An anoxic-aerobic system combined with integrated vertical-flow constructed wetland to highly enhance simultaneous organics and nutrients removal in rural China.

The biggest problem in the treatment of rural domestic sewage is that the existing treatment projects require the big investment and the high operation and maintenance costs. To overcome this problem, cost-effective, low-consuming, resource-recovering and easy-maintenance technologies are urgently demanded. To this end, a novel anoxic-aerobic system combined with integrated vertical-flow constructed wetland (IVFCW) with source separation was proposed for treating rural sewage in this study. The anoxic-aerobic system contained the anoxic filter (ANF), two-stage waterwheel driving rotating biological contactors (ts-WDRBCs). Key parameters of ts-WDRBCs were identified to be 0.6 m drop height and 4 r/min rotational speed found on oxygenated clean water experiments. Then, the optimal operating parameters were determined to be 200% reflux ratio and 3 h hydraulic retention time of ts-WDRBCs. During the 80-day operation, 91.58 ± 1.86% COD, 96.17 ± 0.92% NH4+-N, 82.71 ± 3.92% TN and 92.28 ± 2.78% TP were removed under the optimal operating parameters. Compared with other treatment technologies, this combined bio-ecological system could achieve the higher simultaneous organics and nutrients removal. The effluent NO3--N/NH4+-N concentration ratio of ts-WDRBCs was 2.15 ± 0.54, which was proved to be beneficial for plants growth. The microbial communities coexisted in each section ensured the desired removal performance of combined bio-ecological system. Summarily, high performance together with low investment costs and cheap operation costs are characteristics that make this system a promising and competitive alternative for rural sewage treatment.

Achieving phosphorus recovery at pilot-scale anaerobic anoxic/nitrifying-induced crystallization (A2N-IC) process: Performance, assessment, and challenges.

A pilot-scale anaerobic-anoxic/nitrifying/induced crystallization (A2N-IC) process was established for phosphorus (P) recovery and nutrient removal from municipal wastewater with a treatment capacity of 80 m3d-1. Results show that the A2N-IC process can operate stably on a pilot scale; the recovery efficiency of influent P reached 62.2%, and the total P removal efficiency of the IC section was 65.4%. The IC section had little effect on the removal of chemical oxygen demand (COD) and nitrogen (N), and the P removal efficiency was improved. Soluble non-reactive P (sNRP) was the key factor affecting P recovery efficiency. Although P recovery increases the construction and maintenance costs, the process can be profitable if a market for P recovery products is established. To improve the P recovery efficiency, attention should be paid to the effects of sNRP and dissolved organic matter (DOM) on P recovery, and P-rich sludge should be considered.

Mining phosphorus from waste streams at wastewater treatment plants: a review of enrichment, extraction, and crystallization methods.

Two interrelated problems exist: the non-renewability of phosphate rock as a resource and the excess phosphate in the water system lead to eutrophication. Removal and recovery of phosphorus (P) from waste streams at wastewater treatment plants (WWTPs) is one of the promising solutions. This paper reviews strategies for P recovery from waste streams in WWTPs are reviewed, and the main P recovery processes were broken down into three parts: enrichment, extraction, and crystallization. On this basis, the present P recovery technology was summarized and compared. The choice of P recovery technology depends on the process of sewage treatment and sludge treatment. Most P recovery processes can meet the financial requirements since the recent surge in phosphate rock prices. The safety requirements of P recovery products add a high cost to toxic substance removal, so it is necessary to control the discharge of toxic substances such as heavy metals and persistent organic pollutants from the source.

Kinetics and capacities of non-reactive phosphorus (NRP) sorption to crushed autoclaved aerated concrete (CAAC).

With growing interest in resource recovery and/or reuse, waste materials have been considered a promising alternative for phosphorus (P) adsorption because they are low-cost and easily accessible. Crushed autoclaved aerated concrete (CAAC), as representative construction waste, has been extensively studied for P removal in ecological technologies such as treatment wetlands. However, most of the previous studies focused on the adsorption of orthophosphate, namely reactive phosphorus, and lacked attention to non-reactive phosphorus (NRP) which is widely present in sewage. This study presents the first investigation on the potential and mechanism of CAAC removing four model NRP compounds. Adsorption isotherm and kinetics of NRP onto CAAC indicate that the removal of NRP was a chemisorption process and also involved a two-step pore diffusion process. The desorption experiment shows that different NRP species showed varying degrees of desorption. Most NRP was irreversibly adsorbed on CAAC. Among the model compounds considered in this study, the adsorption capacity and hydrolysis rate of organophosphorus were much less than that of inorganic phosphorus. Moreover, the adsorption of different NRP species by CAAC in the mesocosm study was different from the results of laboratory adsorption experiments, and the possible biodegradation was essential for the conversion and removal of NRP. The findings confirmed the validity of CAAC for NRP removal and the potential advantages of CAAC in terms of costs and environmental impact. This study will contribute to a better understanding of NRP conversion and environmental fate and that can be the basis for a refined risk assessment.

Transformation and fate of non-reactive phosphorus (NRP) in enhanced biological phosphorus removal process with sidestream phosphorus recovery.

Recovery of phosphorus (P) from wastewater can help establish a new P cycle. However, there are many P forms in wastewater, not always in reactive forms, which are the most suitable for direct recovery. The enhanced biological phosphorus removal process with sidestream phosphorus recovery (EBPR-SPR) is an effective way to remove and recover P resources in wastewater, but there is a lack of research on the transformation and fate of non-reactive phosphorus (NRP) in it. This study selected four model NRP to investigate their transformation and fate in an EBPR-SPR process. The transformation of NRP in pure water and activated sludge under anaerobic and aerobic conditions were compared. The effects of Ca/P ratio and pH on NRP recovery were studied, and the recovery products of NRP were characterized. It was found that NRP containing phosphoanhydride and phosphoester bonds were more easily hydrolyzed to reactive P (RP) than that containing PC bonds. NRP will be adsorbed and accumulated by activated sludge, and activated sludge will accelerate the conversion of NRP to RP. Tripolyphosphate can form complex precipitation with Ca2+. When multiform P co-existed, Ca2+ preferably complexed with polyphosphate, which harmed RP recovery. The conversion of NRP should be strengthened to recover more P in wastewater. The effect of NRP should be considered when recovering P from wastewater.

Enhanced Nutrient Removal in A2N Effluent by Reclaimed Biochar Adsorption.

The excessive nitrogen and phosphorus discharged into the water environment will cause water eutrophication and thus disrupt the water ecosystem and even exert biological toxicities. In this study, the absorption removal of nitrogen and phosphorus from the anaerobic tank in an anaerobic−anoxic/nitrifying system using four different kinds of biowaste-reclaimed biochars were investigated and compared. The effects of temperature and pH on nutrient adsorption removal were further investigated. The four kinds of biochar were successfully prepared and well characterized using a scanning electron microscope, fourier transform infrared spectroscopy, X-ray diffraction and Brunner−Emmet−Teller methods. Generally, there was no significant change in chemical oxygen demand (COD) and NH4+-N removal efficiencies when treated by the different biochars, while the activated sludge biochar (ASB) displayed the highest total phosphorus (TP) removal efficiency. The initial TP concentrations (<40 mg/L) displayed no remarkable effects on the TP adsorption removal, while the increase of temperature generally enhanced TP and NH4+-N adsorptions on the ASB. Besides, the increase of pH significantly promoted NH4+-N removal but depressed TP removal. Moreover, the adsorption process of TP by the ASB complies with the secondary kinetic model, suggesting the chemical precipitation and physical electrostatic interaction mechanisms of TP adsorption removal. However, the adsorption of NH4+-N conformed to the inner-particle diffusion model, indicating that the NH4+-N adsorption was mainly involved with pore diffusions in the particles.

Effects of dissolved organic matter on phosphorus recovery via hydroxyapatite crystallization: New insights based on induction time.

Recovery of phosphorus from sewage can help establish a new phosphorus cycle and hydroxyapatite (HAP) crystallization is a promising way. HAP crystallization is an amorphous calcium phosphate (ACP) mediated process, and its induction time reflects the rate of HAP nucleation, and seriously affects the efficiency of phosphorus recovery. In this study, the effects of different types of dissolved organic matter (DOM) on the induction time and phosphorus recovery performance of ACP-mediated HAP phosphorus recovery were studied, and the mechanism was analyzed by X-Ray Diffraction, Fourier transform infrared spectroscopy, and scanning electron micrograph with energy dispersive spectrometry. The results show that DOM greatly prolongs the induction time of ACP-mediated HAP crystallization and leads to an increase in the yield of microcrystals, thus leading to a decrease in phosphorus recovery efficiency. DOM inhibits ACP-mediated HAP crystallization by complexing lattice ions and occupying active growth sites on the crystal surface. Pre-removal of DOM can not only improve the speed and efficiency of phosphorus recovery by the HAP crystallization process but also improve product quality.

Evaluation of phosphorus removal in floating treatment wetlands: New insights in non-reactive phosphorus.

Excess phosphorus (P) in surface runoff has significant deleterious impacts on water quality through eutrophication. Commonly, P is transported via non-point pollution and the proportion of easily plant-available reactive P (RP) among other P forms may vary significantly. Non-reactive P (NRP) can potentially contribute to the eutrophication of waterbodies, however the cleavage into bio-available P forms and eventually their biological uptake remains uncertain. This holds also true for floating treatment wetlands (FTWs) which became established as nutrient mitigation measures for surface waters in recent years. However, little information is available about the conversion and removal of NRP in FTWs. In this study, the conversion and removal of different forms of P in FTWs were investigated. Experiments were operated in batch mode and treatments consisted of (1) two concentration levels: a high P concentration of 3.0 mg/L and a low P concentration of 1.0 mg/L, and (2) four mesocosm treatments: (a) artificial roots only, (b) substrates only, (c) plants only, (d) plants and substrates. The results showed that RP removal mainly depended on sedimentation, substrate sorption, and biological assimilation. The removal of NRP mainly depended on hydrolysis, microbial-mediated conversion, and biological absorption. The combination of plant and substrate provided stable and efficient phosphorus removal performance in high P conditions, while plants were important for P removal in low P conditions. Living plants were indispensable and greatly affected the performance of FTWs. The specific enrichment and culling of microorganisms by plants resulted in the formation of specific rhizosphere microbial communities and promoted the removal of NRP. Pseudomonas, Enterobacter, Acidovorax might be responsible for P mineralization in the FTWs. Comprehensive analysis indicated that the conversion and removal pathways of P in the FTWs were not mutually independent, and the plant-microbe-substrate interactions cannot be underestimated.

Recent developments and applications of floating treatment wetlands for treating different source waters: a review.

Most water bodies around the world suffer from pollution to varying degrees. Floating treatment wetlands (FTWs) are a simple and efficient ecological treatment technology and have been widely studied and applied as a sustainable solution for different source waters. Based on the analysis of abundant literature in the last ten years, this paper systematically reviews the history and the latest development of FTWs. Meanwhile, the treatment performance and pollutant removal mechanisms of FTWs on the natural water, stormwater, domestic wastewater, industrial wastewater, and agricultural runoff are analyzed. In particular, very interesting information is provided, such as water depth, water surface coverage, the ratio of dissolved to total phosphorous (DRP/TP), the ratio of nitrogen to phosphorous (N/P), BOD/COD ratio, and its effects on the efficiency and removal mechanisms of FTWs. This information will provide useful references and guidance for optimizing the design of FTW and pollutant treatment efficiency of different source waters. This paper also provides an objective review of the limitations of FTWs. Subsequently, the enhancements of FTW technology which are recognized to be effective, including aeration, adding functional fillers or obligate degrading bacteria, and construction of hybrid FTWs, are summarized and recommendations are made for further research.

Characteristics of rural domestic wastewater with source separation.

Rural domestic wastewater (RDW), one of the non-point pollution sources, has become a significant object related to sanitation improvement and water pollution control in Taihu Lake Basin, China. Current research on RDW characteristics and management with source separation is limited. In this study, a source-separated investigation into the characteristics of RDW was conducted, and the management suggestions were proposed. The results showed that the average RDW production coefficient was 94.1 ± 31.6 (range: 71.8-143.0) liters per capita (person) per day. Household-level wastewater generation peaked two or three times daily, and the synchronous fluctuation could cause hydraulic loading shocks to treatment facilities. The population equivalents of chemical oxygen demand, ammonium nitrogen (NH4+-N), total nitrogen (TN), and total phosphorus (TP) in RDW were 78.7, 3.7, 4.12, and 0.8 g/(cap·d), respectively. Blackwater from water closet source accounted for 30.4% of the total wastewater amount, contributing 93.0%, 81.7%, and 67.3% to loads of NH4+-N, TN, and TP, respectively. Graywater from the other sources with low nutrient-related pollutant concentrations and loads, accounting for 69.6% of the total wastewater amount, was a considerable alternative water resource. The quantitative and qualitative characteristics indicated that GW and BW had the potential of being reused in relation to water and nutrients, respectively.

Nitrous oxide emission during denitrifying phosphorus removal process: A review on the mechanisms and influencing factors.

Excessive emissions of nitrogen (N) and phosphorus (P) pollutants are leading to increased eutrophication of water bodies. Biological N and P removal processes have become a research priority in the field of sewage treatment with the aim of improving sewage discharge standards in countries worldwide. Denitrifying P removal processes are more efficient for solving problems related to carbon source competition, sludge age conflict, and high aeration energy consumption compared to traditional biological N and P removal processes, but they are easy to produce nitrous oxide (N2O) in the process of sewage treatment. N2O is a greenhouse gas with a global warming potential approximately 190-270 times that of CO2 and 4-21 times that of CH4, which was produced and released into the environmental in denitrifying P removal systems under conditions of a low C/N ratio, high dissolved oxygen, and low activity of denitrifying phosphorus accumulating organisms (DPAOs). This paper reviews the emission characteristics and influencing factors of N2O during denitrifying P removal processes and proposes appropriate strategies for controlling the emission of N2O. This work serves as a basis for the development of new sewage treatment processes and the reduction of greenhouse gas emissions in future wastewater treatment plants.

Review: recent developments of substrates for nitrogen and phosphorus removal in CWs treating municipal wastewater.

Substrates are the main factor influencing the performance of constructed wetlands (CWs), and especially play an important role in enhancing the removal of nitrogen and phosphorus from CWs. In the recent 10 years, based on the investigation of emerged substrates used in CWs, this paper summarizes the removal efficiency and mechanism of nitrogen and phosphorus by a single substrate in detail. The simultaneous removal efficiency of nitrogen and phosphorus by different combined substrates is emphatically analyzed. Among them, the reuse of industrial and agricultural wastes as water treatment substrates is recommended due to the efficient pollutant removal efficiency and the principle of waste minimization, also more studies on the environmental impact and risk assessment of the application, and the subsequent disposal of saturated substrates are needed. This work serves as a basis for future screening and development of substrates utilized in CWs, which is helpful to enhance the synchronous removal of nitrogen and phosphorus, as well as improve the sustainability of substrates and CWs. Moreover, further studies on the interaction between different types of substrates in the wetland system are desperately needed.

Comparison of floating-bed wetland and gravel filter amended with limestone and sawdust for sewage treatment.

Advancements in the design and technology of constructed wetlands for efficient removal of wastewater contaminants are ever in progress to develop situation-based economical systems. Here, we entrenched two horizontal sub-surface flow constructed wetlands (HSFCW) with either chemical, viz. limestone (HSFCW-LS) or organic, viz. sawdust (HSFCW-SD) substrates, and compared them with biological method, viz. growing of water spinach in floating-bed-constructed wetland (FBCW-WS) to enhance the performance of CWs. Same sewage wastewater was used as influent in each fortified CW replicated thrice. Sewage was replaced weekly, for a total of 12 weeks of experimentation. Sampling of raw sewage from influent was undertaken at the inlet in the beginning, and that of treated effluent from the outlet after a week of treatments. Quality of raw sewage used weekly during experimentation remained almost uniform and near to the wastewater standards. Cumulative data of treated wastewater depicted that the FBCW-WS achieved the highest performance in the removal of total nitrogen (TN), [Formula: see text]-N, and total phosphorus (TP) with average removal efficiencies of 75.9, 90.5, and 94.3%, respectively. Whereas, HSFCW-SD performed better for [Formula: see text]-N, FC, and TSS with corresponding removal efficiency of 77.5, 64.3, and 74.2% while HSFCW-LS showed average performance. This study concludes that performance of biological method of macrophyte cultivation (FBCW-WS) is significantly superior to chemical and organic substrates, so it could be more effective, economical, and sustainable approach for sewage treatment.

Modeling and performance improvement of an anaerobic-anoxic/nitrifying-induced crystallization process via the multi-objective optimization method.

The trade-off between energy savings and emission reductions of an activated sludge process is a multi-objective problem relating to several potentially conflicting objectives. Therefore, the optimal modification of an anaerobic-anoxic/nitrifying/induced crystallization (A2N-IC) process by multi-objective optimization method was studied in this work. The multi-objective optimization model comprised three evaluative indices, (effluent quality (EQ), operation cost (OC), and total volume (TV) of structures), and 14 process parameters (decision variables) solving by non-dominated sorting genetic algorithm II (NSGA-II) in MATLAB. The trade-off relationships among EQ, OC, and TV were investigated under 30 days of dynamic influent with different constraint conditions. A series of Pareto solutions were obtained, and one Pareto solution was selected for further analysis. Results showed improved effluent concentrations of chemical oxygen demand (COD), total nitrogen (TN), ammonia-nitrogen (NH4+-N), and total phosphorous (TP) under the optimized strategy compared to the original strategy, where the average effluent concentrations decreased by 2.22, 0.47, 0.13, and 0.02 mg/L, respectively. The values of EQ and OC decreased from 0.015 kg/day and 0.15 ¥/m3 to 0.0023 kg/day and 0.12 ¥/m3, respectively, while the TV increased from 0.31 to 0.33 m3. These results indicated that the multi-objective optimization method is useful for modifying activated sludge processes.

Nitrogen and phosphorus removal processes under different aeration strengths in the principal-type tank of alternate multiple tanks system and process control.

The alternate multiple tanks (AMT) system was a new highly compact sewage treatment with cost-effective balance and automated management. Dissolved oxygen (DO) and oxidation-reduction potential (ORP) as monitoring indicators are closely linked to contaminants and controlling strategy. Thus, this paper was to investigate the N and P removal mechanisms and the real-time control under different aeration strengths through the principal-type tank. Results showed that N and P removal could reach an optimal balance when the air-water ratio (Rs) was 30 with 540L-air/h and 18L-water/h. Under such a condition, the TP, TN, ammonia and nitrate remained 0.4 mg/L (88.4% of removal efficiency), 5.0 mg/L (84.7%), 0.5 mg/L (98.4%) and 4.0 mg/L, respectively, during the discharge. However, restrained aeration reduced the P release and uptake efficiencies and retarded the ammonia oxidation and nitrification, and excessive aeration broke the balance of P release and uptake, prevented denitrification, and had no more effect on nitrification. Inadequate aeration caused residual DO low and insensitive to controlling critical points, while redundant aeration was of no extra effect compared to moderate aeration. An enhanced practical real-time response was obtained, where characteristic points of TP and nitrogen were sensitive to the DO, ORP and systematic manipulation.

Preparation and application of modified zeolites as adsorbents in wastewater treatment.

Natural zeolite has been recognized as a useful adsorbent for wastewater treatment for removing cations. Natural zeolite is a kind of porous material with large specific surface area but limited adsorption capacity. In recent years, emphasis has been given to prepare the surface modified zeolite using various procedures to enhance the potential of zeolite for pollutants. Modification treatment for zeolite can greatly change surface chemistry and pore structure. The article describes various modification methods of zeolite, and introduces the removal mechanisms of common pollutants such as ammonium, phosphorus and heavy metals. In addition, this review paper intends to present feasibility of applying modified zeolite to constructed wetlands which will be beneficial to achieve higher removal effect.

A comprehensive review of phosphorus recovery from wastewater by crystallization processes.

The presence of phosphorus (P) in discharged wastewater can lead to water pollution events and eutrophication. Given the increasing consumption of phosphate (PO43-) rocks, wastewater containing large quantities of P is deemed as a potential source of P recovery. Crystallization of P is an ideal way to recover P because of its simple design, ease of operation, high efficiency, and limited environmental impact. This paper provides a comprehensive review of P recovery by crystallization processes with respect to the mechanisms involved, operational parameters that influence the quality of the crystal, and available seed materials for inducing crystallization. Various operational parameters including pH, molar ratio of participating ions, mixing intensity, reactor type, and seeding conditions, were detailedly investigated. Different kinds of seeds were reviewed critically with regard to their principal properties, application, and long-term prospects. Crystallized products with a high P content can be used directly as slow-release fertilizers for agricultural production, and some test methods have been developed to determine their efficiency as a fertilizer and to evaluate their availability for plants. Further, the feasibility of P recovery by crystallization was evaluated in terms of economic benefits and environmental sustainability. This work serves as a basis for future research of P recovery by crystallization processes and responses to the increasingly stringent problems of eutrophication and the growing depletion of P resources.

Surface-flow constructed wetlands dominated by Cladophora for reclaiming nutrients in diffuse domestic effluent.

In this work, a surface-flow constructed wetland (SFCW) dominated by Cladophora was used to remove and reclaim nutrients in diffuse domestic effluent (DDE) discharged from rural regions around Taihu Lake, a eutrophic shallow lake in China. Growth rate of Cladophora was investigated and linked to ambient factors and nutrient consuming rates. The growth performances of Cladophora and animal-feed microbes were studied during the commissioning of SFCW. Results show that the growth rate of Cladophora was closely correlated with field temperature and surface irradiance, while surface coverage was suitable for the manipulation of SFCW. Harvest of Cladophora along with animal-feed microbes and removal of nutrients in DDE could be achieved by manipulating surface coverage to drag growth rate back at the end of linear zone and to quickly restore Cladophora biomass in the mid zone of surface growth rate. Among four stages of the commissioning, concentrating stage experienced the majority species of animal-feed microbes and maximal nutrient removal; during decomposing stage, however, the reproduction of animal-feed microbes and nutrient removal were lower, whereas the density of pathogens was higher.

Efficient model calibration method based on phase experiments for anaerobic-anoxic/nitrifying (A2N) two-sludge process.

A systematic calibration and validation procedure for the complex mechanistic modeling of anaerobic-anoxic/nitrifying (A2N) two-sludge system is needed. An efficient method based on phase experiments, sensitivity analysis, and genetic algorithm is proposed here for model calibration. Phase experiments (anaerobic phosphorus release, aerobic nitrification, and anoxic denitrifying phosphate accumulation) in an A2N sequencing batch reactor (SBR) were performed to reflect the process conditions accurately and improve the model calibration efficiency. The calibrated model was further validated using 30 batch experiments and 3-month dynamic continuous flow (CF) experiments for A2N-SBR and CF-A2N process, respectively. Several statistical criteria were conducted to evaluate the accuracy of model predications, including the average relative deviation (ARD), mean absolute error (MAE), root mean square error (RMSE), and Janus coefficient. Visual comparisons and statistical analyses indicated that the calibrated model could provide accurate predictions for the effluent chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total nitrogen (TN), and total phosphorus (TP), with only one iteration.

Effects of calcium on the performance, bacterial population and microbial metabolism of a denitrifying phosphorus removal system.

A sequencing batch reactor was operated to study the effects of influent Ca2+ on the efficiency, bacterial population, and microbial metabolism of denitrifying phosphorus removal system. Results showed that high Ca2+ loading (≥80mg/L) significantly inhibited the performance of simultaneous nitrogen and phosphorus removal. The abundance of phosphorus removal-related organisms (Dechloromonas and Candidatus Accumulibacter) decreased with increasing Ca2+ concentration from 20 to 140mg/L, while the abundance of glycogen-accumulating organisms and other bacteria increased. Metabolomic analyses revealed that the metabolic profiles of microbial community were also affected by high influent Ca2+ concentrations. 3-Hydroxybutyrate, acetate, alanine, and glutamate were the main differentiated metabolites in the system. An accumulation of amino acids and a reduction of nucleotides and amines were important response to high Ca2+ loading. Long-term Ca2+ loading had a reversible effect on the denitrifying phosphorus removal system as it could revive after a 50-day recovery process.