For centuries, water was considered a commodity product and used with little or no restriction. In today’s modern society, water is now being considered a precious resource and considerable effort is put into protecting, recycling, and restricting its use freely. State and Federal agencies have put in place guidelines and regulations restricting its use, especially for industrial use.
According to USGS that power plants in U.S. withdraw over 165 billion gallons of fresh water per day, which is used for plant cooling cycle. The same volume of fresh water is used by the irrigation industry. However, it should be noted that considering the large withdrawal of fresh water by thermo electric power, only 3% of the water is actually consumed, compared against the 81% consumed by the irrigation industry.
The water withdrawn by power plants adversely impacts the aquatic life, migratory and the endangered fish species. New guidelines imposed by the Clean Water Act section 316(b) further limits the withdrawal of water for cooling and restricts the use of Once Through Cooling for many new power stations. Despite the new additional power generation due to increase in population and industrial growth; studies conducted by U.S. DOE indicates the withdrawal of fresh water by power generation industry will remain the same as it is today since the old plants continue to be replaced with new ones, using the more efficient cooling system. Furthermore, the new plants will use alternatives, such as, cooling tower, dry cooling or an alternative source of water for cooling.
It is also predicted by USGS, as well as, other U.S. land and water management agencies that the consumption of fresh water in the U.S. will increase by 30-50% by the year 2030 due to population growth and additional water needed by the irrigation industries. Considering the increase in demand for fresh water there will be additional pressure on power generation market to further reduce its fresh water consumption and attempt to locate other alternative sources of water for use. Currently, for once through cooling system, 20-50 gallons of water are typically required to generate each Kw-hr of electricity. Using a re-circulating cooling tower can reduce the water needed to 0.2 to 0.6 gallons of water to generate each Kw-hr electricity (Veil, 2007).
Considering the continuing pressure by State and Federal agencies to reduce water intake by thermo electric power stations, the use of cooling tower or cooling ponds may become increasingly attractive. As the use of fresh water or seawater for cooling tower make-up water becomes restrictive or unavailable, other water sources are needed to be considered. One potential alternative for cooling is the use of treated municipal waste water as the source for make-up water. Treated municipal waste water generated by Publically Owed Treatment Works (POTW) is commonly available and is widespread across the U.S. According to U.S. Department of Energy, 81% of the newly proposed power plants for construction reported by the Energy Information Administration (EIA) have enough cooling water supply from 1 or 2 POTW within a 10 mile radius. The availability of water is improved to 97% if the radius is extended to 25 miles from the proposed plant location. Similar encouraging availability is reported for many of the existing plants, if the decision is made to convert from Once Through Cooling to a re-circulating cooling tower or pond type cooling water system.
Studies and pilot testing sponsored by U.S. Department of Energy, National Energy Technology Laboratory and the work conducted by University of Pittsburg, indicates that secondary treated municipal waste water can be a reliable source of cooling water make-up, which is widely available. In some cases, tertiary treatment may be required and it may still remain cost effective, given the scarcity of fresh water for cooling in some regions of the U.S. Using reclaimed water for industrial water use started several decades ago. Only a small number of plants in FL, TX, CA and a few other states have used reclaimed water for make-up water. However, the interest in using this water source is growing and a larger number of plants are looking at this alternative. A number of challenges are presented when using the secondary treated municipal waste water. Since the water contains high nutrients content, it allows the formation and growth of slim, algae, bacteria and other living organisms; which result in bio-fouling within the cooling system. The presence of Phosphorus, ammonia-nitrogen, and other dissolved salts contribute to scaling and corrosion of the equipment within the cooling water circuit, including the condensers and heat exchangers. Additionally, the presence of pathogens and other toxins in the water will require treatment of the water prior to use, as well as, the treatment of the blow down.
Considering well established technologies available for physical, chemical, and biological treatment of water, each application and water source can be evaluated and proper course of treatment suitable for the plant can be determined.
In almost all states permitting guidelines have already been established. Such guidelines include the established level of various chemicals and biological constituents in the cooling tower water, drift and discharge.
Treatments considered most often when using reclaimed water for cooling tower or boiler feed make-up water includes processes such as,
- Screening & Filtration, using coarse screens, multimedia filtration or membrane technologies, such as, micro or ultra-filtration.
- Sedimentation uses clarifiers or Dissolved Gas Floatation
- Softening of the water using Lime softening or Alum precipitation
- Purification using membrane technologies, such as, MBR, RO or Ion Exchange, EDI, etc.
- Treatment for bacteria, odor etc. using activated Carbon, UV, H2O2, O3 etc.
- Separation management and removal of solid waste and sludge using press filter, belt filter, sludge compactor etc.
- Chemical dosing and pH control
- Using mechanical system, such as, re-circulating Rubber Ball type or Brush and Basket type system to maintain clean condensers and heat exchanger tubes.
For each application the above treatment technologies need to be considered and the most effective solution needs to be selected. The following successful installation demonstrate the feasibility and the application of some of the treatment processes for the use of treated waste water from POTW for use as Cooling Tower make-up and Boiler Feed water make-up.