Pretreatment Of Waste Activated Sludge Research Articles

Deep insights into the roles and microbial ecological mechanisms behind waste activated sludge digestion triggered by persulfate oxidation activated through multiple modes

Persulfate oxidation (PS) is widely employed as a promising alternative for waste activated sludge pretreatment due to the capability of generating free radicals. The product differences and microbiological mechanisms by which PS activation triggers WAS digestion through multiple modes need to be further investigated. This study comprehensively investigated the effects of persulfate oxidation activated through multiple modes, i.e., ferrous, zero-valent iron (ZVI), ultraviolet (UV) and heat, on the performance of sludge digestion. Results showed that PS_ZVI significantly accelerated the methane production rate to 12.02 mL/g VSS. By contrast, PS_Heat promoted the sludge acidification and gained the maximum short-chain fatty acids (SCFAs) yield (277.11 ± 7.81 mg COD/g VSS), which was 3.41-fold compared to that in PS_ZVI. Moreover, ferrous and ZVI activated PS achieved the oriented conversion of acetate, the proportions of which took 73% and 78%, respectively. MiSeq sequencing results revealed that PS_Heat and PS_UV evidently enriched anaerobic fermentation bacteria (AFB) (i.e., Macellibacteroides and Clostridium XlVa). However, PS_Ferrous and PS_ZVI facilitated the enrichment of Woesearchaeota and methanogens. Furthermore, molecular ecological network and mantel test revealed the intrinsic interactions among the multiple functional microbes and environmental variables. The homo-acetogens and sulfate-reducing bacterial had potential cooperative and symbiotic relationships with AFB, while the nitrate-reducing bacteria displayed distinguishing ecological niches. Suitable activation modes for PS pretreatments resulted in an upregulation of genes expression responsible for digestion. This study established a scientific foundation for the application of sulfate radical-based oxidation on energy or high value-added chemicals recovery from waste residues.

Synergistic effects and mechanisms of plasma coupled with peracetic acid in enhancing short-chain fatty acid production from sludge: Motivation of reactive species and metabolic tuning of microbial communities

Suitable waste activated sludge (WAS) pretreatments that boost short-chain fatty acid (SCFA) production from anaerobic fermentation are essential for carbon emission reduction and sludge resource utilization. This study established an efficient WAS pretreatment process combining atmospheric pressure plasma jet (APPJ) with peracetic acid (PAA). The maximum SCFA production (6.5-fold that of the control) largely increased under the optimal conditions (PAA dosage = 25 mg/g VSS (volatile suspended solids), energy consumption = 20.9 kWh/m3). APPJ/PAA pretreatment enhanced the production of multiple reactive species (including OH, CH3C(O)O, 1O2, ONOO–, O2–, and eaq−) and strengthened the effects of H2O2, heat, and light. This synergistically solubilized WAS and released organic substrates for SCFA-producing microbes. In addition, the enrichment of SCFA-producing bacteria and the decrease in SCFA-consuming bacteria favored SCFA accumulation. The key genes encoding for the main substrate metabolism and SCFA production in the metabolic pathway of fermentation were also enhanced.

Heat-enhanced sulfite pretreatment improves the release of soluble substances and the stimulation of methanogenic pathways for anaerobic digestion of waste activated sludge

Waste activated sludge pretreatment is effective in improving sludge hydrolysis rate and methane production. Our previous study revealed that sulfite could be a viable method for sludge pretreatment. To further improve methane production, this study employed an integrated strategy by combining sulfite and heat pretreatment. WAS from a local full-scale plant was pretreated with heat-enhanced sulfite pretreatment (HESP) at 25, 35, 55 °C. Compared with the control (no sulfite, 25 °C), soluble substances (e.g., chemical oxygen demand, protein, polysaccharides, nitrogen and phosphorus) could be significantly enhanced by the absence of sulfite and the increasing temperature. A variation of − 8.32–19.90% in cumulative methane production was observed. Moreover, methane production could be significantly enhanced by 14.15% at sulfite of 300 mg S/L and temperature of 35 °C, while kinetic analysis showed that the highest methane production potential (1.43 times of the control) was obtained at this level. Besides, the microbial structure and pathways reveal that HESP promotes the enrichment of methanogenic bacteria and facilitates methanogenic metabolism. The findings of this study suggest that the HESP could be promising in enhancing methane production, particularly considering the reuse of sulfite-laden industrial wastes and the heat as the by-products of the anaerobic digestion plant with biogas power generation, but certain experimental conditions should be verified case by case.

Process configuration of combined ozonolysis and anaerobic digestion for wastewater treatment

Abstract Industrial activities and increased human population have made wastewater streams not entirely amenable to conventional treatment methods. Anaerobic digestion (AD) can treat such wastewaters with the advantage of bioresource recovery. However, the presence of solids and recalcitrant compounds in most wastewater streams may affect the AD process. Thus, combining AD with advanced oxidation processes (AOPs) such as ozonolysis is necessary. Ozonolysis can improve the biodegradability of wastewater substrates or eliminate biorecalcitrant pollutants that escape the AD process. This study combined ozonolysis with AD to treat waste activated sludge (WAS) and distillery wastewater (DWW). When applied as a pre-treatment, ozonolysis caused the rigid cell walls in WAS to rupture and solubilised the extracellular polymeric substances (EPS), leading to increased biodegradability. For the DWW, ozonolysis pre-treatment reduced the biorecalcitrant aromatic compounds to simple aliphatic compounds, thereby increasing biodegradability. In the ensuing anaerobic process, the WAS pre-treatment improved TSS and COD reductions and a 230% increase in cumulative biogas production. For the DWW, the ozonolysis pre-treatment did not significantly impact COD reduction or biogas production; however, ozonolysis as a post-treatment removed the color causing biorecalcitrant melanoidins from the anaerobically digested effluent and solubilised the sludge (TSS) washed out from the AD unit. Therefore, the AD-ozonolysis process configuration depends on the substrate being treated. Ozonolysis is best applied pre-AD for WAS treatment and post-AD for DWW.

Low-temperature thermal pre-treated sewage sludge for feeding of black soldier fly (Hermetia illucens) larvae: Protein, lipid and biodiesel profile and characterization

Previous work showed that low-temperature thermal pre-treatment of waste activated sludge (WAS) under different temperatures (30, 60, 75, 90 °C) and durations (2, 4, 8, 16 h) prior to black soldier fly larvae (BSFL) rearing could improve the larval growth. In this work, the biochemical profiles of WAS-fed BSFL and the characteristics of biodiesel produced were reported. The BSFL fed with pre-treated WAS had high protein contents (44%–54%) and moderate lipid contents (21%–25%). The WAS pre-treated at 90 °C for 16 h produced BSFL with the highest protein and lipid yields, achieving 69% and 71% of increments, respectively, as compared with the controlled WAS. Subsequently, high conversion of lipid into biodiesel (>90%) could be achieved through the acid-catalyzed transesterification process. The BSFL biodiesel was dominated by saturated fatty acid methyl esters (FAME), with C12:0 being the abundance. This unique FAME profile enabled BSFL biodiesel to have low iodine value, high cetane number and more oxidatively stable as compared with other reported biodiesels like palm oil and rapeseed oil biodiesels. The BSFL biodiesel also satiated the biodiesel standards of EN 14214 and ASTM D6751, except for the flash point, which was still higher and safer than petroleum diesel. In short, the thermal pre-treatment of WAS was effective in promoting BSFL growth and its lipid yield, ultimately leading to higher biodiesel production.

Sulfite pretreatment enhances the medium-chain fatty acids production from waste activated sludge anaerobic fermentation

Production of high-value medium chain fatty acids (MCFAs) from anaerobic fermentation of waste activated sludge (WAS) has been considered as a promising alternative for renewable energy resources. However, the low biodegradability of WAS greatly limits the anaerobic fermentation performance. This study proposed and demonstrated a novel approach, sulfite pretreatment, to efficiently produce MCFAs through anaerobic fermentation of WAS. Pretreatment of WAS at a sulfite concentration of 100–500 mg S/L for 24 h effectively improved the MCFAs production and MCFAs selectivity and the promotion effect was positively correlated with the sulfite concentration used in pretreatment (Pearson's R > 0.9). The maximum MCFAs production of 6.84 g COD/L and MCFAs selectivity of 39.1 % were both achieved under 500 mg S/L sulfite pretreatment, which accounts for 2.6 times and 2.4 times of the control, respectively (MCFAs production of 2.62 g COD/L and MCFAs selectivity of 16.4 % in the control). Sulfite pretreatment also enhanced the WAS degradation from 25 ± 2 % in the control to a maximum of 39 ± 2 % under 500 mg S/L sulfite pretreatment. The electron transfer efficiency and COD flows from the substrate to products were enhanced by up to 25 % due to the sulfite pretreatment, which supports the enhanced WAS degradation. Sulfite pretreatment also promoted the solubilization, hydrolysis, and acidification processes during the anaerobic fermentation by up to 200 %, 60 %, and 45 %, respectively, which subsequently makes more substrates available for MCFAs production. The findings from this study provide a potential solution of using industrial sulfite-laden wastes for WAS pretreatment, to enhance the MCFAs production at a minimized cost.

Microbial community structure and potential function associated with poly-3-hydroxybutyrate biopolymer-boosted activation of peroxymonosulfate for waste-activated sludge decontamination

Excess waste-activated sludge (WAS) is a major biosolid management problem due to its biohazardous and recalcitrant content of phthalate esters (PAEs). This study aimed to assess the combined use of biopolymer, poly-3-hydroxybutyrate and peroxymonosulfate to degrade PAEs and decontaminate WAS. Poly-3-hydroxybutyrate was biosynthesized by Cupriavidus sp. L7L. The combined poly-3-hydroxybutyrate and peroxymonosulfate process removed 86 % of PAEs from WAS in 12 h. The carbonyl groups of poly-3-hydroxybutyrate were conducive to peroxymonosulfate activation leading to PAE degradation followed the radical pathway and surface-mediated electron transfer. Poly-3-hydroxybutyrate and peroxymonosulfate also enriched the PAE-biodegrading microbes in WAS. The microbial population and the functional composition in response to peroxymonosultate treatment was identified, with the genus Sulfurisoma being the most abundant. This synergistic treatment, i.e., advanced oxidation process, was augmented by highly promising microbial polyesters, exhibited important implications for WAS pretreatment toward circular bioeconomy that encompasses carbon–neutral biorefinery and mitigate pollution.

Moderate potassium ferrate dosage enhances methane production from the anaerobic digestion of waste activated sludge

: The annual increase of waste activated sludge (WAS) has become an urgent problem to be solved in sewage plants worldwide. Anaerobic digestion (AD) of WAS is an attractive choice to maximize the resource utilization rate. Nevertheless, the disintegration of sludge complex polymers is difficult, resulting in low bioconversion rate. Potassium ferrate (PF), as a green oxidant with strong oxidizing property, has been attracted great attention in WAS pretreatment recently. The effects of PF pretreatment on WAS hydrolysis and its dosage-response on methane production were investigated in the present study. Results show that as PF dosage raise from 0 to 50 g-K2FeO4/ kg-TS (total solids), the methane yield enhanced significantly by 40.3% from 0.083 to 0.12 L/g-VSadded (volatile solids). Nevertheless, the further increase of PF dosage resulted in the decreased methane production. Especially with the PF dosage of 500 g-K2FeO4/ kg-TS, methane production even slightly lower than the control reactor without PF oxidation. The mechanism analysis showed that although the dissolution of polysaccharides and proteins were enhanced with the high dosage of PF, the accompanying released humic-like substances and high concentration of ferric ions should be the main reasons inhibiting methane production.

Distinct effects of typical sludge pretreatment approaches on the antibiotic resistance genes variations, associated bacterial community dynamics and metabolic activities during anaerobic fermentation process

Anaerobic fermentation is effective for waste activated sludge (WAS) disposal to realize resource generation and pollutants reduction, and various pretreatments were commonly applied to improve the performance. This work mainly investigated the effects of typical WAS pretreatment approaches on the antibiotic resistance genes (ARGs, as emerging contaminants) removal during anaerobic fermentation processes and unveiled the underlying mechanisms. The results indicated that all the pretreatment strategies exhibited evident effects on the overall ARGs removal with the order of Fe2+ activated persulfate (PS/Fe2+) > pH 10 > Ultrasonication > Heat, and showed selective removal tendency for the specific ARGs (namely easily removed (aadA1 and sul1) and persistent ARGs). Mechanistic analysis demonstrated that the pretreatments disrupted the extracellular polymeric substances (EPS) and rose the cell membrane permeability (particularly for PS/Fe2+ and Heat). Then the increased ARGs release benefitted the subsequent reduction of mobile genetic elements (MGEs) and extracellular ARGs (especially for PS/Fe2+ and pH10), resulting the ARGs attenuation. Pretreatments significantly shifted the microbial community structure and the abundances of potential ARGs hosts (i.e., Sulfuritalea, and Denitratisoma). Also, the different pretreatments exhibited distinct effects on the microbial metabolic traits related with ARGs proliferation (i.e., ABC transporters, two-component system and bacterial secretion systems), which also contributed to the ARGs attenuations during WAS fermentation. The partial least-squares path modeling (PLS-PM) analysis indicated that the bacterial community (total effects = 0.968) was key factor determining ARGs fates.

Urine pretreatment significantly promotes methane production in anaerobic waste activated sludge digestion

Methane production of waste activated sludge (WAS) in anaerobic digestion is hindered due to the rate-limited hydrolysis process and the low methane potential of WAS. Pretreatment of WAS is a common and appealing strategy to improve methane production in anaerobic digestion. In this study, we proposed to use urine, an easily obtained human waste with high ammonium concentration and pH, as a novel pretreatment strategy for anaerobic WAS digestion. Urine pretreatment at levels of 5–30 % (Vurine/Vurine+WAS) could substantially enhance methane production by 5–35 % in biochemical methane potential (BMP) tests, with the highest methane production of 179.6 ± 3.3 mL/g volatile solids (VS) achieved under the highest level of urine (i.e. 30 % urine addition). Based on the model analysis, the biochemical methane potential (B0) and hydrolysis rate of WAS (k) rose from 131.9 mL/g VS and 0.19 d−1 in the control without pretreatment to 136.3–178.2 mL/g VS and 0.22–0.30 d−1, respectively, after the urine pretreatment (5–30 % addition). Urine pretreatment with 5–30 % addition also improved the degradation extent (Y) of WAS by 3–35 %. The promising results indicate that urine pretreatment in anaerobic digestion is a promising technology to improve the efficiency of anaerobic digestion with environmental and economic benefits.

Impact of divalent cations on lysozyme-induced solubilisation of waste-activated sludge: Perspectives of extracellular polymeric substances and surface electronegativity

Lysozyme hydrolysis can accelerate waste-activated sludge (WAS) solubilisation, which can significantly shorten the process and promote the efficiency of anaerobic digestion. This study investigated the impact of divalent cations on lysozyme-induced solubilisation of WAS. The performance of lysozyme pretreatment was dramatically inhibited by Mg2+ and Ca2+. Compared to the control group, the amount of net SCOD, protein, and polysaccharides released to the supernatant were reduced by 36.6%, 44.7%, and 35.8%, respectively, in the presence of divalent cations. The extracellular polymeric substance (EPS) matrix became tightly bound, resulting in fewer proteins and polysaccharides being extracted from loosely-bound EPS (LB-EPS) with divalent cations, which was detrimental to the solubilisation of WAS. Divalent cations decreased the surface electronegativity of sludge particles and prolonged the adsorption of lysozymes by sludge flocs. More than 16.6% of total lysozymes remained in the liquid phase of WAS after 240min Mg2+ and Ca2+ strengthened the binding among proteins and polysaccharides and promoted the intermolecular cross-linking of polysaccharides. The EPS matrix formed a dense spatial reticular structure that blocked the transfer of lysozymes from the EPS matrix to the pellet. As a result, the lysozymes accumulated in LB-EPS rather than hydrolysing the microorganism's cell wall. This study provides a new perspective on the restriction of WAS pretreatment with lysozymes and optimises the method of lysozyme-induced solubilisation of WAS.

Removal of 4-nonylphenol in activated sludge by peroxymonosulfate activated with sorghum distillery residue-derived biochar

The presence of 4-nonylphenol (4-NP), an endocrine disrupting chemical, waste activated sludge (WAS) or biosolids at elevated content requires effective method for 4-NP reduction in total sludge management. Herein, sorghum distillery residue-based biochar-activated peroxymonosulfate (SDRBC/PMS) system was studied as pretreatment of WAS. Results indicated 91% of 4-NP removal at pH 6.0 in the presence of 3.1 × 10−6 M and 0.8 g L−1 PMS and SDRBC500, individually. The synergetic effects of singlet oxygen (1O2) and the abundant functional sites (C = O/C–O content) of SDRBC significantly improved 4-NP degradation. The decreased fluorescent dissolved organic matter (DOM) in the sludge also enhanced the pretreatment efficiency. Moreover, the enrichment of the Nitrospira functional bacteria in the microbial community yielded the highest 4-NP degradation in the SDRBC/PMS-pretreated sludge. The SDRBC/PMS system functions mainly via nonradical-mediated oxidation pathway in pretreating WAS in particular and potentially by combined advanced oxidation and biodegradation processes for wastewater treatment in general.

Production of volatile fatty acids concomitant with phosphorus removal and lignin recovery by co-fermentation of waste activated sludge and black liquor

Waste activated sludge (WAS) and black liquor (BL) are the stubborn solid waste and wastewater in the industry that need high investment and operational costs for treatment. In this study, co-fermentation of WAS and BL was performed following a novel process for the production of volatile fatty acids (VFAs) concomitant with phosphorus removal and lignin recovery. BL offers alkaline liquor for WAS pretreatment, and the organic carbon in BL can be used for fermentation. The results showed that the semi-continuous fermentation test at pH 10 and the addition of 800 mg/L of Mg2+ were the optimal conditions for the production of VFAs (approximately 15,000 mg/L) and P removal (≈94%). Additionally, Mg2+ was suited to remove P due to the weak adsorption of Mg2+ by BL under alkaline conditions. Moreover, the composition of VFAs was affected by the pH, for example, high ratios of acetate and propionate were obtained under conditions of high and low pH, respectively. The dominant functional bacteria of Alkalibacterium, Propionibacterium, Proteiniphilum, and Tessaracoccus were found through the analysis of microbial diversity to be responsible for the production of VFAs. The bioconversion of small molecular lignin was also demonstrated through the characterization analyses. Co-fermentation was suggested as an economic and eco-friendly strategy for the treatment and resource utilization of WAS and BL via the overall assessment of the technical process.

Elucidating the production and inhibition of melanoidins products on anaerobic digestion after thermal-alkaline pretreatment

The refractory organics released from waste activated sludge (WAS) are unwanted produced in thermal-alkaline pretreatment, which are not well documented. In this study, we refer to them as melanoidins products (MPs) with characteristics of high molecular weight and inhibition to microbes. The results showed that these MPs from thermal-alkaline (80 °C and pH 10) pretreatment of WAS were identified with a broad molecular weight (>1000 Da). Dark-colored MPs were further verified from glucose and tryptophan as the model components, with values of UV280 and UV420 increasing. The produced MPs with a molecular weight of 1220, 6835, and even 21,200,000 Da were confirmed by SEC-HPLC. Unexpectedly, MPs were found to be electroactive with higher redox peak values than that of humic acids, which were almost not degraded by anaerobes as revealed by SEC-HPLC and 3D-EEM spectra. For the first time, the results demonstrated that MPs delayed volatile fatty acids production and reduced the methane yield (22–26% lower), which was likely attributed to the toxicity and/or electrons competition with anaerobes such as Methanosaeta. Thus, it is clear that MPs negatively impact anaerobic digestion after thermal-alkaline pretreatment, which shall be re-evaluated to minimize MPs when producing biochemicals from WAS.

Improved energy balance at a municipal wastewater treatment plant through waste activated sludge low-temperature alkaline pretreatment

Sludge management has a significant impact on operating costs in municipal wastewater treatment plants (WWTPs). The current trend is to optimize the sludge management in order to enhance the energy independence from external energy suppliers, while reducing the operating costs of the WWTP. One of the methods is through the pretreatment of waste activated sludge (WAS). In this work, low-temperature alkaline pretreatment of WAS was examined as a method of improving the economic balance of the WWTP. In the initial pretreatment tests, the impacts of NaOH dose, temperature and reaction time on COD release as well as the degree of sludge disintegration were examined. Semi-continuous anaerobic digestion (AD) trials were then performed for pretreated WAS in selected conditions with low NaOH dosage for which no pH adjustment was required (16 g of NaOH/kgTS of WAS, 60 °C, 60 min). The tests showed an increase in methane yield from 210.4 ± 10.9 mL CH4/gVS to 248.8 ± 13.1 mL CH4/gVS and 267.1 ± 14.3 mL CH4/gVS for HRT 15 d and HRT 21.5 d, respectively. Energy balance and economic calculations for a full-scale WWTP were conducted based on the obtained results. It was concluded that it would be possible to increase electricity production from methane originated from AD of pretreated WAS up to 27% as well as to reduce the amount of dewatered sludge up to 8.7%. Energy balance and cost-benefit analyses indicated a possibility to reduce the operating cost by over 300,000 EUR/year in the case of a 1,200,000 people-equivalent WWTP.

Enhancing the decomposition of extracellular polymeric substances and the recovery of short-chain fatty acids from waste activated sludge: Analysis of the performance and mechanism of co-treatment by free nitrous acid and calcium peroxide

At present, the bioproduction of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) has attracted worldwide attention due to the demand of carbon neutrality during waste treatment. Calcium peroxide (CaO2) has been reported to be an effective method for the solubilization of WAS and the accumulation of SCFAs, but the high reagent cost limits its industrial application. Therefore, free nitrous acid (FNA) was introduced into the WAS pretreatment system to assist with CaO2 for enhancing the disruption of extracellular polymeric substances (EPS) and the subsequent acidogenesis process. The results showed that FNA and CaO2 synergistically enhanced EPS decomposition and the release of biodegradable organic compounds during pretreatment. The highest soluble chemical oxygen demand (3.1- and 2.6-fold higher compared to individual pretreatments at the same concentrations) after pretreatment and the highest SCFAs accumulation (2.0- and 6.4-fold compared to individual pretreatments at the same concentrations) after a 2-day fermentation period was observed in the FNA + CaO2 (0.15 g/g VSS) co-treated group. Therefore, the FNA + CaO2 (0.15 g/g VSS) co-treatment was determined to be the optimal strategy for ensuring the disintegration of the EPS matrix and enhancing the accumulation of SCFAs in pretreated sludge during anaerobic digestion.

Insight into the enhancing short-chain fatty acids (SCFAs) production from waste activated sludge via polyoxometalates pretreatment: Mechanisms and implications

Polyoxometalates (POMs), a versatile and environmentally-friendly inorganic material, have been extensively studied and applied in chemical catalytic oxidation and biological nutrients removal processes. However, little is known about effects of POMs pretreatment on anaerobic sludge fermentation. This study thereby filled such knowledge gap and provided insights into the underlying mechanisms. Results demonstrated the maximal short-chain fatty acids (SCFAs) production increased by 6.18 times with POMs rising from 0 to 0.05 g/g TSS. Mechanistic investigations revealed that the oxidation stress of POMs as well as reactive oxygen species (ROS) activated by POMs were responsible for the disintegration of waste activated sludge (WAS). More importantly, POMs pretreatment improved the biodegradability of organics released, providing more biodegradable substrates for SCFAs generation. Furthermore, the inhibition of POMs to SCFAs producers was less severe than that to SCFAs consumers, leading to SCFAs accumulation. Microbial community analysis exhibited that increased the population of hydrolysis (i.e., Longilinea) and SCFAs generation microbes (i.e., Acinetobacter and Fusibacter). Further evaluation showed that the POMs-based technology is economically and environmentally attractive for the pretreatment of WAS. Finally, a “closed-loop” concept of the reutilization of renewable POMs may provide an important implication of WAS management in the future.

Calcium hypochlorite enhances the digestibility of and the phosphorus recovery from waste activated sludge

Waste activated sludge (WAS) can be treated using anaerobic digestion (AD) for biogas recovery and volume reduction. However, the poor digestibility and hydrolysis of WAS limit AD applications. The current study investigated the feasibility of applying calcium hypochlorite as a WAS pretreatment strategy to improve AD treatment efficiency using laboratory reactors. The results showed that pretreatment with 5 – 20% Ca(ClO)2 (total suspended solids basis) significantly enhanced WAS anaerobic digestibility, and led to significantly enhanced methane production rate and biomethane yield comparing to the AD of raw WAS (P < 0.05). Low Ca(ClO)2 pretreatment (5 – 10%) significantly enhanced digestion efficiency, which can be attributed to the development of fermentative and syntrophic bacteria. However, high Ca(ClO)2 doses (>20%) reduced microbial activities, leading to slow release of dissolved organic compounds and prolonged methane production lag phase. In addition, high Ca(ClO)2 removed 82.7% of the initial phosphate by calcium-phosphate binding, reducing the phosphorus in liquid digestate.

Effective methane production from waste activated sludge in anaerobic digestion via formic acid pretreatment

Waste activated sludge pretreatment is indispensable as sludge hydrolysis is limited during anaerobic fermentation. In this study, a new pretreatment method by adding formic acid has been proposed to improve carbon recovery from waste activated sludge. With formic acid addition, carbon was recovered as methane or/and volatile fatty acids and the recovery efficiency was higher than HCl pretreatment at the same concentration. The highest carbon recovery efficiency (almost 40%) was shown in the group of 0.1 mol/L formic acid pretreatment which might be attributed to isoelectric-point pretreatment. That was, the produced methane with the purity of 80% and little volatile fatty acids were detected with 0.1 mol/L formic acid pretreatment. However, the carbon recovery efficiency did not increase with the increasing acids concentration. Thus, 0.1 mol/L formic acid was regarded as the optimum pretreatment approach for anaerobic fermentation of waste activated sludge. The results of microbial communities showed that enrichment of hydrogen-producing bacteria Petrimonas and hydrogenotrophic methanogens was triggered, and then might improve carbon recovery from waste activated sludge.

Digestion liquid based alkaline pretreatment of waste activated sludge promotes methane production from anaerobic digestion

This work proved an efficient method to significantly increase methane production from anaerobic digestion of WAS. This method is to reflux proper of digestion liquid into waste activated sludge pretreatment unit (pH 9.5 for 24 h). The yield of maximum methane improved between 174.2 ± 7.3 and 282.5 ± 14.1 mL/g VSS with the reflux ratio of digestion liquid increasing from 0% to 20%. It was observed that the biodegradable organics in the digestion liquid did not affect the biological processes related to anaerobic digestion but increased methane production through reutilization. The ammonium in the digestion liquid was the main contributor to the increase in methane production via promoting sludge solubilization, but refractory organics were the major inhibitors to anaerobic digestion. It should be emphasized that the metal ions present in the digestion liquid were beneficial rather than harmful to the biological processes in the anaerobic digestion, which may be connected with the fact that certain metal ions were involved in the expression and activation of key enzymes. In addition, it was found that anaerobes in digestion liquid were another potential contributor to the enhanced anaerobic digestion.