Zero (liquid) discharge under Clean Water Act compliance

April 10, 2017

The CWA’s long-range goal is to reach zero discharge of pollutants, but the permits do not typically say “no discharge.”

In an ideal world, there would be no waste production and no releases of wastes to the environment. We will never live in an ideal world, but strategies like zero discharge can be meaningful for some industrial waste management operations. The Clean Water Act (CWA) makes the discharge of pollutants from a point source (a man-made conveyance, such as a pipe, ditch, tank or vehicle) to waters of the U.S. illegal, except in accordance with a permit.

The CWA’s long-range goal is zero discharge of pollutants, but the permits do not typically say “no discharge.” Instead, they limit the amounts of different pollutants a source can discharge during a specific period. However, if eliminating liquid discharges to surface water is possible, that obviates the need to obtain the National Pollutant Discharge Elimination System (NPDES) permit and provides some cost avoidance.

Some waste streams, such as municipal wastewater and some industrial waste streams, essentially can be recycled 100 percent. These concepts are subject to need, technical feasibility, costs and regulatory requirements. The principal CWA elements that could directly involve a role for zero liquid discharge include:

  • Meeting ambient water quality criteria and standards
  • The NPDES permits that cover point sources of pollution discharging into a surface water body
  • Effluent guidelines that describe technologies for industries to use to meet discharge requirements

Water quality criteria and standards require all receiving waters to have use designations and numeric or narrative criteria to support those uses. Industry-specific effluent guidelines establish performance expectations for treatment technologies for categories of industrial dischargers. If water quality standards are not met while using existing NPDES permits, a compliance strategy, including total maximum daily loads, could be applied with a link to potential zero discharge requirements, if appropriate.

Two U.S. Environmental Protection Agency (EPA) regulations that include elements of zero liquid discharge are: the Steam Electric Power Generating Effluent Guidelines,1 which calls for zero discharge of pollutants from the fly ash and bottom ash waste streams, and the Effluent Limitations Guidelines and Standards for the Oil and Gas Extraction Point Source Category,2 which prohibits the discharge of oil and gas pollutants to publicly owned treatment works.

Steam electric power generating effluent guidelines

These effluent guidelines apply to wastewater discharge from plants that generate electricity, using fossil fuels or nuclear fuels with a steam-water thermodynamic cycle, for distribution and sale. The primary targets are discharges associated with coal fuels that include ash-handling operations and wastewater from flue gas desulfurization (FGD) air pollution control systems, which are the primary sources of pollutants discharged by steam electric power plants.

The waste streams involve wastewaters from wet FGD systems, fly ash and bottom ash handling, coal pile runoff, condenser cooling, equipment cleaning, and leachate from landfills and impoundments. Wastewaters from flue gas mercury control systems (that is, when the dry mercury capture residues are transported by a wet fly ash handling system to ash ponds) and regeneration of the catalysts are used for selective catalytic reduction (SCR). Nitrogen oxide controls were identified as potential new waste streams that warrant attention, but the EPA was unable to obtain adequate characterization data for these wastes. The EPA estimated annual compliance costs at $480 million versus estimated benefits of $451 million to $566 million, so the guideline cost/benefit ratio appears to be a wash at best.

Wet FGD systems for sulfur dioxide emissions have increased significantly, since the prior effluent guidelines were last revised in 1982, as power plants tried to meet federal and state air pollution control requirements. FGD wastewaters contain significant levels of metals (including arsenic, mercury and selenium) as well as chloride, total dissolved solids (TDS), total suspended solids (TSS) and nutrients. The most commonly used treatment systems for managing FGD wastewater include settling ponds, chemical precipitation systems, biological treatment systems (anaerobic and aerobic), constructed wetlands and vapor-compression evaporation systems.

Settling ponds can effectively remove suspended solid and metal particulates from FGD wastewater. However, they do not effectively remove dissolved metals. Other treatment systems, such as chemical precipitation and biological treatment systems, remove certain dissolved metals from FGD wastewater.

Coal-fired power plants manage bottom ash and fly ash using either wet- or dry-handling techniques. For wet handling systems, the plants typically sluice the fly ash and/or bottom ash to surface impoundments or settling ponds where most of the solids settle out of the water. Some plants recycle some or all the settled ash pond effluent, but most plants discharge the pond overflow. Untreated ash transport waters contain significant concentrations of TSS and metals. The treated effluent from ash ponds generally contains low concentrations of TSS; however, metals are still present predominantly in a dissolved form.

Most newer electric-generating units operate dry fly ash handling systems because of new source performance standards that require “no discharge of wastewater pollutants from fly ash transport water” (40 CFR Part 423.15). These dry fly ash handling systems use a vacuum or blower to transport the fly ash to a storage silo where it is typically sold for beneficial use or sent to a landfill. The dry bottom ash handling process typically consists of quenching the bottom ash in water and conveying it to a dewatering system.

Many pollutants are found in wastewaters generated at coal-fired power plants that can impact the environment, including metals (arsenic, selenium and mercury); TDS; and nutrients. The primary ways that coal combustion wastewater impact the environment are discharges to surface waters, leaching to groundwater, and surface impoundments and constructed wetlands that can be attractive nuisances that increase wildlife exposure to the pollutants in the systems. EPA concluded that coal combustion wastewaters have caused a wide range of environmental effects to aquatic life.

The EPA also investigated other electric power and steam generating activities similar to the processes regulated in the Steam Electric Power Generating Point Source Category but which are not subject to the effluent guidelines. Such activities include electric generating units fueled by non-fossil or non-nuclear fuels (municipal solid waste and biomass); electric generating units at industrial facilities (chemical plants and petroleum refineries); plants that produce steam for distribution and/or sale but do not generate electric power; and facilities that provide a combination of electric power and other utility services.

The EPA compared the volume and characteristics of wastewaters generated by these activities to the plants regulated by the steam electric effluent guidelines and determined that these processes may generate similar wastewaters. However, the volume of the wastewaters generated are much smaller than those generated at plants regulated by the effluent guidelines. Detailed information is available in EPA’s report at EPA 821-R-09-008.

Oil & gas guidelines

This rule is intended to prevent the discharge of wastewater from onshore oil and gas production facilities to publicly owned treatment works (POTWs) and is directly aimed at oil- and gas-producing wells and hydraulic fracturing processes. Most POTWs are not equipped to process the types of waste and contaminants produced by these sources, so they are not permissible point source discharges. However, it does not preclude transport to centralized POTWs that are designed to process these types of wastes.

Many state and federal regulations address the management and disposal of wastes from these oil- and gas-producing facilities. Current industry practice involves about 98 percent of the fluid wastes being recharged underground for disposal or well pressurization, so the new rule is a reaffirmation of those practices and has minimal additional economic impact.

Zero discharge technologies

Zero discharge has many connotations. Zero discharge technologies tend to eliminate the wet waste stream discharge altogether as opposed to treating to zero and then discharging. However, several technologies in combination with appropriate pretreatments, provide the liquid removal and production of dry or highly concentrated waste for land disposal.

Cooling waters used for cooling tower blowdown can be sent for evaporation in ponds to achieve zero discharge. When applicable, reverse osmosis, with evaporators and crystallizers, can achieve zero liquid discharge and recover water for reuse. Changes in processes can also reduce demands on liquid discharge controls.


  1. “Steam Electric Power Generating Effluent Guidelines – 2015 Final Rule.” EPA.
  2. “Effluent Limitations Guidelines and Standards for the Oil and Gas Extraction Point Source Category.” Federal Register.
  3. “Steam Electric Power Generating Point Source Category: Final Detailed Study Report.” EPA.

Joseph Cotruvo, Ph.D., BCES, is president of Joseph Cotruvo and Associates LLC, water, environment and public health consultants, and technical editor of Water Technology. He is a former director of both the EPA Drinking Water Standards and the Risk Assessment Divisions

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