Study On Excess Activated Sludge Reduction By Microbial Predation

The activated sludge process is the most widely used biological wastewater treatment for both domestic and industrial plants. One of the drawbacks is high sludge production which is difficult to treat and dispose. With the strictly enforced environmental and legislative requirements on the discharge of excess sludge, the cost of excess sludge treatment and disposal has accounted up to about 60% of the total operating cost in municipal wastewater treatment plants. There is considerable impetus to explore strategies and technologies for reducing excess sludge production in biological wastewater treatment processes which represents a rising challenge for wastewater treatment plants. One way to reduce excess biomass production would be to encourage the growth of organisms higher in the food chain which feed on sludge. During transfer from a low level to a high trophic level, energy is lost due to inefficient biomass conversion. More and more attention has been paid to this method because it is energy saving and brings no secondary pollution.A.hemprichi was inoculated in batch and continuous experiments to investigate sludge characteristics contributing on the growth of A.hemprichi and sludge reduction in this study. It was found the sludge which could be predated by A.hemprichi was determined by sludge concentration and the floc structure. The specific growth rate of A.hemprichi increased with increasing the amount of available sludge. It was concluded that the Monod equation could be adopted to simulate the growth of A.hemprichi at sludge limited conditions. The maximum density of A.hemprichi increased with elevated available sludge concentration under sludge sufficient conditions. The growth potential was determined by the sludge concentration to the density of A.hemprichi. Sludge reduction could be well fitted by Pirt maintenance equation at high specific growth rate. When the specific growth rate was lower than 0.4, Increasing energy produced by predation was used for maintenance instead of for growth with the decreasing ofμ.The growth characteristics of A.hemprichi in unsterilized activated sludge were similar with that in sterilized sludge. The specific sludge reduction rate was between 0.75～2.0×10-3mgind.-1d-1 with long sludge retention time and low organic loading. The average sludge reduction rate with predation was 41.36% higher than that without predation. Sludge reduction rate was obviously improved by the predation of A.hemprichi, but the sludge concentration at the end of the experiment was not different between the two reactors. The predating action of A.hemprichi destroys the floc structure and cause the sludge volume index to increase, but it was not observed at the normal organic loading operation in SBR reactor.Sludge yield was obviously affected by the changing density of A.hemprichi in SBR reactor with operation at same reactor volume and organic loading. The disperse growth of sludge was influenced by the hydrolic retention time of the disperse reactor. So did to the sludge reduction. At HRT of 8h and 12h, the yield of predating system was reduced by 37.75% and 43.3% respectively, comparing with the reference system. Sludge reduction was increased by 25.76% and 25.51%, although the effluent concentration of NH4+-N and NO3-N was also affected. Comparing with the predation systems which have been studies before, A.hemprichi was inoculated in the tests. HRT used in the disperse stage was longer than before to improve the CODcr removal because the floc size was not much limited on A.hemprichi than that on other smaller size microfauna. With inoculating A.hemprichi, the predating system could be used to reduce sludge production for more wastewater treatment plants with low concentration of CODcr. The sludge yield of predating system was greatly reduced comparing with the conventional aeration sludge wastewater treatment.