Water Treatment During Drought
While seasonal changes produce variances in water quality, drought also affects water treatment programs. During droughts, suspended and dissolved solids concentrate in lake water, which may mean using more chemicals to bring water quality up to the level necessary for industrial purposes. As water levels drop, in-creased sediment and dissolved solids as well as bacteria, algae blooms and other microorganisms decrease process cooling makeup water quality.
Water is not an unlimited natural resource. Industries that use large amounts of water must be prepared not only to pay higher prices for lower quality water but also to do more with less. New water-use programs should be designed to accommodate water restrictions. Proper water treatment planning and program design can help make this change easier.
Water has been used as a heat transfer medium for many years. When water, fuels and discharge costs were inexpensive, the use of these resources was not regulated. Costs have risen, and the need for a more efficient approach to resource management is in demand. Three areas are critical when developing an effective water treatment program:
- Corrosion/scale tendencies.
- Water quality variance.
- Water availability.
The primary goals of any water treatment program are:
- Prevent the development of corrosion, scale and microorganisms to ensure a safe working environment.
- Reduce energy and water usage.
- Preserve valuable assets and reduce labor costs.
All water exhibits either a corrosive or scaling tendency with varying degrees of severity. Some types and sources of water are of higher quality than others, but perfectly stable water does not exist. Water quality can vary from one location to another. The selection of a particular formulation and the amount of chemical used is directly related to the quality of water available at that site. Even within a relatively small geographic area, variances in water quality can require a water treatment program tailored for each particular site. This is especially true during a drought. Different water sources such as wells or rivers display variances in hardness, alkalinity and other factors. If operators do not make adequate periodic adjustments, serious consequences could arise.
It is essential that the correct water treatment chemicals, product concentrations and field service be purchased for the plant. The application expertise, laboratory support, research and development, and technical support behind those products also are important. In a cooling water system, water is used as a heat transfer medium. Cooling systems involve the removal of heat from water and are continually saturated with oxygen, creating the perfect environment for corrosion. A small amount of scale can shut down an entire cooling system.
Cooling tower makeup water contains suspended particles consisting of metal oxides, decaying organic matter, dirt and minerals that are concentrated during the evaporative cooling process. Some of this material becomes nutrients for algae and bacteria growth. Deposit-control mechanisms include dispersants, surfactants, precipitants, filtration and proper maintenance.
In an open-recirculating cooling tower system, water continuously is recirculated and open to the air. Water moving through a heat source such as a condenser, chiller or evaporator increases in temperature and is cooled by evaporation in the tower. Makeup water must be added continuously to compensate for evaporation, leaks and blowdown. When makeup water quality is poor to begin with, as in drought conditions, the contaminant level is higher than normal and must be reassessed. Air is continuously passing through a cooling tower and introducing into the system a variety of gases, including additional oxygen, as well as environmental debris. The open-recirculating cooling tower system has the greatest potential for problems associated with deposits, corrosion and microbiological growth for the following reasons:
- Higher water temperatures.
- Introduction of environmental debris.
- Continuous makeup.
- Continuous high levels of oxygen.
Once-through systems pass water through heat exchange equipment (or other operating equipment) and then discharge the water. These systems need large quantities of low cost, low temperature water. Typical water sources include lakes, rivers and wells. Because the water comes from a variety of environments, problems to be aware of include pitting, corrosion, flow restriction, scale and sediment buildup, heat transfer loss, iron transport and product contamination.
In a closed-recirculating system, corrosion and scale control are simplified if the system is running in a normal and healthy condition with little makeup water. Little, if any, evaporation should take place, and without evaporation, scale normally is not an issue. However, drought situations require more attentiveness.
What Can Contaminate Your SystemScale and Deposition. The term scale refers to the deposits produced by the crystallization or precipitation of salts from solution. Scale on a heat-exchanger surface presents a serious interference in heat transfer. Basic conditions for scaling are met in areas of the system that have the highest water temperature or when the water simply becomes oversaturated with silica or any other scale-forming compounds. Three standard methods can be used to prevent scale formation:
- Remove scaling minerals from the water
prior to use.
- Inhibit the precipitation of scaling compounds.
- Allow impurities to precipitate as a removable sludge rather than a hard deposit.
Within a system having soft water and low alkalinity, controlling tower bleed rate is an effective means of keeping dissolved solids in solution. However, in systems with hard water and high alkaline conditions, inhibitors are needed to control scale. Inhibitors control scale through threshold inhibition, dispersion and crystal modification. Threshold inhibition uses inhibitors to keep highly concentrated solutions from precipitating into scale-forming compounds. Dispersants prevent the formation of scale-forming compounds by manipulating their affinity and keeping them suspended in solution. Crystal modification allows both the formation and precipitation of scale-forming compounds but modifies the compound structure so it produces a sludge that is managed through blowdown or by filtration.
Corrosion. Corrosion in cooling systems is the actual loss of metal through one or more of several mechanisms. It can cause premature system failure, resulting in downtime and the need to replace cooling equipment. Deposition of corrosion products on heat exchanger surfaces also decreases system efficiency by decreasing the flow rate and hindering heat transfer.
Cooling System Fouling. Fouling is the deposition of inorganic or organic solids suspended in the cooling water. Suspended solids, biological growths and makeup water contaminants can gather in areas of the cooling system where fluid flow is blocked or the normal fluid flow velocity is decreased. As air passes through a tower, the water absorbs dirt, silt and microorganisms from the air. The tower's warm and humid environment is ideal for biological growths such as algae, fungi and bacteria, whose colonies produce the biological slimes that can foul heat exchangers, decrease heat exchanger efficiency or plug tubes. Excessive fungal growth may penetrate the timbers in towers, digesting the wood and ultimately causing tower collapse. Slimes can occur throughout the entire recirculating system and may not be visible.
Biological Control Can Protect Your SystemChemical treatment is a reliable method of biofouling control. A biocide supplement can be added to open, closed or once-through systems as a preventive measure. The loss of corrosion inhibitors due to system leakage or biological organisms in the system can increase the cost of maintaining proper chemical residuals. It also can increase potential for corrosion and deposition problems.
Biocides are added to cooling towers, processes and chilled water systems to prevent three potential problems that stem from biological growth:
- Impeded heat transfer, which results in
- Structural material destruction.
- An environment conducive to diseases and personnel hazards.
Good tower system housekeeping is important for an effective biocide program. When water conservation is implemented in cooling systems, an excessive buildup of dirt and debris can absorb the biocides, making them ineffective at the dosage levels allowed. To prevent these problems and uncontrolled growths, the following areas should be addressed:
- The proper biocide must be selected for the specific operation. Considerations include tower size, location, water quality and load.
- All towers should be cleaned at least twice a year. Some require more frequent cleanings.
- Top distribution pans should be covered. Because algae require light for photosynthesis, simply covering these pans can reduce biocide demand.
- The biocide should be added at the proper time of the day. If the operation is comfort cooling, biocide feed at night during low-load periods will allow longer retention and a better kill.
Proper cooling tower maintenance is important because towers are open and they wash large amounts of dirt and debris out of the air. Typical environmental debris in towers includes sand, insect bodies, clothing fibers, paper, leaves, grease and pollen. These materials cannot be dissolved chemically, and trying to do so will harm the tower. Chemical treatment may be able to help keep these materials from adhering to heat transfer surfaces, but they eventually will accumulate in the system's basin, sump or in low-flow areas. Dirt and debris will accelerate corrosion, scale and biological growth.
All towers should be cleaned manually whenever noticeable debris accumulates in the basin. Even if the tower basin remains clean, all towers should be shut down and undergo complete cleaning and inspection at least once a year to ensure that the interior is properly maintained. Realistically, many towers require such service two to four times a year as well as having condensers opened, inspected and cleaned of deposits and debris.
With climatic variances in many regions of the country becoming more frequent, it is imperative that water treatment systems al-ready in place be reanalyzed for effectiveness under drought conditions. By monitoring and making simple adjustments, there will be fewer detrimental effects and costly corrective measures will not be necessary. If a technically sound water treatment program is planned and properly implemented, the water usage reduction mandated in many drought situations can be achieved in spite of decreases in water quality.