Abstract

This report documents the activities performed and the results obtained from the one-year arsenic removal treatment technology demonstration project at the Big Sauk Lake Mobile Home Park (BSLMHP) in Sauk Centre, MN. The objectives of the project are to evaluate (1) the effectiveness of Kinetico’s Macrolite® pressure filtration process in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 μg/L, (2) the reliability of the treatment system, (3) the required system operation and maintenance (O&M) and operator skill levels, and (4) the capital and O&M cost of the technology. The project also is characterizing water in the distribution system and process residuals produced by the treatment system.

BSLMHP provided water to 37 mobile homes with an average daily demand of 7,500 gal. Source water contained 27.5 μg/L (on average) of total arsenic, 2,385 μg/L of total iron, and 130 μg/L of total manganese. Because of the reducing condition with the source water, almost all iron and manganese existed in the soluble form and over 80% (on average) of arsenic existed as soluble As(III). The remainder of arsenic was present as soluble As(V) (13%) and particulate arsenic. The source water also contained, on average, 3.3 mg/L of total organic carbon (TOC), 1.2 mg/L of ammonia (as N), and 417 μg/L of phosphorous (as P).
The Macrolite® CP-213f arsenic removal system evaluated consisted of a KMnO4 feed system, two 36-in × 57-in contact tanks (205 gal each), and four 13-in × 54-in pressure filters (two for each duplex unit) arranged in parallel. Potassium permanganate (KMnO4) was used to oxidize As(III) and Fe(II) prior to Macrolite® pressure filtration. KMnO4 was selected over chlorine due to the presence of elevated TOC and ammonia in source water. Each pressure filter contained 20 in (or 1.5 ft3) of Macrolite®, a low-density, spherical media (40 × 60 U.S. Standard Mesh) designed for a filtration rate two times higher than a conventional gravity filter. The design flowrate was 20 gal/min (gpm), which yielded 20 min of contact time prior to filtration and 5.4 gpm/ft2 of hydraulic loading to the Macrolite® filters. Because of the on-demand operation, the actual flowrates ranged from 1 to 15 gpm, corresponding to 27 to 412 min of contact time and 0.3 to 4.1 gpm/ft2 of hydraulic loading.

From July 13, 2005, through October 1, 2006, the well operated for a total of 2,052 hr at approximately 4.6 hr/day. The system treated approximately 2,017,000 gal of water with an average daily demand of 4,523 gal. KMnO4 effectively oxidized As(III) in source water even in the presence of TOC, as evidenced by reducing its concentrations from 21.9 μg/L (on average) to 1.0 μg/L after contact tanks and forming an average of 22.7 μg/L of particulate arsenic with arsenic presumably bound to iron particles.

During the performance evaluation study, total arsenic levels in the treated water were reduced to an average of 6.4 μg/L mainly in the soluble form. Out of 60 sampling events, arsenic concentrations in treated water exceeded the 10-μg/L MCL for a total of 13 times, mostly due to particulate breakthrough from the Macrolite® filters. To address particulate arsenic breakthrough, the backwash frequency was increased incrementally from every 2,743 gal to every 916 gal of throughput for each filter.

With an average soluble iron to soluble arsenic ratio of 88:1, there was sufficient natural iron present in the source water for effective arsenic removal. Soluble iron was oxidized by KMnO4 to form iron particles, which adsorbed and/or co-precipitated with arsenic before being removed by the filters. Total iron concentrations in the treated water ranged from <25 to 2,363 μg/L and averaged 204 μg/L. An increase in particulate iron correlated with an increase in particulate arsenic, indicating particulate breakthrough from the Macrolite® filters.

The high levels of TOC in the source water appeared to have inhibited the formation of filterable manganese solids. Before November 15, 2005, with the addition of 1.4 to 3.8 mg/L of KMnO4, manganese concentrations after the contact tanks were present primarily in the “soluble” and/or colloidal form that passed through 0.45-μm disc filters, with levels ranging from 416 to 1,126 μg/L. A series of jar tests were conducted in the laboratory to determine if higher KMnO4 dosages might help overcome the TOC effect and form larger filterable MnO2 solids. Based on the results of the jar tests, the KMnO4 dosage was increased to 5.2 mg/L. After November 15, 2005, with the addition of 4.4 to 5.8 mg/L of KMnO4, soluble manganese concentrations after the contact tank, as determined by the use of 0.45-μm disc filters, were reduced to as low as 35 μg/L (on average during February 3 through June 15, 2006) with total manganese concentrations remaining as high as 1,179 μg/L. Meanwhile, total and soluble manganese concentrations, as determined, again, by the use of 0.45-μm disc filters, were reduced, on average, to 163 and 78 μg/L, respectively, during the same test period.

During the 15-month performance period, the control valve on top of each duplex unit was changed out five times to increase the backwash frequency in order to control the particulate arsenic, iron, and manganese breakthrough. The backwash frequency was increased from the initial field setting of every 2,743 gal to every 916 gal per tank. Thereafter, except for three events with elevated arsenic and iron concentrations detected in treated water, the treatment system was working properly as indicated by nine consecutive sampling events where both arsenic and iron were below their respective MCL and secondary maximum contaminant level (SMCL).

The backwash water contained, on average, 130 μg/L of total arsenic, 19.5 mg/L of total iron, and 7.2 mg/L of total manganese. Total suspended solids (TSS) levels ranged from 22.0 to 150 mg/L, averaging 72 mg/L. Based on 72 mg/L of TSS in 130 gal of backwash wastewater produced by one tank, approximately 35.4 g (0.078 lb) of solids were discharged to the septic system and then to a sanitary sewer, containing 63.7 mg of arsenic, 9.6 g of iron, and 3.5 g of manganese. Arsenic, iron, and manganese levels in the backwash solids averaged 2.03 mg/g (or 0.2%), 190 mg/g (or 19%), and 136 mg/g (or 13.6%), respectively.

In general, with the exception of manganese, the water quality in the distribution system has improved after installation of the treatment system, as evidenced by the reduced arsenic and iron concentrations and little or no changes to the pH, alkalinity, lead, and copper. For example, after the treatment system began operation, arsenic and iron concentrations decreased from average baseline levels of 23.4 and 2,791 μg/L to 8.1 and 173 μg/L, respectively. Manganese concentrations increased from average baseline levels of 130 to 397 μg/L due to the additon of KMnO4. Lead concentrations remained fairly constant and averaged 0.6 and 1.6 μg/L before and after system operation (except for a spike of 25.2 μg/L at DS3 on June 14, 2006). Copper concentrations increased from the baseline level of 1.8 to 18.5 μg/L, including a spike of 228 μg/L. Alkalinity and pH concentrations remained fairly constant.
The capital investment cost was $63,547, which included $22,422 for equipment, $20,227 for engineering, and $20,898 for installation. Using the system’s rated capacity of 20 gpm (28,800 gal/day [gpd]), the capital cost was $3,177/gpm ($2.21/gpd). The O&M cost for the Macrolite® CP-213f system included only incremental cost associated with the chemical supply, electricity consumption, and labor. The O&M cost was estimated to be $0.36/1,000 gal during the entire performance evaluation period.

Contact

Thomas J. Sorg

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