In this article we will discuss about the characteristics and quantity of sludge produced in various sewage treatment processes.
Characteristics of Sludge:
Sludge consists of organic and inorganic solids present in the raw sewage and removed in the primary settling tanks plus organic solids produced during secondary treatment of sewage and removed in the secondary settling tanks. The characteristics and the quantity of sludge produced depend on the character of the raw sewage and the treatment process employed.
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The characteristics of sludge produced in various sewage treatment processes are as indicated below:
(i) Sludge from Primary Settling Tanks:
Sludge from primary settling tanks is usually gray-coloured, slimy (or sticky) slurry of settleable solids accounting for about 50 to 60% of the applied suspended solids and tank skimmings. This sludge has an extremely offensive odour and it does not drain easily. It can be dewatered readily after chemical conditioning because of fibrous and coarse nature. Typical solids concentration in this sludge is about 6 to 8% and the portion of volatile solids varies from 60 to 80%. It can be readily digested under suitable conditions of operation.
(ii) Chemical-Precipitation Sludge:
Sludge from chemical precipitation tanks is usually dark in colour, though its surface may be red if it contains much iron. Its odour may be objectionable, but not as bad as odour from sludge obtained from primary settling tanks. While it is somewhat slimy, the hydrate of iron or aluminium in it makes it gelatinous. If it is left in the tank, it under goes decomposition like the sludge from primary settling tanks but at a slower rate. It gives off gas in substantial quantities and its density is increased by standing.
(iii) Activated Sludge:
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Activated sludge generally has a brown flocculants appearance. If the colour is quite dark, it may be approaching a septic condition. If the colour is lighter than usual, there may have been under-aeration with a tendency for the solids to settle slowly. Sludge in good condition has an inoffensive characteristic odour.
It tends to become septic rather rapidly and then has a disagreeable odour of putrefaction. This sludge contains the largest percentage of moisture (about 98-99%) and is the most voluminous amongst the sludges. Thickness of return sludge is 0.5 to 2% suspended solids with a volatile fraction 0.7 to 0.8. Waste activated sludge can be thickened effectively by floatation or centrifugation, though chemical addition may be needed to ensure high solids capture in the concentrating process. Further the activated sludge will digest readily alone or mixed with fresh sewage solids.
(iv) Trickling Filter Sludge:
Trickling filter sludge also called humus is brownish, flocculants, and relatively inoffensive when fresh. It generally undergoes decomposition more slowly than other undigested sludges. However, during unloading period of filters that are operated at low rates, humus contains many worms and its odour becomes quite offensive.
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The suspended particles are fragments of biological growth washed from the filter media. Although they exhibit good settleability, the precipitate does not compact to a high density. For this reason, and the fact that sloughing is irregular, underflow from the final clarifier containing filter humus is returned to the wet well for mixing with the inflowing raw sewage.
Thus humus is settled with raw organics in the primary settling tank. The combined sludge has a solids content of 4 to 6% which is slightly thinner than residue with raw organics only. The trickling filter sludge is readily digested.
Quantity of Sludge:
The quantity of sludge produced varies depending on the degree of removal of suspended solids in primary and secondary settling tanks, moisture content and specific gravity of the sludge. The quantity of sludge produced from a primary settling tank may be about 20 m3 per million litres of sewage flow.
Similarly the chemical-precipitation sludge may also be about 20 m3 per million litres of sewage flow. The quantity of activated sludge produced may be between 35 to 50 m3 per million litres of sewage treated. The trickling filter sludge produced is lesser in quantity being about 5 m3 per million litres of sewage treated. The volume of sludge depends on its water content and the volume of the solids.
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In the case of sludge continuously removed from primary settling tanks, solids generally do not exceed 4 to 5% while in secondary settling tanks they range from 0.5 to 1%. When the secondary sludge is combined with the primary sludge, the solids in the mixed sludge range from 2.5 to 5%. The solids content of digested sludge is usually in the range of 6 to 13% when reduction in bulk takes place due to separation of supernatant.
The quantity of Suspended Solids (SS) in raw sewage is typically 90 gm per day per capita and its concentration varies from 200 to 250 mg/l depending upon per capita sewage flow. Out of these suspended solids about 60% solids are removed in primary settling tanks while the remaining suspended solids are either oxidized in secondary settling tank or incorporated in the biological mass.
The solids generated in the biological processes are about 0.1 to 0.2 kg per kg of BOD5 applied in suspended growth process and 0.4 to 0.5 kg per kg of BOD5 applied in attached growth process depending on the sludge age.
Typical values of the solids that may be present in the sludges produced in various sewage treatment processes are given in Table 16.1.
Volume-Mass Relationship:
If the solid matter in a sludge is composed of fixed (or mineral or inorganic) solids and volatile (or organic) solids, the specific gravity of all of the solid matter can be computed from the following expression:
Thus if 33% of the solid matter in a sludge is composed of fixed mineral solids with a specific gravity 2.5, and 67% is composed of volatile solids with a specific gravity of 1.0, then the specific gravity of all solids Ss is determined by equation 16.1 (a) as-
If the sludge contains 90% water and 10% solids, then taking the specific gravity of water as 1.0 the specific gravity of the sludge is determined by equation 16.2 as-
Once the specific gravity of sludge and percentage of solids in the sludge are known the volume of the sludge can be computed both for raw sludge as well as digested sludge from the following expression-
For approximate calculations for a given solids content, remembering that the volume of sludge varies inversely with the percent of solid matter contained in the sludge as given by-
Equation 16.4 shows the variation in the volume of sludge with its solid content.
If a sludge contains m% water or moisture content by weight, then the solids will be (100-m)% by weight (dry basis). Hence in a given sludge if water or moisture content varies from m1% to m2% then the percentage of solids will vary from (100 – m1)% to (100 – m2)%. Thus-
Equation 16.5 shows the variation in the volume of sludge with its water or moisture content.