Water Treatment

Water treatment involves several processes that remove contaminants from water, ensuring it is safe for human consumption and suitable for environmental discharge.

The main stages are sedimentation, flocculation, filtration, chlorination, fluoridation, and pH adjustment.

Stages of Water Treatment

Screening

• Water is passed through a large wire mesh to remove large debris such as leaves, plastic bags, and other waste materials.

Flocculation

• Flocculating agents, like aluminium sulfate, are added to the water to cause small suspended particles to coagulate and form larger clumps (flocs), which can then be easily removed.
• This process helps clarify the water.

Sedimentation

• The water is allowed to stand in large tanks where gravity causes the flocs to settle at the bottom.
• This removes the bulk of suspended solids from the water.

Filtration

• Water is passed through layers of sand and gravel, which trap finer particles that were not removed during sedimentation.

Chlorination

• Chlorine or chlorine compounds are added to the water to kill microorganisms, such as bacteria and viruses.
• This is essential to prevent waterborne diseases.

Fluoridation

• Fluoride ions are added to the water in small quantities to help prevent tooth decay and strengthen enamel.
• This is a common practice in many public water systems.

pH Adjustment

• The pH of water is adjusted using calcium hydroxide to raise the pH or sulfuric acid to lower it.
• This ensures the water is neither too acidic nor too alkaline, which can cause corrosion of pipes or discomfort to consumers.

Sewage Treatment

Sewage treatment processes are designed to clean wastewater before it is released back into rivers or the environment.

The stages include primary, secondary, and tertiary treatment.

Primary Treatment

• Similar to water treatment, sewage is screened to remove large solids and debris.
• The water is then sent to settling tanks where heavier particles settle to the bottom, forming sludge.

Secondary Treatment

• In this stage, bacteria are used to decompose organic matter in sewage.
• This process is called the activated sludge process.
• Oxygen is continuously pumped into large tanks (aeration tanks) to support the growth of microorganisms, which break down organic waste.

Tertiary Treatment

• This stage removes nitrates and phosphates, which can cause eutrophication (over-enrichment of water with nutrients, leading to oxygen depletion).
• Phosphates are removed by precipitation, while nitrates are removed using biological and ion exchange methods.
• Cost of Tertiary Treatment: Tertiary treatment is the most expensive stage of sewage treatment because it requires advanced technology and chemicals to remove specific pollutants like nutrients.

Water Pollution and Eutrophication

Eutrophication

• Eutrophication occurs when excess nutrients (especially nitrates and phosphates) from sources like fertilisers enter water bodies.
• This causes overgrowth of algae, which depletes dissolved oxygen when they die and decompose, harming aquatic life.

Pollution by Heavy Metal Ions

• Water can be polluted by heavy metal ions such as lead ($Pb^{2+}$, mercury ($Hg^{2+}$), and cadmium ($Cd^{2+}$).
• These ions are toxic to both humans and the environment.
• Removal: Heavy metals can be removed from water by precipitation.

Polluting Potential of Batteries

• Batteries that contain heavy metals pose a significant pollution risk if not recycled properly.
• Lead and cadmium from discarded batteries can leach into water supplies, causing contamination.

EU Limits on Water Contaminants

The European Union (EU) has set limits for various chemical species in water to ensure safety. Examples include:

• Nitrates: Excess levels can lead to health problems such as "blue baby syndrome" and contribute to eutrophication.
• Phosphates: High concentrations accelerate algal blooms, leading to eutrophication.