In this article we will discuss about:- 1. Meaning of Sludge 2. Sources of Sludge 3. Properties and Quantity 4. Necessity of Treatment 5. Digestion 6. Concentration 7. Thickening 8. Dewatering 9. Elutriation 10. Heat-Drying and Incineration 11. Disposal.
Contents:
- Meaning of Sludge
- Sources of Sludge
- Properties and Quantity of Sludge
- Necessity of Treatment of Sludge
- Digestion of Sludge
- Concentration of Sludge
- Thickening of Sludge
- Dewatering of Sludge
- Elutriation of Sludge
- Heat-Drying and Incineration of Sludge
- Disposal of Sludge
1. Meaning of Sludge:
The sludge is unsafe because it contains pathogens and it decomposes rapidly and serves as food for saprophytic organisms. Therefore, the primary objective of the sludge treatment should include its safe removal from the treatment works at reasonable economy and safety of the public health.
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The principal objective of sludge digestion is to subject the organic matter present in the settled sludge of the primary and final sedimentation tanks to an aerobic decomposition so as to make it innocuous and amenable to dewatering on sand beds or mechanical filters before final disposal on land, lagoon or sea.
Regardless of the processes used, the sludge digestion brings about a reduction in its volume. While anaerobic digestion of sludge produces gas which can be utilised whenever feasible, aerobic digestion does not produce any utilisable by-product other than the well stabilized sludge.
2. Sources of Sludge:
In waste water the sources of sludge are the treatment plants.
In short, the following are the sources of sludge:
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(i) Primary settling tank.
(ii) Chemical coagulation plants.
(iii) Trickling filter humus.
(iv) Activate sludge process units.
3. Properties and Quantity of Sludge:
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The properties and the quantity of the sewage depends on the following:
(i) The solids present in the initial stage.
(ii) Quantity of the coagulants and other materials added in the sewage during treatment.
(iii) The biomasses generated by living organisms from the nutrient organic matter during treatment.
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For determining the properties of the sludge the tests should be done to determine the following:
(a) Density, viscosity rigidity and other flow characteristics.
(b) Response to concentration or thickening and to filtration.
(c) Fuel value of the sludge after drying.
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(d) Digestibility and drainability.
(e) Fertilizer value if used for growing crops.
(J) Percentage of water present.
(g) pH-value.
The quantities of solids produced in various sewage treatment processes are measured routinely. The estimation of the quantity of waste water sludge in various units is done on the analysis of the waste water and the expected removals by the specific treatment processes.
The properties and the amount of sewage sludge produced in various treatment processes are briefly as follows:
1. Plain Sedimentation (Primary Treatment):
It contains fine silt, vegetable parrings and faecal matter, which have grey colour. This sludge sticks to the floor of the settling tanks and cannot be drained easily. The fresh sewage contains mostly settleable solids and about 60% of the suspended solids of the raw sewage. If the anaerobic action has taken place, about 67% of the volatile matters are destroyed out of which small portion is converted into fixed solids.
2. Chemical Precipitation (Primary Treatment):
The fresh sewage includes the precipitated chemicals and entangled solids in more or less stoichiometric amount. It is similar to the sludge obtained from the plain sedimentation the only difference being in the percentage of water. It mainly consists of 70 to 90% of the suspended solids in the sewage depending on the effectiveness of the chemical dose.
Table 18.1 shows the moisture content and the approximate quantity of sewage obtained from various types of sewage treatment processes.
3. Trickling filters (Secondary Treatment):
This sludge contains most of the dissolved organic matter and some non-settleable solids which are settleable by adsorption and biological flocculation on the trickling filter film. By the time the film is removed, it starts decomposition. The trickling filter sludge includes 50 to 60% of the non-settleable suspended solids reaching low rate filters. The trickling filter humus is usually added with primary solids for digestion.
4. Activate Sludge Process (Secondary Treatment):
It is golden brown in colour and has a pleasant earthly colour. It contains 98% to 99% of water. It is normally richer in organic matter. During formation and recirculation 5 to 10% of the transferred sludge is mineralized, which depends on the proportion of solids returned and the time of aeration. The excessive activated sludge is allowed to settle with the primary solids.
Table 18.1 gives the approximate quantities and properties of the sewage sludge obtained from various sewage treatment units.
4. Necessity of Treatment of Sludge:
The sludge obtained from the primary and secondary treatment units is foul in nature.
Due to the following necessity, the sewage treatment is done:
(i) To destroy all the pathogens, that it may not pollute the receiving water and make than unsafe for use.
(ii) To reduce the volume of sewage sludge, so that it can be easily disposed off.
(iii) To reduce the percentage of water for facilitating its handling and transporting upto disposal places.
(iv) To stabilise the organic matter so that it may not cause difficulties later on
(v) To recover the industrial value of the sludge.
Fig. 18.1 illustrates the possible flow paths for handling sewage sludge in various ways:
5. Digestion of Sludge:
The sludge digestion breaks the organic matter of the sludge into liquid and simple compounds which are stable and unfoul in nature. A portion of the solids is converted into liquid and gases due to which the volume of the sludge is reduced by 60-75 percent. The moisture content of the sludge cannot be reduced easily, but after its digestion it can be removed without any difficulty.
The gases produced can be used as fuel and the digested sludge has very good fertilizing value. Sludge digestion process removes coliforms by 99.8% by digestion for 30 days at 95-100°F temperature. Mostly sludge digestion takes place under anaerobic conditions which result from the activities of two groups of bacteria.
One group of bacteria attacks the organic substance such a fats, carbohydrates and proteins and converts them into organic acids and alcohols. The second group of bacteria acts under anaerobic conditions and forms methane and carbon dioxide gases using the acids and other products formed by the first group of bacteria. The acids formed by the first group are also neutralized by the alkaline substances present in the sewage.
The sludge digestion also takes place under uncontrolled and slow natural conditions. The first step of digestion is called as period of ‘acid production’, in which stage the group of bacteria acts on the organic matter and produce fatty acids and lower the pH-value to about 6.2.
The second stage is known as period of ‘acid regression’. Another type of organisms act in this stage and anaerobic sludge digestion takes place and various gases such as methane, carbon dioxide etc. In the second stage the pH- value increases and reaches up to about 6.8.
The last or the third stage of sludge digestion is known as ‘period of intensive digestion’ and it takes long time. The more resistant organic matters such as protein amino acids etc. are acted upon by another group of bacteria and the pH- value further rises to about 7.4. Under natural conditions the total time taken for the complete sludge digestion is about three months.
Process of Anaerobic Sludge Digestion:
If the sludge is not aerated vigorously, its digestion proceeds anaerobically. The process of sludge digestion can be written as Bacteria + Organic matter
= bacteria + residual, resistant organic matter + CH4+ CO2+ H2O
The efficiency of the sludge digestion process depends on the environment provided by the engineer of the work of bacteria.
The past experience shows that the sludge digestion process under the following conditions in the best way:
(i) The digestion process should be made continuous rather than intermittent or discontinuous process. Fresh sewage should enter the digestion from one end at the effluent should be drawn from the outlet. As the bacteria convert the sludge into gases and liquid, therefore the discharge at inlet and outlet are not the same.
The organic matters which remain in the tank do the work of further decomposition of the fresh sludge. The effluent contains mostly dissolved and non-settleable substances.
In continuous process the old sludge, which remains in the tank, serves the following function:
(i) It provides seeding organism for the start of decomposition of fresh sludge.
(ii) It gives buffering capacity, so that the produced organic acids may not lower the pH-value of the digesting sludge.
(iii) The operational process should be uniform so that the incoming solids may be dispersed in the digesting sludge quickly and uniformly. The sludge should be thoroughly mixed up by stirring it. But the produced rising gases do this work and provide the optimum contact opportunity between the organic food and the bacteria.
(iv) The activities of bacteria largely depend on the suitability of thermal conditions. Therefore, the process should be accelerated by heating the sludge from an outside source. The methane produced during the process can be utilized for this heating work.
Digestibility of Sludge:
The digestibility of the sludge is determined in the laboratory by performing experiments in it.
The experiments should be done under various circumstances, few of which are:
(i) With or without seeding it from digested sludge.
(ii) At various temperatures to know the optimum temperature at which the bacteriological activities are maximum.
(iii) With or without chemical treatments.
(iv) With or without stirring or through mixing.
(v) With or without aeration.
The results obtained by the experiments under various different conditions will furnish information’s for the economical design of digestion units. During the test the performance of continuous digestion units may be assumed to equal that of a single daily charge of sludge during the period of required sludge storage.
Conventional Digestion:
In this type of digestion the fresh sludge is brought in the middle depth of the tank, where it is mixed up with the digestion solids and is seeded and buffered by them. The digester liquors are taken out by 3-4 outlets which are provided between the higher solids level and the lowest scum-solids level.
Unless the materials inside the tanks are vigorously stirred, they will have the following layers:
(i) Scum layer at the top, which is inactive.
(ii) Central portion of digester liquor from which solids have settled.
(iii) Bottom layer of inactive stable solids.
(iv) Active layer of digesting solids between the stable solid layer and digestion liquor layer.
Multistage Digestion:
In this method two or more sludge digestion tanks are provided in series. Hence less capacity of the tank is required than conventional digestion. The digester liquor is clearer and has no odour which can be easily disposed off. In two-stage digestion two tanks of equal size are provided in series, the first one is heated while the second may or may not be heated.
Actually the digestion process proceeds rapidly in the first tank whereas the second tank serves the purpose of storing and compaction of the digested sludge obtained from the first tank. The sewage gases of first tank are also collected in the second one, from where they are taken.
High-Rate Digestion:
In conventional digestion a detention period of40 to 50 days is provided, due to which larger capacity tanks are required. Even during cold and rainy season it becomes difficult to dispose off the digested solids, which also require more capacity of the tank to retain them in such times. The speed of the digesters can be only increased if the conditions permit.
Now a days high-rate digestion tanks are used, in which the speed is increased:
(i) By mixing the incoming sludge by mechanical stirrers thoroughly with the tank contents and keeping it well-stirred,
(ii) By heating the sludge upto optimum temperature, so that the bacteriological activities may be maximum,
(iii) By continuous operation of the tank,
(iv) By providing multistage digestion.
Method of Starting New Digestion Tank:
For starting a new or cleaned tank, first the tank is filled with water or sewage. If the tank is provided with floating cover, it should be filled such that the cover starts floating. The temperature inside the tank should be maintained constantly nearly 95°F.
If possible, the digested sludge or supernatant liquor of other working tanks should be added in the new tank. The quantity of the sides of active tank which are added in the new tank should be 0.5% (dry weight) of the weight of liquid in the new tank.
After sometime, the production of the gas will start. When the gas produced is about 50% of the estimated one, the load on the tank is increased constantly with slow speed. The load is increased by increments equal to 50 to 100 per cent of the initial load.
The regular tests of the sludge are also done. If the pH- value fails due to the more production or organic acids, the hydrated lime should be added in the tank to bring the pH-value within the desired limits. Sometimes in place of hydrated lime, the milk of lime is added in the raw sludge.
Digester Gas:
In the previous articles, it has been seen that methane and carbon dioxide gases are produced during the sludge digestion processes. The methane gas is highly combustible in nature and can be utilized for various purposes. Sometimes hydrogen sulphide, hydrogen and nitrogen are also produced in the digestion tanks. Out of these hydrogen is combustible whereas hydrogen sulphide is an explosive and toxic gas.
Therefore while using the digester gas, care should be taken against explosion or accidents. The amount of gas produced varies from 0.08 to 0.1 cu.m per day per capita, which is fairly a good quantity and can be utilized.
Digester gas can be commonly utilized for the following:
(i) For heating the plants of digesters, buildings, incinerators and hot water supply.
(ii) For plant power production-pumping, air and gas compressors.
(iii) For gas supply to small factories and institutions.
(iv) Motor municipal cars and trucks.
The collection, storage and utilization of sludge gas is only economical when the treatment plant is large enough. Mostly in India this gas is not utilized.
6. Concentration of Sludge:
This method is also called sludge thickening or sludge conditioning and is the process of removing water contents from the sludge. In the simple method the sludge can be concentrated by adding chemical coagulants in it and resettling in sedimentation tanks.
The dimensions of tank, detention time required, rate of stirring and the optimum dose of coagulant can be determined by actual tests. To prevent the sludge from being septic, the rate of introduction should be more. If the rate is small, then it should be chlorinated.
The water content can also be reduced by vacuum filter or a filter press. To regulate the pH-value lime is added.
The sludge thickening or dewatering is adopted for reducing the volume of sludge or increasing the solid concentration for the following purposes:
(i) To permit increased loading to sludge digesters,
(ii) To increase feed solids concentration to vacuum filters.
(iii) To economise on transport costs as in ocean barging in case of raw sludge
(iv) To minimise the land requirements as well as handling costs when digested sludge has to be transported to disposal sites, and
(v) To save on the auxiliary fuel that may otherwise be needed when incineration of sludge is practised.
7. Thickening of Sludge:
Thickening of sludge by gravity is the most common practice for concentration of sludge. This is adopted for primary sludge or combined primary and activated sludge but is not suitable for alone activated sludge. This process is also not suitable or successful when the activated sludge exceeds 40% of the sludge weight.
Gravity thickners are either continuous flow or fill and draw type, with or without due of chemicals. Continuous flow tanks are deep circular tanks with central need and overflow at the periphery. These are designed for a hydraulic loading of 20,000 to 25,000 lpd/m2. If the loading rates are below 12,000 lpd/m2, they may create odour problems. If the normal sludge contain more solids, the sludge may be diluted with the plant effluent.
The recommended solid loading for primary, mixture of primary and trickling filter and mixture of primary and activated sludge (60 : 40 wt. ratio) are 100,60,40 kg/d/m2 respectively. With these loadings the underflow solids concentration shall be about 8-10% with primary, 6-8% with primary and trickling and 2-6% with primary and activated sludge.
The continuous thickness are mostly circular having 3 m water depth. Concentration of underflow solids is governed by the depth of sludge blanket upto 1m beyond which there is very little influence of the blanket.
Underflow solids concentration is increased with increased sludge detention time, 24 hours being required to achieve maximum compaction. The sludge blanket depths may be varied with fluctuation in solids production to achieve good compaction. During peak conditions, lesser detention times will have to be adopted to keep the sludge blanket depth sufficiently below the overflow weirs to prevent excessive solids carry over.
8. Dewatering of Sludge:
The Dewatering of concentrated sludge is done by continuous centrifugal process. In this process, the outer casing of a centrifuge contains a revolving solids bowl, in which a screw conveyor rotates at slightly lower speed. The sludge enters the bowl through the hub of the conveyor. When the cylinder revolves, the centrifugal force causes the solids to deposit on the wall of the bowl, from where it can be removed.
The sewage sludge can also be dewatered by pressing, aspirating or draining moisture from it. The various methods employed for this are filter presses, vacuum filters and drying beds. Out of these methods, the vacuum filters and drying beds are commonly used. Fig. 18.7 shows the vacuum filter, which essentially consists of a filter drum.
The outer surface of the drum is of copper mesh, over which filter cloth is stretched and wired. There is an inner, solid shell inside the drum. The inner space between the solid shell and the outer shell subdivided into compartments as shown in Fig. 18.7.
Each subdivided section can be connected to the vacuum pump. When the air is sucked from the compartment after immersing it in sludge reservoir, the sludge is lifted and comes on the filter cloth while the sludge liquor is filled in the section. When the drum rotates the sludge blanket is removed from the surface by means of scraper.
9. Elutriation of Sludge:
It is the process of washing the sludge to remove the organic and fatty acids. During the sludge digestion, the volatile acids, alcohols and organic acids are developed, which if not removed, will interfere with coagulation process during dewatering.
If elutriation of sludge is done before the dewatering or concentration, it will much reduce the quantity of coagulants. Sludge elutriation is carried out in single or multiple tanks by washing the sludge with water. During washing the solids are continuously kept in suspension by air or mechanical agitation.
There are three methods of elutriation viz. single stage, multistage and counter current washing, the water requirement being dependent upon the method used. For a given alkalinity reduction, single stage elutriation requires 2.5 times as much water as the two stage and 5 times as much water as counter current washing. Therefore single stage washing is used only for small plants. Counter current washing, although higher in initial cost, is adopted in all large plants.
Water requirement also depends on alkalinity of dilution water, alkalinity of elutriated sludge and desired alkalinity of elutriated sludge. Sludge and water are mixed in a chamber with mechanical mixing arrangement, keeping 20 sees, detention period. The sludge is then settled in settling tanks and excess water decanted. The maximum surface loading on settling tank may be 40 m3/m2/day with a detention period of 4 hours.
Counter current elutriation is generally carried out in twin tanks similar to sedimentation tanks, in which sludge and water enter at opposite ends. Piping and channels are so provided that wash water entering the second stage tank comes first in contact with sludge already washed in the first stage. About 2-3 times quantity of wash water is required than sludge volume elutriated.
The dosage of chemicals, detention period and flow of conditioned sludge to mechanical dewatering, units are automatically controlled by float switches so that these variables are adjusted on the basis of performance and the quality cake coming out.
10. Heat-Drying and Incineration of Sludge:
The dewatered sludge is in the shape of a thin paste and is offensive and unsafe for health. In this form, it cannot be supplied on a commercial basis for use as fertilizer. To do this, the sludge is heat-dried and the moisture contents are reduced less than 10%. The sludge is dried in rotary heat dryers kilns (direct or indirect) or cage mill (flash dryer). The drying can be done more efficiently if it is mixed up previously with dry sludge to reduce the moisture contents.
Several methods such as sludge drying under controlled heat, flash drying, rotary kiln, multiple hearth furnace etc. have been used in combination with incineration devices. Drying is brought about by directing a stream of heated air or other gases at about 350°C.
The hot gases dust and ash released during combustion are to be removed by suitable control mechanisms to minimise air pollution. The dried sludge removed from the kiln is granular and clinker-like which may be pulverized before use as soil conditioner.
If the fertilizing value on the sludge is not to be preserved, the incineration of the sludge may be done. For incineration, first the sludge is heat-dried and then it is kept in furnace. In practice multiple-health furnace is employed for incineration of the sludge.
In this furnace, the sludge cakes are fed from the top of the hearth which moves downward. During their descent the cakes lose moisture, ignite, burn and cool. Hearth temperature is maximum in the centre of the incinerator. The efficiency of the furnace varies from 45 to 70%.
11. Disposal of Sludge:
The disposal of sludge is usually done on hand as manure to the soil, or as a soil conditioner or dumping into sea. Sludge burial is usually resorted to form small quantities of putrescible sludge. Commonly sludge is used as a fertilizer. Ashes from incinerated sludge is used as a landfill.
In some cases wet sludge, raw or digested as well as supernatant from digester can be lagooned as a temporary measure but such practice may create problems like odour nuisance, ground water pollution and other hazards to public health. Wet or digested sludge can be used as sanitary land fill or for mechanised compositing with city refuse.
The use of raw sludge as a fertilizer directly on land for raising crops as a means of disposal is not desirable since it is fraught with health hazards.
Following points should be kept in mind while using sludge as fertilizer:
(a) Sludge from open air drying beds should not be used on soils, where it may come in direct contact of vegetables and fruits grown.
(b) Sludge from open air drying beds should not be used on soils, before raising crops. Soil should not be top dressed with sludge.
(c) Dried sludge may be used for lawns and for growing deep rooted cash crops and fodder grasses, where direct contact of edible part is low.
(d) From public health point of view, heat dried sludge is the safest, though deficient in humus, it is convenient for handling and distribution. It can be used along with farmyard manure.
(e) Raw or digested liquid sludge should not be used, because it is unsafe. It is also unsatisfactory as fertilizer or soil conditioner. If used, it must be thoroughly incorporated into the soil and the land should be given rest, so that biological transformation of organic material takes place. It should be used in such a way as to avoid all possible direct human contact.
In general, digested sludge are of moderate but definite value as a source of slowly available nitrogen and some phosphate. They are comparable to farmyard manure, but it has less percentage of potash.
These contain many essential elements of plant life and minor nutrients, in the form of trace metals. The sludge humus also increases the water holding capacity of soil and reduce soil erosion making an excellent soil conditioner specially in arid zones by making available needed humus content which results in greater fertility.
Sludge lagoons may be used for storage, digestion, dewatering and final disposal of dried sludge may be adopted in isolated locations where the soil is fairly porous and when there is no chance of ground water contamination. Under no circumstances drain water should be allowed to enter the lagoon.
The area and depth of water should be about double of that required for sand drying under comparable conditions. Depth ranges from 0.5 to 1 .5 m. On fill and draw basis lagoons are used for regular drying of sludge, then levelled out and used as lawns. These are also used for emergency storage, when digesters have to be emptied for repairs. But normally lagoons should not be used because they present an ugly sight and cause odour and mosquito breeding.
