| Simple Methods for the Treatment of Drinking Water |
| Technologies |
1 Rapid Filtration
1.1 Principle Mechanisms
1.2 Range of Application
1.3 Types of Rapid Filters
1.4 Conventional (Downflow) Filters
1.5 In Upflow Filter
1.6 Coarse Filters
1.7 Household Size Rapid Filter
2 Slow Sand Filtration
2.1 Mechanisms of Filtration
2.2 Range of Application
2.3 Design of a Slow Sand Filter
2.4 Construction
2.5 Operation and Maintenance
2.6 Modifications
Filtration is the deliberate passage of polluted water through a porous medium, thus utilizing the principle of natural cleansing of the soil. This widely used technique in water treatment is based on several simultaneously occurring phenomena:
- mechanical straining of undissolved suspended particles (screening effect);
-charge exchange, flocculation adsorption of colloidal matter (boundary layer processes);
- bacteriological-biological processes within the filter.
Filters may be divided into two principally different types:
- slow sand (or biological) filtration (v = 0.1 to 0.3 m/h),
- rapid filtration (v = 4 to 15 m/h).
In-between types also exist. Depending on the filtration rate, different mechanisms are operative within the filter. Resulting from this is a variety of possible applications of the various types of filters. Several of them are discussed in the subsequent sections.
Generally, a filter consists of the following components:
- filter. medium (inert medium: quartz sand; or chemically activated medium: burnt material),
- support bed (gravel) and under-drain system,
- influent and effluent pipes, -wash and drain lines, -control and monitoring appurtenances.
1.1 Principle Mechanisms
Rapid filtration is mainly based on the principle of mechanical straining of suspended matter due to the screening effect of the filter bed (sand, gravel, etc.). The particles in the water pass into the filter bed and lodge in the voids between grains of the medium. It is because of this phenomenon that rapid filters are sometimes called space filters. The cleaning of the rapid filter is facilitated by backwashing i.e., by reversing the flow direction; a backwash may be conducted simply with water or by use of a water-air mix (upward air scour). The impurities are thus dislodged and removed from the filter bed. Also operative to some degree in rapid filters are boundary layer and biological mechanisms - their extent largely depends on the filtration rate' filter medium, depth of the filter bed, and quality of the raw water.
The performance of a rapid filter regarding the removal of suspended matter is determined by the following filtration process variables and parameters:
- filtration rate (v),
- influent characteristics, i.e., particle size, distribution, etc.,
- filter medium characteristics which control the removal of the particles and their release upon backwashing, respectively.
Generally, it is true that the treatment effect can be improved by:
- reduced filtration rates,
- smaller granulation size of the filter medium,
- increasing depth of the filter bed, -increasing size of the floes,
- decreasing concentration of particles to be retained.
The range of application of rapid filtration and its performance when combined with other treatment processes is illustrated in Table 9.
There is a large variety of possibilities as re-yards setup and operation of rapid filters. They are generally divided into two categories. The majority of filters used for the treatment of drinking water are open, usually concrete built, filters. They operate with atmospheric pressure and at filtration rates between 4 and 8 m/h. Pressure filters are enclosed and usually made of metal. They operate under (higher than atmospheric) pressure at filtration rates between 8 and 1 5 m/in.
Both types can again be classified into subcategories, according to the flow of the water:
- vertical, downward filtration, downflow, -vertical, upward filtration, upflow,
- horizontal, axial or radial,
- biflow or dual flow.
Finally, the types of filter beds may be classified according to the structure of the filter media:
- single medium, fine grain (deff = 0.5-1.5 mm) or coarse grain (deff 610 mm),
- single medium, decreasing grain size in the direction of the flow,
- multiple media, bed stratification with decreasing grain size in the direction of the flow.
The range of common filter beds is between 1 and 2 m. The operating head is between 1.5 and 2.5 m. The required filter surface area can tee' determined according to the following relationship:
A: surface area (m²), v: filtration rate (m³/ m² . h) = (m/h); Q: throughput of water per hour (m³/h); a: operating hours per day.
Table 9: Treatment Effect of Rapid Filters and Possible Combinations with Other Unit Processes
|
Water Quality Parameters | Purification Effect |
|
Coarse particles of organic origin up to 250 mg/l | Removal at high filtration rates, using coarse filter material (backwashing is simple). |
| High turbidity due to gravel, sand or mud. |
Removal by rapid filtration, preceding sedimentation is recommended. |
|
Low turbidity up to max. 100 NTU |
Direct rapid filtration. |
|
Colloids |
Difficult to remove; |
|
- low concentration |
Addition of coagulant to inflowing water prior to sedimentation; flocs are retained by the filter; backwashing is difficult. |
|
- high concentration |
Preceding coagulation/flocculation and sedimentation in separate tank, rapid filtration |
|
Bacteria of fecal origin, eggs of parasites | Removal of some 50 % at low filtration rate and fine material, subsequent disinfection is required. |
|
Iron and manganese contents up to 25 mg/l | Precipitated compounds are removed upon aeration. |
1.4 Conventional (Downflow) Filters
Rapid filtration is a rather complex process. It is demanding and expensive in design and operation. This is due to the need for frequent filter washing which requires elaborate backwashing systems. Additional complexities associated with the generation of pressure arise for pressure filters. Monitoring, operation and maintenance of these filtration plants require well-trained personnel. Combined with coagulation, flocculation and sedimentation, rapid filtration is a very efficient treatment process for the removal of impurities. However, it should only be used in larger plants and at well equipped sites.
For smaller plants in rural areas, simple rapid filters -without backwashing capabilities - are recommended. A number of filter types operating at filtration rates lower than those for conventional filers are discussed hereinafter. Generally, they serve as pretreatment units to reduce the turbidity of the water. The removal of pathogens requires, in addition, either slow sand filtration and/or disinfection.
In upflow filters, the direction of flow of the raw water is upwards through the filter bed. Backwashing is done by abrupt reversal of the flow direction. The effect of the filter depends on the type of the filter medium, the filtration rate, and possible preceding aeration or addition of a coagulant. For coarse organic and inorganic substances, the filter may act as a simple screen. Or else it may retain precipitated iron compounds.~At low filtration rates and sufficient oxygen content of the raw water, biological activity can be observed.
The advantages of upflow filters as compared with gravity rapid filters are:
- can be constructed from locally available materials,
- quality requirements (uniformity and gradation) and volume of the filter medium are lower. Instead of sand, gravel, crushed bricks, coconut and other type fibers can be used,
- longer filter runs,
- better turbidity removal.
Upflow filters can be constructed at a variety of degrees of complexity. A rather simple type can be built from a 200 a-drum. It can be equipped with a raw water inlet pipe, a somewhat larger size drain at the bottom, and an outlet pipe for the clarified water near the top of the drum (see Fig. 13).
Filtration effect: Reduction of between 50 and 70% of organic and inorganic coarse and fine particles, slight reduction of bacteria.
Filter output: up to 230 a/h.
Filtration rate: 0.5 to 1.5 m/in.
Filter medium: Coarse sand, grain size between 3 and 4 mm diameter.
Filter bed depth: 0.3 m.
Support layer and underdrain: gravel covered by perforated metal tray. Cleaning:
Shut off of the inlet. Quick removal of drain stopper so that supernatant as well as water in the filter bed drain out together with retained particles.
Cost: for drain, send, pipes, tap and stopper.
As a rule, cleaning of the filter which takes no more than ten minutes should be done every day. This is a simple means of preventing the filter bed from clogging. The 200 l drum has a capacity to filter up to 230 l/h. As bacteria cannot be sufficiently removed, subsequent disinfection is indispensible in case of bacterial water contamination. This filter can also be combined with the slow sand filter introduced in section 2.6.
Hence; the performance and technical complexity of this simple upflow filter can be increased as much as one likes. It must be noted though that higher filtration rates result in higher buoyancy forces on the filter medium. The top layer of the sand may be spewn up. This can be avoided by covering the filter bed with a metal grate or by raising the depth of the filter bed. In the latter case' though, backwashing by means of simply draining the water in a reversed direction may become increasingly impossible. Conventional backwashing capability may have to be added.
Better results may be obtained by using smaller grains and stratified filter beds with decreasing grain size from bottom to top (e.g., 0.7 to 2 mm over a depth of I to 1.5 m).
Rapid filters preceding slow sand filters are frequently used to retain coarse particles and to sufficiently reduce turbidity. Coarse sand, gravel or plant fibres are used as a filter medium. It can be replaced upon cleaning.
Such prefiltration can be done either horizontally or vertically. The filtration rates for a coarse filter are lower than those for a conventional rapid filter.
Gravity rapid filter as coarse filter (Fig. 14)
Filtration effect:
Reduction of turbidity by between 50 and 80% (max. load 250 NTU)
Filtration rate: 0.5 to 1.0 m/h
Filterbox: same as slow sand filter
Operating head: 1.0 to 1.5 m.
Filter medium: coarse sand, gravel, shredded coconut fibres.
Two or more layers of different material possible (coarser material up top and finer material below).
Filter bed depth: 0.8 to 1.4 m.
Drainage system: same as slow sand filter.
Cleaning: replace medium completely when head loss exceeds certain value, i.e., when too big (approximately once every 3-4 months).
Horizontal flow coarse filter
This type of treatment process unit which has the water flowing horizontally through the filter medium exhibits a combination of filtration and sedimentation effects. The concentration of suspended particles in the raw water can be reduced significantly. The water thus attains a quality which is satisfactory for subsequent slow sand filtration. Moreover, after a certain time of maturation' a biological film forms on the surface of the stones.
Filtration effect: Reduction of turbidity by between 50 and 70% (max load 150 NTU), reduction of bacteria by approx. 80%.
Filtration rate: 0.5 to 1.5 m/h (max 2.0 m/h).
Filter box: rectangular, similar to settling tank (design: see 3.3.3).
Length: 4 to 10 m width, according to Q/L v = B, floor slope toward drain 1:100.
Filter medium: crushed stone and gravel, divided into zones of different grain size, sequentially graded in coarse-fine-coarse pattern (diameters between 4 and 30 mm).
Cleaning: Since clogging of the filter builds up rather gradually, cleaning may only be necessary after several years of operation. The filter may be cleaned by removing the medium, washing it and putting it back in place.
1.7 Household Size Rapid Filter
Household filters can be made from sand or gravel of different grain sizes, from ceramics, porcelain or other fine porosity materials. They basically operate on the principle of mechanical straining of the particles contained in the water The filter performance depends on the porosity of the filter medium. Through additives in the filter material, additional effects can be obtained (adsorption, disinfection).
Multiple layer filter
Using metal drums, plastic containers or clay vessels and filling them with several layers of sand, gravel or charcoal, simple household filters can be put together. They do not perform well at removing pathogens, though. After filtration, the water therefore needs to be disinfected.
Charcoal adsorbs organic substances which cause disagreeable color and taste.* This effect can only be sustained, however, if the charcoal is frequently renewed. If this is not possible, for whatever reason, or if the filter (empty or filled with water) is left unused for some time, the charcoal can become a breeding ground for bacteria. The result is that the filtered water exhibits a higher bacteria count than the raw water. Monitoring of the filter condition is rendered more difficult by the fact that there is no visual indication given for the point when the charcoal should be replaced. Charcoal cannot be regenerated. It is for these reasons that the use of filters with charcoal media is not recommended.
Ceramics filter
On the household level ceramics filters may be used for the purification of drinking water. If there are native potters, the filter can be manufactured locally. Otherwise they can be readily obtained from various commercial manufacturers.
The purifying agent is a filter element, also called candle, through which the water is passed. Suspended particles are thus mechanically retained, and, depending on the size of the pores, also pathogens. Ceramics filters should only be used if the water is not too turbid, as the pores clog rather quickly.
Ceramics filter elements can be made from various different material compositions (e g., diatomaceous earth, porcelain); they have pore sizes of between 0.3 and 50 ,u. If the pore size is smaller than or equal to 1.5,u all pathogens get removed with certainty. Post treatment of the water prior to consumption is rendered unnecessary.
Filters with larger pores only retain macroorganisms such as cysts and worm eggs. The filtered water must be boiled subsequently or otherwise disinfected
The impurities held back by the candle deposit on the candle's surface. At regular intervals, this coating can be brushed off under running water. After the cleaning, the candle should be boiled. Candles made from diatomaceous earth which contain silver, have the advantage that recontamination of purified water due to infestation of the filter material with bacteria laden washing water can be avoided (see also section 3.6.5).
Depending on their type, ceramics filters can be operated in the following ways:
- gravity filter (Fig. 17 and 19),
- siphon filter (Fig. 18),
- pump filter,
- pressure filter.
Filters operating at atmospheric pressure exhibit a very slow rate of percolation. This can be increased considerably by forcing the water through the medium. Ceramics filters must be handled with care. From time to time they must be checked for fissures so as to prevent the water from passing through the medium without being filtered.
Clay fillers treated with silver (Fig. 19)
This small household filter is manufactured in local potteries in Central America. It consists of a cylindrical clay vessel (diameter 28 cm, height 40 cm) equipped with a lid and an insert (holds 7.2 a). Tests carried out in Guatemala yielded excellent results as regards the removal of bacteria. Two alternative filter elements with different material composition are available.
The raw water, poured into the insert, trickles through its walls. The filtered water is collected in the lower part of the vessels, where it can be released at will.
Filter elements:
Alternative A:
Filtration rate: 2.14 a/day
Filter element (composition): 55 -65% loam, 30 -35% crushed feldspar, 5-10% sawdust
Treatment: colloid silver (3.2% Ag)
Longevity: 1 year
Cost of the filter (Guatemala 1980): U.S. $ 7.70
Alternative B:
Filtration rate: 1.97 a/day
Filter element (composition): 55-70% loam, 20-40% sand, 5-10% sawdust
Treatment: Colloid silver (3.2% Ag)
Longevity: 1 year
Cost of the filter (Guatemala 1980): U.S. $ 7.63
The filter insert can be treated as follows: Prepare a solution of 6.1 ml colloid silver in 200 ml of clean water and lay it on the filter element by means of a brush or a sponge. Finally, let the filter dry for 24 hours. The first two filter runs are to be discarded.
If feldspar is available, it is recommended to follow alternative A, to produce the filter, since its filtration rate is higher. In this case, silver is the only component which must be imported. Though it represents the most expensive part of the filter, it is needed to achieve disinfection.
Cartridge microfilter
Besides ceramics filters, other microfilters made from fine porosity materials are also available: synthetics, paper, felt-like material (pore size between 25 and 50 ,u). They are inserted into a bell-like filter device, which is mounted on the top of a water pipe. When the filter material becomes clogged, i.e., used up, it must be discarded and replaced new. Even though these filters are cheaper to purchase than ceramics filters, their use is more expensive, since the filter material cannot be regenerated.
Slow sand filtration is accomplished by passing raw water slowly - driven by gravity through a medium of fine sand. On the surface of the sand bed, a thin biological film develops after some time of ripening (different from the rapid filter). This film consists of active microorganisms and is called "Schmutzdecke", or filter skin. It is responsible for the bacteriological purification effect. The slow sand filter is therefore also called "surface filler'' or biological filter.
The principle purification processes taking place during slow sand filtration are:
Sedimentation:
The water body sitting on top of the filter bed acts as a settling reservoir. Settleable particles sink to the sand surface.
Mechanical straining:
The sand acts as a strainer. Particles too big to pass through the interstices between the sand grains are retained.
Adsorption:
The suspended particles and colloids that come in contact with the surface of the sand grains by following the passage of the water are retained by:
- adhesion to the biological layer (Schmutzdecke),
- physical mass attraction (Van der Waals force), and
- electrostatic and electrokinetic attractive forces (Coulomb forces).
On account of these forces, an agglomerate of opposite charged particles forms within the top layer of sand. This process needs some time of ripening to fully develop.
Biochemical processes in the biological layer:
- partial oxidation and breakdown of organic substances forming water, CO2 and inorganic salts,
- conversion of soluble iron and manganese compounds into insoluble hydroxides which attach themselves to the grain surfaces,
- killing of E. Coli and of pathogens.
Organic substances are deposited on the upper layer of sand, where they serve as a breeding ground and food for bacteria and other types of microorganisms (assimilation and dissimilation). These produce a slimy, sticky, gelatinous film which consists of active bacteria, their wastes and dead cells and partly assimilated organic materials. The dissimilation products are carried away by the water to greater depth. Similar processes occur there. The bacterial activity gradually decreases with depth. Different types of bacteria are normally found at various depths.
Algae can contribute to the breakdown of organic material and bacteria. They can improve the formation of the biological layer (filter skin). In uncovered filters, growth of algae is driven by photosynthesis. The presence of large amounts of algae in the supernatant reservoir of a filter generally impedes the functioning of the filter. Dead cell material may clog the filter. Increased consumption of oxygen due to the presence of dead cell material increases the possibility that anaerobic conditions will occur. There is always a diurnal variation in the oxygen content due to growth and decay of the algae mass. When algae growth is strong, the algae must be either removed regularly or the filter must be covered.
The conditions necessary for those biochemical processes are:
- sufficient ripening of the biological layers,
- uniform and slow flow of water through the filter, approx. 0.1 to 0.3 m/in,
- a depth of the filter bed of 1 m (0.5 m is needed solely for the biochemical process) of specific grain sizes,
- sufficient oxygen in the raw water (at least 3 mg/l) to induce biological activity.
Table 10: Range of Application of Slow Sand Filters According to Raw Water Quality
* At MPN-Contents Greater than 1000 E. Coli/100 ml, Raw Water Should Subsequently Be Disinfected
|
Water Quality Parameters |
Purification Effect |
| Bacteria | Pathogenic bacteria and E. Coli removed at 99 -99.9 %*; cysts, helminth-eggs and Schistosoma-larvae removed completely. |
|
Viruses |
Complete removal. |
|
Organic substances |
Complete removal. |
Color |
Partial removal. |
|
Turbidity |
Significant reduction; average turbidity of raw water should not be greater than 10 NTU. At higher turbidity, pretreatment necessary to prevent clogging of filter. |
|
Substances difficult to degrade biologically | e.g., detergents, phenoles, pesticides.Only minor degradation possible. |
Reference is made to Table 10.
It is worth noting that microbiological processes and chemical activity are very sensitive to changes in temperature. Both slow down under conditions of low temperature. A reduction in filtration rate can compensate for this effect. Under prolonged cold conditions, the filter should be covered to prevent heat loss, and subsequent disinfection should be provided.
2.3 Design of a Slow Sand Filter
1. Determine the daily demand for treated water, Q (m3/d, m3/h, peak flows),
2. Choice of the filtration rate v (m3/m² + h = m/h).
3. Determination of the number of daily operating hours, a. Aside from shutting down the filter completely (overnight), it is possible to operate it for a few hours a day (factor b), while the inlet valve is closed and the outlet valve is open (mode of decreasing filtration rate
4. Parameters a and b are related to the total filtration area as follows:
b = 0 for continuous operation,
b = 0.5 for 8 hours of daily uninterrupted operation,
b = 0.7 for 16 hours of daily uninterrupted operation.
The ratio of length to width should be in the range between 1 and 4.
5. Determine the number of filters n. There should be at best two filters, so as to have a reserve during down time of one (due to cleaning or ripening period).
The required area per filter is thus obtained by dividing the total area A by the number of (equal size) filters, A/n. The filtration rate for each filter for parallel operation is given by
6. The sizing of the subsequent storage capacity and of the distribution system is to be carried out in accordance with the daily water demand.
Filter box
The smaller the size of a filter unit, the simpler its construction. It must be noted, however, that both the risk of leakage (along edges) and initial capital cost per square meter decreases with the size of the unit. For filter lengths greater than 20 m, the design becomes more complicated because of the hydrostatic pressure. The walls must be watertight.
Table 11: Construction Characteristics of Various Tank Geometries
Form |
Tank Location |
Size (m) |
Slope | Walls Material |
Thickness (m) |
| Earth basin | ø 1-10 | Vertical | Concrete or Masonry | 0.2-0.3 | |
| Round |
ø 1-5 |
Vertical | Ferro-cement | 0.06-0.12 | |
| In/above ground | All sizes | Vertical | Reinforced concrete | 0.15-0.2 | |
|
Rectangular or square |
Earth basin |
L and B | Sloped |
Masonry |
0.1 |
2-20 |
Sealed earth |
0.05 |
|||
Concrete | 0.08 | ||||
|
Sand/cement mix |
0.08 | ||||
| Rectangular or square | In/above ground | AH sizes | Vertical | Reinforced concrete | 0.25 |
|
Earth basins |
Small sizes |
Vertical |
Masonry, concrete |
0.2-0.3 |
Table 11 shows design characteristics for different filter geometries. It must be noted that:
- Earth tanks with sloped side walls have the advantage of lower initial costs. No particular skills are required for the workers to do the excavation. At high groundwater levels, the walls must be absolutely watertight (mainly to prevent the flow of potentially contaminated groundwater). Access to pipework and appurtenances is relatively more difficult.
- Tanks with vertical walls should extend at least 0.3 m into the ground and another 0.5 m above ground. The deeper the tanks reach into the ground, the more favorable the pressure balance that acts on the walls. - Circular shapes are used for small units. Rectangular tanks lend themselves to forming batteries of filters. They are therefore well suited for expandable larger systems.
- It is important for the tank to have a rigid base. The edges between base slab and walls must be watertight. Artificial roughening of the inner wall faces greatly reduces the risk of raw water leaking past the sand.
- Provisions should be made for the tank to receive a cover, if necessary, in order to control algal growth and prevent pollutants from entering due to rain, wind, vermin, etc.
See Table 12regarding filter beds.
Table 12: Filter Medium -Structure and Materials
|
SUPERNATANT |
Depth: At least 1 m, up to 1.5 m. |
|
FILTERBED | |
| Medium: | Sand (washed), or other locally available material (e.g., rice husks), several layers possible. |
|
Depth: |
At least 0.7 m, better: 1.0-1.5 m. |
|
Grain Size: |
Effective size (E.S.): 0.15-0.35 mm. |
| Uniformity coefficient (UC): 2, max. 5. | |
|
Larger sizes reduce the effectiveness and increase the required depth of the filter bed. | |
|
SUPPORT LAYER | |
| Material: | Coarse sand or gravel: several layers with grain size increasing with depth. Prevents escape of filter medium into drainage system, and blocking. |
| Depth: |
0.1-0.4 m (in accordance with drainage system). |
|
DRAINAGE SYSTEM |
Collection of filtered water towards outlet, alternatively: |
| - layer of gravel or crushed rock; grain size 25-50 mm; depth, 0.15 m | |
| - system of bricks, concrete slabs or porous material. See Fig. 22: lateral drains and main drain sloped toward outlet. | |
- system of perforated pipes, water and pressure-proof materials: PVC, cast iron, asbestos cement, locally available porous material (Fig. 23). |
Inlet Zone
The inlet zone of the tank should be designed such that the entering raw water spreads out evenly over the filter bed. Turbulence must also be avoided in order not to stir up the biological layer. This can be achieved best by admitting the water just above the filter bed at a velocity of 0.1 m/in. To prevent scouring near the inlet, a concrete plate may be placed on top of the filter bed (see Fig. 24, b).
If no extra provisions are made, the inlet of the raw water can also serve as the drain for the supernatant for the purpose of cleaning. Since for each cleaning of the filter, the top layer is scooped off, the surface of the filter bed drops more each time. It is therefore more practical to have a vertically adjustable sill along the inlet trough to control inflow and head over the filter (see Fig. 24, a).
The width of the inlet should not be less than Q/20. Sufficient aeration of the entering water can be obtained by means of uniformly spraying or trickling of the water over cascades.
Outlet Zone
The outlet zone is generally arranged so that a weir controls the effluent. It is common that the crest of the weir is placed some 0.1 m above the level of the filter bed (Fig. 25). The purpose of the weir is, among other things, to prevent the filter from running dry. The filtration rate can be controlled by valve F. The effluent weir also serves the purpose of aerating the filtered water. In case of an enclosed weir chamber, adequate ventilation must be provided for air to enter and for gases to escape.
A major advantage of slow sand filters is that operation and maintenance of a well-designed and constructed filter is rather simple. Unskilled personnel can be easily trained. The references in the following sections pertain to Fig. 20.
Initial commissioning of a filter
1. First, with all outlet valves closed, the filter must be charged with filtered water, introduced from the bottom (G) to drive out the air from the voids of the filter bed. This is continued until the whole bed is covered sufficiently (0.1 m) to prevent its being scoured or disturbed by turbulence from the admission of raw water through A.
2. Backfilling valve G is closed, raw water is admitted through A, until the desired working level for the supernatant is reached.
3. Valve J is opened to release filtered water at a filtration rate of one-fourth of the design rate (controlled by efluent regulating valve F).
4. During the start-up period, while ripening of the biological layer proceeds and reaches its full effect, the filtration rate is gradually increased by way of valve F until the desired rate v is attained. The cleaner the raw water, the longer the ripening process will take.
5. From time to time, chemical and bacteriological analyses of raw water and effluent must be taken to monitor the ripening process of the filter.
6. When the filter is in full working condition (see from analyses -from a few days to several weeks) valve J may be closed and valve I opened to feed the clear well. Until then, the water is either run to waste or returned to the raw water.
Normal operation
1. Normal throughflow: The filtration rate is controlled jointly by valves E and F. Initially, F is all but closed. It is opened gradually as the filter head loss increases so as to maintain a constant rate of filtration. The increase in bed resistance is due to a gradual accumulation of retained impurities in the interstices of the filter bed.
2. Operation at decreasing throughfow: This mode of operation' which is well suited for overnights, reduces the required number of personnel and related costs. The raw water inlet is closed, and the outlet remains open. Consequently, the head of the supernatant drops and the filtration rate decreases. The efluent weir should be fixed at such a height as to prevent the supernatant from dropping below a certain minimum depth (e.g., 0.2 m) above the filter skin (Schmutzdecke). When this period is terminated, raw water should be admitted quickly.
3. Temporary shutdown: Close both inlet and outlet valves. (The necessary quick-closing valves must be provided.) It is preferable to continue filtration and divert the effluent to waste or other use since a shutdown of the filter causes a deterioration of the quality of the biological agents (filter skin, etc.).
Filter Cleaning
1.When the filtration rate starts to drop at fully opened regulating valve F, it is time to clean the filter bed.
2. A, I, F valves are closed, C opened to allow the supernatant to drain off. Alternatively, the foregoing mode of operation for decreasing throughflow could be chosen.
3. By opening valves F and particularly D (waste valve) the water within the bed is lowered still further until it is some 0.2 m below the surface.
4. The filter skin and the surface sand adhering to it (top 1.5 to 2 cm of filter) are stripped off quickly and carefully so as not to pollute or disturb the filter to a greater depth.
5. Refilling the filter box follows the pattern described for initial commissioning. Only a day or two will be necessary for reripening (water analysis).
Resanding
Since for each cleaning, the top layer of the filter is removed, the depth of the filter material drops until the minimum design level is reached. This is typically about 0.6 m above the supporting gravel. The filter must then be resanded. The sand is to be washed thoroughly to remove all impurities (especially organic coating). This can be rather difficult (use of washing machine). If readily available, new sand may be better used instead. Also, the reuse of the old sand replenished by new material has its economic merits [79].
The procedures and characteristics discussed in the preceding sections represent a complete scheme necessary to achieve the best possible purification effects. There is room, however, to modify this scheme sufficiently to scale it down to the household level. Examples are:
- substitution of sand by alternative filter material (see example Fig. 29),
- reduction of the depth of the supernatant reservoir,
- effluent discharge via rising pipe (Fig. 26) rather than by a weir. Mounted on the effluent pipe is a stop cock to regulate the filtration rate and to shut off the outflow during cleaning.
Further design alternatives, e.g., for the effluent collection and discharge system, were discussed in earlier sections. Some selected modified slow sand filters are introduced in the following paragraphs. Too drastic a simplification of the full scale scheme may reduce the filter efficiency. It may give rise to the danger of insufficient biological effectiveness, necessary conditions for which are slow inflow and uniform throughfow. A pure and clean appearance of filtered water is no assurance of sufficient bacteriological quality.
Horizontal sand filter
This type of filter (Fig. 27) is constructed by excavation of an earth basin which is subsequently filled with sand. A biological skin develops at the surface of the sand around the inlet point. The filtration rate of the water percolating through the sand body is controlled by the filter resistance and the head differential between inflow and outflow. The retention time in such filters is between 36 hours and 30 days.
Filtration rate: 0,2 to 0.4m/h
Filtration effect: reduction of bacteria count, turbidity, organic content
Filter basin: excavation, watertight lining (e.g., with plastic sheets); depth between 0.5 m and 1.0 m; length 5 m; bottom slope 1: l0 to 1:20
Cleaning: When the filter starts clogging, the point of inflow is simply switched. As soon as the water has drained from the clogged inflow trough, the top sand layer is scraped off. The point of inflow can then be switched back. This technique offers the possibility of uninterrupted operation.
Slow sand filter of household size
A household filter can be simply made from a used metal drum (Fig. 28). A thorough cleaning and disinfection (e.g., with NaOCl) is necessary prior to its use as a filter casing. A drum previously filled with oil or chemicals should not be used.
Filter casing: 200 a metal drum, 0.5 m diameter
Depth of supernatant: 0.1 to 0.3 m so as to facilitate steady flow conditions
Filter medium: sand
Filter bed depth: at least 0.6 m, better 0.75 m
Support layer and outlet: Collection of the filtered water in a gravel layer. Effluent discharge via riser pipe, which is partly perforated. The effluent pipe mounted with a stop cock rises just above the level of the filter bed so as to prevent the filter from running dry.
Filter output: 60 a/h (as compared to up to 230 a/h for the rapid version)
Operation: setting of the filtration rate through effluent stop cock
Cleaning: necessary whenever filtration rate below certain specified value (at fully open valve)
In case of high turbidity, pretreating the water is recommended, by means of an upflow rapid filter (section 3.5.1.5).
Two-stage coconut fiber/burnt rice husk filter (Fig. 29)
This type of filtration plant was developed and tested in Southeast Asia where it is widely used. Two filters are operated sequentially. The first one acts as a coarse filter while the second one operates similarly to a slow sand filter (see Fig. 14). The filtrate is free of color, disagreeable odor and taste. The turbidity is greatly reduced, surplus iron and manganese is removed. Since pathogen removal is not as high as using a slow sand filter, subsequent disinfection (e.g., chlorination in the storage tank) is recommended.
The circumstance that the plant is mostly made from locally available materials and residues keeps the initial capital cost and the operating cost low. For filter vessels, clay jars or containers made of concrete, metal or zinc-plated sheet metal can be used. Feasible operating capacities range between 1 and 15 m3/h, depending mainly on the size of the system.
Coarse filter (dispersible if raw water turbidity is low)
Filter medium: shredded fibers of coconut shells (washed)
Filtration effect: Reduction of turbidity by 60 to 70%. Removal of dissolved particles; due to certain superficial phenomenon coagulation-like effects are achieved by the medium. At high concentrations of colloidal particles (turbidity > 300 NTU) the addition of a coagulant is recommended.
Depth of filter bed: 0.6 m to 0.8 m; depth of supernatant water 1 m above filter bed
Cleaning: Replacement of entire medium, when the supernatant reaches the rim of the tank (every 3 to 4 months).
Slow filter
Filtration rate: 1.25 to 1.5 m/h
Filter medium: burnt rice husks (washed, deff between 0.3 and 0.5 mm; UC between 2.3 and 2.6
Filtration effect: Removal of residual turbidity up to 95%, reduction of coliform bacteria by 60 -90%, removal of iron and manganese up to 90%, removal of color, odor and objectionable taste through adsorptive effect of the activated carbon of the burnt medium
Depth of filter bed: 0.6 to 0.8 m; depth of supernatant 1 m
Supporting layer: 0.05 to 0.1 m of gravel
Drainage: perforated drain pipe Cleaning:
Necessary when supernatant reaches rim of tank (approx. every 3-4 months). After draining of the tank, a layer of 5-10 cm of the filter medium is removed from the top. A refill of the medium is called for when the depth of the filter bed has dropped to a minimum of 0.6 m.