Encyclopedia of Environmental Science and Engineering, Volume I and II

1316 WATER TREATMENT

Flocculent settling

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Discrete settling

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Zone settling

Combined Settling Pattern

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FIGURE 5

Settling Screening

Biological

Growth

Filtration Phenomena

FIGURE 6

will overtake and coalesce with smaller particles to form

even larger particles, which in turn increase the overall set-

tling rate. Clearly, the greater the liquid depth, the greater

will be the opportunity for this type of contact. There is no

mathematical relationship which can be used to determine

the general effect of flocculation on sedimentation, and

empirical data is still required by studying individual labora-

tory cases. As a result, in flocculent settling the removal of

suspended matter depends not only on the clarification rate

but also on the depth.

This is one of the significant differences between non-

flocculent and flocculent settling.

Zone settling The previous two types of settling described

have one property in common, and that is that they both

deal with dilute suspensions. Zone settling, on the other

hand, deals with very concentrated suspensions where it is

assumed that one particle will in fact interfere with the set-

tling rate of another particle. It is clear that in the type of dis-

crete settling, where the particles are somewhat non-reactive

and usually quite dense such as sand, the difference between

dilute suspensions and concentrated or hindered suspensions

is less apparent, so the zone settling phenomenon is usually

considered for the flocculating materials. When the particles

reach the vicinity of the bottom of the settling tank, a more

concentrated suspension zone will be formed and the settling

particles will tend to act in concert and reduce the overall

rate of subsidence.

It can clearly be seen that in a water treatment plant,

particularly if coagulation is applied to remove turbidity, all

three types of settling will occur and any settling tank which

is designed must take into account all three types (Figure 5).

Filtration

As described earlier, it has been found even in the early

Egyptian days that passing water through sand resulted in a

reduction in suspended and colloidal matter, and resulted in a

further clarification of the water. Water which is on occasion

extremely turbid should, of course, first of all be treated by

some coagulation or settling or combination of both. However,

water which is normally not too turbid may be directly applied

to filters or water which has previously been treated by sedi-

mentation and/or coagulation may also be applied to filters to

provide the final polishing and the production of clear, aes-

thetically acceptable water.

The filtration process actually consists of three phenom-

ena occurring simultaneously (Figure 6).

Settling takes place in the small settling basins which

are provided between the particles. Screening takes place

where particles which are larger than the interstices will

be retained simply physically because they cannot pass

through. And finally, a biological action takes place through

bacterial growth which may occur on the particles of the

filter which may occur on the particles of the filter which

grow at the expense of the soluble organic carbon passing

through in the water. This latter phenomenon is not a very

satisfactory way of removing organic carbon, because it

does tend to plug up the filter fairly rapidly and reduce its

effectiveness.

Filters have been developed through the ages through a

series of steps which are mainly related to their operating

characteristics or the material which is used as a filtering

medium.

Slow sand filter The slow sand filter is, as it suggests,

a process whereby water is allowed to pass very slowly

through the system at rates of 2.5 to 7.5 million gallons per

acre per day.

Although this type of filter has been used traditionally

and has been very effective in the past, it has certain operat-

ing disadvantages in that it cannot readily be cleaned. While

some of these filters are still in use in some parts of the

Orient, in Europe and North America, where labor tends to

be more costly, other types of filters have been developed.

When the difference in water level between the outlet and

the water over the filter becomes too great, the filter is taken

out of service and the top inch or two of sand is removed

from the bed and may or may not be replaced with fresh sand

before the filter is put back into operation (Figure 7).

Rapid sand filter A far more popular and common process

for the filtration of water is the rapid sand filter. Instead

of sitting on a sand bed of approximately three feet, as is

the case in the slow sand filter, the bed is twelve to thirty

inches thick and supported on a layer of gravel or other

coarse grain, heavy material six to eighteen inches thick.

Filtration rates on the rapid sand filter are of the order of

three to four gallons per square foot per minute. Occasionally

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