Chicago, IL Projects

Studies of the Application and Removal of PAC at the Jardine & South Water Purification Plants

Addition, Persistence and Removal of PAC

The City of Chicago used less than 3000 kilograms (kg) of powdered activated carbon (PAC) in water treatment in 1992. However, in response to ‘taste-and-odor’ complaints from consumers, PAC use was progressively increased to 876,000 kg (1993), 3,300,000 kg (1994) and 3,400,000 kg (1995).

The present study was conducted in mid-October, 1996 when PAC feeds at the Jardine and South Plants were 10 and 20 pounds per million gallons, respectively. PAC is applied to Lake Michigan water in the mixing basins of both plants along with alum coagulant. After mixing, the waters are flocculated and settled, resulting in the early removal of a portion of the PAC applied. One of the objectives of the study was to determine the degree of removal of the PAC by alum flocculation and settling.

Following settling, additional solids removal is accomplished by sand filtration.

Samples were taken for microscopic particle analysis from sampling lines from the respective rapid mix basins (Mixing Basin Effluent), settling tanks (Settling Basin Effluent) and after filtration (Clear Well).

The reduction of the PAC mass concentration to an average of 0.09 mg/l in the settled water would indicate highly effective coagulation and settling. The efficiency of coagulation and settling for PAC removal was found to be 97.91%. There appears to be relatively little PAC ‘carryover’ to the filters.

PAC Particles in the Finished Water

Of the PAC particles found in Chicago’s finished water samples, approximately 95% were particles 3 µm or smaller. Such small particles provide little surface area for attachment of bacteria or larger organisms. The likelihood of these PAC particles providing potentially pathogenic organisms protection against disinfection would appear to be small.

Assuming that only particles larger than 3 µm might provide protection against disinfection, approximately 1.4 PAC particles per milliliter in the finished water would be considered particles of potential health significance.

References

Chicago Water Department
1000 East Ohio Street
Chicago, IL 60611-3416
312-744-7733

Relevance

• Determination of removal of applied PAC during water treatment to estimate PAC residence time.
• Microscopic enumeration and sizing of PAC particles in finished water.