Arsenic Removal—Meeting Expectations
With the Environmental Protection Agency’s (EPA) recent implementation of a lower limit for arsenic in drinking water, many homeowners and water treatment companies alike are trying to answer the tough question: “What approach should I use for arsenic removal, point-of-entry (POE) or point-of-use (POU)?” POE treats an entire water supply while POU treats water used for drinking and cooking. Many factors are taken into account when determining the right approach. Understanding each contributing factor will help you make an informed decision.
Common Arsenic Issues
State and local regulations. Each state enforces different rules, regulations and guidelines. Be sure to consult with them before implementing any removal technology or system approach at the end user’s location.
Feedwater analysis. To effectively design any water system, it is necessary to know the influent contaminants. Analysis of the background water chemistry is recommended to determine the incoming arsenic levels, pH and other dissolved ions. Depending on which technology is selected, competing ions will have an impact on the performance of media and certain membrane technologies.
Effluent water specification. Getting a complete overview of the user’s water needs is required in the selection process. This includes instantaneous flow rate, daily amount of treated water for arsenic only, total daily amount of entire potable treatment and post-treatment arsenic limit (some states are lower than 10 ppb).
Interferences. Each removal technology has interferences that impact its ability to remove the arsenic or reduce the usable life; for example, pH, silica, phosphates and other oxy-anions affect most adsorptive technologies (such as metal-based and hybrid media). Sulfate and total dissolved solids (TDS) concentrations impact both the removal efficiency and capacity of a strong base anion (SBA) resin unit. Hardness, iron and total suspended solids can foul almost any arsenic removal system. Though they don’t directly interfere with arsenic removal, technologies such as reverse osmosis (RO), SBA or adsorptive media are affected. Be sure to consult the system or technology manufacturer for application guidelines in all these areas.
Speciation: As+3 vs. As+5. The most common theme among all of these removal technologies is the amount of arsenate (As+5) and arsenite (As+3) present in raw water. Technologies such as SBA resin, RO and activated alumina only remove the As+5 species. For these technologies, most or all of the As+3 species passes through. Other technologies, such as iron and other metal oxide adsorbants, have limited capacity for As+3, and prefer As+5. Consult the manufacturer for the amount of capacity each species has for the technology. If you are not sure or don’t know the speciation of the arsenic, it is recommended that pretreatment be put into place to convert all of the arsenic into a known species.
Pretreatment oxidation. The most common practice for converting As+3 to As+5 is the use of oxidation upstream of the removal technology. There are multiple approaches to accomplish this task, but the most common include chlorine/bleach and aeration. Redox media and ozonation have also been used.
Lead/lag configurations. A lead/lag configuration uses at least two vessels on line, in series, at all times. The primary bed, referred to as the worker bed, is doing most of the removal. This initial column is removing the contaminant of concern, usually to acceptable levels. The second column, referred to as the polisher vessel, is acting as a safeguard against premature leakage or exhaustion of the primary vessel. The obvious advantage that a lead/lag system has over a single vessel system is the added safety blanket. The final polishing vessel can maintain effluent water quality in the event of premature leakage or exhaustion of the primary bed (see Figure 1).
Monitoring. Whether it is a POU or POE, the system design should also include ways to monitor performance. Key items include water sampling ports and a water meter to track throughput. This allows the user or service provider to determine system performance at any given time. It is important to test the water on a regular basis and implement a monitoring schedule.
Deciding Factors for POU vs. POE
Although sometimes subjective, the user must decide which areas will need arsenic removal. Studies have shown that arsenic can be adsorbed through the skin in showers and bathing, which will require POE treatment. To minimize the size of POE treatment, non-potable areas like irrigation, lavatories and clothes washing may have a bypassed line of untreated water. If contact of untreated water can be tolerated, then the use of a POU system may be considered.
Cost is always a factor. Initially, POU systems are much less expensive than POE systems, which make them attractive. Low-cost small systems may not be the answer versus a large single system. Sometimes utilizing a combination of the two approaches may be the most cost-effective. Make sure the end user’s requirements are fulfilled when treating the problem.
Selecting POE Systems
There are many technology options when selecting a POE system. The goal is to treat the entire water source and fulfill the user’s needs, while minimizing the costs. The answers to the questions of flow rate, water usage, and water chemistry help shape and mold the best approach.
It is important to review the limitations of each arsenic removal technology and compare them to the information collected, specifically the water chemistry. At this point, the overall system does not just include the potential arsenic removal technology, but also includes the necessary pretreatment. Water storage should also be considered.
Many times technologies can be ruled out due to the user’s and/or location’s requirements, such as system footprint, waste discharge/disposal issues and chemical handling. The water chemistry can also rule out possible treatment technologies. Interferences or water conditions can cause poor throughputs. To compensate, system sizes may need to be larger, or additional pretreatment is required.
The amount of day-to-day attention the system requires also needs to be considered. In most cases, the system will need to be self-sufficient and have little interaction with the user. The system provider also may be contracted to service and monitor the system; thus, it is in his best interest to implement a technology he is familiar with and knows how to service.
Selecting POU Systems
If state and local regulations accept POU systems as a viable option for arsenic removal, then the investigation process begins for selecting the right system. Because the water usage for POU is much less than a centralized POE approach, cartridge-based systems and RO units are the predominate configurations.
When discussing a cartridge-based platform, it is recommended to stage the filtration process based on the results of the water analysis. If a removal technology such as activated alumina, hybrid anion resins or iron adsorbents are selected, the water analysis should be reviewed with the manufacturer. Once this has been selected, staged filtration includes, but is not limited to, the following steps: Stage one will typically include a pretreatment step of oxidation media with possible enhancements of carbon and particle filtration. Because of the safety benefits of a lead/lag relationship, stages two and three will use the selected arsenic removal media (see Figure 2). Post-filtration may be offered but is not a necessary step. To protect the user from over-run of the cartridge system, the automatic shut-off valve should be used. The outlet valve has a flow totalizer meter, and the valve will automatically shut off once the set point is reached. Arsenic removal cartridges should meet the TCLP requirements for proper disposal.
RO membranes have been effective for arsenic removal. The user will gain additional benefits of TDS reduction with the arsenic, but the system will require a separate drain connection. Similar to other technologies, As+3 is not easily removed and oxidation pretreatment may be required. Because most POU RO systems use low-pressure membranes, flow rate will be low and temporary storage is required. Usage profile should be evaluated to determine if RO is a viable option.
When considering whether to use POE or POU for arsenic removal, it is important to take into consideration all factors surrounding each specific situation—the local regulations, the user’s needs, requirements and the limitations of each potential removal technology. A successful arsenic removal system take an objective approach, apply the knowledge gained from answering the right questions and apply the best technology to fulfill the expectation of the customer.