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Water quality refers to the characteristics of water—for example, clarity, mineral content, and contaminants—that help determine how good a water source is for use by people. Water quality would be simple to quantify if water were either drinkable or not (if it contained potentially harmful substances); however, the majority of Earth’s water does not fit neatly into either category. This is because many substances can affect water quality and concentrations of these substances can range from very low to very high.   

The substances that affect water quality are called contaminants or pollutants. These may include sediment, chemicals, and disease-causing microorganisms, which can enter water in many different ways. They can come from specific sources, such as sewer or factory outflow pipes that empty into a body of water. Some pollutants come from sources that are widely spread and thus cannot be tied to a specific point of origin. Typically, these pollutants are carried along in rainwater runoff from a farm field, a street, or a construction site. The chemicals, sewage, or toxic materials eventually either flow into a lake or river or seep into groundwater and are carried far away. This makes it difficult to trace the exact source of this pollution.

Naturally occurring substances also affect water quality. Even raindrops are not pure water. As they fall, they pick up tiny particles of dust and other substances in the atmosphere. Gases in the atmosphere (e.g., carbon dioxide and sulfur dioxide) can dissolve in raindrops to form acid rain. When the rainwater comes in contact with soil and rock material, some of that material dissolves in the water. Substances that are dissolved in water are called solutes. The concentration of natural solutes depends mainly on two factors: the composition of the soil and rock material and how long the water is in contact with that material. Calcium makes water “hard” though not harmful. Hard water has a noticeable taste, and it can leave deposits inside pipes and tanks. The softest and purest water comes from areas that are far from where humans live: these areas have rock such as granite or quartz sandstone that does not dissolve easily in water.

The water for municipal water systems comes mainly from three sources: streams and rivers, natural lakes or artificial reservoirs, and groundwater, pumped from large wells. Lakes and reservoirs that are located in unpopulated areas far from cities and towns usually have the highest-quality water. This is also true for streams and small rivers in unpopulated areas. Large rivers generally have lower-quality water because of pollution from upstream areas. Groundwater is contained in underground materials called aquifers. The quality of groundwater varies from place to place, depending on the quality of the surface water that supplies the aquifers. 

After water is removed from a source, such as a lake or river, it typically enters a water treatment plant. Some sources of water are of such high quality that not much treatment is needed. Usually, an addition of a small amount of chlorine is sufficient to kill any harmful bacteria or other microorganisms. Other water sources, especially large rivers, have higher levels of pollution. Such sources require more to bring the water up to the needed level of quality. River water generally contains fine sediment particles in suspension. The water can be passed through filtration materials, such as sand, to remove bacteria. Another way of removing the fine sediment is to let the water sit in large basins while the sediment slowly settles to the bottom. Sometimes this settling process is speeded up by adding a chemical such as alum. This causes the fine sediment particles to clump together into larger particles. The heavy clumps sink to the bottom of the large basins faster than the original fine particles.

Most of the water that is used in homes and businesses comes out polluted to some extent, from such uses as washing clothes or dishes, bathing, and flushing toilets. The water goes into either municipal sewers or home septic systems.

Home septic systems consist of a large underground tank that collects and cleans waste water from a household. Wastewater flows from the house into the tank, where the solids sink to the bottom. Anaerobic bacteria (those that do not need oxygen) gradually consume most of these wastes. The remaining liquid water flows from the tank into a group of porous underground pipes called a leach field. The buried pipes distribute the water enabling it to soak into the ground. This water still contains pollutants and harmful microorganisms. Some of these are removed as the water flows through soil and rock, but in many places they reach groundwater supplies and add to problems of water pollution.

Municipal sewage is treated in special wastewater treatment plants. When water reaches a treatment plant, it is cleaned in two different ways. First, it goes through a series of steps known as primary treatment. In primary treatment, the water is put in large tanks or ponds to let the solid material, called sludge, either float to the surface or settle to the bottom. The water is then usually chlorinated, and the sludge is treated and disposed of in various ways. Most wastewater undergoes secondary treatment as well. At this point, the water is sprinkled or trickled over a bed of sand or gravel. As the water filters downward, it is put into contact with oxygen and microorganisms, which work together to break down the organic matter in the water. Sometimes the water undergoes tertiary treatment, which involves a variety of processes to purify the water even further. After treatment, the water is finally released into a water source—a stream, lake, or the ocean. 

Some modern homes and apartment buildings might have two separate drainage systems. One is for toilets, which contain sewage. This water is treated at regular water treatment plants. The other drainage system is for sinks and showers. This water is known as “gray water” and can be recycled and used for irrigation of plants and lawns.