Introduction to the Raw Water Treatment Process
Water is an essential resource that plays a crucial role in various industrial processes. However, before it can be utilized, it is imperative to undergo a comprehensive water treatment process. This process ensures that the water meets the required quality standards, making it safe for industrial use.
In this guide, we will explore the basic concepts of raw water treatment, understand the steps involved, and explore the importance of water treatment in industries.
What are the objectives of water treatment?
The objective of any raw water treatment process is changing the composition of water means removing impurities from the water as per requirements like drinking purposes or for any intended end use in industrial purposes.
In industries, water serves as an essential component for various operations such as:
Treatment systems for Raw water
Cooling tower treatment
Treatment systems Boiler feed water
Treatment systems for Wastewater
However, using untreated water directly from the source can lead to several issues.
Raw water often contains impurities, such as suspended solids, organic matter, and harmful microorganisms that can adversely affect the efficiency and durability of industrial equipment. Therefore, the implementation of the water treatment process is essential to ensure the quality and safety of the water used in industries.
The main sources of raw water available for industries are surface water and groundwater. Surface Water is found in reservoirs, lakes, and rivers. Groundwater lies below the earth’s surface, where it travels through and fills openings in the rocks. The rocks that store and transmit groundwater are called aquifers. Ground water must be pumped from the aquifer to the surface of the earth for use.
Understanding the Water Treatment Process Steps
The raw water treatment process consists of multiple steps, each serving a specific purpose in the removal of impurities. Although the exact process may vary depending on the water source and the desired quality, the following steps are commonly involved in the water treatment process.
Scheme – 1 (Treatment for Hard ground Water)
- Raw water
- Softening by lime soda
- Disinfection by Chlorination
This scheme is normally used for drinking water treatment from hard groundwater supplies.
Scheme – 2 (Treatment for Turbid Surface Water)
- Raw water
- Pre sedimentation
- Mixing, flocculation & Settling
- Disinfection by Chlorination
As mentioned above schemes are only representative. There are many other factors on which the treatment method is decided
Raw Water Sources & Their Peculiarities
|Source||Dissolved Impurities||Suspended Impurities||Organic Impurities|
|Deep Well Water||High||Low||Low|
|Shallow Well Water||Moderate||Low||Moderate|
|River||Low||High – Seasonal||Moderate|
Now we discuss each step of the basic raw water treatment system commonly employed in industrial environments.
Pre-treatment for Raw Water
Pre-treatment processes are the initial steps in the water treatment process. They aim to remove large particles and substances that may interfere with subsequent treatment steps. Pre-treatment often involves the use of physical processes such as screening and sedimentation. Screening helps remove larger debris and solid particles, while sedimentation allows the settling of heavier particles through gravity.
Aeration is commonly the first process when treating hard groundwater or water which may have a considerable amount of iron and manganese. The Aeration process aims to remove Taste & odor, removal of Carbon Di Oxide, Removal of Volatile Organics, and Oxidation of Iron & Manganese Di Oxide
Normally, the following types of aerators are used aeration process for water treatment.
Here gravity aerators and spray aerators are commonly used for raw water treatment & Diffusers, Mechanical aerators are used for the wastewater treatment process.
The efficiency of aeration can be determined using the following formula:
Efficiency (%) = [(DO₂ – DO₁) / (DO₂ – DO₃)] x 100
DO₂ is the initial dissolved oxygen concentration.
DO₁ is the dissolved oxygen concentration after aeration.
DO₃ is the dissolved oxygen concentration in saturated water at the same temperature.
Coagulation and Flocculation Process
Impurities in water in the form of solids are classified based on size. Coarse solids settle by gravity because of their size and are easily removed by the sedimentation process. Finer particles did not settle by gravity and passed along with water. For effective treatment, these particles are to be combined into large, settleable solids.
Coagulation and flocculation are essential processes that aid in the removal of suspended solids and colloidal particles from water. Coagulation is the process of disrupting colloidal particles to remove them. This process involves the addition of chemical coagulants. Mainly, the following three coagulants are used in raw water treatment.
1.Aluminum sulphate (Filter Alum). The optimal pH range for Aluminum sulphate is 5.5 to 7.5.
2.Ferrous Sulfate (Copperas) The optimal pH range for ferrous sulfate is 8.0 to 11.0.
3.Ferric Salt. The optimal pH range for Ferric sulfate is 8.0 to 11.0.
These coagulants are neutralizing the negative charges on the particles. This neutralization allows the particles to come together and form larger, weightier particles called flocs. Flocculation involves gentle mixing or stirring to facilitate the formation of larger flocs, making it easier for them to be removed during the sedimentation process.
Sedimentation and Filtration in Water Treatment
Sedimentation is one of the important step in the raw water treatment process that allows the flocs to settle at the bottom of a sedimentation tank. As the flocs settle, the clarified water is separated and collected for further treatment.
Sedimentation can be simple or aided by coagulants and flocculents. Plain sedimentation occurs by natural force alone, i.e., by gravity. Normal sedimentation is used as a primary process or per-treatment process to reduce heavy sediment loads and follow subsequent treatment processes such as coagulation or filtration. Sedimentation is also done after chemical treatment to remove settable solids that have been purified and made more settable by the addition of coagulants to remove organic, colour and turbidity.
The following factors influence the sedimentation process:
1. Impurities, particle size, shape, and weight
2. Viscosity and temperature of particles
3. Surface overflow, surface area, and velocity of flow in the sedimentation process
4. Retention period sedimentation process
5. The effective depth of basins and inlet and outlet arrangements
6. Flocculation characteristics of the suspended material.
7. Dissolved substance in water and environmental conditions (such as wind effects etc )
Generally, the design and construction of sedimentation are classified into four types of settings that are based on the concentration of the particles to interact. Each of these arises in both raw water and wastewater treatment.
Type 1 – Discrete, Type 2 – Flocculated, Type 3 – Hindered & Type 4 – Compressed
Filtration is then employed to remove any remaining suspended solids and smaller particles that may have escaped during sedimentation. This process typically involves passing the water through various layers of filter media, such as sand, gravel, and activated carbon, which effectively trap and remove impurities.
Coagulation and lime softening
We have already clear about the coagulation process. Lime softening is the process of removing hardness from water. It can be done in two ways lime softening and ion exchange.
Now discuss hardness removal by lime softening here.
Hardness is due to multivalent cations, mostly calcium and magnesium. Others that may be present but not in significant amounts are iron and manganese. Lime soda softening is done to reduce the hardness of water. If the hardness is due to the Carbonate of Calcium & Magnesium, then the addition of lime alone is generally sufficient. But if hardness due to sulfate and chlorides is also present, then Soda Ash is also required.
Hardness is of two types:
a) Carbonate or temporary hardness
b) Non-Carbonate Hardness or Permanent Hardness
Precipitation Softening also known as lime soda softening, is the process by which raw water hardness, alkalinity, silica, and other constituents are reduced by precipitation and followed by filtration. The process may be varied according to the composition of the raw water and the end-user requirements. The water is treated with lime or maybe a combination of soda ash and lime.
Filtration is a process to remove particulate matter and floc from the water. This process is treated by passing the water through a bed of porous material such as granular substances like sand, coal.
The classification of filters is done in many ways.
d)Dual media filter
Filtration efficiency is often measured using the formula:
Filtration Efficiency (%) = [(Initial Suspended Solids – Final Suspended Solids) / Initial Suspended Solids] x 100
Disinfection and sterilization are significant steps in the water treatment process, as they help eliminate harmful microorganisms and prevent the growth of bacteria and viruses.
The main aim of the disinfection process is to kill harmful bacteria. This can be done either physical or chemical. Various disinfection techniques are employed, including chlorination, UV (ultraviolet) rays process, and ozonation.
Chlorination involves the addition of chlorine or chlorine-based compounds to the water to kill or deactivate microorganisms. UV ray process utilizes UV light to disrupt the DNA of microorganisms, rendering them unable to reproduce. Ozonation involves the injection of ozone gas into the water, which acts as a powerful oxidant, effectively destroying microorganisms. The major methods mostly employed today are Chlorination & UV ray’s process.
The required dosage of chlorine can be determined using the following formula:
Chlorine Dosage (mg/L) = (Required Ct Value) / (Contact Time in minutes)
The term “Required Ct Value” refers to the product of the desired residual concentration of a disinfectant and the contact time between the disinfectant and the water being treated.
The Ct value is a measure of the effectiveness of a disinfection process.
“C” is the desired residual concentration of the disinfectant (usually measured in milligrams per liter, mg/L). “t” is the contact time between the disinfectant and the water (usually measured in minutes)
Raw water treatment for industrial environmental involves several fundamental steps, including aeration, lime soda softening, filtration, chlorination, sedimentation, coagulation, and lime softening. Each step plays an important role in removing impurities and ensuring the water meets the required quality standards.