🚰Water

Drinking Water

Step 1 - Collection of Rainwater

All drinking water begins as rainwater. Rainwater either flows into rivers and streams or filters through the earth to form "groundwater".

This water is then pumped from sources such as rivers, groundwater aquifers, reservoirs, and in some unique places, directly from the sea. The treatment process may vary based on the source of the water.

There are limits on the amount of water that can be pumped out of rivers and underground sources to prevent harm to the environment.

Step 2 - Storage

Before processing, natural or artificial reservoirs are used to store water. Ponds and lagoons as well as water storage tanks may also be used to provide storage. This happens for two reasons:

1. To provide a store or reserve of water, so if it hasn’t rained for a while or if water can't be pumped as much from the rivers or groundwater sources, water is available to put through the treatment process. Some reservoirs within Thames Water have enough water to last parts of the region for 90 days.

2. Storing the water in reservoirs starts the natural clean-up process, as heavier particles settle to the bottom, meaning that they don’t have to be taken out.

Step 3 - Screening

This involves the removal of large elements of debris within the incoming flow of water. The screen catches items such as any branches, leaves or rubbish. This is crucial to protect downstream equipment and ensure the efficiency of subsequent treatment processes.

Screening may be carried out by water intake structures, large grates or screens located at the intake, trash racks or raking mechanisms.

Step 4 - Removing Particles

Coagulation

Coagulation involves the addition of chemicals to destabilise and aggregate suspended particles in water. By adding chemicals (such as Aluminium Sulphate or Ferric Sulpahte) larger particles are formed to make it easier to remove impurities at a later stage.

Typically coagulants are added at the inlet of the water treatment plant, within the flocculation basins. Coagulents are mixed aggresively within the water initially to ensure effectiveness.

Deep Dive:

Chemicals used for Coagulation carry a positive charge and when these are mixed with the particles in the water, such as clays and silts (which mainly carry negative charges), this causes them to repel each other and remain in suspension.

Microflocs are then produced as the particles congeal together, which are small, loosely bound clusters of particles. These then settle more rapidly in the water due to the increase in size and mass.

More info: https://www.stwater.co.uk/content/dam/stw/my-severn-trent/documents/Water-treatment-process-poster.pdf

Flocculation

After coagulation, flocculants (such as ploymers) are added to the water to assist in congealing the particles together. During flocculation, the water is typically gently mixed to allow the particles to come together to create larger and more visible masses, also known as flocs.

Sedementation Basins

This is where a large number of the flocs settle and lower to the bottom of the basin as sludge which is then removed and disposed of. Clean water at the top of the tank then flows over weirs to the next stage of processing.

Dissolved Air Flotation

These may be used instead of sedemntation basins. In Dissolved Air Flotation, air is pumped into the bottom of the basin causing bubbles to be formed which take the floc particles with them to the top of the basin. A film of floc (sludge) is then formed on the surface of the basin where a sweeper arm then pushes the floc into a collection basin where it is then diposed of.

In Dissolved Air Flotation basins, clean water is taken from the bottom as this is where the least flocs reside.

Filtration

At this stage, the water may look clear, however, there will still be bacteria and very small solids located within it. The filtration process is designed to remove these bacteria and solids to ensure the water meets required standards.

Sand Filters

This is a basin of sand (fine, medium and/or coarse) that filters the water. Water can either be filtered top to bottom or bottom to top, however, the most common is for water to flow from the bottom to the top. As the water flows through the sand filters, remaining particles are trapped prior to any final treatments.

Granulated Activated Carbon

Some treatment plants may also use activated carbon particles. which have 100s of tiny pores, to remove further particles from the water. Although this isn't a mandatory step in the processing, it does help to improve smell and taste of the water.

Disinfection

The final step within the process is to add a very small amount (less than one milligram per litre) of chlorine to the processed drinking water to kill any remaining organisms or bacteria and to keep the water safe until it reaches a consumer's tap. Suplhur dioxide is also added prior to distrubution to reduce chlorine levels to a safe amount.

Step 5 - pH Adjustment and Storage

The treated water is then tested to ensure that it is is not too acidic (as this may corrode metal pipes) or alkaline (as deposits may form inside pipes) and then stored in covered reservoirs before being pumped through pipes and pumping stations to consumers.

At all stages of the water treatment process tests are carried out on the water. More info: https://www.wessexwater.co.uk/your-water/how-we-test

The treatment process takes about three and a half hours from abstraction to distribution. Source: https://www.stwater.co.uk/content/dam/stw/my-severn-trent/documents/Water-treatment-process-poster.pdf

Waste Water

Step 1 - Screening/Pre-Treatment

After wastewater is pumped via sewers to a wastewater (or sewage) treatment plant, it is first filtered by bar screens. These are large grates that capture larger solid materials that reside in the wastewater, such as nappies, wet wipes and sanitary items. This waste is then sent to landfill.

During this phase, grit is also removed from the wastewater by slowing the flow rate by passing it through channels or chambers which then allow the grit to sink to the bottom. Organic solid matter (such as human waste) remains and must then be seperated.

Step 2 - Primary Treatment

In this stage, the wastewater is then put into large settlement tanks to remove suspended solids (human waste). This is achieved as the solids (or sludge) sinks to the bottom of the tank to be removed and treated while primary sludge and fats flot to the top to be scraped by large arms or scrapers then which push the sludge towards the centre, allowing it to be pumped away for further treatment.

Cleaner water then passed over a wall at the top of the tank to flow to the next stage of the treatment process.

The sludge removed in this process may then be treated and used to generate renewable electricity or for fertilising fields. See more: https://www.stwater.co.uk/content/dam/stw/my-severn-trent/documents/Sewage-treatment-poster.pdf

Step 3 - Secondary Treatment

At this stage, all visible bits of sludge should have been removed, however, smaller and sometimes invisble orgamisims must also be removed as well.

Aeration Basins (or lanes) are then used. Here, air is pumped into the water encouraging the useful bacteria to break down and consume the harmful bacteria within the water. The more that the helpful bacteria consume the harmful bacteria, the more the helpful bacteria multiply.

Step 4 - Final Treatment

A final settlement tank is then used to allow the useful bacteria from the previous stage to settle to the bottom of the tank to form more sludge. This is then recycled back into stage 4. The clean water then passes over a wall near the top of the tank where it then may be filtered through sand beds (in larger works) or reed beds (in smaller works) to capture any remaining particles.

Once the water is at the suitable stabdard for discharge, it can be pumped back into watercourses.