Effects of Polyaluminium Chloride dosing on the performance of Coagulation (a case study of Barekese Water Treatment Plant)

In Ghana, the commonly used coagulant in water treatment processes is the Alum. However, its use has been associated with a number of disadvantages including the production of large volumes of post-treatment sludge, high post-treatment iron residue, reduction of water pH (requiring pH adjustment using lime), limited coagulation pH range of 5.5 to 6.5, etc. These shortfalls on the use of Alum have led to the synthesis of improved Aluminium-based chemicals, called polyelectrolyte (polymer), as alternative coagulant. Among the many polymers synthesized, what has been adopted for use at the Barekese Water Treatment Plant is the Polyaluminium Chloride (PAC). However, information obtained from the BWTP indicates that no detailed preliminary studies was conducted to determine optimal operational conditions such as the coagulant dose, mixing speed, mixing time, retention time and pH of the raw water that influence effective performance of the PAC. This study is thus concerned with identifying such optimum operational conditions necessary to enhance efficient performance of the PAC in water purification. The effects of three different mixing speeds on the purification process were investigated (i.e 180:40, 180:25 and 150:25 revolutions per minute). The 180 and 150 rpm in the later parenthesis represent the fast mixing speeds whiles the 40 and 25 rpm represent the slow mixing speeds. The duration of mixing the dosed PAC with the water (mixing time) was also varied to ascertain its effects on the contaminant removal. In one set of the experiment, the fast mixing time was 5 minutes and the slow mixing time, 10 minutes (i.e 5:10). In the other set, the fast mixing time was reduced to 2 minutes and the slow mixing time, 5 minutes (2:5). The pH range within which this study was carried out was 6.0 to 8.5, at interval of 0.5. From the study, the mixing speed that was found to promote efficient coagulation was the 150 rpm as fast mixing speed and 25 rpm as the slow mixing speed (i.e 150:25). Using the reduced mixing time (2 minutes) for the fast speed and 5 minutes for slow mixing speed, the treated water recorded a better water quality. The optimum dose of the PAC that promoted best contaminant removal was 15 ppm. Within the raw water pH range of 6.0 to 8.5 tested it was realized that the effect of pH changes on the performance of the PAC was minimal. A pH 6.0, the PAC-treated water had a turbidity of 4.0 NTU was obtained whiles at pH 8.5; the treated water recorded a turbidity of 3.11. Generally, it was realized that 15 minutes settling time was adequate to sufficiently remove turbidity, colour and iron from the raw water samples. The recorded residual Aluminium in the flocculated water was below the acceptable guideline value of 0.2 mg/l. The cost benefit analysis conducted revealed that the production cost can be reduced by 50% when this optimal PAC dose determined and the conditions recommended are applied. It is therefore proposed that before PAC is adopted as the key coagulant at any water treatment plant, a thorough preliminary study be conducted to determine the optimal operational conditions and dosage.

Source: KNUST