下水處理水의 3次處理에 關한 硏究

Abstract

This study was performed to search the appropriate processes for the water reuse from sewage effluent in Gwangju metropolitan city. Water quality in influent and effluent of Gwangju sewage plant Ⅰ stage 2 was investigated from January to December 2004. Concentrations of the average BOD5, COD, SS, T-N and T-P in sewage influent were 172.1 ㎎/L, 111.1 ㎎/L, 235.2 ㎎/L, 37.765 ㎎/L and 5.028 ㎎/L, respectively, in 2004. Concentrations of suspended solids and organic carbon were generally high during March to August in a year. Concentrations of the average BOD5, and COD in effluent were 3.8 ㎎/L and 9.4 ㎎/L and removal rates of BOD5, COD, SS, T-N, T-P, PO4-P, and NH3-N in effluent, treated by biologically from influent, 97.8%, 91.5%, 98.8%, 63.6%, 47.9%, 47.3%, and 47.5% respectively. The average concentrations of color, turbidity, Fe, Mn, pH, Cl-, hardness, and conductivity were 26 degree, 0.8 NTU, 0.030 ㎎/L, 0.040 ㎎/L, 7.1, 83 ㎎/L, 78 ㎎/L, 595 ㎲/㎝, respectively, and harmful heavy metals as Hg, As, Pb, Cd, Cr were not nearly detected. Algal species investigated in the secondary treated sewage effluent was totally 45 species during 2003 to 2004 and the organism appeared during summer season was 38 species including Cyanophyceae 5, Bacillatiophyceae 7, Chlorophyseae 14, Euglenophyceae 3, Protozoan 3, and et al. Poly aluminium chloride (PAC) showed the optimal coagulant, with effective dose of 80㎎/L, in treating sewage effluent and followed by Alum with 100㎎/L in the range of 0 ～ 350 ㎎/L and ferric chloride was not the proper coagulant because water quality of the effluent was not bad compare to contaminated stream water. The treatment efficiencies of color, total phosphorus, and total organic carbon(TOC) were proportionally increased with coagulant dose of Alum and PAC up to 300㎎/L and 350㎎/L, in which the treatment rate of color were 60% and 66%, each. The removal rates of total phosphorus(T-P) were 90% and 92% when Alum and PAC dose of 100㎎/L were used at each system and the optimal pH was 5.0～7.0 in coagulation process. Two kinds of adsorbents as PWWL, coal base, and PWPL, coconut base, were used to execute adsorption isotherm experiment by checking the removal rate of TOC. There was some discrepancy between the data and the theoretical result in predicting data using ideal adsorbed solution theory (IAST) and Freundlich isotherm. By using 0.4% NaOCl, 4% NaOH, 1% HCl solution, membrane cleaning was performed according to the solution's properties. Powdered activated carbon(PAC) and calcium chloride were used to avoid membrane fouling. The membrane life time was prolonged 10 and 30 times after treating coagulation and coagulation/adsorption each and it was also longer, when pH was 10 of alkali state, than acidic treated situation, because zeta potential reached to around zero. After treating coagulation and membrane, weight average molecular weight was decreased from 950Da to 465 and 396Da. This suggests that materials over 400Da were easily removed by coagulation and adsorption. However, some materials under 400Da were not nearly removed by the two processes. That means natural organic matter(NOM), not removed, can affect to the formation of harmful disinfectant byproducts(DBPs) after Cl2 treatment. Using this hybrid system, it can be possible to use the sewage effluent for water reuse after solving the problem of nitrogen and microbial.