For

Drinking Water Clarification

Abstract

Alum or alum-polymer blends are often used for raw water clarification. The major historical advantage of alum type chemistry was its low purchase cost and availability. Major disadvantages include relatively high use cost (high doses are typically used), pH and alkalinity suppression requiring the addition of pH adjustment chemicals, and high sludge production. This case study introduces the use of, aluminum chlorhydrate, as an alternative clarification chemical with advantages over alum and alum/polymer blends. Although used extensively around other parts of the country, aluminum chlorhydrate was not readily available in Ohio and surrounding states due to its high freight costs from distant manufacturing plants. Now that the product is manufactured in northern Ohio, its availability and use cost have made it an attractive alternative to alum in both performance and cost savings.

Following is data gathered during a 12 day trial that supports the advantages and cost savings associated with the use of aluminum chlorhydrate over alum.   This trial was conducted during 2007 before the steep rise in alum and caustic prices.

Drinking Water Clarification

A drinking water facility, located in northern Ohio, processes 4-6 million gallons per day of Lake Erie water for potable use. The raw water intake is occasionally influenced by a nearby river depending on wind direction. The treatment process consists of initial screening, chemical addition with rapid mix, slow mix, clarifiers, and gravity filters.  Alum or an alum-polymer blend (depending on performance) is used as the primary clarification treatment.

The typical alum-polymer dose prior to the start of the trial was in the 30-50ppm (as dry alum) range, feeding approximately 3000lbs (270 gal) of liquid alum-polymer per day. Typical polymer dosage with the alum-polymer blend is 7% of the coagulant dosage or 2.0-3.5 ppm. Treatments were dosed to the rapid mix section of the process.

Aluminum Chlorhydrate was dosed at approximately 12 ppm (as liquid), feeding 500-540lbs (45-48 gal) per day. DADMAC polymer was fed separately at 0.5-1.0 ppm. It was determined that the addition of polymer aids in the treatment process due to the influence of a river.

Because of the large pH reduction associated with high alum feed, the raw water pH was consistently reduced from ~8.0 to ~7.1. Caustic was added to the filtered water to raise the pH to at least 8.3 to minimize distribution system corrosion. Caustic use was typically 1400lbs per day.  

Turbidity, pH, UV254, and cost data were gathered to benchmark the performance of the aluminum chlorhydrate chemistry. The following sections discuss the performance of each benchmark.

During the initial 24 to 48 hours after the start of the trial the final filtered water turbidity climbed slightly and then returned to the typical filtered water turbidity or slightly better. The initial increase in turbidity was determined to be from the effect the change in clarified water pH, which now was operating in the 7.8-7.9 range, had on the sand filters. Once each sand filter was backwashed, the filtered water quality improved. Most likely residual aluminum from the original alum program was being cleaned from the filter media with the change in pH.

Note the trend lines that indicate even though the raw water turbidity trended upwards, the finished water turbidity improved by trending downward. 

The high basicity of aluminum chlorhydrate minimizes its effect on treated water pH. Where alum type chemistry suppresses pH, aluminum chlorhydrate has minimal effect on treated water pH, eliminating or greatly reducing the amount of pH adjustment chemistry needed. In this particular application, daily caustic usage was reduced by 50% resulting in cost savings.  

Note the gradual increase in clarified water pH at the onset of the trial. Once the pH leveled off it mimicked the raw water pH, following within 0.1-0.2 pH units. Once the trial was completed and alum was returned to the system the pH quickly dropped to previous levels. Caustic use was reduced by ~700lbs per day during the trial period.

UV-254 (ultraviolet absorption at 254 nm) typically is used to quantify DBP (Disinfectant By-Products) precursors. UV254-absorbance testing is typically done as an indicator to measure NOM (Natural Organic Matter) removal. Although not used as a direct measure of TOC (Total Organic Carbon), UV254-absorbance measurements can be used to gauge removal performance.

Organic removal as measured by UV254 remained consistently in the 70-90% removal range and gradually improved near the trial end. Organic removal remained consistent with that of alum and did not require pH suppression, which is typically done for effective removal with alum.

Cost analysis was done comparing the two programs quantifying the savings associated with the use of aluminum chlorhydrate.

Conclusion

Plant personnel were pleased with results and have implemented a full-scale change in the treatment chemistry. The ease of use and cost savings associated with the aluminum chlorhydrate program makes it an attractive option to typical alum programs and is rapidly becoming the coagulant of choice in the clarification of raw water.