A conventional water treatment plant (WTP) typically involves coagulation-flocculation processes to remove suspended particles and colloids in raw water. The process generates a large volume of alum sludge with high aluminum content, which is discharged into a river with improper treatment. The sludge may cause a potential risk to human health, and disrupt the life of river biota. The aims of this study were to determine the physical and chemical characteristics of alum sludge from Surabaya WTP, and to compare them with those of alum sludge from other plants in Indonesia and developing countries. The alum sludge sample was obtained from the Surabaya WTP in Indonesia. The results showed that the alum sludge had a pH value of 7.47, volatile solids of 12,696 mg/L, total suspended solids of 12,511 mg/L, chemical oxygen demand (COD) 9666.7 mg/L, biochemical oxygen demand (BOD) 1082.5 mg/L, and sludge volume index 114.18 mL/g. The sludge had high aluminum and iron concentrations. The aluminum content of the sludge was 1194 mg/L, iron 515 mg/L, chromium 0.217 mg/L, and copper 0.559 mg/L. Having a BOD/COD ratio of 0.1, the alum sludge contained high level of nonbiodegradable organic matter.

Untreated alum sludge from Surabaya water treatment plant (WTP), which contained high concentration of alum was directly discharged into Surabaya River. It might cause problems because of the accumulation of aluminum in the lower part of the river. Alum could be recovered from the drinking water sludge using the electrolysis method. Aims of this study were to determine the optimum pH and electrical current for electrolysis using carbon-silver electrodes to recover aluminum coagulant from the sludge, and to determine the amount of the recovered alum. The sludge was acidified prior to electrolysis. Acidification was done by adding sulfuric acid at pH 3 and 4. Polarization test was conducted at 100, 200, and 300 mA, in order to determine the optimum electrical current. The electrolysis was performed in one compartment batch recirculation reactor, using silver as cathode and carbon as anode for 10 hours. Values of pH were measured every hour. The precipitated matter in the cathode was weighed, and analysed by means of Inductively Coupled Plasma. The optimum conditions of the electrolysis were achieved at initial pH 3 and electrical current 300 mA. The electrolysis resulted in the highest precipitate of 2.6112 g in the cathode.

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Coagulation process using aluminum sulfate (Al2SO4) or alum as coagulant at a drinking water treatment plant generates a large volume of alum sludge. The alum sludge contains high concentrations of aluminum and organic matter, which are harmful to the environment. Organic contaminants can decrease the conductivity and mobility of ions in electrolyte, so it can reduce the effectiveness of electrolysis to remove the aluminum. Aims of this study are: (1) to determine the aluminum and COD removal in electrolysis process using carbon-silver electrodes and (2) to measure the efficiency of COD removal by H2O2 oxidation and UV light exposure. The sludge was acidified to pH 3 and 4 and centrifuged prior to electrolysis. The electrical current was 300 mA, based on polarization test using carbon silver as anode and cathode electrodes. Electrolysis was carried out in a batch recirculation one-compartment reactor for 10 hours. Deposited matter of alum at cathode was weighed and analyzed by Inductively Coupled Plasma. The electrolysis pretreatment was meant to remove high organic contaminants in electrolyte. The pretreatment was carried out by oxidation with addition of H2O2 with a ratio of [H2O2]/[COD] = 1/2 and 254 nm UV light exposure for 4 hours. This study resulted in low aluminium deposit of 3.19 mg at the cathode with COD removal of 11.11% at initial pH 3 using electrolysis. The highest COD removal efficiency by oxidation with addition of H2O2 and UV light exposure was 75%. It can be concluded that H2O2 oxidation and UV light exposure could be applied prior to electrolysis for optimizing aluminum removal.