| The waste water which accounts for3~7%of the water consumption and was generated duringthe treatment process in water plants includes the filter back wash water and sludge. The directlydischarge of waste water with great amount of inorganic particles not only pollutes theenvironment, but also easily cause the waste of water resources. The general treatment ofrecycling after sludge-thickening and dewatering is less economically feasible due to the extraoperation and management cost. Additionally, the general method of treating raw water with lowturbidity like increasing coagulant and polymer flocculent dosage or dosing clay to improve andenhance the coagulation efficiency also increase the operation cost while the grain composition ofclay can not agree well with the raw water, all of which brings technical and economicirrationality.The sludge of the sedimentation tank contains high concentrations of suspended colloidparticles and metal hydroxide of aluminum and iron due to the previous treatment stages likechemical dosing and flocculation-precipitation reaction, which is generally believed that haspositive impacts on the enhancement of the low turbidity raw water. Whether the means that thedirect sludge recirculation of the sedimentation tank has positive impacts on the enhancement oflow turbidity water treatment is firstly verified, then the optimum recirculation conditions andrules is s studied.Firstly, the relationship between G value and rotate speed was established through the formula.The G value of the reaction was optimized through orthogonal experiments. The optimumhydraulic condition and dosage of the static coagulation-flocculation jar test was determined.Based on the results of the static experiments, the optimum recycling conditions and rules isthen studied. Whether the sludge recirculation of the sedimentation tank has positive impacts onthe enhancement of low turbidity water treatment was primarily verified. However, it was noticedthat the turbidity removal rate was below50%without PAC dosage, which indicated that theeffective removal of turbidity can not be realized solely through the sludge recirculation. Based onthe study of the PAC dosage and the turbidity removal rate, the rules of how recycling affected theeffluent quality were researched by the controlled tests between the flocculation-sedimentationreaction with and without sludge recycling. The relation curve of the PAC dosage and the residualturbidity both of the raw water and the mixed water under the fixed standard turbidity of0.5NTUwas modified by cubic polynomials, the results of which showed that the turbidity of the mixedwater should be controlled under33.90NTU when using the method of recirculation (thecorresponding turbidity of the recycled sludge is491.53NTU). The PAC saving effect was nolonger existed when the limit was exceeded. Finally, the relation curve of the mixed waterturbidity and the residual turbidity was also modified by cubic polynomials, which indicated thatthe optimum mixed water turbidity of recirculation was16.75NTU (the corresponding turbidity ofthe recycled sludge is212.9NTU) and the PAC saving rate was up to22.5%。The optimum operating condition and the influencing factor (raw water turbidity, pH value,sludge retention time, backflow points) of the reflux was then observed based upon theIntermittent running flocculation-sedimentation model. It is worth noting that the sludgerecirculation technology can improve the turbidity removal efficiency of low turbidity raw water especially, which remained ideal but would get to certain limited extent when the raw waterturbidity continue to rose. The turbidity removal rate was up to91%at pH value of8.0in thecondition of sludge recirculation, which was8.2%higher compared with the conventional process.The sludge retention time should be controlled within3days, the range over which the effect ofcoagulation enhancement no longer existed. The results indicated that the1#backflow points(before the rotation of300r/min) was the optimum operating condition under which the effluentturbidity came to the lowest.Based on the complex waste water treatment project of WISCO outfall A, the influence ofsludge reflux ratio on effluent quality through was investigated the static experiments of sludgereflux to the pre-coagulation unit. The sludge reflux ratio should be controlled around10%andless than15%.Finally, the waste water recirculation economic efficiency is preliminary assessed though takingan instance of a water supply plant, the waste water quantity and production scale of which is5%and20,0000t/d. |
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