Microorganisms: The effects of on-site water treatment
by Abigail Cantor, P.E.
This article is part of a series discussing the growth of microorganisms in plumbing systems. As previously noted, many types of on-site water treatment equipment create conditions for microorganisms to grow and thrive by increasing the residence time of the water in the plumbing system with extra water storage volume, providing additional surface area for microorganisms to form biofilms on, and removing or using up any available disinfection to fight microorganisms.
The best practice in plumbing design is to provide on-site water treatment only when it is absolutely necessary to do so. It is also important to select the appropriate treatment system, to determine a proper location for the equipment in the plumbing system, and to size the equipment so that volume and surface area are minimized. Finally, automatic clean-in-place systems or manual cleaning protocols must be utilized to keep the equipment free of biofilms.
Determining Necessary Water Treatment
The first step in plumbing design is to determine specifically what contaminants, if any, are of concern in a building’s water source. To do this, one must consider that there are numerous chemical compounds and types of microorganisms that can potentially contaminate drinking water. Contaminants are identified and regulated in the United States with separate standards for municipal water systems, private water systems, and bottled water. See Table 1.
If the building is connected to a public water system, the water has already been rigorously tested for the list of contaminants listed in Tables 2 and 3. The results of those tests are public record, available at the regulatory agency that governs the state or territory where the building is located. The results are also sent to each water utility customer annually in the Consumer Confidence Report. There might be local issues to be concerned about, such as increasing concentrations of an industrial chemical in a public well. In that case, the property owner should keep track of water utility plans to resolve the problem and attend water commission meetings, read the water utility website, or call the water quality manager. If a building owner is not comfortable with the utility’s approach to removing the contaminant threat, then an on-site water treatment device should be used for removal. It would be a rare and special case to need such a device.
It is possible for building plumbing systems to receive debris from municipal water distribution system piping. Debris occurs in the distribution system when particles, like sand, settle out and when dissolved chemicals in the water, like manganese, iron, or aluminum, chemically precipitate out. The possibility varies with the nature of the water and the water utility’s pipe cleaning and replacement program. Debris can temporarily be entrained in the water during utility or road construction; it can happen seasonally due to water main flushing and other routine maintenance activities. If a property owner experiences discolored water at an intolerable frequency, then on-site removal of the debris may be desired.
There is also the possibility that a building’s plumbing system leaches contaminants into the water. Lead, copper, and iron are known to transfer from piping materials into water to varying degrees depending on characteristics of the individual water system.
If the property owner owns the water source, such as a private well, they must take the responsibilities of a water utility manager. After complying with any state regulations on water quality for private water sources, the property owner must decide what other contaminants they might want to test for and, if significant, remove. See Tables 2 and 3. A common issue for private wells is high iron concentrations which can precipitate out and stain sinks and laundry.
For both private water sources and municipal water, the hardness of the water can be an issue. Water hardness is mostly a measure of calcium and magnesium concentrations in the water. Depending on other characteristics of the water, including temperature, the calcium and magnesium can precipitate out of the water as solid compounds. The solids can cover heating surfaces in hot water heating systems, which in turn, will require more energy to heat the water; the buildup of solids will also reduce the life of the hot water heating tank. For this reason, it is more economical to remove hardness from water where hardness is greater than about 120 mg/L as calcium carbonate (7 grains of hardness). Many people argue that hard water for cold domestic use should be softened; they state that hard water will clog pipes, create spots on glass shower doors, and react with soap so that it will not lather. These are debatable arguments. Even in locations with very high hardness (300 to 500 mg/L as calcium carbonate or 17.5 to 30 grains), cold un-softened water does not typically cause these severe problems. Older residences in those locations only use softened water for the hot water system. It is only recently, with modern plumbing practices, that cold water is also softened.
Determining Type of Water Treatment System
When specific contaminants have been identified in the water, then the proper contaminant removal technique can be selected. The proper technique is the one that removes all contaminants of concern at the highest efficiency for the lowest financial and environmental costs. Below are descriptions of common on-site treatment techniques.
Activated carbon filters
Activated carbon is carbon, typically from charcoal, that has been processed to make it very porous. The more pores in the carbon, the higher the surface area. The higher the surface area, the more specific contaminants can be pulled from the water to adhere to the carbon, a process called “adsorption”.
Different chemicals have different attractions to the carbon. For example, heavier compounds have a greater attraction than lighter compounds. This means carbon filters will not remove every type of contaminant. In addition, the carbon can become saturated with contaminants and stop removing them. Most importantly, just before the saturation point, the concentration of contaminants in the water flowing out of the filter begins increasing at a rapid rate. Therefore, a carbon filter must be removed before the “breakpoint” of the least adsorptive contaminant or else the consumer will be drinking high levels of the contaminant that they intended to remove. Filter manufacturers make assumptions as to what contaminants might typically be in water and set a common time when filters should be changed. This may or may not be applicable in individual water systems.
Some carbon filters, such as certain ones that attach to sink faucets, are manufactured so that they combine treatment techniques within a small block of carbon. Like other activated carbon filters, they have their limitations as to what contaminants can be treated and for how long. In addition, the filters, themselves, can add contaminants to the water, based on compounds in the manufactured filter material. There are research projects looking into this phenomenon.
Reverse osmosis and other membrane technologies
Reverse osmosis is a treatment technique that places a membrane barrier in the water. The membrane is made of synthetic organic materials that do not have straight-through pores like a filter. Instead, the pores are like a microscopic maze that can prevent many dissolved contaminants from passing through. High pressure on the upstream side pushes the water, minus many of the contaminants, through the membrane.
There are other membrane technologies where the pores are straighter but very small. Those technologies remove specific contaminants at lower pressures than reverse osmosis.
Membrane technologies prevent a percentage of the incoming water from going through the membrane, and instead, the water is sent down the drain to waste with the rejected contaminants. The technique is not practical when it is too expensive to waste a percentage of the available water. In addition, the synthetic organic membranes can dissolve in contact with some chemicals that might be in the water. Chlorine used for disinfection is one of the chemicals and it is typically removed in a carbon filter upstream of the membrane.
Physical filters provide a physical barrier that can remove particles from water. The filters can be made of sand or flossy material that will not allow particles of a certain size to pass through.
Ion exchange/water softeners
Ion exchange is a treatment process where one ion is taken out of the water and others are put into the water in its place. An ion is an atom or molecule with either additional electrons or missing electrons; this gives the atom or molecule a negative or positive electric charge. Water softeners are an example of an ion exchange process. Here, calcium and magnesium, dissolved in the water as positively-charged ions, are “stuck to” negatively-charged ions. When in contact with the ion exchange material, they are attracted and adhere to the material. In exchange, the material releases two sodium ions for every calcium or magnesium ion; the sodium ions, then, form a union with the negatively-charged ions that were left behind in the water. In the case of softening, the sodium concentration increases in the water.
At certain intervals, the ion exchange material must be cleaned to knock off the exchanged ions and replenish the original type of ions on the material. In the case of water softeners, a solution of sodium chloride (brine) is used to flush out the ion exchange material. This regeneration process creates a waste stream of chloride-laden water that is sent down the drain and out to the wastewater treatment plant.
Iron and manganese removal
Dissolved iron and manganese in the water eventually react with an oxidant like oxygen or chlorine in the water and precipitate out as a solid on pipe walls, sinks, and laundry. To remove dissolved iron and manganese before it drops out elsewhere, an oxidant is pumped or bubbled into the water. After a certain contact time, the iron and manganese are oxidized to a solid form and the particles are filtered out in a sand filter.
The filter must be backwashed periodically to clean the solids out and send them in a waste stream down the drain.
Sequestering is used to hold metals like iron and manganese in the water and prevent them from precipitating out as solids. Traditionally, polyphosphate chemical products have been used in water systems to hold the metals in the water. This is especially done when a small water utility or private property owner cannot afford a treatment process to remove iron and manganese.
It is now known that the use of polyphosphates carries negative side effects. The polyphosphates not only can hold iron and manganese in the water but can also pull lead, copper, and iron from pipes and hold those metals in solution as well. The consumer drinks any concentration of the metals being held in the water. Polyphosphates also provide an essential nutrient, phosphorus, for the growth of microorganisms and in doing so, can aid in biofilm formation. Finally, as the phosphorus from the polyphosphates eventually flows to waste, the wastewater treatment plants struggle with meeting stringent phosphorus discharge limits.
This series of articles on the growth of microorganisms and the formation of biofilms in piping systems has emphasized that disinfection of water is a main weapon against microorganisms. With a clean piping system, it typically only takes a low dose of disinfection (0.3 to 0.5 mg/L free chlorine) to fight off intruding microorganisms and keep the piping system clean.
When on-site water treatment systems remove or use up disinfection in the incoming water, a dosing system should be added to replenish the disinfection in the water. For owners of private water sources, continuous disinfection of the water before and after any treatment should be considered.
Some people complain about the taste of chlorine in their water. If that is in issue, then drinking water can be left in a big pot open to the air or with cheesecloth covering it to allow the chlorine to transfer from the water into the air. Alternatively, chlorine can be removed by a carbon filter at the drinking water faucet. (Refer to the carbon filter discussion above.)
A more serious negative effect of disinfection is the possible formation of carcinogenic disinfection by-products. This can occur when the water has high naturally-occurring organic carbon compounds that react with the chlorine. If water is received from a municipal water system, disinfection by-products are tracked and minimized by regulation (Table 2); the property owner should re-chlorinate water within the concentration boundaries of the municipal utility. For private water sources, the owner should become familiar with the disinfection by-product forming potential in the water and chlorinate accordingly.
Determining the Location of the Water Treatment System
Treatment systems located at the point in the plumbing where the water enters the building is called a point-of-entry water system. Treatment systems located at the drinking water faucet are called point-of-use systems. The location of on-site water treatment in a building’s plumbing system is a critical design decision.
Point-of-entry systems treat the complete water flow to the building and are subsequently larger in size than point-of-use systems. This creates a greater possibility of biofilm formation in the treatment equipment from increased surface area and retention of water. It also increases the volume of water needed to clean and maintain the treatment system. Many point-of-entry systems remove or use up incoming disinfection and so all of the piping downstream of the treatment system is not protected from the growth of microorganisms unless a disinfection dosing system is added.
Although smaller with little or no waste streams, point-of-use treatment systems must be installed for every drinking water faucet, while point-of-entry systems are installed at one location only.
Water treatment equipment for specific needs, such as water softeners for hot water, should be located as close to the specific need as possible. Water softeners are typically located in a mechanical room adjacent to the water heating system.
Sizing Water Treatment Equipment
The goal of proper modern plumbing design should be to minimize volume of water retained and surface area of the treatment equipment. The larger the size of the water treatment equipment, the more volume of water is retained on-site and the more surface area is available for biofilm formation. As already discussed, one way to minimize volume is to eliminate treatment equipment unless absolutely necessary. In addition, the volume of water to be treated should be carefully considered. Divide the estimated total water use into: water for drinking/cooking, water for cleaning, and water for any significant purpose such as filling large bathtubs. Design separate pipelines and treatment strategies for each purpose.
Cleaning Water Treatment Equipment
Various types of water treatment systems have cleaning cycles. Sand filters must be backwashed to remove trapped solids. Ion exchange material must be backwashed to remove solids and must be regenerated to replace ions. Water treatment of this type has automatic clean-in-place cycles. The cleaning water can be chlorinated to disinfect and fight developing biofilms routinely. It is critical to work with the equipment manufacturer in setting up a cleaning water disinfection system; chlorine in too high a dose can destroy the treatment material.
Filters that require replacement of filter cartridges should be changed before or at the time recommended by the manufacturer to prevent breakthrough of contaminants and the development of biofilms.
This article continues the series warning against the growth of microorganisms and the formation of biofilms in plumbing systems. On-site water treatment systems can contribute to the growth of microorganisms by increasing the retention time of water, increasing the surface area where biofilms can form, and by removing or using up disinfection in the water.
The first step in plumbing system design is to determine which contaminants in the water are essential to remove on-site. In many buildings that receive municipal water, additional water treatment is not necessary. There is a greater need for on-site water treatment when the building is served by a private water source where the property owner must manage their own personal water utility. There are also special needs for water treatment such as the need to soften hard water before it enters a hot water heating system.
After it is determined what contaminants must be removed, the best removal system must be selected. Every treatment system has advantages and disadvantages and has specific removal efficiencies for each individual contaminant. The sizing of the water treatment equipment and its location in the plumbing system are also critical design choices affecting the growth of microorganisms.
Finally, all water treatment equipment must be cleaned and disinfected or filter cartridges replaced routinely to clean out and prevent the formation of biofilms.
Maintaining a high water quality, including the elimination of microbiological growth, is a very delicate balancing act that should be given the highest priority when designing a plumbing system.