Study On Treatment Of Domestic Sewage By Combined Biofilm Reactor With Recycling

In this dissertation, a new wastewater treatment process-a combined biofilm reactor was introduced, which was based on the traditional Contact Oxidation Method and BAF and was added anoxic zone. For enhancing nitrogen removal, nitrifying fluid return could be carried out. According to the order of the sewage flow, it was divided into three zones:the anaerobic zone (A), the contact oxidation zone (O) and the BAF (B). Both A and O zones were packed with polypropylene multi-faceted hollow balls and B zone was packed with ceramsite. A lab-scale experiment treating domestic sewage in the combined reactor was conducted. Contaminants removal was investigated and the optimum operational parameters were determined. The mechanics of contaminants removal were analyzed in detail, and kinetic models of COD removal were deduced and inspected. In addition, the mechanics, styles and intensity of backwashing were studied in this experiment.According to the experimental results under different conditions, the optimum operational parameters were determined as following:HRT was 6.5 h, DO concentrations of O zone and B zone were 3～4 mg/L and 6～7 mg/L, respectively, and nitrifying fluid return ratio(R) was 150%. Under the optimum conditions, with average influent COD of 290.8 mg/L and ammonia of 42.2 mg/L, the average removal efficiency of COD, ammonian, total nitrogen and turbidity were 90.83%,98.97%, 76.27% and 88.66%, respectively, and accordingly, the effluent concentrations were 22～31 mg/L,0.2～0.75 mg/L,9.6～13.3 mg/L and 6～10 NTU, respectively, so their average effluent concentrations were 26.4 mg/L,0.43 mg/L,11.15 mg/L and 8 NTU, respectively, all meeting the A-level standard in GB18918-2002. And some of the four indexes met water quality standard for urban miscellaneous water consumption (GB/T18920-2002) and for scenic environment use (GB/T18921-2002). The removal of total phosphorus was relatively poor, and the average removal efficiency of 37.98% and total phosphorus in effluent of 1.4～1.9 mg/L was just attained. The experimental results showed that, the changes of HRT, DO, R, temperature, COD concentration and ammonia concentration in influent could influence the removal efficiency of contaminants in varying degrees. The change of HRT made a great effect on ammonia removal, and the removal rate of ammonia dropped greatly with the decrease of HRT. The change of DO made little effect on the removal of contaminants in O zone but made a great effect on the removal of ammonia in B zone. The removal rate of ammonia decreased dramatically when the DO was less than 6 mg/L in B zone. The removal rate of total nitrogen was most obviously influenced by the change of return ratio, presenting the trend of first increase then decrease with the increase of return ratio. With HRT of 6.5 h, the removal rates of COD and ammonia increased with the increase of temperature, while with HRT of 4.25 h, the removal rate of COD increased slightly with the increase of temperature, and the removal of ammonia was hardly affected. COD concentration in influent made different effects on removal rates of COD and ammonia in A-O zone. With the increase of COD concentration, the removal rate of COD ascended firstly, and then descended slightly, but the removal rate of ammonia decreased constantly on the whole. The removal of contaminants in B zone was affected slightly by COD concentration in influent. Ammonia concentration in influent made a great effect on ammonia removal in A-O zone, rendering ammonia concentration in effluent to increase linearly with the increase of ammonia concentration in influent.By the combined deduction of theory and experiment, the kinetics models of COD removal were achieved as following:C= C0e-(-0.117R2+0.2958R-0.0037T12+0.1559T1+0.2361)h1 (in A-O zone), C=C1e-(0.861R2-0.2837R+0.1834T22-0.5895T2+0.8856)h2 (in B zone). Based on the above two models, the kinetics model C2=C0e0.1377R2-0.2789R-0.0205T2-0.019T-1.3904 of combined biofilm reactor was deduced.By analyzing in theory and experimental testing, the styles and intensity of backwashing were determined. They were as following:for A-O zone, firstly, simultaneous backwashing with air and water for 5min, under the backwashing intensity of air and water of 12 L/(m2·s) and 4 L/(m2·s), respectively; then, backwashing with water for 9min, under its intensity of 4L/(m2-s). While for B zone, firstly, backwashing with air for 4min, under its intensity of 18 L/(m2-s); then, simultaneous backwashing with air and water for 8 min, under the backwashing intensity of air and water of 18 L/(m2·s) and 4 L/(m2·s), respectively; lastly, backwashing with water for 3min, under its intensity of 4 L/(m2·s).The experimental results showed the combined biofilm reactor could achieve good removal efficiency of COD, ammonia, total nitrogen and SS, and relatively poor removal efficiency of total phosphorus. And some other advantages, like small footprint, simple operation, good capability of counteracting influent loading fluctuation, steady effluent quality, and especially prominent nitrogen removal were also presented. It was proved that it was feasible for the combined biofilm reactor to be applied in the distributed wastewater treatment for water reuse.