Performance Of System Consisting Of Vertical Flow Trickling Filter And Horizontal Flow Multi-soil-layering Reactor For Treatment Of Sewage

Decentralized wastewater treatment systems, characterized in treatment and reuse on-site, can avoid network construction and specialized managements, which makes the technology applicable very much to the treatment of rural wastewater in general rural area of China. However, few of them have a good performance on the removal of total nitrogen (TN).In order to have a great improvement in total nitrogen removal for rural wastewater treatment, a novel two-stage hybrid system consisting of a vertical flow trickling filter (VFTF) and a horizontal flow multi-soil-layering (HFMSL) bioreactor was developed. The VFTF packed with zeolite mainly aimed at fully nitrifying, while the HFMSL composed of soil mixture blocks (SMB) and permeable layers (PL) in alternated form primarily worked as a denitrification bioreactor. Simulated septic tank effluent was pumped intermittently into the system using a submerged pump controlled by a time switch, dispersed evenly onto the filter media in VFTF through a set of perforated pipes, and finally gravitated into HFMSL at hydraulic loading rate (HLR) of 660 L·m-2·d-1. The specific objectives were to access the system performance under conditions of different C/N ratios and water supply frequencies, such as C/N at 2:1,5:1,8:1 and frequency at 16s/60min,8s/30min,4s/15min.The results indicated that (1) the performance of apparatus was observed under conditions of different carbon-nitrogen (C/N) ratios and water supply frequencies. Total removal efficiencies of chemical oxygen demand (CODCr), total phosphorus (TP), Ammonium (NH4+-N) and total nitrogen (TN) were 90.8%,90.8%,94.2%, 85.4%, corresponding to mean final effluent concentrations of 20.6,0.34,2.8,7.2 mg·L-1, which reached the class one A standard in "National Discharge Standards for Wastewater Treatment Plant of Cities and Towns" (GB 18918-2002); (2) the optimal operating conditions were speculated at C/N ratio of 7:1, water supply frequency of 8s/30min in which the TN removal rate was high at 92.8% corresponding to the maximum nitrogen mass removal rate of 30.2 g·N·m-2·d-1; (3) having a better nitrogen removal efficiency for the current system, it was important to ensure the sufficient nitrification in the upper VFTF stage and meanwhile keep the organic substrates adequate enough in the lower HFMSL stage via adjusting influent C/N ratio and water supply frequency, however, alternatives for sustainable denitrification should be adding more effective carbonaceous media into SMBs with economic and technical considerations; (4) the design of MSL into the format of horizontal flow made the facility a satisfactory denitrification reactor which was able to provide relatively long hydraulic retention time (HRT), ideal anoxic condition as well as adequate organic substrates; (5) the higher water supply frequency benefited for the intermittent feeding system in pollutants removal when the HRT was kept at an equal level.Therefore the hybrid system could operate well even against the hydraulic shock loadings, and it is really a promising technology in application in the future.