Water conservation via water reuse is environmentally and economically sound. Currently, water reuse is practiced in the United States and other countries. More facilities are reusing water so that fresh water can be made available for potable purposes while reducing wastewater discharge. This is particularly important with the current drought and water shortages throughout the United States.

Water reuse is the use of any water that previously has been used. Used water may have been used once or many times. It can come from many different operations within a facility (referred to as internal used water) or from operations outside of a facility (identified as external used water). Internally and externally used waters can be reused individually or collectively in any process requiring water use or consumption.

Typical internal used waters include pump seal water, sample line water, steam condensate, softener or deionized backwash and rinse water, reverse osmosis reject water, boiler or cooling tower blowdown and treated wastewater. Typical external used waters include reclaimed water (treated municipal waste), industrial wastewater and agricultural runoff.

Used waters vary in quality and quantity. Many can be utilized with no further treatment. However, others will require treatment to remove suspended solids, metals, oils, etc. Specific processes in different industries generate used waters with varying water quality and volume.

Reused waters can be utilized in almost any process that requires water. Although some used waters are utilized in processes without requiring further treatment or purification, initially, used water quality must be determined to identify if it is suitable for reuse. Some processes require relatively high purity water. Thus, certain used waters must be treated prior to reuse.

Processes that can handle used waters without treatment prior to use include:

• Cooling tower makeup.
  
• Boiler water makeup.
  
• Demineralizer/filter backwash.
  
• Demineralizer rinse water.
  
• Reverse osmosis reject.
  
• Turbine gland seal condenser water.
  
• Sample system water.

Conservation and Reuse

Cooling tower systems have been targeted as potentially large water savers in a number of states. Because they evaporate water, they are water consumers that can reduce discharge. Water savings can be achieved by simply raising cycles of concentration one or more units (say, from two to three or two to four). The water savings can be as much as 30 to 50%.

Cooling towers also are good systems for reuse water consumption. Reuse can satisfy a portion or the entire makeup water requirement. Reuse water will require a change in the cooling water treatment program to protect the equipment from corrosion, deposition and biological problems.

Utilizing used waters requires a systematic approach that is common to all facilities. The approach starts with a survey of water users, generators and consumers. Next, a database should be developed that contains the quantity and quality of the used water. The database also should include the quality of water required for the system in which it will be used.

The water database should list the typical water constituents for each used water. Often, these include conductivity, hardness, alkalinities, iron, chlorides, silica, suspended solids and organics. Other values such as oil, bacteria, barium, strontium and copper also may be considered. The volume available, if it is continuous or periodic, and how often it is available also should be included in the database.

Next, determine what water quality the receiving system requires and what impact will occur relative to deposition, corrosion, etc. Should these used waters be added to cooling tower systems as part of the makeup water, for example, then identify if more or less water treatment is required. This evaluation is critical.

Combining Existing and Used Water

Do not overlook the possibility of utilizing some used waters with existing waters even if that water is of poor quality. The key to successful water reuse is to identify the quality and quantity of used water and their impact on the process that will use it. As an example, reverse osmosis reject water commonly is used as part of the cooling tower makeup water even though the mineral content is many times that of the normal makeup water quality. This is possible because less than 10% of the total makeup water is reverse osmosis reject used water. The impact on the process using it often is insignificant. However, for each application, a study is needed to determine the impact and acceptability for water reuse.

Extensive reuse of recycle waters, also known as reclaim or gray water, is occurring primarily in California, Texas, Arizona, Nevada and Florida, and soon it will begin in Colorado. It is being used successfully as cooling tower makeup water in utilities, refineries and chemical plants.

Water reuse is rapidly growing, particularly in areas lacking water due to drought, depletion or need to supply more potable water. Successful reuse requires a practical approach of matching water quality and quantity with water users.