A Study On The Properties Of Water Treatment Plant Sludges And Effects Of Coagulants Conditioning On Their Dewatering Performance

The costs for the sludge transportation, treatment and disposal were large due to the high water contents in water treatment sludges. It is difficult for the water to be recycled and reused due to the high concentration of organic mater, turbidity and colour. It wasted a lot of water, and the direct emissions will cause serious pollution to environment. In order to solve these problems, many water treatment plants used flocculants to achieve the reduction of sludge.The effects of properties of sludge and coagulants on the sludge dewatering performances were investigated with several sludge samples in different water treatment processes and with different raw water characteristics (low temperature and turbidity, high turbidity, high Alga, high concentration of organic matter). The experimental results are as follows.(1) The concentration of organic matter in the sludge was related to the concentration of DOC in the raw water, and the concentrations of organic matter were quite different. For example, the concentration of organic matter in the sludge samples were low in Shanghai, Nantong and Daqing water treatment plants sludges. The concentration of protein in sludge samples were low, while the concentration of organic matter for the other three sludge samples were high.(2) The results integrated sludge specific resistance, settlement ratio and sunction time indicated that the sludge dewatering performances for Shanghai, Nantong water treatment plant were better than other water treatment plants. The particle size and Zeta potentials had significant effects on the conditioning performances.(3) Many inorganic ions such as Al, Fe, Ca, Mg were existed in the sludge, and Al and Fe were mainly produced by the dosage of coagulants. Ca and Mg ion mainly came from some particulate matter in raw water. The concentration for Cu and Zn in Tianjin sludge samples exceeded the standard, and the concentration of Hg in Nantong and Beijing sludge samples also exceeded. This will seriously hinder the resource utilization of sludges.(4) The results of conditioning indicated that the sludge dewatering performances were improved after treated by coagulants. For example, in Shanghai and Nantong water treatment plants sludges, high-efficiency polyaluminium chloride (HPAC) had more obvious advantages which had better electrical neutralization and flocculation ability. Anionic polyacrylamide (PAM) was relatively weak to improve the dewatering performance; the residual concentration of DOC was increased due to the residual PAM. As for the other water treatment plants, the concentrations of organic matter were larger, and the organic matter had significant effects on the coagulation performances. Polyaluminium chloride (PAC) and HPAC were not good in improving the dewatering performance of these sludges. The anionic PAM would increase the concentration of DOC, but not only it could counteract the negative charge on the surface of the sludge particles, but also its strong ability of adsorption bridging had significant effects on aggregation of particles and settlement of flocs. So it made the filtration and dewatering properties of sludge increases more, and showed the optimal effect of coagulants conditioning in the process of the experiment.