Wastewater treatment processes produce large quantities of sludge commonly containing over 99% water, the sludge transportation is difficult and cost is high because the high sludge moisture content is often accompanied by a large sludge volume. Therefore, the development of sludge dewatering technology has become one of the important topics for the research of environmental protection workers in China. The traditional mechanical dewatering method has limited effect on sludge dewatering, and the application of electric field assist on the basis of mechanical dewatering can often achieve deep dewatering effect.
In this paper, the EPS composition and distribution characteristics at both electrodes in sludge electro-dewatering process were studied under different operating conditions ( voltage, pH and ionic strength). Meanwhile, the sludge cake porosity at both electrodes were analyzed, the correlation between EPS distribution and sludge cake porosity and sludge electro-dewatering rate was clarified. On this basis, the inorganic chemical conditioning agent was used to pre-condition the sludge, and the influence mechanism of different chemical agents on the sludge electro-dewatering effect was investigated. The migration and transformation of harmful heavy metals in the process of electro-dewatering process was studied, and use the complexing agent citric acid to simultaneously enhance the electro-dewatering effect and remove heavy metal content. Main contents and results of this dissertation are as follows:
1. In this study, the effects of voltage, pH and ionic strength on sludge electro-osmotic dewatering performance and electrochemical effects were investigated. The solubilization and degradation of EPS were analyzed by examining the variation of dissolved organic matter (DOM) in the filtrate, and the relationships between microstructural properties of sludge cake and DOM and electro-osmosis dewatering performance in electro-dewatering process was examined. It was found that electro-dewatering properties were improved by raising the operating voltage or decreasing the pH value, while dewatering rate initially increased at low ionic strength it decreases with increased ionic strength. In addition, the porous structure of cathodic cake was more plentiful than that at the anode. At the cathode, the EPS dissolution was mainly related to alkalization, while the oxidation and acidification were responsible for release of EPS at the anode. Meanwhile, electrophoresis effect was able to promote migration of EPS toward the anode. The average electro-osmotic dewatering rate at the anode and at the cathode strongly correlated with the volume of pore of sludge cake. There was no correlation between the total content of anodic DOM and electro-osmotic dewatering rate at the anode, however, the content of cathodic DOM negatively correlated with average electro-osmosis dewatering rate of cathode. Since cathode is the main water-permeable side in sludge electro-dewatering, and the sticky biopolymers (proteins and humic subtances) could not be converted into small molecules, higher EPS release was associated with worse sludge filterability. As for the anode, the biopolymers were degraded into small molecules due to electrochemical oxidation, which greatly reduced the impact of DOM on dewatering effect.
2. The effects of pretreatment of three typical inorganic coagulants (FeCl3, PAC and HPAC) in the process of electro-dewatering were analyzed. The results showed that the electro-dewatering rate at cathode were increased with the raised of the coagulants dosage, and the rate of electro-dewatering increased the most after conditioning by FeCl3. As the dosage of HPAC, PAC and FeCl3 increased, the zeta potential of the sludge raised. Sludge flocs formed by FeCl3 conditioning are denser than PAC and HPAC, and the increase of sludge electroosmotic strength is the largest. The inorganic coagulant compresses the EPS structure of sludge, which reduces the soluble EPS content and relieves the filter cloth blockage. FeCl3 has the strongest compression effect on EPS structure, and it has a more important effect on improving the sludge dehydration effect than improving the ionic strength EPS compression of sludge system
3. The present study investigated the sludge electro-dewatering behavior and the heavy metal migration and transformation at varying operation conditions (voltage, ionic strength and chelating agent) during waste activated sludge electro-dewatering. Results indicated that increasing voltage improved the mobility of heavy metals, and electro-dewatering performance was improved by raising the ionic strength, The alkalization was enhanced at cathode and promoted the dissolution of EPS, which intensify the mobility of heavy metals bound with biopolymers, and the transformation ability of heavy metals in sludge cake from stable state to active state was enhanced. The addition of inorganic salts and chelating agents (citrate acid) promoted electro-dewatering of sludge, and changed the existing states of heavy metals in sludge and facilitate harmful heavy metal removal in sludge electro-dewatering. In addition, the content of heavy metals in cathodic filtrate is higher than that in anodic filtrate under different operating conditions due to the free heavy metals ions and positively charged complex heavy metals moved to the cathode chamber due to electro-migration.