Experimental Study On Suspended Carrier Oxidation Ditch Process To Treat Urban Sewage

Sewage treatment plants in three gorges reservoir area are facing function upgradeto protect the water environment. As a representative of oxidation ditch, Sewagetreatment plant of Changshou district had some problem in operation. Such as thecontaminant loading in complex water quality was high, and the TN and TP of effluentcouldn’t match the first level B criteria in GB18918-2002simultaneously.Though nitrogen and phosphorus removal of activated sludge process likeoxidation ditch are mature, there are contradictions between phosphate accumulatingbacteria and denitrifying bacteria because of the different SRT and the competition forthe carbon source. This paper is for the study of suspended carrier oxidation ditchprocess, which combines biofilm process with oxidation ditch process, addingsuspended carrier in oxidation ditch. In the experiments, the treatment efficiency of pilotoxidation ditch in different conditions is studied, the nitrogen and phosphorus removalmechanism are analyzed, the optimal conditions are determined, and the systemperformance under the optimal operating condition are verified.The oxygenation capacity of suspended organism carrier, suspended ball andpolyhedral ball was researched by experiment. The results showed that these carriers allcould improve the oxygen transfer coefficient under dosing ratio of30%, and suspendedorganism carrier was observably better than others. Suspended organism carrier wasused in follow-up experiments.Oxidation ditch dissolved oxygen (DO), sludge return ratio (R) and sludgeretention time (SRT) were chosen for orthogonal experiment. According to the rangeanalysis and variance analysis of orthogonal experiment, DO had significant influenceon decarburization, nitrogen and phosphorus removal; R had significant influence onnitrogen and phosphorus removal; SRT had significant influence on phosphorusremoval. Sludge return ratio optimizing experiment of75%,85%and95%wasconducted. The results showed that pilot suspended carrier oxidation ditch had goodefficiencies of COD, NH4+-N, TN and TP in three conditions, and the effluents werebetter than first level B criteria in GB18918-2002steadily. R=75%was determined asthe optimal operating condition parameter. Other parameters were DO=0.8~1.2mg/L,SRT=11~15d, dosing ratio30%, HRT (anaerobic tank)=2h, HRT (ditch)=8h, andMLSS=2400~2800mg/L. The efficiency of COD, NH4+-N, TN and TP was η(COD)=82.26~85.91%, η(NH4+-N)=84.16~87.18%, η(TN)=63.10~66.77%,η(TP)=75.46~78.87%, respectively. Effluent quality was ρ(COD)=27~44mg/L,ρ(NH4+-N)=2.3~4.8mg/L, ρ(TN)=8.42~14.75mg/L, ρ(TP)=0.54~0.79mg/L. COD,NH4+-N and TN of the effluent met the first level A criteria in GB18918-2002, TP metthe first level B criteria.The study confirmed suspended carrier oxidation ditch had good treatment effect tourban sewage, and provided theoretical basis and technical reference for applying thetechnology in improvement of oxidation ditch.