Purification and Management of Domestic Water Supply

The Importance of Clean and Safe Domestic Water Supply

A clean and safe water supply is fundamentally critical to the survival and well-being of mankind. When the water supply becomes inadequate or contaminated, several significant problems arise that can devastate communities. One of the most immediate impacts is food shortages and famine; this occurs because crops require a consistent supply of clean water to grow, and without it, agricultural production fails. Additionally, poor sanitation resulting from a lack of clean water leads to the rapid spread of bacteria and disease, particularly when drinking water sources become infected.

Water for domestic use is typically harvested from several primary sources. These include surface water from rivers and reservoirs, as well as underground water sources, which are referred to as groundwater. A specific geological formation, such as a rock that stores water, is known as an aquifer.

Understanding Impurities in Untreated Water

Untreated water contains a variety of substances that must be removed before it is safe for human consumption. These impurities are categorized into two main types: insoluble and soluble.

Insoluble impurities are substances that do not dissolve in water. Examples provided include soil, small pieces of plants, and other types of organic matter. Soluble impurities, on the other hand, are substances that have dissolved into the water. These include dissolved calcium, various metallic compounds, and inorganic pollutants that may have entered the water system from the environment or industrial activity.

The Multi-Stage Water Treatment Process

The purification of water involves several distinct stages designed to remove specific types of contaminants in a logical order.

The first step in the purification process is known as screening. This stage involves the removal of large insoluble objects that could interfere with later treatment steps or damage equipment. Examples of items removed during screening include rocks, plastic bags, and tree branches. After screening, the water is transferred to a sedimentation tank. In this tank, the water is allowed to sit undisturbed so that heavy particles like soil and sand can drop to the bottom of the container, forming a layer of sediment.

Following sedimentation, the water undergoes filtration. This process is specifically used to remove large insoluble particles that did not settle during sedimentation. The water is passed through several layers of filters made of sand, gravel, and charcoal. These layers effectively trap larger particles. However, filtration is not sufficient to make water completely safe because bacteria and other microorganisms are too small to be trapped by these physical filters.

To address the biological contaminants, chlorination is employed. This final major step involves the careful addition of chlorine to the water supply. Chlorine acts as a disinfectant, killing bacteria and other unwanted microorganisms. Failure to treat water properly can lead to the outbreak of serious bacterial diseases, such as cholera and typhoid, which are spread through the consumption of contaminated water.

Beneficial Dissolved Substances in Natural Water

Water obtained from natural sources is rarely just pure $H_2O$; it contains many dissolved substances. Some of these are highly beneficial to life and ecosystems.

One of the most important dissolved substances is oxygen ($O_2$). Dissolved oxygen is essential to support both animal and plant life in aquatic environments. The specific level of oxygen present determines the number and variety of living organisms an environment can support. Oxygen enters the water through two primary methods: as a byproduct of photosynthesis performed by aquatic plants, and through the diffusion of oxygen from the atmosphere into the water surface. Conversely, oxygen is removed from the water through the process of respiration in both plants and animals. The exact amount of oxygen that can remain dissolved in water is dependent on two factors: the temperature of the water and its salinity (whether it is saltwater or freshwater).

Metallic compounds, often referred to as minerals, also dissolve into water as it passes over and through various rock formations. A variety of metals are needed by the human body in trace amounts to maintain health. These include:

• Group I metal ions: Sodium ($Na^+$) and potassium ($K^+$).
• Group II metal ions: Calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$).
• Transition metal ions: Iron ($Fe$), cobalt ($Co$), nickel ($Ni$), copper ($Cu$), zinc ($Zn$), and chromium ($Cr$).

Specific examples of their biological importance include calcium, which supports the health of teeth and bones, and iron, which is essential for the production of haemoglobin used in red blood cells.

Harmful Substances and Contaminants in Water

While some minerals are beneficial, natural water can also be contaminated with potentially harmful metallic compounds. Heavy metals such as lead ($Pb$) and mercury ($Hg$) are particularly dangerous. These metals can enter water systems from sources such as mining operations, metal smelting, improper waste disposal, corrosion of infrastructure, and industrial metal processing plants. The health impacts are severe: lead exposure can cause liver and kidney damage, while mercury has been linked to significant damage to the nervous system.

Sewage, or human wastewater, introduces a different range of contaminants. In modern infrastructure, sewage is carried by underground pipes called sewers to wastewater treatment plants. At these facilities, solids are either filtered out or biologically digested. The remaining treated liquid, known as effluent, is then returned to rivers or the sea. However, natural disasters like earthquakes or extreme weather events can cause leaks in the sewage system. If sewage leaks into drinking water supplies, it spreads harmful microbes that cause diseases such as diarrhoea, cholera, dysentery, typhoid, and polio.

The Impact of Fertilisers and Agricultural Run-off

To increase crop yields, farmers use NPK fertilisers, which provide three essential plant nutrients: nitrogen ($N$), phosphorous ($P$), and potassium ($K$). These fertilisers are made from water-soluble salts so that plant roots can easily absorb them. However, if heavy rain occurs shortly after the fertiliser is spread, the compounds do not enter the plants. Instead, they are washed over the soil surface and into nearby waterways in a process called run-off.

When these fertilisers enter streams and rivers, they trigger a biological chain reaction:

1. High levels of nitrates and phosphates cause the rapid growth of algae, known as algal blooms.
2. These blooms cover the surface of the water, blocking out sunlight.
3. Aquatic plants living below the surface are unable to perform photosynthesis without sunlight and subsequently die.
4. The decay of these plants leads to a sharp drop in dissolved oxygen levels.
5. The lack of oxygen causes the death of aquatic animals, such as fish.

Nitrates and phosphates are the most common pollutants resulting from run-off. Aside from fertilisers, phosphates can also enter water through domestic waste; for instance, pentasodium triphosphate ($Na_5P_3O_{10}$) is a common ingredient in biological washing powders.

Chemical Tests for Identifying Water

There are two primary chemical tests used to confirm the presence of water, both involving a color change in a chemical compound upon hydration.

First, Cobalt(II) chloride can be used, typically in the form of cobalt chloride paper. This substance turns from blue (anhydrous) to pink in the presence of water. The chemical equation for this reaction is:

$CoCl_2\,(s) + 6H_2O\,(l) \rightarrow CoCl_2.6H_2O\,(s)$

Second, Anhydrous copper(II) sulfate can be used. This substance is naturally white but turns blue when water is added. The chemical equation for this reaction is:

$CuSO_4\,(s) + 5H_2O\,(l) \rightarrow CuSO_4.5H_2O\,(s)$

Industrial and Domestic Uses of Water

Water is a versatile resource used extensively across both industrial and domestic sectors.

Water in Industry:

• Coolant: Used to reduce temperatures in industrial processes, such as in nuclear power plants.
• Agriculture: Used for watering crops to ensure food production.
• Solvent: Acts as a medium for many chemical production processes.
• Energy: Used in hydroelectric power stations to generate electricity.
• Raw Material: Serves as a primary reactant in chemical processes, such as the production of ethanol from ethene and steam.

Water in the Home:

• Hygiene and Consumption: Used for drinking, cooking, and washing.
• Sanitation: Essential for general household sanitation systems.
• Automotive: Used in car radiators to prevent engine overheating.
• Gardening: Used for the maintenance of gardens and household plants.