This article originally appeared in Water Quality Products March 2020 issue as "A Tale as Old as Time"
The existence of life on Earth is largely owed to the presence of water, more particularly presence of clean and freshwater. Water, the major component of all living things, available in Earth in many forms, such as surface water, undergroundwater, seawater, rainwater, etc. More than two-thirds of Earth’s surface is covered by water, of which 97.4 % water is available as seawater, and the rest includes 1.98% glaciers, 0.59% groundwater, 0.014% freshwater lakes. Roughly 99% of water available on Earth is not suitable for daily use. The remaining 1% of water is available as Earth’s freshwater for all day-to-day uses.
In primitive days, water from ground and lakes was believed to be pure and drinkable. With the evolution of human civilization, definition of drinking water quality, its effects on health and methods for water purification underwent dramatic changes. Economic growth backed by industrialization has led to significant improvement in human development index; however, it has also been coupled with many environmental issues which have largely been ignored. One of the major and most widespread issues that humanity is facing in the 21st century is related to water quantity and water quality. This will be more critical in the future with issues such as urbanization, global warming, climate change and droughts. According to the United Nations, 844 million people around the world still lack even basic access to drinking water. Unsafe water kills more people than disasters and conflicts. Hence, there is a definite need to treat contaminated water and increase availability of drinkable water. Many bodies, such as the U.S. EPA, World Health Organization (WHO) and European Union (EU) have played a key role in developing regulations for many toxic species found in drinking water.
Treatment of contaminated water is realized from early days. In early civilizations, the commonly practiced measure to check purity was the taste of the water. The perception and understanding of water purity started changing after the discovery of the microscope in the 17th century. With the help of the microscope, people were able to see suspended particles, such as the tiny material particles to the micro-organisms in contaminated water. With time, as water became more contaminated, people took measurements to ensure they were drinking clean water. The ancient Greeks and Romans lined their drinking water pottery with silver believing silver would remove all contaminants and ensure clean water.
Contaminated water causes serious health risk and leads to many waterborne diseases, which can be prevented by taking appropriate measures even at the household level. Providing safe drinking water for all is a challenging task. Continued research efforts in this area over the past few decades resulted in many processes/technologies to counter the problem. Often the suitable technology to treat contaminated water is based on raw water characteristics (e.g. type of contamination and to what extent they are present in raw water), infrastructure, affordability/cost, as well as acceptability.
Aesthetic Water Quality
The causes of water contamination include a wide range and aesthetic parameters. Aesthetic water contamination is usually not considered harmful to health but is important in public satisfaction and perception of drinking water quality. Presence of aesthetic components can change the taste and look of the water and may cause it to be undrinkable by some standards. The aesthetic aspects of drinking water may include taste, color, odor, turbidity, pH, salinity, hardness, softness, temperature and more.
One of the most common and noticeable taste and odor concerns that we perceive from municipal water systems is the taste and odor of chlorine. Many public water systems treat water with chlorine to disinfect it. Although this is an important step in the treatment process, excess chlorine and chloramine tastes can often cause customer dissatisfaction and lead the customer to choose home treatment or a water purifier. In some cases, water with a high mineral concentration may have an unpleasant metallic taste. Metallic tastes can be caused by the presence of iron or manganese in the source water or may come from pipe corrosion. Presence of sulphides, such as hydrogen sulphide in water, produces an objectionable “rotten egg” smell. The presence of organic compounds in water also causes taste and odor related problems.
Another aesthetic quality of water is turbidity. Water turbidity is due to the presence of fine suspended mineral particles, suspended microscopic algae and animals. These fine particles cause the scattering and adsorption of light rays, which gives the water a cloudy appearance. Water turbidity beyond a certain limit can cause staining of sinks and fixtures and the discoloration of laundered fabrics. Apparent color is another aesthetic quality and is usually the result of the presence of colored particulates.
The most effective and economic way to remove most of these aesthetic problems is by use of a sediment and activated carbon filter. Sediment filter can remove most of the solid particulates from water reducing the turbidity. Owing to its high adsorption efficacy, activated carbon filters can remove a lot of soluble organic compounds, colored inorganic materials and taste and odor problems like those that occurs from excess amounts of chlorine. Other commonly noticed aesthetic issues, such as salinity, can be removed by reverse osmosis. Hardness can be removed by ion-exchange treatment and softness can be managed by pH adjustments.
Another class of contaminants affecting drinking water quality is microorganisms. Every year, 1.8 million people die from diarrheal diseases, 90% are children under five, mostly in developing countries. Improved water supply and sanitation reduces diarrhea morbidity to great extent. However, improving water quality is a challenging and expensive task in areas where the level of contamination is high.
There are numerous pathogenic bacteria and viruses that can contaminate water and can result in typhoid, dysentery, cholera and gastroenteritis. Some non-pathogenic bacteria, although not harmful, may cause taste and odor problems. There are a number of ways of removing microorganisms from water. In most countries, the municipal water system uses chlorine or chlorinated compounds (e.g. chlorine dioxide) as the disinfectant to effectively kill a large variety of microbial waterborne pathogens.
In the U.S., more than 98% of all water utilities that disinfect drinking water use chlorine as the disinfectant. Filtration can be employed as a treatment method against most microorganisms. Types of filtration methods are mostly depending on the pore size of the filter, amount of the contaminant, particle size of the contaminant and charge of the contaminant particle. Sometimes, a combination of filtration and chemical disinfection can be used to achieve improved reduction efficiency. Some recent techniques for water disinfection include the use of ultraviolet light (UV), use of ozone/hydrogen peroxide, activated carbon and more.
Heavy Metal Contamination
Heavy metal toxicity and the dangers associated with its presence in drinking water have been known of for a long time. Heavy metals are released into the environment through different routes such as industries, mining activities and agricultural activities. Among all heavy metals, arsenic contamination in natural water and wastewater is a worldwide problem. More than 150 million people worldwide are estimated to be exposed to drinking water with arsenic concentration above the WHO guideline of 10 ppb. Chronic arsenic ingestion from drinking water is known to have an adverse effect on human health. Humans and animals can be exposed to arsenic contamination through food, direct consumption of water containing metal or via inhalation. Existing arsenic removal technologies commonly in use are oxidation, precipitation, coagulation, membrane separation, adsorption and ion exchange. In most treatment plants, these technologies are used either at the same time or in sequence.
Another commonly found heavy metal in water is lead. Lead enters into drinking water when the pipes that contain lead corrode, especially where the water has high acidity or low mineral content. Effective treatment methods of lead-contaminated water are reverse osmosis (RO), adsorption and activated carbon filtration.
Inorganic mercury is another toxic heavy metal widely used in the chlor-alkali industry, in the electrochemical process of manufacturing chlorine, and in gold mining. Inorganic mercury is converted to organic compounds, such as methyl mercury, which is stable and accumulates in the food chain and water. Humans consume mercury through contaminated food or water.
Other heavy metals, including cadmium, chromium, copper and selenium, are also found at low concentrations in water. Most of them can be removed effectively through filtration, adsorption and RO.
Another class of contaminants commonly known as emerging contaminants (ECs) are substances not commonly regulated in the environment and known or suspected to have adverse ecological or human health effects. This group includes compounds such as pharmaceutical and personal care products (PPCPs), pesticides, hormones, and endocrine disrupting compounds (EDCs). The U.S. EPA has reported the presence of a total of 116 different ECs in public drinking water systems. These contaminants at a low concentration in drinking water are unlikely to pose any measurable risk to human health immediately but have the potential to cause long-term adverse effects to living organisms and the ecosystem. Consequently, the concern over the extensive use of various ECs and their contribution to environmental pollution is emerging as a serious concern.
Removal of ECs and their metabolites from water is a significant and challenging task. Most of the ECs are water soluble and non-biodegradable, which makes it difficult to remove them effectively by conventional water treatment methods. Advanced treatment technologies have been developed over the past few years for effective removal of ECs from water. These include adsorption, membrane separation, advance oxidation, photocatalysis and more. The selection of a specific treatment depends on several factors, such as cost of the process, operating conditions, selectivity, types of contaminants, their chemical properties and concentration in the effluent stream.