Use of Treated Municipal Wastewater as Power Plant Cooling System Makeup Water: Tertiary Treatment vs. Expanded Chemical Regimen for Recirculating Water Quality Management

Introduction

Every day, water‐cooled thermoelectric power plants in the United States withdraw from 60 billion to 170 billion gal of freshwater from rivers, lakes, streams, and aquifers, and consumes from 2.8 billion to 5.9 billion gal of that water. Freshwater withdrawals for cooling in thermoelectric power production account for about 40% of all withdrawals, essentially the same amount as withdrawals for agricultural irrigation, as documented by the U.S. Geological Survey. Sustained droughts nationwide underscore the critical need to think about using treated municipal wastewater (MWW) for use in cooling in electric power generation. It needs a great deal of water for electric power production, to condense stream in the power plant stream cycle. Air cooling is possible, but it is more costly and less efficient. Water will continue to be the preferred coolant for new thermoelectric power plants (Dzombak 2013).

Motivation for the project: Increase in demand for electricity brings with it an increase in water needed for cooling. The cooling of thermoelectric power plants accounts for 41% of all freshwater withdrawal in the United States, i.e. approximately the same amount of water as is withdrawn for agricultural irrigation. Some areas of the United States have little or no freshwater available for use. Alternative sources of water are needed for new electric power production. The U.S. Department of Energy has been conducting and sponsoring research to investigate the feasibility and costs of using alternative sources of water for power plant cooling, especially in recirculating cooling systems which are required for most new power production in the United States.

Goals and highlights of the project: Treated MWW is a common, widely available alternative source of cooling water for thermoelectric power plants across the United States, as determined in a predecessor DOE project (2006–2009) by the project team. However, the biodegradable organic matter, ammonia–nitrogen, carbonate, and phosphates in the treated wastewater pose challenges with respect to enhanced biofouling, corrosion, and scaling, respectively. The overall objective of this study was to evaluate the benefits and LCC of implementing tertiary treatment of secondary treated MWW prior to use in recirculating cooling systems.

The study comprised bench‐and pilot‐scale experimental studies with three different tertiary treated MWWs, and LCC and environmental analyses of various tertiary treatment schemes. Sustainability factors and metrics for reuse of treated wastewater in power plant cooling systems were also evaluated. The three tertiary treated wastewaters studied were secondary treated MWW subjected to acid addition for pH control (MWW_pH); secondary treated MWW subjected to nitrification and sand filtration (MWW_SF); and secondary treated MWW subjected to nitrification, sand filtration, and GAC adsorption (MWW_NFG).

Tertiary treatment was determined to be essential to achieve appropriate corrosion, scaling, and biofouling control for use of secondary treated MWW in power plant cooling systems. The ability to control scaling, in particular, was found to be significantly enhanced with tertiary treated wastewater compared to secondary treated wastewater. MWW_pH treated water (adjustment to pH 7.8) was effective in reducing scale formation, but it increased corrosion and the amount of biocide required to achieve appropriate biofouling control. Corrosion could be adequately controlled with tolytriazole addition (4–5 ppm TTA), however, which was the case for all of the tertiary treated waters. For MWW_NF treated water, the removal of ammonia by nitrification helped to reduce the corrosivity and biocide demand. Additional GAC adsorption treatment, MWW_NFG, yielded no net benefit. For all of the tertiary treatments, biofouling control was achievable, and most effectively with preformed monochloramine (2–3 ppm) in comparison with NaOCl and ClO₂.

LCC analyses were performed for the tertiary treatment systems studied experimentally and for several other treatment options. A public domain conceptual costing tool (LC3 model) was developed for this purpose. MWW_SF (lime softening and sand filtration) and MWW_NF were the most cost‐effective treatment options among the tertiary treatment alternatives considered because of the higher effluent quality with moderate infrastructure costs and the relatively low doses of conditioning chemicals required (Dzombak 2013).

Life cycle inventory (LCI) analysis along with integration of external costs of emissions with DC was performed to evaluate relative emissions to the environment and external costs associated with construction and operation of tertiary treatment alternatives. Integrated LCI and LCC analysis indicated that MWW_NF and MWW_SF alternatives exhibited moderate external impact costs with moderate infrastructure and chemical conditioner dosing, which makes them (especially MWW_NF) better treatment alternatives from the environmental sustainability perspective since they exhibited minimal contribution to environmental damage from emissions (Dzombak 2013).

Key Points

1. This study undertook a comprehensive, integrated approach by looking at all aspects of the water quality control problem in a recirculating cooling system employing treated MWW as makeup water, and we determined optimal approaches for tertiary treatment considering both direct economic costs and environmental impacts of alternative water treatment/management approaches in an integrated manner.
2. The work included pilot‐scale demonstrations in the field, in addition to laboratory and modeling work, in partnership with a MWW treatment facility.
3. In regard to originality and innovation, this study was the first comprehensive research study in the public domain of the use of treated MWW as makeup water in recirculating cooling systems of electric power plants.
4. The challenges of using treated MWW in power plant cooling systems are many and include the technical challenges of controlling corrosion, scaling, and biofouling in a recirculating cooling system employing a low‐quality makeup water; the challenge of determining capital and operating costs in a complex operational environment in which the water is being concentrated four times or more in the recirculating cooling system; and the challenge of assessing environmental and social risks and benefits for use of treated wastewater as cooling system makeup water. It was a complex problem, and further complicated by the economic, social acceptance, and sustainability issues involved.
5. Alternative sources of water are needed for new power production in regions without new sources of freshwater available. Our research will help advance the use of treated MWW in electric power production and thus help contribute to economic development. Further, in this research they evaluated explicitly the relative sustainability of various water treatment/management alternatives by considering environmental impact and social acceptance factors in addition to direct economic costs of the alternatives.