A Point-of-Entry (POE) system is more appropriate if a contaminant is present that poses a health threat from general use as well as from consumption. Volatile organic compounds and radon are examples of this type of contaminant. These contaminants may get into the indoor air when water is used for showering and washing. In this
case, it is more economical to have a large POE system that treats water as it enters the home than to have POU systems at each tap.
AC filters used for home water treatment contain
either granular activated carbon (GAC) or powdered block carbon. The amount of AC in a filter is one of the most important characteristics affecting the amount and rate of
pollutant removal. More carbon in a cartridge means more capacity for chemical removal, resulting in longer cartridge lifetime. This means fewer cartridge changes and less chance of drinking contaminated water. Particle size
will also affect the rate of removal; smaller AC particles generally show higher adsorption rates.
Rust, scale, sand or other sediments can clog any AC filter. A solution to this problem is to place foam or cotton filters (often called sediment or fiber filters) between the cartridge and incoming water. When sediment filters become clogged, they need to be replaced or they will cause water pressure to drop.
An AC filter must be deep enough so that the
pollutants will adsorb to the AC in the time it takes the water to move through the filter. The appropriate filter depth depends on the flow rate of water through the filter. The slower the flow rate, the better the removal. The poor performance of some end-of-faucet devices is
probably due to improper filter depth.
Physical and chemical characteristics of the water will also affect performance. The acidity and temperature can be important. Greater acidity and lower water temperatures tend to improve the performance of AC filters. AC filters have a limited lifetime. Eventually, the surface of the AC will be saturated with adsorbed pollutants and no further purification will occur. This is called breakthrough --- the pollutants have broken through the filter to emerge in the treated water. When this occurs, it is possible that the contaminant concentrations in the treated water will be even higher than those in the untreated water. At this time, the cartridge needs to be replaced. Knowing when breakthrough
will occur and thus when to replace the cartridge is a major problem with AC treatment.
Some cartridges are sold with predictions about their longevity. These are generally only crude estimates since they do not take into consideration factors that are characteristic to a specific water source, such as pollutant concentration. The retailer you purchase the treatment device from can make better estimates of the filter's useful lifetime based on water usage (flow rate) and pollutant concentrations shown in the chemical analysis. Hence, to get the most accurate estimates, you should learn what these amounts are before purchasing the system. Note that if pollutant concentrations increase over time and testing is not performed to reveal this change, such estimates may turn out to be not very practical or useful.
Unfortunately, AC filters can be excellent places for bacteria to grow. Conditions for bacterial growth are best when the filter is saturated with organic contaminants, which supply the food source for the bacteria, and when the filter has not been used for a long period of time. It is still unclear whether the bacteria growing on the
carbon poses a health threat. Some manufacturers have placed silver in the AC in order to prevent the growth of bacteria. The effectiveness of this procedure has not been independently verified. In addition, silver may contaminate the drinking water.
The above considerations have led public health
officials to consider AC home treatment a temporary solution to be used only until the source of contamination can be eliminated and the water supply is safe. Even with proper installation, maintenance and operation,
malfunction of home water treatment systems can occur.
AC Filter Guidelines
- Make sure the filter contains AC.
- Know the quantity of AC in the filter since this will determine the amount and rate of pollutant removal.
- Use prefilter to add life to AC filters.
- Replace prefilters and AC filters regularly.
- Determine appropriate intervals for replacement of AC filters based on contaminant concentration, water characteristics, water flow rate, depth of filter, type and amount of AC and prefilter. Retailers can help in this analysis.
|
How-To: Information about Activated Carbon Water Treatment
