A Shock to the System
A new, stricter arsenic rule is starting to affect small and large water supplies
On Jan. 23, 2006, the arsenic rule was implemented with a new limit of 10 parts per billion (ppb) (the old standard was 50 ppb). The new rule has a broad reach; it affects large and small drinking water treatment systems, including non-community water supplies.
While a number of different arsenic removal methods are available, facilities must consider evaluation criteria, such as installed and operating costs, operation complexity, waste volume and type, and interaction with competing contaminants. To date, single-use adsorption media and iron co-precipitation are two of the most common removal processes installed.
Single-use adsorption media and co-precipitation are cost-effective and easy-to-operate arsenic removal methods. Moreover, the two treatment methods can benefit both small and large systems.
Single-use Adsorption Media Removal
Single-use adsorption media was developed partly to address lower European and U.S. arsenic regulations. Single-use adsorption media are capable of removing arsenite (As III) and arsenate (As V) from water, typically without requiring pre-oxidation. This type of media is placed in vessels where water passes through it to adsorb arsenic. Once adsorption capacity is reached, single-use media is replaced with new media, thus eliminating on-site regeneration chemicals. Other benefits of single-use adsorption media include minimal equipment, low and infrequent liquid waste volume, and low operator input requirements. The media is tested prior to disposal to ensure compliance with landfill requirements. State and local testing requirements and results determine if the exhausted media is disposed of at a non-hazardous or hazardous material landfill.
Single-use media provides a simple treatment solution for utilities currently without process treatment, or for those that have difficulty disposing of liquid waste residuals. Media adsorption capacity and the cost to replace media are important considerations in determining cost of operation.
When naturally occurring iron or manganese is present, a co-precipitation process may be utilized to cost effectively remove all contaminants in one treatment process. Common chemical oxidation methods for iron and manganese are also effective for oxidizing arsenite. Of arsenic's two valence states, arsenite is more difficult to remove from water; however, it can be readily oxidized with chlorine or potassium permanganate to form negatively charged (and more easily removable) arsenate. Arsenite must be oxidized prior to being removed by co-precipitation. If naturally occurring iron cannot sufficiently remove the arsenic, ferric-based coagulant may be added prior to filtration. Similar to iron-based adsorption media, the resulting ferric-arsenic sludge is safe for landfill disposal.
The term "co-precipitation" is a very general process term that indicates arsenic is being adsorbed by the formed ferric hydroxide floc. The final process design varies with source water conditions. Oxidation, coagulant addition, and clarification may also be incorporated into the overall process design. All co-precipitation processes include a filtration step to separate arsenic-bound floc from the finished water.
Granular media and membrane filtration are quite efficient for the separation process. As a result, either may be used. Membrane filtration generally must be operated at a high flux to remain cost-effective against granular media filtration in this application. Backwash waste from the filtration process will regularly generate a solids-laden liquid waste; therefore, co-precipitation is best applied where liquid waste can readily be disposed. Bench- and pilot-scale testing the co-precipitation process provides insight to determine removal efficiency and overall process design.
Several small and large arsenic removal systems have been installed and are in operation. Many of those incorporate adsorption media and co-precipitation processes.
Small System Adsorption
A military installation in the western United States required arsenic removal for a 20 gallons per minute (gpm) supply specific to one particular area of the base. As 31 ppb of arsenic was the only contaminant that needed to be removed, it required a simple-to-operate and -maintain system.
The client selected an arsenic removal system with adsorption media that included two 24-inch-diameter by 72-inch-high fiberglass treatment vessels placed in series, along with backwash controllers and piping to and from the units. During series operation, water flows through the lead vessel and then into the next (lag) vessel.
With a series-operated system, the majority of removal occurs in the lead vessel while the lag vessel is used as a safety net to ensure treated water quality. Once the lead vessel reaches adsorption capacity, the media is replaced and the vessel is placed in the lag position by changing the piping connections. To facilitate a simplified media change-out process, the entire vessel is replaced with a new vessel containing new media. The vessel containing exhausted media may be used as the containment vessel for disposal of the media.
Small System Co-precipitation
A small midwestern community with a 100 gpm supply needed to remove approximately 1.0 parts per million (ppm) iron, 0.13 ppm manganese and 42 ppb arsenic from its water supply. The initial process was selected because it could remove all contaminants in a single process stage. It was also based on the oxidation-filtration process, with additional provisions for ferric chloride if needed.
An on-site pilot study was conducted to ensure proper process and equipment selection. During the study, chemical feeds of 8.0 milligrams per liter (mg/L) ferric chloride, 0.25 ppm potassium permanganate, 2.5 ppm sodium hypochlorite, and 0.1 ppm cationic polymer produced effluent water quality of less than 0.010 ppm for both iron and arsenic while manganese was reduced to less than 0.050 ppm.
The original process was altered to incorporate a clarification stage to reduce solids load being placed directly on the filter system. The clarification step reduces influent filter solids and thus extends filter run time. This stage also provides additional detention time for completion of chemical reactions.
Final design of the plant incorporated two packaged treatment units, each designed for 50 gpm. The packaged systems incorporate adsorption clarification followed by granular media filtration in factory-assembled units. The "plug-and-play" systems arrived on site completely wired and piped to minimize installation time and cost.
The benefits of pilot testing prior to final design selection are very evident in this project. It allowed the owner and engineer assurances of process selection and defined the requirements for chemical feed and waste handling.
Large System Adsorption
A Southwest utility was meeting almost all water quality parameters within regulatory guidelines. It only needed to remove 11 to16 ppb of arsenic to comply with the arsenic rule.
The utility previously only had well pumps and chlorination equipment. Minimal operating requirements and low waste volume generation at this site convinced the utility to select adsorption media for the treatment process. Two 14-foot-diameter, series-operated arsenic removal vessels provide treatment, with effluent arsenic at non-detectable levels at system start-up. Like other series-operated systems, either vessel can be operated as the lead or lag vessel. When media replacement is required, the lead vessel will be changed out and placed in the lag position. Treatment will continue for a period of time prior to the new lead vessel requiring media change-out.
Large System Adsorption Polishing
A project on the East Coast had a reverse osmosis (RO) system in place to remove multiple contaminants, including arsenic. While the system provided acceptable treatment, the new arsenic rule required it to have further treatment. To meet the new 10 ppb requirement, an on-site pilot study was conducted using single-use adsorption media to polish the permeate from the RO units.
The pilot study results showed a media capacity of approximately three years before media change-out would be required. This offered the least amount of process changes and allowed the new arsenic removal system to be installed while the existing plant remained in operation. The lower pH of the RO permeate provides excellent water for adsorption media treatment.
The full-scale plant went into service in 2005, with six 14-foot-diameter adsorption vessels. The vessels are arranged for series-operation, with three vessels in lead mode and three vessels in lag mode. To minimize its handling requirements, the utility contracted with the manufacturer to perform future media change-outs.
Large System Co-precipitation
This southwestern project required treatment of a total 6,000 gpm flow rate from three well supplies, with an average arsenic concentration of 17 ppb. Both adsorption media and coagulation-filtration methods were evaluated during the initial project design, as the water conditions were suitable for either process. Ferric chloride coagulant was added prior to filtration after the evaluation showed this to be the most cost-effective system based on installed and operating costs.
Three 10-foot-diameter by 40-foot-overend, single-cell horizontal pressure filters were installed for the filtration process. Use of pressure filtration equipment allowed treatment without the need for re-pumping to the distribution system. Backwash waste generated from the pressure filters consists of arsenic bound tightly with the ferric chloride sludge. This waste is thickened prior to disposal, allowing recycling of the supernatant prior to the sludge being disposed of at a landfill facility.
Large System Co-precipitation-Clarification-Filtration
An East Coast project's extremely high arsenic concentrations of 100 to 130 ppb necessitated that a co-precipitation process including clarification be used. At the time of design and installation, arsenic adsorption media was not commercially available.
However, adsorption would not have been a good option anyway, due to multiple contaminants and elevated arsenic concentration. In addition to arsenic, elevated levels of iron and manganese were present at this 6,300-gpm plant.
Clarification was included to improve overall operating efficiency, with an upflow system removing the bulk of solids through adsorption and clarification prior to filtration. Naturally occurring iron and manganese were oxidized with potassium permanganate. Iron-based coagulant was added to the water prior to the adsorption clarification system to increase arsenic removal. Effluent arsenic is consistently below 10 ppb, and iron and manganese are below secondary standards. Waste is produced from both periodic flushing of the adsorption clarifier and filter backwashing.
As the new arsenic rule starts to be enforced, many systems are still being designed, installed, and started in operation. Fortunately, operating plants like those described above serve as a road map for evaluation of processes such as single-use adsorption media and iron co-precipitation prior to plant design.
This article originally appeared in the 06/01/2006 issue of Environmental Protection.