The sewage filters that are commonly employed are of the following types: 1. Intermittent Sand Filters 2. Contact Beds 3. Trickling Filters 4. Dunbar Filters 5. Magnetite Filters 6. Rapid Sand Filters
Type # 1. Intermittent Sand Filters:
The intermittent sand filter is the earliest type of sewage filter used in the biological treatment of sewage. It consists of a rectangular tank with a specially prepared bed of sand. The tank is constructed below the ground surface by excavating the earth and it is without any lining on sides and bottom.
The tank is about 1 to 1.25 m deep and 1000 to 4000 m2 in area with ratio of length to width about 3 to 4. The filtering media consists of sand with an effective size of 0.2 to 0.5 mm and uniformity coefficient of 2 to 5. The sand is filled in the tank and a sand bed of depth about 0.75 to 1 m is prepared.
Below the sand bed, a bed of gravel of thickness about 0.15 to 0.30 m is provided in order to facilitate the drainage of the effluent. To carry off the effluent, open jointed drain pipes are laid in the gravel bed. Usually 3 to 4 beds are provided adjacent to each other so that they can work in rotation. A dosing tank with a siphon is provided to serve all the beds. Fig. 14.1 shows the details of intermittent sand filter with three beds.
Operation:
Sewage effluent from primary settling tank is applied intermittently on each bed through the dosing tank provided with a siphon. The quantity of sewage applied each time is such that the depth of sewage flooding the entire filter bed is about 50 to 100 mm. During the period the sewage remains standing on the filter bed, it gradually percolates through the bed.
As the sewage percolates through the bed, the suspended organic matter present in the sewage gets trapped in the voids of the top portion of the sand bed. Thus the organic matter is first removed by the straining action of the filter.
After the entire quantity of applied sewage has percolated down, the filter bed is kept at rest for some time before the sewage is again applied. During the rest period the trapped organic matter is acted upon by the aerobic bacteria present in the filter layer. These aerobic bacteria flourish well in the presence of free oxygen available from the atmosphere during the rest period.
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The effluent from the intermittent sand filter is quite clear, well nitrified and stable. The effluent contains suspended solids less than 10 ppm and also the BOD of the effluent is less than 5 ppm. However, in order to maintain the efficiency of the filter, the topmost layer of depth about 25 mm should be raked at regular intervals to break up the materials caught in the top part of the filter. The sand of the filter should also be renewed from time to time.
Rate of Filtration:
The rate of filtration of an intermittent sand filter depends mainly on the following three factors:
(i) Depth and size of filtering material;
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(ii) Nature of the influent; and
(iii) Quality of effluent required.
Table 14.1 shows the typical rates of filtration for different sizes of filtering materials with different types of sewage.
As indicated in Table 14.1 finer sand will result in lower rate of filtration but it will result in better quality effluent. Coarser sand will permit higher rate of filtration but in this case the penetration of solids will be too deep. Generally top 15 cm layer is kept of finer sand than the rest which is kept of relatively coarser size.
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Advantages of Intermittent Sand Filters:
1. Quality of effluent obtained from the intermittent sand filters is very clean and stable and it generally does not require any further treatment before disposal except chlorination in some cases.
2. Oxidation of organic matter of sewage is brought about by aerobic bacteria as the sewage is applied intermittently, there are less chances for anaerobic conditions to develop, and hence there is no trouble of odour, flies, etc.
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3. The operation is very simple, requiring no mechanical equipment except for dosing and hence does not require constant skilled supervision.
4. Small head is required for applying the sewage on the surface of the filter.
5. There is no secondary sludge which is to be disposed of, except for the occasional sand scraping.
Disadvantages of Intermittent Sand Filters:
1. The rate of filtration, and hence that of loading is very small, per unit surface area of the filter. Hence they cannot be employed for medium and large size sewage treatment plants.
2. Since a sand filter of about one hectare area is required per 2500 population, they require large areas of land and larger quantity of sand on account of which their construction is costly.
Due to above indicated limitations, intermittent sand filters are nowadays not employed except for very small plants such as those for hotels, hospitals, or other social centres located in remote areas where land and sand are easily available.
Type # 2. Contact Beds:
Contact beds also called contact filters, are in general similar to intermittent sand filters. A contact bed consists of a watertight tank filled with filtering media. The tank is usually constructed below the ground surface by excavating the earth and it is provided with a lining of cement concrete or watertight cement plaster on masonry, on sides as well as on bottom.
The filtering media in this case is gravel, ballast or broken stones. The size of the particles of filtering media varies from 15 to 40 mm. The depth of the filter bed is kept between 1 and 1.8 m, the common value being 1.2 m.
The area of the filter bed is usually less than 2000 m2. Usually 3 or 4 beds are provided adjacent to each other so that they can work in rotation. A dosing tank with a siphon is provided to serve all the beds. Fig. 14.2 shows the details of contact bed sewage filter with three beds.
Sewage effluent from primary settling tank is applied on each bed through the dosing tank provided with a siphon. The sewage after passing over the coarse filtering media is collected at the bottom and conveyed through the under drainage system to the effluent pipe which may be taken to the secondary settling tank for settling out the oxidized organic matter.
The sewage is uniformly applied over the entire surface of the filter bed by means of distributing troughs having perforations/outlets at regular interval. The rate of loading is slow and may vary from 4000 to 6000 m3/hectare/day.
Operation:
The complete cycle of operation of a contact bed consists of the following four operations:
(i) Filling:
The outlet valve of the underdrain is closed and the tank is slowly filled with sewage effluent from the primary settling tank through the dosing tank. The depth of sewage applied may be 50 to 100 mm over the top of the bed. This filling may take about 1 to 2 hours.
(ii) Contact:
The dosing tank outlet is then closed, and the sewage admitted over the contact bed is allowed to stand for about 2 hours. During this period the fine suspended, colloidal and dissolved organic matter present in the sewage gets transferred to the filter media, and comes in contact with the bacterial film covering the filter media.
(iii) Emptying:
The outlet valve of the underdrain is then opened and the sewage present in the contact bed is withdrawn slowly without disturbing the organic film of the bed. This operation may take about 1 to 2 hours.
(iv) Oxidation:
The contact bed is then allowed to stand empty for about 4 to 6 hours. During this period of rest, atmospheric air enters the void spaces of the contact media, thus supplying oxygen to the aerobic bacteria, resulting in the oxidation of the organic matter present in the film.
This completes the cycle of operations, which may be repeated after every 8 to 12 hours. Thus each contact bed is given two to three loadings per day. The stabilized organic matter from the voids is finally washed down the contact material in the next emptying operation of the tank.
However, since the flow of sewage from the primary settling tank is continuous while the bed is operated intermittently, a number of units are constructed side by side, so that sewage can be applied in turn to different units.
The voids inside the filtering media go on reducing continuously due to accumulation of solids in them. Hence after every 4-5 years, the filtering media is taken out, washed, dried and refilled. Similarly the drain pipes are also washed and cleaned after every 4-5 years.
The effluent received from contact bed is usually non-putresible, but it is turbid and high in bacterial content. In general, 85 to 90% of suspended matter, 60 to 80% of organic matter and 50 to 75% of bacteria are removed by contact bed filters. Hence the effluent from contact beds is passed through secondary settling tank.
If effluent of better quality is required, the contact beds may be arranged in series as shown in Fig. 14.3, in which case the effluent from one contact bed is admitted to the next one for further treatment, and for obtaining effluent of better quality.
Advantages of Contact Beds:
(i) Contact beds can be operated without exposing the sewage effluent to view.
(ii) Contact beds can work under small heads.
(iii) There is no nuisance of filter flies.
(iv) The problem of undesirable odour is less as compared to that in the case of trickling filters.
Disadvantages of Contact Beds:
(i) Rate of loading is much less in comparison to trickling filters.
(ii) Large areas of land is required for their installation.
(iii) Operation of contact beds requires skilled supervision.
(iv) Cost of contact beds is more as compared to that of trickling filters.
(v) Contact beds require long rest periods.
(vi) There is relatively high incidence of clogging.
Because of the above indicated limitations contact beds are not commonly used except for small institutions. Moreover, the development of trickling filters and other methods of sewage treatment has nowadays made contact beds of historical importance only.
Type # 3. Trickling Filters:
Trickling filters are also known as percolating filters or sprinkling filters. The first trickling filter was adopted in England in 1893 and in the United States of America in 1908 and since then, they have been widely used for biological treatment of sewage. The concept of a trickling filter came up from the use of contact bed filters.
The modern trickling filter consists of a bed of highly permeable media of coarse, rough and hard material. Sewage is applied to the surface of the bed intermittently by sprinkling through rotary distributors and trickles downward through the bed to underdrains, where it is collected and discharged through an outlet channel, and conveyed to a conduit leading to a settling tank.
A gelatinous microbial or biological film or slime layer is formed around the particles of the filtering media. The colour of this film is blackish, greenish and yellowish. The film mainly consists of aerobic bacteria and besides bacteria it may consist of fungi, algae, lichens, protozoa, etc.
For the existence of this film, oxygen is supplied by the provision of suitable ventilation facilities in the body of the filter and also by the intermittent working of the filter. The fine suspended, colloidal, and dissolved organic matter present in the sewage collect on this film, where biochemical oxidation of the organic matter is accomplished by aerobic bacteria.
A new filter when first put into use will usually be quite ineffective for about two weeks until a satisfactory gelatinous film has formed on the particles in the bed. The film eventually becomes quite thick with accumulated organic matter and will slough off (or unload) from time to time and be discharged with effluent.
The effluent from trickling filters therefore require sedimentation to remove the solids that pass the filter. Thus trickling filter is always followed by a secondary settling tank to remove from the filter effluent the settleable organic solids produced in the filtration process.
The trickling filter is always preceded by primary sedimentation along with skimming devices to remove scum. This will prevent the clogging of the filter with the larger suspended solids present in the sewage. Many trickling filter installations are designed so that a portion of the effluent can be recirculated back through the filter.
Recirculation provides a longer effective contact time in the filter and odours are reduced since the waste is diluted in the recirculation. In addition, the rate of flow through the filters can be kept constant by varying the proportion which is recirculated.
The trickling filter serves both to oxidize and bioflocculate the organic matter in sewage and their efficiency is assessed on the total reduction in BOD effected through the filter and the subsequent settling tank, since the effluent quality is reckoned after the settlement of the bioflocculated solids.
Trickling filters are used for the biological treatment of domestic sewage and those industrial wastes which are amenable to aerobic processes. They find use for complete treatment of moderately strong wastes and as roughing filters for very strong wastes prior to activated sludge units. Trickling filters possess a unique capacity to handle shock loads and provide dependable performance with a minimum of supervision.
Biological Process in a Trickling Filter:
As sewage trickles through the filter media a microbial or biological film or slime layer is formed on the filter media (see Fig. 14.4). The organic matter present in the sewage is adsorbed onto the biological slime layer. In the outer portion of the biological slime layer aerobic conditions prevail and hence in this portion of the slime layer the organic matter present in the sewage is degraded by aerobic micro-organisms.
As the organic matter abstracted from the flowing sewage is synthesized into new microbial cells the biological slime layer grows in thickness. As the thickness of the slime layer increases the diffused oxygen is consumed before it can penetrate the full depth of the slime layer, and hence in the lower portion of the slime layer near the surface of the filter media an anaerobic environment is established.
Thus the grown slime layer which usually has a total thickness between 0.1 and 2.0 mm consists of both aerobic and anaerobic layer. The thickness of the aerobic portion of the slime layer is limited by the depth of penetration of oxygen into microbial layer which depends on the coefficient of diffusivity of oxygen in the film, the concentration of oxygen at the solid-liquid interface and the overall oxygen utilization rate of microorganisms present in the slime layer.
As the slime layer increases in thickness, the adsorbed organic matter is metabolized before it can reach the micro-organisms near the surface of the filter media. As a result of having no external organic source available for cell carbon, the micro-organisms near the surface of the filter media enter into an endogenous phase of growth and lose their ability to cling to the surface of the filter media.
Eventually there is scouring of the slime layer due to flowing liquid and a new slime layer begins to grow on the filter media. This phenomenon of scouring of the slime layer is called sloughing or unloading of the filter and is primarily a function of the organic and hydraulic loading on the filter.
The organic loading accounts for the rate of metabolism in the slime layer, and the hydraulic loading accounts for shear velocities. Filter sloughing aids ventilation by keeping the filter media open. It also continuously renews the bio-mass, maintaining it active which is essential for efficient functioning of the filter.
Type # 4. Dunbar Filters:
The Dunbar filter is so named because it was first developed and used at Dunbar, a town in Germany. This filter is similar in construction and operation to an intermittent sand filter except that the filter media in this filter is graded from top to bottom from I mm to 150 mm size.
The filter is loaded intermittently. At each dose of sewage, the filter is flooded to a depth of about 50 to 100 mm at the top, and then allowed to rest. The process is repeated until the filter is clogged. The surface of the filter is then cleaned to make the filter ready for use again. The rate of surface loading may be about 25 million litres per hectare of surface area per day. The BOD removal is to the extent of about 85% or so. However, such filters are useful only for very small isolated populations such as of tubercular or other sanatoria.
Type # 5. Magnetite Filters:
The magnetite filter consists of a bed of magnetite sand (crushed magnetite iron ore) about 80 mm thick, supported on a fine screen of non-corrosive, non-magnetic metal wire such as phosphor-bronze wire screen. The magnetite sand is of about 0.85 mm size. Fig. 14.13 shows the section of a magnetite filter.
When the sewage applied to the filter bed percolates through the layer of magnetite sand, the suspended solids present in the sewage are retained in the voids purely by mechanical straining action. The rate of loading usually varies from 120 to 300 litres per m2 per minute. The maximum permissible head loss is between 75 to 100 mm, and the filter is required to be cleaned when the head loss exceeds this value.
Filter is usually cleaned automatically when the head loss exceeds the above value, through the operation of a float switch. The cleaning operation is done by means of a travelling electromagnet along the total length of the tank. The energized magnet lifts the magnetite sand clear of the supporting screen, and the partially treated sewage rushes through the bed. On demagnetization, the sand falls back on the wire screen.
Alternate action of magnetization and demagnetization cleans the magnetite sand particles. The dirty water is removed through a small centrifugal pump provided in the filter tank. These filters are, however, not in common use.
Type # 6. Rapid Sand Filters:
Rapid sand filters used for treatment of sewage are similar to those used for treating raw water for public water supply. These filters are adopted in treatment of sewage for reclaiming the used water, and water thus reclaimed can be used for purposes other than drinking, such as for gardening, etc. However, these filters get clogged in very short time, and are therefore not generally used.