Study of coagulant effective dose for water treatment plant in Semarang City

Water is an important component which community need, especially for clean water consumption. Water treatment plant is one of the solutions as fulfilment of clean water needs that guaranteed quality, quantity and continuity. Determination of the effectiveness of the type of coagulant is used to guarantee the quality of raw water according to the requirements of water quality of drinking of health minister of Republic Indonesia 492/2010 law. Blorong River is one of the water sources in Semarang City which has characteristics of turbidity of 41.1 NTU; pH 7.11 and TSS 110 mg/L. The aim of this study is to determinate effective type and dose from several coagulants. The method uses the Jar Test method with a variety of velocity gradients. The first variation is 550/sec and 100/sec, the second variation is 650/sec and 80/sec, and a third variation 700/sec and 60/sec. The types of coagulants used are Poly Aluminium Chloride (PAC), Aluminium Sulphate and Chemical Mixing Alum with parameters observed are turbidity, pH, total suspended solids (TSS), and flock size. This study resulted coagulant PAC as an effective coagulant with an optimal dose of 10 ppm resulting in a pH of 6.71, turbidity 1.65 NTU, TSS 34 mg/L and flock size 1.7 cm at the third velocity gradient. Based on the planning discharge of 200 L/sec, the PAC coagulant costs IDR100,000 L/day.