Reduction System Of Waste Activated Sludge With Worm-feeding Reactor Using CFD:Simulation,Optimal Design And Economic Evaluation

With the increase of population and economic development in China,there is a sharp rise in the production of waste sludge and the cost of its treatment has become a bottleneck in waste water treatment.In recent years,a sludge reduction technology based on predation of micro faunal has been paid more and more attention due to its ecological approach and free of secondary pollution.However,the stability of worm-reactor cannot be guaranteed at present,and the growth of worms could be mainly affected by the change of flow field and distribution of dissolved oxygen.Hence,the effects of reactor aeration rate and worm placement on sludge reduction by using the methodology of numerical simulation combined with experimental verification were investigated.Furthermore,the economic evaluation is carried out on sewage sludge conventional process connected with or without worms feeding.The distribution of velocity field,volume fraction of air and turbulent intensity in worm reactor were numerically simulated by the models of Eulerian two-phase flow and Standard k-ε turbulence provided by ANSYS FLUENT 16.0 software.The optimal worm placement was obtained and the better sewage sludge reduction effect could be achieved when the aeration rate was about 0.2m3/h in this worms reactor.The results of s simulation above were subsequently verified by the municipal waste water treatment train which integrated with sequencing batch reactor(SBR)and worm reactor.It was confirmed that the sludge reduction rate could be achieved by the maximum of 23 wt% when the aeration rate was 0.2m3/h with optimal worm placement in this reactor.Furthermore,the effect of three factors(aeration rate,worm placement and sludge concentration)on worm sludge reduction was also investigated and the results had an agreement on numerical simulation with its reliability.The cost of SBR-worm sludge reduction system was evaluated with its optimal operating conditions(aeration rate of 0.2m3/h and optimal worm placement in P2)in comparison to the common SBR sludge treatment system.Total nitrogen and total phosphorus increased by 4% and 12% respectively due to the release of Worms faeces,which may lead to the fact that the operating cost of SBR-worm waste water treatment increases by about 130.076 yuan/m3 accordingly.However,due to the improvement of waste activated sludge dewaterability with worms feeding,the expense of following sludge digestion,dewatering and drying will be significantly decreased.Compared with the common sludge treatment,the total construction and operating cost of sludge treatment with worms feeding can save 190,000 yuan/t and 4.6 yuan/t respectively.In addition,the daily worms can create economic value of about 460 yuan/t,which may fully offset the increasing cost of SBR treatment system with sewage sludge reduction.Therefore,sludge reduction by worms feeding will be ecologically and economically justified.