28 Wastewater Treatment

Dr. C. P. Singh Chandel

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Contents

  1. Introduction
  2. Sedimentation
  3. Coagulation and Flocculation
  4. Filtration
  5. Water Disinfection
  6. Wastewater Treatment
  7. Advanced Oxidation Processes(AOPs) for Wastewater Decontamination
  8. Properties of Hydroxyl Radicals
  9. Production of hydroxyl radicals
  10. Application of Advanced Oxidation Processes
  11. References

 

Introduction

The tremendous use of water in homes, factories, textile industry and a in numerous other works generates huge amount of wastewater. If it is discharge into water bodies without treatment, it will lead to sewer water pollution. It will pollute drinking water sources, rivers, oceans, ground water and even agricultural crops. It is therefore necessary that the efforts be made to treat the wastewater. There are several techniques, which are used in treatment of drinking and wastewater. These are discussed briefly below.

 

Sedimentation

Water obtained from different sources such as lake, river, sea, etc always has suspended particles, which are produced by erosion of rock and soil. Whereas the large particle due to high mass settled down quiet rapidly, the fine clay particle remains suspended for a long time. The suspended particles or sediments make the water turbid.

In the treatment of wastewater, the suspended particles must be may settled down. Sedimentation is based on gravity to remove suspended solids from water. This process is also called settling or clarification.

Settling of solids depends on following factors:

  1. The gravitational force (with and without externally added chemicals) of the particle
  2. Physical characteristics such as diameter, density, etc
  3. Medium temperature, viscosity, density, etc.

 

Coagulation and Flocculation

Coagulation & Flocculation is necessary part of drinking water/ wastewater treatment. The water obtained from natural sources as well as wastewater contained small particulates which are colloidal in nature. They remain suspended and do not settle down under gravity. These colloidal particles carry the same charge. The repulsion between particles prevents them from combining into larger particulates to settle down. To precipitate these particles, the coagulation is carried out.

Coagulation is commonly achieved by adding different types of chemicals (coagulants) to the wastewater to promote destabilization of the colloid dispersion and agglomeration of the resulting individual colloidal particles by neutralization of charge. Such compounds are called coagulants. In water treatment, the commonly used reagents as coagulants are ferrous sulfate (FeSO4.7H2O), aluminum sulfate (Al2 (SO4)3.18H2O) and ferric chloride (FeCl3).

 

Filtration

Filtration is the process of removing small particles from water by passing it through a porous medium. The small particles also include pathogens. The filtration techniques used are many. The most common type filter is used is granular media gravity filter.

In the pressure filter water is pumped under pressure. The filter media are artificial membranes, nets, sand filter, and high technological filter systems. The choice of filters depends on the required filtering speed and the cleanness requirement. The flow required for filtration can be achieved using gravity or pressure. In pressure filtration, one side of the filter medium is at higher pressure than that of the other so that the filter plane has a pressure drop. Some portion of this filter type must be enclosed in a container.

 

Water Disinfection

The drinking water must be free from harmful bacteria and viruses. It should also not contain fecal matter from animals and humans both. This is done by disinfection of water. The process requires an oxidant stronger than O2. The different disinfection agents used are as follows.

Chlorination

It is done by passing chlorine in water. The process is cheaper and rapid and also most common. The chlorine reacts with water to produce hypochlorous acid at moderate pH.

The disinfectant is HOCl.

 

Ozonation

Ozone, which is a strong oxidant is often used as a disinfectant. Ozone is produced at the site and bubbled through the water. It produces highly reactive radicals such as hydroxyl(OH) and hydroperoxy(HO2). These destroy the harmful germs.

 

Wastewater Treatment: Primary, Secondary and Tertiary

Primary Treatment

The purpose of primary treatment is to remove suspended and floating solids. This can be done by using both physical and chemical methods using some of the following steps.

Screening: The different types of screens, viz., mesh and bar are used. The screens remove floating and suspended coarse particles when water is passed to screen.

Sedimentation: In this process, the fine suspended particles which do not settle down under gravity due to their colloidal nature are coagulated by addition of alum etc. The coagulated particles are filtered out.

Removal of Slit and Grit: The grit, which comprises sand and other heavier particles, is removed by slow passage of water through grit chambers.

Removal of Greasy and Oily Matter: For this, water is kept in a skimming tank. When the compressed air is blown, oil and grease start floating on the water surface and are skimmed off.

Secondary Treatment

The secondary treatment of wastewater is mainly concerned with oxidation in order to purify wastewater.

Biofiltration: This method utilizes different type of filters such as sand filters, contact filters, or trickling filters to remove the remaining sediment from wastewater.

Aeration: In the wastewater after sedimentation process, activated sludge, which contains micro organisms and aerobic bacteria, is mixed. Now the air is blown for aeration. The organic matter is oxidized under these aerobic conditions. This treated water is sent to a separate tank, where sludge is allowed to settle down. The purified water is collected.

 

Tertiary Treatment

This is the third and last step in the wastewater purification. It involves the following important steps.

  1. Removal of phosphates and ammonia: For removal of phosphate, lime is added whereby the following reaction takes place and calcium phosphate settles down as precipitate.
  1. The last traces of suspended matter are removed by use of filter beds filled with gravels, coarse and fine sand.
  2. Disinfection: Harmful pathogenic bacteria are removed by using disinfectant such as chlorine, ozone and ultraviolet radiation.

 

Advanced Oxidation Processes (AOPs) for Wastewater Decontamination

Advanced Oxidation Processes(AOPs) and Advanced Oxidation Technologies(AOTs) are now used for wastewater decontamination . AOPs were first proposed by WH Glaze and coworkers for drinking-water treatment with ozone, hydrogen peroxide and ultraviolet radiation (Environmental Science & Technol. 1987;21:224–30)

Fundamentally, AOPs are based on the use of highly reactive radicals such as

  1. Hydroxyl radicals (OH·).
  2. Sulfate radicals (SO4 ·− ).

These reactive species are the strongest oxidants that can oxidize almost any compound present in the water. These reactions are rapid and the rate constants are very high.

 

Properties of Hydroxyl Radicals

  1. It is highly reactive due to presence of an unpaired electron.
  2. It has very high redox potential ~ 2.89 volt (pH = 0) –1.95 volt (pH = 14).
  3. . OH radical is very nonselective and rapidly reacts with numerous species at bimolecular rate constants, k, of the order of ≈108 – 1010 M−1 s −1 .
  4. These radicals have very short lifetime and are produced in situ.
  5. Hydroxyl radicals attack organic pollutants through four basic pathways:

i) Radical addition

ii) Hydrogen abstraction

iii) Electron transfer

iv) Radical combination

 

Basic Hydroxyl Reaction Mechanism

The most common reaction of hydroxyl radical is the abstraction of hydrogen atom. This initiates a radical chain oxidation:

Organic compounds produce carbon-centered radicals such as R or R–OH. These carbon-centered radicals react with O2 to form peroxyl radicals (ROO).

All these radicals further react and result in the formation of reactive species such as H2O2 and super oxide (O2− ), leading to chemical degradation and even mineralization of organic compounds.

 

Table-1 Bimolecular rate constants (M–1 s–1) of hydroxyl radicals

Compounds Rate Constants
Chlorinated alkenes 109–1011
Phenols 109–1010
N-containing organics 108–1010
Aromatics 108–1010
Ketones 109–1010
Alcohols 108–109

Organic nitrogen is oxidized to nitrate /N2 and organic sulfur is oxidized to the sulfate. Cyanide is oxidized to cyanate, which is then further oxidized to CO2 and NO3/N2).

 

Production of Hydroxyl Radicals

Advanced oxidation methods for generation OH radicals employ several techniques some of which are mentioned below.

  1. Non-photochemical

i) Ozonation at elevated pH (>8.5)

ii) Ozone + hydrogen peroxide (O3/H2O2)

iii) Ozone + catalyst (O3/CAT)

iv) Fenton system (H2O2/Fe2+)

 

2. Photochemical methods

i) O3/UV

ii) H2O2/UV

H2O2 + UV → 2·OH

iii) O3/H2O2/UV

iv) Photo-Fenton/Fenton-like system

v) Photocatalytic oxidation (UV/TiO2)

TiO2   +UV  → e− + h+

 

Irradiation of the photocatalytic surface leads to an excited electron (e−) and positive hole(h+). The hydroxyl radical is formed by the reaction.

Ti(IV) + H2O ⇌ Ti(IV)-H2O

Ti(IV)-H2O + h+ ⇌ Ti(IV)-·OH + H+

 

Precise, pre-programmed dosages, sequences and combinations of above reagents are used to obtain a maximum OH yield. In general, under appropriate conditions, AOPs are able to reduce the concentration of contaminants from several hundred ppm to less than 5 ppb. Obviously, AOPs bring out a significant reduction in COD and TOC of wastewater. AOPs are now called as water treatment processes of the 21st century.

 

Applications of Advanced Oxidation Processes

UV, UV/H2O2, or O3/H2O2 systems are being used worldwide.

3. An O3/H2O2 system has been used for atrazine removal from the water of the River Seine in Paris at a scale of 5000 m3/h, etc.

4. UV disinfection is found in drinking water treatment plants throughout Europe and USA. Many bacteria and viruses are inactivated and many organic compounds undergo photolysis in the presence of UV radiation.

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References:

  • Colin Baird(1998), Environmental Chemistry, W. H. Freeman and Company, New York
  • A. K. De(2014), Environmental Chemistry, New Age International Publishers, Delhi
  • Mark J. Hammer and Mark J. Hammer, Jr. (2015), Water and Wastewater Technology, PHI Learning Private Limited, Delhi
  • O G Palanna(2009), Engineering Chemistry, Tata McGraw Hill Education Private Limited, New Delhi
  • James E. Girard(2011), Principles of Environmental Chemistry, Second Edition, Jones and Bartlett India Pvt. Ltd, New Delhi
  • Gilbert M. Masters, Wendell P. Ela(2013), Introduction to Environmental Engineering and Science, PHI Learning Private Limited, Delhi
  • P. S. Sindhu(2002), Environmental Chemistry, New Age International Publishers, New Delhi
  • Stanley E. Manahan, Environmental Chemistry, Seventh Edition, Lewis Publishers, New York
  • https://en.wikipedia.org/wiki/Advanced_oxidation_process
  • Yang Deng, Renzun Zhao(2015), Advanced Oxidation Processes (AOPs) in Wastewater Treatment in Current Pollution Reports,1, 167–176