Water is essential for human’s life. A person can live only three days without water. Since water is the primary resource, water quality is vital. The technological progress brought in the significant advances to the humanity. However, water pollution became the by-product of it. People use natural water resources for their own purposes, as well as plants and different production operations. As a result, natural water resources are polluted by various chemicals that differ in toxicity, chemistry, and origin. Therefore, it is vital to understand the risks to human health. In this paper, we will consider three inorganic and three organic substances and the health risks they bring. For every pollutant we will define the effective water treatment methods.
In low concentrations, some of them are present in the environment, for example, fluoride or nitrate ions. Though, they are toxic in high concentrations. Others, such as mercury, phenol, polychlorinated biphenyls (PCB), dibenzodioxins (PCDD), etc., are the result of anthropogenic activity.
Mercury is found in drinking water in the form of inorganic (Hg+ or Hg22+) and organic compounds (methyl mercury, found in rare cases). Maximum contaminant level (MCL) established by Environmental Protection Agency (EPA) is 2 ppb (0.002 mg/l). Mercury affects the nervous system, bringing up nervousness, irritability, and difficulties with memory and changes in hearing or vision. It also causes kidney damage. Young children are more sensitive to inorganic mercury than adults. Typically, mercury does not occur on drinking/tap water in the U.S. in concentrations that can seriously affect the health. For example, mercury concentrations in the ground and drinking water in the Washington State measured by EPA are 0.3 ng/l. Since a concentration of mercury is very low, it can be removed from water by highly selective methods described below.
Nitrates are the salts of nitric acid and are present in water as NO3- (nitrate ion) or NO3N (nitrate-nitrogen). The nitrate is one of nutrients required for plants high yields. Nitrate ions are highly mobile and can easily migrate to groundwater. EPA states the MCL of nitrate in water as 10mg/l. It’s higher concentration in water cause methemoglobinemia or blue baby syndrome. Acid in baby’s stomach is not as strong as adults’; so, nitrate ion is converted to nitrite (NO2-) that turns hemoglobin into methemoglobin. Blood hemoglobin transports oxygen, while methemoglobin is not able to transport it effectively. Severe case methemoglobinemia can cause damage to the brain or death. Pregnant woman and people with reduced stomach acidity are exposed to risk of methemoglobinemia too. The symptoms of nitrate poisoning are difficulties in breathing, dizziness, headache, and weakness. Although nitrates are rare in urban water supply systems, rural water supply contain water-bearing formations, which are, sometimes, polluted.
Fluoride (F-) is an anion of hydrofluoric acid. Intentional water fluoridation has been widely used over 60 years to reduce tooth decay. However, the excess concentrations of fluoride bring potential risks to human health. Chronic exposure to high concentrations of fluoride in water may cause pain and tenderness of the bones and mottled teeth for children. Le%u0301vy presents the research results that deny the bone fractures and skeletal fluorosis caused by fluoride in water. Numerous researches presented in the work state that there are no reproductive or developmental health effects of water fluoradisation. Thus, EPA determines the MCL for fluoride as 4 mg/l. EPA position for water fluoridation is neutral; so, it neither stimulates the process, nor states its harmfulness.
Water treatment methods
Since mercury, nitrate, and fluoride are present in drinking water in ion form, the methods for their removal are alike. Distillation is a water treatment process that requires water boiling with subsequent collection and condensation of vapor. The salts (fluoride and nitrate) remain in precipitate. This method is not suitable for mercury removal, since mercury vapors even at room temperatures. Reverse osmosis can remove these three inorganic pollutants. During the process, water is filtering through the selective membrane while pressure is applied. Large molecules, as Hg2+, F-, NO3- cannot go through the membrane, but water does. Ion exchange is applicable in three cases as well. Cation-exchange material (H-ionite filter) removes mercury ions, while anion-exchange material (Cl-ionite filter) effectively removes nitrates and fluoride from drinking water. The popular charcoal filters proved to be not effective for fluoride and nitrate, and they can remove only organic forms of mercury. Adsorption can be applied for mercury removal only if specific adsorbents containing sulfur groups are used.
Phenol can appear in drinking water through the leakage from landfills and wastewater from municipal waste incinerators and petroleum fueled facilities. The consumption of water containing 2 mg/l of phenol during lifetime period does not cause any harmful effects, as well as exposure to 6 mg/l during 10 days. In his work, Baker studied the health effects of phenol and concluded the incidents of nausea, diarrhea, mouth sores, and irritation of the oral cavity. There is a hidden risk of phenol presence in drinking water. If water is chlorinated for disinfection purposes, phenol is transformed to chlorophenol, which is carcinogenic.
Phenol is fairly well destroyed by bacteria. So, biochemical treatment is the suitable method. Water that contains organic compounds can be purified by using a charcoal filter or adsorbents. Phenol can be removed by oxidation process during ozonation. Distillation can be used as well.
The polychlorinated biphenyls are the substances that do not appear in the natural environment. There are 209 congeners that do not go through biodegradation, but accumulate in water, sediments, fish, etc. The MCL for PCB is 0.5mkg/l. The higher levels proved to bring severe health effects. PCBs cause cancer, bring effects on immune, reproductive, nervous, and endocrine systems. Since PCBs are lypophylic, they accumulate in adipose tissue and act as an immune suppressor, sometimes called ‘chemical HIV’. The removal from water and monitoring of water supply reservoirs is a challenging task, since PCBs acceptable levels are low. Therefore, one-stage treatment is insufficient. PCBs can be removed from water when coagulation, charcoal adsorption, and sand filtration are combined. Also, some bacteria strains can decompose PCBs.
PCDDs are polychlorinated molecules with a structure of two benzene rings bound together by two oxygens, creating a third ‘dioxin’ ring. There are 75 congeners, known as dioxins. As PCBs, PCDDs are anthropogenically generated and thus, do not go through the biodegradation chain. The MCL of dioxin is 0.00000003 mg/l, which is the lowest for all pollutants listed in the National Primary Drinking Water Regulations (National Service Center for Environmental Publications 1995). Consumption of water with PCDD causes complications of the reproductive system and significantly increases the cancer risk. Furthermore, dioxin effects have not been researched well, and that is the reason for a low value of MCL. Dioxins can be removed from water by selective methods, such as adsorption on selective adsorbents or ion-exchange on modified resin. Two or three stages of treatment assure better result. Though MCL is low, treatment quality assessment requires specific analytical methods and procedures. In some cases, quality assessment cannot be performed due to the expensiveness of the analysis.
Water pollutants are an issue of a great interest of society. Both organic and inorganic pollutants can be dangerous and bring high risks of adverse health effects, even if their concentrations in drinking water are low. The difficulties of water quality monitoring are low concentrations of pollutants. Modern treatment technologies provide a choice of means for purifying water. However, typically, methods are specific for every substance, and if the task requires removal of two or three compounds, two or three stages of the process are required. The main water treatment methods used are ion exchange, specific adsorption, distillation or hyperfiltration.