MEMORANDUM 

TO: Bill Swietlik, EPA 

FROM: Lydia Lambert, Kristi Bubb, ERG 

DATE: September 2, 2010 

SUBJECT: Drinking Water Treatment Technologies that Can Reduce Metal and 

Selenium Concentrations Associated with Discharges from Steam Electric 

Power Plants (DCN SE02154) 

 In support of the development of revised effluent guidelines for the steam electric power 

industry, EPA will conduct an environmental assessment (EA) analysis to evaluate the 

environmental impact of identified pollutants of concern under current (i.e., baseline) discharge 

practices and assess the potential environmental improvement from reductions in pollutant loads 

under varying regulatory option scenarios. As part of this analysis, EPA may consider the 

potential impact steam electric discharges pose to downstream drinking water treatment plants 

and the potential reduction in treatment costs associated with lower pollutant loads entering 

surface waters. As a preliminary assessment for the drinking water impact analysis, EPA directed 

ERG to conduct a literature search to determine if drinking water treatment processes can reduce 

or eliminate metals (e.g., arsenic, mercury, chromium, copper, and manganese) or other 

pollutants of concern (e.g., selenium) commonly detected in wastewater discharges from steam 

electric power plants. This memorandum presents the drinking water standards for the potential 

pollutants of concern, common drinking water treatment technologies that treat heavy metals and 

selenium, drinking water source water monitoring programs, and potential next steps for the 

drinking water impact analysis. 

1.0 DRINKING WATER STANDARDS FOR POTENTIAL POLLUTANTS OF CONCERN 

 The nation’s drinking water is regulated under the Safe Water Drinking Act (SDWA) of 

1974 and its 1986 and 1996 amendments. Under the SDWA, EPA is required to set enforceable 

national health-based drinking water standards to control the level of contaminants in drinking 

water and its sources (i.e., rivers, lakes, reservoirs, springs, and ground water wells). In addition 

to the primary drinking water standards, EPA has set secondary drinking water standards, which 

are non-enforceable guidelines for contaminants that may cause cosmetic effects (e.g., skin and 

tooth discoloration) or aesthetic effects (e.g., taste and odor). Drinking water treatment plants 

are not required by EPA to adopt these secondary standards; however, states may choose to 

adopt and enforce them (U.S. EPA, 2004). 

 Under the drinking water standards, EPA sets two types of enforceable standards for a 

pollutant: the Maximum Contaminant Level (MCL) and the Treatment Technique (TT). EPA 

strives to set a MCL, which is the maximum amount of a contaminant allowed in water delivered 

to a public water system that can be achieved using the best available treatment technology. 

However, a TT is set instead of a MCL if a reliable method that is economically and technically 

feasible to measure a contaminant at particularly low concentrations is unavailable (U.S. EPA, 
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September 2, 2010 

Page 2 

2004). Table 1-1 presents the drinking water treatment standards for the potential pollutants of 

concern in steam electric power plant discharges. Drinking water treatment technologies must be 

designed to effectively treat these pollutants to the following drinking water standard 

concentrations in the produced water from the treatment facility. However, the effectiveness of 

the treatment operations are impacted by the speciation of the pollutant (i.e., total or dissolved), 

which is dependent on the characteristics of the plants’ operations. 

Table 1-1. Drinking Water Treatment Standards for Potential Pollutants of Concern in 

Steam Electric Power Plant Discharges 

Analyte Unit 

Drinking Water 

Standard Value 

Type of Drinking 

Water Standard 

Potential Health Effects 

from Ingestion of Water 

Selenium, Total mg/L 0.05 MCL  Hair or fingernail loss. 

 Numbness in fingers or toes. 

 Circulatory problems. 

Arsenic, Total mg/L 0.010 MCL  Skin damage. 

 Problems with circulatory 

systems. 

 Increased risk of cancer. 

Mercury, Total 

(inorganic) 

mg/L 0.002 MCL  Kidney damage. 

Chromium, Total mg/L 0.1 MCL  Allergic dermatitis. 

Copper, Total mg/L 1.3 TT 

(Corrosion 

Control) 

 Short term exposure: 

Gastrointestinal distress. 

 Long term exposure: Liver 

or kidney damage. 

Manganese, Total mg/L 0.05 Secondary 

Standard 

 Brownish-red discoloration 

of water. 

 Off-taste or odor of water. 

Source: U.S. EPA, 2009. 

2.0 DRINKING WATER TREATMENT TECHNOLOGIES

Conventional wastewater treatment unit processes are used to remove the potential 

pollutants of concern from drinking water. Typically, these unit processes are designed to treat 

to the MCL or TT levels. Conventional wastewater treatment is a combination of physical, 

chemical, and biological processes and operations that remove solids, organic matter, and/or 

nutrients from wastewater. Conventional wastewater treatment processes are defined in order of 

increasing treatment level: primary, secondary, and tertiary (commonly referred to as advanced 

wastewater treatment). 

Conventional treatment trains used to remove metals from drinking water typically begin 

with coagulation followed by flocculation. After flocculation, drinking water source water is 

then treated by either sedimentation or filtration or a combination of the two processes. 

Descriptions of these unit processes are as follows: 

 Coagulation. A unit process that involves the addition of one or more chemicals (i.e., 

coagulants) to promote the aggregation of particles in the drinking water source water. 
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September 2, 2010 

Page 3 

Chemicals commonly used for coagulation of heavy metals include inorganic metal 

compounds (e.g., aluminum sulfate and ferric chloride), prehydrolyzed metal salts, and 

synthetic organic chemicals (MWH, 2005). 

 Flocculation. A unit process that involves the mixing of chemically treated water to 

induce coagulation. This process causes very small suspended particles to collide and 

form larger heavier particles (i.e., floc) that can settle out by gravity. The purpose of 

flocculation is to aggregate particles in the source water to a size that can be efficiently 

removed from suspension by sedimentation and filtration treatment systems. To optimize 

solids removal, organic polymers are commonly used as flocculant aids to enhance 

settleability and filterability of floc particles (Viessman and Hammer, 2005). 

 Sedimentation. A unit process that involves the separation of settleable solids by the 

force of gravity. This unit process involves the introduction of drinking water source 

water to a large inactive basin (i.e., sedimentation basin) for an extended period of time. 

Over time, the suspended particles settle to the bottom of the basin, separating the solids 

from the source water. This unit process commonly follows coagulation and flocculation 

treatment to remove flocculated impurities and precipitates (MWH, 2005). 

 Filtration. A unit process that involves the separation of non-settleable solids from 

drinking water source water by passing the liquid through a porous medium, which traps 

and removes the solids. The primary goal of filtration is to completely remove the 

targeted solids from suspension. The most common forms of filtration are granular 

filtration and membrane filtration. The two types of membrane filters primarily 

employed by drinking water treatment facilities are pressure-driven microfiltration (0.1-

μm sized pores) and ultrafiltration (0.01-μm sized pores), which utilize the semipermeable membrane as a mechanism for separation (MWH, 2005). 

Principal advanced treatment unit operations and processes for removal of metals and/or 

selenium include the following: 

 Chemical Precipitation. A unit process that is the most commonly operated treatment 

technology for the removal of heavy metals from drinking water. This unit process 

involves the addition of chemicals to water to alter the physical state of dissolved and 

suspended solids and facilitate their removal by sedimentation. The precipitation of 

substances in water depends primarily on the solubility of the complexes formed in the 

drinking water source water, which is generally controlled by the pH level. Typically, 

metals precipitation occurs at a high pH (i.e., 9 through 9.5). For example, heavy metals 

found in source water typically form hydroxide and sulfide complexes, which have low 

solubility in water at a high pH. Therefore, the formation of insoluble complexes causes 

the metals to precipitate out of solution. The precipitation of heavy metals in source 

water is usually accomplished by the addition of chemicals, such as alum, lime or iron 

salts, and polyelectrolytes (AWWA, 1999). 

 Ion exchange. A unit process in which an ion in a liquid phase is exchanged with an ion 

in a solid phase. The solid phase (i.e., an ion exchange resin) is an insoluble material that 
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September 2, 2010 

Page 4 

varies depending on exchange capacity, particle size, and stability. When the resin comes 

in contact with the liquid solution, the contaminant ion migrates out of solution to the 

charged functional group, being replaced by the counterion in solution. In drinking water 

treatment applications, ion exchange is primarily used for water softening and 

demineralization (e.g., removal of mineral content); however, the use of ion exchange for 

other applications has increased due to a greater concern for health effects from other 

contaminant ions (i.e., arsenic and selenium) (MWH, 2005). 

 Reverse Osmosis (RO). A unit process that separates solutes from drinking water source 

water by utilizing the physical and chemical properties of the solutes for separation. 

During RO, a high-pressure stream is directed across the surface of a semi-permeable 

membrane opposite in direction and greater than the osmotic pressure imposed across the 

membrane. Due to the pressure differential between the sides of the membrane, the flow 

of the stream travels from the more concentrated to the less concentrated side of the 

membrane. As water passes through the membrane, solutes are rejected and the reject 

stream becomes more concentrated (MWH, 2005). Typically, the RO process is used for 

the treatment of arsenic in source water. 

 Distillation. A unit process that separates metal contaminants from drinking water source 

water through a combination of vaporization and condensation. During this unit process, 

the water is heated to its boiling point until the water vaporizes. The vaporized water is 

removed from its heating source, fed to a new location, and condensed to its original 

liquid form. Contaminants with a higher boiling point than water will remain in their 

original location to yield contaminant-free source water in the new location. (Metcalf & 

Eddy, 2003). 

 Electrodialysis. A unit process that separates metal ions in drinking water source water 

through the use of an electrical voltage-driven membrane. This membrane process is 

driven by the electrical potential between two electrodes, which generate an electric 

current that passes through the source water. The electric current causes the negatively 

charged ions to be drawn toward the positively charged electrode and the positively 

charged ions to be drawn toward the negatively charged electrode, resulting in ion 

removal from the source water (ASCE/AWWA, 2005). 

3.0 DRINKING WATER SOURCE WATER MONITORING

 A database containing nation-wide drinking water source water monitoring data would be 

beneficial to the drinking water impact analysis. The data would allow ERG to identify pollutant 

concentrations upstream of drinking water intake locations, in order to determine if the 

concentrations have increased significantly enough to impact the effectiveness of drinking water 

treatment processes. In an effort to locate the data, ERG conducted research on source water 

protection regulations and practices. ERG determined that EPA does not require states to 

monitor pollutant concentrations in source water; however, the SDWA Amendments of 1996 

require states to develop and implement source water assessment programs. Under these 

programs, states develop a source water assessment for each public water system. This 

assessment includes a susceptibility analysis to determine how likely a water supply is to be 

contaminated by identified potential sources of contamination. Although the susceptibility 
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September 2, 2010 

Page 5 

analysis does not require states to collect monitoring data of pollutant concentrations in the 

source water areas, EPA encourages states to undertake monitoring to accurately identify 

potential significant sources of contamination, which could be utilized for the EA analysis (U.S. 

EPA, 1997). 

4.0 POTENTIAL NEXT STEPS FOR THE DRINKING WATER IMPACT ANALYSIS

 Drinking water treatment options are typically designed to treat for the removal of the 

pollutants of concern in the steam electric industry (i.e., arsenic and mercury); therefore, ERG 

would suggest the following next steps to determine if steam electric power plant discharges 

have a significant impact on pollutant concentrations in the source water entering the drinking 

water treatment facilities. Potential next steps for the drinking water impact analysis include: 

 Proximity Analysis. ERG will use outfall latitude and longitude data requested in the 

steam electric questionnaire to determine the proximity of steam electric outfalls 

discharging flue gas desulfurization (FGD) scrubber wastewater and ash wastewater 

upstream from drinking water intake locations. 

 Review Available Source Water Assessment Documentation. ERG will contact Lee Kyle 

at EPA’s Office of Ground Water and Drinking Water to determine if the data 

documented in source water assessments are accessible to the public. 

 Hotspot Analysis. If ERG determines that the source water assessments are readily 

available to the public, ERG will conduct a hotspot analysis to examine waterbodies that 

receive FGD wastewater discharges from multiple power plants to determine if the 

pollutant concentrations are significantly greater in these areas. 

 Review Sampling, Monitoring, and Questionnaire Data. ERG will review data collected 

through the sampling episodes, long-term monitoring, and questionnaire to calculate 

pollutant concentrations in FGD scrubber wastewater and ash wastewater discharges by 

accounting for dilution based on cooling water flows. ERG will develop a methodology 

to predict effluent concentrations discharged from the plant to determine if a subset of 

discharges exceeds MCL or TT levels at end-of-pipe. 

 Estimate Source Water Concentrations. ERG will use sampling data, monitoring data, 

questionnaire data, and stream flow data from the NHD Plus system to estimate the 

potential concentration of pollutants of concern reaching downstream drinking water 

intake structures. 

5.0 REFERENCES

American Society of Civil Engineers (ASCE)/American Water Works Association (AWWA). 

2005. Water Treatment Plant Design (4th ed.). New York, NY: McGraw Hill. 

American Water Works Association (AWWA). 1999. Water Quality and Treatment (5th ed.). 

New York, NY: McGraw Hill. 
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September 2, 2010 

Page 6 

Metcalf & Eddy. 2003. Wastewater Engineering: Treatment and Reuse (4thed.). New York, NY: 

McGraw Hill. 

MWH. 2005. Water Treatment: Principles and Design (2nd ed.). Hoboken, NJ: John Wiley & 

Son, Inc. 

U.S. EPA. 1997. State Source Water Assessment and Protection Programs: Final Guidance.

Office of Water. Washington, DC (August). Available online at: 

http://www.epa.gov/safewater/sourcewater/pubs/swpguidance.pdf 

U.S. EPA. 2004. Drinking Water Standards & Health Effects. Office of Ground Water and 

Drinking Water. Washington, DC (June). Available online at: 

http://www.epa.gov/safewater/sdwa/pdfs/fs_30ann_standards_web.pdf 

U.S. EPA. 2009. National Primary Drinking Water Regulations. Office of Ground Water and 

Drinking Water. Washington, DC (May). Available online at: 

http://www.epa.gov/safewater/consumer/pdf/mcl.pdf

Viessman, Jr., W, and Hammer, M.J. 2005. Water Supply and Pollution Control (7th ed.). Upper 

Saddle River, NJ: Pearson Education, Inc.