Water | Wastewater Treatment Basics, Reasons

Wastewater or water should be treated if water is contaminated with pollutants. There are different methods and technology to treat wastewater. And also, there are defined parameters, characteriscs and standards in wastewater treatment and they should be considered in any water treatment process. In this tutorial, we will learn basics of wastewater treatment.

Why do we need to treat water?

Water is one of the most abended materials in the world. But about 97% of water from the total is saline water. From freshwater, around 69% in ice caps and glaciers, 30% contains as groundwater and less than 0.27% contain in rivers and lakes. Most rivers and lakes are polluted due to many natural and anthropogenic (people done things) reasons. Therefore, there is a very small amount of water is on the earth that we can use as it is.



Categories and diseases related to water

If we used polluted water, it may cause many health impacts. Diseases related to water can be categorized as below.

  • Waterborne diseases - Waterborne diseases are illnesses caused by drinking contaminated water. Pathogenic microorganisms, viruses and chemicals such as NH4+ caused these diseases. Typhoid fever, Cholera are common diseases. Drinking NH4+ contaminated water causes blue baby bone.
  • Water washed diseases - Water washed diseases are illnesses caused by washings and bathing using contaminated water.
  • Water-based diseases - Inflection with parasites. For an example, Schistosomiasis is a water-based disease and it is a parasitic infection.
  • Water-related diseases - Transmission by animals (insects) related to water. Dengue fever transmits through mosquitos.

More than 5 million people die every year due to these diseases. Therefore, the quality of water is very important. If we have not enough pure water in nature, we have to treat water before use.


Why do we need to treat wastewater?

Wastewater generation is increasing day by day. Discharging wastewater to the environment without causing any adverse effects is the biggest challenge in every industry. Most of the time wastewater is treated and discharged into this irrigation system.

Various environmental legislations and standards are available according to the type of industry and discharge method place of wastewater discharge.

Some standards for discharge of effluents are

  • Tolerance limit for the discharge of industrial waste into inland surface water
  • Tolerance limit for the discharge of industrial waste on land for irrigation purpose
  • Tolerance limit for industrial and domestic waste discharge into an inland marine coastal area
  • Tolerance limit for waste from rubber factories being discharge into inland water surfaces
  • Tolerance limit for waste from the textile industry being discharge into inland water surfaces
  • Tolerance limit for waste being discharged from tanning industries
  • Tolerance limit for discharge of effluent into public sewers with central treatment plants.

These tolerance limits are introduced to protect the environment. If you dispose of wastewater to the environment without following these standards you will be guilty and offense according to the law. Not only that it will cause to interrupt equilibrium in ecosystems. Therefore, we have to treat wastewater before disposed to the environment.


Impurities in water

To understand wastewater/water treatment, understanding impurities is very important because we used their properties to remove them from water.

Impurities in water can be divided into three categories as below.

  1. Physical impurities
  2. Chemical impurities
  3. Biological impurities

Physical impurities

  • Dissolved organics
  • Colloidal particles
  • High temperature

Chemical impurities

  • Dissolved organics
  • Colloidal particles
  • High temperature

Physical impurities

  • Salts (CaCO3, CaHCO3, MgCO3)
  • Cations (Fe2+, Mn2+ )
  • Anions (Cl-, F-, SO42-)
  • Heavy metals (Cd, Hg, Pd)

Biological impurities

  • Microorganisms
  • Pathogenic organisms


Unit operations in waste water treatment

Treatment of wastewater means

  1. partial reduction
  2. complete removal of excessive impurities presents in wastewater

For that, different methods and techniques are used. Selections of proper unit operation is one of the most important factors in design. Influent water quality, required effluent water quality, cost of treatment method and space availability for the treatment plant are the most important factors to select unit operations. The treatment unit operations can be divided into three categories as below.




Physical unit operations

  • Screening
  • Comminution
  • Sedimentation
  • Flotation
  • Granular medium filtration

Chemical unit operations

  • Chemical precipitation
  • Adsorption
  • Disinfection
  • Dichlorination
  • Other chemical application

Biological unit operations

  • Activate sludge process
  • Aerated lagoon
  • Trickling filters
  • Rotating biological contractors
  • Pond stabilization
  • Anaerobic digestion
  • Biological nutrient removal

Suitable treatment methods according to the impurities in wastewater

Unit operations use for the treatment are depend on impurities present in water. See table 1 to get a basic understanding of treatment methods that we can use according to impurities.

suitable treatment methods according to the impurities in wastewater

Table 1: Suitable treatment methods according to the impurities in wastewater


Treatment Sequence

There is a series to use these unit operations. According to that treatment steps are categorized as preliminary, primary, secondary, tertiary and advance. The wastewater treatment processes main steps are described in table 2.

Steps of wastewater treatment process

Table 2: Steps of wastewater treatment process



The typical process flow diagram shows in figure 1.

Typical flow diagram of wastewater treatment

Figure 1: Typical flow diagram of wastewater treatment



Questions





How do I know whether the wastewater treatment plant is being operated smoothly and properly?

You should do continuous testing such as COD, BOD, TSS etc. for water after treatment processes. If one or few parameters are deviated from given ranges, immediate attention should be given to identify where the problem is.








Lewis structures of Molecules

P2O5 lewis structure OH- lewis structure Ammonium ion (NH4+) lewis structure H2CO3 lewis structure