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Chignik Lake, Alaska is a small community located on the south side of the Alaska Peninsula. The community's only potable water source is one groundwater well, which is located within close proximity to potential contamination sources. Due to this threat of contamination and per the recommendations contained in the sanitation facilities master plan completed on behalf of the community in 2000, local leaders desired to provide a new groundwater source that was not susceptible to the noted contaminant sources. Chignik Lake's proposed new potable water source consists of two groundwater wells that were installed in August 2003 by the Alaska Native Tribal Health Consortium (ANTHC). The purpose of the water treatment plant (WTP) pilot testing activities was to evaluate and compare the performance of different pressure filtration alternatives and to determine the optimum water treatment process(es) for Chignik Lake's new water source.

Pressure filtration has the capability of removing a wide variety of particulate contaminants. The pressure filter treatment process typically consists of a steel pressure vessel(s) with water to be filtered entering under pressure and leaving under a slightly reduced pressure due to headloss within the vessel (AWWA, 1999). Pressure filtration is commonly utilized in small water systems like Chignik Lake. Additionally, since the filtrate is leaving the vessel under pressure it can be distributed to the community system without the need for additional pumping.

The purpose of the WTP pilot testing was to perform the following:

Evaluate different pre-treatment chemicals to promote effective contaminant removal in the pressure filters prior to distribution to the community.

Select and optimize operating parameters including chemical type, chemical dosage and sequencing of chemical addition to meet the State of Alaska Department of Environmental Conservation (ADEC) primary and secondary drinking water quality standards in the most economical manner (maximizing filter run times).

The contaminants of concern for the pilot test were as follows:

Arsenic (As);

Manganese (Mn); and,

Hydrogen Sulfide (H2S [taste and odor]).

Prior to and during the pilot testing activities, a series of jar tests were conducted to evaluate the contaminant removal effectiveness and dosage rates for several different water treatment chemicals. Jar testing is the most common method for obtaining data on contaminant removal effectiveness for a water treatment process short of full-scale pilot testing. Jar testing mimics full-scale operation by simulating both flow through oxidation and solids removal processes. Following jar testing activities, the pilot testing process was modified based on the resulting data.

The presumptive treatment methodology for the removal of arsenic from drinking water supplies consists of adsorptive processes. This methodology, for the purposes of the Chignik Lake pilot testing consisted of chemical addition (i.e. Ferric Chloride [FeCl3], polymers, etc.) followed by filtration. Typical H2S removal processes consist of oxidation via either chemical addition or mechanical aeration. Removal of Mn is most commonly performed through the continuous regeneration, manganese greensand process. This process consists of potassium permanganate (KMnO4) injection followed by greensand filtration. This occurs via KMnO4 oxidizing Mn into an insoluble precipitate which is then readily filterable.

Typical manganese greensand filter arrangements consist of anthracite to extend filter run lengths, manganese greensand zeolite as the filter media, and filter garnet to support the filter media. Manganese greensand zeolite aids in filtration, adsorbs excess KMnO4, acts as an ion exchange media, and oxidizes reduced metals. In addition to the above discussed factors, considerations including filter surface loading rate and the presence of dissolved organics, which may limit filtration, oxidation and ion exchange ability of the manganese greensand media, were considered during the pilot testing activities.

Based on the results from the pilot testing activities, the contaminants of concern: As and Mn; can be effectively treated to the applicable ADEC water quality standards via KMnO4 and FeCl3 dosing followed by pressure filtration. Additionally, the aesthetic concerns associated with H2S can also be neutralized from the finished water using this treatment process. Results from the testing activities indicated that a single column filter operation outperformed a dual column filter operation.
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Document Type: Research Article

Publication date: 2007-01-01

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