Novel Iron-Dependent Technology For Nitrate And Phosphorus Removal

The pollution from nitrogenous and phosphorus compounds is an increasingly serious problem around the world. Thanks for the effective control of organic pollution, the management of wastewater with low C:N:P molar ratio appears to be a new challenge. Hence, autotrophic technologies are urgently required to solve this bottleneck. Nitrate-dependent anaerobic ferrous oxidation (NAFO) is an autotrophic process using ferrous iron as electron donor to convert nitrate to nitrogen gas under anoxic conditions. Based on NAFO, a new autotrophic denitrification technology could be developed. Meanwhile, the product (ferric salts) of NAFO process was proposed to be used in phosphate removal from wastewaters.In this research, the autotrophic nitrate removal technology based on NAFO process and the enhanced phosphorus removal technology based on partitionable-space enhanced coagulation (PEC) reactor were developed, respectively. The research with respects mainly to the working performances and process characteristics of the technologies was conducted. Taking NAFO process as a link, the combined technology for nitrate and phosphorus removal was also investigated. The major results are as follows.1) The autotrophic nitrate removal technology based on NAFO process was developed, and its working performance and operation conditions were investigated.It was proved that NAFO system was feasible for autotrophic denitrification. The volumetric loading rate (VLR) and volumetric removal rate (VRR) under steady state were 0.161±0.008 kg-N/(m3·d) and 0.076±0.006 kg-N/(m3·d), respectively. In NAFO system, temperature and effluent pH had obvious effects on nitrate removal with optimal value of 30.15 ℃ and over 6.0, respectively. Nitrate concentration was preferred to be less than 130 mg-N/L. Organic matters had little influence on NAFO performance. The effluent pH was suggested as an indicator which demonstrated a good correlation with nitrogen removal.2) The physical and ecological characteristics of the NAFO sludge, including the morphology, structure, element contents and microbial community were studied.It was discovered that the NAFO sludge showed good settleablity. The surface mean diameter (SMD), volumetric mean diameter (VMD), density and settling velocity were remarkably increased compared with inoculum sludge, with values of 1468.98±64.53 μm,2389.00±131.52μm,1580±80 kg/m3 and 390.07±161.31 m/h, respectively. The NAFO sludge aggregated and formed a rougher surface, and the color changed from black to yellow. The microbial cells were covered by precipitates with crystalline, needlelike structures on the surface. Abundant iron compounds were discovered to accumulate in NAFO sludge with peak value of 51.73% (m/m). The XRD results showed the iron compounds referred to be hematite, magnetite and iron hydroxide. The MiSeq high-throughput sequencing results revealed that the predominant bacterial populations in NAFO sludge belonged to a-Proteobacteria, β-Proteobacteria, y-Proteobacteria, Clostridia, Thermoleophilia and Actinobacteria.3) The effects of key operation conditions on performance of phosphorus removal with ferrous salt were investigated, and the optimal values and process characteristics were revealed.It was proved that the "Priority/Combination" model was suitable for optimization of phosphorus removal with ferrous salt. The effect of individual operation parameter on phosphorus removal was followed as the order of Fe(Ⅱ)/P>pH>FMS (flash mixing speed). The combined effects of the operation parameters were negative, which decreased in the order of Fe(Ⅱ)/P-FMS>pH-FMS>Fe(Ⅱ)/P-pH. The optimal operation parameters for wastewater with initial phosphorus concentration of 100 mg/L were Fe(II)/P=3.32, pH=7.49, FMS=182 r/min. It was revealed that excessive agents were usually required in order to meet the discharge standard of phosphorus. And the dosage increased with the decrease of effluent phosphorus concentration. For the control of wastewater with initial phosphorus concentration of 100 mg/L, the agent dose required was up to 4.12-5.83 kg-Fe(II)/kg-P. It was clarified that the process of phosphorus removal by ferrous salt was an unbalance reaction. And it was ascribed partly to the requirement for a higher residual Fe(II) concentration to get a lower concentration of residual phosphorus in the solution, and partly to the competition between Fe3(PO4)2 precipitation and Fe(OH)2 precipitation.4) The enhanced phosphorus removal technology based on PEC reactor was developed, and its working performance and process characteristics were investigated.It was proved that the novel PEC reactor was feasible for phosphorus control with obvious advantage of high efficiency and low cost. The VRR reached up to 2.86-0.04 kg-P/(m3·d) with a phosphorus removal rate of over 97%. The precipitant consumption was reduced to 2.60-2.76 kg-Fe(Ⅱ)/kg-P with low operational cost of $ 0.632-0.673/kg-P. The peak phosphorus content in precipitate was up to 30.44%by P2O5, which revealed the benefit of the phosphorus resource recycling. The excellent performance of PEC technology was mainly attributed to the partitionable-space and ’flocculation filter’. The partition limited the trans-regional back-mixing of reagents along the reactor, which promoted the precipitation process. The’flocculation filter’ retained the microflocs, enhancing the flocculation process.5) The performances of phosphorus removal by NAFO sludge and system effluent were studied, and the combined technology for nitrate and phosphorus removal was also discussed.It was discovered that the NAFO sludge was capable for phosphorus removal. The VRR was up to 1.48 kg-P/(m3-d) (18.46 mg-P/g-VS), which was a high level compared with values in previous literature. The effluent from the NAFO system was also suitable for phosphorus removal with capability of 3.42 kg-Fe/kg-P. The mix of ferrous iron and ferric iron in the liquid attributed to the advantages of high efficiency and low cost. A novel device named "Integrated facility for phosphorus removal chemically utilizing self-productive ferric salt and nitrogen removal biologically utilizing nitrate-dependent anaerobic ferrous oxidation microorganism" was invented for nitrate and phosphorus removal. In the combined technology, the biological process for nitrate removal based on NAFO process was clarified to be the bottleneck, which should be enhanced.