Blow-Down Water Introduction
The water that is drained from cooling equipment to remove mineral build-up is called “blow-down” water or “bleed” water. The cooling equipment that requires blow-down is most often: cooling towers, evaporative condensers, evaporative coolers, evaporative cooled air-conditioners, and central boilers (both steam and hot water). These cooling systems rely on water evaporation to garner the cooling effect (latent heat of evaporation). As the water evaporates, the mineral content (calcium carbonate, magnesium, sodium, salts, etc) of the remaining water increases in concentration of minerals. If left undiluted, these minerals will cause scaling on equipment surfaces; possibly damaging the system. The blow-down water is usually dumped into the wastewater drain, yet in some cases, this water can be reused for irrigation and other selected uses. For more information on these cooling systems and related water use, please see:
Evaporative Cooling Introduction
Cooling Tower Introduction
Blow-down water has a much higher mineral content than potable water supplied by the local water utility; often 2 to 5 times more minerals. The concentration of mineral is measured as Total Dissolved Solids (TDS). Most potable water in the US is has a TDS level of 100 to 350 parts per million (ppm), though some potable water can be as high as 500ppm. Depending on the supply water and the cooling system operation; the blow-down water TDS can range from 500 to 1300 ppm.
Besides minerals, the water might also contain algea, bacteria, or pathogens. There are often chemical additives put in the water to impede scaling, reduce ph levels, and kill biological contaminates. The possible presence of pathogens suggest the water should never come in contact humans or sprayed above ground. A review of the on-site water treatment regimen might reveal other possible hazards.
Uses for the Water
There is somewhat of a paradox in using blow-down water for irrigation. If the cooling system is efficiently using water, the blow-down water will have a TDS level too high to be suitable for irrigating plants. In general, most plants cannot survive where the irrigation water exceeds 1,000ppm of TDS; though this somewhat depends on the type of minerals in the water. (The irrigation industry generally recommends turf be irrigated with water containing less than 500ppm of TDS.) A poorly maintained cooling tower will dump blow-down water at less than 400ppm of TDS; very suitable for many plants and turf grasses.
A well maintained cooling system, using water very efficiently, will drain blow-down water with TDS levels too high for irrigating most all landscape plants. The water can, and should be used when sufficiently diluted with other water sources having lower TDS levels, such as condensate, potable water, reclaimed water, harvested rainwater, etc. We advise the cooling system be operated at peak water efficiency standards; never reducing the cycles of concentration just to allow the undiluted blow-down water be used for irrigation.
Irrigating with blow-down water is also affected by local climates and soil conditions. The continual use of high TDS water will cause a build-up of these minerals and salts in the soil. If these minerals are not periodically flushed from the soil, they will greatly impair root growth of plants, and can kill the plants over time. It should be noted that plants prefering acidic soils (pine trees, azaleas, berries, hydrangeas, etc) should never be irrigated with blow-down water, as it often has a high salt content. In climates of heavy seasonal rains, these minerals are usually rinsed out of the soil, especially in sandy loams. In more arid climates, the landscapers have found it necessary to occasionally flush the soil by irrigating with softer water (potable or reclaimed). The blow-down water should only be applied subsurface, unless the water is additionally treated to remove any threat of pathogens.
There may be other uses for the water, beside irrigation. Care must be taken to account for the very high mineral content of the blow-down. In general, the undiluted blow-down water will quickly cause a build-up of scale (mineral deposits) on most pipes, spray heads, valves and plumbing fixtures. While we fully support innovative ways to reuse the water, we suggest users investigate possible damage to equipment, fixtures, and/or plants before implementation. Never use water with TDS levels above 500ppm in hardware, unless you verify its applicability with the manufacturer of the equipment, appliance, fixture or fitting.