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

A review of iron use and recycling in municipal wastewater treatment plants and a novel applicable integrated process

Chemical methods are expected to play an increasingly important role in carbon-neutral municipal wastewater treatment plants. This paper briefly summarises the enhancement effects of using iron salts in wastewater and sludge treatment processes. The costs and environmental concerns associated with the widespread use of iron salts have also been highlighted. Fortunately, the iron recovery from iron-rich sludge provides an opportunity to solve these problems. Existing iron recovery methods, including direct acidification and thermal treatment, are summarised and show that acidification treatment of FeS digestate from the anaerobic digestion-sulfate reduction process can increase the iron and sulphur recycling efficiency. Therefore, a novel applicable integrated process based on iron use and recycling is proposed, and it reduces the iron salts dosage to 4.2 mg/L and sludge amount by 80%. Current experimental research and economic analysis of iron recycling show that this process has broad application prospects in resource recovery and sludge reduction.

Acute impact of salinity and C/N ratio on the formation and properties of soluble microbial products from activated sludge

The present study investigated the shock of NaCl and C/N ratio on properties of soluble microbial products (SMPs), focusing on their sized fractions. The results indicated that the NaCl stress increased the content of biopolymers, humic substances, building blocks, and LMW substances in SMPs, while the addition of 40 g NaCl L^-1 significantly changed their relative abundance in SMPs. The acute impact of both N-rich and N-deficient conditions accelerated the secretion of SMPs, but the characteristics of LMW substances differed. Meanwhile, the bio-utilization of SMPs has been enhanced with the increase of NaCl dosage but decreased with the increase of the C/N ratio. The mass balance of sized fractions in SMPs + EPS could be set up when NaCl dosage <10 g/L and C/N ratio >5, which indicates the hydrolysis of sized fractions in EPS mainly compensated for their increase/reduction in SMPs. Besides, the results of the toxic assessment indicated that the oxidative damage caused by the NaCl shock was an important factor affecting the property of SMPs, and the abnormal expression of DNA transcription cannot be neglected for bacteria metabolisms with the change of C/N ratio.

Conductive materials enhance microbial salt-tolerance in anaerobic digestion of food waste: Microbial response and metagenomics analysis

Previous studies have shown that high salinity environments can inhibit anaerobic digestion (AD) of food waste (FW). Finding ways to alleviate salt inhibition is important for the disposal of the growing amount of FW. We selected three common conductive materials (powdered activated carbon, magnetite, and graphite) to understand their performance and individual mechanisms that relieve salinity inhibition. Digester performances and related enzyme parameters were compared. Our data revealed that under normal and low salinity stress conditions, the anaerobic digester ran steady without significant inhibitions. Further, the presence of conductive materials promoted conversion rate of methanogenesis. This promotion effect was highest from magnetite > powdered activated carbon (PAC) > graphite. At 1.5% salinity, PAC and magnetite are beneficial in maintaining high methane production efficiency while control and the graphite added digester acidified and failed rapidly. Additionally, metagenomics and binning were used to analyze the metabolic capacity of the microorganisms. Some species enriched by PAC and magnetite possessed higher cation transport capacities and were to accumulate compatible solutes. PAC and magnetite promoted direct interspecies electron transference (DIET) and syntrophic oxidation of butyrate and propionate. Also, the microorganisms had more energy available to cope with salt inhibition in the PAC and magnetite added digesters. Our data imply that the promotion of Na+/H+ antiporter, K+ uptake, and osmoprotectant synthesis or transport by conductive materials may be crucial for their proliferation in highly stressful environments. These findings will help to understand the mechanisms of alleviate salt inhibition by conductive materials and help to recover methane from high-salinity FW.

Potential of anaerobic co-fermentation in wastewater treatments plants: A review

Fermentation (not anaerobic digestion) is an emerging biotechnology to transform waste into easily assimilable organic compounds such as volatile fatty acids, lactic acid and alcohols. Co-fermentation, the simultaneous fermentation of two or more waste, is an opportunity for wastewater treatment plants (WWTPs) to increase the yields of sludge mono-fermentation. Most publications have studied waste activated sludge co-fermentation with food waste or agri-industrial waste. Mixing ratio, pH and temperature are the most studied variables. The highest fermentation yields have been generally achieved in mixtures dominated by the most biodegradable substrate at circumneutral pH and mesophilic conditions. Nonetheless, most experiments have been performed in batch assays which results are driven by the capabilities of the starting microbial community and do not allow evaluating the microbial acclimation that occurs under continuous conditions. Temperature, pH, hydraulic retention time and organic load are variables that can be controlled to optimise the performance of continuous co-fermenters (i.e., favour waste hydrolysis and fermentation and limit the proliferation of methanogens). This review also discusses the integration of co-fermentation with other biotechnologies in WWTPs. Overall, this review presents a comprehensive and critical review of the achievements on co-fermentation research and lays the foundation for future research.

Volatile Fatty Acids (VFA) Production from Wastewaters with High Salinity—Influence of pH, Salinity and Reactor Configuration

The hydrocarbon-based economy is moving at a large pace to a decarbonized sustainable bioeconomy based on biorefining all types of secondary carbohydrate-based raw materials. In this work, 50 g L−1 in COD of a mixture of food waste, brine and wastewater derived from a biodiesel production facility were used to produce organic acids, important building-blocks for a biobased industry. High salinity (12–18 g L−1), different reactors configuration operated in batch mode, and different initial pH were tested. In experiment I, a batch stirred reactor (BSR) at atmospheric pressure and a granular sludge bed column (GSBC) were tested with an initial pH of 5. In the end of the experiment, the acidification yield (ηa) was similar in both reactors (22–24%, w/w); nevertheless, lactic acid was in lower concentrations in BSR (6.3 g L−1 in COD), when compared to GSBC (8.0 g L−1 in COD), and valeric was the dominant acid, reaching 17.3% (w/w) in the BSR. In experiment II, the BSR and a pressurized batch stirred reactor (PBSR, operated at 6 bar) were tested with initial pH 7. The ηa and the VFA concentration were higher in the BSR (46%, 22.8 g L−1 in COD) than in the PBSR (41%, 20.3 g/L in COD), and longer chain acids were more predominant in BSR (24.4% butyric, 6.7% valeric, and 6.2% caproic acids) than in PBSR (23.2%, 6.2%, and 4.2%, respectively). The results show that initial pH of 7 allows achieving higher ηa, and the BSR presents the most suitable reactor among tested configurations to produce VFA from wastes/wastewaters with high salinity.

Thermophilic bacteria combined with alkyl polyglucose pretreated mariculture solid wastes using as denitrification carbon source for marine recirculating aquaculture wastewater treatment

In marine recirculating aquaculture systems (RAS), efficient nitrogen removal is challenging due to the high NO₃^--N concentration, low organic matters content, and high salinity. In this study, mariculture solid wastes (MSW) acidogenic liquid pretreated by thermophilic bacteria (TB) combined with alkyl polyglucose (APG) was first used as carbon source for denitrification to remove NO₃^--N. TB + APG pretreatment could accelerate the hydrolysis of MSW, and the highest volatile fatty acids (VFAs) yield (40.3%) was obtained with TB + 0.2 g/g VSS APG pretreatment. MSW acidogenic liquid pretreated by TB + 0.2 g/g VSS APG was a reliable carbon source for denitrification, and the optimum COD/NO₃^--N ratio (C/N) was 8 with no residue of NO_x^--N. VFAs were more effectively utilized by denitrifiers than carbohydrate and protein. The high denitrification potential (P_DN) and denitrification rate (V_DN) indicated the higher denitrification ability at C/N of 8 using MSW acidogenic liquid as carbon source. The outcomes of this work could provide useful information for promoting technological innovation in marine RAS wastewater treatment.

Strategies for the valorisation of a protein-rich saline waste stream into polyhydroxyalkanoates (PHA)

Saline Mussels Cooking Wastewater was valorised to produce PHA with Mixed Microbial Cultures (MMC). Due to the high protein content (1.8-5.7 g COD_PROT/L), PHA accumulating capacity was below 10%, so several strategies were tested. In the acidification unit, Na(HCO₃) was added, increasing protein conversion into Volatile Fatty Acids (VFA) from 10.3% to 69.2% and subsequent PHA accumulation from 6.9 to 14.7%. In the enrichment unit, the incorporation of a settling stage after the feast phase provoked a shift in the proteins' oxidation from the feast to the famine phase, where the nitrogen released in the famine is used by the MMC for growth. This increased the biomass concentration and the tolerated COD (from 1.6 to 4.2 g VSS/L and from 2.2 to 4.38 g COD/L). Finally, varying the proteins/VFA ratio for MMC acclimation to proteins allowed increasing PHA accumulation from 8.8 to 41.5%.

Effect of sodium dichloroisocyanurate treatment on enhancing the biodegradability of waste-activated sludge anaerobic fermentation

In the present study, a novel oxidant (sodium dichloroisocyanurate, NaCl₂(NCO)₃; SDIC) combined with microorganisms was employed to achieve a higher performance of waste-activated sludge (WAS) anaerobic fermentation. Four concentrations of SDIC (0, 0.3, 0.6, and 1.0 mg SDIC/mg SS) were studied in WAS fermentation systems. The results showed that the release of proteins and polysaccharides was enhanced by the addition of SDIC with values of 1002.25 mg COD/L and 680.25 mg COD/L, respectively, and these values increased 14.46-18.07 times (proteins) and 3.74-7.40 times (polysaccharides) compared with that of the blank test. Additionally, the short-chain fatty acids also increased 2.24 times. The rate of extraction of organic substances from the sludge increased from 3.03% to 33.33%. Furthermore, the fermented sludge with the SDIC treatment had higher hydrolytic acidification efficiencies for bovine serum albumin and glucose, increasing from 4.558% to 9.91% and 2.976%-6.764%, respectively. However, SDIC treatment of the conventional fermented sludge resulted in lower hydrolytic acidification efficiencies with values of 4.978%-1.781% and 3.334%-0.582%, respectively. Biological enzyme analysis also showed that SDIC enhanced α-glucosidase and protease activity but inhibited dehydrogenase, alkaline phosphatase, and acid phosphatase activity. Proteobacteria and Comamonas were the main microbial communities observed in the WAS anaerobic fermentation.

Effect of increasing salinity and low C/N ratio on the performance and microbial community of a sequencing batch reactor

The purpose of this study was to investigate the effects of increasing salinity on the performance and microbial community structure in a sequencing batch reactor (SBR) treating low C/N ratio wastewater. The SBR was subjected to a gradual increased salinity from 0 wt% to3.0 wt% under low Chemical Oxygen Demand (COD)/N ratio, operating for 80 days. The study results indicated that high salinity decreased the removal efficiency of ammonium (NH4+-N) from 77.09% (1.0 wt%) to 45.7% (3.0wt%). The organic matter removal are not significantly affected by the high salinity. Non-metric Multi-Dimensional Scaling (NMDS) analysis showed that the gradual increased salinity altered the overall bacterial community structure, and low salinity (1wt%) promoted the bacterial diversity, while high salinity (2 and 3 wt%) significantly decreased the bacterial diversity in low C/N ratio activated sludge system. Further analysis revealed that two genera related to nitrification process (unclassified-Nitrosomonadales and g-Nitrospira) were inhibited, while a genus related to organic removal (Piscicoccus) and three genera related to denitrification (Rodobacteraceae, Denitromonas and Hyphomicrobium) increased significantly at a salinity of 3 wt%. This study provides insights of shifts in the bacteria community under the stress of high salinity in low C/N ratio of activated sludge systems.

Enhanced anaerobic fermentation of waste activated sludge by reverse osmosis brine and composition distribution in fermentative liquid

This study reported a "treating waste by waste" strategy to dispose waste activated sludge (WAS), i.e. reverse osmosis (RO) brine-enhanced anaerobic fermentation. RO brine was hazardous by-product from seawater desalination process, which contains numerous Na⁺. After 4-day RO brine-enhanced anaerobic fermentation at Na⁺ concentration of 0.33 mol/L, 5.0 g/L VSS reduction (37.9% of VSS) was achievable, leading to considerable soluble chemical oxygen demand (SCOD) release of 349.6 mg/g VSS. Acetic acid was predominant component in SCOD (31.1%), followed by propionic, butyric, valeric acids and proteins (14.0-17.6%). Sludge solubilization and SCOD composition in the enhanced anaerobic fermentation with RO brine and NaCl agent were similar, whereas less nutrient release and extracellular polymeric substance (EPS) disruption were achieved by RO brine, attributing to the Ca²⁺&Mg²⁺-caused skeleton strengthening on EPS matrix. Such RO brine-based strategy provided environmental and economic benefits, e.g. none chemical consumption, synchronous disposal of WAS and RO brine.

Effect of pH on volatile fatty acid production from anaerobic digestion of potato peel waste

In this study, potato peel waste was used as feedstock to produce volatile fatty acids (VFAs) by anaerobic digestion. The effects of different pH levels (pH 5.0, pH 7.0, pH 11.0, and uncontrolled pH) on VFA concentration and composition, intermediate products, and metabolic state were evaluated. The results showed that the highest total VFA production was achieved with pH 7.0 (41.9 g COD/L and 632.2 mg COD/g VS_fed), followed by that with uncontrolled pH. Butyric acid was the dominant product under acidic pH, whereas acetic acid dominated under alkaline pH. The type of acidogenic fermentation at pH 7.0 was the mixed-acid type. The change in NADH level in the mixed-acid type of fermentation consisted of small fluctuations, enhancing the stability and efficiency of fermentation. The enzymatic activities of acetate kinase and butyrate kinase were slightly inhibited at pH 5.0 and 11.0, resulting in relatively low VFAs production.

Innovatively employing magnetic CuO nanosheet to activate peroxymonosulfate for the treatment of high-salinity organic wastewater

Magnetic CuO nanosheet (Mag-CuO), as a cheap, stable, efficient and easily separated peroxymonosulfate (PMS) activator, was prepared by a simple one-step precipitation method for the removal of organic compounds from salt-containing wastewater. The experiments showed that the removal efficiencies of various organic pollutants including Acid Orange 7, Methylene Blue, Rhodamine B and atrazine in a high-salinity system (0.2 mol/L Na₂SO₄) with the Mag-CuO/PMS process were 95.81%, 74.57%, 100% and 100%, respectively. Meanwhile, Mag-CuO still maintained excellent catalytic activity in other salt systems including one or more salt components (NaCl, NaNO₃, Na₂HPO₄, NaHCO₃). A radical-quenching study and electron paramagnetic resonance analysis confirmed that singlet oxygen (¹O₂) was the dominant reactive oxygen species for the oxidation of organic pollutants in high-salinity systems, which is less susceptible to hindrance by background constituents in wastewater than radicals (^•OH or SO₄^•-). The surface hydroxylation of the catalyst and catalytic redox cycle including Cu and Fe are responsible for the generation of ¹O₂. The developed Mag-CuO catalyst shows good application prospects for the removal of organic pollutants from saline wastewater.

Effect of alkyl polyglycosides on the performance of thermophilic bacteria pretreatment for saline waste sludge hydrolysis

In this study, alkyl polyglycosides (APG) was used to further accelerate the hydrolysis of saline waste sludge with thermophilic bacteria (TB) pretreatment. In the presence of 0.4 g/g TSS APG, the concentrations of soluble chemical oxygen demand (SCOD), soluble carbohydrate and soluble protein in dissolved organic matters (DOM) were 0.4, 2.4 and 1.3 times of that without APG addition, respectively. Excitation emission matrix (EEM) fluorescence spectroscopy revealed that the addition of APG led to the increase of soluble microbial materials and the decrease of fulvic acid-like substances in DOM, which was beneficial for the subsequent process of anaerobic digestion. Using APG promoted the releasing of enzymes trapped in saline waste sludge and improved the activity of enzymes during hydrolysis. The activities of α-glucosidase and protease increased by 8.8% and 21.3% respectively in the presence of 0.4 g/g TSS APG comparing no APG addition.

Enhancing the hydrolysis of saline waste sludge with thermophilic bacteria pretreatment: New insights through the evolution of extracellular polymeric substances and dissolved organic matters transformation

Recently, the treatment and utilization of saline waste sludge has drawn growing attention because large amounts of saline waste sludge were generated with the increase of saline wastewater discharge. In this study, thermophilic bacteria (TB) pretreatment was applied to accelerate the hydrolysis of saline waste sludge and the efficiency of hydrolysis at different salinities was evaluated. Compared with the group without salinity, the releasing of carbohydrate (up to a 67.0% decrease) in extracellular polymeric substances (EPS) was inhibited at the salinity ranging from 1.0% to 2.5%, and the releasing of protein (up to a 17.6% decrease) was inhibited under salinity conditions. Excess salinity (4.0%) caused the cell lysis, and the content of soluble chemical oxygen demand (SCOD), soluble carbohydrate and protein in dissolved organic matter (DOM) increased by 44.9%, 38.8% and 20.8% than that obtained without salinity, respectively. According to the excitation-emission matrix (EEM) fluorescence spectroscopy, the biodegradability of sludge was improved at 2.0% salinity. At 2.0% salinity, the maximum fluorescence intensity of soluble microbial byproduct substances (76,358.9 (au)) and the minimum fluorescence intensity of humic acid-like substances (173,424 (au)) were obtained. The increased salinity was beneficial for the sludge stabilization and was disadvantageous for the sludge reduction.

Removal of tridecane dicarboxylic acid in water by nanoscale Fe0/Cu0 bimetallic composites

In this study, nanoscale zerovalent Fe⁰/Cu⁰ bimetallic composites were synthesized by liquid-phase reduction of Fe(II)/Cu(II) and applied for decomposition of tridecane dicarboxylic acid (DC13). The removal performance of Fe⁰/Cu⁰ bimetallic composites for DC13 in terms of Fe/Cu ratios, addition amount, reaction time and initial pH were studied. The as-prepared nanoscale composites were characterized by a transmission electron micrographs (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), BET surface area, fourier transform infrared spectroscopy (FT-IR) and inductively coupled plasma-atomic emission spectrometry (ICP). Finally, the degradation mechanisms of DC13 utilizing the Fe⁰/Cu⁰ nanocomposites were investigated by using mass spectrumetry (MS). The results indicated that the Fe⁰/Cu⁰ bimetallic composites exerted a remarkable removal capacity for DC13 through the multiple reactions, e.g., coagulation, adsorption and •OH reduction in the Fe⁰/Cu⁰ system. XPS indicated that the Fe⁰/Cu⁰ reduction reaction of hydroxyl radicals (•OH) system played a significant role in degradation of DC13 and the LC-MS result suggested that DC13 was degraded into inorganic small molecules by •OH radicals generated from the corrosion of Fe⁰. The experimental results indicated that the nanoscale Fe⁰/Cu⁰ could be used as a potential material to remove DC13 because of its remarkable degradability.

Effect of salinity on removal performance and activated sludge characteristics in sequencing batch reactors

The removal performance, activated sludge characteristics and microbial community in sequencing batch reactors (SBRs) were studied at salinity ranging from 0 to 20 g/L. Results showed that salinity deteriorated the removal performance. Removal rate of ammonium (NH₄⁺-N), total phosphorus (TP) and chemical oxygen demand (COD) were gradually dropped from 95.34%, 93.58% and 94.88% (0 g/L) to 62.98%, 55.64% and 55.78% (20 g/L), respectively. The removals of NH₄⁺-N and TP were mainly influenced during aerobic phase. Besides, salinity increased the extracellular polymeric substances (EPS) content of activated sludge, decreased the content of protein (PN) and loosely bound extracellular polymeric substances (LB-EPS) which led to better settleability of activated sludge. Moreover, salinity inhibited the dehydrogenase activity (DHA) of activated sludge. Sequence analysis illustrated Zoogloea and Thioclava were predominant at 0 and 20 g/L salinity, respectively. The difference of microbial community under high salinity was likely caused by the variation of richness.