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Electro-dewatering of sewage sludge: Effect of near-anode sludge modification with different dosages of calcium oxide

Wei, Yijun, Zhou, Xingqiu, Zhou, Lang, Liu, Changyuan, Liu, Jiangyan
Environmental research 2020 v.186 pp. 109487
anodes, calcium oxide, dewatering, electrical conductivity, electrical resistance, electrolysis, energy, pH, protons, sewage sludge, water content, zeta potential
The efficiency of common sludge electro-dewatering (EDW) is restrained by the following issues: 1, the near-anode sludge dries out quickly, causing a rapid increase in electrical resistance; 2, the pH at anode decreases by the accumulation of H⁺ from the electrolysis of moisture, resulting in a decrease in Zeta potential (ζ). Alleviating the negative impact of these problems is the key to improving the dewatering efficiency of EDW. Therefore, in this study, calcium oxide (CaO) was used for near-anode sludge modification to increase its pH and electrical conductivity. With increasing CaO dosage, pH rose from 6.0 to 12.2, electrical conductivity increased from 368 ± 16 μS/cm to 6285 ± 21 μS/cm and the ζ declined from −15.3 ± 0.6 mV to −8.8 ± 0.4 mV. The EDW tests were conducted at 30 V and 25.5 kPa. The results indicate near-anode sludge modification with CaO weighing 3%–5% mass of raw sludge (mu(RS)) improved the EDW effect, while the energy consumption increased slightly. When 3%–5% mu(RS) of CaO was added, the final moisture content of sludge was 54.5%–44.3%, below that of the blank group (no CaO added), which was 57.9%; the time to obtain target moisture content (60%) was 910 s–590 s, lower than the blank group's 1060 s; and the energy consumption to obtain target moisture content was 0.233 kW h/kg H₂O–0.271 kW h/kg H₂O, higher than the blank group's 0.157 kW h/kg H₂O. A quantitative criterion (KsiEDW) was adopted to assess the feasibility of EDW. Economically and energetically, the experiment with 4% mu(RS) of CaO added for near-anode modification was the optimal condition in this research, due to its second smallest KsiEDW, the best sludge reduction effect (67.2%), lower final moisture content (46.2%) and less time (640 s) to obtain target moisture content. The results show some mechanisms of EDW and provide experience for practical application.