| Wastewater treatment plants(WWTP)can produce large volumes of activated sludge with high moisture content by biological process.The moisture content of sludge is still as high as 80% after the mechanical dewatering process.Thus,it is necessary to deeply dewater the mechanically dewatered sludge to meet the sludge disposal requirements.Electro-dewatering can quickly reduce the moisture content from 80% to60%,which is a new high-efficiency sludge deep dewatering technology.In recent years,electro-dewatering technology has been rapidly developed,but there are still some bottleneck problems,such as the low efficiency at the later stage of electro-dewatering and the ultimate moisture content of electro-dewatered sludge being difficult to less than 50%.Those bottleneck problems limit the industrial application of electrodewatering technology.Based on this,this paper firstly studied the electrical conduction and water holding mechanism of activated sludge.The variations of apparent electrical resistivity and water holding capacity of sludge during the electro-dewatering process were investigated,and the fundamental factors leading to the low efficiency in the later stage of electro-dewatering were revealed.Then,the traditional electro-dewatering process was improved and optimized.The dewatering characteristics and mechanism of electro-dewatering processes,such as the electro-dewatering of sludge near the anode and the cathode,and the different iron forms(ZVI vs.Fe(II))/ peroxydisulfate(PDS)combined vertical electro-dewatering(VED)(ZVI/PDS-VED and Fe(II)/PDSVED),were investigated.The electro-dewatering efficiency was further improved and the energy consumption was decreased,promoting the development of electrodewatering technology.The variation characteristics of apparent electrical resistivity of sludge during the electro-dewatering process were studied.It was found that the apparent electrical resistivity of sludge was lower than 5000 Ω/m at the early stage of electro-dewatering.When the moisture content was below 60%,the apparent electrical resistivity of sludge gradually increased from the cathode to the anode,and the highest value was71750±8602 Ω/m.The formation of the nonionic chemical systems and the gas barrier layer increased the apparent electrical resistivity of sludge near the anode,and the decrease of moisture content and conductivity leads to the increase of the apparent electrical resistivity of sludge near the cathode.Moreover,the release of electrolytes and the electro-migration had a significant effect on the apparent electrical resistivity of sludge.The variation characteristics of the water holding capacity of sludge during the electro-dewatering process were studied.It was found that compared with the sludge near the anode,the sludge near the cathode had lower filterability and higher saturated moisture content,showing a higher water holding capacity.The protein/polysaccharide ratio of loosely bound extracellular polymeric substances(LB-EPS)was significantly positively correlated with the specific resistance to filtration(SRF)of sludge(r=0.891,p<0.01).At the later stage of electro-dewatering,the voltage of 40 V could destroy the microstructure of the sludge near the anode,resulting in a decrease in water holding capacity.It was found that a great difference in the electrical conduction and water holding characteristics of the sludge near the two electrodes during the electro-dewatering process.Thus,the electro-dewatering performance and energy consumption of sludge near the anode and the cathode were further studied.The results indicated that when the average moisture content of sludge was 70%,the dewaterability of raw sludge was higher than that of sludge near the two electrodes.The energy consumption of the sludge near the cathode was only 20.3% of that of the raw sludge and 43.4% of that of the sludge near the anode at the end of dewatering.When the average moisture content of sludge was 60%,the sludge near the anode was unable to be dewatered,and the dewatering energy consumption of the sludge near the cathode increased significantly.The moisture on the cathode side of raw sludge and the sludge near the cathode were difficult to be removed by electro-dewatering.The degradation of protein in EPS was an important factor to decrease the moisture content of electro-dewatered sludge.The above studies showed that the existence of organic matters with high water holding capacity such as EPS was the main factor affecting the electro-dewatering performance of activated sludge.Therefore,to further destroy the EPS,the sludge was conditioned with different iron forms(ZVI vs.Fe(II))/PDS and then electro-dewatered.The effects of ZVI/PDS-VED and Fe(II)/PDS-VED processes on the dewatering performance of activated sludge were studied.The results indicated that with the 0.3g/g DS of iron activators,0.583 g/g DS of PDS,and 30 V of voltage,the moisture contents of sludge after ZVI/PDS-VED and Fe(II)/PDS-VED reached the minimum values of 50.6±1.2% and 32.1±1.5%,respectively.The ZVI/PDS and Fe(II)/PDS conditioning decreased the difference in sludge moisture content between the anode and the cathode during the electro-dewatering,facilitating the water homogenization in the sludge.ZVI/PDS-VED and Fe(II)/PDS-VED could effectively decrease the bound water and the free water.Free water content was positively correlated with α-helix(r=0.999,p<0.05)and C=O(r=0.998,p<0.05).Compared with the traditional VED and the ZVI/PDS-VED,the Fe(II)/PDS-VED had a greater degradation efficiency of EPS,which could reach 82.6%.It improved the surface hydrophobicity of sludge,and thus significantly improved the dewatering performance of sludge.The studies of this paper extended the basic theory of the electrical conduction and water holding mechanism of activated sludge in the electro-dewatering process,and explored many methods to improve the electro-dewatering performance of activated sludge,providing support for the industrial application of the electro-dewatering technology. |
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