Intensified nitrogen and phosphorus removal by embedding electrolysis in an anaerobic-anoxic-oxic reactor treating low carbon/nitrogen wastewater

High-efficient nutrient removal in a single-stage electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) for low C/N sanitary sewage treatment

To efficiently remove nutrients from low C/N sanitary sewage by conventional biological process is challenging due to the lack of sufficient electron donors. A novel electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) was established to promote nitrogen and phosphorus removal for sanitary sewage with low C/N ratios (3.5-1.5). Highly efficient removal of nitrogen (>79%) and phosphorus (>97%) was achieved in the E-SBBR operating under alternating anoxic/electrolysis-anoxic/aerobic conditions. The coexistence of autotrophic nitrifiers, electron transfer-related bacteria, and heterotrophic and autohydrogenotrophic denitrifiers indicated synergistic nitrogen removal via multiple nitrogen-removing pathways. Electrolysis application induced microbial anoxic ammonia oxidation, autohydrogenotrophic denitrification and electrocoagulation processes. Deinococcus enriched on the electrodes were likely to mediate the electricity-driven ammonia oxidation which promoted ammonia removal. PICRUSt2 indicated that the relative abundances of key genes (hyaA and hyaB) associated with hydrogen oxidation significantly increased with the decreasing C/N ratios. The high autohydrogenotrophic denitrification rates during the electrolysis-anoxic period could compensate for the decreased heterotrophic rates resulting from insufficient carbon sources and nitrate removal was dramatically enhanced. Electrocoagulation with iron anode was responsible for phosphorus removal. This study provides insights into mechanisms by which electrochemically assisted biological systems enhance nutrient removal for low C/N sanitary sewage.

Enhanced nitrogen removal in sewage treatment is achieved by using kitchen waste hydrolysate without a significant increase in nitrous oxide emissions

Kitchen waste hydrolysate (KWH) is an effective replacement for commonly used carbon sources such as sodium acetate (NaAc) and glucose (Glu), in wastewater treatment plants (WWTPs) to enhance the total nitrogen (TN) removal efficiency in sewage and reduce the operating cost of WWTPs. However, KWH utilization introduces complex organic matter that may lead to increased nitrous oxide (N2O) emissions, compared with that of NaAc and Glu, causing significant damage to the atmosphere. Therefore, this study aims to compare the effects of KWH, Glu, and NaAc on N2O emissions in sewage treatment. The results indicated that KWH introduction did not lead to a significant increase in N2O emissions, with a conversion rate of only 5.61 %. Compared with raw sludge, the addition of only Glu and NaAc significantly increased the abundance of the nar G gene, indicating that the readily degradable carbon sources initiated denitrification at a faster rate than KWH. When KWH was added, there was a notable increase in the abundance of genes associated with partial nitrification and denitrification (nir K, hzo, and nos Z). In contrast, Glu and NaAc did not have a significant effect on the nos Z gene. The results suggested that KWH supplementation was more effective to reduce N2O to N2. Moreover, the KWH addition significantly increased the microbial diversity in the sludge and promoted the presence of shortcut nitrification and denitrification bacteria (Comamonadaceae) and denitrification bacteria (Rhodobacteraceae), further indicating the potential of KWH for enhanced denitrification and reduced N2O emissions. Overall, to the best of our knowledge, this is the first study that demonstrated KWH, as a novel and complex organic carbon source, can be safely used in sewage treatment processes to improve the pollutant removal efficiency without causing a significant increase in N2O emissions.

Exopolysaccharides from marine microbes with prowess for environment cleanup

A variety of both small and large biologically intriguing compounds can be found abundantly in the marine environment. Researchers are particularly interested in marine bacteria because they can produce classes of bioactive secondary metabolites that are structurally diverse. The main secondary metabolites produced by marine bacteria are regarded as steroids, alkaloids, peptides, terpenoids, biopolymers, and polyketides. The global urbanization leads to the increased use of organic pollutants that are both persistent and toxic for humans, other life forms and tend to biomagnified in environment. The issue can be addressed, by using marine microbial biopolymers with ability for increased bioremediation. Amongst biopolymers, the exopolysaccharides (EPS) are the most prominent under adverse environmental stress conditions. The present review emphasizes the use of EPS as a bio-flocculent for wastewater treatment, as an adsorbent for the removal of textile dye and heavy metals from industrial effluents. The biofilm-forming ability of EPS helps with soil reclamation and reduces soil erosion. EPS are an obvious choice being environmentally friendly and cost-effective in processes for developing sustainable technology. However, a better understanding of EPS biosynthetic pathways and further developing novel sustainable technologies is desirable and certainly will pave the way for efficient usage of EPS for environment cleanup.

Phosphorus removal and recovery: state of the science and challenges

Phosphorus is one of the main nutrients required for all life. Phosphorus as phosphate form plays an important role in different cellular processes. Entrance of phosphorus in the environment leads to serious ecological problems including water quality problems and soil pollution. Furthermore, it may cause eutrophication as well as harmful algae blooms (HABs) in aquatic environments. Several physical, chemical, and biological methods have been presented for phosphorus removal and recovery. In this review, there is an overview of phosphorus role in nature provided, available removal processes are discussed, and each of them is explained in detail. Chemical precipitation, ion exchange, membrane separation, and adsorption can be listed as the most used methods. Identifying advantages of these technologies will allow the performance of phosphorus removal systems to be updated, optimized, evaluate the treatment cost and benefits, and support select directions for further action. Two main applications of biochar and nanoscale materials are recommended.

Enhancing nitrate removal efficiency of micro-sized zero-valent iron by chitosan gel balls encapsulating

The activity of micro-sized zero-valent iron (MZVI) material for nitrate removal in neutral pH and low C/N ratios water needs to be improved. In this study, micro-sized zero-valent iron@chitosan (MZVI@CS) material was synthesized through embedding MZVI particles into chitosan (CS) gel by sol-gel method, and was used for deep removal of NO₃^--N in the absence of organic carbon sources and neutral pH. The NO₃^--N removal rate of MZVI@CS was 0.37 mg-N·L^-1·d^-1 (dosage of 1%, initial pH = 7, 25 °C, initial nitrate concentration = 15 mg-N·L^-1), which was 11.33 times higher than that of MZVI. The apparent activation energy (E_a) of MZVI@CS with nitrate was 38.23 kJ·mol^-1. MZVI@CS can remove nitrate effectively at a low concentration (15 mg-N·L^-1). A stable denitration rate (0.37-2.28 mg-N·L^-1·d^-1) could be maintained under weak acidic, neutral and alkaline conditions (pH = 5-9). More than 80% of reduced nitrate was converted to N₂, and only a small amount was converted to NH₄⁺ or NO₂^-. The gel structure of MZVI@CS eliminated the agglomeration between MZVI particles while the forming of Fe-CS chelates reduced the formation of iron oxide and solved the problems of passivation, hence successfully strengthened the NO₃^--N removal efficiency of MZVI. Therefore MZVI@CS has great application potential in NO₃^--N deep removal of water bodies with neutral pH and low C/N ratios.

Recovery of phosphate and ammonia from wastewater via struvite precipitation using spent refractory brick gravel from steel industry

Spent refractory brick (SRB) generated from the steel industry has a high magnesium content. In this study, a procedure was developed to utilize SRB gravels for efficient recovery of phosphate and ammonia from high strength wastewater via struvite (MgNH₄PO₄∙6H₂O(s)) precipitation. Mg²⁺ and Ca²⁺ were first leached from SRB gravels using nitric acid solution. Ca²⁺ in the solution could inhibit struvite precipitation and was sequestered by dosing SO₃^2- to form calcium sulfite (CaSO₃(s)). The resulting Mg²⁺-rich solution was then employed to initiate struvite precipitation for phosphate and ammonia recovery. The optimal precipitation was achieved with a molar ratio of [Mg²⁺]:[NH₃-N]:[PO₄³-P] = 2:1:2 at pH 9.5. The residual phosphate in the solution can be further removed via the precipitation of calcium phosphate minerals. Overall, 99.6% phosphate and 98.2% ammonia could be recovered and the treated wastewater could meet the discharging standards of ammonia and phosphate. The resulting solids, including calcium sulfite, struvite and calcium phosphate can be potentially used in the cement industry and agriculture sector to achieve sustainable recycle of spent materials.

Kitchen waste hydrolysate enhances sewage treatment efficiency with different biological process compared with glucose

Insufficient carbon source is the primary factor that limits biological nitrogen and phosphorus removal during sewage treatment. This study investigates the feasibility and biological process of kitchen waste hydrolysate (KWH) replacing glucose to improve pollutant removal efficiency. It was found that using KWH as carbon source achieved better removal effect than glucose during sewage treatment. And more than 96% of total nitrogen (TN), total phosphorus (TP), and the chemical oxygen demand were removed after 48 h of acclimation. Nitrogen and phosphorus introduced by adding KHW had no negative effect on the effluent quality. Compared with glucose, KWH decreased the diversity of bacteria and significantly promoted the accumulation of acid-producing bacteria (Propionibacterium) and denitrifying bacteria (Rhodobacteraceae). Moreover, KWH significantly improved the relative abundance of the amo A, nap A, and nos Z genes. This result further indicated that KWH was beneficial for denitrification and was a favorable external carbon source.

Solar-driven, self-sustainable electrolysis for treating eutrophic river water: Intensified nutrient removal and reshaped microbial communities

River ecosystems are the most important resource of surface freshwater, but they have frequently been contaminated by excessive nutrient input of nitrogen (N) and phosphorus (P) in particular. An efficient and economic river water treatment technology that possesses the capacity of simultaneous N and P removal is urgently required. In this study, a solar-driven, self-sustainable electrolytic treatment was conducted in situ to intensify N and P removal from eutrophic river water. Solar panel was applied to provide the electrolysis setups with energy (voltage 10 ± 0.5 V), and the current density was controlled to be 0.06 ± 0.02 mA cm^-2. Results indicated that the average removal efficiencies of total N (TN) and total P (TP) under electrolysis conditions reached 72.4 ± 11.7 and 13.8 ± 5.3 mg m^-2 d^-1, which were 3.7- and 4.7-fold higher compared to untreated conditions. Enhanced TN removal mainly reflected the abatement of nitrate N (NO₃^--N) (80.6 ± 4.1%). The formation of ferric ions through the electro-dissolution of the sacrificial iron anode improved TP removal by coprecipitation with SPS. Combined high-throughput sequencing and statistical analyses revealed that electrolysis significantly reshaped the microbial communities in both the sediment-water interface and suspended sediment (SPS), and hydrogenotrophic denitrifiers (e.g., Hydrogenophaga) were highly enriched under electrolysis conditions. These findings indicated that in situ electrolysis is a feasible and effective technology for intensified nutrient removal from river water.

Nitrate removal from low C/N wastewater at low temperature by immobilized Pseudomonas sp. Y39-6 with versatile nitrate metabolism pathways

For solving the challenge in nitrate removal from low C/N wastewater at low temperature, Pseudomonas sp. Y39-6 was isolated and used in nitrate removal. It showed aerobic-heterotrophic denitrification with rate of 1.77 ± 0.31 mg/L·h and unusual aerobic-autotrophic nitrate removal (rate of 0.324 mg/L·h). The aerobic-autotrophic nitrate removal mechanisms were deep investigated by analyzing the nitrate removal process and genomic information. At aerobic-autotrophic condition, the strain Y39-6 could assimilate nitrate to amino acid (NO₃^- + PHA + CO₂ → C₅H₇O₂N) with the carbon source from Polyhydroxyalkanoic acid (PHA) degradation and CO₂ fixation. Flagella motivation, swarming activity and extracellular polymeric substances (EPS) production regulated Pseudomonas sp. Y39-6 forming biofilm. Carriers immobilized with Pseudomonas sp. Y39-6 were used in moving bed biofilm reactor (MBBR) and achieved 24.83% nitrate removal at C/N < 1 and 4 °C. Results of this study provided a practical way for nitrogen removal from low C/N wastewater in cold region.

Simulating a combined lysis-cryptic and biological nitrogen removal system treating domestic wastewater at low C/N ratios using artificial neural network

In this study, a combined alkaline (ALK) and ultrasonication (ULS) sludge lysis-cryptic pretreatment and anoxic/oxic (AO) system (AO + ALK/ULS) was developed to enhance biological nitrogen removal (BNR) in domestic wastewater with a low carbon/nitrogen (C/N) ratio. A real-time control strategy for the AO + ALK/ULS system was designed to optimize the sludge lysate return ratio (R_SLR) under variable sludge concentrations and variations in the influent C/N (⩽ 5). A multi-layered backpropagation artificial neural network (BPANN) model with network topology of 1 input layer, 3 hidden layers, and 1 output layer, using the Levenberg-Marquardt algorithm, was developed and validated. Experimental and predicted data showed significant concurrence, verified with a high regression coefficient (R² = 0.9513) and accuracy of the BPANN. The BPANN model effectively captured the complex nonlinear relationships between the related input variables and effluent output in the combined lysis-cryptic + BNR system. The model could be used to support the real-time dynamic response and process optimization control to treat low C/N domestic wastewater.

Effect of carbon source on pollutant removal and microbial community dynamics in treatment of swine wastewater containing antibiotics by aerobic granular sludge

Aerobic granular sludge sequencing batch reactor (AGSBR) is a promising approach for wastewater treatment. In the paper, the effects of methanol, starch and sucrose as carbon sources on the treatment of swine wastewater (SW) containing antibiotics by aerobic granular sludge (AGS) were studied. The results revealed that the carbon sources could affect the morphology, biomass, and settleability of AGS, and AGS could maintain a better sludge performance when sucrose was used as carbon source. The pollutants (ammonium nitrogen (NH+ 4-N), organic matter and total phosphorus (TP)) in SW also had a good removal effect, and the removal rates reached 81.14%, 96.83% and 97.37% respectively. The removal efficiencies of tetracycline (TC) and oxytetracycline (OTC) from SW were the best when sucrose as co-metabolic matrix by microorganisms. The analysis of miseq pyrosequencing demonstrated that carbon sources with methanol, starch and sucrose improved the diversity of microbial community in AGS, and the dominant bacteria also changed. The dominant groups involved in TC and OTC, removal at different classification levels suggested that the formation of bacterial communities was determined by carbon sources.

Metagenomic analysis reveals enhanced nutrients removal from low C/N municipal wastewater in a pilot-scale modified AAO system coupling electrolysis

The conventional biological nutrients removal process is challenged by insufficient organic carbon in influent. To cross such an organic-dependent barrier, a pilot-scale electrolysis-integrated anaerobic/anoxic/oxic (AAO) process was developed for enhanced removal of nitrogen (N) and phosphorus (P) from low carbon/nitrogen (C/N) municipal wastewater. Average removal efficiencies of total nitrogen (TN) and total phosphorus (TP) in the electrolysis-AAO reached to 77.24% and 95.08% respectively, showing increases of 13.88% and 21.87%, as compared to the control reactor. Spatial variations of N and P showed that NH₄⁺-N removal rate was promoted in aerobic zone of electrolysis-AAO. The intensified TN elimination, which was mostly reflected by abatement of NO₃^--N with the concomitant slight accumulation of NH₄⁺-N and NO₂^--N, mainly occurred in anoxic2 compartment as the electrons supplied by electrolysis. Furthermore, minor P contents were measured and remained almost unchanged along the reaction units, indicating that chemical precipitation should be the dominant mechanism of P-removal in electrolysis-AAO. From the metagenomic-based taxonomy, phylum Actinobacteria was dramatically inhibited, and phylum Proteobacteria dominated the electrolysis-AAO. Particularly, nitrifying bacteria and multifarious autotrophic denitrifiers were enriched, meanwhile, a significant evolution of heterotrophic denitrifiers was found in electrolysis-AAO compared to control, which was mostly reflected by the inhibition of genus Candidatus Microthrix. Batch tests further confirmed that autotrophic denitrifiers using H₂ and Fe²⁺ as essential electron sinks were mainly responsible for the electrolysis-induced denitrification. Differential metabolic capacities were revealed from the perspectives of functional enzymes and genes, and network analysis allowed insight of microbial taxa-functional genes associations and shed light on stronger relevance between autotrophic denitrifiers and denitrification-associated genes in the electrolysis-AAO system.

Recent development of advanced biotechnology for wastewater treatment

The importance of highly efficient wastewater treatment is evident from aggravated water crises. With the development of green technology, wastewater treatment is required in an eco-friendly manner. Biotechnology is a promising solution to address this problem, including treatment and monitoring processes. The main directions and differences in biotreatment process are related to the surrounding environmental conditions, biological processes, and the type of microorganisms. It is significant to find suitable biotreatment methods to meet the specific requirements for practical situations. In this review, we first provide a comprehensive overview of optimized biotreatment processes for treating wastewater during different conditions. Both the advantages and disadvantages of these biotechnologies are discussed at length, along with their application scope. Then, we elaborated on recent developments of advanced biosensors (i.e. optical, electrochemical, and other biosensors) for monitoring processes. Finally, we discuss the limitations and perspectives of biological methods and biosensors applied in wastewater treatment. Overall, this review aims to project a rapid developmental path showing a broad vision of recent biotechnologies, applications, challenges, and opportunities for scholars in biotechnological fields for "green" wastewater treatment.

Granulation of activated sludge using butyrate and valerate as additional carbon source and granular phosphorus removal capacity during wastewater treatment

As an efficient and low-cost phosphorus (P) removal method from wastewater, enhanced biological phosphorus removal process always faces the insufficient carbon source issue. In this study, two identical sequencing batch reactors were used to cultivate aerobic granular sludge, in which butyrate (Rb) and valerate (Rv), two major volatile fatty acids that can be produced from anaerobic fermentation of waste biomass, were respectively applied as additional carbon source. Both reactors exhibited almost same excellent organics and total nitrogen removals during 120 days' operation, about 95.2-95.7% and 67.9-68.0% respectively with noticeable difference in P removal. Compared to the granules in Rv (24.3 mg P/g-total solids), bigger and more stable ones with higher P removal capacity (11.5 mg P/g-volatile solids∙d) were finally achieved in Rb, containing higher P content (36.0 mg P/g-total solids) with more orthophosphate and polyphosphate accumulated. Microbial community analysis reflected more polyphosphate-accumulating organisms (Rhodocyclus-related bacteria and Actinobacteria) in the granules from Rb.

Single-stage denitrifying phosphorus removal biofilter utilizing intracellular carbon source for advanced nutrient removal and phosphorus recovery

Advanced nutrient removal of municipal wastewater has insufficient carbon source, and resource recovery is neglected. In this study, a single-stage biofilter based on denitrifying phosphorus removal (DPR) was proposed for advanced nutrient removal and phosphorus recovery, which was operated under alternating anoxic/anaerobic mode with no extracellular carbon source in anoxic period. The results showed that the biofilter achieved efficient and stable performance with low carbon consumption (C/N ≈ 3.7). The average removal efficiency of NO₃^--N, TN and PO₄^3--P were 74.81%, 71.08% and 91.15%, respectively. DPR primarily occurred in the middle of the filtration bed and nutrient removal was driven by intracellular polymers, which was the main carbon source. High-throughput sequencing indicated that Dechloromonas was enriched and contributed to DPR while Zoogloea was responsible for endogenous denitrification. Denitrifying polyphosphate accumulating organisms and endogenous denitrifiers synergistically enhanced the nutrient removal capacity. The study further provides research perspectives for improving nutrient removal.

Overview of strategies for enhanced treatment of municipal/domestic wastewater at low temperature

Biological wastewater treatment has been widely applied to municipal/domestic wastewater treatment systems. However, low temperature significantly decreases process performance. Furthermore, increasingly stringent effluent discharge standards are causing wastewater treatment facilities to have to improve and maintain contaminants removal under low temperature. Hence, this review aims to summarize strategies for enhanced treatment of municipal/domestic wastewater at low temperature. First, mechanisms of the effects of low temperature on wastewater treatment, including physiological characteristics, microbial growth rate, microbial activity, microbial community structure and sludge settleability, are analyzed. Strategies for performance intensifications at low temperature, mainly operational parameters regulation, bioaugmentation, biofilm technology, chemical phosphorus precipitation and application of novel process technologies, are then reviewed. Finally, future directions to address low temperature wastewater are highlighted. A special emphasis is given to the application of novel process/technology configurations to enhance process performance at low temperature in practical engineering.

Response of algal-bacterial granular system to low carbon wastewater: Focus on granular stability, nutrients removal and accumulation

The effect of influent chemical oxygen demand to nitrogen (COD/N) ratio on the granular stability, nutrients removal and accumulation of the algal-bacterial AGS was investigated. Two sequencing batch reactors were operated under different influent COD/N ratio, i.e., R₁ (control, COD/N = 8) and R₂: (COD/N = 8, 4, 2, and 1 through stepwise decrease of COD). Results showed that the integrity coefficient of the granules in R₂ stabilized at 0.7-5.4% during the whole operation. Significantly enhanced dissolved inorganic carbon (DIC) uptake and the faster growth of algae indicated the great potential for reduction in greenhouse gases (GHGs) emission by using the algal-bacterial AGS system. The algal-bacterial AGS biomass contained high phosphorus (P) and N contents as well as extremely high P bioavailability (up to 98%) which could be easily used for resource recovery. Loosely bound extracellular polymeric substances (LB-EPS) might be the key factor to control the deterioration of granular stability in this system.