In commercial and institutional buildings, cooling systems are not exactly top of mind when it comes to conservation and the use of “green” technologies. Yet the potential for significant cost and water savings and reduced environmental impact exists. With more-stringent effluent and chemical-handling regulations, increasing drought conditions, and fluctuating chemical prices, facility owners increasingly are looking to cooling towers — more specifically, the treatment of cooling water — to meet the objectives of sustainable cooling: more efficient equipment performance, saving makeup and discharged water, and reducing or eliminating the use of chemicals.

To effectively manage a cooling tower, one must understand it in terms of:

• Water use

  Exactly how much water does the system use on a yearly basis?

• The type and amount of chemicals used

  TRADITIONAL METHODS

  Are the chemicals potentially hazardous to workers? What is the actual cost of use? Are there more effective alternatives? How volatile are prices?

• Discharge regulations

  What discharge limits are imposed by the local publicly owned treatment works (POTW)? Is the facility in violation?

• Environmental impact

  Are harmful chemicals being discharged into the environment?

• Operational efficiency

  Poor water treatment can adversely affect tower efficiency (e.g., scale formation, biofouling) and impact heat transfer.

• Water-reuse options

  Can discharge water be used for other purposes (e.g., landscape irrigation, equipment washing, dust control) on site?

• Worker safety

  Is the current treatment technology a health or safety hazard? Does it require moving heavy supplies?

More than 95 percent of the time, chemicals are used to treat cooling water for scale, corrosion, and bacteria. A partial list of commonly used chemicals and their potential issues is given in Table 1.

As dissolved-solids levels increase, so do scale-deposition tendencies. With chemical treatment of cooling water, cycles of concentration are limited by an inability to prevent scale formation. This is attributable to the way in which most chemicals work. They prevent the formation of calcium carbonate by sequestering calcium, preventing crystal growth, or keeping the material in solution. Ultimately, a saturation limit is reached, and the addition of more chemicals becomes ineffective. This is in contrast to non-chemical technologies, which form calcium-carbonate solids and remove them from water, increasing cycles of concentration.

A recent trend in chemical treatment is the use of dry products, which can be used for scale prevention, corrosion protection, and microbial control. Because they usually are concentrated (up to 100-percent active), they are significantly smaller by volume, lighter, and much easier and safer to handle, store, and transport than their liquid equivalents.

With a properly designed treatment program and continuous monitoring, chemicals can be an effective means of maintaining good water quality in a cooling tower. However, facility managers are beginning to seek alternatives because of environmental concerns.

DISCHARGE OF CHEMICALLY TREATED WATER

Most of the chemically treated water discharged from a cooling tower is classified as wastewater. Depending on the nature of the discharge and the route of disposal, regulatory requirements may apply. The four routes of disposal of cooling water are:

• Holding tanks for transport to a treatment facility.

• Groundwater discharge.

• Sewer discharge.

• Surface-water discharge.

Disposal via holding tanks and groundwater is uncommon and will not be discussed here.

Local and state permits for sewer discharge generally are less stringent than those for surface-water discharge because water goes through a sewage-treatment plant prior to being discharged to the environment. However, most sewage-treatment plants, in an attempt to conserve treatment capacity, prohibit — or at least discourage — the discharge of cooling water. A local POTW should be contacted about its regulations and permit requirements.