Removing Contaminants with Chemistry
Water treatment has been around far longer than most would think, dating back to 2000 B.C.E.; several methods were used for purifying water, such as boiling and filtering the water through sand and even charcoal. During these times, the reasons for purifying water were related to taste, as there was no concept of the various micro-biological and chemical contaminants that we see today.
The advances of human civilization prompted the need for further water treatment, as well as better technology to treat water. Water treatment has many applications, from residential to urban to industrial, and the problems and treatments vary based on the desired outcome. For example, large cities need to treat water to meet various regulations in order to protect public health, whereas industry may have more stringent and specific water quality needs.
Water treatment is often complicated because it deals with the chemical makeup of the water, which varies widely. The first step is to determine what contaminants should be removed. Unfortunately, this is not always a straightforward process. Many contaminants make themselves known in obvious ways, such as iron or hydrogen sulfide, which can cause taste and odor issues. Other contaminants, such as arsenic or pesticides, are only uncovered through a sophisticated laboratory analysis.
It is important to test water so that the right type of treatment system can be selected. Keep in mind, however, that more than one analysis may be necessary to get a baseline because water quality fluctuates. More information on possible contaminants in a specific area can be obtained by talking to residents, the local health department, the water department or the state geological society. In addition, your laboratory should be able to recommend appropriate tests based on a customer’s concerns and the information provided about the water source. It is important not only to address customers’ concerns, but also to educate them about potential contaminants they may not have considered.
Removing Hardness Minerals
One of the most common treatment methods is water softening. Water soft-eners remove hardness minerals, such as calcium and magnesium, by a process called ion exchange. Ions are simply charged particles. There are cations, which are positively charged, and anions, which are negatively charged. Ion exchange utilizes either cation or anion resins depending on what contaminants are to be removed; cation resins will remove elements that have a positive charge, while anion resins will remove those with a negative charge.
If the softener contains a cation exchange resin, which is saturated with sodium chloride, as the hard water passes through the resin, it exchanges sodium chloride for the hardness minerals. Once the resin is full of hardness minerals, it must be regenerated with a brine solution. This means the hardness minerals are flushed out and replaced again with sodium chloride to start the process all over again.
This process sounds simple enough, but this is where the chemical makeup of the water becomes important in sizing and setting up the equipment, as well as deciding whether pretreatment is needed. At least accurate levels for hardness, iron, manganese and pH should be known. If the pH level is too low, a neutralizer may be needed to bring the pH within a range the softener can handle. Some other items to consider testing for would include cadmium, copper, zinc, lead, barium and radium, as these elements are also removed by cation exchange, and significant amounts of any could affect the efficiency of the treatment unit. One other element to test for is sodium, as most water contains some level of sodium, and if present in high amounts, such as when salt water intrusion occurs, it can reduce the efficiency of the softener.
Removing Nitrates & Arsenic
Anion exchange is commonly used to remove nitrates, and it is also being analyzed for arsenic removal. Anion exchange works just like cation exchange in that is uses salt for regeneration. The biggest difference is that the resin is specifically formulated to remove contaminants with a negative charge. When installing an ion exchange system to remove contaminants such as arsenic or nitrate, it is extremely important to get a complete and accurate analysis to ensure proper removal. This is important because if competing ions are present, there is a possibility they will be attracted more strongly to the anion resin than the contaminant you are trying to remove. For example, when removing nitrates, it is important to know the amount of sulfates present, as some resins would preferentially remove the sulfate instead of the nitrate.
Removing arsenic with ion exchange becomes a little more difficult because species begin to play a role. There are two main species, As III and As V, which are of concern in drinking water. As III commonly occurs in a compound called arsenite, which is prevalent in ground-water and cannot be removed through ion exchange. An oxidation step is necessary to convert arsenite into arsenate, which can be removed easily with anion exchange. There are a variety of oxidants that can be used, such as free chlorine, sodium hypochlorite, ozone or potassium permanganate. Additionally, it is still important to know the levels of any potential competing ions, such as nitrates, sulfates, phosphates, molybdenum, selenium and vanadium. Other adsorptive materials are being explored for arsenic removal, but again, it is the chemistry of the water that will determine the efficiency and removal of this harmful contaminant.
Removing Inorganic Contaminants
Reverse osmosis is an ever-popular way of reducing many inorganic contaminants. Essentially, contaminated water is pushed through a semi-permeable membrane, which traps the contaminants but allows the pure water through. The membrane, however, can become plugged easily if the water contains large amounts of minerals, such as calcium, magnesium, iron or manganese. Using a softener for pretreatment may be necessary to reduce the potential for plugging. A good analysis should include the items mentioned previously as well as pH, conductivity, TDS, sodium, potassium, chloride, sulfate, carbonate bicarbonate, fluoride, phosphate, silica, arsenic, ammonia and nitrates. Knowing these levels will help in appropriately sizing the equipment to get maximum performance and life.
The Bottom Line
These are just a couple of treatment methods. There are many more, and the chemistry will get even more complicated as new contaminants and treatment methods are discovered. Chemistry is the bottom line when it comes to treating the water. Have an accurate analysis of the water completed before attempting to decide which treatment system will work best, and you will save yourself time and money in the long run. Unfortunately, there is not a one-size-fits-all solution when it comes to water treatment. The real professionals use chemistry to ensure the systems they install will work correctly and efficiently.