AWWA ACE70799

The objective of this project was the optimization of removal of dissolved organic carbon (DOC) inthe water treatment process at a typical lime softening plant (Sanford, Manitoba, Canada). The DOC of La Salle River, Sanford's raw water supply, varied from 8.9 mg/L to 31.8 mg/L in 2005 to 2008. The treated water DOC from the plant varied from 6.1mg/L to 6.8 mg/L. The plant uses gas chlorine for disinfection; the chlorine dose applied at the plant is about 5.0 mg/L. The THM level in the plant effluent varies from 122.0 ppb to 163.0 ppb, which exceeds the THM regulations (100 ppb in Canada). THM levels in the distribution system ranges between 100.0 ppb to 200.0 ppb. This is due to the extended contact time of DOC and chlorine within the distribution system, and additional chlorine dosage is added at the local water storage reservoirs. Currently, the DOC is not effectively removed at the plant as both coagulation and lime softening takes place in the same reactor where pH is above 11. Optimal removal of DOC in chemical coagulation requires much lower pH ranging from 5.5 - 6.5. One of the objectives of this research was to reduce the dissolved organic carbon (DOC) in a separate optimized coagulation reactor prior to lime addition. Another goal of this research was to investigate the relationship between THM precursors. Coagulation tests were performed with alufer S25 (a mixture with 25% alum and 75% ferric sulfate) in the summer and ferric chloride in the winter, followed by softening with lime and soda ash. Softened water was acidified to pH 8.5 with H₂SO₄ and filtered through 0.45 um glass filters, followed by chlorination with 7% sodium hypochloride (NaClO) solution. The results of laboratory jar testing indicate that the optimized coagulation reduces effluent DOC to 3.2 mg/L in the summer and 5.9 mg/L in the winter. The effluent THM level in the laboratory coagulated and filtered summer water was 58.5 ppb and 73.5 ppb with the addition of 8mg/L and 10 mg/L chlorine, respectively. The TTHM Formation Potential (TTHMFP) in the studied water was 160 ppb. Includes 32 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2009

Number of Pages:

32

File Size:

1 file , 1.4 MB