Thermo-alkaline pretreatment of waste activated sludge at low-temperatures: effects on sludge disintegration, methane production, and methanogen community structure

Hydrothermal carbonisation of waste-activated sludge: Possible route to improve sludge management in municipal wastewater treatment facilities

This study investigated the integration of hydrothermal carbonisation (HTC) and anaerobic digestion (AD) of waste-activated sludge (WAS) for sustainable sludge management. HTC were performed at 180 °C, 210 °C, and 260 °C for 30, 60, and 120 min. Hydrochar and aqueous hydrothermal liquor (AHL) were tested for biochemical methane potential. Increasing HTC severity reduced solids and carbon content, altered H/C and O/C ratios and lowered hydrochar yield while increased its stability. Under the mildest condition (180 °C for 30  min), AHL achieved the highest methane yield (387 NmL CH4 g-1 VS), 33 % higher than raw WAS, while hydrochar produced lower yields. Heavy metals were mainly retained in hydrochar with most within regulatory limits except cadmium, leaving AHL relatively clean. Although total biogas production output decreased due to carbon partitioning into hydrochar, AHL required only 86 % of WAS digestion capacity. HTC improves methane yield per unit of AHL, enables better use of existing plant capacity, and generates value-added byproducts.

How does alkaline-thermal pretreatment followed by anaerobic digestion affect the content of polyethylene terephthalate and polyamide 66 microplastics?

Microplastics (MPs) are ubiquitous and increasing in quantity, causing raising concern. Wastewater treatment plants (WWTPs) are a point source for both aquatic environments and soil, through the use of sludge in agriculture. Understanding the fate of MPs within the wastewater and sludge lines of a treatment plant and, possibly, enhancing their removal will improve the safe reuse of sludge and water effluent and the wastewater biorefinery concept application. This study investigates the effects of alkaline-thermal pretreatment of sludge, followed by anaerobic digestion, on the physical and chemical characteristics of polyethylene terephthalate (PET) and polyamide 66 (PA(66)) contained. Experiments were conducted to evaluate the influence of different NaOH concentrations, temperatures, and reaction times on the degradation of the MPs in anaerobic digestion. PET MPs exhibited relevant mass reduction and structural changes in relation to the NaOH concentration and temperature. PA(66) MPs showed limited chemical alterations, indicating higher resistance to degradation. Batch anaerobic digestion tests of pretreated samples did not modify them further. Chemical characterization of MPs was performed using both Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) and Focal Plane Array-Fourier Transform-Imaging-Micro-Spectroscopy (FPA-μFTIR-Imaging), revealing distinct trends between surface-level and bulk material changes in the MPs. The results highlighted that ATR-FTIR recorded lower carbonyl index values compared to FPA-μFTIR-Imaging. These findings emphasized the importance of using complementary analytical techniques to thoroughly understand MPs degradation. The outcomes suggest that tailored pretreatment strategies are essential to enhance MPs removal in WWTPs, ensuring safer sludge reuse within a circular economy framework.

Synergistic effect of electrooxidation and anaerobic digestion of waste-activated sludge for microbial inactivation

Electrochemical pretreatment and anaerobic digestion (AD), as well as a combination of both processes, were studied for the treatment of waste-activated sludge (WAS) to evaluate microbial inactivation, for faecal coliforms, Salmonella spp., bacteriophages, and helminth eggs. Electrooxidation (EO) of WAS was performed in a commercial cell with boron-doped diamond electrodes. 1 L of WAS (3% total solids) was fed to the electrochemical cell in recirculation mode. The conditions tested were 19.3 mA/cm2, 30 min, and 3.8 L/min. For AD tests, raw and pretreated WAS were digested in an OxiTop® OC 110 apparatus for 15 days. Inactivation of faecal coliforms, Salmonella spp., and bacteriophages reached more than 5 logs when EO was combined with AD. In contrast, EO alone did not inactivate these parameters, while AD achieved eliminations around 3 logs. Moreover, the combined process inactivated 91% of the initial viable helminth eggs, considerably higher than AD (29%) and EO (0%). The results suggest that EO separates extracellular polymeric substances and segregates particles, including microorganisms, that are exposed to environmental factors (e.g., volatile fatty acids or ammonia) during AD, showing a synergistic effect.

Enhanced performance and economic feasibility of sewage sludge digestion using a two-stage anaerobic digestion with a dynamic membrane and alkaline-thermal pretreatment

This study suggests a high-rate sewage sludge anaerobic digestion (AD) process. An alkaline-thermal pretreatment and a dynamic membrane (DM) were used to enhance AD efficiency and economic feasibility in a two-stage system. The effect of pretreatment on volatile fatty acid (VFA) production in the acidogenic phase was investigated at various hydraulic retention times (HRT). After optimizing the acidogenic phase condition (HRT of 3 days), single- and two-stage AD processes with DM modules were operated simultaneously to compare performance. The highest methane production rates of 0.69 L/L/d for single-stage AD and 1.10 L/L/d for two-stage AD were observed at a total HRT of 12 days. Phase separation enhanced the growth of acetoclastic methanogens. A techno-economic analysis showed that the two-stage AD system would achieve a positive net present value within 2 years. This study demonstrated the feasibility of high-rate AD systems for sewage sludge using DM, alkaline-thermal pretreatment, and phase separation.

Investigation of thermal alkaline pretreatment of primary and waste activated sludge on the energy efficiency of sludge digestion

Because of the naturally limited anaerobic degradability and limited biogas yield of raw sludge (RS), this study aims to increase the biogas production of primary sludge (PS) and waste activated sludge (WAS) by the integration of thermal alkaline process (TAP). PH 11 is confirmed to be the most suitable pH value for the TAP of both sludges. Moreover, with the pretreatment at pH 11 and 160 °C (6 bar) for 30 min, the investigated PSs and WASs achieved an increased biogas production of up to 81 % and 72 %, respectively. The improved net electricity production of WASs after TAP varied between 15-43 % compared to conventional WAS digestion. However, the TAP of PS at pH 11 enhanced the biogas production by 1-81 %, which did not constantly contribute to an improved net electricity production.

Sewage sludge management and enhanced energy recovery using anaerobic digestion: an insight

Sewage sludge (SS) is a potential source of bioenergy, yet its management is a global concern. Anaerobic digestion (AD) is applied to effectively valorize SS by reclaiming energy in the form of methane. However, the complex floc structure of SS hinders hydrolysis during AD process, thus resulting in lower process efficiency. To overcome the rate-limiting hydrolysis, various pre-treatment methods have been developed to enhance AD efficiency. This review aims to provide insights into recent advancements in pre-treatment technologies, including mechanical, chemical, thermal, and biological methods. Each technology was critically evaluated and compared, and its relative worth was summarized based on full-scale applicability, along with economic benefits, AD performance improvements, and impact on digested sludge. The paper illuminates the readers about existing research gaps, and the future research needed for successful implementation of these approaches at full scale.

The flow pattern effects of hydrodynamic cavitation on waste activated sludge digestibility

The disintegration of raw sludge is of importance for enhancing biogas production and facilitates the degradation of substrates for microorganisms so that the efficiency of digestion can be increased. In this study, the effect of hydrodynamic cavitation (HC) as a pretreatment approach for waste activated sludge (WAS) was investigated at two upstream pressures (0.83 and 1.72 MPa) by using a milli-scale apparatus which makes sludge pass through an orifice with a restriction at the cross section of the flow. The HC probe made of polyether ether ketone (PEEK) material was tested using potassium iodide solution and it was made sure that cavitation occurred at the selected pressures. The analysis on chemical effects of HC bubbles collapse suggested that not only cavitation occurred at low upstream pressure, i.e., 0.83 MPa, but it also had high intensity at this pressure. The pretreatment results of HC implementation on WAS were also in agreement with the chemical characterization of HC collapse. Release of soluble organics and ammonium was observed in the treated samples, which proved the efficiency of the HC pretreatment. The methane production was improved during the digestion of the treated samples compared to the control one. The digestion of treated WAS sample at lower upstream pressure (0.83 MPa) resulted in higher methane production (128.4 mL CH4/g VS) compared to the treated sample at higher upstream pressure (119.1 mL CH4/g VS) and control sample (98.3 mL CH4/g VS). Thus, these results showed that the HC pretreatment for WAS led to a significant increase in methane production (up to 30.6%), which reveals the potential of HC in full-scale applications.

Feasibility and mechanism of recycling carbon resources from waste cyanobacteria and reducing microcystin toxicity by dielectric barrier discharge plasma

Recycling carbon resources from discarded cyanobacteria is a worthwhile research topic. This study focuses on the use of dielectric barrier discharge (DBD) plasma technology as a pretreatment for anaerobic fermentation of cyanobacteria. The DBD group (58.5 W, 45 min) accumulated the most short chain fatty acids (SCFAs) along with acetate, which were 3.0 and 3.3 times higher than the control. The DBD oxidation system can effectively collapse cyanobacteria extracellular polymer substances and cellular structure, improve the biodegradability of dissolved organic matter, enrich microorganisms produced by hydrolysis and SCFAs, reduce the abundance of SCFAs consumers, thereby promoting the accumulation of SCFAs and accelerating the fermentation process. The microcystin-LR removal rate of 39.8% was obtained in DBD group (58.5 W, 45 min) on day 6 of anaerobic fermentation. The toxicity analysis using the ECOSAR program showed that compared to microcystin-LR, the toxicity of degradation intermediates was reduced. The contribution order of functional active substances to cyanobacteria cracking was obtained as eaq- > •OH > 1O2 > •O2- > ONOO-, while the contribution order to microcystin-LR degradation was eaq- > •OH > •O2- > 1O2 > ONOO-. DBD has the potential to be a revolutionary pretreatment method for cyanobacteria anaerobic fermentation.

Superparamagnetic Iron Oxide Nanoparticles as Additives for Microwave-Based Sludge Prehydrolysis: A Perspective

Wastewater treatment plants are critical for environmental pollution control. The role that they play in protecting the environment and public health is unquestionable; however, they produce massive quantities of excess sludge as a byproduct. One pragmatic approach to utilizing excess sludge is generating methane via anaerobic digestion. For this, a prehydrolysis step can significantly improve digestion by increasing biogas quality and quantity while decreasing final sludge volumes. One of the many prehydrolysis approaches is to deliver heat into sludge via microwave irradiation. Microwave-absorbing additives can be used to further enhance thermal degradation processes. However, the implications of such an approach include potential release of said additive materials into the environment via digested sludge. In this perspective, we present and discuss the potential of superparamagnetic iron oxide nanoparticles (SPIONs) as recoverable, hyperreactive microwave absorbers for sludge prehydrolysis. Due to their size and characteristics, SPIONs pack spin electrons within a single domain that can respond to the magnetic field without remanence magnetism. SPIONs have properties of both paramagnetic and ferromagnetic materials with little to no magnetic hysteresis, which can enable their rapid recovery from slurries, even in complicated reactor installations. Further, SPIONs are excellent microwave absorbers, which result in high local heat gradients. This perspective introduces the vision that SPION properties can be tuned for desirable dielectric heating and magnetic responses while maintaining material integrity to accomplish repeated use for microwave-enhanced pretreatment.

Exploration of browning reactions during alkaline thermal hydrolysis of sludge: Maillard reaction, caramelization and humic acid desorption

The browning reaction produces melanoidins, 5-hydroxymethylfurfural (HMF) and humic acids which influence subsequent anaerobic digestion and protein recovery. This paper systematically evaluates the variation of organics that make sludge browning with heating temperature and reaction time, the effect of browning organics on protein recovery and anaerobic digestion, and finally proposes a pathway for the occurrence of the Maillard reaction (MR) in the sludge environment. The results show that the browning of sludge hydrolysate is related to the comprehensive influence of the MR, caramelization and humic acid desorption. The increase of temperature (80 °C-150 °C) and pH (9-13) will promote the extent of browning of sludge hydrolysate, and the sludge browning reaction basically stabilizes at the reaction time of 1 h. Humic acid and melanoidin could co-precipitate with the protein, thereby reducing the purity of the recovered protein. The inhibition of anaerobic digestion starts when the melanoidin concentration is 8.01 mmol/L. The three-dimensional fluorescence, GC-MS and FT-IR analysis show that melanoidins have the same functional groups and fluorescence properties as humic acid does, and the humic acid in the supernatant of the sludge treated with ATH was not only converted at its adsorbed state, but also possibly generated by the reaction of the dissolved proteins with polysaccharides. Finally, LC-MS/MS was used to identify the intermediate products of the MR and the possible structural formula of melanoidin. This study further clarifies the browning reaction in hydrothermal sludge treatment and provides help for the accuracy of subsequent studies.

Untangling the effect of solids content on thermal-alkali pre-treatment and anaerobic digestion of sludge

Total solids (TS) content is critical for thermal hydrolysis and anaerobic digestion (AD) performance, but its role in thermal-alkaline pre-treatment (TAP) is unclear. Therefore, this study aimed to reveal the key role of TS content in TAP and AD of waste activated sludge. The results showed that the optimum TS content of TAP (at 90 °C for 1 h, pH = 10) was 8 %. Sludge disintegration and methane production increased from 19.7 ± 2.2 % to 34.3 ± 2.9 % and from 167.4 ± 4.2 to 246.0 ± 6.2 mL/g volatile solids, respectively, when TS content were increased from 2 % to 8 %. A high TS content will likely promote sludge disintegration since it will reduce heat loss and improve heating efficiency. Additionally, increasing TS content from 2 % to 10 % minimized the production of intracellular reactive oxygen species by 30.4 ± 0.7 % and increased cell viability by 11.5 ± 2.6 %. In contrast, excessive TS content (i.e., ≥10 %) deteriorated the fluidity of sludge, which prevents it from disintegration. Once TS reached 10 %, the accumulation of ammonia nitrogen and volatile fatty acids reached 812.7 ± 27.4 and 1932.0 ± 5.3 mg/L, respectively, which reduced the activity of acidulase and coenzyme F420 and shifted the archaeal community from acetylotrophic to hydrogenotrophic methanogens. This article provides new insights into the TS content in TAP and AD technology.

Mechanism of dielectric barrier plasma technology to improve the quantity and quality of short chain fatty acids in anaerobic fermentation of cyanobacteria

The recycling of high value carbon resources from cyanobacteria has become a research hotspot. This work investigated the possibility of dielectric barrier discharge (DBD) plasma pretreatment to improve the anaerobic fermentation performance of cyanobacteria. The maximum accumulations of short-chain fatty acids (SCFAs) and acetic acid in DBD group were 3.30 and 1.49 times of that in control group. The physical effects of DBD plasma and the oxidative stress response of cyanobacteria cells could improve the solubilization of cyanobacteria polymer. The destruction of humus by DBD plasma can reduce the negative impact of humus on the early stage of anaerobic fermentation, thus facilitating the rapid start of anaerobic fermentation. The contents of Bacteroidetes, Firmicutes and Chloroflexi in DBD group were higher than those in control group, while the content of Proteobacteria was on the contrary, which was conducive to the hydrolysis and acidification process. The decrease of Methanosaeta sp. and Methanosarcina sp. abundance in DBD group might be another reason for the increase of acetic acid ratio. Under the joint action of plasma chemical oxidation and microbial degradation, the degradation effect of microcystin-LR in the anaerobic fermentation supernatant of DBD group was better than that of the control group, which was conducive to the recycling of cyanobacteria anaerobic fermentation supernatant. Therefore, DBD pretreatment was conductive to recycling valuable carbon source from cyanobacteria and can be further developed as a potential new pretreatment technology.

Principles and potential of thermal hydrolysis of sewage sludge to enhance anaerobic digestion

Sewage sludge is rich source of carbon, nutrients, and trace elements and can be subjected to proper treatment before disposal to fulfill government legislation and protect receiving environments. Anaerobic digestion (AD) is a well-adopted technology for stabilizing sewage sludge and recovering energy-rich biogas and nutrient-rich digestate. However, a slow hydrolysis rate limits the biodegradability of sludge. In the present study we have attempted to explain the potential of thermal hydrolysis to enhance anaerobic digestion of sewage sludge. Thermal pretreatment improves biodegradability and recycling of the sludge as an excellent energy and nutrients recovery source at reasonable capital (CAPEX) and operational (OPEX) costs. Other pretreatments like conventional (below/above 100 °C), temperature-phased anaerobic digestion (TPAD), microwave and chemically mediated thermal pretreatment have also been accounted. This review provides a holistic overview of sludge's characterization and value-added properties, various techniques used for sludge pretreatment for resource recovery, emphasizing conventional and advanced thermal pretreatment, challenges in scale-up of these technologies, and successful commercialization of thermal pretreatment techniques.

Fate and effects of polyethylene terephthalate (PET) microplastics during anaerobic digestion of alkaline-thermal pretreated sludge

Plastics are resilient, hard to degrade materials that can persist in nature for centuries. Microplastics (MPs) exhibit similar tough character and hold the potential to harm marine and terrestrial ecosystems upon their release into the environment. Most modern wastewater treatment plants remove MPs from wastewater with over 90% efficiency but unfortunately concentrate them in sludge. Recent studies have reported MPs' impact on the performance of sludge treatment systems, including anaerobic digesters. Despite its resilience, polyethylene terephthalate (PET) has inherent weaknesses against alkaline and thermal conditions and becomes more prone to further degradation if exposed to such stress conditions. Sludge pretreatment practices aiming to increase biogas production by disrupting floc structure show great similarity with the stress factors mentioned. Thus, this study aims to integrate pretreatment with anaerobic digestion and investigate the fate and effects of PET MPs during these processes. For this purpose, waste activated sludge samples spiked with different doses of PET (0, 1, 3, 6 mg/g TS) in sizes of 250-500 µm were pretreated by 0.5 M alkali for two days and then thermally hydrolyzed at 127 °C for 120 min. Pretreated and unpretreated sludges were digested in a 60-day biochemical methane potential test. The results showed that the spiking of PET MPs into sludge posed a positive impact on the methane yield of unpretreated reactors at statistically significant levels. Integrating pretreatment increased the methane yield by 22.0% and made the impact of MPs on digester efficiency no longer observable. Also, PET exposed to pretreatment and 60-day digestion experienced remarkable changes in surface morphology, crystallinity and carbonyl index, which can further impact their fate and effects on the environment.

Microalgae and Cyanobacteria Biomass Pretreatment Methods: A Comparative Analysis of Chemical and Thermochemical Pretreatment Methods Aimed at Methane Production

Anaerobic digestion of microalgae and cyanobacteria was first proposed as a destination for algal biomass accumulated on stabilization ponds since it could not be disposed of directly in the environment. Now, the versatility of algal biomass makes them a suitable candidate to produce biofuels and other biomolecules in biorefineries. Anaerobic digestion of biomass is advantageous because it does not require the extraction of specific cellular constituents or drying of the biomass. Nevertheless, challenges remain regarding biomass concentration and their resistant cell walls, which are factors that could hamper methane yield. Many pretreatment methods, including chemical and thermochemical, have been proposed to break down the complex polymers present on the cell wall into smaller molecules. Unfortunately, the relationship between biomass solubilization and methane yield is not well defined. This article intends to review the anaerobic digestion of algal biomass and the role of chemical and thermochemical pretreatments in enhancing methane production. Several pretreatment conditions selected from the scientific literature were compared to verify which conditions actually improve methane yield. The severity of the selected pretreatments was also assessed using the combined severity factor. Results suggest that thermochemical pretreatment in less severe conditions is the most efficient, leading to a greater increase in methane yield. Only enzymatic pretreatments and some thermal pretreatments result in a positive energy balance. The large-scale implementation of pretreatment methods requires technological innovations to reduce energy consumption and its integration with other processes in wastewater treatment plants.

Energy feasibility and life cycle assessment of sludge pretreatment methods for advanced anaerobic digestion

Energy sustainability is one of the critical parameters to be studied for the successful application of pretreatment processes. This study critically analyzes the energy efficiency of different energy-demanding sludge pretreatment techniques. Conventional thermal pretreatment of sludge (∼5% total solids, TS) produced 244 mL CH₄/gTS, which could result in a positive energy balance of 2.6 kJ/kg TS. However, microwave pretreatment could generate only 178 mL CH₄/gTS with a negative energy balance of -15.62 kJ/kg TS. In CAMBI process, the heat requirements can be compensated using exhaust gases and hot water from combined heat and power, and electricity requirements are managed by the use of cogeneration. The study concluded that <100 ℃ pretreatment effectively enhances the efficiency of anaerobic digestion and shows positive energy balance over microwave and ultrasonication. Moreover, microwave pretreatment has the highest global warming potential than thermal and ultrasonic pretreatments.

Optimization of supercritical carbon dioxide explosion for sewage sludge pre-treatment using response surface methodology

The present work was conducted to assess whether the implementation of Supercritical Carbon dioxide Explosion (SCE) is an efficient approach for sewage sludge pre-treatment. In this context, SCE was optimized with the aim to develop a method attempting to increase the biodegradability of sewage sludge's organic matter content, and thus, to enhance the subsequent anaerobic digestion and methane production. The statistical tool of response surface methodology was applied to evaluate the effects of the main pre-treatment parameters (i.e. temperature and time) and their interactions on methane yield, which was defined as the response. Temperature was found to be the most significant variable, having the greatest effect on methane yield. Following this, an optimum set of pre-treatment conditions corresponding to a temperature of 115 °C and time of 13 min, was determined. Under these optimum conditions, the predicted response value was 300 mL CH₄/g of volatile solids. The corresponding experimental value obtained from the validation experiment fitted well with this value, clearly demonstrating the effective use of response surface methodology in optimizing SCE. Additionally, under optimum conditions, the methane yield presented a statistically significant increment of 8.7%, compared to untreated sludge. This revealed the impact of SCE as an effective and alternative way for the efficient pre-treatment of sewage sludge. Finally, thermal pre-treatment, alkaline and acidic hydrolysis were also applied to the already pre-treated sludge. It was concluded that the combined pre-treatment techniques contributed to a further increase of methane production compared to raw (untreated) substrate.

Critical evaluation of heat extraction temperature on soluble microbial products (SMP) and extracellular polymeric substances (EPS) quantification in wastewater processes

While soluble microbial products (SMP) and extracellular polymeric substances (EPS) in wastewater bioprocesses have been widely studied, a lack of standard quantification procedures make it difficult to compare results between studies. This study investigated the effect of temperature on SMP and EPS profiles for biological nutrient removal (BNR) sludges and aerobic membrane bioreactor sludge by adapting the commonly used heat extraction and centrifugation scheme, followed by colorimetric quantification of the carbohydrate and protein fractions using the phenol-sulfuric acid (PS) and the bicinchoninic acid (BCA) methods, respectively. To overcome known inconsistencies in colorimetry, total carbon (TC), total nitrogen (TN), and fluorometry analyses were performed in tandem. SMP samples marginally benefitted from heat extraction, owing to their mostly soluble nature, while EPS profiles were greatly influenced by temperature. 60 °C appears to be a suitable general-purpose extraction temperature near the lysis threshold for the sludges tested. The PS method's misestimation due to lack of specificity was observed and contrasted by TC analyses, while the TN analyses corroborated the BCA assays. Fluorometry proved to be a sensitive and rapid analytical method that provided semi-quantitative information on SMP and EPS constituents, particularly its proteinaceous components, with positive implications for robust wastewater process control.

Sludge disintegration and anaerobic digestion enhancement by alkaline-thermal pretreatment: Economic evaluation and microbial population analysis

Alkaline-thermal pretreatment was examined for waste activated sludge (WAS) disintegration and subsequent anaerobic digestion (AD). Pretreatment at 60 °C was estimated to provide better economic benefits than higher temperature conditions. The maximum methane yield of 215.6 mL/g COD was achieved when WAS was pretreated at 60 °C and pH 10 for 24 h, which was 46.6% higher than untreated WAS. The pretreatment condition also provided the maximum net savings. The degree of sludge disintegration, considering both loosely bound-extracellular polymeric substance and soluble COD, would be a better indicator to predict anaerobic digestibility than the solubilization rate that considers soluble COD alone. Microbial analysis implied that pretreatment facilitated the growth of hydrolytic bacteria, phyla Bacteroidetes and Firmicutes. In addition, sludge pretreatment enhanced the growth of both acetoclastic and hydrogenotrophic methanogens, genera Methanosaeta and Methanobacterium. The mild AT-PT would be useful to enhance the digestion performance and economic benefit of WAS digestion.

Current advances and future outlook on pretreatment techniques to enhance biosolids disintegration and anaerobic digestion: A critical review

Waste activated sludge (biosolids) treatment is intensely a major problem around the globe. Anaerobic treatment is indeed a fundamental and most popular approach to convert organic wastes into bioenergy, which could be used as a carbon-neutral renewable and clean energy thus eradicating pathogens and eliminating odor. Due to the sheer intricate biosolid matrix (such as exopolymeric substances) and rigid cell structure, hydrolysis becomes a rate-limiting phase. Numerous different pretreatment strategies were proposed to hasten this rate-limiting hydrolysis and enhance the productivity of anaerobic digestion. This study discusses an overview of previous scientific advances in pretreatment options for enhancing biogas production. In addition, the limitations addressed along with the effects of inhibitors in biosolids towards biogas production and strategies to overcome discussed. This review elaborated the cost analysis of various pretreatment methods towards the scale-up process. This review abridges the existing research on augmenting AD efficacy by recognizing the associated knowledge gaps and suggesting future research.

Black liquor increases methane production from excess pulp and paper industry sludge

The aim of the study was to use black liquor produced during the soda pulping process in a pulp and paper mill to increase methane production during pulp and paper industry sludge treatment and decrease the treatment cost. The effects of black liquor on sludge solubilization and methane production were assessed and the economic feasibility of the process was evaluated. Black liquor and NaOH were found to be equivalent in the thermochemical pretreatment process to solubilize sludge and disintegrate flocs. However, adding black liquor increased the background chemical oxygen demand and volatile fatty acid concentration and increased the amount of methane produced by approximately 7-30%. A start-up delay was emphasized by first-order kinetics model due to black liquor addition while methane production remained stable. Economic assessments of five scenarios were performed. It was found to be economically feasible to use black liquor to replace NaOH for the thermal pretreatment process. The surplus methane generated suggested that co-digestion of sludge and black liquor allows surplus bioenergy to be produced during the thermochemical pretreatment anaerobic digestion process.

Improvement of anaerobic co-digestion of plant waste and excess sludge using calcium peroxide

Plant waste (PW) and excess sludge (ES) are two main organic matters of municipal solid waste. However, there are few reports on their anaerobic co-digestion. In this work, the mixed proportion of PW and ES anaerobic co-digestion was first optimized at mesophilic temperature, and then the anaerobic co-digestion of PW and ES was enhanced with strong oxidant calcium peroxide (CP). The results showed that the optimal mixing ratio of PW and ES was 1/1 (in terms of volatile solids), the C/N of mixed digestion substrate was 23.5/1, and the maximum methane production was 172.6 mL/g (in terms of volatile solids). CP could enhance methane production from anaerobic co-digestion of PW and ES. When the content of CP was 0.2 g/g (in terms of total suspended solids), the maximum methane production was 234.8 mL/g, about 1.4 times of the blank. The mechanism investigation showed that CP promoted the release of organic matter during the co-digestion, and the higher the content of CP, the greater the release of soluble chemical oxygen demand. The presence of appropriate amount of CP promoted the activities of key enzymes in anaerobic fermentation process, and then increased the efficiency of methane production. The results of this work provide an alternative strategy for the resource utilization of PW and ES.

Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability of waste activated sludge

Thermal alkaline pretreatment (TAP) of waste activated sludge (WAS) before anaerobic digestion (AD) was reviewed. Focus of the review was on impact of TAP process parameters on biomethane yield (BY) and kinetics of AD and downstream dewatering. With higher initial biodegradability of untreated WAS, effect of TAP on BY decreases. Depending on initial biodegradability, BY increase of 22-97% is expected. Treatment temperatures below 100 °C showed to be as effective as temperatures higher than 100 °C in terms of BY increase. Alkali dosage and resulting initial pH have a significant effect on BY increase and showed to have an optimum range of 40-60 mg NaOH per g total solids (TS) of sludge. It is advised that alkali is dosed based on solids content in WAS and monitored by pH. Treatment time of 1.5-5 h is sufficient for an effective low temperature TAP (T < 100 °C), with longer treatment times showing no positive impact on BY increase. Load of sludge liquor with organics and nutrients increases with more intensive TAP conditions. Despite kinetic enhancement of hydrolysis step in AD, more research is needed to clarify if TAP improves kinetics of entire AD process which determines required digester volume. Impact of TAP on dewaterability of digestate is ambiguous and needs more investigation using standardized methods, also with regards to potential effects on polymer demand. Findings of experimental studies were reflected against available data from commercialized TAP process of Pondus®, throughout review. Finally, important process design parameters of TAP such as input TS and point of alkali dosage are discussed and recommendations for future research are presented.

The Impact of Antimicrobial Substances on the Methanogenic Community during Methane Fermentation of Sewage Sludge and Cattle Slurry

This study showed the effect of amoxicillin (AMO), and oxytetracycline (OXY) at a concentration of 512 µg mL−1, and sulfamethoxazole (SMX), and metronidazole (MET) at a concentration of 1024 µg mL−1 on the efficiency of anaerobic digestion (AD) of sewage sludge (SS) and cattle slurry (CS). The production of biogas and methane (CH4) content, and the concentration of volatile fatty acids (VFAs) was analyzed in this study. Other determinations included the concentration of the mcrA gene, which catalyzes the methanogenesis, and analysis of MSC and MST gene concentration, characteristic of the families Methanosarcinaceae and Methanosaetaceae (Archaea). Both substrates differed in the composition of microbial communities, and in the sensitivity of these microorganisms to particular antimicrobial substances. Metronidazole inhibited SS fermentation to the greatest extent (sixfold decrease in biogas production and over 50% decrease in the content of CH4). The lowest concentrations of the mcrA gene (106 gD−1) were observed in CS and SS digestates with MET. A decline in the number of copies of the MSC and MST genes was noted in most of the digestate samples with antimicrobials supplementation. Due to selective pressure, antimicrobials led to a considerably lowered efficiency of the AD process and induced changes in the structure of methanogenic biodiversity.

Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on anaerobic digestion and impact on dewaterability and refractory COD

Thermal alkaline pretreatment (TAP) of waste activate sludge (WAS) was carried out in pilot-scale over a year to investigate its seasonal effects on anaerobic digestion and its impact on dewaterability, sludge liquor quality and formation of soluble refractory COD (sCOD_ref). Temperature of TAP was set at 65-70 °C and pH was increased by initial dosing of sodium hydroxide [NaOH] (50% w/w, 1-2.5 mL/L sludge) as alkali agent following 2-2.5 h reaction time. Pilot digesters were fed with primary sludge (PS) and hydrolyzed WAS (HWAS) and compared to a reference digester fed with PS and untreated WAS. Biogas yield increase due to TAP of WAS showed a sinusoidal trend throughout the year with maximum in summer (+42%), minimum in winter (+3%) and average of +20%, indicating a strong seasonal effect on TAP efficiency. Ammonium [NH₄⁺-N], orthophosphate [PO₄^3--P] and sulphate [SO₄^2-] in sludge liquor increased by 34.6%, 17.0% and 21.6% with TAP, respectively. Centrifugation tests showed no significant difference in dewaterability of both digestates with respect to total solids of sludge cake. Normalized capillary suction time of digestate increased due to TAP, indicating a lower capability for water release. Furthermore, detected sCOD_ref after batch aerobic biodegradation tests showed an increase of 30.3% with TAP. Hence, implementation of TAP of WAS in full-scale will potentially lead to an increase of 0.8-1.1 mg/L of sCOD_ref in effluent of six wastewater treatment plants (WWTP) in Berlin. In conclusion, TAP of WAS leads to increase in biogas production with a slighter negative impact on effluent COD quality than high-temperature thermal hydrolysis.

Process optimization and effect of thermal, alkaline, H2O2 oxidation and combination pretreatment of sewage sludge on solubilization and anaerobic digestion

BACKGROUND

This study investigated the feasibility of enhancing anaerobic digestion of sewage sludge with triple, dual, and individual pretreatment of waste activated sludge with heat, alkalinity, and hydrogen peroxide. These pretreatments disrupt sludge flocs, organisms' cell walls, extracellular polymeric substance, and intracellular organic matter, which increase biodegradability and hydrolysis rate of activate sludge. In addition, the influence of various variables on methane production was analyzed using the response surface methodology with the quadratic model. Eventually, an optimized temperature and chemical concentration for the highest methane production and lowest chemical usage is suggested.

RESULTS

The highest amount of methane production was obtained from the sludge pretreated with triple pretreatment (heat (90 °C), alkaline (pH = 12), and hydrogen peroxide (30 mg H2O2/g TS)), which had better performance with 96% higher methane production than that of the control sample with temperature of 25 °C approximately and a pH = 8. Response surface methodology with a quadratic model was also used for analyzing the influence of temperature, pH, and hydrogen peroxide concentration on anaerobic digestion efficiency. It was revealed that the optimized temperature, pH, and hydrogen peroxide concentration for maximizing methane production and solubilization of sludge and minimizing thermal energy and chemical additives of the pretreatments are 83.2 °C, pH = 10.6 and 34.8 mg H2O2/g TS, respectively, has the desirability of 0.67.

CONCLUSION

This study reveals that triple pretreatment of waste activated sludge performed better than dual and individual pretreatment, respectively, in all desirable output parameters including increasing methane production as the most important output, increasing in COD solubilization, protein and polysaccharide, and decreasing in VSS solubilization.

Comparison of two advanced anaerobic digestions of sewage sludge with high-temperature thermal pretreatment and low-temperature thermal-alkaline pretreatment

Semi-continuous experiments were conducted to compare the performances and energy efficiencies of two advanced anaerobic digestions (AAD) of sewage sludge with high-temperature thermal pretreatment (HTTP, 160 ± 1 °C and 0.55 MPa for 30 min) and low-temperature thermal-alkaline pretreatment (LTTAP, 60 ± 1 °C and pH 12.0 ± 0.1 for 30 min), which had similar sludge disintegration degree (9.44-9.48%). At the steady period of a SRT 20 d, the two AAD had similar methane production (150.22 ± 9.55 ml/L/d and 151.02 ± 12.56 ml/L/d) and organic matter removals (22.54 ± 2.84% and 23.15 ± 2.46% for volatile solids-VS). The results of high-throughput sequencing showed that the methanogenic pathways of the two AAD were strictly hydrogenotrophic (AAD with HTTP) and hydrogenotrophic/acetoclastic methanogenesis (AAD with LTTAP), respectively. The energy balance analysis suggested that the AAD with LTTAP was superior to that with HTTP because the former had a higher energy efficiency (1.610) than the latter (1.358).

Development of an alkaline/acid pre-treatment and anaerobic digestion (APAD) process for methane generation from waste activated sludge

Anaerobic sludge digesters are biorefineries for energy recovery from waste activated sludge (WAS) via methane production, in which disintegration of floc structure and microbial cells is a major challenge in releasing extracellular polymeric substances (EPS) and cytoplasmic macromolecules for subsequent hydrolysis and fermentation. Here, we developed a new process combining alkaline/acid pre-treatments and anaerobic digestion (APAD) to improve sludge digestion. Both alkaline and acid pre-treatments effectively disintegrated the floc structure and microbial cells to release sludge organic contents. Under the optimized alkaline/acid pre-treatment condition, carbon removal achieved 52.8 ± 1.7% in APAD digesters, in contrast to 30.9 ± 2.2% and 42.4 ± 1.6% in anaerobic digesters fed with fresh WAS (control-AD) and thermal pre-treated sludge (thermal-AD), respectively. Both alkaline/acid and thermal pre-treatments largely shifted sludge community composition and function, but in distinct ways, possibly due to their different sludge constitutes (i.e., dissolved organic matter and NaCl). Correspondingly, microbial network analysis identified three modules with varied keystone taxa and interaction patterns in the three digesters. Life cycle assessment showed the comparable environmental impacts of APAD, thermal-AD and control-AD. In all, this study provided a new solution for WAS treatment and insights into impact of sludge pre-treatments on sludge digestion microbiome.

Additional function of pasteurisation pretreatment in combination with anaerobic digestion on antibiotic removal

Pasteurisation pretreatment (PP) in combination with anaerobic digestion (PPAD) is one of the best ways for sterilisation of pathogenic microorganisms in manure. However, the effect of antibiotic residues in manure on PPAD has not been studied. This study investigated the function of PPAD on antibiotic removal and the effect of antibiotic on PPAD performance. Results demonstrated that chlortetracycline (CTC) and oxytetracycline (OTC) concentrations decreased from 17.236 and 183.446 to 0 and 17.348 mg/kg·TS using PPAD, respectively. PPAD for swine manure containing CTC and OTC increased methane production from 244.0 ± 7.6 to 254.0 ± 6.1 mL/g·VS and reduced technical digestion time (T₈₀) from 30 to 25 days compared with AD process. Moreover, PPAD affected only archaeal communities, whereas PP affected bacterial/archaeal communities. Thus, PPAD can be used to treat antibiotic-containing manure and reduce the negative effects of antibiotics.

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.

Comparison between Thermo-Alkaline and Electro-Fenton Disintegration Effect on Waste Activated Sludge Anaerobic Digestion

Disintegration of municipal waste activated sludge (WAS) using thermo-alkaline (TA) and electro-Fenton (EF) methods was investigated and compared in terms of the efficiency of sludge solubilisation and enhancement of anaerobic biodegradability. Performance of organic matter solubilisation (soluble COD, proteins, polysaccharides) of sludge pretreated with EF was proved to be better than that with TA pretreatment, which resulted in the enhancement of anaerobic biodegradability. Comparison of results indicated that percentages of PN and PS release obtained after EF pretreatment (68.95 and 65.22%) were higher than those obtained by TA method (45.25 and 35.22%) respectively. An improvement of biogas potential about 2 and 1.6 times was achieved respectively by EF and TA pretreatment in comparison to raw sludge. During semi-continuous fermentation study in continuous stirred tank reactor, EF pretreated sludge gave the best biogas yield (0.6 L biogas/g COD) at an OLR of 2.5 g COD/L. d in comparison to TA pretreated sludge (0.3 L biogas/g COD), where low biogas yield about 0.1 L biogas/g COD was registered by raw sludge in the same CSTR. Therefore, the integration of EF process to anaerobic digestion might be a promising process for sludge reduction and biogas recovery.

Optimization of the wastewater treatment plant: From energy saving to environmental impact mitigation

This paper outlines a multi-objective, integrated approach to analyze various possibilities for increasing energy efficiency of the largest Italian wastewater treatment plant (WWTP) at Castiglione Torinese. In this approach, wastewater and sludge treatment units are thoroughly investigated to find the potential ways for improving the energy efficiency of the system. Firstly, a multi-step simulation-based methodology is proposed to make a full link between treatment processes and the energy demand and production. Further, a scenario-based optimization approach is proposed to find the nondominated and optimized performance of the WWTP. The results prove a potential to save up to 5000 MWh of the annual energy consumption of the plant, in addition to improve the effluent quality through operational changes only. Even for what concerns the sludge line a model was proposed for the optimization of the energy recovery from the processes that in a WWTP are devoted to the management of sewage sludge. The obtained results show that the introduction of an advanced thickening stage and sludge pre-treatment would have a positive impact on the energy and greenhouse gas balance of the plant.

Thermal-alkaline pretreatment of polyacrylamide flocculated waste activated sludge: Process optimization and effects on anaerobic digestion and polyacrylamide degradation

Deterioration of anaerobic digestion can occur with the presence of polyacrylamide (PAM) in waste activated sludge, and little information on mitigating this deterioration is currently available. In this study, simultaneous mitigation of PAM negative effects and improvement of methane production was accomplished by thermal-alkaline pretreatment. Under the optimized pretreatment conditions (i.e., 75 °C, pH 11.0 for 17.5 h), the biochemical methane potential of PAM-flocculated sludge increased from 100.5 to 210.8 mL/g VS and the hydrolysis rate increased from 0.122 to 0.187 d^-1. Mechanism investigations revealed that the pretreatment not only broke the large firm floccules, improved the degradation of PAM, but also facilitated the release of biodegradable organics from sludge, which thereby provided better growth environment and enough nutrients to anaerobic microbes for methane production. The activities of key enzymes responsible for methane production and PAM degradation were greatly improved in pretreated reactor, with the accumulation of acrylamide being avoided.

How does free ammonia-based sludge pretreatment improve methane production from anaerobic digestion of waste activated sludge

Previous studies reported that free ammonia (FA) pretreatment could improve methane production from anaerobic digestion of waste activated sludge (WAS) effectively. However, details of how FA pretreatment improves methane production are poorly understood. This study therefore aims to reveal the underlying mechanisms of FA pretreatment affecting anaerobic digestion of WAS through a series of batch tests using either real sludge or synthetic media as the digestion substrates at different pH values. At pH 8.5 level, with an increase of FA level from 18.5 to 92.5 mg/L (i.e., NH+ 4-N: 100-500 mg/L; pH 8.5) the maximum methane yield varied between 194.0 ± 3.9 and 196.9 ± 7.7 mL/g of VSS (25 °C, 1 atm). At pH 9.5 or 10 level, however, with an increase of initial FA level from 103.2 to 516.2 mg/L, the maximal methane yield increased linearly. The mechanism studies revealed that FA pretreatment at high levels not only accelerated the disintegration of WAS but also enhanced the biodegradability of WAS. Although pH in the digesters was adjusted to 7.0 ± 0.1, the high levels of NH+ 4-N added or released led to substantial levels of residual FA ranging from 4.4 to 11.6 mg/L. It was found that this level of FA inhibited homoacetogenesis and methanogenesis significantly, though hydrolysis, acidogenesis, and acetogenesis processes were unaffected largely. Further analyses showed that the inhibition constant of FA to substrate degradation was in the sequence of dextran > glucose > hydrogen > acetate, indicating the methanogenesis process was more sensitive to FA.

Effect of Mild-Temperature Thermo-Alkaline Pretreatment on the Solubilization and Anaerobic Digestion of Spent Coffee Grounds

Mild-temperature thermo-alkaline pretreatment of spent coffee grounds (SCG) was studied to improve its solubilization and methanation. The simultaneous effects of NaOH concentration (0–0.2 M) and temperature (60–90 °C) were investigated. Significant solubilization of SCG was achieved by the pretreatment, particularly under high-NaOH-concentration and high-temperature conditions. However, adding NaOH above a certain concentration adversely affected the methane production. Therefore, the degree of solubilization (SD) correlated poorly with methane yield (Ym). Response surface models of SD and Ym were successfully generated. The maximum response of SD (36.4%) was obtained at 0.18 M NaOH and 90.0 °C, while that of Ym (263.31 mL CH4/g COD added) was obtained at 0.13 M NaOH and 70.5 °C. Hydrogenotrophic Methanospirillum species were the dominant methanogens in all the SCG digestion tests. It is likely that NaOH concentration had a more significant influence on the development of microbial community structure, particularly of methanogens than temperature.

Dissolving organic matter from low-organic sewage sludge for shortening the anaerobic digestion time

Pretreatments have been successfully used to shorten the HRT of anaerobic digestion (AD) of sewage sludge, but they become ineffective for low-organic sewage sludge, with HRT > 10 days. Herein, a new process using alkaline hydrolysis-anaerobic digestion has been developed to solve this problem. Firstly, maximum organic matter in the sludge was dissolved by strong alkaline hydrolysis (pH > 11) in a two-stage alkaline hydrolysis system (TSAHS). Secondly, only the supernatant of the sludge that contained most of the methane potential was applied for AD. The operational conditions were optimized and the process mechanism was also analyzed. The results showed that under optimum operational conditions, above 19% of the organic matter in the sludge was released into the supernatant after alkaline hydrolysis in TSAHS, and the supernatant for AD achieved a methane production of 392 mL CH₄ per g COD. The process attained a methane production of 0.26 m³ CH₄ per kg VS and a VS reduction of 43.5%, while the HRT was only 12 h. The advantage of the mechanism was that the alkaline neutralization capacity of the sludge maintained a proper pH value for the supernatant from TSAHS, which benefited subsequent AD. It is concluded that the new process based on the dissolution of organic matter can attain a short digestion time for low-organic sludge.

Pretreatments have been successfully used to shorten the HRT of anaerobic digestion (AD) of sewage sludge, but they become ineffective for low-organic sewage sludge, with HRT > 10 days. A new process below was developed to solve this problem.

Improved sludge dewaterability and hydrolysis performance after pretreatment with Fenton's reagent

The dewaterability of excess sludge significantly improved upon pretreatment with Fenton's reagent in this study. After 0.9 g/L of Fe²⁺ and 5.0 g/L of H₂O₂ were added to the sludge, and reacted for 2 h at pH = 4, the specific resistance to filtration (SRF) of the excess sludge decreased from an initial value of 29.74 × 10¹² m/kg to 6.49 × 10¹² m/kg. The factors that affected this improvement in sludge dewaterability as evaluated by SRF reduction showed the following order: H₂O₂ > pH > Fe²⁺ > reaction time. Furthermore, the hydrolysis performance of the sludge under the optimal reaction conditions was investigated. The results indicated that the concentration of soluble chemical oxygen demand in the supernatant increased almost 14 times compared to raw sludge, and the contents of soluble protein and soluble polysaccharide were more than 8 and 17 times higher, respectively, than for the untreated situation. However, the amounts of ammonia nitrogen (NH₄⁺-N) and phosphate (PO₄^3--P) released from the sludge showed different trends: NH₄⁺-N increased by 200%, while PO₄^3--P decreased by 82%. The production of volatile fatty acids (VFAs) from the treated sludge showed that total VFAs increased by 66%, and iso-butylacetic acid was the dominant product among the total VFAs.

Stepwise hydrolysis to improve carbon releasing efficiency from sludge

Based on thermal alkaline hydrolysis (TAH), a novel strategy of stepwise hydrolysis was developed to improve carbon releasing efficiency from waste activated sludge (WAS). By stepwise increasing hydrolysis intensity, conventional sludge hydrolysis (the control) was divided into four stages for separately recovering sludge carbon sources with different bonding strengths, namely stage 1 (60 °C, pH 6.0-8.0), stage 2 (80 °C, pH 6.0-8.0), stage 3 (80 °C, pH 10.0) and stage 4 (90 °C, pH 12.0). Results indicate stepwise hydrolysis could enhance the amount of released soluble chemical oxygen demand (SCOD) for almost 2 times, from 7200 to 14,693 mg/L, and the released carbon presented better biodegradability, with BOD/COD of 0.47 and volatile fatty acids (VFAs) yield of 0.37 g VFAs/g SCOD via anaerobic fermentation. Moreover, stepwise hydrolysis also improved the dewaterability of hydrolyzed sludge, capillary suction time (CST) reducing from 2500 to 1600 s. Economic assessment indicates stepwise hydrolysis shows less alkali demand and lower thermal energy consumption than those of the control. Furthermore, results of this study help support the concepts of improving carbon recovery in wastewater by manipulating WAS composition and the idea of classifiably recovering the nutrients in WAS.

Control of the red tide dinoflagellate Cochlodinium polykrikoides by ozone in seawater

The inactivation of C. polykrikoides, a red tide dinoflagellate, by ozonation was investigated in seawater by monitoring numbers of viable and total cells. Parameters affecting the inactivation efficacy of C. polykrikoides such as the ozone dose, initial cell concentration, pH, and temperature were examined. The viable cell number rapidly decreased in the initial stage of the reaction (mostly in 1-2 min), whereas the decrease in total cell number was relatively slow and steady. Increasing ozone dose and decreasing initial cell concentration increased the inactivation efficacy of C. polykrikoides, while increasing pH and temperature decreased the cell inactivation efficacy. The addition of humic acid (a promoter for the ozone decomposition) inhibited the inactivation of C. polykrikoides, whereas bicarbonate ion (an inhibitor for the ozone decomposition) accelerated the C. polykrikoides inactivation. Observations regarding the effects of pH, temperature, humic acid, and bicarbonate ion collectively indicate that the inactivation of C. polykrikoides by ozonation is mainly attributed to oxidative cell damages by molecular ozone, rather than by hydroxyl radical, produced during the ozone decomposition. At high ozone dose (e.g., 5 mg/L), hypobromous acid formed by the reaction of bromide with ozone may partially contribute to cell inactivation. The use of ozone of less than 1 mg/L produced 0.75-2.03 μg/L bromate.

Continuous fermentation of food waste leachate for the production of volatile fatty acids and potential as a denitrification carbon source

This study investigated the simultaneous effects of hydraulic retention time (HRT) and pH on the continuous production of VFAs from food waste leachate using response surface analysis. The response surface approximations (R(2)=0.895, p<0.05) revealed that pH has a dominant effect on the specific VFA production (PTVFA) within the explored space (1-4-day HRT, pH 4.5-6.5). The estimated maximum PTVFA was 0.26g total VFAs/g CODf at 2.14-day HRT and pH 6.44, and the approximation was experimentally validated by running triplicate reactors under the estimated optimum conditions. The mixture of the filtrates recovered from these reactors was tested as a denitrification carbon source and demonstrated superior performance in terms of reaction rate and lag length relative to other chemicals, including acetate and methanol. The overall results provide helpful information for better design and control of continuous fermentation for producing waste-derived VFAs, an alternative carbon source for denitrification.

Mild-temperature thermochemical pretreatment of green macroalgal biomass: Effects on solubilization, methanation, and microbial community structure

The effects of mild-temperature thermochemical pretreatments with HCl or NaOH on the solubilization and biomethanation of Ulva biomass were assessed. Within the explored region (0-0.2M HCl/NaOH, 60-90°C), both methods were effective for solubilization (about 2-fold increase in the proportion of soluble organics), particularly under high-temperature and high-chemical-dose conditions. However, increased solubilization was not translated into enhanced biogas production for both methods. Response surface analysis statistically revealed that HCl or NaOH addition enhances the solubilization degree while adversely affects the methanation. The thermal-only treatment at the upper-limit temperature (90°C) was estimated to maximize the biogas production for both methods, suggesting limited potential of HCl/NaOH treatment for enhanced Ulva biomethanation. Compared to HCl, NaOH had much stronger positive and negative effects on the solubilization and methanation, respectively. Methanosaeta was likely the dominant methanogen group in all trials. Bacterial community structure varied among the trials according primarily to HCl/NaOH addition.

Performance and microbial communities of a batch anaerobic reactor treating liquid and high-solid sludge at thermophilic conditions

HS-AD with TS of 20% achieved similar reactor utilization efficiency as that in L-AD, and enriched hydrogenotrophic methanogensMethanoculleus.

Improvement of anaerobic digestion of waste-activated sludge by using H₂O₂ oxidation, electrolysis, electro-oxidation and thermo-alkaline pretreatments

Disintegration of municipal waste-activated sludge (WAS) is regarded as a prerequisite of the anaerobic digestion process to reduce sludge volume and improve biogas yield. Pretreatment of WAS using thermo-alkaline (TA), H2O2 oxidation, electrolysis and electro-oxidation (EO) processes were investigated and compared in term of COD solubilization and biogas production. For each pretreatment, the influences of different operational variables were studied in detail. At optimum conditions, EO gave the maximum COD solubilization (28 %). The effects of pretreatments under the optimum conditions on anaerobic digestion were experienced with biochemical methane potential assay. Significant increases in biogas yield up to 78 and 40 % were observed respectively in the EO and TA pretreated samples compared to raw sludge. Results clearly revealed that the application of EO is a significant alternative method for the improvement of WAS anaerobic digestion.

Effects of different sludge disintegration methods on sludge moisture distribution and dewatering performance

A key step in sludge treatment is sludge dewatering. However, activated sludge is generally very difficult to be dewatered. Sludge dewatering performance is largely affected by the sludge moisture distribution. Sludge disintegration can destroy the sludge structure and cell wall, so as change the sludge floc structure and moisture distribution, thus affecting the dewatering performance of sludge. In this article, the disintegration methods were ultrasound treatment, K2FeO4 oxidation and KMnO4 oxidation. The degree of disintegration (DDCOD), sludge moisture distribution and the final water content of sludge cake after centrifuging were measured. Results showed that three disintegration methods were all effective, and K2FeO4 oxidation was more efficient than KMnO4 oxidation. The content of free water increased obviously with K2FeO4 and KMnO4 oxidations, while it decreased with ultrasound treatment. The changes of free water and interstitial water were in the opposite trend. The content of bounding water decreased with K2FeO4 oxidation, and increased slightly with KMnO4 oxidation, while it increased obviously with ultrasound treatment. The water content of sludge cake after centrifuging decreased with K2FeO4 oxidation, and did not changed with KMnO4 oxidation, but increased obviously with ultrasound treatment. In summary, ultrasound treatment deteriorated the sludge dewaterability, while K2FeO4 and KMnO4 oxidation improved the sludge dewaterability.

Methane production and microbial community structure for alkaline pretreated waste activated sludge

Alkaline pretreatment was studied to analyze the influence on waste activated sludge (WAS) reduction, methane production and microbial community structure during anaerobic digestion. Methane production from alkaline pretreated sludge (A-WAS) (pH = 12) increased from 251.2 mL/Ld to 362.2 mL/Ld with the methane content of 68.7% compared to raw sludge (R-WAS). Sludge reduction had been improved, and volatile suspended solids (VSS) removal rate and protein reduction had increased by ∼ 10% and ∼ 35%, respectively. The bacterial and methanogenic communities were analyzed using 454 pyrosequencing and clone libraries of 16S rRNA gene. Remarkable shifts were observed in microbial community structures after alkaline pretreatment, especially for Archaea. The dominant methanogenic population changed from Methanosaeta for R-WAS to Methanosarcina for A-WAS. In addition to the enhancement of solubilization and hydrolysis of anaerobic digestion of WAS, alkaline pretreatment showed significant impacts on the enrichment and syntrophic interactions between microbial communities.

Effect of ultrasound, low-temperature thermal and alkali pre-treatments on waste activated sludge rheology, hygienization and methane potential

Waste activated sludge is slower to biodegrade under anaerobic conditions than is primary sludge due to the glycan strands present in microbial cell walls. The use of pre-treatments may help to disrupt cell membranes and improve waste activated sludge biodegradability. In the present study, the effect of ultrasound, low-temperature thermal and alkali pre-treatments on the rheology, hygienization and biodegradability of waste activated sludge was evaluated. The optimum condition of each pre-treatment was selected based on rheological criteria (reduction of steady state viscosity) and hygienization levels (reduction of Escherichia coli, somatic coliphages and spores of sulfite-reducing clostridia). The three pre-treatments were able to reduce the viscosity of the sludge, and this reduction was greater with increasing treatment intensity. However, only the alkali and thermal conditioning allowed the hygienization of the sludge, whereas the ultrasonication did not exhibit any notorious effect on microbial indicators populations. The selected optimum conditions were as follows: 27,000 kJ/kg TS for the ultrasound, 80 °C during 15 min for the thermal and 157 g NaOH/kg TS for the alkali. Afterward, the specific methane production was evaluated through biomethane potential tests at the specified optimum conditions. The alkali pre-treatment exhibited the greatest methane production increase (34%) followed by the ultrasonication (13%), whereas the thermal pre-treatment presented a methane potential similar to the untreated sludge. Finally, an assessment of the different treatment scenarios was conducted considering the results together with an energy balance, which revealed that the ultrasound and alkali treatments entailed higher costs.