A Comparative Study of Metallic Coagulants for Rawal Lake Filtration Plant

Abstract

Rawal Lake Filtration Plant (RLFP) was commissioned in 1962 by JICA with Aluminum Sulphate [Al2(SO4)3.18H2O] as the sole coagulant. Since its commissioning the water quality of Rawal Lake has been affected by influx of polluted streams and waste disposal by visitors, yet the management never attempted to revisit the coagulation process or the coagulant. The total treatment capacity of the RLFP is 24 MGD out of which about 22.5 MGD is supplied to cantonment area and some other parts of Rawalpindi city and remaining to NIH and surrounding areas. In general, the quality of raw water to RLFP deteriorates after wet spells and improves in dry season except for off-flavor complaints in summer. Thus there was a serious need to check some other coagulants for Rawal Lake Filtration Plant to overcome these problems. For this purpose three metallic coagulants: Aluminium Sulphate (Alum) [Al2(SO4)3.18H2O], Ferric chloride (FCL) [FeCl3] and Polyaluminium Chloride (PACl) [Aln(OH)mCl(3n-m))x] were selected. An attempt was made to add secondary coagulants such as cationic polyelectrolytes but was discontinued due to enormous rise in cost of treatment. In addition to determining optimum individual dose of each selected coagulant, a combination was also tried. Keeping in view the wide variations in turbidity and pH of raw water over the year, the turbidity and pH of lake water samples was modified between 40 – 120 NTU and pH from 6 to 8. Raw water 13 samples for this study were taken from the inlet of Rawal Lake Filtration Plant. The main experimental variables were coagulant type and dose, turbidity and pH of raw water and the order with which the coagulants were dosed. Whilst turbidity reduction was used as base-line criteria, other variables such as changes in pH, residual coagulant concentration and alkalinity consumption by coagulant were also used for comparison. Residual pH, color, residual alkalinity, fecal coliform, residual coagulant concentration, Electrical Conductivity (EC) and Total Dissolved Solids (TDS) were also investigated. Finally a cost analysis was conducted to examine long term economic implications of the change in coagulant type/dose. Results show that PACl exhibits the highest overall turbidity removal efficiency as compared to Alum and FCL at the lowest dose. PACl consumes less alkalinity when compared with other two coagulants. Residual coagulant concentration in case of PACl was also lower than the Alum and FCL. PACl also gives excellent removal efficiency for coliforms. PACl results at natural conditions (natural pH and turbidity) were far superior to the other two coagulants used, except that it was relatively expensive. Alum and PACl at the ratio of 90:10 & 95:5 with PACl as order 1st gives better results than Alum alone. At these combinations the cost of the coagulants is quite comparable with Alum alone.