Visible light photocatalytic disintegration of waste activated sludge for enhancing biogas production

Enhanced ammonia nitrogen and phenol removal by immobilized bacteria through composite mycelium pellet-driven quinone redox cycle

The composite mycelium pellet (CMP) was coupled with Pseudomonas sp. Y1 (CMP-Y1) to remove phenol and ammonia nitrogen (NH4+-N). The CMP was formed by the self-assembly of fungal mycelium with sponge iron (SIO), gallic acid (GA), and oxalic acid. The results showed that CMP with abundant pore size and successful internal loading of sponge iron containing iron nanoparticles. CMP could induce GA redox cycle to form Fenton-like reaction and thus achieve efficient phenol removal (93.32%, 24 h). Meanwhile, the removal efficiencies of phenol, NH4+-N, and chemical oxygen demand (COD) using CMP-Y1 at 12 h were 93.71, 92.40, and 89.00%, respectively. The increase in the electron transfer activity of strain Y1 by the addition of CMP could facilitate the nitrogen removal processes. In addition, high-throughput sequencing results indicated the abundance of antioxidant and repair genes was increased, which might be a strategy of strain Y1 to cope with oxidative stress. This strategy provided the possibility for the practical application of the combination of advanced oxidation and biological treatment, and offered new insights into the symbiotic system of fungi and bacteria.

Integrated microbial electrolysis with high-alkali pretreated sludge digestion: Insight into the effect of voltage on methanogenesis and substrate metabolism

Integrated microbial electrolysis with anaerobic digestion is proved to be an effective way to improve methanogenesis efficiency of waste activated sludge (WAS). WAS requires pretreatment for efficient improvement of acidification or methanogenesis efficiency, but excessive acidification may inhibit the methanogenesis. In order to balance these two stages, a method for efficient WAS hydrolysis and methanogenesis has been proposed in this study by high-alkaline pretreatment integrated with microbial electrolysis system. The effects of pretreatment methods and voltage on the normal temperature digestion of WAS have also been further investigated with emphasis on the effects of voltage and substrate metabolism. The results show that compared to low-alkaline pretreatment (pH = 10), high-alkaline pretreatment (pH > 14) can double the SCOD release and promote the VFAs accumulation to 5657 ± 392 mg COD/L, but inhibit the methanogenesis process. Microbial electrolysis can alleviate this inhibition effectively through the rapid consumption of VFAs and speeding up of the methanogenesis process. The optimal methane yield of the integrated system is 120.4 ± 8.4 mL/g VSS at the voltage of 0.5 V. Enzyme activities, high-throughput and gene function prediction analysis reveal that the cathode and anode maintain the activity of methanogens under high substrate concentrations. Voltage positively responded to improved methane yield from 0.3 to 0.8 V, but higher than 1.1 V is found to be unfavorable for cathodic methanogenesis and results in additional power loss. These findings provide a perspective idea for rapid and maximum biogas recovery from WAS.

Enzymatic Treatments for Biosolids: An Outlook and Recent Trends

Wastewaters are nutrient-rich organic materials containing significant concentrations of different nutrients, dissolved and particulate matter, microorganisms, solids, heavy metals, and organic pollutants, including aromatic xenobiotics. This variety makes wastewater treatment a technological challenge. As a result of wastewater treatment, biosolids are generated. Biosolids, commonly called sewage sludge, result from treating and processing wastewater residuals. Increased biosolids, or activated sludge, from wastewater treatment is a major environmental and social problem. Therefore, sustainable and energy-efficient wastewater treatment systems must address the water crisis and environmental deterioration. Although research on wastewater has received increasing attention worldwide, the significance of biosolids treatments and valorization is still poorly understood in terms of obtaining value-added products. Hence, in this review, we established some leading technologies (physical, chemical, and biological) for biosolids pretreatment. Later, the research focuses on natural treatment by fungal enzymes to end with lignocellulosic materials and xenobiotic compounds (polyaromatic hydrocarbons) as a carbon source to obtain biobased chemicals. Finally, this review discussed some recent trends and promising renewable resources within the biorefinery approach for bio-waste conversion to value-added by-products.

Multiple environmental risk assessments of heavy metals and optimization of sludge dewatering: Red mud-reed straw biochar combined with Fe2+ activated H2O2

In this study, Fe-rich biochar (RMRS-BC) was prepared from red mud and reed straw to improve sludge dewatering and transformation of heavy metals (HMs, including Cd, Cr, Cu, Pb, and Zn). The optimal concentrations of RMRS-BC, Fe²⁺, and H₂O₂ to promote sludge dewaterability were identified by response surface methodology (RSM). The optimal dosages of RMRS-BC, Fe²⁺, and H₂O₂ were 74.0, 104.9, and 75.7 mg/g dry solids (DS), respectively. The corresponding capillary suction time (CST) and water content of sludge cake were 14.3 s and 51.25 wt%. For the improvement mechanism, heterogeneous and homogeneous Fenton reactions occurred due to RMRS-BC and Fe²⁺ activating H₂O₂. The extracellular polymeric substances (EPS) decomposed into dissolved organic matter (proteins and polysaccharides), thereby promoting the transformation of bound water to free water and further reducing the water content of the sludge cake. The research quantitatively assessed the environmental risk of heavy metals in the conditioned sludge cake based on bioavailability and ecotoxicity, pollution levels and potential ecological risks. Compound conditioning using RMRS-BC, Fe²⁺, and H₂O₂ could significantly improve the solubility and reduce the leaching toxicity of HMs. In general, RMRS-BC combined with Fe²⁺ to activate H₂O₂ provided an effective method to enhance sludge dewaterability and reduce HMs risk.

Hydrogen Production by the Thermophilic Dry Anaerobic Co-Fermentation of Food Waste Utilizing Garden Waste or Kitchen Waste as Co-Substrate

Multicomponent collaborative anaerobic fermentation has been considered a promising technology for treating perishable organic solid wastes and producing clean energy. This study evaluated the potential of hydrogen production by thermophilic dry anaerobic co-fermentation of food waste (FW) with garden waste (GW) or kitchen waste (KW) as co-substrate. The results showed that when the ratio of FW to GW was 60:40, the maximum cumulative hydrogen production and organic matter removal rate reached 85.28 NmL g−1 VS and 63.29%, respectively. When the ratio of FW to KW was 80:20, the maximum cumulative hydrogen production and organic matter removal rate reached 81.31 NmL g−1 VS and 61.91%, respectively. These findings suggest that thermophilic dry anaerobic co-fermentation of FW using GW or KW as co-substrate has a greater potential than single-substrate fermentation to improve hydrogen production and the organic matter removal rate.

Mild hydrogen peroxide interceded bacterial disintegration of waste activated sludge for efficient biomethane production

Regardless of the issue of sludge management all over the world, the role of phase separated pretreatment prior to anaerobic digestion are more promising in terms of energy efficient biomethane production. However, the effect of phase separated pretreatment (dissociation of extracellular polymeric substances (EPS) followed by biological pretreatment in a two-step process) must be sensibly evaluated from various perceptions to consolidate its effectiveness in sludge management and bioenergy recovery. In this study, mild hydrogen peroxide induced bacterial pretreatment (H₂O₂-BP) was employed as phase separated pretreatment to investigate the effectiveness of EPS dissociation prior to biological pretreatment on sludge solubilization and biomethanation. The novelty of this study is the application of mild dosage of hydrogen peroxide at sludge pH for the removal of EPS layer with lesser formation of recalcitrant substances which thereby enhances the disintegration by enzyme secreting bacterial and methane generation. The outcome confirmed that the higher EPS dissociation was achieved at H₂O₂ dosage of 8 μL per 100 mL of sludge with negligible cell lysis. An extractable EPS of 172.8 mg/L was obtained after H₂O₂ treatment. The higher sCOD solubilization of 22% and the suspended solid reduction of 17.14% were achieved in hydrogen peroxide followed by bacterial pretreatment (H₂O₂-BP) as compared to of bacterial pretreatment alone (BP) (solubilization-11% and suspended solids reduction-9.3%) and control (C) sludges (solubilization-5% and suspended solids reduction-4.3%). The methane generation for H₂O₂-BP sludge is 0.174 L/gCOD which is higher than BP (0.078 L/gCOD,) and C sludge (0.02175 L/gCOD). A higher biomass solubilization and increased biomethanation in H₂O₂-BP revealed that dissociation of EPS prior to bacterial pretreatment increases the surface area for bacterial pretreatment facilitating easier accessibility of substrate and enhanced biomethanation.

Enhancing methane production from dewatered waste activated sludge through alkaline and photocatalytic pretreatment

Waste activated sludge generated from wastewater treatment plants makes an abundant source of biomass. Its effective utilization through anaerobic digestion (AD) requires pretreatment to disintegrate the sludge matrix and increase organic matter availability. In this study, dewatered waste activated sludge (DWAS) was subjected to alkaline, photocatalytic, and alkaline-photocatalytic pretreatment for its disintegration and subsequent methane production using different concentrations of sodium hydroxide and titania nanoparticles. Individual pretreatment resulted in maximum disintegration degree (DD_sCOD) of 11.3 and 5.2% at 0.8% NaOH and 0.6 gTiO₂/L, respectively. Alkaline-photocatalytic pretreatment yielded 37% DD_sCOD at 0.8% NaOH-0.4 g/L TiO₂. As compared to control, AD at 0.4% NaOH and 0.5 g/L TiO₂ pretreatments yielded maximum methane, which was 50.4 and 32.6% higher. Similarly, alkaline-photocatalytic pretreatment at 0.4% NaOH-0.5 g/L TiO₂ yielded methane as 462 N mL/g VS, which was 71.1% higher. Modified Gompertz model fitted the methane yield data well.

Removal of pathogen and antibiotic resistance genes from waste activated sludge by different pre-treatment approaches

In wastewater treatment plants, most of the pathogens and antibiotic resistant genes (ARGs) transferred into and concentrated in waste activated sludge (WAS), which would cause severe public health risks. In this study, the capabilities of several WAS pre-treatment approaches to inactivate coliforms/E. coli and ARGs, as well as the subsequent regrowth of coliforms/E. coli and ARGs/intI1 in treated sludge were investigated. The results showed that electro-Fenton (EF), with continuous hydroxyl radical generation, could efficiently inactivate coliforms/E. coli in 60 min (about 4 log units), followed by methanol (MT), anode oxidization (AO), and acidification (AT). Kinetic analysis showed that the inactivation mainly occurred in the first 10 min. However, the efficiencies of all studied pre-treatment approaches on inactivating ARGs/intI1 (<2 log units) were lower than coliforms/E. coli, whilst EF still had the highest efficiency of ARGs/intI1 reduction. Mechanical ultrasound treatment (ULS) could not inactivate coliforms/E. coli in WAS, but it could efficiently reduce ARGs/intI1. High regrowth rates of coliforms/E. coli were observed in the treated WAS in 10 days, but the abundances of ARGs/intI1 continuously reduced during the after-treatment incubation. Our study showed that EF could efficiently disinfect potential pathogens, however, the reduction of ARGs/intI1 in WAS need further investigation.

The experimental optimization and comprehensive environmental risk assessment of heavy metals during the enhancement of sewage sludge dewaterability with ethanol and Fe(Ⅲ)-rice husk

The optimal concentrations of ethanol, Fe³⁺ and rice husk (RH) to enhance sludge dewaterability were determined by response surface methodology (RSM). Results showed the optimal concentrations of ethanol, Fe³⁺ and RH were 22.2 g/g DS, 239.9 mg/g DS and 348.9 mg/g DS, respectively, and the CST reduction efficiency reached 72.3%. The transformation behavior and mechanism of the heavy metals (HMs) during conditioning process were determined in terms of total HMs content, leaching tests, and fraction distribution. The environmental risk of HMs was quantitatively evaluated after conditioning in terms of bioavailability and ecotoxicity, potential ecological risks, and pollution levels. Results showed that the high ecological risk of HMs in raw sludge cake is primarily dominated by Cd and the use of Fe³⁺ alone negatively affected the immobilization of HMs and reduction of leaching toxicity. However, after repeated conditioning with Fe³⁺ and ethanol, the total HMs content reduction values in sludge cake were 75%, 93%, 100%, 91%, and 74% for Pb, Cr, Cd, Zn, and Cu, respectively. The potential ecological risk index (PERI) and geoaccumulation indicated low or no overall environmental risk after repeated conditioning. Particularly, the risk of Cd was reduced from high risk to low risk after repeated conditioning according to the PERI. Ethanol/Fe³⁺-RH can effectively reduce HMs risk from the sludge cake in the dewatering tests.

Enhancement of the sewage sludge dewaterability by using ethanol and Fe(III)-rice husk

Proteins of extracellular polymeric substances (EPS) in sewage sludge play a key role in the sludge dewatering. Ethanol can denature proteins and improve sludge dewaterability. In this study, ethanol was used to precondition and combined with Fe³⁺ and rice husk (RH) for dewatering enhancement. The experimental results of the capillary suction time (CST) reduction efficiency indicated that the sewage sludge pretreated with ethanol and Fe³⁺-RH revealed well cooperative formation mechanism with regard to dewatering performance. Using the response surface methodology (RSM) determined that CST reduction efficiency of sewage sludge reached 78.5% under optimal conditions of ethanol 25.21 g/g dry solid (DS), Fe³⁺ 185.70 mg/g DS, and RH 406.02 mg/g DS, respectively. Moreover, the results showed that the composite conditioner is effective for the specific resistance to filtration decreased from initial 1.66E + 13 m/kg to 2.44E + 11 m/kg. The analysis of EPS showed that extractable proteins in EPS increased to maximum when the sludge was pretreated by Fe³⁺-RH because EPS were destroyed and proteins in EPS were released. After the addition of ethanol, extractable protein content was reduced because of protein precipitation and released interstitial water and bound water. The sludge morphology analysis indicated that the RH as a skeleton builder provided the outflow passages, which enhanced the dewatering performance of sludge. From these results, the combination treatment of ethanol and Fe³⁺-RH is a promising synergetic strategy to enhance the dewaterability of sewage sludge.

High-Rate Anaerobic Digestion of Waste Activated Sludge by Integration of Electro-Fenton Process

Anaerobic digestion (AD), being the most effective treatment method of waste activated sludge (WAS), allows for safe disposal. The present study deals with the electro-Fenton (EF) pretreatment for enhancing the WAS biogas potential with low-cost iron electrodes. The effect of pretreatment on the physicochemical characteristics of sludge was assessed. Following EF pretreatment, the pH, conductivity, soluble chemical oxygen demand (SCOD), and volatile fatty acids (VFA) increased to 7.5, 13.72 mS/cm, 4.1 g/L, and 925 mg/L, respectively. Capillary suction time (CST) analysis highlighted the dewaterability effect of EF on WAS, as demonstrated by the decrease in CST from 429 to 180 s following 30 min of pretreatment. Batch digestion assays presented an increase in the biogas yield to 0.135 L/g volatile solids (VS) after 60 min of EF pretreatment in comparison to raw sludge (0.08 L/g VS). Production of biogas was also found to improve during semi-continuous fermentation of EF-pretreated sludge conducted in a lab-scale reactor. In comparison to raw sludge, EF-pretreated sludge produced the highest biogas yield (0.81 L biogas/g VS) with a high COD removal rate, reaching 96.6% at an organic loading rate (OLR) of 2.5 g VS/L. d. Results revealed that the EF process could be an effective WAS disintegration method with maximum recovery of bioenergy during AD.

Basic and procedural requirements for energy potential from biogas of sewage treatment plants

The objective of this study is to evaluate the potential for electricity generation in the State of São Paulo (SP) from the sewage treatment. A sewage treatment plant (STP) with domain in the production of biogas from wastewater treatment plant (WWTP) is the basis for this case study. The basic premise is that the very generation of electricity in STPs is advantageous for companies in the sanitation sector in Brazil, resulting in cost reductions of the treatment process. Gains at the end of the process are found in two levels, namely: (i) economic, by generating 165% of electricity from biogas burning in relation to the expend; (ii) energy, by adding a new sustainable and storable energy source equivalent to 4% of natural gas offered in the State of SP and 0,5% of electricity produced from biogas burning in relation to electricity consumption. In conclusion, the potential of electricity production linked to the biogas at STPs is capable of supply its domestic demand and export the surplus to other segments of the state and national economy.

Enhancing methane production of excess sludge and dewatered sludge with combined low frequency CaO-ultrasonic pretreatment

Methane production of excess sludge (ES) and dewatered sludge (DS) were investigated with low frequency CaO-ultrasonic pretreatment. The results showed that the concentrations of SCOD and VFAs in pretreated ES (P-ES) and DS (P-DS) were 212.11% and 75.26%, 270.30% and 159.52% higher than those of untreated ES and DS, respectively. The contents of acetic acid and ethanol comprised 83.87%-92.88% of the total VFAs. The cumulative methane production (CMP) of P-ES and P-DS were 167.08 and 162.96 mL/g·VS, respectively, which resulted in 40.45% and 36.94% higher than those of untreated ones. The biodegradability of P-ES was 87.65%, which was close to theoretical value. Low frequency CaO-ultrasonic pretreatment could not only improve the performance of anaerobic digestion (AD), but also accelerate the decomposition rate of two kinds of sludge. Therefore, this study provided meaningful insight for exploring efficient pretreatment strategy to stabilize and enhance AD performance for practical application.

Disintegration of Wastewater Activated Sludge (WAS) for Improved Biogas Production

Due to rapid urbanization, the number of wastewater treatment plants (WWTP) has increased, and so has the associated waste generated by them. Sustainable management of this waste can lead to the creation of energy-rich biogas via fermentation processes. This review presents recent advances in the anaerobic digestion processes that have led to greater biogas production. Disintegration techniques for enhancing the fermentation of waste activated sludge can be apportioned into biological, physical and chemical means, which are included in this review; they were mainly compared and contrasted in terms of the ensuing biogas yield. It was found that ultrasonic- and microwave-assisted disintegration provides the highest biogas yield (>500%) although they tend to be the most energy demanding processes (>10,000 kJ kg−1 total solids).