Final Report: Arsenic Removal System for Residential and Point-of-Use ApplicationsEPA Contract Number: 68D02078
Title: Arsenic Removal System for Residential and Point-of-Use Applications
Investigators: Turchi, Craig S.
Small Business: ADA Technologies Inc.
EPA Contact: Manager, SBIR Program
Project Period: October 1, 2002 through July 31, 2003
Project Amount: $99,993
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text | Recipients Lists
Research Category: Water and Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)
Arsenic contamination in groundwater is a severe health risk to populations throughout the world. In the United States, the problem is most pronounced in the West, parts of the Midwest, and the Northeast. In response, the U.S. Environmental Protection Agency (EPA) announced a tougher drinking water standard for arsenic, lowering the standard from 50 ppb to 10 ppb. This change is expected to impact 10 percent of the Nation’s community drinking water systems. Although several technologies are readily amenable to incorporation in large water treatment processes, fewer options are available for small water systems, particularly those serving less than 500 users.
The goal of this research project is to develop and commercialize a complete arsenic removal system for point-of-use and point-of-entry (POU/POE) drinking water systems. The unit will combine a highly effective arsenic sorbent with an arsenic-monitoring sensor and alarm to alert the user that the bed requires replacement. The units will feature ADA Technologies, Inc.'s (ADA) new Amended Silicate™ sorbent—a material that exhibits high capacity and the ability to remove both forms of arsenic (arsenite and arsenate) that commonly are found in well water. The integrated arsenic sensor and alarm will indicate when the sorbent requires replacement, much as a home smoke alarm alerts the occupants to the threat of fire. The system will provide easy-to-maintain hardware for individual home use or deployment in small, centrally managed water systems.
Under Task 1 of Phase I, the ADA team examined the potential of arsenate detection via quartz crystal microbalance (QCM) technology. In this approach, an arsenate-selective sorbent material, the receptor, is bound to the QCM. As the receptor collects arsenate, its mass increases, and this is detected as a change in the vibration frequency of the quartz crystal. During Phase I, the team updated the sensor and test assembly hardware, tested arsenic-receptor coatings for the QCM, improved the QCM precleaning technique to prevent delamination, and tested the sensor with arsenate and phosphate solutions. The team demonstrated that the QCM technology could detect arsenate and that pH and temperature bias can be eliminated through the use of a dual-head sensor.
ADA examined electrocoagulation (EC) and acidic electrolyzed water (AEW) pretreatments as means of prolonging sorbent life. These two methods utilize low-voltage direct current potentials to modify the incoming water chemistry. "Dosage" control is provided by adjusting the voltage and current to electrodes in the water stream; no liquid additives are used. The purpose of the pretreatment is to minimize problems associated with interfering ions, fouling, or microbial growth in the sorbent columns. Tests showed that EC and filtration can remove arsenic by itself without the need of downstream sorbent columns. In a different configuration, the EC system was able to remove high concentrations of silica, a species known to foul packed columns, prior to sorbent columns.
ADA defined the AEW conditions necessary to add approximately 1 ppm hypochlorous acid to drinking water. This pretreatment serves to lower pH, kill microorganisms, and oxidize As(III), all of which enhance the performance of iron-based arsenic sorbents. The two pretreatment techniques subsequently were applied during the small-scale columns tests to determine if they enhanced sorbent performance. The EC process removed arsenic, but provided little benefit to downstream sorbent columns. The AEW process increased sorbent capacity by approximately 30 percent.
Throughout the period of this project, ADA continued to improve the performance of the Amended Silicate sorbent by modification of the formulation process. Amended Silicate recipes were compared to commercial granular ferric hydroxide (GFH). Isotherm capacity tests were performed using water designed to simulate Alamosa, CO, groundwater, and the "arsenic challenge water" based on the NSF/ANSI 53 test protocol. The Amended Silicate sorbent removed As(III) and As(V) species with equal efficiency and was unaffected by pH within the tested range of 6.0 to 8.1. Formulation UV4-80/20/2, used for the small-scale columns tests with the arsenic challenge water, exhibited capacity matching or exceeding GFH. Further improvements led to the formulation 7162003-V4, a sorbent that exceeded GFH capacity by a factor of seven in isotherm testing. Analysis of the sorbent using Mössbauer spectroscopy confirmed that changes to the amendment process were creating the favored iron mineral phase for arsenic sorption. Even more encouraging was the fact that the iron content of the 7162003-V4 formulation was lower than either UV4-80/20/2 or GFH, indicating that a highly efficient Fe(OH)3 phase was produced, and that greater capacity may be achieved by increasing the iron loading. Comparative results are shown in Figure 1.
Figure 1. Isotherm data for two Amended Silicate formulations and commercial GFH (US Filter). Data obtained with "arsenic challenge water" at pH 8.5.
Small-scale column tests compared the performance of UV4-80/20/2 with GFH, with and without pretreatment processing. Columns were filled with equal volumes of the two sorbents, but due to differences in bulk density, roughly four times more GFH was used by weight. Column life ranged from 3,500 to more than 5,000 bed volumes when tested with the arsenic challenge water at pH 8.5. The EC process removed arsenic to less than 10 ppb by itself, but provided little benefit to the downstream sorbent columns. Use of AEW pretreatment extended column life by approximately 30 percent.
Subsequent dissection of the columns revealed a clear arsenic front within the GFH column but showed that the Amended Silicate was poorly utilized. The different morphology of the Amended Silicate material will require different column configurations than are recommended for GFH. Hence, effective utilization of the Amended Silicate capacity will depend on proper column design—a key element of Phase II. Both materials passed Toxicity Characteristic Leaching Procedure testing on the arsenic-saturated sorbent; maximum arsenic concentration in the leachate for both sorbents was 100-fold lower than the EPA allowable limit of 5 mg/L.
Amended Silicate cost is estimated at $0.50/lb, versus $3-$4/lb for GFH. Cost estimates were made assuming 100 ton/year production. ADA and CH2M Hill (Denver, CO) recently announced the formation of a joint venture company, Amended Silicates, LLC, to promote, produce, and market Amended Silicate sorbents.
The Phase I project has shown clear promise of the Amended Silicate material and has verified the feasibility of QCM detection of arsenate. Isotherm tests have shown that the Amended Silicate can achieve seven-fold higher capacity versus commercial GFH on a mass basis (150 percent of the capacity on a volume basis) when tested in the arsenic challenge water. This, coupled with the lower estimated cost of the Amended Silicate, indicates a clear economic advantage for the new sorbent. Electrochemical pretreatment can enhance the performance of arsenic sorbents, but the economic value will depend strongly on incoming water quality. EC treatment may be suitable as a primary treatment process for arsenic removal, and can be configured to remove silica from drinking water.
Key issues for Phase II include: (1) defining the proper hydraulic loading and contact time to take advantage of the Amended Silicate capacity, (2) testing the QCM sensor under typical drinking water conditions, (3) interfacing the sensor with a user alert function, and (4) verifying performance in field trials. In addition to ADA's relationship with CH2M Hill on Amended Silicate production, the company has teamed with a major POU/POE hardware supplier to address the issue of column design and aid with sensor integration activities. Partnering with an established hardware provider is the most efficient method to introduce the technology into the marketplace. Definition of the teaming relationship will be formalized at the outset of Phase II.