Handbook of Water and Wastewater Treatment Plant Operations - Chapter 15 ppt

In regards to a potential potable water supply, the key words are quality and quantity.. In this chapter we discuss the surface water and groundwater hydrology and the mechanical compone

Part IV

Water and Water Treatment

Potable Water Source

Because of huge volume and flow conditions, the quality

of natural water cannot be modified significantly within the body of water Accordingly, humans must augment nature’s natural processes with physical, chemical, and biological treatment procedures Essentially, this quality control approach is directed to the water withdrawn, which is treated, from a source for a specific use.

15.1 INTRODUCTION

Before presenting a discussion of potential potable water

supplies available to us at the current time, it is important

that we define potable water:

Potable water is water fit for human consumption and domestic use, which is sanitary and normally free of min-erals, organic substances, and toxic agents in excess of reasonable amounts for domestic usage in the area served, and normally adequate in quantity for the minimum health requirements of the persons served.

In regards to a potential potable water supply, the key words are quality and quantity If we have a water supply

that is unfit for human consumption, we have a quality

problem If we do not have an adequate supply of quality

water, we have a quantity problem

In this chapter we discuss the surface water and groundwater hydrology and the mechanical components

associated with collection and conveyance of water from

its source to the public water supply system for treatment

We also discuss development of well supplies

To better comprehend the material presented in this chapter, we have provided the following list of key terms

and their definitions

15.1.1 K EY T ERMS AND D EFINITIONS

Surface water the water on the earth’s surface as

dis-tinguished from water underground (ground-water)

Groundwater subsurface water occupying a saturated

geological formation from which wells and springs are fed

Hydrology the applied science pertaining to

proper-ties, distribution, and behavior of water

Permeable a material or substance that water can pass

through

Overland flow the movement of water on and just

under the earth’s surface

Surface runoff the amount of rainfall that passes over the surface of the earth

Spring a surface feature where without the help of man, water issues from rock or soil onto the land or into a body of water, the place of issu-ance being relatively restricted in size

Precipitation the process by which atmospheric moisture is discharged onto the earth’s crust Precipitation takes the form of rain, snow, hail, and sleet

Water rights the rights, acquired under the law, to use the water accruing in surface or groundwater for a specified purpose in a given manner and usually within the limits of a given time period

Drainage basin an area from which surface runoff or groundwater recharge is carried into a single drainage system It is also called catchment area, watershed, and drainage area

Watershed a drainage basin from which surface water

is obtained

Recharge area an area from which precipitation flows into underground water sources

Raw water the untreated water to be used after treat-ment for drinking water

Caisson large pipe placed in a vertical position

Impermeable a material or substance water will not pass through

Contamination the introduction into water of toxic materials, bacteria, or other deleterious agents that make the water unfit for its intended use

Aquifer a porous, water-bearing geologic formation

Water table the average depth or elevation of the groundwater over a selected area The upper surface of the zone of saturation, except where that surface is formed by an impermeable body

Unconfined aquifer an aquifer that sits on an imper-vious layer, but is open on the top to local infiltration The recharge for an unconfined aquifer is local It is also called a water table aquifer

Confined aquifer an aquifer that is surrounded by for-mations of less permeable or impermeable material

Porosity the ratio of pore space to total volume That portion of a cubic foot of soil that is air space and could therefore contain moisture

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438 Handbook of Water and Wastewater Treatment Plant Operations

Static level the height to which the water will rise in

the well when the pump is not operating

Pumping level the level at which the water stands

when the pump is operating

Drawdown the distance or difference between the

static level and the pumping level When the

drawdown for any particular capacity well and

rate pump bowls is determined, the pumping

level is known for that capacity The pump

bowls are located below the pumping level so

that they will always be underwater When the

drawdown is fixed or remains steady, the well

is then furnishing the same amount of water as

is being pumped

Cone of depression as the water in a well is drawn

down, the water near the well drains or flows

into it The water will drain further back from

the top of the water table into the well as

draw-down increases

Radius of influence the distance from the well to the edge of the cone of depression; the radius of a circle around the well from which water flows into the well

Annular space the space between the casing and the wall of the hole

Specific yield the geologist’s method for determining the capacity of a given well and the production

of a given water-bearing formation, it is expressed

as gallons per minute per foot of drawdown

15.1.2 H YDROLOGIC C YCLE

To attain a better understanding how water is made avail-able, an understanding of the hydrologic cycle (water cycle) is necessary (see Figure 15.1) The hydrologic cycle

is a cycle without a beginning or an end It transports the earth’s water from one location to another As shown in Figure 15.1, it consists of precipitation, surface runoff, infiltration, percolation, and evapotranspiration

Lancaster, PA, 2001.)

OCEAN Estuary

River

Hills

Lake

Evaporation

Transpiraton

Hills

Hills

Foliage Precipitation

Clouds

Atmospheric Water Clouds

Evapotranspiration (from plants and inland waters)

Potable Water Source 439

In the hydrologic cycle, water from streams, lakes,

and oceans evaporated by the sun, together with

evapora-tion from the earth and transpiraevapora-tion from plants, furnishes

the atmosphere with moisture Masses of warm air laden

with moisture are either forced to cooler upper regions or

encounter cool air masses, where the masses condense and

form clouds This condensed moisture falls to earth in the

form of rain, snow, and sleet Another part of the

precip-itation runs off to streams and lakes, while a third part

enters the earth to supply vegetation and rises through the

plants to transpire from the leaves, and part seeps or

per-colates deeply into the ground to supply wells, springs,

and the baseflow (dry weather flow) of streams

The cycle constantly repeats itself — a cycle without

end

to return to the atmosphere varies tremendously

After a short summer shower, most of the rainfall

on land can evaporate into the atmosphere in

only a matter of minutes A drop of rain falling

on the ocean may take as long as 37,000 years

before it returns to atmosphere, and some water

has been in the ground or caught in glaciers for

millions of years

15.2 SOURCES OF WATER

Approximately 40 million mi3 of water cover or reside

within the earth The oceans contain about 97% of all

water on earth The other 3% is freshwater: (1) snow and

ice on the surface of earth contain about 2.25% of the

water, (2) usable groundwater is approximately 0.3%, and

(3) surface freshwater is less than 0.5%

In the U.S., for example, average rainfall is

approxi-mately 2.6 ft (a volume of 5900 km3) Of this amount,

approximately 71% evaporates (about 4200 cm3), and 29%

goes to stream flow (about 1700 km3)

Beneficial freshwater uses include manufacturing,

food production, domestic and public needs, recreation,

hydroelectric power production, and flood control Stream

flow withdrawn annually is about 7.5% (440 km3)

Irriga-tion and industry use almost half of this amount (3.4% or

200 km3/year) Municipalities use only about 0.6%

(35 km3/year) of this amount

Historically, in the U.S., water usage is increasing (as

might be expected) For example, in 1990, 40 billion gal

of freshwater were used In 1975, the total increased to

455 billion gal Projected use in 2002 was about 725 billion

gal

The primary sources of freshwater include the following:

1 Captured and stored rainfall in cisterns and water jars

2 Groundwater from springs, artesian wells, and drilled or dug wells

3 Surface water from lakes, rivers, and streams

4 Desalinized seawater or brackish groundwater

5 Reclaimed wastewater Current federal drinking water regulations actually define three distinct and separate sources of freshwater They are surface water, groundwater, and groundwater under the direct influence of surface water (GUDISW) This last classification is the result of the Surface Water Treatment Rule (SWTR) While the definition of what conditions constitute GUDISW is specific, it is not obvi-ous This classification is discussed in detail later

15.3 SURFACE WATER

Surface waters are not uniformly distributed over the Earth’s surface In the U.S., for example, only about 4%

of the landmass is covered by rivers, lakes, and streams The volumes of these freshwater sources depend on geo-graphic, landscape, and temporal variations, and on the impact of human activities

Surface water is that water that is open to the atmo-sphere and results from overland flow (i.e., runoff that has not yet reached a definite stream channel) In other words, surface water is the result of surface runoff

For the most part, surface (as used in the context of this text) refers to water flowing in streams and rivers It also refers to the following:

1 Water stored in natural or artificial lakes,

2 Man-made impoundments, such as lakes, made

by damming a stream or river

3 Springs that are affected by a change in level

or quantity

4 Shallow wells that are affected by precipitation

5 Wells drilled next to or in a stream or river

6 Rain catchments

7 Muskeg and tundra ponds

15.3.1 A DVANTAGES AND D ISADVANTAGES

The biggest advantage of using a surface water supply as

a water source is that these sources are readily located; finding surface water sources does not demand sophisti-cated training or equipment Many surface water sources have been used for decades and even centuries (e.g., in the U.S.), and considerable data are available on the quan-tity and quality of the existing water supply Surface water

is also generally softer (not mineral-laden), which makes its treatment much simpler

440 Handbook of Water and Wastewater Treatment Plant Operations

The most significant disadvantage of using surface water as a water source is pollution Surface waters are

easily contaminated (polluted) with microorganisms that

cause waterborne diseases and chemicals that enter the river

or stream from surface runoff and upstream discharges

Another problem with many surface water sources is turbidity, which fluctuates with the amount of

precipita-tion Increases in turbidity increase treatment cost and

operator time

Surface water temperatures can be a problem because they fluctuate with ambient temperature, making consistent

water quality production at a waterworks plant difficult

Drawing water from a surface water supply might also present problems; intake structures may clog or become

damaged from winter ice, or the source may be so shallow

that it completely freezes in the winter

Water rights cause problems as well; removing surface water from a stream, lake, or spring requires a legal right

The lingering, seemingly unanswerable question is who

owns the water?

Using surface water as a source means that the pur-veyor is obligated to meet the requirements of the SWTR

and Interim Enhanced Surface Water Treatment Rule

(IESWTR) (Note: This rule only applies to large public

water systems [PWSs] that serve more than 10,000 people

It tightened controls on disinfection by-products (DBPs)

and turbidity and regulates Cryptosporidium.)

15.3.2 S URFACE W ATER H YDROLOGY

To properly manage and operate water systems, a basic

understanding of the movement of water and the things

that affect water quality and quantity are important In

other words, a basic understanding of hydrology is

essen-tial A discipline of applied science, hydrology includes

several components, including the physical configuration

of the watershed, the geology, soils, vegetation, nutrients,

energy, wildlife, and the water

The area from which surface water flows is called a drainage basin or catchment area With a surface water

source, this drainage basin is most often called in

nontech-nical terms a watershed (When dealing with groundwater,

we call this area a recharge area.)

quan-tity and quality of surface water is called the drainage basin or watershed

When you trace on a map the course of a major river from its meager beginnings on its seaward path, the fact

that its flow becomes larger and larger is apparent While

every tributary brings a sudden increase, between

tributar-ies, the river grows gradually from overland flow entering

it directly (see Figure 15.2)

Not only does the river grow its whole watershed or drainage basin, but basically the land it drains into grows

as well, in the sense that it embraces an ever-larger area The area of the watershed is commonly measured in square miles, sections, or acres When taking water from

a surface water source, knowing the size of the watershed

is desirable

15.3.3 R AW W ATER S TORAGE

Raw water (i.e., water that has not been treated) is stored for single or multiple uses, such as navigation, flood control, hydroelectric power, agriculture, water supply, pollution abatement, recreation, and flow augmentation The pri-mary reason for storing water is to meet peak demands and to store water to meet demands when the flow of the source is below the demand Raw water is stored in natural storage sites (such as lakes, muskeg, and tundra ponds) or

in man-made storage areas such as dams

The photos depicted in Figure 15.3A through Figure 15.3D show one man-made raw water source con-trol method for agricultural and other uses that is currently being used Figure 15.3A, shows Middle Two Medicine Lake that is snow and ice fed and connected by river to the Smaller Two Medicine Lake (not shown) Between the two lakes are many breathtaking waterfalls Figure 15.3B shows Running Eagle Falls that plunges into the 157-mi Two Medicine River shown in Figure 15.3C Figure 15.3D shows the man-made spillway downriver from Running Eagle Falls The spillway controls flow at a set level for recreational, agricultural, grazing, and other uses

Figure 15.4A through Figure 15.4D show another example of how raw water supplies are stored Figure 15.4A and Figure 15.4B show views of Lake Whitehurst, Norfolk, VA Lake Whitehurst is the primary potable water raw water supply reservoir for Norfolk and other local customers Figure 15.4C and Figure 15.4D show the man-made spillway that controls the volume and level of water contained in the lake The spillway is impor-tant to the homeowners bordering the lake, because it acts

as a flood control mechanism, protecting properties from high water level damage Lake Whitehurst not only pro-vides a potable water source for Norfolk customers, but

it also provides a pristine recreation area within the city limits

As mentioned and shown in Figure 15.4C and Figure 15.4D, the spillway is man-made Man-made spill-ways and dams are either masonry or embankment dams

If embankment dams are used, they are typically con-structed of local materials with an impermeable clay core

Potable Water Source 441

PA, 2001.)

Wastewater Operator Certification, Technomic Publ., Lancaster, PA, 2001.)

Watershed divide

Spring

Groundwater seepage Reservoir

River

Mouth of Watershed

Surface runoff Rain storm

Creek Melting snow

442 Handbook of Water and Wastewater Treatment Plant Operations

15.3.4 S URFACE W ATER I NTAKES

Withdrawing water from a river, lake, or reservoir so that

it may be conveyed to the first unit process of the treatment

process requires an intake structure Intakes have no

stan-dard design and range from a simple-pump suction pipe

sticking out into the lake or stream to expensive structures

costing several thousands of dollars Typical intakes

include submerged intakes, floating intakes, infiltration

galleries, spring boxes, and roof catchments Their primary

functions are to supply the highest quality water from the

source and to protect piping and pumps from, or clogging

as a result of, wave action, ice formation, flooding, and

submerged debris A poorly conceived or constructed

intake can cause many problems Failure of the intake

could result in water system failure

On a small stream, the most common intake structures

used are small gravity dams placed across the stream or

a submerged intake In the gravity dam type, a gravity line

or pumps can remove water behind the dam In the

sub-merged intake type, water is collected in a diversion and

carried away by gravity or pumped from a caisson

Another common intake used on small and large

streams is an end-suction centrifugal pump or submersible

pump placed on a float The float is secured to the bank, and the water is pumped to a storage area

Often the intake structure placed in a stream is an infiltration gallery The most common infiltration galleries are built by placing well screens or perforated pipe into the streambed The pipe is covered with clean, graded gravel When water passes through the gravel, coarse fil-tration removes a portion of the turbidity and organic material The water collected by the perforated pipe then flows to a caisson placed next to the stream and is removed from the caisson by gravity or pumping

Intakes used in springs are normally implanted into the water-bearing strata They are then covered with clean, washed rock and sealed, usually with clay The outlet is piped into a spring box

In some locations, a primary source of water is rain-water Rainwater is collected from the roof of buildings with a device called a roof catchment

After determining that a water source provides a suit-able quality and quantity of raw water, choosing an intake location includes determining the following:

1 Best quality water location

2 Dangerous currents

3 Sandbar formation

4 Wave action

5 Ice storm factors

6 Flood factors

7 Navigation channel avoidance

8 Intake accessibility

9 Power availability

10 Floating or moving object damage factors

11 Distance from pumping station

12 Upstream uses that may affect water quality

15.3.5 S URFACE W ATER S CREENS

Generally, screening devices are installed to protect intake pumps, valves, and piping A coarse screen of vertical steel bars, with openings of 1 to 3 in placed in a near-vertical position excludes large objects It may be equipped with

a trash truck rack rake to remove accumulated debris A finer screen, one with 3/8-in opening, removes leaves, twigs, small fish, and other material passing through the bar rack Traveling screens consist of wire mesh trays that retain solids as the water passes through them Drive chain and sprockets raise the trays into a head enclosure, where the debris is removed by water sprays The screen travel pattern is intermittent and controlled by the amount of accumulated material

be employed, the most important consideration

is ensuring that they can be easily maintained

Park, Montana (From Spellman, F.R., The Handbook for

Waste-water Operator Certification, Technomic Publ., Lancaster, PA,

2001.)

Potable Water Source 443

15.3.6 S URFACE W ATER Q UALITY

Surface waters should be of adequate quality to support

aquatic life and be aesthetically pleasing, and waters used

as sources of supply should be treatable by conventional

processes to provide potable supplies that can meet the

drinking water standards Many lakes, reservoirs, and

riv-ers are maintained at a quality suitable for swimming,

water skiing, boating, and drinking water

Whether the surface water supply is taken from a river, stream, lake, spring, impoundment, reservoir, or dam,

sur-face water quality varies widely, especially in rivers,

streams, and small lakes These water bodies are not only

susceptible to waste discharge contamination, but also to

flash contamination (can occur almost immediately and not

necessarily over time) Lakes are subject to summer/winter stratification (turnover) and algal blooms Pollution sources range from runoff (agricultural, residential, and urban) to spills, municipal and industrial wastewater dis-charges, recreational users, as well as from natural occur-rences Surface water supplies are difficult to protect from contamination and must always be treated

15.4 GROUNDWATER

As mentioned, part of the precipitation that falls on land infiltrates the land surface, percolates downward through the soil under the force of gravity, and becomes ground-water Groundwater, like surface water, is extremely important to the hydrologic cycle and to our water supplies

Certification, Technomic Publ., Lancaster, PA, 2001.)

for Wastewater Operator Certification, Technomic Publ., Lancaster, PA, 2001.)

444 Handbook of Water and Wastewater Treatment Plant Operations

Almost half of the people in the U.S drink public water

from groundwater supplies Overall, more water exists as

groundwater than surface water in the U.S., including the

water in the Great Lakes Sometimes pumping it to the

surface is not economical, and in recent years, pollution

of groundwater supplies from improper disposal has

become a significant problem

We find groundwater in saturated layers called

aqui-fers under the earth’s surface Three types of aquiaqui-fers

exist: unconfined, confined, and springs

Aquifers are made up of a combination of solid

mate-rial such as rock and gravel and open spaces called pores

Regardless of the type of aquifer, the groundwater in the

aquifer is in a constant state of motion This motion is caused by gravity or by pumping

The actual amount of water in an aquifer depends upon the amount of space available between the various grains of material that make up the aquifer The amount

of space available is called porosity The ease of movement through an aquifer is dependent upon how well the pores are connected For example, clay can hold a lot of water and has high porosity, but the pores are not connected, so water moves through the clay with difficulty The ability

of an aquifer to allow water to infiltrate is called perme-ability

The aquifer that lies just under the earth’s surface is called the zone of saturation, an unconfined aquifer (see

Technomic Publ., Lancaster, PA, 2001.)

Technomic Publ., Lancaster, PA, 2001.)

Potable Water Source 445

Figure 15.5) The top of the zone of saturation is the water

table An unconfined aquifer is only contained on the

bottom and is dependent on local precipitation for

recharge This type of aquifer is often called a water table

aquifer

Unconfined aquifers are a primary source of shallow

well water (see Figure 15.5) These wells are shallow (and

not desirable as a public drinking water source) They are

subject to local contamination from hazardous and toxic

materials — fuel and oil and septic tanks and agricultural

runoff providing increased levels of nitrates and

micro-organisms These wells may be classified as groundwater

under direct influence of surface water (GUDISW) and

require treatment for control of microorganisms

A confined aquifer is sandwiched between two imper-meable layers that block the flow of water The water in

a confined aquifer is under hydrostatic pressure It does not have a free water table (see Figure 15.6)

Confined aquifers are called artesian aquifers Wells drilled into artesian aquifers are called artesian wells and commonly yield large quantities of high quality water An artesian well is any well where the water in the well casing would rise above the saturated strata Wells in confined aquifers are normally referred to as deep wells and are not generally affected by local hydrological events

A confined aquifer is recharged by rain or snow in the mountains where the aquifer lies close to the surface of the earth Because the recharge area is some distance from

Certification, Technomic Publ., Lancaster, PA, 2001.)

Operator Certification, Technomic Publ., Lancaster, PA, 2001.)