Preliminary Treatment
 

Source and Intake

Introduction

We will trace water from that source into the treatment plant and through the first steps of treatment

First the water must make its way from the source to the treatment plant.  The water treatment plant operator may need to deal with problems in the reservoir, such as algae and aquatic plants, and with the intake, or structure through which water passes from the source to the water treatment plant.

 

 

 

Algae and Aquatic Plants

Before the raw water even leaves the source, we often run into problems.  In lakes and reservoirs, the most prevalent problems are algae and aquatic plants. 

Algal blooms are primarily caused by excess nutrients in the reservoir.  The algae are prompted to multiply by the surplus of food.  The greatest problems caused by algal blooms are taste and odor and filter clogging, though a variety of other problems are associated with the episodes. 

The primary method used to control algae in the reservoir is the addition of copper sulfate.  Alternatively, powdered activated carbon and pond covers can be used to block out sunlight, without which the algae cannot survive.  Aquatic plants can also be encouraged to grow in the reservoir, competing with the algae for nutrients and resulting in smaller algal populations.

As you can see, aquatic plants can be beneficial in a reservoir.  However, they can also cause problems of their own, such as increasing sedimentation, blocking intakes, and creating breeding sites for mosquitoes.  Control of aquatic plants can be either physical (removing the plants by hand or by machine), biological (introducing insects, pathogens, and fish to keep the growth of the problem plants in check), or chemical (introducing herbicides.)

 

 

 

Intake

In addition to dealing with raw water quality at the source, we can affect the quality of water entering the treatment plant based on location of the intake.  The intake type and location will depend on the water source.  Here we will consider only surface water sources. 

In a lake or reservoir, multi-leveled intakes are best.  These intakes allow you to choose the level from which you will take the raw water.  Since lakes and reservoirs often have varying water quality at different levels, a well regulated multi-leveled intake can greatly enhance water quality.

In a river or stream, stratification of water typically does not occur, so a single-level intake structure is sufficient.  When choosing the location for an intake in a river, the important factor is to reduce sand and silt buildup around the intake.  If possible, the intake should be placed on the outside of a curve where the water velocity is high and thus prevents sediment buildup. 

 

 

 

Preliminary Treatment

Introduction 

Preliminary treatment, also known as pretreatment.  Preliminary treatment is any physical, chemical or mechanical process used on water before it undergoes the main treatment process.  The purpose of preliminary treatment processes is to remove any materials which will interfere with further treatment.  Pretreatment may include screening, presedimentation, chemical addition, flow measurement, and aeration.

 

 

 

Screens

Screening may occur once or a series of times as the water flows from intake to treatment plant.  The screens are used to remove rocks, sticks, leaves, and other debris.  Very small screens can even be used to screen out algae in the water.  All objects are removed by physical size separation - if they are small enough to pass through the holes in the screen, objects flow with the water into the treatment plant.  If the objects are too large to pass through the holes, then they are caught by the screen. 

Screens should be angled for easy cleaning.  In addition, some screens are constructed so that they are self-cleaning.  Screens on the outside of intakes are often cleaned by flushing water from the treatment plant backwards through the intake, knocking off any debris which has accumulated there. 

There are two primary types of screens - bar screens and wire-mesh screens.  The two types differ in function and in the size of holes in the screens. 

A bar screen, such as those shown below, is used to remove large debris from the water.  The spaces between the bars are typically two to four inches wide. 

Bar screen.

 

 

A wire-mesh screen, in contrast, is used to remove smaller debris.  As you can see in the picture below, the gaps are typically about half an inch wide.  Water must be flowing relatively slowly in order to pass through a wire-mesh screen - velocities should be no greater than 3.5 inches per second.

 

Wire-mesh screen

 

 

It should be noted that screening is often minimal or absent in groundwater systems.  In essence, the water is screened as it trickles down through the earth into the groundwater. 

 

 

Presedimentation

When raw water has a high turbidity level, the water is passed through a presedimentation basin to settle out sand, grit, and gravel.  The presedimentation stage is similar to the sedimentation stage which occurs after flocculation and coagulation.  Here in pretreatment, the purpose of sedimentation is to make the chemical treatment phase of the water treatment process more efficient by removing sediment from the raw water. 

While in the presedimentation basin, activated carbon may be added to the basin for taste, odor, and color problems.  Chemicals may also be added to control the growth of algae.

 

 

 

Aeration

Aeration removes carbon dioxide and hydrogen sulfide from the water.  It also oxidizes the iron and manganese.  

 

 

 

Flow Measurement

Flow measurement is an important part of the pretreatment process.  By measuring the rate at which water flows into the water treatment plant, the operator is able to adjust chemical feed rates, calculate detention times, and monitor the amount of water being treated.

Flow is often measured using a Parshall flume, such as the one shown below.  The Parshall flume is a device used to measure the flow in an open channel.

 

  Parshall flume


When the inflow is within a pipe, the flow is often measured using a pressure differential meter.  The pressure differential meter consists of a primary element, a transmitter, and a receiver.  The primary element, shown below, consists of a tube with a constriction in it. When water flows through the constriction, the velocity of the water increases, as a result, the pressure of the water decreases. 

 

Pressure differential meter

 

A transmitter measures the pressure of the water before and after the constriction in the pipe and calculates the difference in pressure between the two locations.  The transmitter in turn sends the reading to a receiver which converts the signal from pressure to flow to be read by the operator.   

 

 

 

Other Monitoring

As the raw water enters the treatment plant, it is also monitored for a variety of characteristics including pH, turbidity, total alkalinity, temperature, and coliform bacteria.  These tests determine the amounts of alkali, coagulants, and other chemicals which must be added to the water during the treatment process. 

The pH and total alkalinity of the water will influence the amount of alkali to be added and can also influence the flocculation conditions.  The level of turbidity will influence the amount of polymer (coagulant) added to the water.  Temperature is also measured since cold water does not floc as well as warm water and requires the addition of more polymer.  Based on the results of the pretests, feed rates are set for various chemicals to be added to the water.   

 

 

Review

Before beginning primary treatment, water must pass through a variety of stages.  At the reservoir, factors such as algae and aquatic plants must be dealt with.  Then the water enters a single- or multi-level intake to be passed from the source to the plant.

At the plant, preliminary treatment may include some or all of the following:

         screening - to remove rocks, sticks, leaves, etc.

         presedimentation - to remove sand, grit, and gravel

         aeration - to remove carbon dioxide and hydrogen sulfide; to oxidize iron and manganese

         monitoring - to measure flow, pH, turbidity, total alkalinity, temperature, and coliform bacteria