Stringent Discharge Standards Research Articles

Efficient nitrogen and phosphorus removal performance and microbial community in a pilot-scale anaerobic/anoxic/oxic (AOA) system with long sludge retention time: Significant roles of endogenous carbon source

Stringent wastewater discharge standards require wastewater treatment plants (WWTPs) to focus on enhancing nitrogen (N) and phosphorus (P) removal efficiency. Increasing sludge concentration by regulation of sludge retention time (SRT) would enhance wastewater treatment loads. However, phosphorus-accumulating organisms (PAOs) would be outcompeted by glycogen-accumulating organisms (GAOs) under long SRT, leading to a collapse of P removal. In this work, pilot-scale anaerobic-oxic-anoxic (AOA) and anaerobic-anoxic-oxic (AAO) systems with long SRT (30 d) were parallelly established for actual urban wastewater treatment. The results indicated that sludge reflux ratio, temperature, and C/N ratio significantly impact N and P removal performance of AOA and AAO systems with long SRT, and removal efficiency of AOA system significantly exceeded that of AAO system. AOA system with long SRT achieved the optimal performance at sludge reflux ratio of 200%, temperature of 25 °C, and C/N ratio of 8, with COD, NH4+-N, TN, and PO43--P removal ratio of 92.80 ± 2.24%, 97.38 ± 0.89%, 88.97 ± 2.47%, and 94.33 ± 3.27%, respectively. In addition, compared to AAO system, AOA system could save 23.08% of the aeration volume. This work highlighted that AOA system with long SRT included multiple coupled nitrogen and phosphorus removal pathways, such as autotrophic/heterotrophic nitrification, anoxic/oxic denitrification, endogenous denitrification, and denitrifying phosphorus removal. Among these, the synergistic effect of endogenous denitrification and denitrifying phosphorus removal driven by internal carbon sources contributed to satisfactory nitrogen and phosphorus removal efficiency in AOA system with long SRT.

A Novel Coupling of a Biological Aerated Filter with Al3+ Addition-O3/H2O2 with Microbubble for the Advanced Treatment of Proprietary Chinese Medicine Secondary Effluent

The advanced treatment of proprietary Chinese medicine secondary effluent (PCMSE) was strongly needed with the recent implementation of a more stringent discharge standard. Based on the features of PCMSE and the reuse of Al3+from wastewater from soaking of Pinellia Ternata with alumen (WSPTA), three new combined processes were designed for the advanced treatment of PCMSE on a larger pilot scale. A pilot scale study showed that compared with two other combined processes, the new coupling of a biological-aerated filter with Al3+ addition (BAFA)-O3/H2O2 with microbubble (OHOMB) (CBAFAOHOMB) obtained the maximum pollutant removal (with removals of 91.71%, 94.64%, and 82.32% being observed for color, total phosphorus (TP), and chemical oxygen demand (COD), respectively) and acquired the lowest Al3+residual in the effluent. During CBAFAOHOMB treatment of PCMSE, the vast majority of TP elimination, 35.20% of COD removal, and 49.40% of color removal were achieved by BAFA; OHOMB obtained 64.80% of COD removal and 60.60% of color removal, and biofilm activity in BAFA slightly changed under a 10 mg/L Al3+ dose. Furthermore, microbubble aeration was more efficient in removing organics than conventional bubble aeration during O3/H2O2 oxidation, and suspended solid (SS) relatively significantly lowered oxidation ability in the OHOMB system. These results indicated that CBAFAOHOMB markedly integrated advantages of BAFA and OHOMB, and was a proposed process for the advanced treatment of PCMSE. Meanwhile, it was feasible that WSPTA was reused for PCMSE treatment as an Al3+ source.

Inhibition of phosphorus removal performance in activated sludge by Fe(III) exposure: transitions in dominant metabolic pathways.

Simultaneous chemical phosphorus removal process using iron salts (Fe(III)) has been widely utilized in wastewater treatment to meet increasingly stringent discharge standards. However, the inhibitory effect of Fe(III) on the biological phosphorus removal system remains a topic of debate, with its precise mechanism yet to be fully understood. Batch and long-term exposure experiments were conducted in six sequencing batch reactors (SBRs) operating for 155 days. Synthetic wastewater containing various Fe/P ratios (i.e., Fe/P = 1, 1.2, 1.5, 1.8, and 2) was slowly poured into the SBRs during the experimental period to assess the effects of acute and chronic Fe(III) exposure on polyphosphate-accumulating organism (PAO) growth and phosphorus metabolism. Experimental results revealed that prolonged Fe(III) exposure induced a transition in the dominant phosphorus removal mechanism within activated sludge, resulting in a diminished availability of phosphorus for bio-metabolism. In Fe(III)-treated groups, intracellular phosphorus storage ranged from 3.11 to 7.67 mg/g VSS, representing only 26.01 to 64.13% of the control. Although the abundance of widely reported PAOs (Candidatus Accumulibacter) was 30.15% in the experimental group, phosphorus release and uptake were strongly inhibited by high dosage of Fe(III). Furthermore, the abundance of functional genes associated with key enzymes in the glycogen metabolism pathway increased while those related to the polyphosphate metabolism pathway decreased under chronic Fe(III) stress. These findings collectively suggest that the energy generated from polyhydroxyalkanoates oxidation in PAOs primarily facilitated glycogen metabolism rather than promoting phosphorus uptake. Consequently, the dominant metabolic pathway of communities shifted from polyphosphate-accumulating metabolism to glycogen-accumulating metabolism as the major contributor to the decreased biological phosphorus removal performance.

The urgent need to reduce phosphorus discharges for sustainable mangrove wetland management

Phosphorus affects microbial metabolic activity, nitrogen and carbon cycling in mangrove sediment, but its influence on carbon stability and greenhouse gases emission remains unclear. This study compared greenhouse gases (CO2, N2O, and CH4) emissions from mangrove sediment receiving wastewater containing various phosphorus concentrations, and evaluated its long term effect on sediment carbon flux when phosphorus pollution is eliminated. Significant increases in greenhouse gases flux and decrease of total organic carbon and readily oxidizable organic carbon in the sediment were observed after phosphorus discharge. Specifically, the N2O flux was reduced significantly at high phosphorus levels while the CO2 flux and the microbial biomass organic carbon was increased. The copy numbers of ammonia oxidation (AOA-amoA, AOB-amoA) gene, denitrification (narG, nirK) gene and methanogenesis (mcrA) gene increased with the increasing phosphorus concentration. During the wastewater discharge period for 70 days, the global warming potential of sediment flux at high phosphorus discharge condition was more than 4 times that of the control group, and the loss of total organic carbon and readily oxidizable organic carbon was 4.66 % and 7.1 %, respectively. During the remediation period (71–101 days), the greenhouse gases flux decreased rapidly, ends up with a similar level of the control group. Our results indicate that using mangrove wetland for pollution minimization in the coastal aquaculture industry could increase greenhouse gases emisison significantly, it is therefore essential to reduce phosphorus discharges from various anthropogenic activities, and local authorities must set up more stringent discharge standards in the future.

Fe(II)-driven spatiotemporal assembly of heterotrophic and anammox bacteria enhances simultaneous nitrogen and phosphorus removal for low-strength municipal wastewater

The mainstream anaerobic ammonium oxidation (anammox) faces considerable challenges with low-strength municipal wastewater. A Fe(Ⅱ)-amended partial denitrification coupled anammox (PD/A) process was conducted and achieved a long-term and efficient nitrogen and phosphorus removal, yielding effluent total nitrogen and phosphorus concentrations of 1.97 ± 1.03 mg/L and 0.23 ± 0.13 mg/L, respectively, which could well meet more stringent effluent discharge standard of some wastewater treatment plants in specific geographical locations, e.g., estuaries. Fe(Ⅱ)-driven vivianite formation provided key nucleuses for the optimization of the spatial distribution of heterotrophic and anammox bacteria with enhanced extracellular polymeric substances as key driving forces. Metagenomics analysis further revealed the increase of key genes, enhancing anammox bacteria homeostasis, which also bolstered the resistance to environmental perturbations. This study provided a comprehensive sight into the function of Fe(Ⅱ) in mainstream PD/A process, and explored a promising alternative for synergetic nitrogen and phosphorus removal for low-strength municipal wastewater treatment.

Effect of Photocatalytic Pretreatment on the Membrane Performance in Nanofiltration of Textile Wastewater

Traditional methods like biological treatment, flocculation-coagulation, adsorption, and advanced oxidation are commonly employed for textile wastewater treatment, but their sustainability is hindered by issues such as the adverse impact of textile wastewater on microorganisms and the requirement for substantial chemical usage. In response to increasingly stringent legal discharge standards, membrane technologies are emerging as prominent alternatives for effective textile wastewater treatment. The application of photocatalysis as a pretreatment to improve effluent quality and treatment performance has shown effective results in the treatment of textile wastewater by nanofiltration (NF). However, innovative solutions are needed to improve the efficiency of UV photocatalytic reactors. Here, the TiO2/halloysite nanotube (HNT) photocatalyst was shown to completely remove dyes under UV illumination. Two wastewater samples from photocatalytic (PC) pretreatment were treated using innovative NF membranes with different contents. The study examined the impact of PC pretreatment on the flux of wastewater from a textile factory heat recovery tank, which increased from 18.32 to 27.63 L/m2.h. The membranes achieved > 98% removal in COD, while bare membrane achieved 95% removal in conductivity. The addition of s-DADPS as monomer and HNT as nanoparticles to the membranes with different compositions affected the cross-linking in the TFC layer. During the tests conducted on the water extracted from the dyeing tank, the color was completely eliminated without any loss of flux. Additionally, improvements in COD removal were observed.

Analysis of Microbial Community in Circulating Cooling Water System of Coal Power Plant during Reagent Conversion

The use of phosphorus-containing chemical corrosion and scale inhibitors has been found to result in excessive phosphorus discharge and an inability to reduce the high concentration of CODcr in the circulating cooling water, thereby making it challenging to comply with increasingly stringent sewage discharge standards. This study aims to assess the practicality of utilizing biological corrosion and scale inhibitors in coal power plants’ operation, as well as investigating the correlation between water quality indicators and microbial communities during the conversion period. The data illustrates that, in comparison to the chemical method, there is a decrease in turbidity of the circulating water from 19.44 NTU to 9.60 NTU, a reduction in CODcr from 71.55 mg/L to 45.47 mg/L, and a drop in TP from 2.35 mg/L to 0.38 mg/L. Microbial community analysis during the transition period reveals that microorganisms rapidly establish a new equilibrium in the circulating water, sediment, and fiber ball, resulting in significantly different microbial community structures. The relative abundance of corrosive microorganisms such as Flavobacterium, Pedomicrobium, and Hydrogenophaga is significantly diminished in the circulating water, whilst the abundance of anaerobic microorganisms like Anaerolineaceae and Rhodopseudomonas in the sediment also declines. Conversely, there is an increased presence of microorganisms associated with contaminant degradation, such as CL500-3 and SM1A02. These findings suggest a decrease in the risk of system corrosion and an enhancement in contaminant degradation capability. This study provides evidence supporting the replacement of chemical agents with biological agents in circulating cooling water systems, contributing to more environmentally friendly and sustainable practices.

Effect of macronutrients (carbon, nitrogen, and phosphorus) on the growth of Chlamydomonas reinhardtii and nutrient recovery under different trophic conditions.

More stringent discharge standards have led to the development of an alternative nutrient recovery system from wastewater. Microalgae cultivation in wastewater treatment works has presented considerable promise from the perspective of sustainable resource management. Growth kinetics models are useful tools to optimize nutrient recovery from wastewater by algal uptake. Therefore, this research aims to identify the growth kinetics of Chlamydomonas reinhardtii under both heterotrophic and phototrophic conditions with different nutrient concentrations that typify those found in wastewater treatment works. In addition, the effects of macronutrients (C, N, and P) on heterotrophic and phototrophic microalgae growth and nutrient recovery were studied. Greater specific growth rates were achieved under heterotrophic conditions than in phototrophic cultivation. The maximum specific growth rates and nutrient recovery efficiencies were achieved at 5 mg P L-1 under both heterotrophic and phototrophic growth conditions. Nitrate was the preferred form of nitrogen source under heterotrophic conditions, while nitrogen sources did not present any significant influences in the phototrophic cultivation. Specific growth rates reported for both heterotrophic and phototrophic microalgae at lower carbon concentrations (3.10 d-1 and 0.46 d-1, sequentially) were higher than those at higher carbon concentrations (1.95 d-1 and 0.22 d-1, respectively). C. reinhardtii presented an extreme capacity to adapt and grow at all experimental conditions tested in heterotrophic and phototrophic cultivations.

Nanoscale Zero-Valent Iron (nZVI) for Heavy Metal Wastewater Treatment: A Perspective

Industries such as non-ferrous metal smelting discharge billions of gallons of highly toxic heavy metal wastewater (HMW) worldwide annually, posing a severe challenge to conventional wastewater treatment plants and harming the environment. HMW is traditionally treated via chemical precipitation using lime, caustic, or sulfide, but the effluents do not meet the increasingly stringent discharge standards. This issue has spurred an increase in research and the development of innovative treatment technologies, among which those using nanoparticles receive particular interest. Among such initiatives, treatment using nanoscale zero-valent iron (nZVI) is one of the best developed. While nZVI is already well known for its site-remediation use, this perspective highlights its application in HMW treatment with metal recovery. We demonstrate several advantages of nZVI in this wastewater application, including its multifunctionality in sequestrating a wide array of metal(loid)s (> 30 species); its capability to capture and enrich metal(loid)s at low concentrations (with a removal capacity reaching 500 mg·g−1 nZVI); and its operational convenience due to its unique hydrodynamics. All these advantages are attributable to nZVI’s diminutive nanoparticle size and/or its unique iron chemistry. We also present the first engineering practice of this application, which has treated millions of cubic meters of HMW and recovered tons of valuable metals (e.g., Cu and Au). It is concluded that nZVI is a potent reagent for treating HMW and that nZVI technology provides an eco-solution to this toxic waste.

Coupled pyrite and sulfur autotrophic denitrification for simultaneous removal of nitrogen and phosphorus from secondary effluent: feasibility, performance and mechanisms

The discharge standards of nitrogen (N) and phosphorus (P) in wastewater treatment plants (WWTPs) have become increasingly strict to reduce water eutrophication. Further reducing N and P in effluent from municipal WWTPs need to be achieved effectively and eco-friendly. In this study, a carbon independent pyrite and sulfur autotrophic denitrification (PSAD) system using pyrite and sulfur as electron donor was developed and compared with pyrite autotrophic denitrification (PAD) and sulfur autotrophic denitrification (SAD) systems through batch and continuous flow biofilter experiments. Compare to PAD and SAD, PSAD was more effective in simultaneous removal in N and P. At hydraulic retention time (HRT) 3 h, average effluent concentrations of total nitrogen (TN) and total phosphate (TP) of 1.40 ± 0.03 and 0.19 ± 0.02 mg/L were achieved when treating real secondary effluent with 20.65 ± 0.24 mg/L TN and 1.00 ± 0.24 mg/L TP. The improvement in simultaneous removal of N and P was attributed to the coupling of PAD and SAD in enhancing the transformation of sulfur and iron and enlarging the reaction zone in the pyrite and sulfur autotrophic denitrification biofilter (PSADB) system. Therefore, more biomass was accumulated and the microbial denitrification functional stability, including electrons transfer and consumption was enhanced on the surface of pyrite and sulfur particles in the PSADB system. Moreover, autotrophic denitrifiers (Thiobacillus and Ferritrophicum), sulfate-reducing bacteria (Desulfocapsa) and iron reducing bacteria (Geothrix), acting as contributors to microbial nitrogen, sulfur and iron cycle, were specially enriched. In addition, the leaching of iron ions was promoted, which facilitated the removal of phosphate in the form of Fe3(PO4)2·8H2O and Fe3PO4. PSADB has proven to be an efficient technology for simultaneous removal of N and P, which could meet increasingly stringent discharge standards effectively and eco-friendly.

Optimal pathways for upgrading China's wastewater treatment plants for achieving water quality standards at least economic and environmental cost

Wastewater treatment plants (WWTPs) in China must be upgraded to meet new discharge standards, but this incurs both economic and environmental costs and benefits. To select the optimal upgrade pathway, we developed ten upgrade paths based on two common decision-making scenarios for WWTP upgrade in developing countries. Using model simulation, life-cycle assessment, life-cycle cost, and multiple-attribute decision-making, we incorporated the full costs and benefits associated with the construction and operation into the decision-making process. We used a weighting scheme of attributes for the three regions and ranked the upgrade paths using the technique for order preference by similarity to an ideal solution (TOPSIS). The results showed that constructed wetlands and sand filtration were advantageous in terms of lower economic costs and environmental impacts, while the denitrification filter pathways required less land. Optimal pathways differed by region, highlighting the importance of a detailed and integrated assessment of the costs and benefits of WWTP upgrade options over the full life cycle. Our findings can inform decision-making on upgrading China's WWTPs to meet stringent discharge standards and protect inland water and coastal environments.

Advanced treatment of low-pollution and poor biodegradability sewage by combined process

A system was developed for the treatment of low-pollution sewage with poor biodegradability and a low ratio of chemical oxygen demand (CODCr) to nitrate-nitrogen (COD/NO3−-N (C/N)) ratio to achieve high efficiency. This system combines the anaerobic-oxic (AO) process with the biological aerated filter (BAF) and introduces the Fenton advanced oxidation process. A pilot test system was established, and feasibility investigations were conducted using a continuous flow of 500 L/h. The results showed that the optimal conditions for removing nitrate from sewage were achieved with a C/N ratio of 5.5–6 and a hydraulic retention time (HRT) of 4 h. Under these conditions, the average nitrate removal efficiency was 83.36%, and the average concentration of total nitrogen (TN) in the effluent was reduced to 6.68 mg/L. Additionally, the Fenton oxidation treatment was effective in reducing the COD content of sewage, with an average removal efficiency of 82.47% and a decrease in CODCr from 55.24 mg/L to 9.68 mg/L. The combined process successfully met the stringent discharge standards after advanced treatment. Microbial community analysis revealed that the dominant genera in anoxic BAFs were Trichococcus, Comamonadaceae, Lentimicrobium, and Thauera, while the dominant genera in aerobic BAFs were mainly Acinetobacter, Thiothrix, Gemmobacter, and Caldilineaceae. The synergistic effect of various nitrifying and denitrifying bacteria in BAFs effectively removed nitrate from the sewage. Furthermore, GC-MS analysis indicated that the Fenton oxidation significantly reduced the concentration of refractory organic compounds in the sewage, including long-chain organic compounds and acrylic acid maleic acid copolymer (MA/AA). After upgrading and renovating the factory's sewage treatment facilities, the impact of the discharged effluent on the receiving drainage basin will be minimized.

Comparison of Soil Nitrate and Phosphorus Concentrations Prior to and Five to Eleven Years after Recycled Water Irrigation

Increasing demand on water supplies in western US and more stringent wastewater discharge standards have made recycled water a common water source for irrigating urban green spaces. Studies are needed to evaluate nitrate-N leaching potential and phosphorus (P) movement along soil profile when recycled wastewater is used for irrigation. We collected and analyzed soil samples at the commencement and 5 and 11 years after recycled water irrigation on 2 golf courses, 5 metropolitan parks, and 1 school ground. Samples were taken at 0-20, 20-40, 40-60, 60-80, and 80-100 cm depths on golf courses and at 0-20 and 20-40 cm depths at other locations. Soil samples were tested for soil pH, soil nitrate, and AB-DTPA extracted P level. No increase in soil nitrate-N was observed over 5 and 11 years with recycled water irrigation, suggesting leaching of nitrogen to the groundwater was not a great concern. Soil P concentration at the surface soil depth was the highest 11 years after recycled water irrigation. Moreover, soil P increasing below the surface layer was observed on sites with sandy soil, suggesting long-term recycled water irrigation could impose some risk for P leaching on sandy soil.

Assessing groundwater recharge rates, water quality changes, and agricultural impacts of large-scale water recycling

The over-exploitation and insufficient replenishment of groundwater (GW) have resulted in a pressing need to conserve freshwater and reuse of treated wastewater. To address this issue, the Government of Karnataka launched a large-scale recycling (440 million liters/day) scheme to indirectly recharge GW using secondary treated municipal wastewater (STW) in drought-prone areas of Kolar district in southern India. This recycling employs soil aquifer treatment (SAT) technology, which involves filling surface run-off tanks with STW that intentionally infiltrate and recharge aquifers. This study quantifies the impact of STW recycling on GW recharge rates, levels, and quality in the crystalline aquifers of peninsular India. The study area is characterized by hard rock aquifers with fractured gneiss, granites, schists, and highly fractured weathered rocks. The agricultural impacts of the improved GW table are also quantified by comparing areas receiving STW to those not receiving it, and changes before and after STW recycling were measured. The AMBHAS_1D model was used to estimate the recharge rates and showed a tenfold increase in daily recharge rates, resulting in a significant increase in the GW levels. The results indicate that the surface water in the rejuvenated tanks meets the country's stringent water discharge standards for STW. The GW levels of the studied boreholes increased by 58–73 %, and the GW quality improved significantly, turning hard water into soft water. Land use land cover studies confirmed an increase in the number of water bodies, trees, and cultivated land. The availability of GW significantly improved agricultural productivity (11–42 %), milk productivity (33 %), and fish productivity (341 %). The study's outcomes are expected to serve as a role model for the rest of Indian metro cities and demonstrate the potential of reusing STW to achieve a circular economy and a water-resilient system.

Nitrogen recovery from wastewater as nitrate by coupling mainstream ammonium separation with side stream cyclic up-concentration and targeted conversion

A sustainable strategy to recover nitrogen as nitrate by ammonium ion exchange and regeneration (AIR) from low-strength domestic wastewater was proposed to reduce nitrogenous pollutants discharge and alleviate nitrogen loss from atmosphere. In the AIR module, ammonium is rapidly swapped by ion exchangers from the wastewater, and then converted and concentrated as nitrate by cyclic regeneration and side stream nitrification. Nitrate is further concentrated by electrodialysis, and recovered as NaNO3. Then chemically enhanced primary treatment, AIR and membrane bioreactor were coupled to construct an efficient and compact Chemically enhanced primary treatment - Ammonium Ion exchange and Regeneration - Membrane bioreactors (CAIRM) process for nitrogen recovery and synchronous pollutants removal from domestic wastewater under stringent discharge standards. Pilot-scale CAIRM efficiently removed pollutants (nitrogen removal of 93.7%) and recovered 77.1% ammonium during long-term operation without damage to performance of ion exchangers. Compared with the anaerobic/anoxic/aerobic process, CAIRM process reduced land occupancy by 70.1%, decreased upfront investment and treatment cost by 29.3% and 40.7% after resource recovery, and cut greenhouse gas emission by 52.5%. Application of the proposed strategy is expected to save energy of nitrate production and convert global nitrogen circular pollution to circular economy.

Lanthanum oxide-based nanomaterials synthesized by using carbon aerogel template for efficient phosphate capture

Phosphate is a chemical compound that contains phosphorus, an essential nutrient for the growth of all organisms on Earth. However, the overuse of fertilizers has created a serious environmental problem worldwide, because the discharge of wastewater containing excess phosphate leads to eutrophication. Therefore, proper management of phosphate from wastewater is very important. In this study, lanthanum oxide-based nanomaterials (LONs) were synthesized by a hard-template method using porous carbon aerogels, and their potential as adsorbents for efficient phosphate removal was evaluated. The use of carbon aerogels offers facile production of porous adsorbents because simple calcination under air can remove the template, which is more suitable for environmental and industrial applications than the use of toxic chemicals for conventional silica template removal. The morphology and porosity of the LONs were controlled by adjusting the synthesis conditions, including calcination and oxidation. The phosphate removal capacity of LONs strongly depends on the porous structure and surface area because the adsorption process is governed by monolayer adsorption of phosphate-forming lanthanum phosphate. LONs with an optimized structure exhibited a high adsorption capacity of 135.14 mg P g−1 at 28 °C (qm by the Langmuir isotherm model). LON adsorbents have great potential for efficient phosphate removal to meet increasingly stringent discharge standards and to prevent the pollution of water resources.

A comprehensive assessment of upgrading technologies of wastewater treatment plants in Taihu Lake Basin

To meet the increasingly stringent discharge standards of wastewater treatment plants (WWTPs) in the Taihu Lake Basin, the Chinese government successively established the National Special Water Project Program to develop new technologies to retrofit and upgrade existing wastewater treatment processes during the 11th, 12th, and 13th Five-Year Plans. However, there is a lack of systematic sorting of the existing research outcomes, and thus hinders the application and promotion of the upgrade technologies. Based on the outcomes of the National Special Water Project and a field survey, this research analyzed the current status of wastewater treatment in the Taihu Lake Basin and systematically integrated the retrofitting measures of WWTPs in terms of achieving the Grade IA of the national standard and local stricter discharge standards (DB 32/1072–2018 and DB 33/2169–2018). In particular, the boundary conditions, design parameters, specific recommendations of the technologies, and some typical engineering cases were provided accordingly. Finally, this study discussed the future development directions of WWTPs during the upgrade process from the perspective of carbon neutrality and digitalization. The present work will hopefully assist in retrofitting and constructing WWTPs to achieve the stricter effluent discharge criteria and help optimize the design and construction of WWTPs in the best way.

Deep purification of copper from Cu(II)-EDTA acidic wastewater by Fe(III) replacement/diethyldithiocarbamate precipitation

Cu(II)-EDTA is a highly stable typical metal-organic complex in a wide pH range (3.0–12.0) and it is difficult to deeply purify Cu(II) by conventional precipitation methods. In this study, Fe(III) replacement/diethyldithiocarbamate (DDTC) precipitation combined process is proposed as a promising strategy to achieve the deep purification of Cu(II) from Cu(II)-EDTA acidic wastewater. The underlying mechanism has also been systematically elucidated by chemical equilibriums, experiments, and density functional theory (DFT) calculations, laying a foundation for the development and application. Chemical equilibriums show that Fe(III) replacement favors the stoichiometric release of Cu(II) from Cu(II)-EDTA and the formation of Fe(III)-EDTA complex under acidic conditions. Experimentally, Cu(II) is removed (over 99.99%) and deeply purified (under 0.008 mg/L) under the optimal conditions, which is lower than the most stringent discharge standards of copper ions in electroplating effluent (<0.5 mg/L, China). DFT calculations reveal that DDTC could further precipitate the released free copper ions via the carbon disulfide (–C(=S)–S) chelating group while exhibiting a slight effect on the Fe(III) in Fe(III)-EDTA. Considering these results, the electronic structures of Cu(II) and Fe(III), as well as their interaction with EDTA and DDTC ligands, are discussed to understand the mechanism of Fe(III)/DDTC process. By introducing a low dosage of Fe(III), the DDTC could efficiently purify Cu(II) from the Cu(II)-EDTA acid wastewater and realize the near-zero discharge of metal pollutants in metal-organic complex wastewater. It is believed that the main findings may benefit the water pollution reduction and comprehensive recycling of metal resources.

Nitrogen Advanced Treatment of Urban Sewage by Denitrification Deep-Bed Filter: Removal Performance and Metabolic Pathway.

This study aimed to explore the performance of denitrification deep-bed filter (DN-DBF) to treat municipal sewage for meeting a more stringent discharge standard of total nitrogen (TN) (10.0 mg L–1). A lab-scale DN-DBF was conducted to optimize operation parameters and reveal the microbiological mechanism for TN removal. The results showed that more than 12.7% TN removal was obtained by adding methanol compared with sodium acetate. The effluent TN concentration reached 6.0–7.0 mg L–1 with the optimal influent carbon and nitrogen ratio (C/N) and hydraulic retention time (HRT) (3:1 and 0.25 h). For the nitrogen removal mechanism, Blastocatellaceae_Subgroup_4 and norank_o_JG30-KF-CM45 were dominant denitrification floras with an abundance of 6–10%. Though large TN was removed at the top layer of DN-DBF, microbial richness and diversity at the middle layer were higher than both ends. However, the relative abundance of nitrite reductase enzymes (EC1.7.2.1) gradually increases as the depth increases; conversely, the relative abundance of nitrous oxide reductase gradually decreased.

Achieving biological nutrient removal in an old sewage treatment plant through process modifications – A simulation and experimental study

Modifying old sewage treatment plants (STP) to meet revised stringent discharge standards is of great concern. Process modeling can serve as an essential tool for this. This work reports a BioWin©-activated sludge model/anaerobic digestion for a 55 million litres per day STP located in South India. Fifty state variables and eighty process equations were used in the model to achieve biological nutrient removal. The model was calibrated and was validated with plant data. The proposed modification was to incorporate an intermediate virtual anoxic zone to achieve simultaneous nitrification-denitrification and total dissolved phosphorus (TDP as PO4) removal. Several scenarios, namely, different hydraulic residence time (HRT) between 1.5 to 2.5 h, dissolved oxygen (DO) between 2.5 to 4.5 mg/L, and mixed liquor suspended solid (MLSS) between 2500 to 4500 mg/L in the bioreactor, were studied to identify optimum conditions. The optimum DO and MLSS levels were identified as 4 mg/L and 4000 mg/L, respectively. The optimum HRT of 2 h in aeration zone, 1 h in anoxic, and 3 h in reaeration zone was identified. Implementation of these modifications in the STP, with minimal operational interventions and no capital costs, improved its performance as predicted by the model. The total nitrogen and TDP (as PO4) reduced from 20 mg/L to 8 mg/L and 3.5 mg/L to 0.9 mg/L, respectively, and met the revised discharge standards. This intervention gave a cost saving of approximately 5.6 million USD.