Question

Discuss in detail how coagulation and flocculation are performed at a drinking water treatment plant. Describe how the optimum coagulant dose can be determined to treat fresh surface waters for drinking water supply. For removal of flocs from drinking water, when should settling basins be used and when should dissolved air flotation be applied? Why are GAC and PAC used in treating drinking water

Answer 1

Coagulation and flocculation practices are essential pre treatments for many water purification systems. Coagulation-flocculation is a chemical water treatment technique typically applied prior to sedimentation and filtration to enhance the ability of a treatment process to remove particles Dissolved and suspended particles are present in most of natural waters

In conventional coagulation-flocculation a coagulant is added to the source water to create an attraction among the suspended particles. The mixture is slowly stirred to induce particles to clump together into flocs. The water is then moved into a quiet sedimentation basin to settle out the solids.

Drinking Water Treatment-Flocculation. ... The water flows into a tank with paddles that provide slow mixing and bring the small particles together to form larger particles called flocs. Mixing is done quite slowly and gently in the flocculation step.

When water contain light particles such such as algae, which are difficult to settle. dissolved air flotation (DAF) can also usually achieve lower effluent turbidity than settling, typically less than 0.5 NTU. It is not as sensitive to temperature, especially cold temperatures as is common in settling, and the start up time is very short, approximately 30 minutes. Further, DAF does not need to generate heavy floc for settling, so lower coagulant dosage and shorter flocculation time can be used.

GAC A filter with granular activated carbon (GAC) is a proven option to remove certain chemicals, particularly organic chemicals, from water. GAC filters also can be used to remove chemicals that give objectionable odors or tastes to water such as hydrogen sulfide (rotten eggs odor) or chlorine.  

PAC Polyaluminum Chloride (PAC) compounds were developed to provide better performance than alum could offer. While they accomplished this goal, they also provide additional cost benefits when compared to alum. PAC has a minimal impact on pH and therefore minimized the need to feed adjustment chemicals. And it is able to do a better job while using 30-60% less aluminum on average. This translates roughly to a similar percent reduction in the amount of sludge produced.