Effects of alkali types on waste activated sludge (WAS) fermentation and microbial communities

EDTA-enhanced alkaline anaerobic fermentation of landfill leachate-derived waste activated sludge for short-chain fatty acids production: Metals chelation and EPSs destruction

Alkaline anaerobic fermentation (AAF) of waste activated sludge (WAS) has been demonstrated to be promising for short-chain fatty acids (SCFAs) recovery. However, high-strength metals and EPSs in the landfill leachate-derived WAS (LL-WAS) would stabilize its structure, suppressing AAF performance. To improve sludge solubilization and SCFAs production, AAF was coupled with EDTA addition for LL-WAS treatment. The results show that sludge solubilization at AAF-EDTA was promoted by 62.8% than AAF, releasing 21.8% more soluble COD. The maximal SCFAs production of 477.4 mg COD/g VSS was thus achieved, i.e., 1.21 and 6.13 times those at AAF and the control, respectively. SCFAs composition was also improved with more acetic and propionic acids (80.8% versus 64.3%). Metals bridging EPSs were chelated by EDTA, which significantly dissolved metals from sludge matrix (e.g., 23.28 times higher soluble Ca than AAF). EPSs tightly bound with microbial cells were thus destructed (e.g., 4.72 times more protein release than alkaline treatment), causing an easier sludge disruption and subsequently a higher SCFAs production by hydroxide ions. These findings suggest an effective EDTA-supported AAF for metals and EPSs-rich WAS to recover carbon source.

Application and improvement methods of sludge alkaline fermentation liquid as a carbon source for biological nutrient removal: A review

Alkaline fermentation can reduce the amount of waste activated sludge and prepare sludge alkaline fermentation liquid (SAFL) rich in short-chain fatty acids (SCFAs), which can be used as a high-quality carbon source for the biological nutrient removal (BNR) process. This review compiles the production method of SAFL and the progress of its application as a BNR carbon source. Compared with traditional carbon sources, SAFL has the advantages of higher efficiency and economy, and different operating conditions can influence the yield and structure of SCFAs in SAFL. SAFL can significantly improve the nutrient removal efficiency of the BNR process. Taking SAFL as the internal carbon source of BNR can simultaneously solve the problem of carbon source shortage and sludge treatment difficulties in wastewater treatment plants, and further reduce the operating cost. However, the alkaline fermentation process results in many refractory organics, ammonia and phosphate in SAFL, which reduces the availability of SAFL as a carbon source. Purifying SCFAs by removing nitrogen and phosphorus, directly extracting SCFAs, or increasing the amount of SCFAs in SAFL by co-fermentation or combining with other pretreatment methods, etc., are effective measures to improve the availability of SAFL.

Sewage sludge acidogenic fermentation for organic resource recovery towards carbon neutrality: An experimental survey testing the headspace influence

Volatile fatty acids (VFAs) produced by acidogenic digestion of sewage sludge are very interesting bio-products which can contribute to carbon neutrality of wastewater treatment plants. Studies on the production of VFAs from sewage sludge from fermenters with membrane are limited. In view of above, VFAs from a fermenter pilot plant equipped with a membrane bioreactor and fed with real sewage sludge has been monitored. The effect of headspace volume (HdV) on VFA production was studied for the first time to elucidate the optimal operation conditions. Specifically, three fermenter HdV values (namely, 20, 40 and 60% of the total volume) have been investigated. Results revealed that the HdV of 20% ensured the highest sCOD production (900 mgCOD/L) and VFA/COD ratio (45.4%). High value of HdV (namely, 40 and 60%) strongly decreased the acidogenic fermentation performance in terms of VFA production.

Response of anaerobic digestion of waste activated sludge to types of alkalis: Contribution identification of metal ions

Alkaline pretreatment is one promising strategy for promoting anaerobic digestion of waste activated sludge (WAS). This study selected three types of alkalis with monovalent (NaOH and KOH), divalent (Ca(OH)₂ and Mg(OH)₂), and trivalent (Fe(OH)₃ and Al(OH)₃) cations to reveal the roles of metal ions on short chain fatty acids (SCFAs) production. The enhanced production potentials of SCFAs were reduced by order of alkalis with monovalent, divalent, and trivalent cations. Na⁺, K⁺, Ca²⁺, and Mg²⁺ did no contributions on SCFAs production, while Fe³⁺ and Al³⁺ performed better than control, especially the latter. The mechanism analysis proved that Na⁺, K⁺, Ca²⁺, and Mg²⁺ did no significant effects on solubilization, hydrolysis, acidification and methanogenesis stages, while the first three stages were improved by Fe³⁺ and Al³⁺ and the methanogenesis stage was inhibited. The findings may provide some new insights when using alkalis or residual metal ions to improve anaerobic digestion of WAS.

A review of enhanced municipal wastewater treatment through energy savings and carbon recovery to reduce discharge and CO2 footprint

Municipal wastewater treatment that mainly performed by conventional activated sludge (CAS) process faces the challenge of intensive aeration-associated energy consumption for oxidation of organics and ammonium, contributing to significant directly/indirectly greenhouse gas (GHG) emissions from energy use, which hinders the achievement of carbon neutral, the top priority mission in the coming decades to cope with the global climate change. Therefore, this article aimed to offer a comprehensive analysis of recently developed biological treatment processes with the focus on reducing discharge and CO2 footprint. The biotechnologies including "Zero Carbon", "Low Carbon", "Carbon Capture and Utilization" are discussed, it suggested that, by integrating these processes with energy-saving and carbon recovery, the challenges faced in current wastewater treatment plants can be overcome, and a carbon-neutral even be possible. Future research should investigate the integration of these methods and improve anammox contribution as well as minimize organics lost under different scales.

Removal and recovery of heavy metals from sewage sludge via three-stage integrated process

Heavy metal contamination of sewage sludge is one of the concerns preventing its land application. Traditional processes applied for stabilization of sewage sludge are still inadequate to serve sustainable solutions to heavy metal problem. In this study, fermentation and bioleaching potentials of sewage sludge were investigated in anaerobic reactors for either non-pretreated or ultrasonicated sludge at three different pH regimes (free of pH regulation, acidic, and alkaline). The results of the study revealed that combination of ultrasonication pretreatment and alkaline fermentation performed the best among the other cases, resulting in 33.7% hydrolysis, 10.5% acidification, 11-33% metal leaching, and up to 25% reduction in bioavailability of potentially toxic heavy metals. Bioleaching effluent obtained from the best performing reactor was subjected to membrane-based metal recovery. A supported liquid membrane impregnated with a basic carrier successfully recovered soluble metals from the bioleaching effluent with an efficiency of 39-68%. This study reveals that the proposed three-stage process, ultrasonication pretreatment-alkaline fermentation-supported liquid membrane, effectively produces stable sludge with reduced heavy metal toxicity and recovers metals from organic waste streams.

Synergistic pretreatment of CaO and freezing/thawing to enhance volatile fatty acids recycling and dewaterability of waste activated sludge via anaerobic fermentation

To avoid the generally deteriorated dewaterability of sludge in waste activated sludge (WAS) anaerobic acidogenesis, the combination of varied calcium oxide (CaO) dosage (i.e., 0.01-0.07 g/g TS) and freezing/thawing pretreatment (5 F/T cycles) was investigated for concurrently improving the production of volatile fatty acids (VFAs) and dewatering performance of sludge. The highest release of soluble chemical oxygen demand (SCOD) (1836 ± 96 mg/L) and accumulation of VFAs (448.0 mg COD/g VS) were reached through the co-pretreatment of CaO (0.07 g/g TS) and F/T (50 h at -24 °C) (i.e., 0.07 CaO-F/T). Meanwhile, optimal dewaterability of sludge was also achieved in 0.07 CaO-F/T co-pretreated WAS fermentation, which was reflected by large particle size (98.32 μm), low capillary suction time (41.6 s), decreased specific resistance to filtration (SRF, reduced 47.5% against blank) and high zeta potential (-9.59 mV). Co-pretreatment of CaO and F/T reduced the species number of total microbial population, but improved the abundance of acid-producing bacteria. Increased abundance of Bacteroides, Macellibacteroides, Petrimonas, Prevotella, Clostridium, and Megasphaera was positively relevant to the high yields of VFAs. The economic analysis indicated that CaO-F/T was economically acceptable with considerable estimated net profits, which provided a feasible and efficient alternative for further WAS treatment at large scale.

Performance and stability of biogas recirculation-driven anaerobic digestion system coupling with alkali addition strategy for sewage sludge treatment

Wastewater treatment plants are particularly challenging with the treatment and disposal of sewage sludge produced from the treatment units due to its high costs and environmental hazards. In this study, a biogas recirculation-driven anaerobic digestion (AD) system was developed with upward shear force being provided by biogas recirculation coupled with the alkali addition strategy, targeting biogas upgrading, sludge stabilization, and sludge flocculation simultaneously, thus reducing the sludge management costs. Compared to the conventional AD system, the novel biogas recirculation-driven AD system could achieve biogas upgrading with 10% higher CH₄ content. Besides, the combination of NaOH and Ca(OH)₂ addition strategy obviously improved sludge settleability and dewaterability compared to the single NaOH addition strategy. Owing to the attraction between negatively charged sludge particles and Ca²⁺ ions, the available Ca²⁺ in the former AD system may facilitate the re-flocculation and P immobilization in solid digestate, fix partial CO₂ with less CO₂ emission, and bridge with some sludge flocs. Moreover, 12.6% lower net cost for sludge management was achieved by this biogas recirculation-driven AD system together with the combination alkali addition strategy, which is regarded as a promising integrated multi-purpose system for sludge treatment.

Extent of bioleaching and bioavailability reduction of potentially toxic heavy metals from sewage sludge through pH-controlled fermentation

Utilization of anaerobically stabilized sewage sludge on arable lands serve as a renewable alternative to chemical fertilizers as it enables recycling of valuable nutrients to food chain. However, probable presence of heavy metals in sewage sludge restricts the use of stabilized sludge on lands. In this study, a novel approach based on pH-controlled fermentation and anaerobic metal bioleaching was developed to reduce ecotoxicity potential of fermented sludge prior to its land application. Sewage sludge was subjected to pH-controlled fermentation process at acidic, neutral, and alkaline pH levels with the aim of increasing metal solubilization and decreasing bioavailable metal fractions through anaerobic bioleaching. Alkaline reactor performed the best among all reactors and resulted in 3-fold higher hydrolysis (34%) and 6-fold higher acidification (19%) efficiencies along with 43-fold (in average) higher metal solubilization than that of neutral pH reactor. As a result of alkaline fermentation, 32-57% of the metals remained as bioavailable and 34-59% of the metals were encapsulated as non-bioavailable within solid fraction of fermented sludge (biosolid), whereas 8-12% of total metal was solubilized into fermentation liquor. Our results reveal that anaerobic bioleaching through alkaline fermentation enables biosolid production with less metal content and low bioavailability, facilitating its utilization for agricultural purposes.

Migration and transformation mechanism of phosphorus in waste activated sludge during anaerobic fermentation and hydrothermal conversion

This study investigated migration and transformation mechanism of P in waste activated sludge (WAS) during anaerobic fermentation (AF) process and the subsequent hydrothermal conversion (HTC) process. Control of pH during the AF processes was found to be significant, whereby the use of acidic (pH = 5.5) or alkaline conditions (pH = 9.5) facilitated the release of either apatite phosphorus (AP) or non-apatite inorganic phosphorus (NAIP) and organic phosphorus, respectively. At the same pH of 9.5, NaOH promoted the transfer of P into liquid phase, and P in the solid phase was mainly in the form of NAIP. In contrast, Ca(OH)₂ enhanced the incorporation of P into the solid products, with the P mainly in the form of AP. The subsequent HTC process promoted the NAIP transferred to AP, and the bioavailability of P in the HTC solid products was decreased. The P K-edge X-ray absorption near edge structure analysis provided detailed information about the phosphates. It demonstrated that the conversion of Ca₈H₂PO₄·6.5H₂O to Ca₅(PO₄)₃·OH was facilitated by HTC under the alkaline condition. This study sheds lights on transformation mechanism of P speciations during AF and HTC processes, which would provide fundamental information for effective utilization of P in bio-wastes.

Effect of ozone pretreatment on characteristics of dissolved organic matter formed in aerobic and anaerobic digestion of waste-activated sludge

The characteristics of dissolved organic matter (DOM) formed in aerobic and anaerobic digestion of waste-activated sludge (WAS) after ozone pretreatment were investigated with three ozone dosages (4.72, 10.96, and 13.8 mg O₃/min) and four ozonation times (0, 10, 20, and 30 min) using six aerobic and six anaerobic digestion reactors. High decreasing rate of volatile suspended solid/total suspended solid indicated enhanced destruction of volatile solids and efficient sludge reduction. The results of TOC and UV absorbance indicated that increasing ozone dosage and time significantly enhanced hydrolysis and degradation of DOM. Data analysis with a first-order sequential reaction model revealed that, for aerobic digestion, k_h increased in the range of 0.00049-0.00154 day^-1; and for anaerobic digestion of WAS, k_h increased in the range of 0.00302-0.00796 day^-1 and k_d increased in the range of 0.24910-0.54548 day^-1. Detailed analysis of the composition of DOM showed that ozone pretreatment increased irreversible membrane resistance (IMR) and enhanced the breakdown of large DOM molecules, the formation of aromatic proteins, and tryptophan- and tyrosine-aromatic amino acids, as well as the accumulation of humic acid- and fulvic acid-like substances.

Insights into microbial interaction profiles contributing to volatile fatty acids production via acidogenic fermentation of waste activated sludge assisted by calcium oxide pretreatment

This first-attempted study illustrated the calcium oxide (CaO) agentia-pretreatment for prompting waste activated sludge (WAS) solubilization and enhancing volatile fatty acids (VFAs) bio-production through acidogenic fermentation. The 15-h CaO pretreatment was capable to produce a soluble chemical oxygen demand (SCOD) yield of ca. 153.17 mg COD/g VS and VFAs generation efficiency of 327.8 mg COD/g VS with adding dosage of 0.07 g/g TS. The relative frequencies corresponded to metabolic functions profiling were promoted obviously by CaO pretreatment and contributed to biosolid decomposition/VFAs production in sludge fermentation. Main genera of Azonexus, Arcobacter, Acinetobacter, Thauera, Petrimonas, Clostrium and Macellibacteroides cooperated synergically toward triggering concurrent VFAs generation/biosolid biodegradation. Finally, the CaO-pretreatment displayed positive merits in terms of sludge biosolid decomposition/recoverable resource harvest as compared with other alkali pretreatments. This study might shed lights on enriching intensification strategy for WAS management toward high-efficiency of recoverable resource harvest with lower cost.

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.

Enhancing nitrogen removal from wastewater in sequencing batch reactors (SBRs) using additional carbon source produced from food waste acidogenic fermentation at different temperatures

Fermentation slurry from food waste (FSFW) produced at different temperatures (20, 37, and 55 °C) was utilized as external carbon source for promoting nitrogen removal in this study. It was found that high temperatures improved the hydrolysis rate by promoting the hydrolytic enzyme activity. Mesophilic temperature (37 °C) was favorable for organic acid (especially lactic acid) production by selectively enriching the Lactobacillus (with a relative abundance of 90.6%), while thermophilic temperature (55 °C) would restrict the acidogenesis rate (18.9%) and result in the accumulation of carbohydrate in the fermented slurry. Organic acids in the FSFW act as easily biodegradable carbon sources, but the macromolecular and particulate organic components can be utilized as slowly biodegradable carbon sources in the denitrification processes. Using the FSFW as carbon sources to enhance nitrogen removal from wastewater in sequencing batch reactors (SBRs) for more than 150 days, the FSFW produced at thermophilic temperature could significantly promote the microbial metabolic capacity of the activated sludge and improve the nitrogen and phosphate removal efficiencies.

Enhanced short-chain fatty acids production from waste activated sludge with alkaline followed by potassium ferrate treatment

This study reported an efficient approach, i.e., alkaline followed by potassium ferrate (PF) pretreatment, to enhance short chain fatty acids (SCFAs) production from waste activated sludge anaerobic fermentation process. The optimum condition was initial pH of 10.0 and PF dosage of 28 mg Fe(VI)/g total suspended solid, with the highest SCFAs production of 382 mg chemical oxygen demand/g volatile suspended solid, which was 2.03 and 2.06 times higher than that of corresponding sole treatments. It was found that the alkaline + PF treatment could provide more soluble substrates for subsequent acidification process by accelerating disruption of both microbial cells and extracellular polymeric substances. And the alkaline + PF treatment also benefited to the activity promotion of specific hydrolases and inhibition of methanogens. Besides, the abundances of microorganisms related to SCFAs production, such as Proteiniclasticum and Macellibacteroides, were increased greatly, whereas the main SCFAs consumer, Proteobacteria, was decreased from 29.1% to 14.4%.

Effect of basic oxygen furnace slag addition on enhanced alkaline sludge fermentation and simultaneous phosphate removal

This study presents a promising approach that enhances the sludge fermentation by using basic oxygen furnace (BOF) slag as an alkaline source for the first time. BOF slag added to the reactors could maintain a stable alkaline condition due to continuous release of Ca(OH)₂ from slag. The reactor pH could be adjusted to a target value by the choice of the BOF slag dose. Concentrations of soluble chemical oxygen demand (sCOD) and short-chain carboxylates (SCCs) were substantially increased in the presence of BOF slag. At a BOF slag mass to sludge volume ratio of 1/10 g slag/L sludge, the reactor pH was maintained at 10 and the concentration of SCCs produced was the highest (i.e., 3510 mg COD L^-1 from 14,000 mg VS L^-1 of sludge mixture), followed by B/S ratios of 1/20, 1.50, 1/5, and 1/2.5 g slag L^-1 sludge with reactor pH of 9.4, 8.9, 10.5, and 11, respectively. Our data suggest that the pH value that best facilitates the degradation of sludge into SCCs and inhibit the conversion of SCCs into biogas is around 10. Interestingly, compositions of the accumulated SCCs varied greatly depending on the BOF slag dose. BOF slag showed phosphorus removal ability due to enhanced precipitation of Ca-PO₄^3--P complexes, which significantly lowered PO₄^3- concentration of the reactor effluent.

Alkaline fermentation of waste activated sludge with calcium hydroxide to improve short-chain fatty acids production and extraction efficiency via layered double hydroxides

Ca(OH)₂ addition was proposed to improve short chain fatty acids (SCFAs) production via alkaline fermentation of waste activated sludge (WAS), with CO₃^2- and PO₄^3- being removed simultaneously, so that the SCFAs extraction efficiency could be improved. Alkaline fermentation of WAS with Ca(OH)₂ and NaOH addition were performed respectively, and the efficiencies of SCFAs extraction from the fermentation liquids via in situ layered double hydroxides (LDHs) were investigated. The results showed that, the SCFAs production was much more improved with NaOH addition than that with Ca(OH)₂ addition. However, the SCFAs contents in the synthesized SCFAs-LDH were 6.7 ± 0.4 mg COD/L (control), 12.4 ± 0.5 mg COD/L (with NaOH addition) and 17.4 ± 0.4 mg COD/L (with Ca(OH)₂ addition), respectively. This means that Ca(OH)₂ addition is an effective way to improve SCFAs extraction from fermentation liquid of WAS.

An integrated process for struvite electrochemical precipitation and ammonia oxidation of sludge alkaline hydrolysis supernatant

This study reported two-phase electrochemical processes, including struvite electrochemical precipitation and ammonia electrooxidation, for the treatment of supernatant from the hydrolysis sludge. The results showed that in phase I, the removal efficiencies of 92.3% PO₄^3--P and 50.1% NH₄⁺-N could be achieved in electrochemical precipitation with magnesium sacrificial anode at pH 9.0 and 40 mA after 120-min electrolysis, and slightly increased to 95.1% and 57.3%, respectively, when current further increased to 120 mA, while the energy consumption (ECS, from 0.6 to 6.7 kWh m^-3) and specific energy consumption [SECS, from 2.7 to 29.9 Wh g (PO₄^3--P)^-1] sharply increased. In phase II, the residual NH₄⁺-N is further indirectly electrooxidized to nitrogen with modified Ti anode (Ti/SnO₂-Sb-Pd). With the generation of active chloride, about 83.2% NH₄⁺-N was removed with the molar ratio of Cl/N 5:1 at 50 mA after 120-min treatment, and slightly increased to 92.2%, when current increased to 125 mA, while SECS significantly increased [from 0.027 to 0.117 kWh g (NH₄⁺-N)^-1]. The results indicated that current were the crucial factors; meanwhile, lower current and longer reaction time may be the optimal options in electrochemical process with higher efficiency and lower energy consumption. Finally, the integrated process was conducted at the optimal conditions (pH = 9.0, I = 40 mA in phase I; Cl/N = 5, I = 50 mA in phase II) with the supernatant of the alkaline hydrolysis sludge. Removal of ammonia nitrogen (79.3%) and removal of phosphorus (94.3%) were achieved, confirming the feasibility of practical application for the simultaneous phosphorus recovery and ammonia removal.

Efficient production of short-chain fatty acids from anaerobic fermentation of liquor wastewater and waste activated sludge by breaking the restrictions of low bioavailable substrates and microbial activity

An efficient approach of bioconverting the organic wastes in liquor wastewater (LW) and waste activated sludge (WAS) to valuable short-chain fatty acids (SCFAs) via anaerobic fermentation was explored. The maximal SCFAs concentration was 5400 mg COD/L with approximate 80.0% acetic and propionic acids under optimized conditions (LW/WAS ratio 1:1, pH 8 and fermentation 4 d). Mechanisms investigation found that the fermentation of LW/WAS made up the drawbacks of sole WAS fermentation by improving the bioavailable substrates and low C/N ratio to stimulate the microbial activities. The bioconversion efficiency of substrates for SCFAs generation was therefore enhanced. The humic acids present in LW could also play positive roles in SCFAs promotion. Moreover, the performance of LW/WAS fermentation was highly correlated with appropriate fermentation pH. The fermentative bacteria responsible for SCFAs production were highly enriched and the activities of key hydrolases, acid-forming enzymes and ATP concentration were greatly improved at pH 8.

Deep exploitation of refractory organics in anaerobic dynamic membrane bioreactor for volatile fatty acids production from sludge fermentation: Performance and effect of protease catalysis

Volatile fatty acids (VFAs) production from waste activated sludge fermentation could be improved in anaerobic dynamic membrane bioreactor (ADMBR) by retaining residual organics within the reactor and prolonging their reaction time. However, the accumulation of refractory organics made it operate unstably. Therefore, protease catalysis was adopted to deeply exploit those refractory organics in sludge. By combining with dynamic membrane retention, protease catalysis indeed presented a good performance. VFAs yield was further enhanced by over 40% in ADMBR. Membrane fouling was slightly relieved due to protein and polysaccharide degradations in the sludge of dynamic membrane. It was also interestingly found that not only protease activity of sludge was improved from 5 to 21 U/ml, but also β-GLC activity was enhanced from 13 to 20 μmoL/L/h. Microbial community analysis showed protease addition could reduce bacterial richness and evenness in sludge, and accelerate the growth of polysaccharides-hydrolyzing bacteria, as well as inhibit some proteolytic bacteria.