By Dominicus Brendler, Kemwater (Thailand) Ltd
Dec. 6, 2002 -- Pathum Thani Water Works are operated by BJT Water Co., Ltd, the operations arm of Thames Water International in Thailand.
At present the plant produces around 120,000 to 140,000 m3 of potable water per day. The raw water comes from the Chao Praya river, which also supplies most of the raw water for the Bangkok metropolitan area.
This raw water has a typical pH of around 7.5 and is of medium alkalinity. Due to the variation of the tropical Thai climate between rainy season and dry season, the turbidity values of the incoming raw water vary greatly. Whereas in the dry season turbidity values generally fall to between 20 and 50 NTU, peaks of 300 NTU can occur in the rainy season.
The treatment process at Pathum Thani was designed for polymer-assisted coagulation with alum. Pre-liming and post-liming adjust the pH to suitable levels for coagulation and distribution respectively. Due to the acidic nature of alum and its narrow coagulation range, pH liming is constantly required, which results in high maintenance costs. In the rainy season especially high amounts of lime and alum accumulate in the sludge production.
Kemwater proposed PAX-XL13, a highly basic aluminium chlorosulphate for a trial at this plant. Its lower coagulant requirement and the elimination of separate lime dosing would offer a substantial benefit. The tabel below gives a comparison of the typical specifications of the two coagulants.
PAX-XL13 provides several advantages, such as rapid floc formation, very low residual aluminium in the treated water and good dose efficiency, along with a wide coagulation pH. Adjustments of pH are not therefore required in the normal pH range found in potable water applications.
The respective doses of alum and PAX-XL13 in full-scale runs at the Pathum Thani Water Works during the rainy season can be seen in the graph, table 3 (below). Turbidity measurements were recorded before and after sand filters. During the period the raw water turbidity increased from 104 NTU during the initial period to a peak of 278 NTU.
When using PAX-XL13 no lime was required and the organic flocculant dose could be reduced. In fact higher flocculant doses resulted in increased turbidity. In addition PAX-XL13 showed much better dose sufficiency.
Kemwater (Thailand) Ltd. would like to explicitly acknowledge the help received from the staff of BJT Water Co. Ltd. throughout the trial.
Typical analysis Liquid alum PAX-XL13:
Al3+, % w/w 4,23 5,3
SO42-, % w/w 23 4
OH/Al, molar 0 0,9
Specific gravity, g/cm3 1,32 1,24
