Lesson 12_ Overview of the Wastewater Treatment Process

7/29/2019 Lesson 12_ Overview of the Wastewater Treatment Process

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Lesson 12:

Overview of the Wastewater Treatment Process

Objective

In this lesson we will learn the following:

What the different sources of wastewater include.

What the different steps include in the treatment process.

Reading Assignment

Along with the online lesson, read Chapter 1: The Treatment Plant Operator, Chapter 2: Why Treat

Wastes? and Chapter 3: Wastewater Treatment Facilities, in your textbook Operation of

Wastewater Treatment Plants Volume I .

Lecture

Introduction

This lesson begins the second half of ENV 110. This half of the course is concerned with the

treatment of wastewater. Before beginning this lesson, you should have taken Exam 1 and visited

two water treatment plants to conclude the water treatment portion of the course.

Lessons 1 through 11 considered the water treatment process. Water flows from the source

through the treatment plant and to the consumer. But what happens to the water after it reaches the

consumer?

The second half of this course will concern itself with wastewater - the used water and solids from a

community as well as the storm water which runs off streets and other surfaces during storms.

You will remember that water is naturally cleaned and reused as part of the hydrologic cycle in the

outside world. In the human world, water is also cleaned and reused. Our wastewater is

channeled to a wastewater treatment plant where it is cleaned and released back into lakes and

rivers. This water reenters the hydrologic cycle and will eventually be pumped back up by another

water treatment plant to be purified and released to customers.


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Sources of Wastewater

You can classify wastewater as domestic, industrial, or storm, according to its origin. Domestic

sources include water used for normal activity in homes, businesses and institutions. Domestic

wastewater is readily treatable.

The character of industrical wastewater depends on the type of industry using the water. Some

industrial wastewaters can be treated the same as domestic wastes without difficulty. Others maycontain toxic substances or high percentages of organic materials or solids which make treatment

difficult. In such cases, the industrial plant may have to pretreat its wastewater to remove these

pollutants or reduce them to treatable levels before they are accepted into a publicly-owned

treatment facility.

Storm water often goes to a treatment plant, although it is usually low in pollutants. Great amounts

of storm water can interfere with treatment efficiency in two ways: Storm water may cause too

much dilution of the wastewater. At the same time, it may cause hydraulic overloading of the plant.

In most cases, wastewater systems now call for separate storm sewers.

In the Treatment Plant

In the treatment plant there are many steps involved in treating wastewater. Below is a quick

overview of the possible steps involved. We will learn more about each step as the course goes on.


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The general principle in wastewater treatment is to remove pollutants from the water by getting

them either to settle or to float, and then removing this material. Some pollutants are easily

removable. Others must be converted to a settleable form before they can be removed. Treatment

facilities are designed in stages. Each stage either removes particles from the wastewater or changes

dissolved and suspended material to a form that can be removed. A modern wastewater treatment

plant may include these stages:

influentprimary treatment

secondary treatment

tertiary treatment

disinfection and effluent discharge


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Influent

Influent is the raw material that has been collected and conveyed to the plant for treatment. It

includes all the water and debris that entered the collection system.

Primary Treatment

To prevent damage to pumps and clogging of pipes, raw wastewater passes through mechanically

raked bar screens to remove large debris, such as rags, plastics, sticks, and cans. Smaller inorganic

material, such as sand and gravel, is removed by a grit removal system. The ligher organic solids

remain suspended in the water and flow into large tanks, called primary clarifiers. Here, the

heavier organic solids settle by gravity. These settled solids, called primary sludge, are removed

along with floating scum and grease and pumped to anaerobic digesters for further treatment.

Secondary Treatment

The primary effluent is then transferred to the biological or secondary stage. Here, the wastewater

is mixed with a controlled population of bacteria and an ample supply of oxygen. The

microorganisms digest the fine suspended and soluble organic materials, thereby removing them

from the wastewater. The effluent is then transferred to secondary clarifiers, where the biological

solids or sludges are settled by gravity. As with the primary clarifier, these sludges are pumped toanaerobic digesters, and the clear secondary effluent may flow directly to the receiving environment

or to a disinfection facity prior to release. There are several variations of secondary treatment,

including:

activated sludge

trickling filtration

rotating biological contactors (RBC)

lagoons and ponds

Tertiary Treatment

Tertiary, or advanced, wastewater treatment is the term applied to additonal treatment that is

needed to remove suspended and dissolved substances remaining after conventional secondary


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treatment. This may be accomplished using a variety of physical, chemical, or biological treatment

processes to remove the targeted pollutants. Advanced treatment may be used to remove such

things as color, metals, organic chemicals, and nutrients such as phosphorus and nitrogen.

Disinfection

Before the final effluent is released into the receiving waters, it may be disinfected to reduce the

disease-causing microorganisms that remain in it. The most common processes use chlorine gas or

a chlorine-based disinfectant such as sodium hypochlorite. To avoid excess chlorine escaping to the

environment, the effluent may be dechlorinated prior to discharge. Other disinfection options include

untraviolet light and ozone.

Review

You can classify wastewater as domestic, industrial, or storm, according to its origin. In the

treatment plant there are many steps involved in treating wastewater. The general principle in

wastewater treatment is to remove pollutants from the water by getting them either to settle or to

float, and then removing this material. A modern wastewater treatment plant may include these

stages:

influentprimary treatment

secondary treatment

tertiary treatment

disinfection and effluent discharge

Assignment

Work the following crossword puzzle that comes from definitions in your textbook. You may either

print the puzzle out, complete it and mail or fax back to the instructor or you may send an email

with the correct answers numbered accordingly
http://water.me.vccs.edu/courses/env110/crosswords/lesson12.pdf


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Quiz

Answer the questions in the Lesson 12 quiz . When you have gotten all the answers correct, print

the page and either mail or fax it to the instructor.
http://water.me.vccs.edu/courses/env110/quiz12.htm

Documents

Lesson 12_ Overview of the Wastewater Treatment Process