Analyses On The Efficiency Of Enhanced Primary Treatment And A～2/O System As Well As Studies On The Optimization Of The Operation Parameters Of Xi'an Wastewater Treatment Plant

In view of the inlet water quality and its treatment system, a lot of experiments were carried out on the biological enhanced primary treatment (BEPT) and the A²/O system; some factors were analyzed in the operation. The results show that:(1) Compared with the ordinary primary treatment (OPT), the BEPT can strengthen the sedimentation of suspended solids (SS), simultaneously, partly possesses the function of biological metabolism. Because of the tow mechanisms, both biological flocculation & adsorption and biological metabolism, the pollutants including SS and others easily joined by SS can be removed effectively, in addition, to a certain extent, some dissolved pollutants can be removed.The tests were done in the lab and on the primary sedimentation tank system, respectively. The experimental results of BEPT are compared with that of OPT, and it is obtained that the removal rate of SS through the BEPT is 5-10% higher than that though the OPT, besides, the better the property of the activated sludge is, the higher the removal rate is; that of COD 7-15% higher; that of BOD₅ 5-10% higher; the removal of COD is obviously improved as the dosage of activated sludge is increased, and it is improved also as the retention time is prolonged. When the dosage of sludge is 1.3% of the total inlet water, and the retention time is 0.5-1.0 hour, the removal rate of SS can reach 50-60%,that of COD 36-45%; but that of TN is very little, as a result of the conversion of the organic nitrogen to the NH₄⁺-N, the content of NH₄⁺-N in the effluent is usually higher than that in the influent to the BEPT, as the hydraulic retention time (HRT) exceeds 30 minutes, the TP in the effluent becomes higher too.Through the experiment, it can be drawn that the HRT in the primary sedimentation pond should be designed 1.0 hour or so, which can meet the demand of good sedimentation.(2) In view of the problem in the operation of the aero tank of the A²/O system , somefactors, such as the temperature, the sludge load (F/M), and the volume of both the returned activated sludge and the returned mixed liquid, are analyzed, these may influence the removal efficiency of nitrogen and phosphorus in the influent to A²/O system. It is drawn that the main causes are both short SRT (only 5-6 days) and low DO (usually less than 1.0mg/L), both of which restrain the nitrification, finally, influencing the denitrification.Thanks to its short SRT and pre-denitrification tank installed, both of which serve a good environment for phosphorus removal, the content of TP in the effluent (usually 0.5 mg/L or so) is rather low, its removal rate can attain to 85%.(3) To realize a good removal of nitrite and phosphorus, with a lot of experiments and calculations done, it is obtained that the SRT of the A²/O system should be controlled around 15 days, and the DO at the outlet of the aero tank about 2 mg/L.(4) After some causes of the occurrence of the sludge bulge were analyzed in the treatment system of Xi' an wastewater plant, it is found: for the filamentous bulge in the traditional aero tank, the main causes are both the high sludge load (F/M) and low DO, in order to insure a good treatment efficiency, it is better to keep the DO at about 2.5 mg/L; for the nonflilamentous bulge in the A²/O system, the main cause is lack of DO, it will be better for the Do to be kept at 2 mg/L.The experimental results above can not only be provided as operation parameters for Xi' an wastewater plant, but also be made as references for the design and operation of other same kind of wastewater plants.