Biotreatment Of Wastewater Research Articles

Insight into the application of biochars produced from wood residues for removing different fractions of dissolved organic material (DOM) from bio-treated wastewater

The primary aim of this study was to conduct a comparative analysis of four distinct biochar types, which were derived from wood residues through varied production methods. To assess their efficacy in removing dissolved organic matter (DOM) from bio-treated wastewater, this study examines the capacity of these biochars to extract DOM fractions from wastewater sourced from landfill leachate. Three types of the biochar were raw gasification biochar , and gasification biochars activated with H2O and CO2 respectively. The other one was a pyrolytic biochar produced from wood residues. To evaluate the biochars’ performance in DOM removal, various wastewater DOM components were analyzed. The study found that the production method significantly impacted the biochars’ ability to remove DOM while the activation technique showed slight impact on the biochars’ performance in DOM removal. Activated gasification biochars exhibited superior DOM removal compared to pyrolytic biochar. 78% of the wastewater COD was removed by activated gasification biochars while the raw biochar only removed 48% of the COD, at 7 gL−1 of the biochars. Pyrolytic biochar could remove 64% of the wastewater COD at 7 gL−1 of the biochar and showed better fluorescent compounds adsorption. Further analysis revealed that pyrolytic biochar was more selective in removing higher molecular weight compounds. In contrast, activated gasification biochars removed compounds with a broad range of molecular weights. The biochars’ adsorption efficiency were attributed to changes in specific biochar characteristics, such as pore size distribution and changes in functional groups. This study suggests that the gasification biochars have great capability in DOM removal.

The modified properties of sludge-based biochar with ferric sulfate and its effectiveness in promoting carbon release from particulate organic matter in rural household wastewater

The scarcity of carbon sources presents a significant challenge for the bio-treatment of rural domestic wastewater (RDW). This paper presented an innovative approach to address this issue by investigating the supplementary carbon source through in-situ degradation of particulate organic matter (POM) facilitated by ferric sulfate modified sludge-based biochar (SBC). To prepare SBC, five different contents of ferric sulfate (0%, 10%, 20%, 25%, and 33.3%) were added to sewage sludge. The results revealed that the pore and surface of SBC were enhanced, providing active sites and functional groups to accelerate the biodegradation of protein and polysaccharide. During the 8-day hydrolysis period, the concentration of soluble chemical oxidation demand (SCOD) increased and peaked (1087–1156 mg L−1) on the fourth day. The C/N ratio increased from 3.50 (control) to 5.39 (25% ferric sulfate). POM was degraded the five dominant phyla, which were Actinobacteriota, Firmicutes, Synergistota, Proteobacteria, and Bacteroidetes. Although the relative abundance of dominant phyla changed, the metabolic pathway remained unchanged. The leachate of SBC (<20% ferric sulfate) was beneficial for microbes, but an excessive amount of ferric sulfate (33.3% ferric sulfate) could have inhibition effects on bacteria. In conclusion, ferric sulfate modified SBC holds the potential for the carbon degradation of POM in RDW, and further improvements should be made in future studies.

Performance of simultaneous carbon and nitrogen removal of high-salinity wastewater in heterotrophic nitrification-aerobic denitrification mode

In this work, heterotrophic nitrification-aerobic denitrification (HN-AD) mode was successfully established after a 105-day acclimation in a sequencing batch reactor (SBR) system using mustard tuber wastewater (MTWW) as substrate. The performance of simultaneous carbon and nitrogen removal (SCNR) was investigated using manual wastewater within the salinity from 1% to 6%. Results indicate that high salinity caused little effect on carbon removal, while it brought a significant influence on nitrogen removal. At the salinity of 1% and 3%, the heterotrophic nitrification rates were 2.83 and 3.96-fold of those of the autotrophic nitrification, respectively. As the salinity was up to 6%, the autotrophic nitrification was significantly inhibited, while the anoxic denitrification was slightly affected and the aerobic denitrification became dominant. The analysis on microbial community structure of the acclimated wastewater sludge reveals that the genera with autotrophic nitrification were gradually eliminated with an increasing salinity during the acclimation. Paracoccus, Pseudomonas, Alcaligenes and Bacillus were the dominant genera related to HN-AD process. The optimal operation parameters for SCNR were investigated, including C/N ratio (25), DO concentration (6 mg/L) and temperature (35 °C). This work provides an insight into the migration of nitrogen removal pathway and the succession of HN-AD microbial community in the process of high-salt wastewater biotreatment.

Microbial ecological responses of partial nitritation/anammox granular sludge to real water matrices and its potential application

Granular sludges are commonly microbial aggregates used to apply partial nitritation/anammox (PN/A) processes during efficient biological nitrogen removal from ammonium-rich wastewater. Considering keystone taxa of anammox bacteria (AnAOB) in granules and their sensitivity to unfavorable environments, it is essential to investigate microbial responses of autotrophic PN/A granules to real water matrices containing organic and inorganic pollutants. In this study, tap water, surface water, and biotreated wastewater effluents were fed into a series of continuous PN/A granular reactors, respectively, and the differentiation in functional activity, sludge morphology, microbial community structure, and nitrogen metabolic pathways was analyzed by integrating kinetic batch testing, size characterization, and metagenomic sequencing. The results showed that feeding of biotreated wastewater effluents causes significant decreases in nitrogen removal activity and washout of AnAOB (dominated by Candidatus Kuenenia) from autotrophic PN/A granules due to the accumulation of heavy metals and formation of cavities. Microbial co-occurrence networks and nitrogen cycle-related genes provided evidence for the high dependence of symbiotic heterotrophs (such as Proteobacteria, Chloroflexi, and Bacteroidetes) on anammox metabolism. The enhancement of Nitrosomonas nitritation in the granules would be considered as an important contributor to greenhouse gas (N2O) emissions from real water matrices. In a novel view on the application of microbial responses, we suggest a bioassay of PN/A granules by size characterization of red-color cores in ecological risk assessment of water environments.

Technological, Ecological, and Energy-Economic Aspects of Using Solidified Carbon Dioxide for Aerobic Granular Sludge Pre-Treatment Prior to Anaerobic Digestion.

The technology of aerobic granular sludge (AGS) seems prospective in wastewater bio-treatment. The characteristics as well as compactness and structure of AGS have been proved to significantly affect the effectiveness of thus far deployed methods for sewage sludge processing, including anaerobic digestion (AD). Therefore, it is deemed necessary to extend knowledge on the possibilities of efficient AGS management and to seek viable technological solutions for methane fermentation of sludge of this type, including by means of using the pre-treatment step. Little is known about the pre-treatment method with solidified carbon dioxide (SCO2), which can be recovered in processes of biogas upgrading and enrichment, leading to biomethane production. This study aimed to determine the impact of AGS pre-treatment with SCO2 on the efficiency of its AD. An energy balance and a simplified economic analysis of the process were also carried out. It was found that an increasing dose of SCO2 applied in the pre-treatment increased the concentrations of COD, N-NH4+, and P-PO43- in the supernatant in the range of the SCO2/AGS volume ratios from 0.0 to 0.3. No statistically significant differences were noted above the latter value. The highest unit yields of biogas and methane production, reaching 476 ± 20 cm3/gVS and 341 ± 13 cm3/gVS, respectively, were obtained in the variant with the SCO2/AGS ratio of 0.3. This experimental variant also produced the highest positive net energy gain, reaching 1047.85 ± 20 kWh/ton total solids (TS). The use of the higher than 0.3 SCO2 doses was proved to significantly reduce the pH of AGS (below 6.5), thereby directly diminishing the percentage of methanogenic bacteria in the anaerobic bacterial community, which in turn contributed to a reduced CH4 fraction in the biogas.

The role of anthraquinone-2-sulfonate on intra/extracellular electron transfer of anaerobic nitrate reduction

To improve the electron (e−) transfer efficiency, exogenous redox mediators (RMs) were usually employed to enhance the denitrification efficiency due to the electron shuttling. Previous studies were mainly focused on how to improve the extracellular electron transfer (EET) by exogenous RMs. However, the intracellular electron transfer (IET), another crucial e− transfer pathway, of biological denitrification was scarcely reported, especially for the relationship between the denitrification and IET. In this study, Coenzyme Q, Complexes I, II and III were determined as the core components in the IET chain of denitrification by using four specific respiration chain inhibitors (RCIs). Anthraquinone-2-sulfonate (AQS) partially recovered the IET of denitrification from NO3−-N to N2 gas when the RCIs were added. Specifically, the generations of N2 gas were improved by 9.68%–18.25% in the experiments with RCIs and AQS, comparing to that with RCIs. nrfA gene was not detected by reverse transcription-polymerase chain reaction, suggesting that Klebsiella oxytoca strain could not conduct dissimilatory nitrate reduction to ammonium. Nitrate assimilation was considered as the main NH4+-N formation way of K. oxytoca strain. The two e− transfer pathways of denitrification were constructed and the roles of AQS on the IET and EET of denitrification were specifically discussed. The results of this study provided a better understanding of the e− transfer pathways of denitrification, and suggested a potential practical use of exogenous RM on bio-treatment of nitrate-containing wastewater.

Effects of bioaugmentation on the performance of industrial-scale activated sludge sequencing batch reactor under load shock of heavy oil refinery wastewater

The stable and efficient operation of the cyclic activated sludge sequencing batch reactor (CASSBR) in heavy oil refineries has become an urgent necessity in wastewater biotreatment. Here, the present study focuses on the selection of inoculated strains, the optimization of process parameters, and the effects of bioaugmentation on the performance in CASSBR under the different rates of heavy oil refinery wastewater shock loading. Results showed that the shock resistance of the CASSBR gradually enhanced and the normal performance was restored in a short time by the addition of a 0.1% biofortifier, and the average removal efficiency of chemical oxygen demand and total nitrogen reached 86% and 55% within 15 days, respectively. The recovery time of the reactor was shortened to 6 days by bioaugmentation measures. This study not only presents a detailed case on the ecological trade-off of the bioreactor and pollutant removal of inoculated strains during bioaugmentation, but also provides a theoretical basis and technical support for evaluating the effects of heavy oil refinery wastewater shock loading on biological wastewater treatment systems.

Efficiency of an alkaline, thermostable, detergent compatible, and organic solvent tolerant lipase with hydrolytic potential in biotreatment of wastewater

Discharging the tannery wastewater into the environment is a serious challenge worldwide due to the release of severe recalcitrant pollutants such as oil compounds and organic materials. The biological treatment through enzymatic hydrolysis is a cheap and eco-friendly method for eliminating fatty substances from wastewater. In this context, lipases can be utilized for bio-treatment of wastewater in multifaceted industrial applications. To overcome the limitations in removing pollutants in the effluent, we aimed to identify a novel robust stable lipase (PersiLipase1) from metagenomic data of tannery wastewater for effective bio-degradation of the oily wastewater pollution. The lipase displayed remarkable thermostability and maintained over 81 % of its activity at 60 °C.After prolonged incubation for 35 days at 60°C, the PersiLipase1 still maintained 53.9 % of its activity. The enzyme also retained over 67 % of its activity in a wide range of pH (4.0 to 9.0). In addition, PersiLipase1 demonstrated considerable tolerance toward metal ions and organic solvents (e.g., retaining >70% activity after the addition of 100 mM of chemicals). Hydrolysis of olive oil and sheep fat by this enzyme showed 100 % efficiency. Furthermore, the PersiLipase1 proved to be efficient for biotreatment of oil and grease from tannery wastewater with the hydrolysis efficiency of 90.76 % ± 0.88. These results demonstrated that the metagenome-derived PersiLipase1 from tannery wastewater has a promising potential for the biodegradation and management of oily wastewater pollution.

The effect and mechanism of molybdenum electron shuttle on perchlorate bioreduction

Here, the enhancement effect and mechanism of MoO3 and silicomolybdic acid (SiMo12) on perchlorate bioreduction were investigated. The bioreduction rates of perchlorate at 36 h were enhanced1.92 and 1.33-fold by SiMo12 and MoO3. According to electrochemical tests, SiMo12 and MoO3 improved the electrochemical activity and reduced the resistance, demonstrating that the electron transfer was accelerated. The secretion of intracellular cytochrome c(Cyt-c) was increased by 1.64-fold in SiMo12 replenished system, while was not increased in MoO3 replenished system. In addition, the secretion of membrane Cyt-c was increased by 2.05 and 1.58-fold in SiMo12 replenished system and MoO3 replenished system. The secretion of extracellular polymeric substance was enhanced 1.72-fold by SiMo12, while it was not enhanced by MoO3. The perchlorate reductase A (PcrA) in perchlorate reductase(Pcr) was promoted 1.41 and 1.21-fold by SiMo12 and MoO3. The mRNA reverse expression levels of perchlorate reductase O (PcrO)and perchlorate reductase Q (PcrQ) were increased by SiMo12, while were not affected by MoO3. This work provided a reliable strategy for accelerating the perchlorate reductase in wastewater bio-treatment.

Spontaneous granulation of moderately halophilic sludge inoculated with saltern sediments from single granule into multi-granule aggregation

There is very limited research on the application of moderate halophiles for biotreatment of hypersaline wastewater widely generated from some industries. This study demonstrated the development of moderate halophiles inoculated from saltern sediments into aerobic granule sludge (AGS) to treat hypersaline wastewater with a salinity of 100 g/L. The granulation of moderate halophiles can occur without applying the settling velocity selective pressure. The saltern sediment initially aggregated into single small granules and finally developed into 1200 ± 50 μm multiparticle granules. The halophiles affiliated in Halomonas was dominant in the granular bacterial community, with a relative abundance of 94.52%. Halomonas ventosae secreted sulfated polysaccharides. The sulfated polysaccharides content accounted for 63.95 ± 2.10% in the polysaccharides (PS), having an adhesive role in connecting single granules. Multiparticle granules showed the clear stratified structure, with α-D-glucopyranose polysaccharides in the inner bounders and β-D-glucopyranose polysaccharides in the outer. The moderately granular sludge showed the stable chemical oxygen demand (COD) removal efficiency of >90% and the aerobic total inorganic nitrogen (TIN) removal efficiency (equal to ammonia removal) of 70 ± 5.00%. This paper contributes new insight into the formation of moderately halophilic granular sludge and accelerates the application of moderately halophilic granular sludge to treat hypersaline wastewater.

Two-stage algal cultivation for the biotransformation of urban wastewater’s pollutants into multiple bioproducts in a circular bioeconomy paradigm

Resource recovery and recycling of urban wastewater have become essential due to rapidly depleting water resources. Here, a newly isolated cyanobacterium Pseudoscillatoria coralii BERC01 was employed for the biotreatment of urban wastewater (UWW) via a two-stage cultivation system followed by biotransformation of the biomass into multiple bioproducts. During the stage-I, the cyanobacterial strain BERC01 removed 100 % suspended solids (9–27 g/L) from the UWW within 3–7 days via biosorption, and the resultant biomass was used as a soil amender which improved the phosphate availability, microbial carbon biomass, and nitrate-nitrogen of the soil by 2.65–5.86-fold, 2.27–3.65-fold, and 3.20–6.57-fold, respectively. During stage-II, re-culturing the BERC01 in the recycled water improved the productivities of carbohydrates and proteins by 10-fold and 3-fold, respectively, when compared to the control. Cultivation of BERC01 also improved the pH of UWW (9.47 ± 0.2 – 10.93 ± 0.16) to reduce the bacterial and/or fungal contamination. The two-stage cultivation of BERC01 in the UWW lowered the total nitrogen, total phosphorous, chemical oxygen demand, and biological oxygen demand of the UWW by 41.94 %, 95 %, 79 %, and 89.64 %, respectively, bringing the quality of treated water within the WHO’s safety limits of agricultural purposes. Cascading processing of the stage-II biomass yielded food-grade pigments (phycocyanin = 148.95 ± 1.81 mg/g, allophycocyanin = 51.92 ± 0.51 mg/g), and 470–490 mg/g of lipids which contained compounds of pharmaceutical interest such as undecanoic acid, pentadecanoic acid, and eicosanoic acid. Finally, the pigment-free and de-oiled residual biomass (PDRB) was completely valorized to mycoprotein and α-amylase through fungal fermentation using 10–50 g/L PDRB as a sole source of nutrition. Resultantly, Aspergillus niger and Aspergillus oryzae produced 98.44 U/mL and 118.56 U/mL of α-amylase in 96 h of incubation, respectively, whereas the protein content of the fermented biomass was improved by 10.42-fold and 7.34-fold, respectively. This novel approach could help in achieving the economic and environmental sustainability of the algal biorefineries.

Recovering rare earth elements via immobilized red algae from ammonium-rich wastewater

Biotreatment of acidic rare earth mining wastewater via acidophilic living organisms is a promising approach owing to their high tolerance to high concentrations of rare earth elements (REEs); however, simultaneous removal of both REEs and ammonium is generally hindered since most acidophilic organisms are positively charged. Accordingly, immobilization of acidophilic Galdieria sulphuraria (G. sulphuraria) by calcium alginate to improve its affinity to positively charged REEs has been used for simultaneous bioremoval of REEs and ammonium. The results indicate that 97.19%, 96.19%, and 98.87% of La, Y, and Sm, respectively, are removed by G. sulphuraria beads (GS-BDs). The adsorption of REEs by calcium alginate beads (BDs) and GS-BDs is well fitted by both pseudo first-order (PFO) and pseudo second-order (PSO) kinetic models, implying that adsorption of REEs involves both physical adsorption caused by affinity of functional groups such as –COO– and –OH and chemical adsorption based on ion exchange of Ca2+ with REEs. Notably, GS-BDs exhibit high tolerance to La, Y, and Sm with maximum removal efficiencies of 97.9%, 96.6%, and 99.1%, respectively. Furthermore, the ammonium removal efficiency of GS-BDs is higher than that of free G. sulphuraria cells at an initial ammonium concentration of 100 mg L−1, while the efficiency decreases when initial concentration of ammonium is higher than 150 mg L−1. Last, small size of GS-BDs favors ammonium removal because of their lower mass transfer resistance. This study achieves simultaneous removal of REEs and ammonium from acidic mining drainage, providing a potential strategy for biotreatment of REE tailing wastewater.

Evaluation of a Microbial Consortium and Selection of a Support in an Anaerobic Reactor Directed to the Bio-Treatment of Wastewater of the Textile Industry

The dyeing processes of the textile industry generate waste products such as unfixed dyes, phenolic surfactants and heavy metals. These constitute an environmental problem for the bodies receiving their wastewater due to the interruption of the lighting in the aquatic environment and the release of toxic molecules by the decomposition of the dyes. There are several treatment methods, of which biological methods are the most feasible. In the current study, the I5-ESPE microbial consortium was obtained and evaluated on the components of textile wastewater, in addition to the selection of a support for an anaerobic reactor that is directed to the treatment of effluents from the textile industry. Two microbial consortia were achieved by exposure to air in Pseudomonas culture medium modified with direct dyes Red 23 and Blue 106, evaluating their removal capacity of the reactive dyes Navy 171, Red 141 and Yellow 84. The consortium I5-ESPE was selected for its greatest action, yielding approximately 95% removal. Its tolerance to phenol was also determined; we reached 98% removal of chromium(VI) and 67% of total chromium under anaerobic conditions and some 25% zinc in aerobiosis. The reduction in the chemical oxygen demand (COD) was evaluated with (57.03%) and without (31.47%) aeration. The species Staphylococcus xylosus, Saccharomyces cerevisiae and Candida tropicalis were identified prior to treatment of textile wastewater, as well as Enterobacter cloacae and Bacillus megaterium after treatment. Bacillus subtilis was present throughout the process. We evaluated coconut shell as a support for an anaerobic reactor, and it demonstrated better physical characteristics than plastic and common rock, in addition to similar results in the reduction in COD of 50%, volatile suspended solids of 2545.46 mg/L and total suspended solids of 282.82 mg/L.

Simultaneous Production of Bio-energy and Bio-treatment of Wastewater Using Photosynthetic Microbial Fuel Cell: Optimization and Kinetic Modeling Approach

Simultaneous Production of Bio-energy and Bio-treatment of Wastewater Using Photosynthetic Microbial Fuel Cell: Optimization and Kinetic Modeling Approach

Characterization of Extracellular Polymeric Substances by Microscopic Imaging Techniques in Wastewater Biotreatment: A Review

Characterization of Extracellular Polymeric Substances by Microscopic Imaging Techniques in Wastewater Biotreatment: A Review

Meso-thermophilic acidogenic biotreatment of mixed wastewater from toilets and coffeeshop: Effect of temperature on the efficiency of organic removal and VFA productions

Meso-thermophilic acidogenic biotreatment of mixed wastewater from toilets and coffeeshop: Effect of temperature on the efficiency of organic removal and VFA productions

Bioenergy recovery associated with treatment of actual meat industry wastewater in blue green algal biocathode based microbial fuel cell: Experimental and modeling study

ABSTRACT The photosynthetic microbial fuel cell (PMFC) being one of the microbial fuel cells (MFCs) type, seems to be as a great promising technology for wastewater treatment accompanied with renewable energy production. This study aimed to evaluate the performance of blue-green algae-based PMFC for simultaneous biotreatment of actual slaughterhouse wastewater (SWW), ammonium removal, and bioenergy recovery. The experimental results demonstrated significant removal efficiency of chemical oxygen demand (COD) and ammonium (NH4 +) of 99.6 ± 0.5%, and 82.7 ± 1.5%, respectively associated with a power output of 469.6 ± 3.0 mW/m2. Butler–Volmer–Monod model was applied to describe the overpotential-polarization curve for the suggested PMFC. A significant agreement was observed between the experimental and predicted results with a determination coefficient (R2) value of 0.974. The observed favorable results reported in this investigation strongly encourage the use of the proposed technique for the treatment of slaughterhouse effluents.

Enzymes immobilized polymeric supports for wastewater treatment application: A short review

Micro-pollutants, with a growing number of hazardous contaminants being introduced into both human and natural ecosystems, have become a major topic of rising environmental concern and the biological method is an up-and-coming wastewater treatment technique in late decades. This review focuses on enzyme immobilization in biological wastewater treatment, in which enzymes are used to biodegrade and remove various types of pollutants. The synthesis of different kinds of polymer substrates, their biocompatibility, stability, reusability and application for the immobilization of enzymes, and their biochemical as well as biotechnological applications which employment of enzymes immobilization for different wastewater treatment processes are reviewed. Various cases of wastewater biotreatment are described, such as removal of pigments and colour in textile industry wastewater, refractory organics wastewater, as well as nitrogen and phosphorous wastewater treatment. The review concludes with a brief discussion of potential research directions in enzyme immobilization on polymeric supports for wastewater treatment application.

Biochar immobilized petroleum degrading consortium for enhanced granulation and treatment of synthetic oil refinery wastewater

This study investigated the effect of biochar immobilized consortium for enhanced biotreatment of oil refinery wastewater in sequencing batch reactors. The control reactor (R0), and reactors with free bacterial consortium (R1), biochar (R2) and biochar immobilized consortium (R3) were operated in parallel. Results illustrated that granulation was faster in R2 and R3, the proportion of granules >0.45 mm was almost 30% higher than the R0 and R1 during the first 45 days. R3 exhibited optimum operational performance as highest removal efficiencies of COD (97.9%), oil (97.4%), NH4+-N (97.2%) and TN (90.2%) at the end of the experiment. Biochar enriched microbial richness and diversity whereas inoculated consortium positively modulated the community structures. Higher abundance of inoculated Rhodococcus (4.81%) bacteria in R3 suggested successful formation of functional strains in the granular sludge. It concludes that the application of biochar immobilized consortium is a better choice for the treatment of refinery wastewater.

Re-evaluation of the environmental hazards of nZnO to denitrification: Performance and mechanism

Numerous studies have shown that zinc oxide nanoparticles (nZnO) have an inhibitory effect on wastewater biotreatment, where doses exceeding ambient concentrations are used. However, the effect of ambient concentrations of ZnO (<1 mg/L) on anaerobic digestion processes is not clear. Herein, this study comprehensively explored the impact of nZnO on the denitrification performance and core microbial community of activated sludge under ambient concentrations. Results showed that only 0.075 mg/L nZnO had shown a beneficial effect on nitrogen removal by activated sludge. When nZnO concentration reached 0.75 mg/L, significant enhancement of nitrate reduction and mitigation of nitrite accumulation were observed, indicating a remarkable stimulatory effect on nitrogen removal. Simultaneously, nZnO could weaken the sludge surface charge and improve the secretion of extracellular polymeric substances, thus enhancing sludge flocculation for denitrification. Microbial community analysis revealed that nZnO exposure increased the relative abundance of denitrifying bacteria, which could contribute to the reinforcement of traditional denitrification. Furthermore, exogenous addition of NH4+ significantly inhibited the accumulation of nitrite, implying that nZnO had a potential to improve the denitrification process via a partial denitrification-anammox pathway. Considering current ambient concentration, the stimulatory effect shown in our work may better represent the actual behavior of ZnO in wastewater biotreatment.