Access to freshwater is one of the largest drivers of creating a modern residential setup. But it is not possible for everyone to get a continuous supply of potable water. The availability of potable water differs from area to area. In many parts of the world, people don’t have accessibility to drinking water. The role of the local government or municipal body is important to tackle this issue. Local authorities may come up with the installation of municipal water treatment plants to ensure a sufficient supply of potable water in every household.
In this article, we will discuss in detail about drinking water treatment plants.
Drinking water treatment plant
Let’s see how exactly does one water treatment plant purifies the dirty river water or well water and turned into the water that is safe to drink.
It involves a lot of processes using chemicals, filters and removing all the toxins and hazards from the source water. A step by step description is given below:
Coagulation and flocculation
All drinking water usually comes from some sources like a fresh lake, river, well or a stream. These water sources contain some amount of sediments as well as organic materials. The sediments may include sand, bacteria dirt, wood, etc. This all must be removed before the water can be given for utility. To achieve this, chemicals called coagulants are added into the water. These chemicals include aluminum sulfate or ferric chloride. The positively charged metal salts of the coagulant react with negatively charged colloids, which stick together to form bigger flocks. These flocks can be easily removed through filtration. Coagulants are often added to the water, right at the inlet of the drinking water plant, where the water then goes into the mixing basins called the Flocculation basin.
In these basins, the solution of water and coagulants are slowly mixed together to form floc particles. After mixing for a certain amount of time determined by the water quality and flocculation basin, the water moves to the sedimentation basin.
In these tanks, the floc particle begins settling out at a faster rate, which is the intended goal of this process. By simply adding and mixing the chemicals into the water and inviting the water to sit, a large portion of sediments settle out to the bottom as sludge. The floc particles are automatically removed from the bottom of the basin, turning the water into clean water. Then it further goes for the next stage of filtration.
Dissolve air flotation tank
In some plants like a commercial RO plant, engineers actually use dissolve air flotation tanks instead of a sedimentation tank. In these cases, the air is pumped into the bottom of the tank to cascade off tiny bubbles. As these bubbles go up, they take floc particles up and make a film of floc particles on the surface of the water. This film will be swiped out and passed to the collection basin where it is taken to the handling facility. In this tank, the clean water is taken from the bottom with the least amount of floc.
After the flocculation followed by the sedimentation, the next step of treatment will start.
The water at this stage will start looking clear but still, there will be some bacteria and very small suspended solids. The filtration process seeks to remove the remainder of the suspended solids and the bacteria to further bring purity to the water.
Every Commercial RO treatment plant uses a sand filter for filtration. Sand filters add extra measures to ensure clean water at the end.
A sand filter can be set up essentially in two different ways, either the water flows from the bottom and exits from the top of the water flows from the top and exits from the bottom. The most common technique is to have water flow from the bottom and exit from the top. Such filtration is highly efficient. The water flow from sand filter need to have clarity of around less than 0.3 Nephelometric turbidity units.
Activated carbon basins
Many water treatment plants pass the water through the activated carbon basins. Activated carbon basins have hundreds of tiny pores which help to remove the stuff sediments and bacteria. This process is not wholly necessary but it helps to improve the taste and odour of the potable water.
At this stage, water is almost crystal clear but still, there are some residual bacteria remains in the water that takes us to the next level of water treatment.
There are usually three methods, which can be used individually or in combination with one another depending on the design
- chlorine or chloramine treatment
- Ozone treatment
- Ultraviolet treatment
- Chlorine treatment: In the U.S. most of the water treatment plants use chlorine or chlorine-based compounds for disinfection. The downside of this method is that this chemical can react with other organic material and produce disinfected byproduct which is harmful to human health.
- Ultra Violet treatment: In this method UV light is passed through the water which scrambles any bacteria DNA. This method does not kill bacteria present in water but makes it impossible for it to reproduce. The main downside of this problem is that any bacteria introduced into the water after this process can’t be treated.
- Ozone treatment: The next method is also called ozonation. This process involves using Ozone in the water. It kills bacteria into the water as well as improves the taste and smell of the water. This process also requires the addition of sodium bisulfate at the end to remove ozone. Sodium bisulfate does not cause any harmful effects to the population.
After the disinfection, the water becomes be ready to be pumped into the cities. The water flowing out of the water treatment plant or any other commercial RO plant is tested for various levels of chemicals, particles or other quality checks outlined by the authorities.