Soluble Chemical Oxygen Demand Research Articles

Effect of extracellular polymeric substances removal and re-addition on anaerobic digestion of waste activated sludge

Extracellular polymeric substances (EPS) play a key role in anaerobic digestion of excess sludge. The influence of EPS removal and re-addition on sludge anaerobic digestion were investigated by batch experiment. The results showed that both EPS removal and re-addition improved sludge anaerobic digestion, the cumulative methane production increased by 14.9 % and 50.4 %. EPS removal promoted sludge hydrolysis and acidification, the soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) concentration increased by 31 % and 29 %. The re-addition of EPS increased the substrate of sludge fermentation, resulting in 1.64 and 1.77 times higher SCOD and VFAs concentration than that of control group. In addition, EPS contained lots of electrochemically active substances, and re-adding EPS increased the electron transfer system activity in anaerobic digestion by 164 %, which enhanced significantly methane production. Both removal and re-addition of EPS altered the microbial community structure in anaerobic digestion, the bacteria with hydrolysis and acidification functions were enriched, such as Chloroflexi and Desulfobacterota. Methanobacterium and Methanosaeta were the dominant methanogens in anaerobic digestion systems, and their relative abundance were increased by removing and re-adding EPS.

Growth of microalgae and cyanobacteria consortium in a photobioreactor treating liquid anaerobic digestate from vegetable waste

This research examines the biological treatment of undiluted vegetable waste digestate conducted in a bubble column photobioreactor. Initially, the bioreactor containing 3N-BBM medium was inoculated with Microglena sp., Tetradesmus obliquus, and Desmodesmus subspicatus mixture with a density of 1.0 × 104 cells/mL and the consortium was cultivated for 30 days. Then, the bioreactor was semi-continuously fed with liquid digestate with hydraulic retention time (HRT) of 30 days, and the treatment process was continued for the next 15 weeks. The change in the microalgal and cyanobacterial species domination was measured in regular intervals using cell counting with droplet method on a microscope slide. At the end of the experiment, Desmonostoc sp. cyanobacteria (identified with 16S ribosomal RNA genetical analysis) as well as Tetradesmus obliquus green algae along with Rhodanobacteraceae and Planococcaceae bacteria (determined with V3–V4 16sRNA metagenomic studies) dominated the microbial community in the photobioreactor. The experiment demonstrated high treatment efficiency, since nitrogen and soluble COD were removed by 89.3 ± 0.5% and 91.2 ± 1.6%, respectively, whereas for phosphates, 72.8 ± 2.1% removal rate was achieved.

Enhanced sludge solubilization by a fluidized electrode: Granular activated carbon promoted electrochemical oxidation

Electrochemical sludge pretreatment is receiving increasing attention because of its small footprint and higher environmental compatibility. However, the limited effective area of electrode plates and the low conductivity of sludge hinder the widespread application of electrochemical pretreatment. In this study, granular activated carbon (GAC) was employed to construct a fluidized electrode electrochemical system (FEE) to promote electrochemical pretreatment. Under the optimal operating parameters, the FEE system could effectively facilitate sludge decomposition, indicated by 126% increase in soluble chemical oxygen demand (SCOD) and 23.1% reduction in sludge volume. Mechanism study revealed that the addition of GAC significantly enhanced the conductivity of sludge, thereby promoting the oxidation capacity of FEE system. Furthermore, continuously generated H2O2 in FEE further promoted sludge solubilization. GAC offered an effectively, green and sustainable enhancement approach for sludge electrochemical pretreatment.

Effect of predatory bacterial mixtures on biolysis of waste activated sludge to improve dewatering performance

ABSTRACTThe generation of surplus sludge during biological wastewater treatment has become a prevalent issue, necessitating the development of a dewatering approach that is efficient, economically feasible, and ecologically sound. Bdellovibrio-and-like-organisms (BALOs) are obligatory parasitic bacteria that prey on an array of bacteria. In this study, different BALO strains were isolated and purified from waste activited sludge (WAS). Anti-predation host strains were applied to screen the BALO strains with different host-range to minimize the overlap of the biolysis prey spectrum. In addition, the BALO strains with different host preferences were mixed for sludge biolysis treatment efficiency comparison. The results indicated that the capillary suction time and the bound water content in the WAS treated with the mixed BALOs were significantly decreased by 25.9% ± 1.7% and 5.2% ± 1.2%, respectively, compared to those treated with the single BALO strain. The soluble chemical oxygen demand concentration in the mixed BALOs treated group was increased by 31.2% ± 0.7% than that treated with the single strain. The findings indicate that the mixed strains used in the treatment process resulted in a notable enhancement of both sludge dewatering performance and lysis degree. In addition, the abundance of Proteobacteria treated with the BALO mixtures decreased by 69.1% than the single strain treated one which demonstrated that the BALO mixture expanded the sludge host lysis spectrum. This study revealed the different effects of single and mixed strains on sludge community structure, suggesting that the BALO host-range expansion is crucial to further improve sludge dewatering performances.

Intermittent energization improves anaerobic digestion of microbial electrolysis cell-assisted nitrogen-rich sludge under mesophilic and thermophilic conditions

Anaerobic digestion of nitrogen-rich sludge is limited by hydrolysis and threatened by high ammonia nitrogen. Anaerobic digestion of microbial electrolysis cell-assisted (AD-MEC) can effectively improve hydrolysis and system stability. However, prolonged energisation can exacerbate energy consumption and lead to a reduction in economic benefits. The effects of AD-MEC on the anaerobic digestion performance of nitrogen-rich sludge under mesophilic and thermophilic conditions with different energization times (0 h, 3 h, 6 h, 12 h and 24 h per day) were investigated. AD-MEC enhances the concentration of anaerobically digested soluble chemical oxygen demand and volatile fatty acids. The AD-MEC digester in 12 h-ON/12 h-OFF energization mode under mesophilic conditions had a methane yield of 133.3 ± 6.1 mL g−1-VSadded, which was enhanced by 23.7% compared with the control. The AD-MEC digester in 12 h-ON/12 h-OFF energized mode under thermophilic conditions promoted the recovery of the destabilized digester with a methane yield of 43.9 ± 2.1 mL g−1-VSadded, which was 14.5% higher compared with the control. Microbial communities showed that mesophilic AD-MEC enriched hydrogenotrophic methanogens dominated by Methanosarcina, and thermophilic AD-MEC enriched Coprothermobacterota phylum bacteria and hydrogenotrophic methanogens promoted recovery of the destabilized digester. Therefore intermittent energization of the AD-MEC using the 12 h-ON/12 h-OFF mode can enhance methane yield and save energy.

Effects of MgCl2 on sludge anaerobic digestion: Hydrolysis, acidogenesis, methanogenesis, and microbial characteristics

Back diffused draw salt affects the anaerobic digestion performance in a forward osmosis membrane bioreactor. Previous studies have established the influence of MgCl2 on sludge anaerobic digestion; however, its effects on individual phases, such as hydrolysis, acidogenesis, and methanogenesis remain unclear. This study investigates the influence of different concentrations of MgCl2 on these stages of anaerobic digestion and compares variations in methane production under different substrates. The findings demonstrate that adding MgCl2 significantly enhances sludge hydrolysis. At a concentration of 50 g/L, the soluble chemical oxygen demand increases by 265.2 %, although with unstable reactions. MgCl2 significantly affects the production of fatty acids. Specifically, the production of acids was enhanced when the concentration of MgCl2 was below 20 g/L, while it was inhibited when the concentration exceeded 50 g/L. The highest concentration of acetic acid recorded is 1991.28 mg/L, which represents an 83.5 % increment. In contrast, methane production shows a higher susceptibility to MgCl2, experiencing inhibition at concentrations exceeding 1 g/L. The inhibitory concentration for methane production is lower than for acid production. The study also observes significant differences in microbial structure, particularly in Methanogen. When using different substrates, the predominant orders of methanogen are Methanobacteriales for tests with acid solution as a feed and Methanosarcinales for tests with waste sludge as a feed, respectively. Analyses of metabolic pathways reveal that carbon dioxide and acetic acid are the most prevalent pathways in terms of abundance. In summary, MgCl2 has varying effects on various stages of anaerobic digestion of sludge.

Tuning of food wastes bioavailability as feedstock for bio-conversion processes by acoustic cavitation and SPC, SPS, or H2O2 as external oxidants

The growing amount of food wastes makes them a suitable source for the generation of bioproducts through anaerobic digestion. Appropriate hydrolysis of the feedstock can enhance the efficiency of production of desired products. In this work, acoustic cavitation (AC) was employed as a pretreatment method to enhance hydrolysis stage by the modification of model (potato-based) food waste for increase in soluble chemical oxygen demand (CODs) and dissolved carbohydrate. For the first time high and low frequency AC was compared for this purpose. The application of sole AC at a frequency of 20 kHz for feedstock loading of 3 % has led to 125 % and 124 % increase in CODs and dissolved carbohydrates, respectively. The combination of AC with external oxidants hydrogen peroxide (H2O2), sodium persulfate (SPS), and sodium percarbonate (SPC) was also studied. This part of the studies revealed that SPS has superior properties for increasing CODs by 258 % and dissolved carbohydrates by 240 %. On the other hand, addition of sodium hydroxide (NaOH) as alternative reagent, leads to a 173 % increase in CODs and 155 % increase in dissolved carbohydrates. Making both ways of processing highly effective to increase the bioavailability of food wastes for further biologic processing.

Acidogenic Fermentation of Kitchen Waste for the Production of Volatile Fatty Acids: Bioaugmentation by Bacillus GIEC

In this study, the lignocellulosic (banana peel, tea residue, and paper towel in a ratio of 1:1:1) and protein (chicken breast) components of kitchen waste (KW) were used as substrates for mesophilic anaerobic fermentation to produce volatile fatty acids (VFAs). The ability of a new strain belonging to Bacillus sp. to improve the degradation of kitchen waste and VFAs production was investigated. The results showed that the addition of Bacillus cell wall depolymerization GIEC (Bacillus GIEC) to the fermentation system could result in higher concentrations of soluble chemical oxygen demand (sCOD), improved the removal rates of volatile solids (VS), and increased yield of VFAs from the substrates. Compared with the control group, the sCOD concentrations of lignocellulosic and protein substrates increased by 132.58% and 18.36%, respectively; the volatile solids removal rates of lignocellulosic and protein substrates increased by 84.96% and 135.53%, respectively; the yield of VFAs of lignocellulosic and protein substrates increased by 61.29% and 35.92%, respectively, reaching 0.31 g/g VSadded and 0.67 g/g VSadded, separately. According to the study, the addition of Bacillus GIEC enhanced the solubilization of solid organic matter during hydrolysis process, further resulting in a higher yield of VFAs compared to the control group. Furthermore, the micro-aerobic test showed that the bioaugmentation ability of Bacillus GIEC has little effect by the presence of oxygen. The Bacillus GIEC has the potential for bioaugmentation of the VFAs production from kitchen waste.

Ozone combined with mechanical agitation to enhance sludge cell lysis and provide an alternative carbon source for nitrogen removal of bioreactors

The activated sludge method results in the production of significant amounts of excess sludge. Moreover, biological nitrogen removal requires an external carbon source, which presents environmental and economic challenges. To address these challenges, this study combined ozone with mechanical agitation to achieve sludge cell lysis. Through both experiments and COMSOL Multiphysics simulations, optimal conditions were identified. These conditions include a six-bladed disk turbine stirrer speed of 800 rpm, an ozone concentration of 40 mg/L, and an ozone dose of 100 mg·g−1 MLSS (Mixed Liquid Suspended Solids). Notably, the sludge solubilization efficiency improved by 20.1 % when compared to using ozone alone. After the sludge dissolution, this paper observed elevated concentrations of soluble chemical oxygen demand (SCOD), total nitrogen (TN) and total phosphorus (TP) in the supernatant, reaching 3126 mg/L, 144.3 mg/L, and 49.6 mg/L, respectively. This study also investigated the feasibility of using this supernatant as an external carbon source in a sequencing batch reactor. The results showed that while the supernatant had minimal impact on COD removal, it significantly enhanced nitrogen removal. Compared to using glucose as an external carbon source, the TN removal rate increased by 16.9 %. However, TP removal was inhibited.

Simultaneous nitrification and denitrification of real domestic wastewater using novel biodegradable Cocos Nucifera fibre as biofilter media and carbon source: Performances and microbial community composition

The simultaneous nitrification and denitrification (SND) performances in biofiltration system are largely governed by the filter media material and also the operating conditions. Moreover, a significant gap exists regarding the influence of different operating conditions on the SND performances in using plant-based waste materials as the biofilter media. Thus, in this study, long-term SND and organic pollutants removal were investigated in a continuous-flow biofiltration system filled with novel biodegradable Cocos nucifera fibre media operating under different dissolved oxygen (DO) concentrations, HRT and backwash intensity. Under best operating conditions of DO 1.5 mg/L, HRT 16.0 h, and airflow and water flow rate of 1.0 L/m2/s, the biofiltration system was able to remove an average of 70.7 % of NH3-N, 98.5 % of NO2-N, 90.3 % of COD, 86.9 % of soluble COD (sCOD), 98.7 % of TSS, 70.4 % of TN and achieved 94.3 % of SND efficiencies. The coexistence of enriched functional microorganisms such as nitrifiers (Nitrosomonas, Nitrospira) and denitrifiers (Dechloromonas, Zoogloea, Acidovorax) were believed to be responsible for the efficient performance of the system working under best operating conditions.

Treatment and optimization of high-strength egg-wash wastewater effluent using electrocoagulation and electrooxidation methods

Egg-washing wastewater contains a high concentration of nutrition and organic matter since eggs are broken during the washing and cleaning processes. Moreover, the wastewater contains small amounts of detergents or sanitizing agents. These contaminants may pose environmental challenges when they are not properly managed or treated. The study scrutinizes the efficiency of electrocoagulation (EO) and electrooxidation (EO) approaches for egg-wash wastewater treatment. The response surface methodology was employed to optimize the operational parameters. The removal efficiencies of soluble chemical oxygen demand (sCOD 90%), ammonia (NH3–N 91%), nitrate (NO3−-N 97%), nitrite (NO2−-N 89.3%), total dissolved nitrogen (TDN 91%), and phosphate (90%) were measured under various treatment conditions. The optimum treatment conditions achieved in the combined EC + EO process were pH 6.0, current density 20 mA cm−2, and electrolysis time of 60 min, respectively. Degradation kinetics of the egg-wash pollutants showed a significant reduction in half-life (t1/2) with EO (after EC-Aluminum) at 15 min, 12 min, 17 min, and 15 min for sCOD, NO2−-N. NO3−-N, and TDN, respectively. Whereas the half-life of NH3–N (18 min) and phosphate (17 min) reduced significantly with the EO (after EC-iron). Al and Fe electrodes coupled with boron-doped diamond were found efficient for pollutant removal.Environmental implication.Egg-wash wastewater has a high protein content and contains nutrients that are essential for living organisms. While these compounds can be valuable for agricultural use by increasing soil phosphate concentration, they can also become an issue if the excess nutrients are not properly managed. The soil has a threshold limit for holding phosphate, and any excess amount may be transported through surface runoff or contaminate groundwater through leachate, potentially affecting aquatic ecosystems and water quality. This study explores the efficiency of electrocoagulation and electrooxidation methods in treating egg-wash wastewater. These methods aim to remove pollutants and reduce their environmental impact.

Dairy manure acidogenic fermentation at hyperthermophilic temperature enabled superior activity of thermostable hydrolytic enzymes linked to the genus Caldicoprobacter

In this study, fermentation experiments were conducted under mesophilic, thermophilic, and hyperthermophilic conditions to investigate adaptation of microbial communities and its effect on extracellular enzyme activities toward degradation of cellulose, hemicellulose and proteins in dairy manure. Hyperthermophilic conditions transformed the microbiome structure and stimulated activity of extracellular proteolytic, cellulolytic, and hemicellulolytic enzymes. Specifically, the activities of protease, cellulose 1,4-β-cellobiosidase, and β-glucosidase secreted by hyperthermophilic microbes were higher by 22%, 47% and 49% compared to those produced by mesophilic and thermophilic communities. Enhanced hydrolytic activity of hyperthermophilic microbes enabled improved feedstock solubilization and production of 39% and 22% more soluble COD than mesophilic and thermophilic microbes, respectively. Connections between hydrolytic function and microbial community structure at various temperatures were assessed using the PICRUSt2 computational tool. Genus Caldicoprobacter was identified as the primary candidate responsible for increased production of thermostable endo-1,4-β-glucanase, β-glucosidase and endo-1,4-β-xylanase, and enhanced hydrolytic performance of hyperthermophilic microbial community.

Comprehending the impact of berberine on anaerobic digestion of waste activated sludge

Berberine is a natural isoquinoline alkaloid performing wide-spectrum antimicrobial and antiviral effects like antibiotics. Its production generates berberine containing wastewater, and berberine adsorbed on waste activated sludge (WAS) will unavoidably enter the anaerobic digestion (AD) system while its impact on the AD process is unknown. Our research found that berberine of 20 mg/L (BBR20) slightly enhanced the methane yield (4.2 ± 0.6%) under mesophilic condition (35.0 ± 1.0 °C). However, 100 and 500 mg/L (BBR100 and BBR500) depressed methane production by 17.3 ± 4.3% and 83.2 ± 0.4%; meanwhile more soluble chemical oxygen demand (SCOD) including volatile fatty acid (VFA), protein, and polysaccharide were left in the fermentation broth, which led to an increase in sludge reduction. 88.3 ± 0.09%–99.1 ± 0.04% of berberine was distributed in the sludge phase and could be efficiently removed even under a high berberine level of 500 mg/L during the AD process. Exposure to different berberine concentrations promoted sludge dissolution and triggered more sludge extracellular polymeric substances (EPS) being dissolved. Lower berberine concentration (20 mg/L) enhanced acidification and methanogenesis steps, resulting in a final methane generation increase. While hydrolysis, acidification and methanogenesis processes were all inhibited by 100 and 500 mg/L berberine. Microbial analysis revealed that the main acid-producing bacteria genera were changed as Bacteroidetes vadinHA17 dominated in control, BBR20 and BBR100 groups, was replaced by Petrimonas in BBR500. Additionally, Methanosaeta, as a strict acetoclastic methanogen, was suppressed under exposure to 100 and 500 mg/L berberine. Accordingly, the declined abundance of archaea genera consuming acetic acid caused more VFA accumulation and less methane production in BBR100 and BBR500 groups.

Peracetic acid pretreatment improves biogas production from anaerobic digestion of sewage sludge by promoting organic matter release, conversion and affecting microbial community

Peracetic acid (PAA) pretreatment is considered as a novel and effective chemical pretreatment method for sludge. However, there is little information available on potential mechanisms of how PAA pretreatment affects sludge anaerobic digestion (AD). To fill the knowledge gap, this study investigated the effects and potential mechanisms of PAA pretreatment on sludge AD systems from physicochemical and microbiological perspectives. Batch experiments resulted that biogas production was enhanced by PAA pretreatment and the highest cumulative biogas yield (297.94 mL/g VS (volatile solid)) was obtained with 2 mM/g VS of PAA pretreatment. Kinetic model analysis illustrated that the PAA pretreatment improved the biogas potential (Pt) of sludge AD, but prolonged the lag phase (λ) of AD. Mechanistic studies revealed that reactive oxygen species (ROS) (HO•, O2−•, 1O2 and CH3C(O)OO•) were the major intermediate products of PAA decomposition. These ROS effectively promoted the decomposition and solubilization of sludge, and provided more biodegradable organic matter for the following AD reactions. 16S rRNA amplicon sequencing showed that some functional microorganisms associated with hydrolysis, acidogenesis, acetogenesis as well as methanogenesis, such as Hydrogenispora, Romboutsia, Longivirga, Methanosarcina and Methanosaet, were significantly enriched in reactors pretreated with PAA. Redundancy analysis and variation partitioning analysis indicated that functional microorganisms were significantly correlated with intermediate metabolites (soluble carbohydrate, soluble protein, soluble chemical oxygen demand and volatile fatty acids) and cumulative biogas production. This study provides a fresh understanding of the effects and mechanisms of PAA pretreatment on sludge AD, updates the insights into the response of functional microorganisms to PAA pretreatment, and the findings obtained might provide a fundamental basis for chemical pretreatment of sludge AD using oxidants.

Effect of pretreatment with alkali on the anaerobic digestion characteristics of kitchen waste and analysis of microbial diversity

Abstract Kitchen waste contains high contents of organic matter and moisture, and it is prone to biodegrade and decompose to give odors. If not collected and transported promptly or treated improperly, it is highly likely to pollute the environment and spread diseases. Because the lipid content in kitchen waste is high and a portion of organic matter is not subject to hydrolysis, the development of anaerobic digestion technology has been greatly limited. Kitchen waste was pretreated with NaOH, KOH, and Ca(OH)2 with different concentrations, and 50 days sequencing batch mesophilic anaerobic digestion experiments were conducted. This study sheds light on the pollution reduction and energy generation of kitchen waste. The results are as follows: (1) The lipid content of kitchen waste could be reduced, and the concentration of dissolved organic matter could be increased by pretreating with alkali. The degradation rate of kitchen waste lipid reached a maximum of 50.51%, if 3% NaOH was added, and the soluble chemical oxygen demand concentration was increased by 235.3%. (2) The cumulative methane (CH4) output and biogas production efficiency were improved in the anaerobic digestion process with kitchen waste pretreated with alkali. The maximum daily gas output of kitchen waste pretreated with NaOH and KOH took place on the 11th to 12th day, with the biogas production efficiency of 40.4 and 45.2 mL·g·VS−1. The cumulative CH4 output was increased from 370.2 mL·g·VS−1 (untreated) to 393.1 and 434.1 mL·g·VS−1, respectively. In addition, the concentration of CH4 in biogas was increased from 54.8% (untreated) to 59.1% and 61.7%, respectively. (3) The Chao1 and Ace values of bacteria were increased first and then decreased. On the 10th day, the diversity of bacteria reached the highest value, and on the 20th day, the diversity of archaea reached its maximum. Therefore, it was verified that the improvement in the hydrolysis acidification efficiency and degree was crucial for the rapid and complete anaerobic digestion reactions.

Sludge composting with self-produced carbon source by phosphate buffer coupled hyperthermophilic pretreatment realizing nitrogen retention

During sludge composting, ammonia emissions is the main pathway of nitrogen loss. In addition to regulating pH, increasing the carbon source is also important for controlling ammonia emissions. Previous studies have focused on additional carbon sources, while this study used phosphate-buffered saline (PBS) and hyperthermophilic pretreatment (HP) to release difficult-to-utilize carbon sources in extracellular polymeric substance (EPS) for composting. The PBS concentration was 0.22 mmol/g total matter (TM), PBS soaking days were 4.74, HP temperature was 149.46 °C, and the soluble chemical oxygen demand (SCOD) was 45.08 g/kg. Compared with the control check, the peak ammonia emission concentration in the pretreated group was reduced by 8.70 % and the total nitrogen loss was reduced by 18.10 %. Changes in EPS, ammonia assimilation activity, and microbial community composition demonstrated that PBS and HP released the carbon source from the sludge, promoted ammonia assimilation, and enriched bacteria related to ammonia assimilation such as Bacillus (16.52 %). HP treatment made Acinetobacter (12.5 %) the dominant genus in the calefactive phase, and the enrichment of Actinobacteriota (27.27 %) promoted the composting maturity. However, total pretreatment reduced the porosity and permeability of the sludge and affected the activity of microorganisms. Considering the cost of energy consumption, partial pretreatment has more practical application prospects.

Improved methane production from ultrasonically-pretreated secondary sedimentation tank sludge and new model proposal: Time series (ARIMA)

Secondary sedimentation tank sludge (SSTS) is a severe type of waste that needs to be treated to prevent human health risks. Anaerobic digestion (AD) alone can be ineffective for treating SSTS due to low substrate biodegradability. This study proposes the use of ultrasonic pretreatment (UP) on SSTS for 0 – 60 min. UP effect on SSTS was analyzed through the actual specific energy input, nominal energy input, and soluble chemical oxygen demand. The highest cumulative methane yield (CMY) of 241.56 mL/g VS obtained during the AD of the pretreated substrate (i.e., 30 min) was 58.4% higher than that of untreated SSTS. The resulting CMY curves were subsequently simulated via the modified Logistic and Gompertz, Artificial Neural Network, Auto-Regressive Integrated Moving-Average (ARIMA) models. As a novel proposal for predicting CMYs, the ARIMA model is recommended due to its simplicity and better performance than sigmoidal models (R2 = 0.9975 – 0.9988).

Substituting microalgae with fecal sludge for biohythane production enhancement and cost saving through two-stage anaerobic digestion

Finding a low-cost, simple technology for producing high-quality bioenergy (e.g., biohythane: bioH2 and bioCH4) from waste biomass is challenging. Production of cleaner energy, such as biohythane, than methane from microalgae (MA), is a suitable approach. However, having a low C-N ratio of microalgae limits scaling up the process. Therefore, this study took the opportunity to use fecal sludge (FS) as a co-substrate for C-N ratio optimization to improve biohythane production while reducing cost. The biomass hydrothermal pretreatment enhanced soluble chemical oxygen demand (sCOD) from 390 mgL−1 to 1085 mgL−1, resulting volatile fatty acid (VFA) accumulation rate of 1853 mgL−1 at a mixing ratio of 50:50. The results of two-stage anaerobic digestion (TSAD) of 50-50 MA-FS showed a cumulative biohydrogen yield of 24.5 mLg−1-VS (Volatile Solid) and biomethane yield of 281 mLg−1-VS with a synergetic effect of 52.7%. The techno-economic analysis suggests a return on investment (ROI) of 4.8 for utilizing a 50-50 biomass ratio. In addition, the input cost could be reduced to 75.6 % if the ratio of MA and FS was adjusted to 25–75. Therefore, the TSAD of MA and FS is a promising option to enhance the biohythane yield and its high economic potential.

Treatability of high-strength real sheep slaughterhouse wastewater using struvite precipitation coupled Fenton's oxidation: The MAPFOX process

Struvite (MAP, magnesium ammonium phosphate hexahydrate, MgNH4PO4.6H2O) precipitation-aided Fenton's OXidation (MAPFOX process) was explored in the treatment of high-strength real sheep slaughterhouse wastewater (RSSW) for the first time under a comprehensive soft-computing-based modeling study. The experimental results showed that under the highest-efficiency conditions (chemical combination of MgCl2.6H2O + NaH2PO4.2H2O, a molar ratio of Mg2+:NH4+-N:PO43–P = 1.2:1:1, a reaction pH of 9.0 ± 0.10, [NH4+-N]0 = 240 ± 20 mg/L, and a reaction time of 15 min), MAP precipitation could effectively remove more than 80 %, 60 %, 55 %, and 70 % of color, total chemical oxygen demand (TCOD), soluble COD (SCOD), and ammonium nitrogen (NH4+-N) from the raw RSSW. The results of the Fenton's oxidation used as the post-treatment unit of the proposed MAPFOX system indicated that the integrated advanced oxidation process (AOP) was able to reduce the residual pollutant levels in the MAP-pretreated RSSW to the relevant discharge standards. Under the subsequent condition of [Fe2+]0 = 90 mmol/L, [H2O2]0 = 180 mmol/L, reaction pH = 3.25, and total reaction time = 60 min, more than 97 % of color, TCOD, SCOD, and NH4+-N could be removed from the RSSW via the Fenton's oxidation after the MAP-based physicochemical treatment. According to SEM micrographs, surface morphology of dehydrated struvite exhibited irregular-shaped and overlapping sharp-edged particles of various sizes with an average size of about 50.9 μm. The Fourier Transform Infrared (FTIR) spectroscopy confirmed the active functional groups and type of bonds for the high-strength RSSW-oriented struvite (heated) within the spectral range of 4000–450 cm−1. Thermogravimetric Analysis (TGA), Derivative Thermogravimetry (DTG), Differential Thermal Analysis (DTA), and Differential Scanning Calorimetry (DSC) of the dehydrated struvite revealed that the weight loss occurred in three temperature zones, the maximum weight loss rate of 0.252 mg/min was recorded at around 224 °C and at time of 20.83 min, and the sample had strong endothermic and medium exothermic peaks at about 241 °C and 679 °C, respectively. The predictive successes of the implemented soft-computing approaches were benchmarked in terms of various statistical goodness-of-fit parameters. The performance assessment indices corroborated the superiority of the support vector machines-Pearson VII universal kernel function (SVM-PUKF)-based model (correlation coefficient (CC) = 0.9999–1.0000), mean absolute error (MAE) = 0.0222–0.0389 %, mean absolute percentage error (MAPE) = 0.0270–0.0506 %, root mean squared error (RMSE) = 0.0258–0.0415 %, coefficient of variation of RMSE (CV(RMSE) = 0.0003–0.0008, Nash–Sutcliffe efficiency (NSE) = 0.9998–1.000, Legates and McCabe's index (LMI) = 0.9894–0.9952) over other data-intelligent models in predicting the pollutant removal efficiencies. The computational results also indicated that the narrowest uncertainty bands (±1.96Se = 0.0537–0.1483 %) and the lowest amounts of expanded uncertainty (U95 = 3.1224–5.3124 %) values for all efficiency sets were achieved for the applied SVM-PUKF-based strategy. This study demonstrated the first-ever and successful application of the proposed MAPFOX process in treatability of the RSSW and capability of the implemented soft-computing strategy in modeling a highly nonlinear treatment system.

Anaerobic digestion of thickened secondary sludge after microwave advanced oxidation pretreatment

A microwave-enhanced advanced oxidation process (MW-AOP) was used to treat thickened waste secondary sludge prior to anaerobic digestion (AD). A large amount of soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) was formed in the solution after pretreatment. Both single- and two-phase systems loaded up to 18.2 g volatile solids/(l day) while achieving high methane production, as well as high overall treatment efficiency. Both the upflow anaerobic blanket reactor and the continuous stirred tank reactor could operate at very short hydraulic retention times as short as 2 days without any evidence of souring. The results indicated that an acid reactor in the two-phase system was not required, due to very high levels of SCOD and VFAs produced in the MW-AOP-pretreated solution. As such, a single AD should be able to handle this easily biodegradable substrate following microwave pretreatment.