Filters Used in Sewage Treatment | Filtration | Sanitary Engineering

The filters which are mostly used in sewage treatment can be classified as follows: 1. The Intermittent Sand Filters 2. The Contact Beds 3. The Trickling Filters.

1. Intermittent Sand Filters:

These are the early development of sewage treatment units. These are similar in Construction to the slow-sand filter of water treatment. These require larger area, due to which these are not commonly employed in modern sewage treatment works.

Fig. 16.1 shows an intermittent sand filter. It consists of layers of sands with an effective size of 0.2 to 0.5 mm and of uniformity coefficient 2-5. If the soil itself is sandy, there is no need of providing extra sand. But if the soil is of other variety, sands of the above specifications are laid in a depth of about 100-120 cm.

To carry off the effluent the open joint drainage pipes are laid in the bottom of the sand bed in 90 to 120 cm depth. Their drainage pipes are surrounded with layers of coarse stone and gravel graded from coarse to fine, to keep and the sand out. In some cases when the soil itself-sandy, the percolating effluent may reach the ground water table, and no effluent may reach the drainage pipes.

The sewage is applied evenly on the surface of the sand bed by influent waste water troughs as shown in a Fig. 16.1. The distribution trough has side openings to distribute the sewage uniformly. To prevent the scouring and displacement of sand the distribution trough is kept on concrete apron or protective stone. While applying the sewage the flooding is done from 3 to 10 cm depth after an interval of 24 hours. The capacity of these filters is 0.8 x 10⁶ to 1.1 x 10⁶ litres/hectare per day.

The effluent from the intermittent sand filters is very clear and contains suspended solids less than 10 ppm which is well nitrified and stable. The effluent also has B.O.D. less than 5ppm and is free from odours. Therefore, the plant works without creating any nuisance at the site.

If the quantity of sewage is more 3 to 4 such beds can be constructed in parallel. For cleaning these filters, the sand from the top is scraped from time to time and are refilled with fresh clean sand.

The following are the advantages of intermittent sand filters:

(i) Operation is simple, only mechanical equipment is required for dosing.

(ii) The effluent is very clean and can be directly disposed of in natural watercourses without any further treatment.

(iii) There is no trouble of odour and insects.

(iv) Smaller head is required.

(v) There is no secondary sludge, which is to be disposed of except the occasional sand scraping.

The following are the disadvantages of intermittent sand filters:

(i) Their rate of loading is very small.

(ii) They require large area and much quantity of sand in their construction which makes them uneconomical.

(iii) They cannot treat large quantity of sewage, therefore cannot be employed at big plants.

The intermittent sand filters are most suitable for hospitals, institutions, small towns and factories, where it is not possible to dispose of the effluent of septic tanks on the ground surface.

2. Contact Beds:

In ancient time contact beds were very popular in the treatment of sewage, but now a days these are similar in construction to the intermittent sand filters, the only difference being in the filtering media. The filtering media consists of 2 to 2.5 cm size broken stone ballast or brick ballast.

The depth of the filtering media is between 90-150 cm. The sewage is uniformly applied over the whole surface of the filtering media, by means of distribution troughs and is collected at the bottom by means of a system of under drain pipes.

Fig. 16.2 illustrates the essential of double-contact beds with dosing and draining siphons.

The operation of the contact beds includes the following:

(i) Filling:

In this operation the sewage is applied on the surface of contact beds as quickly as possible by means of dosing siphon. The sewage is allowed to be filled 5-10 cot above the surface of contact beds. The filling may take one hour or so.

(ii) Contact:

In this operation, the dosing is stopped and the applied sewage is allowed to come in contact for about an hour with the bacterial film covering the filter medium. Within this time the soluble contents of sewage are absorbed by the organic film and are stabilized.

(iii) Emptying:

The contact beds are then slowly emptied and drained. So that the absorbed soluble contents of the sewage are not washed out with the sewage, which is being drained.

(iv) Resting:

After emptying, the contact beds are allowed to remain at rest for 5-6 hours. Within this period the atmospheric air enters in the voids of the contact media and makes it ready for taking another sewage load. By supplying oxygen to the aerobic bacteria, which oxidize the organic matter present in the sewage which is transferred by sewage on the surface of the filtering media.

The complete cycle of operation takes 8-12 hours. As these contact beds are intermittent in action, therefore more numbers of units are constructed in parallel and the sewage is applied in turn to each unit. For this purpose continuous supervision is required.

The effluent obtained from these beds is also clear and odourless. These beds remove 80 to 90% suspended solids and 60 to 75% B.O.D. The rate of loading is very low 4500 to 6500 m³/hectare/day. The voids inside the filtering media continuously go on reducing due to accumulation of the solids in them.

After 4-5 years the filtering media is taken out, washed, dried and filled again. Similarly the under drain pipes are also washed and cleaned after 3-4 years. These are also not common these days.

Generally, the contact beds are also intermittent in their operation. The continuous operation of contact beds is possible by blowing air into the waste water flowing through them in sufficient quantity to keep the water and slime surface aerobic and in sufficient intensity to tear away ageing slime accumulation of solids on the surface.

3. Trickling Filters:

Trickling filters are used for the biological treatment of domestic sewage and industrial wastes, which are amenable to aerobic biological processes. These are used for the complete treatment or moderately strong wastes and as roughing filter for strong wastes prior to activated sludge units. They possess a unique capacity to handle shock loads and provide dependable performance with minimum supervision.

The trickling filter is always preceded by primary sedimentation so that the settleable solids in the sewage may not clog of the filter. The sedimentation tanks should have skimmers to remove the scum. The trickling filter is always followed by a final settling tank to remove from the filter effluent, the settleable organic solids produced in the filtration process.

It is always advantageous to provide skimming devices for the final settling tanks also. They serve both to oxidise and bio-flocculate the organic material in sewage and their efficiency is assessed on the total reduction in B.O.D. effected through the filter and the subsequent settling tank. Since the effluent quality is reckoned after the settlement of the bio-flocculated solids.

As the sewage trickles through the filter media a biological slime consisting of aerobic bacteria and other biota builds up around the media surfaces, normally in two weeks period, making filter ready for use. Organic material in the sewage is absorbed on the biological slime, where they are partly degraded by the biota thus increasing the weight and thickness of the slime.

Eventually, there is a scouring of the slime and a fresh slime layer begins to grown on the media. This phenomenon of scouring of the slime is called sloughing or unloading of the filter. Filter sloughing helps ventilation by keeping the filter media open.

It also continuously reviews the biota, maintaining it active for the efficient functioning of the filter. The degree of filter sloughing to be given depends on the organic loading and the hydraulic loading which will influence its scour. Details of Trickling Filter have been shown in Fig. 16.3.

Types of Trickling Filters:

The trickling filters are classified as low rate and high rate trickling filter depending on the organic and hydraulic loadings over them, as follows:

In the trickling filters the hydraulic loading is the total flow of sewage, including recirculation applied on unit area of the filter in a day, while the organic loading rate is the 5- days 20 °C B. O. D. a day excluding the B.O. D. of the recirculant, applied per unit volume in a day. Usually recirculation is not generally adopted in low rate filters.

Media depths for low rate filters range from 1.8 ta3.0 m. They require larger media volumes than high rate filters and have high initial costs. However, they are easy in operation and give consistently good quality effluent and are preferred when plant capacities are smaller in the case of institutions.

In comparison to low rate filters in high rate filters, a part of the settled or filter effluent is recycled through the-filter. Recirculation has the advantage of bringing the organic matter in the waste in contact with the biological slime more than once, thus increasing the efficiency of the filters. Due to higher hydraulic loading, there are less chances of filter clogging and aids uniform distribution of organic load over the filter surface.

It also helps to dampen the variation in the strength and the flow of sewage applied on the filter. Recirculation ratios usually range from 0.5 to 3 and the values exceeding 3 are considered to be uneconomical in the case of domestic sewage but the ratios of 8 and above have been used with industrial wastes.

Two stage filters consist of two filters in series with a primary settling tank, an intermediate settling tank which may be omitted in certain cases and a final settling tank. Recirculation facilities are provided for each stage.

Trickling Filter Operation and Loading:

The sewage is given primary treatment before passing through the trickling filter. In the trickling filter the sewage trickles over the filtering media by means of moving arms. After getting trickled over the filtering media, sewage starts percolating downward through the contact media. While moving down it is acted upon by the bacteria.

The dissolved and colloidal organic matters are precipitated and flocculated by the action of the bacteria and form a gelatinous layer at the surface of the contact media. The aerobic bacteria eat up this gelatinous layer and oxidize it and convert it to sulphate, phosphates, nitrates etc. With the time the gelatinous slime become thicker and heavier and move towards the bottom of the trickling filter and pass out with the effluent and settles in the secondary settling tank.

In the normal rate filters the sewage loading is done between 11,500 to 33,500 m³/ hectare and 250 to 320 kgm of B.O.D. per 100 sq.m of the contact media. Some hydraulic pressure is maintained for flushing the filter bed and for preventing its clogging due to gelatinous film.

The biological load in the trickling filters should not exceed the permissible limits, otherwise the efficiency of those filters shall be reduced and effluent will get deteriorated. This directly indicates that the efficiency goes on reducing with the increase of the load, as shown in Fig. 16.5.

The normal trickling filter removes 90% B.O.D.. the effluent is well nitrified and quite stable. The sludge from the secondary settling tanks is thick (moisture content 95%) which is heavy and easily digestible. The filter is very strong and durable and can take any intermittent shock loads and it cannot cause any detrimental effect on it. This filter is very suitable for medium towns and cities.

High Rate Filters:

The normal rate trickling filter have high efficiency in removing B.O.D. and other organic matters. The effluent coming out of the filter is highly stabilized and the slope formed is easily digestible. The main drawback of the normal trickling filters is their high initial cost, requirement of large area for construction and large quantity of filtering media.

To overcome the above drawbacks the experiments were carried out by increasing the rate of sewage flow through the filters and the following observations were made:

(i) When the rate of sewage flow is increased, the thickness of the gelatinous film is reduced and the materials deposited in the filtering media are continuously washed away with the effluent.

(ii) It was observed that thinner gelatinous film is more efficient and supplies more continuous food to the aerobic bacteria.

(iii) The precipitation and biological coagulation of the dissolved and colloidal matters is more or less of the same degree as in normal rate filters.

(iv) As the time available in the contact period for the bacteria to act upon the sewage is less, less oxidation of the organic matter takes place.

(v) The load on the secondary settling tanks is increased, because the efficiency of high rate trickling filter is less as compared with normal rate trickling filters and large quantity of unloaded putrescible organic material reaches the secondary settling tank.

(vi) The quality of sludge produced is not so easily digestible as the sludge of low rate trickling filter. The quality of sludge obtained is in between the quality of activated sludge and low rate trickling filter sludge.

(vii) When the rate of application of sewage is increased 3-4 times the rate of application in slow rate trickling filter, the efficiency is about 85% of B.O.D.

(viii) The high rate trickling filters are much economical in initial cost (cost of construction and land etc.) than normal rate trickling filters.

The B.O.D. loading in high rate trickling filters can be950to 1000 cu.m per 100of the contact media. The hydraulic load of the filter can be increased of 3,35,000 cu.m./day/ hectare. It should not be kept less than 45,000 cu.m./day/hectare of the contact surfaces for proper flushing of the gelatinous film.

Due to the above advantages, the high rate trickling filters are very common now a days.

Following modifications are done over the slow rate trickling filters for getting better results in high rate filters:

(a) The depth of the filtering media is reduced to 170 to 120 cm, for better aeration for high rate of biological activity.

(b) The size of the undertrains are increased and their slopes are made more steeper for quick collection of effluent.

(c) The speed of the rotating arms is increased to 2 R. P. M. for supplying sewage at high rate.

(d) The size of the secondary settling tanks is increased for collecting more quantity of sewage and flocculent solids coming with the effluent of the trickling filter.

Bio Filters:

This is also called two-stage treatment. It is nothing but two high rate trickling filter connected in series. Previously, it has been such that the efficiency of the filtration is increased by recirculation. In recirculation the effluent is again allowed to pass in the same trickling filter, but in biofilters the effluent is passed in the another trickling filter, connected in series to the first one. In the biofilter units the depths of tanks are reduced to 1.2 to 0.9′ metre only resulting in the economy in construction.

The following are the advantages of biofilters:

(i) There is more flexibility in operation. When concentrated sewage is to be treated, the filters are joined in series and when light sewage is to be treated they may work in parallel.

(ii) By adjusting the quantity of sewage and effluent entering the tanks, they can be uniformly loaded.

(iii) As the depths of the tanks have been reduced, they are economical.

(iv) The size of the filters is reduced due to two units in place of one.

If a high capacity or roughing filter is followed by an intermediate settling tank, a normal rate filter and a final settling tank, very good results will be obtained. The B.O. D. load on the roughing filters can be increased upto 74,000 kgm/hectare metre/day. The load on the normal trickling filters can be between 2200 to 3000 kgm/hectare metre/day.

Humus Tanks:

These are also known as secondary settling tanks and are similar to sedimentation tanks. It has been observed that the effluent of trickling filters contains finely-divided suspended solids whose only character has been changed. Due to the biological oxidation of these suspended solids, they are converted into easily settleable solids.

The effluent from the trickling filter is passed through the humus tanks in which a detention period upto 2 hours is provided, due to which all the suspended matter settles in the bed and is removed. The capacity of these tanks in 30 to 50 cm³/m²/day.

The weir loading in the humus tanks should be kept less than 225 cu.m./m of weir length. If the recirculation of the effluent is to be done, the size of the tank should be designed accordingly to provide adequate settling time for the total combined flow of sewage and recirculated effluent.

Comparison of low rate and high rate trickling filters:

Table 16.4 shows the main comparative difference between the low rate and high rate trickling filters.