuous stirred tank reactors (ALK-2-CSTRs) were implemented to explore its efficiency in co-digesting WCO and sewage sludge with stepwise increase of WCO in the co-substrates. Results demonstrate that the ALK-2-CSTRs effectively promoted methane yield from the co-substrates via promoting hydrolysis, long chain fatty acids (LCFAs) degradation and protecting methanogens from exposure to high concentration of LCFAs directly. The maximum methane yield of the ALK-2-CSTRs is 39.2% higher than that of a single stage CSTR system at the optimal feed mixture of 45:55 (WCO:SS [VS]). The thermophilic operation applied to the stage-1 of the ALK-2-CSTRs failed to improve the methane yield when the methanogenic performance was stable; while upon WCO overloaded, the elevated temperature mitigated the deterioration of methanogenesis by stimulating the bioconversion of the toxic LCFAs, especially the unsaturated oleic acid. Microbial community analysis reveals the ALK-2-CSTRs stimulated the growth of lipolytic bacteria and hydrogenotrophic methanogens, which suggests the hydrogenotrophic methanogenic pathway was promoted. Cost evaluation demonstrates the economical superiority of the ALK-2-CSTR over the prevailing strategies developed for enhancing methane yield from the co-substrates.

Combining thermophilic aerobic reactor (TAR) with mesophilic anaerobic digestion (MAD) improves the degradation of pharmaceutical compounds

The removal efficiency of nine pharmaceutical compounds from primary sludge was evaluated in two different operating conditions: (i) in conventional Mesophilic Anaerobic Digestion (MAD) alone and (ii) in a co-treatment process combining Mesophilic Anaerobic Digestion and a Thermophilic Aerobic Reactor (MAD-TAR). The pilot scale reactors were fed with primary sludge obtained after decantation of urban wastewater. Concerning the biodegradation of organic matter, thermophilic aeration increased solubilization and hydrolysis yields of digestion, resulting in a further 26% supplementary removal of chemical oxygen demand (COD) in MAD-TAR process compared to the conventional mesophilic anaerobic digestion. The highest removal rate of target micropollutants were observed for caffeine (CAF) and sulfamethoxazole (SMX) (>89%) with no substantial differences between both processes. Furthermore, MAD-TAR process showed a significant increase of removal efficiency for oxazepam (OXA) (73%), propranolol (PRO) (61%) and ofloxacine (OFL) (41%) and a slight increase for diclofenac (DIC) (4%) and 2 hydroxy-ibuprofen (2OH-IBP) (5%). However, ibuprofen (IBP) and carbamazepine (CBZ) were not degraded during both processes. Anaerobic digestion affected the liquid-solid partition of most target compounds. Sorbed fraction of pharmaceutical compounds on the sludge tend to decrease after digestion, this tendency being more pronounced in the case of the MAD-TAR process due to much lower concentration of solids.

Influences of Temperature and Substrate Particle Content on Granular Sludge Bed Anaerobic Digestion

Influences of temperature (25–35 °C) and substrate particulate content (3.0–9.4 g total suspended solids (TSS)/L) on granular sludge bed anaerobic digestion (AD) were analyzed in lab-scale reactors using manure as a substrate and through modeling. Two particle levels were tested using raw (RF) and centrifuged (CF) swine manure slurries, fed into a 1.3-L lab-scale up-flow anaerobic sludge bed reactor (UASB) at temperatures of 25 °C and 35 °C. Biogas production increased with temperature in both high- and low-particle-content substrates; however, the temperature effect was stronger on high-particle-content substrate. RF and CF produced a comparable amount of biogas at 25 °C, suggesting that biogas at this temperature came mainly from the digestion of small particles and soluble components present in similar quantities in both substrates. At 35 °C, RF showed significantly higher biogas production than CF, which was attributed to increased (temperature-dependent) disintegration of larger solid particulates. Anaerobic Digestion Model No.1 (ADM1) based modeling was carried out by separating particulates into fast and slow disintegrating fractions and introducing temperature-dependent disintegration constants. Simulations gave a better fit for the experimental data than the conventional ADM1 model.

Resource recovery from pig manure via an integrated approach: A technical and economic assessment for full-scale applications

Intensive livestock farming cannot be uncoupled from the massive production of manure, requiring adequate management to avoid environmental damage. The high carbon, nitrogen and phosphorus content of pig manure enables targeted resource recovery. Here, fifteen integrated scenarios for recovery of water, nutrients and energy are compared in terms of technical feasibility and economic viability. The recovery of refined nutrients with a higher market value and quality, i.e., (NH₄)₂SO₄ for N and struvite for P, coincided with higher net costs, compared to basic composting. The inclusion of anaerobic digestion promoted nutrient recovery efficiency, and enabled energy recovery through electricity production. Co-digestion of the manure with carbon-rich waste streams increased electricity production, but did not result in lower process costs. Overall, key drivers for the selection of the optimal manure treatment scenario will include the market demand for more refined (vs. separated or concentrated) products, and the need for renewable electricity production.

Detection of acidification limit in anaerobic membrane bioreactors at ambient temperature

High-volume, low-strength industrial wastewaters constitute a large potential for biogas production, which could be realized by membrane bioreactors operating at the ambient temperature of the wastewater. However, the start-up of low-temperature anaerobic processes using unadapted inoculum can be sensitive to overloading, which results in acidification. This study assessed if a novel acidification limit test can be used to identify stable organic loading rates as well as process over-loading. The test is based on easy-to-apply batch experiments for determination of the hydrolysis rate constant and the specific methanogenic activity of the acetotrophic and hydrogenotrophic pathways. For evaluation, two anaerobic membrane bioreactors, treating synthetic dairy wastewater at an ambient temperature of 24 °C, were used with a slow or a rapid start-up regime, respectively. Tests for hydrolysis rate and methanogenic activity were performed throughout the experiment and were used to calculate acidification limits for each system throughout the start-up. The acidification limit test was able to successfully identify both stable operation of one reactor and process failure of the other reactor as the organic loading rate increased. The reactor failure was caused by over-loading the acetotrophic pathway and coincided with microbial changes observed in real-time PCR and moving window analysis. Overall, the acidification limit tests seem promising as an easy applicable method for estimating what organic loading rate can be utilized, without risking acidification of anaerobic systems.

Thermophilic sludge digestion improves energy balance and nutrient recovery potential in full-scale municipal wastewater treatment plants

The conventional treatment of municipal wastewater by means of activated sludge is typically energy demanding. Here, the potential benefits of: (1) the optimization of mesophilic digestion; and (2) transitioning to thermophilic sludge digestion in three wastewater treatment plants (Tilburg-Noord, Land van Cuijk and Bath) in the Netherlands is evaluated, including a full-scale trial validation in Bath. In Tilburg-Noord, thermophilic sludge digestion covered the energy requirements of the plant (102%), whereas 111% of sludge operational treatment costs could be covered in Bath. Thermophilic sludge digestion also resulted in a strong increase in nutrient release. The potential for nutrient recovery was evaluated via: (1) stripping/absorption of ammonium; (2) autotrophic removal of ammonium via partial nitritation/anammox; and (3) struvite precipitation. This research shows that optimization of sludge digestion may lead to a strong increase in energy recovery, sludge treatment costs reduction, and the potential for advanced nutrient management in full-scale sewage treatment plants.

Thermophilic versus Mesophilic Anaerobic Digestion of Sewage Sludge: A Comparative Review

During advanced biological wastewater treatment, a huge amount of sludge is produced as a by-product of the treatment process. Hence, reuse and recovery of resources and energy from the sludge is a big technological challenge. The processing of sludge produced by Wastewater Treatment Plants (WWTPs) is massive, which takes up a big part of the overall operational costs. In this regard, anaerobic digestion (AD) of sewage sludge continues to be an attractive option to produce biogas that could contribute to the wastewater management cost reduction and foster the sustainability of those WWTPs. At the same time, AD reduces sludge amounts and that again contributes to the reduction of the sludge disposal costs. However, sludge volume minimization remains, a challenge thus improvement of dewatering efficiency is an inevitable part of WWTP operation. As a result, AD parameters could have significant impact on sludge properties. One of the most important operational parameters influencing the AD process is temperature. Consequently, the thermophilic and the mesophilic modes of sludge AD are compared for their pros and cons by many researchers. However, most comparisons are more focused on biogas yield, process speed and stability. Regarding the biogas yield, thermophilic sludge AD is preferred over the mesophilic one because of its faster biochemical reaction rate. Equally important but not studied sufficiently until now was the influence of temperature on the digestate quality, which is expressed mainly by the sludge dewateringability, and the reject water quality (chemical oxygen demand, ammonia nitrogen, and pH). In the field of comparison of thermophilic and mesophilic digestion process, few and often inconclusive research, unfortunately, has been published so far. Hence, recommendations for optimized technologies have not yet been done. The review presented provides a comparison of existing sludge AD technologies and the gaps that need to be filled so as to optimize the connection between the two systems. In addition, many other relevant AD process parameters, including sludge rheology, which need to be addressed, are also reviewed and presented.

Recovery of energy and nutrient resources from cattle paunch waste using temperature phased anaerobic digestion

Cattle paunch is comprised of partially digested cattle feed, containing mainly grass and grain and is a major waste produced at cattle slaughterhouses contributing 20-30% of organic matter and 40-50% of P waste produced on-site. In this work, Temperature Phased Anaerobic Digestion (TPAD) and struvite crystallization processes were developed at pilot-scale to recover methane energy and nutrients from paunch solid waste. The TPAD plant achieved a maximum sustainable organic loading rate of 1-1.5kgCODm(-3)day(-1) using a feed solids concentration of approximately 3%; this loading rate was limited by plant engineering and not the biology of the process. Organic solids destruction (60%) and methane production (230LCH4kg(-1) VSfed) achieved in the plant were similar to levels predicted from laboratory biochemical methane potential (BMP) testing. Model based analysis identified no significant difference in batch laboratory parameters vs pilot-scale continuous parameters, and no change in speed or extent of degradation. However the TPAD process did result in a degree of process intensification with a high level of solids destruction at an average treatment time of 21days. Results from the pilot plant show that an integrated process enabled resource recovery at 7.8GJ/dry tonne paunch, 1.8kgP/dry tonne paunch and 1.0kgN/dry tonne paunch.

Particle-Scale Modeling of Methane Emission during Pig Manure/Wheat Straw Aerobic Composting

Inefficient aerobic composting techniques significantly contribute to the atmospheric methane (CH4) levels. Macro-scale models assuming completely aerobic conditions cannot be used to analyze CH4 generation in strictly anaerobic environments. This study presents a particle-scale model for aerobic pig manure/wheat straw composting that incorporates CH4 generation and oxidation kinetics. Parameter estimation revealed that pig manure is characterized by high CH4 yield coefficient (0.6414 mol CH4 mol(-1) Cman) and maximum CH4 oxidation rate (0.0205 mol CH4 kg(-1) VS(aero) h(-1)). The model accurately predicted CH4 emissions (R(2) = 0.94, RMSE = 2888 ppmv, peak time deviation = 0 h), particularly in the self-heating and cooling phases. During mesophilic and thermophilic stages, a rapid increase of CH4 generation (0.0130 mol CH4 kg(-1) VS h(-1)) and methanotroph inactivation were simulated, implying that additional measures should be performed during these phases to mitigate CH4 emissions. Furthermore, CH4 oxidation efficiency was related to oxygen permeation through the composting particles. Reducing the ambient temperature and extending the aeration duration can decrease CH4 emission, but the threshold temperature is required to trigger the self-heating phase. These findings provide insights into CH4 emission during composting and may inform responsible strategies to counteract climate change.

The use of laboratory scale reactors to predict sensitivity to changes in operating conditions for full-scale anaerobic digestion treating municipal sewage sludge

Anaerobic digestion of sewage sludge is highly complex and prone to inhibition, which can cause major issues for digester operators. The result is that there have been numerous investigations into changes in operational conditions, however to date all have focused on the qualitative sensitivities, neglecting the quantitative. This study therefore aimed to determine the quantitative sensitivities by using factorial design of experiments and small semi continuous reactors. Analysis showed total and volatile solids removals are chiefly influenced by retention time, with 79% and 59% of the observed results being attributed to retention time respectively, whereas biogas was mainly influenced by loading rate, 38%, and temperature, 22%. Notably the regression model fitted to the experimental data predicted full-scale performance with a high level of precision, indicating that small reactors are subject to the same sensitivity of full-scale digesters and thus can be used to predict changes loading, retention time, and temperature.

Biogas production from wheat straw and manure--impact of pretreatment and process operating parameters

Non-treated or steam-exploded straw in co-digestion with cattle manure was evaluated as a substrate for biogas production compared with manure as the sole substrate. All digestions were performed in laboratory-scale CSTR reactors (5L) operating with an organic loading late of approximately 2.8 g VS/L/day, independent of substrate mixture. The hydraulic retention was 25 days and an operating temperature of 37, 44 or 52°C. The co-digestion with steam exploded straw and manure was evaluated with two different mixtures, with different proportion. The results showed stable performance but low methane yields (0.13-0.21 N L CH4/kg VS) for both manure alone and in co-digestion with the straw. Straw appeared to give similar yield as manure and steam-explosion treatment of the straw did not increase gas yields. Furthermore, there were only slight differences at the different operating temperatures.

Explicit temperature-based model for anaerobic digestion: application in domestic wastewater treatment in a UASB reactor

Temperature is an important environmental variable that can strongly affect the performance of anaerobic reactors working at ambient temperatures. This study presents a mechanistic mathematical model which depends in an explicit way on the operating temperature. The cardinal temperature model function is proposed to describe the temperature dependence of the kinetic parameters and the experimental data from an UASB-degasification system was used to calibrate and validate the model. The performance of the model is compared with the classic Arrhenius approach. The results showed that the temperature-based model of the anaerobic digestion is able to reproduce a long-term reactor operation in terms of biogas production and the concentration of organic matter at fluctuating ambient temperature.