Study On The Gel-like Structure And Drying Technology Of Sewage Sludge

With the economic and urbanization development of China,the sewage sludge has been increasing dramatically.The treatment of sludge has become one of major societal problems over the last few decades.In order to achieve the optimization of the stabilization,harmless disposal and resource utilization of sludge,sludge dewatering and reduction should be realized firstly using efficient conditioning-dewatering and drying techniques.The sludge which was used in this study was the activated sludge(AS)or digested sludge.Rheology and fractal methods were employed to investigate the effects of extracellular polymeric substances(EPS)on the gel-like structure of sludge.The effects of environmental condtions on the gel-like structure of the raw and conditioned AS were discussed as well.On the other hand,sludge dewatering and drying through constructed wetlands and membrane distillation were explored from the technical perspective.The influence of the plant on the drying performance from the sludge treatment wetlands and the drying mechanism from the membrane distillation process were investigated.The results were as followed:Compared with the raw AS,AS after Slime extraction and AS after TB-EPS extraction,AS after LB-EPS extraction produced a higher hysteresis loop area(Hla),limiting viscosity(??),yield stress(?yT??yT),energy of cohesion of network sludge(EcT?EcD),shear modulus(GT)and critical storage modulus(G0'),which indicates that extent of thixotropy for AS after LB-EPS extraction displayed greater and the sludge needed more higher shear stress to reach the optimal opening and orientation in the flow direction.The rigidity of the sludge network exhibited greater and the sludge needed more shear stress to make the bound water move.In addition,the elasticity,cohesion energy and yield stress of the network for sludge after LB-EPS extraction showed greater.AS after LB-EPS extraction exhibited a stronger and denser(mass fractal dimention Df=2.86 through strain amplitude sweep SAS test)gel-like structure and is more difficult to dewater(the surface water and the internal water in the TB-EPS and Pellet accounts for 10%of the total water content).Besides,TB-EPS exhibited a stronger and denser(Dt=2.86 through SAS test)gel-like structure in comparison with Slime and LB-EPS fraction.The abovementoned results indicate that,when sludge has not only a stronger,but a denser structure,the dewatering and drying performance of sludge might not be better.Raw AS and raw AS conditioned with WD960 at the optimum polymer dosage were used.Under different environmental conditions(pH,ionic strength and temperature),the NaCl concentration showed almost no impact on the network structure of the conditioned AS.The conditioned AS displayed a higher G0',that is,a stronger gel structure than the raw AS.Both the raw and conditioned AS displayed the ability to swell and de-swell by absorption and desorption of water.Apart from the phenomenon that the estimated osmotic pressure(?)showed positive beyond the NaCl concentration of 4.2 M,the estimated ? values were negative with the absolute values greater than those of the raw AS,which indicates that the conditioned AS exhibited a stronger trend towars swelling and a greater dewatering performance.This tendency obeyed the Donnan Equilibrium.In the study of sludge dewatering and drying through constructed wetlands,the plants improved the drying performance.The TSS content of the experimental sample(with palnts)reduced by 20%,and the VSS content varied from 60.1%to 45.2%with 14.9%of reduction.While the TSS content of the controlled sample(without plants)only decreased by 10%and the VSS content kept almost constant at 62.1%.The water of the sludge was mainly removed through percolation.22.2%-27.7%of the water content was eliminated by evaporation.63.20%-70%was removed by percolation.In addition,in comparison with the controlled sample,the experimental one displayed greater values of the water removal and the percolate flux.During the drying process,there were three different phrases.Initially,the plants grew and tillered slowly after plant seeds being planted in May.During this month,the plants adjusted to the new environment.Thereafter,in June-July,plants thrived and the biomass increased quickly.Finally,in July-August,plants grew slowly and dead leaves appeared.In the study of sludge dewatering and drying through membrane distillation,the best drying performance was found at Reynolds number Rea of 1065 and the temperatue of 120?.The shortest drying time was found at 12h.During the constant drying rate process,the highest vapor flux of 11.9×10-4kg/(s m2)for the pure water and 6.6×10-4kg/(s m2)for AS were obtained at Rea of 1065.The thickness of the boundary layer affected dyring performance more significantly under different air flow rates.Water partial vapor pressure difference played a more important role in the drying performance under different oven temperatures.The mass transfer should be explained by the Knudsen-molecular diffusion model in this study.In comparison to the membrane diffusion resistance(8.84×10-5m),the resistance of the boundary layer was the dominant part of the overall resistance(0.002?0.014m).For both pure water and AS systems,the heat flux displayed negative values,which indicates that the overall heat transferred from the air side to the feed side.In addition,compared with the pure water system,the AS needed more energy to accomplish the evaporation process.