| Simple Methods for the Treatment of Drinking Water |
| Technologies |
The treatment processes introduced and outlined in this chapter were selected according to their suitability and appropriateness for application in less developed regions. They can be classified as:
- aeration;
- sedimentation;
- coagulation and flocculation;
- filtration;
- disinfection.
In the following the basic features of these methods will be presented to permit an understanding of the underlying physical, chemical and biological processes of water treatment. For a more detailed description, see the literature (appendix). Both abstract flow schemes and examples will be used to demonstrate and emphasize simple versions of the more commonly known treatment methods. Size and capacities of treatment components and of the whole plant are discussed along with aspects of application, performance and combination with other methods. The examples selected stress the possibility of using locally available materials instead of imported ones. The exclusive use of local materials may not always be possible. In particular, if certain treatments of the raw water, such as chlorination are believed necessary, effective local substitutes may not exist. These limitations require eliminating a number of treatment technologies from further consideration - including those which are either too expensive or too complicated, and whose high level of performance may far exceed what is needed. A few technologies which are borderline for our purposes will be discussed briefly (ozonation, uv-radiation).
Potential industrial and agricultural contaminants (chemicals such as oil, phosphates, sulphates, heavy metals, etc.) which end up in water resources in increasing amounts will also be addressed in this chapter. These contaminants must be removed if the water is to be made potable. It must be pointed out that it generally requires more advanced analytic methods to spot these substances in the water, and their removal may be altogether impossible as sophisticated technologies are required. It is therefore recommended that water which is contaminated as described above not be used for the purposes of these projects.
Since treatment generally presents the most demanding component of a water supply system, it must be examined whether alternative methods exist that yield a measure of quality improvement: several such methods include the proper mode of water abstraction***, and protection of the water source from contamination, as well as rehabilitation, upgrading and systematic monitoring of already existing works, and construction of efficient sanitation facilities.
| Simple Methods for the Treatment of Drinking Water |
| Technologies |
1. Range of Application
2. Aerators
The basic purpose of aeration is the reduction of the content of substances which cause unpleasant tastes and odors as well as discoloration. Aeration is frequently used for treatment of groundwater where it also has additional positive side effects (precipitation of iron and manganese). When treating surface water aeration is useful in adding oxygen to the raw water. Aeration always precedes some other treatment process. The combination of treatment components is determined by the desired result of the treatment.
2. Aerators
1. Range of Application
Aeration equipment is used to intensively mix air and water so as to facilitate the transfer of gases into or out of the water. The following effects can be obtained:
- Addition of oxygen; this may be necessary for surface water where the natural oxygen content was depleted due to the presence of large amounts of organic substances. Aeration contributes positively to subsequent biological treatment (e.g. slow sand filtration).
- Removal of dissolved iron and manganese; iron and manganese are oxidized and form nearly insoluble hydroxide sludges. They can be removed in a settling tank or by means of a coarse filter.
- Removal of excess carbon dioxide (CO2) to prevent corrosion of metal and concrete surfaces.
- Reduction of H2S, CH4 and other volatile compounds which produce objectionable taste and-odor.
- Temperature reduction.
1. Range of Application
2. Aerators
Aeration can be done in various ways. In this manual only those methods are discussed which are simple and facilitate gas transfer. Open aeration is possible by means of spraying the water or running it over surfaces multiple tray aerators or trickling aerators consisting of a series of vertical trays with wire mesh bottoms over which water is distributed and made to fall into a collection basin at the base. The water is dispersed in fine droplets of spray which efficiently take in oxygen from the atmosphere. If the trays are filled with coarse material, such as gravel, the efficiency can be increased.
A cascade aerator is another possible aeration device. A simple cascade consists of a lateral sequence of basins (masonry, concrete or timber) at various levels, the water spilling over from one basin to the next lower one. Total height of the cascade may be between 1 and 6 meters. The large water surface thus created allows simple and fast aeration. Baffles obstructing the flow of the water increase the effect.
If there are only small amounts of iron and manganese to be removed, or if the purpose of aeration is the addition of oxygen, it is sufficient to install a small weir just above the downstream clarifying tank so as to feed the water into that tank through a perforated pipe.
A third method of aeration which is the most efficient of all -and the most expensive and complicated - is based on the principle of diffusion. Water is forced into the air through fixed nozzles. Large contact surfaces for gas transfer are commonly set up above a settling tank or a filter.
Aeration of water usually requires. an interruption of the gravity flow of water through -a treatment plant. This means that downstream from the aeration, the water must be lifted once more. Exceptions may be possible in cases of gravity flow with significant differences in altitude (hills).
Small Aerators for Removal of Iron and Manganese
Figure 5 exhibits a simple device for domestic use. It consists of four vertically stacked round concrete pipes (diem. 45 cm) or metal drums (vol. 200 l) which are protected against corrosion. The two top segments are filled with gravel. The third from the top is filled with sand. The bottom consists of wire-mesh or grates. Aeration louvres are placed around the device. A low ph value lime (CaO) is added to the gravel in the upper segments. The device is mounted on a low pedestal made of masonry or concrete.
A handpump lifts the raw water, forcing it through nozzles on to the gravel. The water then trickles through layers of stones and trays. It is collected in the bottom segment and can-be drawn off by means of a faucet. Particles precipitated from the water due to aeration accumulate on the lower sand layer. The latter is to be exchanged once or twice a month.
An aerator of this size is capable of treating some 200 l/h, i.e., some 1400 l/h m².It can be easily modified in size in accordance with the actual needs.
| Simple Methods for the Treatment of Drinking Water |
| Technologies |
Sedimentation is a phenomenon which occurs in nature perpetually. It aids the natural purification of lakes and rivers. Use is made of this physical process in the treatment of water by passing it through settling basins or storage tanks at low and uniform velocities. This constitutes a simple means of reducing the contents of suspended matter and partially of bacteria.
Sedimentation is usually just one of several sequential treatment processes. It can be combined by preceeding it with coagulation and flocculation, and succeeding it with slow sand filtration. Following these procedures, disinfection is required for high bacteria contents.
1. Areas of Application
Turbidity
Under the influence of gravity, suspended matter in rain water settles out if it has a density greater than that of the water itself. The efficiency of a settling basin depends on the nature (shape, size, density) of the particles that are accountable for the turbidity; gravity, sand and silt, which pollute surface waters heavily and settle easily, especially during the rainy season. Colloidal matter which contributes much to turbidity is held in suspension mainly by electrostatic forces and because of its low density.
Colloidal particles, when brought in contact with coagulants, form flocculent material that can be settled or filtered out. Before designing a settling tank, laboratory experiments should be carried out to determine the contents of settleable and nonsettleable matter. Storage tank inlets should be screened to prevent contamination by gross suspended matter. Tanks should also be covered to protect them from birds and small animals.
Pathogenic Organisms
Simple sedimentation by means of passing water through a settling tank does not achieve a significant removal of pathogens. Two to four weeks storage, though, can reduce bacteria populations considerably (50- 90%) by means of biological processes. Storage in excess of one month can reduce the viral count. The degree of purification depends on the severity of pollution and on the presence of other pollutants. Storage induced contamination (mosquito breeding due to algal growth) must be avoided by covering tanks. Schistosoma larvae, infectious agents of Bilharzia, usually cannot survive more than two days of protected storage, provided suitable hosts (snails) are not present.
Color
Removal of color without the use of chemical procedures can only be achieved by very long storage times.
1. Areas of Application
2. Simple Settling Basins
Settling basins can be operated either continuously
or in batch mode. The choice of method may depend
on whether water is readily available and/or must be supplied
continuously. Simple methods are available for either mode. The most
important considerations are discussed here.
Batch Mode
Batch operation is mainly used if only small amounts
of water are to be treated and stor-ed. A settling
tank is filled with water, which is retained for between
two days and several months, depending on water availability, demand
and desired level of purification. When used, the water is
drawn off the top, down to a depth which just
covers the layer of deposits. This sludge layer at the
bottom of the tank is to be removed from time
to time. This can be done manually after the tank
has been emptied. A tank floor sloping towards the drain
greatly simplifies sludge extraction. Tanks can be constructed simply by
raising earth embankments, which have to be sealed to prevent
seepage. On the household level, clay vessels or other locally
available jars can be used. It is important to protect
receptacles from contamination: the water must not be taken out
with soiled jars. Instead, an outlet spout should be provided.
A cover not only protects. Layout and design of settling
and storage tanks are determined by the desired retention time
and the water demand of the consumers.
Continuous Mode
For larger amounts of water, it is
more economical to operate a settling tank continuously. The rain
water is slowly and uniformly passed through the tank either
horizontally or vertically. The through flow velocity must be kept
smaller than the settling velocity of the suspended matter. Horizontal
flow tanks generally achieve higher rates of removal for high
solids concentrations.
The most common geometric form of
sedimentation tanks are circular, square or rectangular. Triangular shapes are
possible. Water inlet and outlet are to be positioned such
that shortcircuiting is prevented, and the detention time of the
water is long enough to allow complete settling of particles.
Circular tanks have radial flow patterns. The water
can be introduced either in the center or around the
periphery. The clarified liquid is then drawn off in a
trough either at the rim or in the center. Rectangular
tanks have horizontal flow patterns. Inlet and outlet troughs are
provided at the head and tail ends of the tank.
Ideally, the tank may be divided into four
distinct zones, each of which acts characteristically different (Fig. 7).
Inlet Zone: In this zone, the entering
water is spread out uniformly and at low turbulence over
the entire cross-section of the tank (Fig. 8).
Settling zone: Portion of the tank where sedimentation occurs.
Outlet zone: Slow uniform draw-off of the
clarified water from the settling zone. The outward progression of
the flow shall not disturb the settling prozess.
Sludge zone: Collection of the deposits. If the sludge is to slide down by
itself, the floor of the tank should be sloped 45º. The draw off occurs at a
sludge drain.
The tanks may be built above ground with sealed masonry, concrete, or reinforced
concrete. Alternatively, earth basins may be used with vertical or inwardly
sloping watertight walls.
3. Design of a Rectangular Settling Tank With Horizontal Flow
4. Effect of Temperature and Salt Content of the Raw
Water and Wind Conditions
1. Areas of Application
3. Design of a Rectangular Settling Tank With Horizontal Flow (Fig. 7)
Settling tanks are designed such
that the reduced flow velocity of the water allows suspended
particles to settle out within the settling zone. Generally, the
smaller the particles, the smaller their settling velocity(s), i.e., the
lower the horizontal flow velocity of the water must be.
The necessary design parameters are determined as follows:
1. Decide on the hourly throughput Q (m³ /h).
2. In a laboratory test, determine
the settling velocity s, also called surface loading rate, of
the suspended matter in the raw water. The settling velocity
is obtained by measuring the time T (detention time) it
takes a particle to drop from the surface to the
bottom of the tank at depth H.
s = H/T
s and T are both dependent
on the nature of the particles to be removed; s
normally ranges between 0.1 and I m/h; for particles with
diameter ° = 0.01 mm, the settling velocity is approx.
s = 0.6 m/in. If flocculation preceded settling, the aggregated
particles settle at a velocity s between 1 and 3
m/in. The detention time T may range between 4 to 12 hours.
3. The volume V of the
tank is then determined by the hourly throughput Q, and
the detention time:
V = H . B
. L = Q . T
This gives
S = Q/B + L, where B L is the
surface area of the basin. The efficiency or flow capacity
of the basin is therefore determined by the ratio of
flow rate and surface area of the basin. Ideally, the
flow capacity is independent of the depth of the basin.
2. Simple Settling Basins
4. Effect of Temperature and Salt Content of the Raw
Water and Wind Conditions
4. The required geometry of the tank can now be calculated. The following ranges should not be exceeded:
|
Depth of the tank |
1:5 m £ H £ 2.5 m |
|
ratio H/L |
1:5 £ H/L £ 1:10 |
|
ratio B/H |
1:4 £ B/H £ 1:8 |
5. The horizontal flow velocity, v0, of the water
v0 = Q/B . H
ranges between 3 and 36 m/in. For suspensions with low densities, lower velocities should be chosen. When flocculation precedes sedimentation, higher velocities may be appropriate. Horizontal velocities should be kept low enough, however, to avoid scouring from the bottom of the basin.
6. The weir loading rate is given by the flow rate Q per unit width of the weir, Q/R (m³ /m x h). It should be chosen in the range between 3 and 10 m²/h. An increase in the width of the weir reduces the effluent velocity.
7. The volume of sludge produced in m³ per m² of tank area and per unit of time depends on the characteristics of the raw water and the design, i.e., efficiency of the tank. From this, in turn, the size and slope of the settling zone and the frequency of sludge removal can be determined.
1. Areas of Application
4. Effect of Temperature and Salt Content of the Raw
Water and Wind Conditions
Unfortunately, settling -tanks seldom perform in
accordance with the theory. A nonuniform density distribution across the
depth of the tank may disturb the settling process. Even
small temperature differences (1º C) or changes in the salt
content (1 g/l and hour) of the entering raw water
will create density currents which reduce the efficiency of the
plant. When designing an open basin, wind conditions should be
examined, since surface currents induced by wind blowing over the
basin affect the basin performance.
2. Simple Settling Basins
3. Design of a Rectangular Settling Tank With Horizontal Flow