Study On Combined Processes Of Chemical-biological Flocculation And Biological Aerated Filter For Urban Sewage Treatment

At present,traditional activated sludge secondary treatment process is commonly adopted by sewage treatment plants in our country.However,the process has shortages,such as large area occupied,low-sludge in aeration tank and high operating costs.As a new sewage treatment technology,biological aerated filter has advantages such as small site area,good effluent quality and low operating costs,which is applied by sewage treatment plants in our country at present.But the process has a higher request for the influent organic and suspended matter.It plays an important role to development a suitable technology for decreasing influent loads and guarantee efficient operation of the biological aerated filter.The combined processes of chemical-biological flocculation and biological aerated filter was adopted for the treatment of urban sewage treatment in this study, and the influence of the parameters such as flocculant dose,DO,sludge return ratio, hydraulic retention time and temperature on the pollutants removal rate in the chemical-biological flocculation process was investigated,respectively.Moreover,the role of biological flocculation in the chemical-biological flocculation process and the influence of operation parameters,conditions of backwashing and low temperature on biological aerated filter was also studied in this thesis.(1) The experimental results of chemical-biological flocculation enhanced first grade treatment process showed:1) The chemical-biological flocculation process could be set up immediately.The removal rate of organic compounds could arrive at 60%at the fourth day by this experiment.2) The optimal condition of the process of chemical-biological flocculation was as follows:influent flow was 88L/h,HRT was 0.5h,dosing quantity of Fe³⁺ was 10mg/L,dissolved oxygen of corridor was respectively 2mg/L,1.5mg/L and lmg/L, sludge return ratio was about 70%.3) The removal rate of COD_Cr,NH₄⁺-N,PO₄^3--P,SS could arrive at 63%,21%, 78%and 75%respectively at the optimal condition by the process of chemical-biological flocculation.4) During the low temperature(7℃～11℃),the changes in removal rate of these contaminants were not obvious by the process of chemical-biological flocculation.5) The chemical-biological flocculation was much more effective than the traditional chemical enhanced first grade treatment process due to the biological flocculation.when the ferric was equal to 10mg/L which was optimal,the removal rates of COD_Cr,NH₄⁺-N,PO₄^3--P and SS were improved 17%,6%,5%and 6% respectively higher than before.(2) The experimental results of biological aerated filter showed:1) The study found that biological aerated filter had been aerated after two weeks at the 18℃～20℃,the system was put up biofilm successfully.2) In the biological aerated filter the removal rate of COD_Cr and NH₄⁺-N could raise up to 77%and 93%under the condition that the HRT was 2.0h,and ratio of gas and water was 3:1.3) The result of two different medias of BAF showed:the removal efficiency of BAF with new china media was equal to that with ceramsite media.But the new china media was light specific gravity and high mechanical strength,which had obvious advantages than ceramsite media when the system was backwashing.4) The performance study of filter height of BAF showed that the sections ranging from the beginning of filter bed to the 1200mm distance after the beginning contributed to the removal of COD_Cr.In the sections,COD_Cr removal rate was 68%, which accounted for about 86%in the total removal rate.At the sections of the filter bed ranging from the 800mm to the 1200mm segment,the NH₄⁺-N removal rate was 76%.5) The way of BAF backwash was a combination of gas and permeate backwashing.Water flushing intensity was 2～4L/(s·m²),air washing intensity was 6～8L/(s·m²),The period of backwashing cycle was 2d,backwashing time was 7～10min.After 5～6h washing filter could be restored to the best state that the removal rate of COD_Cr was more than 60%. 6) During the low temperature(7℃～11℃),COD_Cr and SS could still keep the high removal rate.As a matter of fact,the removal rate of NH4+-N was influenced by low temperature.But low temperature had little effect on the process compared with activated sludge process.The system could prolong the hydraulic retention time to improve the efficiency at low temperature.