The effect of pH on anaerobic fermentation of primary sludge at room temperature

Valorization of organic carbon in primary sludge via semi-continuous dark fermentation: First step to establish a wastewater biorefinery

In this study, lab-scale, bench-scale, and pilot-scale experiments were carried out to optimize short-chain fatty acids production from primary sludge. Batch tests showed the requirement of short retention times and semi-continuous operation mode showed a plateau of maximum daily productivity at 36-hours hydraulic retention time with minimal methanation. Optimization from pH 5 to pH 10 at 36 h-hydraulic retention time under long-term semi-continuous operating mode revealed that production of short-chain fatty acids was pH dependent and highest yields could be achieved at pH 7 by establishing optimum redox conditions for fermentation. Pilot-scale experiments at 32 °C showed that daily productivity (3.1 g∙Lreactor-1∙dHRT-1) and yields (150 mg∙gVS-1; OLR = 21 gVS∙Lreactor-1∙dHRT-1; pH 7) of short-chain fatty acids could be significantly improved, specifically for acetic and propionic acids. From these results, a robust dark fermentation step for recovery of valuable products from the solids treatment step in a biorefinery can be achieved.

Thermo-alkaline pretreatment of excess sludge: Effects of temperature on volatile fatty acids accumulation and microbial community

In order to explore the role of thermal-alkaline pretreatment temperatures (TAPT) in sludge fermentation and the microbial characteristics, five groups (100, 120, 140, 160 °C and control group) were set up and the results showed that the increasing TAPT promoted the dissolution of soluble chemical oxygen demand (SCOD) and VFAs, but had slight influence on the release of NH₄⁺-N and PO₄^3--P. What's more, when it was 120 °C, the SCOD dissolution was comparable to that at 160 °C. Overall, 120 °C was the optimal condition, corresponding to the fact that the maximum release of SCOD was 8788.74 mg/L (2.63 times of the control group), the maximum dissolution of VFAs was 4596 mg/L (about 1.28 times of the control group). The trend of C/N was not significant. High-throughput sequencing showed that Firmicutes and Actinobacteriota were enriched with the temperature increasing, while Proteobacteria and Chloroflexi did not change significantly. Firmicutes was in a stable dominant position. Temperature conditions brought about significant changes in microbial interspecific interaction. Carbohydrate and amino acids had the highest metabolic abundance, especially at 120 °C group. The change rule of amino acid metabolism was similar to that of lipid metabolism, and the abundance of energy metabolism gradually increased with temperature. The protein metabolism was greatly affected by temperature. This study revealed the effect of microbial mechanism of TAPT on the sludge acid production efficiency.

Biobased volatile fatty acids (VFA) production via anaerobic acidogenesis of sugar processing industry effluent

Rapid industrialization and unscientific disposal of industrial wastewaters have resulted in the pollution of water bodies and deterioration of water quality all over the globe. Valorization of industrial wastewaters will help in reducing the negative impact on the environment and will add value to the waste. The present study targets utilization of sugar processing industrial effluent for bio-based production of Volatile fatty acids (VFA) through anaerobic acidogenesis. Batch studies conducted to determine the VFA production potential of sugar processing industry effluent resulted in the VFA yield of 0.70 g/g COD utilized. Further continuous VFA production system was developed and optimization of Organic loading rate (OLR) (2-22 g COD/L^·day) was carried out with constant Hydraulic retention time (HRT) of 1 day. The continuous reactors studies resulted in a maximum VFA yield of 0.72 g/g COD utilized and productivity of 11.04 g COD/L·day at OLR of 14 g COD/L^·day and 22 g COD/L^·day, respectively. The developed process will provide an environmentally safe and efficient method for the conversion of complex industrial wastes to valuable products such as VFA.

Comparing VFA Composition, Biomethane Potential, and Methane Production Kinetics of Different Substrates for Anaerobic Fermentation and Digestion

Solid waste is one of the largest sources of greenhouse gases (GHGs) today. The carbon footprint of landfills also has a large impact on global warming. Therefore, it is becoming more urgent to study the possibility of better environmentally friendly approaches for solid waste management and its safe disposal. The digestion of solid waste is a biological process that breaks down the organic content of the solid waste and thus stabilizes it. It also allows the recovery of valuable resources (such as biogas) and the utilization of stabilized waste in various industries. In this study, six substrates were studied to determine their biomethane potential (BMP) in anaerobic digestion. The substrates were fermented and digested anaerobically, and the biogas production was measured. The methane yield of food waste substrates had a higher methane yield between 354 and 347 mL/g-TCOD, and a biodegradability of 89–87%. Wastewater sludge substrates yielded between 324 and 288 mL/g-TCOD with a biodegradability of 81–73%. A kinetics analysis using first-order and Gompertz models was performed for biodegradation and methane production.

Pilot-scale hydrolysis of primary sludge for production of easily degradable carbon to treat biological wastewater or produce biogas

Organic compounds in wastewater are required for the biological removal of nitrogen, but they can also be used for biogas production. Distribution of the internal organic carbon at the plant is therefore critical to ensure high quality of the treated water, reduce greenhouse gas emissions, and optimize biogas production. We describe a wastewater treatment plant designed to focus equally on energy production, water quality, and reduced emissions of greenhouse gases. A disk filter was installed to remove as much carbon as possible during primary treatment. Primary sludge was then hydrolyzed and centrifuged. The hydrolysate centrate contained volatile fatty acids and was used either for the secondary wastewater treatment or to produce biogas. The yield during hydrolysis was 30-35 g volatile fatty acid per kg dry material or 40-65 g soluble COD per kg total solid. The specific denitrification rate was 20-40 g/(g·min), which is on the same order of magnitude as that for commonly used external carbon sources. Hydrolysis at around 35 °C and pH 7 gave the best results. The hydrolysate centrate can be stored and added to the biological treatment to improve water quality and reduce emissions of nitrous oxide or it can be used to produce biogas to optimize the operation of the plant.

Rapidly achieving partial denitrification from nitrate wastewater in a alkaline fermentation system with primary sludge as inoculated sludge and fermentable substrate

In order to promote practical engineering application of anaerobic ammonium oxidation(anammox) process, reduction of primary sludge(PS) in wastewater treatment plants(WWTPs) and removal of nitrate contaminant, a single-stage simultaneous alkaline fermentation coupled with partial denitrification(SAFPD) system was established successfully in this study. Nitrite production was rapidly achieved from nitrate wastewater with PS as inoculated sludge and fermentable substrate under anaerobic and anoxic operating conditions. During the stable operation period, the primary sludge reduction(PSR) and productivity of organic matters were 27.9% and 483.8mgCOD/gVSS, with nitrate removal of 90.7%, NO₃^- to NO₂^- transformation ratio(NTR) of 80.0%. After 125 days of acclimation, the relative abundance of Thauera, Dechloromonas and Candidatus_Competibacter increased from 0.17%, 0.02% and 0.05% to 11.58%, 4.28% and 5.6% respectively. Above results showed that this SAFPD system not only realized the reduction of PS and nitrate removal, but also laid a solid foundation for anammox process with its high nitrite production.

Enriched sewage sludge from anaerobic pre-treatment in spurring valorization potential of black soldier fly larvae

The valorization of sewage sludge by black soldier fly larvae (BSFL) has gained attentions for sewage sludge management since the sludge can be reduced securely as well as larval biomass can be used for biorefineries application. Nevertheless, the BSFL growth was impeded while assimilating nutrition from sewage sludge due to the presence of extracellular polymeric substances (EPS) that had entrapped the essential nutrients inside. Accordingly, the pre-treatment of sewage sludge via anaerobic digestion at different pH was employed in this work to rupture the EPS structure and release more nutrients for larval growth. The results showed that larvae fed with raw sewage sludge had attained the lowest final larval weight (2.05 ± 0.38 mg/larva) as opposed to batches fed with pre-treated sewage sludges. This was because the soluble carbohydrate (more than 6.81 ± 1.31 mg of glucose/g sewage sludge) in EPS was released after anaerobic pre-treatment, facilitating larval assimilation for growth. Furthermore, it was observed that further increasing of pH for sewage sludge pre-treatment had led to lower final larval weight gained due to the inhibitory effect stemming from ammonia production at higher pH. The anaerobic pre-treatment of sewage sludge being executed at pH 3 for 8 days had achieved the highest final larval weight at 7.34 ± 0.97 mg/larva. The still low quality of sewage sludges after the pre-treatment also offered benefit, where high sewage sludge reduction and waste reduction index were recorded due to the necessity of BSFL to consume more sewage sludge in compensating the nutrients destitution in sludge. Lastly, the possibility of predicting final larval weight was successfully materialized via a statistical model derived from the multiple linear regression method. The derived model incorporated the interactive parameters of anaerobic pre-treated pH and durations at various combinations could predict the final larval weight.

Co-digestion of sewage sludge and wood fly ash

This study uses a new approach for the recycling of plant nutrients by co-digesting sewage sludge with fly ash from a wood combustion. Sewage sludge and fly ash both are enriched with nutrients of the wastewater resp. wood, which makes these products an enhanced source for recycled fertilizers. The effects of the ash addition to the anaerobic digestion are studied in several lab scale experiments including effects on the gas production and microbial activity. Following that, the fertilizing qualities of the digestate are evaluated by plant growth experiments. The results show that the fertilizing qualities of the digested sludge were improved by the ash addition. Next to this, gas production results show that the methane production was not affected by the ash addition, while the total gas release was reduced. The sulphur addition by the ash stimulated sulphate reducing bacteria. The sulphate reducing bacteria did not markedly inhibit the methanogens.

Applicability of two-stage anoxic/oxic shortcut nitrogen removal via partial nitrification and partial denitrification for municipal wastewater by adding sludge fermentation products continuously

Partial nitrification and partial denitrification combined with anammox is a promising process for sewage treatment. In this study, real municipal wastewater was treated in a continuous two-stage anoxic/oxic (A/O) reactor. External mixed sludge fermentation products were added in the anoxic zone, simultaneously achieving partial nitrification and partial denitrification and achieving a high and relatively stable accumulation of nitrite. The maximum accumulation rates of NO₂^--N in A1.2 and A2.1-A2.4 zones of the reactor reached 70% and 61%-37%, respectively, which improved denitrification efficiency and created conditions that supported the coupling of subsequent anammox. The influent nitrogen load of the system was 0.078 kg/(m³•d), and the mean influent and effluent total nitrogen were 51 and 12 mg/L, respectively. The mean total nitrogen removal rate reached 76%. Further analysis revealed that Hyphomicrobium (incomplete denitrifiers) and Nitrosomonas (ammonia oxidizing bacteria) were enriched, which may have facilitated the high nitrite accumulation. Moreover, the batch test showed that adding sludge fermentation during denitrification significantly suppressed nitrite reduction, resulting in the nitrite accumulation.

Anaerobic Membrane Bioreactors for Municipal Wastewater Treatment: A Literature Review

Currently, there is growing scientific interest in the development of more economic, efficient and environmentally friendly municipal wastewater treatment technologies. Laboratory and pilot-scale surveys have revealed that the anaerobic membrane bioreactor (AnMBR) is a promising alternative for municipal wastewater treatment. Anaerobic membrane bioreactor technology combines the advantages of anaerobic processes and membrane technology. Membranes retain colloidal and suspended solids and provide complete solid–liquid separation. The slow-growing anaerobic microorganisms in the bioreactor degrade the soluble organic matter, producing biogas. The low amount of produced sludge and the production of biogas makes AnMBRs favorable over conventional biological treatment technologies. However, the AnMBR is not yet fully mature and challenging issues remain. This work focuses on fundamental aspects of AnMBRs in the treatment of municipal wastewater. The important parameters for AnMBR operation, such as pH, temperature, alkalinity, volatile fatty acids, organic loading rate, hydraulic retention time and solids retention time, are discussed. Moreover, through a comprehensive literature survey of recent applications from 2009 to 2021, the current state of AnMBR technology is assessed and its limitations are highlighted. Finally, the need for further laboratory, pilot- and full-scale research is addressed.

Volatile Fatty Acid Production from Food Waste Leachate Using Enriched Bacterial Culture and Soil Bacteria as Co-Digester

The production of volatile fatty acids (VFAs) from waste stream has been recently getting attention as a cost-effective and environmentally friendly approach in mechanical–biological treatment plants. This is the first study to explore the use of a functional bacterium, AM5 isolated from forest soil, which is capable of enhancing the production of VFAs in the presence of soil bacteria as a co-digester in non-strict anaerobic fermentation processes of food waste leachates. Batch laboratory-scale trials were conducted under thermophilic conditions at 55 °C and different pH values ranging from approximately 5 to 11, as well as under uncontrolled pH for 15 days. Total solid content (TS) and volatile solid content (VS) were observed with 58.42% and 65.17% removal, respectively. An effluent with a VFA concentration of up to 33,849 mg/L (2365.57 mg/g VS; 2244.45 mg/g chemical oxygen demand (COD)-VFA VS; 1249 mg/g VSremoved) was obtained at pH 10.5 on the second day of the batch culture. The pH resulted in a significant effect on VFA concentration and composition at various values. Additionally, all types of VFAs were produced under pH no-adjustment (approximately 5) and at pH 10.5. This research might lead to interesting questions and ideas for further studies on the complex metabolic pathways of microbial communities in the mixture of a soil solution and food waste leachate.

Production of Volatile Fatty Acids in a Semi-Continuous Dark Fermentation of Kitchen Waste: Impact of Organic Loading Rate and Hydraulic Retention Time

The aim of the study was to evaluate the possibility of using the process of dark fermentation to convert kitchen waste into valuable volatile fatty acids in a semi-continuous process at different values of the organic loading rate (2.5 and 5.0 gVS/(L × d)) and hydraulic retention time (5 and 10 d) using anaerobic mixed microbial consortia. The experiments were performed in a bioreactor of working volume 8L with pH control. The maximum volatile fatty acids yield in a steady state (22.3 g/L) was achieved at the organic loading rate of 5.0 gVS/(L × d) and HRT of 10 days. The main products of dark fermentation were acetic and butyric acids, constituting, respectively, 35.2–47.7% and 24.1–30.0% of all identified volatile fatty acids. Additionally, at the beginning of the fermentation and in a steady-state condition, the microbial population analysis (16S rDNA) of the fermentation mixture with the most effective volatile fatty acids generation has been performed to monitor the DF microflora development. The dominant microorganisms at a phylum level in a steady state were Firmicutes (44.9%) and Bacteroidetes (30.1%), which indicate the main role of those phyla in the volatile fatty acids synthesis.

Integrated fermentation and anaerobic digestion of primary sludges for simultaneous resource and energy recovery: Impact of volatile fatty acids recovery

This research assessed the impact of volatile fatty acids (VFA) recovery and biomethane potential in an integrated fermentation-digestion process with a single stage digestion of primary and rotating belt filtration (RBF) sludges. Implementing semi-continuous fermentation at 1, 2, and 4 days solids retention time (SRT) showed a direct impact on the hydrolysis and VFA recovery which increased as SRT increased, while also improving the dewaterability by reducing the concentrated sludge volume index of the processed sludge. pH-controlled fermentation was effective improving the VFA yields by up to 93% and 72% at pH 9 (relative to no pH control), for RBF and primary sludges, respectively; although fermentation at pH 6 (optimum) showed promise for enhancing VFAs while lowering the required chemicals significantly. Although cellulose constituted only 21.0% and 29.5% of the TSS in primary and RBF sludges, it contributed 38-41% of the methane production for the two sludges, respectively. Experimental results of integrated fermentation-digestion and single stage digestion processes were incorporated in techno-economic analysis. Results confirmed the economic viability of fermentation with payback periods of 2.7 ± 1.1 years (RBF), and 3.6 ± 2.7 years (PS), while also revealed that VFA recovery could save up to 7.2 ± 2.0% (RBF), and 7.6 ± 2.7% (PS) of the respective total sludge handling and disposal costs, despite an average of 12.7% and 8.4% decrease in biogas production due to VFA extraction in the integrated systems of RBF and primary sludges, respectively. Overall, the integrated fermentation-digestion system economically outperformed the single stage digestion for both sludge types under all studied scenarios.

Short-chain fatty acids recovery from sewage sludge via acidogenic fermentation as a carbon source for denitrification: A review

Wastewater treatment plants face the problem of a shortage of carbon source for denitrification. Acidogenic fermentation is an effective method for recovering short-chain fatty acids (SCFAs) as a carbon source from sewage sludge. Herein, the most recent advances in SCFAs production from primary sludge and waste activated sludge are systematically summarised and discussed. New technologies and problems pertaining to the improvement in SCFAs availability in fermentation liquids, including removal of ammoniacal nitrogen and phosphate and extraction of SCFAs from fermentation liquids, are analysed and evaluated. Furthermore, studies on the use of recovered SCFAs as a carbon source for denitrification are reviewed. Based on the above summarisation and discussion, some conclusions as well as perspectives on future studies and practical applications are presented. In particular, the recovery of carbon source/bioenergy from sewage sludge must be optimised considering nutrient removal/recovery simultaneously.

Pilot-scale study for phosphorus recovery by sludge acidification and dewatering

Phosphorus recovery from wastewater is a focus area in Denmark; the aim is to recover at least 80% of the phosphorus. In order to extract phosphorus, surplus sludge from wastewater treatment plants was acidified (pH 2-4) to increase the dissolved phosphorus concentration, which then can be precipitated and recovered. Pilot-scale acidification and dewatering tests were done using sludge from three different wastewater treatment plants: plant (1) digested primary and secondary sludge, plant (2) digested primary sludge, and plant (3) non-digested sludge. Treatment of digested sludge gave the highest phosphorus release, but the acid consumption was high due to carbon dioxide stripping. The dry matter content of the acidified dewatered sludge was high (20-40%), but the dry matter content in the filtrate increased with decreasing pH. Approximately half of the dry matter content in the filtrate could be removed by introducing an additional separation step. The optimal pH for phosphorus extraction was 3, where up to 68% of the phosphorus was dissolved. Part of the released orthophosphate was lost with the filter cake but still, 60% of the total phosphorus content in the sludge ends up in the filtrate.

Enhanced production of volatile fatty acids by adding a kind of sulfate reducing bacteria under alkaline pH

Anaerobic digestion could make sludge stable and harmless, and the volatile fatty acids (VFAs) produced from it. The objective of this study was to reduced sludge production and realize the resource utilization of VFAs through enhance anaerobic sludge fermentation by adding sulfate reducing bacteria (SRB) under alkaline pH. Under the neutral and alkaline pH, SRB was added into the sludge fermentation liquid with sole stock solution and sterilization treatment respectively, while the liquid without any additives was used as control. The results indicated that obvious increase of the production of VFAs was observed after adding SRB under alkaline pH. And, more protein and polysaccharide were obtained which were the main substrates for the production of VFAs. The concentration of ammonia nitrogen (NH₄⁺-N) and phosphate (PO₄^3--P) were also increased with the addition of SRB. So, a high yield production of VFAs could be achieved through the addition of SRB + alkaline pH.

Elucidating salinity adaptation and shock loading on denitrification performance: Focusing on microbial community shift and carbon source evaluation

To understand the denitrification efficiency and microbial community shift with increasing salinity in salinity adaptation and shock loading process, nitrate (NO3--N), nitrite (NO2--N) and chemical oxygen demand (COD) removal efficiencies were monitored feeding acetate and primary sludge fermentation liquid. During adaptation process, salinity had little effect on NO3--N removal efficiency (>99.0%) with acetate-fed, while for fermentation liquid-fed, it decreased to around 97% at high salinity (>2.5%). Effluent NO2--N was lower than 0.1 mg/L, though obvious fluctuation of NO2--N was observed with fermentation liquid-fed when salinity change. During shock loading process, denitrification process all had slight decrease when the salinity abruptly increased to 5.0%. Traditional denitrifier of Thauera was the dominant genus, and a specialized microbial community of Azoarcus in salinity adaptation and Paracoccus in shock loading for denitrification showed high salinity tolerant. Meanwhile, microbial diversity was enriched with fermentation liquid-fed at high salinity condition.

Value Proposition of Untapped Wet Wastes: Carboxylic Acid Production through Anaerobic Digestion

Although traditional anaerobic digestion (AD) process to produce methane-rich biogas from wet waste is deep-rooted, high carbon footprint and its low value as compared with other renewable sources demand advanced strategies to avoid its production. An emerging conversion pathway to arrest methanogenesis for producing value-added fuels and chemicals instead of biogas is sought as a sustainable alternative. This research provides a comprehensive analysis on current technology development, process challenges, applications, and economics for producing high-value short-chain carboxylic acids from AD of wet wastes. We show that (1) the theoretical energy yields of acids equal or exceed biogas, and (2) the cost of these acids is competitive with those produced from chemical markets, making this economically viable for mass production. With global abundance of wet waste feedstocks, this process of short-chain acid production provides a promising alternative to conventional biogas production technology, while achieving waste management and carbon mitigation goals.

Enzymatic pre-treatment for enhancement of primary sludge fermentation

This research showed the interrelated impact of cellulase enzyme, temperature, and SRT on enhancement of primary and rotating belt filter (PS, RBF) sludges fermentation. SRTs of 1, 2, and 4-days were tested at 25 °C and 35 °C. Enzymatic enhancement was examined using three different doses of enzyme (i.e. 0.5%, 1%, and 1.5% of the total solids in the feed). The results showed a positive impact of enzyme dose as well as temperature and SRT on VFA and soluble COD production. For the RBF sludge, enzyme addition enhanced the VFA yield of fermentation at room temperature (25 °C) from 52-103 mgCOD/g VS to 93-188 mgCOD/g VS, as compared with increase from 78-192 to 87-202 mgCOD/gVS in PS. Intensification of the fermentation process, particularly for the cellulose-rich RBF sludge, by enzyme addition confirms process viability as an alternative to the extraneous carbon sources for biological nutrient removal in wastewater treatment plants.

Comparation of thermophilic bacteria (TB) pretreated primary and secondary waste sludge carbon sources on denitrification performance at different HRTs

In this study, thermophilic bacteria pretreated primary and secondary waste sludge hydrolysis and acidification liquid were used as denitrification carbon sources at different HRTs (hydraulic retention time). The NO₃^--N removal rate of 99.3%, 99.0%, 99.9% and 99.2% was achieved at the optimal HRT of 8, 8, 4 and 6 h, respectively. Meanwhile, the utilization of COD (Chemical oxygen demand), proteins, carbohydrates, and VFAs (Volatile fatty acids) in carbon source during denitrification was also investigated. High-throughput sequencing technology showed that the microbial community changed with the different sludge carbon sources. And the dominant genus in both reactors was Thauera, which played a key role in denitrification.

Overview of key operation factors and strategies for improving fermentative volatile fatty acid production and product regulation from sewage sludge

In recent years, volatile fatty acid (VFA) production through anaerobic fermentation of sewage sludge, instead of methane production, has been regarded as a high-value and promising roadmap for sludge stabilization and resource recovery. This review first presents the effects of some essential factors that influence VFA production and composition. In the second part, we present an extensive analysis of conventional pretreatment and co-fermentation strategies ultimately addressed to improving VFA production and composition. Also, the effectiveness of these approaches is summarized in terms of sludge degradation, hydrolysis rate, and VFA production and composition. According to published studies, it is concluded that some pretreatments such as alkaline and thermal pretreatment are the most effective ways to enhance VFA production from sewage sludge. The possible reasons for the improvement of VFA production by different methods are also discussed. Finally, this review also highlights several current technical challenges and opportunities in VFA production with spectrum control, and further related research is proposed.

Bioengineering of anaerobic digestion for volatile fatty acids, hydrogen or methane production: A critical review

Anaerobic digestion (AD) is a well-established technology used for producing biogas or biomethane alongside the slurry used as biofertilizer. However, using a variety of wastes and residuals as substrate and mixed cultures in the bioreactor makes AD as one of the most complicated biochemical processes employing hydrolytic, acidogenic, hydrogen-producing, acetate-forming bacteria as well as acetoclastic and hydrogenoclastic methanogens. Hydrogen and volatile fatty acids (VFAs) including acetic, propionic, isobutyric, butyric, isovaleric, valeric and caproic acid and other carboxylic acids such as succinic and lactic acids are formed as intermediate products. As these acids are important precursors for various industries as mixed or purified chemicals, the AD process can be bioengineered to produce VFAs alongside hydrogen and therefore biogas plants can become biorefineries. The current review paper provides the theory and means to produce and accumulate VFAs and hydrogen, inhibit their conversion to methane and to extract them as the final products. The effects of pretreatment, pH, temperature, hydraulic retention time (HRT), organic loading rate (OLR), chemical methane inhibitions, and heat shocking of the inoculum on VFAs accumulation, hydrogen production, VFAs composition, and the microbial community were discussed. Furthermore, this paper highlights the possible techniques for recovery of VFAs from the fermentation media in order to minimize product inhibition as well as to supply the carboxylates for downstream procedures.

Characterization of the refractory dissolved organic matters (rDOM) in sludge alkaline fermentation liquid driven denitrification: Effect of HRT on their fate and transformation

Enhanced biological denitrification for nitrogen removal using sludge alkaline fermentation liquid (SAFL) as an alternative carbon source has been widely reported in previous studies, while limited studies focused on the degradation of the organics presented in SAFL. In this study, an SAFL driven anoxic denitrification sequencing batch reactor (SBR) was established, the mechanism of organics utilization was characterized and the refractory dissolved organic matters (rDOM) was identified. Denitrification could rapidly proceed with the presence of volatile fatty acids (VFAs) initially, while the denitrification rate largely decreased after the VFAs depleted. A great deal of rDOM, which was hard to be utilized by denitrifying microorganism, was found in the effluent. A prolonged hydraulic retention time (HRT) led to the further transformation of particles and colloids to smaller colloids and soluble organics. Extended HRT promoted the degradation of soluble microbial by-product (SMP), but had minor effect on the removal of humic-like, and fulvic acid-like substances. The characterization of the effluent demonstrated the building blocks, were dominated in the rDOM (43.79%-48.78%), followed by high molecular weight protein (HMW-PN) (13.37%-17.39%), HMW polysaccharide (HMW-PS) (12.84%-15.9%), low molecular weight (LMW) neutrals (11.28%-13.65%), and hydrophobic dissolved organic carbon (HO-DOC) (8.0%-12.62%). Moreover, it was found that the building blocks were relatively easy to be degraded with the extension of HRTs, followed by LMW-PS, LMW-PN, LMW neutrals, HMW-PN, and HMW-PS. However, further extended HRT >24 h could not improve the removal of building blocks, LMW-PS and LMW neutrals. This study, for the first time, provided insights into the transformation of organic matters produced by SAFL in a denitrification system and acted as a guide for the subsequent advanced treatment.

A critical review of volatile fatty acids produced from waste activated sludge: enhanced strategies and its applications

This paper reviews the recent achievements in the enhanced production of volatile fatty acids (VFAs) from waste activated sludge (WAS). The enhanced strategies are divided into two approaches. The first strategy focuses on the regulation of carbon-to-nitrogen (C/N) ratio by co-digestion of WAS with carbon-rich substrates, including municipal solid wastes (MSW), marine algae, agricultural residues, and animal manures. The other strategy is to enhance the solubilization and hydrolysis of WAS or inhibit the methanogenesis by applying various pretreatments, such as mechanical, chemical, enzymatic, and thermal pretreatment. Finally, the applications of WAS-derived VFAs are discussed. The future researches in enhancing VFAs production and wide application of the VFAs from both technical and economic perspectives are proposed.

Layered double hydroxides for phosphorus recovery from acidified and non-acidified dewatered sludge

Phosphate, which contains the essential element phosphorous (P), is a necessary fertilizer for agriculture, but the current phosphate deposits are running out and alternative sources are needed. Sludge obtained from wastewater treatment plants contains high concentrations of phosphorus and represents an alternative, sustainable source. In this study, sludge obtained from a wastewater treatment plant with biological and chemical phosphorus removal was acidified (pH = 3, 4, 5 and 6) to release orthophosphate followed by sequestration of the orthophosphate by a zinc aluminum layered double hydroxide (Zn₂Al-LDH). Sulfuric acid (H₂SO₄), nitric acid (HNO₃), and hydrochloric acid (HCl) was tested, which showed that only sulfate anions compete with phosphate and results in reduced phosphate recovery (25-35%). The orthophosphate concentration in the liquid phase increased from 20% (raw sludge) to 75% of the total phosphorus concentration at a pH of 3, which enhanced the phosphate uptake by the ZnAl-LDH from 1.7 ± 0.2% to 60.3 ± 0.6%. During acidification, the competing anion carbonate is degassed as CO₂, which further improved the phosphate uptake. PXRD showed the intercalation of carbonate in the LDH in the raw sludge at pH = 8, whereas orthophosphate was intercalated at lower pH values. ²⁷Al MAS NMR spectroscopy and powder X-ray diffraction (PXRD) proved preservation of the LDH at all pH values. Furthermore, about a fourth of the Al is present as an amorphous aluminum phosphate (AlPO₄) upon exposure to phosphate at low pH (pH = 3 and 5) based on ²⁷Al MAS NMR spectroscopy. At a pH of 6 about a third of the P is present as brushite (CaHPO₄·2H₂O).

In vitro evaluation of nano zinc oxide (nZnO) on mitigation of gaseous emissions

BACKGROUND

Enteric methane (CH4) accounts for about 70% of total CH4 emissions from the ruminant animals. Researchers are exploring ways to mitigate enteric CH4 emissions from ruminants. Recently, nano zinc oxide (nZnO) has shown potential in reducing CH4 and hydrogen sulfide (H2S) production from the liquid manure under anaerobic storage conditions. Four different levels of nZnO and two types of feed were mixed with rumen fluid to investigate the efficacy of nZnO in mitigating gaseous production.

METHODS

All experiments with four replicates were conducted in batches in 250 mL glass bottles paired with the ANKOMRF wireless gas production monitoring system. Gas production was monitored continuously for 72 h at a constant temperature of 39 ± 1 °C in a water bath. Headspace gas samples were collected using gas-tight syringes from the Tedlar bags connected to the glass bottles and analyzed for greenhouse gases (CH4 and carbon dioxide-CO2) and H2S concentrations. CH4 and CO2 gas concentrations were analyzed using an SRI-8610 Gas Chromatograph and H2S concentrations were measured using a Jerome 631X meter. At the same time, substrate (i.e. mixed rumen fluid+ NP treatment+ feed composite) samples were collected from the glass bottles at the beginning and at the end of an experiment for bacterial counts, and volatile fatty acids (VFAs) analysis.

RESULTS

Compared to the control treatment the H2S and GHGs concentration reduction after 72 h of the tested nZnO levels varied between 4.89 to 53.65%. Additionally, 0.47 to 22.21% microbial population reduction was observed from the applied nZnO treatments. Application of nZnO at a rate of 1000 μg g- 1 have exhibited the highest amount of concentration reductions for all three gases and microbial population.

CONCLUSION

Results suggest that both 500 and 1000 μg g- 1 nZnO application levels have the potential to reduce GHG and H2S concentrations.

The relationship between volatile fatty acids accumulation and microbial community succession triggered by excess sludge alkaline fermentation

The volatile fatty acids (VFAs) accumulation pattern and microbial community succession were studied during excess sludge (ES) alkaline fermentation at pH of 10.0 with expanded granular sludge blanket reactor over 5 cyclers. Microbial community shifted conspicuously as ES suffered alkaline fermentation. Both VFAs and acid-producing bacteria increased rapidly during the first 8 days fermentation time, and they showed a quite positive correlation relationship. In addition, soluble chemical oxygen demand (SCOD) also dramatically increased during the first 8 days, which implied 8 day was the optimum sludge retention time (SRT) for ES alkaline fermentation and VFAs accumulation time. Illumina Miseq Sequencing analysis indicated that Clostridium, Bacillus, Amphibacillus and Peptostreptococcaceae were the dominant bacteria genus to produce VFAs. Acetic acid took about 84% in total VFAs because among the total acid-producing bacteria most bacteria could produce acetic acid.

Use of solid phosphorus fractionation data to evaluate phosphorus release from waste activated sludge

Waste activated sludge (WAS) can become an important source of phosphorus (P). P speciation was examined under anaerobic conditions, with different pH (4, 6 and 8) and temperatures (10, 20 and 35 °C). Aqueous P was measured and an extraction protocol was used to find three solid phosphorus fractions. A pH of 4 and a temperature of 35 °C gave a maximum of 51% of total P solubilized in 22 days with 50% of total P solubilized in 7 days. Batch tests indicate that little pH depression is needed to release non-apatite inorganic P (including microbial polyphosphate), while a pH of 4 rather than 6 will release more apatite inorganic P, and that organic P is relatively more difficult to release from WAS. Fractionation analysis of P in WAS can aid in design of more efficient methods for P recovery from WAS.

Phosphorus dissolution from dewatered anaerobic sludge: Effect of pHs, microorganisms, and sequential extraction

Phosphorus (P) and iron mass balance from Limassol Wastewater Treatment Plant showed that the major removal and accumulation occurred at the aerobic secondary stage and at Dewatered Anaerobic Sludge (DWAS), respectively. The purpose of this study was to examine various parameters that effect the P dissolution under low pH from DWAS. The parameters that significantly contribute to P extraction were the exposure to pH 2.5, the anaerobic conditions and the sequential extraction. The addition of chemolithotrophic acidophilic bacteria has negatively influenced P dissolution, whereas the addition of acidophilic Heterotrophic Iron Reducing (HIR) bacteria has slightly increased P dissolution but they contributed to pH maintenance at lower levels compared to no addition of HIR. P fractionation of the residual sludge after sequential extraction pointed out that the organically bound P was hardly dissoluted from DWAS. The residual DWAS after acid treatment generated around 45% less methane compared to the initial DWAS.

In-situ sludge pretreatment in a single-stage anaerobic digester

This study aimed to develop an in-situ sludge pretreatment method by increasing the temperature from thermophilic to extreme thermophilic condition in a single-stage anaerobic digester. The results revealed that a stable performance was obtained within the temperature range of 55-65°C, and the maximum methane yield of 208.51±13.66mL/g VS was obtained at 65°C. Moreover, the maximum extent of hydrolysis (33%) and acidification (27.1%) was also observed at 65°C. However, further increase of temperature to 70°C did not improve the organic conversion efficiency. Microbial community analysis revealed that Coprothermobacter, highly related to acetate oxidisers, appeared to be the abundant bacterial group at higher temperature. A progressive shift in methanogenic members from Methanosarcina to Methanothermobacter was observed upon increasing the temperature. This work demonstrated single-stage sludge digestion system can be successfully established at high temperature (65°C) with stable performance, which can eliminate the need of conventional thermophilic pretreatment step.

The effect of Mg2+ on digestion performance and microbial community structures in sludge digestion systems

The important criteria in anaerobic digestion is the rate-limiting step which decides the fate of value-added products especially from waste-activated sludge (WAS). Hence, the present study investigated the effect of magnesium (Mg²⁺) addition on anaerobic digestion of WAS. The lab-scale experiments were conducted at 25 °C with Mg²⁺ doses ranging from 0.01 to 0.2 mol/L. Maximum total volatile fatty acids (VFAs) production (372.78 mg COD/L) occurred at a Mg²⁺ dose of 0.2 mol/L, which was about eight times higher than the control tests. Further, Mg²⁺ addition facilitated sludge dewaterability and phosphorus removal. The mechanism of improved VFAs generation was analyzed from the view of both chemical and biological effects. Chemical effect significantly enhanced the release of calcium and iron in WAS, resulting in the disintegration of WAS, which benefited hydrolysis and acidification processes. Illumina MiSeq sequencing analysis revealed that enrichment of functional bacteria and the increase of bacterial diversity were obtained in the 0.2 mol Mg²⁺/L experiment, while the influence was negative on the reactor with 0.025 mol/L Mg²⁺. Meanwhile, methanogens were accordantly inhibited in the experiments with Mg²⁺ addition.

Influence of low pH on continuous anaerobic digestion of waste activated sludge

The influence of low pH on single stage continuous anaerobic digestion was evaluated, with the goal of increasing soluble phosphorus (P) concentration to mitigate in-reactor P precipitation. This was performed at pH 5.0, 5.5, 6.0, 6.5 and 7.0 using 1 L stirred-tank mesophilic reactors fed with sewage waste activated sludge. Low pH (5.5) caused a significant (p < 0.01) increase in soluble P concentration up to 79% of the total P, while methane yield was reduced by 50%. Total volatile fatty acids and soluble chemical oxygen demand concentrations increased from 40 to 504 mg L^-1 and 600 to 2017 mg L^-1 respectively, as the pH was reduced from 7.0 to 5.5. Higher concentrations of propionic acid (370-430 mg L^-1) were found at low pH (5.5). The reduction in methane yield was associated with a shift in microbial community and decreased destruction of particulate organics. Acidogens dominated at low pH (< 6.0), while methanogens decreased by 88% at pH 5.5 compared to neutral pH. Apart from the loss in methanogenic and hydrolytic capacity, chemical needs for acid dosing to maintain low pH conditions, and other negative impacts of chemical dosing were identified as key limitations.

Enhanced volatile fatty acids (VFAs) production in a thermophilic fermenter with stepwise pH increase - Investigation on dissolved organic matter transformation and microbial community shift

In this study, a mixture of primary and wasted activated sludge was fermented in a semi-continuous reactor aiming for enhanced volatile fatty acids (VFAs) production. The reactor was subjected to a stepwise pH increase from 7 to 10 during approximately 130 days of operation. The result revealed that the maximum acidification was obtained at pH 8.9 (21%) resulting in the maximum production of VFAs (423.22 ± 25.49 mg COD/g VSS), while the maximum hydrolysis efficiency was observed at pH 9.9 (42%). The high pH was effective in releasing dissolved organic matter (DOM) including protein, carbohydrate, building blocks and low molecular weight (LMW) neutrals. More LMW DOMs were released than high molecular weight (HMW) DOMs fractions at higher pH. pH 9.9 favored hydrolysis of HMW DOMs while it did not enhance the acidogenesis of LMW DOMs. The microbial community analysis showed that the relative abundance of phyla Actinobacteria and Proteobacteria increased with the increased pH, which may lead to the maximum hydrolysis at pH 9.9. At pH 8.9, class Clostridia (59.16%) was the most dominant population where the maximum acidification (21%) was obtained. This suggested that the dominance of Clostridia was highly related to acidification extent. The relative abundance of Euryarchaeota decreased significantly from 58% to 2% with increased pH.

Integrated systems for biopolymers and bioenergy production from organic waste and by-products: a review of microbial processes

Recently, issues concerning the sustainable and harmless disposal of organic solid waste have generated interest in microbial biotechnologies aimed at converting waste materials into bioenergy and biomaterials, thus contributing to a reduction in economic dependence on fossil fuels. To valorize biomass, waste materials derived from agriculture, food processing factories, and municipal organic waste can be used to produce biopolymers, such as biohydrogen and biogas, through different microbial processes. In fact, different bacterial strains can synthesize biopolymers to convert waste materials into valuable intracellular (e.g., polyhydroxyalkanoates) and extracellular (e.g., exopolysaccharides) bioproducts, which are useful for biochemical production. In particular, large numbers of bacteria, including Alcaligenes eutrophus, Alcaligenes latus, Azotobacter vinelandii, Azotobacter chroococcum, Azotobacter beijerincki, methylotrophs, Pseudomonas spp., Bacillus spp., Rhizobium spp., Nocardia spp., and recombinant Escherichia coli, have been successfully used to produce polyhydroxyalkanoates on an industrial scale from different types of organic by-products. Therefore, the development of high-performance microbial strains and the use of by-products and waste as substrates could reasonably make the production costs of biodegradable polymers comparable to those required by petrochemical-derived plastics and promote their use. Many studies have reported use of the same organic substrates as alternative energy sources to produce biogas and biohydrogen through anaerobic digestion as well as dark and photofermentation processes under anaerobic conditions. Therefore, concurrently obtaining bioenergy and biopolymers at a reasonable cost through an integrated system is becoming feasible using by-products and waste as organic carbon sources. An overview of the suitable substrates and microbial strains used in low-cost polyhydroxyalkanoates for biohydrogen and biogas production is given. The possibility of creating a unique integrated system is discussed because it represents a new approach for simultaneously producing energy and biopolymers for the plastic industry using by-products and waste as organic carbon sources.

The use of fermentation liquid of wastewater primary sedimentation sludge as supplemental carbon source for denitrification based on enhanced anaerobic fermentation

Wastewater primary sedimentation sludge was prepared into fermentation liquid as denitrification carbon source, and the main components of fermentation liquid was short-chain volatile fatty acids. Meanwhile, the acetic acid and propionic acid respectively accounted for about 29.36% and 26.56% in short-chain volatile fatty acids. The performance of fermentation liquid, methanol, acetic acid, propionic acid and glucose used as sole carbon source were compared. It was found that the denitrification rate with fermentation liquid as carbon source was 0.17mgNO3(-)-N/mg mixed liquor suspended solid d, faster than that with methanol, acetic acid, and propionic acid as sole carbon source, and lower than that with glucose as sole carbon source. For the fermentation liquid as carbon source, the transient accumulation of nitrite was insignificantly under different initial total nitrogen concentration. Therefore, the use of fermentation liquid for nitrogen removal could improve denitrification rate, and reduce nitrite accumulation in denitrification process.

The effect of salinity on waste activated sludge alkaline fermentation and kinetic analysis

The effect of salinity on sludge alkaline fermentation at low temperature (20°C) was investigated, and a kinetic analysis was performed. Different doses of sodium chloride (NaCl, 0-25g/L) were added into the fermentation system. The batch-mode results showed that the soluble chemical oxygen demand (SCOD) increased with salinity. The hydrolysate (soluble protein, polysaccharide) and the acidification products (short chain fatty acids (SCFAs), NH4(+)-N, and PO4(3-)-P) increased with salinity initially, but slightly declined respectively at higher level salinity (20g/L or 20-25g/L). However, the hydrolytic acidification performance increased in the presence of salt compared to that without salt. Furthermore, the results of Haldane inhibition kinetics analysis showed that the salt enhanced the hydrolysis rate of particulate organic matter from sludge particulate and the specific utilization of hydrolysate, and decreased the specific utilization of SCFAs. Pearson correlation coefficient analysis indicated that the importance of polysaccharide on the accumulation of SCFAs was reduced with salt addition, but the importance of protein and NH4(+)-N on SCFA accumulation was increased.

Combined Effect of Free Nitrous Acid Pretreatment and Sodium Dodecylbenzene Sulfonate on Short-Chain Fatty Acid Production from Waste Activated Sludge

Free nitrous acid (FNA) serving as a pretreatment is an effective approach to accelerate sludge disintegration. Also, sodium dodecylbenzene sulfonate (SDBS), a type of surfactants, has been determined at significant levels in sewage sludge, which thereby affects the characteristics of sludge. Both FNA pretreatment and sludge SDBS levels can affect short-chain fatty acid (SCFA) generation from sludge anaerobic fermentation. To date, however, the combined effect of FNA pretreatment and SDBS presence on SCFA production as well as the corresponding mechanisms have never been documented. This work therefore aims to provide such support. Experimental results showed that the combination of FNA and SDBS treatment not only improved SCFA accumulation but also shortened the fermentation time. The maximal SCFA accumulation of 334.5 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS) was achieved at 1.54 mg FNA/L treatment and 0.02 g/g dry sludge, which was respectively 1.79-fold and 1.41-fold of that from FNA treatment and sludge containing SDBS alone. Mechanism investigations revealed that the combined FNA pretreatment and SDBS accelerated solubilization, hydrolysis, and acidification steps but inhibited the methanogenesis. All those observations were in agreement with SCFA enhancement.

Improved volatile fatty acids anaerobic production from waste activated sludge by pH regulation: Alkaline or neutral pH?

In this study, the anaerobic fermentation was carried out for volatile fatty acids (VFAs) production at different pH (between 7.0 and 10.0) conditions with untreated sludge and heat-alkaline pretreated waste activated sludge. In the fermentation with untreated sludge, the extent of hydrolysis of organic matters and extent of acidification at alkaline pH are 54.37% and 30.37%, respectively, resulting in the highest VFAs yield at 235.46mg COD/gVS of three pH conditions. In the fermentation with heat-alkaline pretreated sludge, the acidification rate and VFAs yield at neutral pH are 30.98% and 240.14mg COD/gVS, respectively, which are higher than that at other pH conditions. With the glucose or bovine serum albumin as substrate for VFAs production, the neutral pH showed a higher VFAs concentration than the alkaline pH condition. The results of terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that the alkaline pH caused low microbial richness. Based on the results in this study, we demonstrated that the alkaline pH is favor of hydrolysis of organic matter in sludge while neutral pH improved the acidogenesis for the VFAs production from sludge. Our finding is obvious different to the previous research and helpful for the understanding of how heat-alkaline pretreatment and alkaline fermentation influence the VFAs production, and beneficial to the development of VFAs production process.

A biologically inspired variable-pH strategy for enhancing short-chain fatty acids (SCFAs) accumulation in maize straw fermentation

This study investigates the feasibility of varying the pH to enhance the accumulation of short-chain fatty acids (SCFAs) in the in vitro fermentation of maize straw. The corresponding hydrolysis rate and the net SCFA yield increased as inoculum ratio (VSinoculum/VSsubstrate) increased from 0.09 to 0.79. The pH were maintained at 5.3, 5.8, 6.3, 6.8, 7.3, and 7.8, respectively. A neutral pH of approximately 6.8 was optimal for hydrolysis. The net SCFA yield decreased by 34.9% for a pH of less than 5.8, but remained constant at approximately 721±5mg/gvs for a pH between 5.8 and 7.8. In addition, results were obtained for variable and constant pH levels at initial substrate concentrations of 10, 30 and 50g/L. A variable pH increased the net SCFA yield by 23.6%, 29.0%, and 36.6% for concentrations of 10, 30 and 50g/L. Therefore, a variable pH enhanced SCFA accumulation in maize straw fermentation.

Phosphorus recycling potential assessment by a biological test applied to wastewater sludge

Phosphorus (P) recycling as mineral fertilizer from wastewater activated sludge (WAS) depends on the amount that can be dissolved and separated from the organic matter before the final crystallization step. The aim of the biological phosphorus dissolution potential (BPDP) test developed here was to assess the maximum amount of P that could be biologically released from WAS prior that the liquid phase enters the recovery process. It was first developed for sludge combining enhanced biological phosphorus removal and iron chloride. Because carbohydrates are known to induce acidification during the first stage of anaerobic digestion, sucrose was used as a co-substrate. Best results were obtained after 24-48 h, without inoculum, with a sugar/sludge ratio of 0.5 gCOD/gVS and under strict anaerobic conditions. Up to 75% of the total phosphorus in sludge from a wastewater treatment plant combining enhanced biological phosphorus removal and iron chloride phosphorus removal could be dissolved. Finally, the test was applied to assess BPDP from different sludge using alum compounds for P removal. No dissolution was observed when alum polychloride was used and less than 20% when alum sulphate was used. In all the cases, comparison to chemical acidification showed that the biological process was a major contributor to P dissolution. The possibility to crystallize struvite was discussed from the composition of the liquids obtained. The BPDP will be used not only to assess the potential for phosphorus recycling from sludge, but also to study the influence of the co-substrates available for anaerobic digestion of sludge.

A new activated primary tank developed for recovering carbon source and its application

A novel activated primary tank process (APT) was developed for recovering carbon source by fermentation and elutriation of primary sludge. The effects of solids retention time (SRT), elutriation intensity (G) and return sludge ratio (RSR) on this recovery were evaluated in a pilot scale reactor. Results indicated that SRT significantly influenced carbon source recovery, and mechanical elutriation could promote soluble COD (SCOD) and VFA yields. The optimal conditions of APT were SRT=5d, G=152s(-1) and RSR=10%, SCOD and VFA production were 57.0mg/L and 21.7mg/L. Particulate organic matter in sludge was converted into SCOD and VFAs as fermentative bacteria were significantly enriched in APT. Moreover, the APT process was applied in a wastewater treatment plant to solve the problem of insufficient carbon source. The outcomes demonstrated that influent SCOD of biological tank increased by 31.1%, which improved the efficiency of removing nitrogen and phosphorus.

Low pH anaerobic digestion of waste activated sludge for enhanced phosphorous release

This paper assesses anaerobic digestion of waste activated sludge (WAS) at low pH to enhance phosphorous solubility. Batch biochemical methane potential tests were conducted at a pH range of 5 to 7.2 in two separate sets (two different WAS samples collected from municipal WWTP). Low pH (<5.7) caused a significant (p = 0.004) decrease in methane potential (B0) up to 33% and 3.6 times increase in phosphorus release compared to neutral pH (7-7.7), but with no major change in methane production rate coefficient (khyd). The loss in methane yield was mainly due to decrease in hydrolytic capability rather than inhibition of methanogenesis with volatile fatty acids being <300 mgCOD L(-1) and soluble COD <1300 mgCOD L(-1) even at low pH. While pH did not influence the acetoclastic community (Methanosaeta dominated), it was the primary driver for the remaining community (p = 0.004), and caused a loss of diversity and shift to Clostridia.

Sludge minimization in municipal wastewater treatment by polyhydroxyalkanoate (PHA) production

An innovative approach has been recently proposed in order to link polyhydroxyalkanoates (PHA) production with sludge minimization in municipal wastewater treatment, where (1) a sequencing batch reactor (SBR) is used for the simultaneous municipal wastewater treatment and the selection/enrichment of biomass with storage ability and (2) the acidogenic fermentation of the primary sludge is used to produce a stream rich in volatile fatty acids (VFAs) as the carbon source for the following PHA accumulation stage. The reliability of the proposed process has been evaluated at lab scale by using substrate synthetic mixtures for both stages, simulating a low-strength municipal wastewater and the effluent from primary sludge fermentation, respectively. Six SBR runs were performed under the same operating conditions, each time starting from a new activated sludge inoculum. In every SBR run, despite the low VFA content (10% chemical oxygen demand, COD basis) of the substrate synthetic mixture, a stable feast-famine regime was established, ensuring the necessary selection/enrichment of the sludge and soluble COD removal to 89%. A good process reproducibility was observed, as also confirmed by denaturing gradient gel electrophoresis (DGGE) analysis of the microbial community, which showed that a high similarity after SBR steady-state had been reached. The main variation factors of the storage properties among different runs were uncontrolled changes of settling properties which in turn caused variations of both sludge retention time and specific organic loading rate. In the following accumulation batch tests, the selected/enriched consortium was able to accumulate PHA with good rate (63 mg CODPHA g CODXa(-1) h(-1)) and yield (0.23 CODPHA CODΔS(-1)) in spite that the feeding solution was different from the acclimation one. Even though the PHA production performance still requires optimization, the proposed process has a good potential especially if coupled to minimization of both primary sludge (by its use as the VFA source for the PHA accumulation, via previous fermentation) and excess secondary sludge (by its use as the biomass source for the PHA accumulation).

Recovery of volatile fatty acids from fermentation of sewage sludge in municipal wastewater treatment plants

This work investigated the pilot scale production of short chain fatty acids (SCFAs) from sewage sludge through alkaline fermentation and the subsequent membrane filtration. Furthermore, the impact of the fermentation liquid on nutrient bioremoval was examined. The addition of wollastonite in the fermenter to buffer the pH affected the composition of the carbon source produced during fermentation, resulting in higher COD/NH4-N and COD/PO4-P ratios in the liquid phase and higher content of propionic acid. The addition of wollastonite decreased the capillary suction time (CST) and the time to filter (TTF), resulting in favorable dewatering characteristics. The sludge dewatering characteristics and the separation process were adversely affected from the use of caustic soda. When wollastonite was added, the permeate flux increased by 32%, compared to the use of caustic soda. When fermentation liquid was added as carbon source for nutrient removal, higher removal rates were obtained compared to the use of acetic acid.

Mechanisms of nitrite addition for simultaneous sludge fermentation/nitrite removal (SFNR)

Simultaneous sludge fermentation and nitrite removal (SFNR) was investigated as a novel sludge/wastewater treatment process with high nitrogen concentrations. The results showed that introducing nitrite improved the primary sludge (PS) fermentation system by improving the chemical oxygen demand (COD) yields and the volatile suspend solid (VSS) reduction. At a nitrite dosage of 0.2 g g SS(-1), the COD production was 1.02 g g VSS(-1) and the VSS reduction was 63.4% within 7-day fermentation, while the COD production was only 0.17 g g VSS(-1) and the VSS reduction was only 4.9% in the blank test. Nitrite contained in wastewater was removed through denitrification process in the SFNR system. The solubility of carbohydrate and protein was substantially enhanced, and their contents reached the peak once nitrite was consumed. In addition, the nutrient release and methane generation were inhibited in the SFNR system, which alleviated the environmental pollution. Unlike traditional fermentation systems, neither alkaline condition nor high free nitrite acid (FNA) concentration affected the PS fermentation in the SFNR system. Molecular weight distribution (MWD) and Live/Dead cell analysis indicated that the sludge disruption by nitrite and the consumption of soluble organic substances in sludge might play important roles in SFNR.

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.

Hydrolysis, acidification and dewaterability of waste activated sludge under alkaline conditions: combined effects of NaOH and Ca(OH)2

Hydrolysis, acidification and dewaterability of waste activated sludge (WAS) were investigated at pH 10 controlled by the addition of NaOH, Ca(OH)2 or their mixtures at various ratios. Similar efficiency of WAS solublisation was observed in all cases, at 38-40%. High volatile fatty acid (VFA) production and good sludge dewaterability could not be achieved simultaneously by adding NaOH or Ca(OH)2 alone, but could be achieved by adding mixtures of NaOH and Ca(OH)2. VFA production in the case with the addition of Ca(OH)2 only (1201 mg(COD)/L) was lower than in the cases with the addition of NaOH or its mixtures with Ca(OH)2 (1813-1868 mg(COD)/L), and the lower VFA production with Ca(OH)2 addition alone could be related to the fact that a higher concentration of Ca(2+) was released into the fermentation liquid, which likely inhibited the hydrolysis process of protein. Furthermore, adding mixtures of NaOH and Ca(OH)2 was more economical for VFA production.

Mosquito biolarvicide production by sequential fermentation with dual strains of Bacillus thuringiensis subsp. israelensis and Bacillus sphaericus using sewage sludge

This study demonstrated the bioconversion of sewage sludge into a composite biolarvicide for mosquito control based on sequential fermentation with dual strains of Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus (Bs). Results showed that sewage sludge was a suitable fermentation substrate for supporting growth, sporulation and mosquitocidal proteins synthesis by Bti and Bs. Through sequential fermentation with dual strains, a 10-L bench-scale fermentor was capable of producing Bti and Bs at a cell concentration of 2.1×10(9) and 6.8×10(8) CFU/mL, respectively. Such sequential fermentation can save half of raw materials and energy consumption comparing with the sludge fermentation with single strain. The toxic activity and persistence of the composite biolarvicide against mosquito larvae in the polluted waters were enhanced by the increased toxin complexity and synergistic interactions. This study, for the first time, validates the technical feasibility of using sewage sludge to produce a cost-effective composite biolarvicide based on Bti and Bs.