Screening and Skimming in the Treatment of Sewage | Sanitary Engineering

In this article we will discuss about:- 1. Introduction to Screening and Skimming 2. Purpose of Screening 3. Types of Screens 4. Factors to be Considered for Designing Screens 5. Disposal of Screenings 6. Removal of Oil, Grease, Etc. 7. Flotation 8. Skimming Tanks 9. Disposal of Skimming.

Introduction to Screening and Skimming:

Sewage contains suspended and floating matters in it. The suspended matters are of large size such as tree leaves, paper, gravel; timber-pieces etc. as well as of small size such as sand, silt etc. The large size suspended and floating matters can be removed by passing sewage through screens.

The process of removing the large matters from sewage by passing it through screens is called screening. The floating matters such as oils, grease etc. are removed from the sewage by skimming tanks and this process of removing is called skimming.

Purpose of Screening:

Screening is an essential step in sewage treatment from removal of materials, which would otherwise damage the plant, interfere with the satisfactory operation of treatment units or equipment or cause objectionable share line conditions if disposed off in sea. Screens are provided ahead of pumping stations, meters and as a first step in all treatment works.

This is the first treatment or operation which is carried out at the treatment plants.

Following are the main purposes of the screening:

(i) To remove the floating solids which may clog the pumps at the treatment plants and outfall.

(ii) To remove the solids from the sewage which will form ugly sludge banks at the site of sewage disposal, when the sewage is given only primary treatment.

(iii) To remove the solids which will clog the trickling filters and will also interfere with the aeration in the activated sludge treatment units.

Types of Screens:

A screen is a device with openings generally of uniform size for removing bigger suspended or floating matter in sewage. The screening element may consist of parallel bars, rods, gratings or wire-meshes or perforated plates and the openings may be of any shape although generally they are circular or rectangular.

All types of screens can be classified as:

(i) Racks or bar-screens.

(ii) Perforated or fine screens.

(iii) Communiters or cutting screens.

The classifications can also be done as:

(a) Coarse, medium or fine screen (on their opening size).

(b) Disc, band, drum, wings or cage etc. (on their shapes).

(c) Hand or mechanical cleaned screens.

(d) Fixed, moving or moveable screens.

(i) Bar-Screens:

The bar-screens may be coarse or medium size screens. The coarse screens essentially consist of steel bars or flats placed vertically or at an inclination to the horizontal. The openings between the bars are 5 cm or above. These racks are placed in the screens chamber provided in the way of sewer line.

The racks remove the rags, sticks, dead animals etc. from the sewage and prevent the sewage pumps against damage. Fig. 13.1 shows the plan and elevation of most common type coarse rack used in the sewage works.

The width of the rack channel should be sufficient so that self-cleaning velocity should be available. In Fig. 13.1 a by-pass channel is shown which prevents the overtopping. The by-pass channel is provided with vertical bar screen. A well-drained trough is provided for storing the impurities while cleaning the racks. These racks can be cleaned manually or mechanically.

Medium size screens are similar to the coarse screens, the only difference being in the size of opening between the bars which is between 2 cm to 5 cm. Bars are usually 10 mm thick on the upstream side and taper slightly to the downstream side. These mechanically racked units are used before all pumps or treatment units.

The bars used for the screens are rectangular in cross-section usually about 10mm x 50 mm and are placed with the larger dimension parallel to the flow. A weir on the side of the screen may be used as an overflow by-pass.

The racks arc designed in such a way that the maximum loss of head after clogging should not exceed 75 cm. To compensate for this loss of head in racks, the floor of the rack chamber is kept 10-15 cm below the bed of the sewer as shown in Fig. 13.1.

(ii) Fine Screens:

When the sewage is disposed of into a body of water, it becomes necessary to remove the unsightly floating matter so that they may not cause sight nuisance in the natural water courses. It is done by passing the sewage through tine screens.

This screening of sewage also increases the efficiency of the chlorination. The fine screens are mostly manufactured from metal plates having slotted perforations. The sizes of these slotted perforations are between 1.5 mm to 0.7 mm × 50 mm spaced closely.

Fine screens may be of the drum or disc type, mechanically cleaned and continuously operated. Fine screens have usually a net submerged open area of net less than 0.05 m² for every 1000 m³ of average daily flow of sewage from a separate system, the corresponding figure being 0.075 m² for combined systems. They are also used for beach protection where sewage without any further treatment is discharged into sea for disposal by dilution.

Fig. 13.2 shows such a fine screen of disc type which is cleaned by brush. These screens are less common now-a-days, because their efficiency is low, have high maintenance cost and difficulty in the disposal of screening materials. These may be of disc type or drum type.

(iii) Communiters:

These are cutting screens, which are very popular in modern sewage works due to their efficiency and less maintenance cost. Fig. 13.3 illustrates a communiter in which a cutting screen drum continuously revolves and removes the fine suspended impurities from the sewage.

Fig. 13.4 illustrates another type of cutting screen, commonly known as Shredder or disintegrator, which is also employed for the same purpose.

Other Types of Screens:

Following are some other types of screens which are not commonly used:

(a) Cage screen:

It mainly consists of a rectangular bar with the sides made of parallel bars with the upstream side of the cage omitted the bottom and the top of the cage-screens are made up of solid or of perforated metal plates.

(b) Band screen:

The band screen is in the shape of a cylinder (or cone) made of perforated plates which rotates on a horizontal axis. Sometimes the cone is also manufactured with wires of non-corrosive metals instead of perforated plates.

Fixed screens are those screens which are permanently fixed in the position and are cleaned by means of rakes or teeth by pulling them over.

Moveable screens are designed in such a manner that when they are not in operation or use, they are lifted or taken out from the sewage for purpose of cleaning or other work. These screens are designed in such a way that they remain in continuous movement during their operation.

Factors to be Considered for Designing Screens:

Total area of screens required can be found out by assuming a suitable, velocity and determining the depth of submergence of the screen at the time of this velocity.

Following factors are to be considered:

(i) The velocity of flow should be kept reasonably constant and it may be taken as about 1 m/sec at an average flow of sewage.

(ii) Excess openings to the tune of 100% should be provided.

(iii) The slope of screens should be between 30° to 60°.

(iv) A perforated platform for temporary dewatering should be provided because the sewage contains about 90% to 95% of water.

(v) The maximum depth at screen should be about 1 m.

(vi) The detention period should be assumed as zero.

Disposal of Screenings:

When sewage is passed through screens, the suspended and floating matters larger than the openings of the screens, cannot pass through the screens and are removed from the sewage. These removed material are continuously deposited on the screens. After some time when the screens become partially clogged, they are cleaned and the screenings are taken away for disposal.

The quantity of the racking and screening depends on the size of the openings through which sewage passes. The courser screens collect less quantity than tine screens. The screening contains more than 80% moisture and the remaining organic and inorganic matter. The screenings obtained from the fine and medium screens are highly offensive and pitiable in nature, therefore require quick disposal.

These are various methods for the disposal of screenings, which may be classified as:

(i) Burial:

The medium size screening can be disposed of by this method. In this method trenches are excavated on the ground and the screenings are buried into them. The trenches are immediately covered with soil after burying. During cleaning the racks, if the screening matter starts giving foul smell, it can be prevented by sprinkling lime powder over it.

(ii) Incineration:

In this method the screening matter is burnt in air. Before burning, it is necessary to dry the screening first, by spreading it over the ground and exposing it to the sun. The partially dry screening can be burnt by adding oil, pulverized coal or other types of fuel in it. This method is costly, therefore, it is not commonly used.

(ii) Digestion:

The screenings obtained from the fine screens can be placed in sludge digestion tanks. In this method, the screening is digested with the sludge. Screenings obtained from communiters and shredder can be disposed of by this method.

Removal of Oil, Grease, Etc.:

Mostly all types of sewage contains floating matters such as grease, oils, fats etc. The sources of these floating matters are kitchens, garages, soap-industries etc. These matters form scum over the surface of the sewage and interfere with various sewage treatment operations.

Due to the following difficulties, the removal of greasy and oily matters from the sewage is necessary:

(i) They form scum in the sedimentation tanks.

(ii) They clog the fine screens.

(iii) They considerably reduce the efficiency of the activated sludge process.

(iv) They decrease the biological growth due to which various bacteria and protozoa cannot perform their work.

(v) They occupy large space resulting in the decrease in the efficiency of sewage treatment process and increase their maintenance cost.

(vi) They form unsightly scum and other odourous matter when the sewage is discharged into natural water courses. The presence of scum reduces the reaeration of the water courses and causes anaerobic conditions.

(vii) They interfere with the operation of the trickling filter and inhibit the biological growth. The biological treatment of the sewage is adversely affected.

Floatation:

All the lighter matters than water float on its surface. If the sewage is allowed to remain quiescent, then oils, fats, grease etc. will rise to the surface of sewage, where they will form scum, which can be easily removed by hand or mechanically. If the sewage is flowing, then most of these materials will remain in suspension or mixed condition, therefore, for their removal its sewage must be first brought in quiescent condition.

If sewage is filled in a tank and is allowed to remain at rest, large oily matters will rise more rapidly than smaller ones. Therefore, the quantity of oily and greasy matters rising at the surface will directly depend upon the detention period.

In practice, it is not possible to allow such a longer detention period, so that all these matters may reach the surface. The efficiency of floatation can be increased if the sewage contains large amount of air bubbles in it, because the air bubbles will rise up and will lift solids upwards. Only smaller bubbles do this work, larger bubbles have no lifting value.

The lighter matters from the sewage can be more efficiently removed by passing compressed air in the bottom of the tank or by applying a vacuum above the sewage surface. The air bubbles impart sufficient buoyancy to lift the oils, grease, fats etc.

Skimming Tanks:

These tanks are used for removing oil, grease and fats of the sewage. This tank is in the form of long, trough-shaped structure. Tank surface is made as large as possible, and the sides narrow down at a steep angle. Detention period of 3 min is provided in these tanks.

To prevent heavy solids from settling in the bed, compressed air is blown through the diffusers placed in the floor of the tank. Two vertical baffle walls are provided in the tank, which divide it into three compartments as shown in Fig. 13.5.

The vertical baffles do not touch the bed of the tank. When compressed air is circulated, the oily matters rise upward and are collected in the side trough, from where they are removed. The sewage enters the tank from inlet, all the oily matters are collected in the through. The outlet conduit is given such a slope that grit is swept up on its slope and is removed out of the tank.

q = rate of flow of sewage in m³/day

V_r = Minimum rising velocity of the oily material to be removed in metre/minute.

= 0.25 m/minutes for most of the cases.

The aeration which is done in the skimming tanks has the following advantages in addition to the removal of the oily matters:

(a) The sewage becomes fresh i.e. oxygen contents are increased.

(b) Objectionable gases such as H₂S are expelled from the sewage.

(c) The flocculation of the colloidal matters takes place and they are easily removed in the sedimentation tanks.

It has been observed that if chlorine gas at the rate of 1.5 mg/litre of sewage is applied along with the compressed air, the percentage of grease removal shall be increased by 300 to 400 percent. Foaming and floatation agents such as oil, resin, grease, glue etc. Help in the promotion of scum formation and also enhance the grease removal. The action of these agents is similar to those of coagulants in the sedimentation with the coagulations.

If the sewage is subjected to the vacuum for 10-15 minutes, the grease can also be removed from sewage.

Disposal of Skimming:

The disposal of the skimming obtained from the skimming tanks can be done for the manufacture of the soap, lubricants, wax, pitch and other non-edible products. The skimming’s are usually disposed of by burning or burying in the ground. If the mineral oils are present in small quantity and more quantity of organic and vegetable oils are present, these can be used in the production of the fuel gas.

In hot countries, it is difficult to skim out the greasy and oily matter, because these do not easily congeal. In Indian climatic conditions, the skimming tanks are not required. But in few industrial towns, they may be provided.