Chapter 4
Water Treatment
Many aquifers and isolated surface waters are high in water quality and may be pumped from the supply and transmission network directly to any number of end uses, including human consumption, irrigation, industrial processes, or fire control.
However, clean water sources are the exception in many parts of the world, particularly regions where the population is dense or where there is heavy agricultural use.
In these places, the water supply must receive varying degrees of treatment before distribution.
Impurities enter water as it moves through the atmosphere and between soil particles in the ground.
These background levels of impurities are often supplemented by human activities.
Chemicals from industrial discharges and pathogenic organisms of human origin may cause health problems.
Excessive silt and other solids may make water aesthetically unpleasant and unsightly.
Heavy metal pollution, including lead, zinc, and copper, may be caused by corrosion of the very pipes that carry water from its source to the consumer.
The method and degree of water treatment are important considerations for environmental engineers.
Generally speaking, the characteristics of raw water determine the treatment method.
Most public water systems are relied on for drinking water as well as for industrial consumption and fire fighting, so that human consumption, the highest use of the water, defines the degree of treatment.
A typical water treatment plant is diagrammed in the following Fig.
Diagram of a typical water treatment facility.
It is designed to remove odors, color, and turbidity as well as bacteria and other contaminants.
Raw water entering a treatment plant usually has significant turbidity caused by colloidal clay and silt particles.
These particles carry an electrostatic charge that keeps them in continual motion and prevents them from colliding and sticking together.
Chemicals like alum (aluminum sulfate) are added to the water both to neutralize the particles electrically and to aid in making them “sticky” so that they can coalesce and form large particles called flocs.
This process is called coagulation and flocculation.
COAGULATION AND FLOCCULATION
Silt particles suspended in water are difficult to remove because they are very small, often colloidal in size, and possess negative charges, and are thus prevented from coming together to form large particles that could more readily be settled out.
The removal of these particles by settling requires first that their charges be neutralized and second that the particles be encouraged to collide with each other.
The charge neutralization is called coagulation, and the building of larger flocs from smaller particles is called flocculation.
A fairly simple but not altogether satisfactory explanation of coagulation is available in the double-layer model.
Charges on a suspended particle, as explained by the double-layer theory.
The solid particle is negatively charged, and attracts positively charged ions from the surrounding fluid.
Some of these negative ions are so strongly attracted that they are virtually attached to the particle and travel with it, thereby forming a slippage plane.
Around this inner layer is an outer layer of ions consisting mostly of positive ions, but they are less strongly attracted, are loosely attached, and can slip off.
The charge on the particle as it moves through the fluid is the negative charge, diminished in part by the positive ions in the inner layer.
The latter is called the zeta potential.
If the net negative charge is considered a repulsive charge, since the neighboring particles are also so charged, the charge may be pictured as in the Fig.
Reduction of the net charge on a particle as a result of the addition of trivalent counterions. (A) Particle carries net negative charge and van der Waals positive charge; energy barrier prevents coagulation.
In addition to this repulsive charge, however, all particles carry an attractive electrostatic charge, van der Waals force, that is a function of the molecular structure of the particle.
The combination of these forces results in a net repulsive charge, an energy barrier, or “energy hill,” that prevents the particles from coming together.
The objective of coagulation is to reduce this energy barrier to 0 so that the particles no longer repel each other.
Adding trivalent cations to the water is one way to reduce the energy barrier. These ions are electrostatically attracted to the negatively charged particle and, because they are more positively charged, they displace the monovalent cations.
The net negative charge, and thus the net repulsive force, is thereby reduced, as shown in the Fig.
Reduction of the net charge on a particle as a result of the addition of trivalent counterions. Addition of trivalent cations reduces energy barrier, and coagulation is possible.
Under this condition, the particles do not repel each other and, on colliding, stick together. A stable colloidal suspension can be destabilized in this way, and the larger particles will not remain suspended.
Alum (aluminum sulfate) is the usual source of trivalent cations in water treatment.
Alum has an advantage in addition to its high positive charge: some fraction of the aluminum ions may form aluminum oxide and hydroxide by the reaction
These complexes are sticky and heavy and will greatly assist in the clarification of the water in the settling tank if the unstable colloidal particles can be made to come in contact with the flocs.
This process is enhanced through an operation known as flocculation.
A flocculator introduces velocity gradients into the water so that the particles in a fast-moving stream can catch up and collide with slow-moving particles.
Such velocity gradients are usually introduced by rotating paddles, as shown in the Fig.
Flocculator used in water treatment.
The power required for moving a paddle through the water is
where
P = power (N/s or ft-lb/s),
D = drag force on paddles (N),
v = velocity of paddle tip (m /s or ft /s), about 75% of the actual paddle speed.
The drag force on the paddle is given by
where
ρ = density of water, (kg/m3 or lb-s2/ft3),
A = paddle area (m2 or ft2), and
CD = drag coefficient. (1.8 for flat blades)
where
P = power (N/s or ft-lb/s),
A = paddle area (m2 or ft2),
p = fluid density (kg/m3 or lb-s2/ft3), and
CD = drag coefficient.
The velocity gradient produced as a result of a power input in a given volume of
water is:
where:
G = velocity gradient (in S-1),
µ = viscosity (in dyne-s/cm2 or lb-s/ft2), and
V = tank volume (in m3 or ft3).
Or
Generally accepted design standards require G to be between 30 and 60 s-l.
Time is also an important variable in flocculation, and the term GT is often used in design, where T is the hydraulic retention time in the flocculation basin.
GT values are typically between 104 and l05.
EXAMPL 1 . A water treatment plant is designed for 30 million gallons per day. The flocculator dimensions are length = 100 ft, width = 50 ft, depth = 16 ft. Revolving paddles attached to four horizontal shafts rotate at 1.7 rpm. Each shaft supports four paddles that are 6 in. wide and 48 ft. long. Paddles are centered 6 ft from the shaft. Assume CD = 1.9, and the mean velocity of water is 35% of the paddle velocity. Find the velocity differential between the paddles and the water. At 50oF, the density of water is 1.94 lb-s2/ft3 and the viscosity is 2.73 x 10-5 lb-s/ft2-. Calculate the value of G and the time of flocculation (hydraulic retention time).
The rotational velocity is
where r = radius in feet and n = rpm, so that
The velocity differential between paddles and fluid is assumed to be 65% of vt, so that
Total power input is
and the velocity gradient is
which is a little low. The time of flocculation is
This is within the accepted range.
EXAMPL 2
A water treatment plant is being designed to process 50000 m3/d of water. Jar test and pilot-plant analysis indicate that an alum dosage of 40 mg/L with flocculation at a Gt value of 4.0 x 104 produces optimal results at the expected water temperatures of 15oC. determine:
1. The monthly alum requirement.
2. The flocculation basin dimensions if three cross-flow horizontal paddles are to be used. The flocculator should be a maximum of 12 m wide and 5 m deep in order to connect appropriately with the settling basin.
3. The power requirement.
4. The paddle configuration.
Solution
1- Monthly alum requirements
40 mg/L = 0.04 kg/m3
And
2- Basin dimensions:
Assume an average G value of 30 s-1
Gt = 4.0 * 104
Volume of the tank is
V = Qt = 50000 m3/d * 22.22 min * 1d/1440 min = 771.5 m3
The tank will contain three cross flow paddles, so its length will be divided into three compartments. For equal distribution of velocity gradient, the end area of each compartment should be square, ie depth equals 1/3 length. Assuming maximum depth of 5m, length is:
L = 5 * 3 = 15 m
And width is
5 * 15 * w = 771.5
W = 10.3 m < 12 m OK.
The configuration of the tanks and paddles should be as follows:
3- Power requirements:
Assume G value tapered as follows:
First compartment, G = 40 s-1
Second compartment, G = 30 s-1
Third compartment, G = 20 s-1
Power requirement for compartments 1, 2 and 3:
4- Paddle configuration
a- Assume paddle design as shown below
Each paddle wheel has four boards 2.5 m long and w wide – three paddle wheels per compartment.
b- Calculate w from power input and paddle velocity
At 15o C
Assume v = 0.67 m/s * 0.75 = 0.5 m/s and CD = 1.8
A = length of board * w * number of boards
3 paddles at 4 boards per paddle = 12 boards
A = 12 * 2.5 * w = 30w
c- Calculate rotational speed of paddles
First compartment:
Second compartment:
Second compartment:
SETTLlNG
When the flocs have been formed they must be separated from the water.
This is invariably done in gravity settling tanks that allow the heavier-than-water particles to settle to the bottom.
Settling tanks are designed to approximate uniform flow and to minimize turbulence.
The two critical elements of a settling tank are the entrance and exit configurations.
The following Figure shows one type of entrance and exit configuration used for distributing the flow entering and leaving the water treatment settling tank.
Settling tank used in water treatment.
Alum sludge is not very biodegradable and will not decompose at the bottom of the tank.
After some time, usually several weeks, the accumulation of alum sludge at the bottom of the tank is such that it must be removed.
The sludge exits through a mud valve at the bottom and is wasted either into a sewer or to a sludge holding and drying pond.
In contrast to water treatment sludges, sludges collected in wastewater treatment plants can remain in the bottom of the settling tanks only a matter of hours before starting to produce odoriferous gases and floating some of the solids.
The water leaving a settling tank is essentially clear. Polishing is performed with a rapid sand filter.
FILTRATION
Soil particles help filter the ground water.
The actual process of separating impurities from carrying liquid by rapid sand filtration involves two processes: filtration and backwashing.
The following figure shows a cutaway of a slightly simplified version of a rapid sand filter.
Rapid sand filter.
Water from the settling basins enters the filter and seeps through the sand and gravel bed, through a false floor, and out into a clear well that stores the finished water.
Valves A and C are open during filtration.
The rapid sand filter eventually becomes clogged and must be cleaned.
Cleaning is performed hydraulically.
The operator first shuts off the flow of water to the filter, closing valves A and C, then opens valves D and B, which allow wash water (clean water stored in an elevated tank or pumped from the clear well) to enter below the filter bed.
This rush of water forces the sand and gravel bed to expand and jolts individual sand particles into motion, rubbing against their neighbors.
The light colloidal material trapped within the filter is released and escapes with the wash water. After a few minutes, the wash water is shut off and filtration is resumed.
The solid impurities in the water are removed by many processes, the most important of which are straining, sedimentation, interception, and diffusion.
Mechanisms of solids removal in a filter: (A) straining, (B) sedimentation, (C) interception, and (D) diffusion.
Straining, possibly the most important mechanism, takes place exclusively in the first few centimeters of the filter medium.
As the filtering process begins, straining removes only particles in the water large enough to get caught in the pores (A in the Fig.).
After a time, these trapped particles themselves begin to form a screen that has smaller openings than the original filter medium. Smaller particles suspended in the water are trapped by this mat and immediately begin acting as part of the screen. Thus, removal efficiency owing to screening tends to increase in some proportion to the time of the filtration phase.
In sedimentation, larger and heavier particles do not follow the fluid streamline around the sand grain, and settle on the grain (B in the Fig.).
Interception occurs with particles that do follow the streamline, but are too large and are caught because they brush up against the sand grain (C in the Fig.).
Finally, very small particles are experiencing Brownian motion and may collide with the sand grain by chance. This process is called diffusion (D in the Fig.).
The first three mechanisms are most effective for larger particles, while diffusion can occur only for colloidal particles.
A typical removal efficiency curve for different sized particles is shown in the following figure .