Preparation Of Rice Husk-based Skeleton Builders And Their Effects And Mechanisms On Sewage Sludge Conditioning And Dewatering

Great amount of sewage sludge is produced in China,and improper treatment and disposal of sewage sludge may cause serious secondary pollution.In sludge treatment and disposal,the sludge dewatering is a key process.However,the sewage sludge is normally hard to be dewatered,so sludge conditioning is necessary to be applied prior sludge dewatering.Meanwhile,vast amounts of rice husk,a waste product from rice processing,need proper treatment in China.To explore safe,effective,environmentally friendly and cheap skeleton builders for sludge conditioning and dewatering,and to utilize rice husk properly,four types of rice husk-based particles were prepared,and the effects of rice husk-based particles on sludge conditioning and dewatering as skeleton builders and the conditioning mechanisms were investigated.Then the effects of these four rice husk-based particles on conditioned sludge characteristics were compared.Finally,the feasiblity of combined process of zoogloea disruption,floc reconstruction and skeleton building for enhancing the sludge dewaterability was investigated.The changes of sludge characteristics were investigated to analyze the synergistic mechanisms for the sludge conditioning and dewatering.The sludge dewaterability was evaluated mainly with the net sludge solids yield(YN).The sludge specific resistance to filtration(SRF),the moisture content of sludge cake and sludge settling velocity(SV30)were the other main index.The effects of rice husk flour size,dosage and adding order on sludge conditioning and dewatering were investigated.The results showed that the optimal size of rice husk flour was 109-150 ?m and the optimal adding order was first rice husk flour then FeCl3.The optimal dosage of rice husk flour and FeCl3 was 70% dry sludge(DS)and 138.09 g/kg DS,respectively.Under the optimal conditions,the sludge SRF decreased by 59.7%,the sludge YN increased by 45.3%,the moisture content of sludge cake increased from 81.57% to 73.37%,and the SV30 decreased from 98% to 88%,compared to that conditioned with FeCl3(138.09 g/kg DS)alone.The effects of preparation temperature,conditioning dosage and adding order of the biochar produced from sludge cake conditioned with rice husk flour and Fe Cl3(Biochar-conditioned)on sludge conditioning and dewatering were investigated.The results indicated that the optimal preparation temperature was 400 °C and the optimal adding order was first Biochar-conditioned then FeCl3.The optimal dosage of Biochar-conditioned and Fe Cl3 was 70% DS and 115.07 g/kg DS,respectively.Under the optimal conditions,the sludge SRF decreased by 63.9%,the sludge YN increased by 39.2%,the moisture content of sludge cake increased from 76.93% to 70.27%,and the SV30 decreased from 91% to 83%,compared to that conditioned with Fe Cl3(115.07 g/kg DS)alone.The microstructure and surface element content of Biochar-conditioned were investigated,and the results illustrated that the Biochar-conditioned prepared at 400 °C contained the sludge-based biochar with high content of iron and the rice husk-based biochar with a lot of pores and drapes,leading to more effective enhancement of sludge dewaterability.The effects of pyrolysis conditions,conditioning dosage and adding order of rice husk biochar(RB)on sludge conditioning and dewatering were investigated.The results indicated that the optimal pyrolysis temperature and time were 500 °C and 2 h,respectively.And the optimal adding order was first rice husk biochar then FeCl3.The optimal dosage of rice husk biochar and Fe Cl3 was 60% DS and 138.09 g/kg DS,respectively.Under the optimal conditions,the sludge SRF decreased by 58.3%,the sludge YN increased by 40.9%,the moisture content of sludge cake increased from 82.46% to 77.37%,and the SV30 decreased from 93% to 84%,compared to that conditioned with FeCl3(138.09 g/kg DS)alone.The effects of modified method and conditioning dosage of rice husk biochar modified by Fe Cl3(MRB-Fe)on sludge conditioning and dewatering were investigated.The optimal modified method was that MRB-Fe was prepared from raw rice husk biochar(RB)modified by Fe Cl3 solution(3 mol/L)for 1 h.The optimal MRB-Fe dosage was 60% DS.Under this dosage,the sludge SRF decreased by 97.9%,the YN increased by 28 times,the moisture content of sludge cake increased from 96.69% to 77.97%,and the SV30 decreased from 96% to 60%,compared with raw sludge.The microstructure and surface element content of MRB-Fe were also investigated,the results illustrated that the negative charge of sludge particles could be counteracted by the positive charge from iron species on MRB-Fe surface,causing the improvement of sludge settleability and dewaterability.The zeta potential of sludge filtrate and the microstructure and coefficient of compressibility of sludge cake were investigated to analyze the sludge conditioning mechanisms of the four types of rice husk-based skeleton builders.On the one hand,all the zeta potentials of sludge conditioned with rice husk-based skeleton builders more approached 0 m V,compared with sludge without rice husk-based skeleton builders,which mean that the conditioned sludge particles with rice husk-based skeleton builders attracted each other and formed an unstable system to cause great settling and dewatering.On the other hand,large cracks were found on the cake of sludge conditioned with rice husk-based skeleton builders and the coefficient of compressibility of conditioned sludge cake was less than that of raw sludge cake,which illustrated that the porosity and permeability of sludge cake was promoted by adding rice husk-based skeleton builders,so the sludge moisture could moved out more easily and the sludge dewaterability was improved.The effects of four types of rice husk-based skeleton builders on sludge characteristics were compared.The results showed that Biochar-conditioned was the best rice husk-based skeleton builder to improve sludge dewaterability and settleability and to decrease the turbidity and SCOD of sludge filtrate.However,the rice husk biochar was the best choose for sludge further disposal especially for agricultural application with the higher organic content,lower heavy metal content because of the lower heavy metal content and higher specific surface area.Biochar-conditioned was combined with KMn O4 and Fe Cl3 for sludge conditioning enhanced sludge dewaterability.When the sludge was firstly destroyed by KMn O4 oxidation(20 g/kg DS),then Biochar-conditioned(70% DS)addition and lastly Fe Cl3(138.09 g/kg DS)flocculation,the sludge SRF decreased by 97.5%,the YN increased by 20.8 times,the moisture content of sludge cake increased from 94.71% to 74.64%,compared with raw sludge.Sludge zoogloea was destroyed by KMn O4 oxidation,the sludge flocs were reconstructed by electrostatic neutralization of Fe Cl3 flocculation,and Biochar-conditioned addition built a sludge cake with incompressible and permeable structure to further improve the sludge dewaterability.