Enhanced biological nitrogen removal in MLE combined with post-denitrification process and EF clarifier

Insight into correlation of advanced nitrogen removal with extracellular polymeric substances characterization in a step-feed three-stage integrated anoxic/oxic biofilter system

As a core component of the biomass, the important role of extracellular polymeric substances (EPS) on treatment performance has been recognized. However, the comprehensive understanding of its correlation with nitrogen removal remains limited in biofilm-based reactors. In this study, the relevance between EPS and advanced nitrogen removal in a novel step-feed three-stage integrated anoxic/oxic biofilter (SFTIAOB) was specifically investigated. The operation showed as high as 81% TN removal was achieved under optimal conditions. Among the whole reactor, 2nd anoxic (A2) zone was the largest contributor for nitrogen removal, followed by the 3rd anoxic (A3) and 2nd oxic (O2) zones. EPS composition analysis found that high content of polysaccharides in tightly bound-EPS (A2 and A3) and protein in loosely bound-EPS and tightly bound-EPS (O2). Fourier transform infrared spectroscopy, three-dimensional fluorescence spectrum further verified stratified EPS subfractions containing different secondary protein structures, while 3-turn helix and tryptophan-like protein was the main reason for nitrogen removal. High-throughput sequencing revealed the co-existence of nitrogen removal-associated genera accomplished nitrification/denitrification combined with aerobic denitrification and anammox. Moreover, the correlation of EPS and microbial composition with nitrogen removal was clarified by redundancy analysis (RDA). Finally, potential mechanism for nitrogen removal was illuminated. This research gives more insight into EPS characteristics in enhancing nitrogen removal during the operation and optimization of a step-feed multi-stage A/O biofilm process.

A continuous plug-flow anaerobic/aerobic/anoxic/aerobic (AOAO) process treating low COD/TIN domestic sewage: Realization of partial nitrification and extremely advanced nitrogen removal

The realization of stable partial nitrification and advanced nitrogen removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes treating domestic sewage. Herein, a novel anaerobic/aerobic/anoxic/aerobic (AOAO) continuous plug-flow reactor, characterized with double sludge reflux and a bypass of anaerobic mixed liquor conveyed to anoxic zone, was first constructed to realize stable partial nitrification in treating domestic sewage. The alternating anoxic/aerobic conditions and longer anoxic sludge retention time might be responsible for the partial nitrification. Nitrite accumulation ratio reached 89.3 ± 3.3% with the maximum activity ratio of AOB to NOB increasing from 0.72 to 8.17. A content total inorganic nitrogen (TIN) removal efficiency (93.7 ± 2.2%) and effluent TIN concentration (2.9 ± 0.9 mg N/L) were obtained after 238 days' operation. Specifically, nitrogen balance of the typical cycle showed that about 30.1% of TIN was removed through simultaneous partial nitrification and denitrification (SND) in aerobic zone and 48.2% by endogenous denitrification in anoxic zone. The AOAO process is an economic treatment for domestic sewage with aerobic hydraulic retention time (HRT) of 4 h.

Simultaneous anaerobic oxidation/partial nitrification-denitrification for cost-effective and efficient removal of organic carbon and nitrogen from highly polluted streams

Laboratory bench-scale anoxic/aerobic reactors with complete mix and continuous flow conditions were operated with high-strength synthetic wastewater to achieve simultaneous COD and nitrogen removal. High concentrations of organic carbon and nitrogen can be found in slaughterhouse, dairy, and food processing wastewaters, and also in some landfill leachates. Therefore, the goal of this study is to find a simple, efficient, reliable, cost-effective, and general solution for organic carbon and ammonia removal from streams with high influent concentrations of more than 5000 mg/L COD and 250 mg/L NH₃-N. The highest COD (97%) and NH₃-N (91%) removal efficiencies were obtained with initial COD and ammonia concentrations of 5211 mg/L and 262.8 mg/L NH₃-N with volumetric loading rates of 11.26 kg COD/m³ d and 0.57 kg NH₃-N/m³ d for COD and ammonia, respectively. Anaerobic oxidation is the main COD removal pathway in a simultaneous anaerobic oxidation/partial nitrification-denitrification (SAO/PND) system, and nitrogen removal significantly occurs via bacterial assimilation and partial nitrification-denitrification pathways. There are several advantages for this proposed SAO/PND system from a practical point of view, such as feasibility of simultaneous COD and nitrogen removal in a single reactor; simple operation; flexibility and practicality of this system as a general solution and cost effectiveness.

Post-denitrification using alginate beads containing organic carbon and activated sludge microorganisms

Nitrate concentration in the final effluent is a key issue in pre-denitrification biological treatment systems. This study investigated post-denitrification with alginate beads containing immobilized activated sludge microorganisms and organic carbon source. A batch study was first performed to identify suitable carbon sources among acetate, glucose, calcium tartrate, starch and canola oil on the basis of nitrate removal and bead stability. Canola oil and starch beads exhibited significantly higher denitrification rates, greater bead stability and lower nitrite accumulation (6 mg/L and 10 mg/L, respectively). Glucose and acetate beads showed longer acclimation phases and degraded faster whereas tartrate beads had higher nitrite build-up (39 mg/L) and degraded due to brittleness. Post-denitrification with canola oil and starch beads was investigated in the final clarifier of a coupled upflow bioreactor and aerobic system treating synthetic dairy farm wastewater, and showed a denitrification efficiency of >90%. Beads faded in 12 days due to alginate degradation. Therefore, enhancement in bead strength or use of more stable nontoxic gel would be required to further prolong the treatment. Moreover, this study was conducted at laboratory scale and further research is needed for application in real systems.

Electroflotation clarifier to enhance nitrogen removal in a two-stage alternating aeration bioreactor

Stringent water treatment criteria and rapidly growing pollutant loads provoke the demand for retrofitting wastewater treatment plants towards a higher capacity. In this study, we assess a two stage alternating aeration (AA) bioreactor equipped with electroflotation (EF) clarifier, for nitrogen removal within a short hydraulic retention time (HRT). The EF under steady solids loading required a minimum unit height and gas: solids ratio of 0.006 for efficient clarification. The separated sludge blanket was further thickened with retaining stability when the cyclic solids loading was smaller than 1.0 kg m(-2). In the continuous operation of the bioreactor, the returned activated sludge concentration increased to more than 18,000 mg L(-1), while the effluent suspended solids concentration was lowered below 5 mg L(-1). Under influent chemical oxygen demand (COD)/total inorganic nitrogen (TIN) concentration of 300/30 mg L(-1), the TIN removal efficiency was near 70% with cycle time ratios of 0.17 and 0.27. Under higher influent COD concentration of 500mg L(-1), TIN removal efficiency was found to be 73.4% at a carbon:nitrogen (C:N) ratio of 10 and even higher (80.4%) at a C:N ratio of 16.6. The increased mixed liquor suspended solids concentrations (> 6000 mg L(-1)) under the high COD loading were efficiently maintained by using the EF clarifier. The results of this study demonstrate that an EF clarifier with a HRT of less than 1 h can support reliable nitrogen removal in the AA process that has a HRT of 6 h, even under increasing influent loadings.