The methods used to treat and dispose sludge are: 1. Thickening of Sludge 2. Digestion or Stabilisation 3. Conditioning 4. Dewatering 5. Drying 6. Incineration or Thermal Reduction 7. Ultimate Disposal.
1. Thickening of Sludge:
Sludge thickening is commonly achieved by the following three methods:
(i) Gravity Thickening:
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Gravity thickening is the most common practice for concentration of sludges as it is the simplest and the least expensive. This is adopted for primary sludge or combined primary and activated sludge but is not successful in dealing with activated sludge independently. Gravity thickening of combined sludge is not effective when activated sludge exceeds 40% of the total sludge weight and other methods of thickening of activated sludge have to be considered.
Gravity thickening is accomplished in a tank similar in design to a conventional sedimentation tank. Gravity thickeners are either continuous flow or fill and draw type, with or without addition of chemicals.
Continuous flow type thickeners are mostly deep circular tanks with a side water depth of about 3 m and a steeply sloping hopper bottom. Dilute sludge is fed to a centre feed well. The feed sludge is allowed to settle and compact, and the thickened sludge is withdrawn from the bottom of the tank.
Conventional sludge collecting mechanisms with deep trusses or vertical pickets stir the sludge gently, thereby opening up channels for water to escape and promoting densification. The continuous supernatant flow that results is returned to the primary settling tank. The thickened sludge that collects on the bottom of the tank is pumped to the digesters or dewatering equipment as required, and hence storage space must be provided for the sludge.
Continuous flow type thickeners are designed for a hydraulic loading of 20 000 to 25 000 lpd/m2. Loading rates lesser than 120000 lpd/m2 are likely to give too much solids to permit this loading and hence it is necessary to dilute the sludge with the plant effluent.
The surface loadings for various types of sludge are given in Table 16.2 along with solids concentration of various types of thickened sludges.
In a gravity thickener three distinct zones occur:
(i) Zone of clear supernatant at the top;
(ii) Feed zone characterized by hindered settling; and
(iii) Compression zone near the bottom where consolidation occurs.
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It has been observed that stirring serves to compact sludge in the compression zone by breaking up the floe and permitting water to escape. Hence better efficiencies can be obtained by providing slow revolving stirrers, particularly with gassy sludges (i.e., sludges full of gases).
In operation, a sludge blanket is maintained on the bottom of the thickener to aid in concentrating the sludge. Concentration of the underflow solids is governed by the depth of sludge blanket upto 1 m 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. 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 carryover.
(ii) Floatation Thickening:
Floatation thickening employs a unit operation called floatation which is used to separate solid or liquid particles from a liquid phase. Separation is brought about by introducing fine air bubbles into the liquid phase. The bubbles attach to the particulate matter, and the buoyant force of the combined particle and gas bubbles is great enough to cause the particle to rise to the surface.
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The particles thus floated to the surface are then collected by a skimming operation. Floatation thickening is used mainly for thickening or concentrating light biological sludges such as waste activated sludges which have a density very close to that of water and are thus readily buoyed to the surface.
Depending upon the method in which air bubbles are added or caused to be formed, there are four basic variations of the floatation-thickening operation, viz. – dissolved-air floatation; vacuum floatation; dispersed-air floatation; and biological floatation. However, only dissolved-air floatation is extensively used for sludge thickening.
In a dissolved-air floatation unit (or pressure type floatation unit) a portion of the subnatant effluent is pressurized from 290 to 490 k Pa (3 to 5 kg/cm2) by means of a pump with compressed air added at the pump suction, and held in a pressure tank for several minutes to allow time for air to dissolve.
The effluent from the pressure tank after passing through a pressure-reducing valve is mixed with unpressurized influent sludge just before it is admitted to the floatation tank. The pressure in the floatation tank being atmospheric, the excess dissolved air comes out of solution in the form of minute bubbles (with diameter upto 80 μ) throughout the entire volume of liquid in the floatation tank.
The air bubbles attach themselves to the sludge particles and rise up, thus carry sludge particles to the top where a blanket of thickened sludge thickness varying from 0.2 to 0.6 m is formed. The thickened sludge is then removed from the surface by skimmer mechanism, while the un-recycled subnatant is returned to the plant.
The supernatant effluent is recycled at the rate of 15 to 120% of the influent sludge.
The recycle ratio depends on:
(i) Feed solids concentration;
(ii) Detention time; and
(iii) Air/solids ratio.
Detention time in the floatation tank is not critical, provided that particles rise rapidly enough and the horizontal velocity does not scour the bottom of the sludge blanket. An air/solids ratio of 0.01 to 0.03 is sufficient to achieve acceptable thickening of waste activated sludge.
Surface overflow rates in floatation thickeners vary from 10 to 45 m3/day/m2 at detention time of 30 minutes to 1 hour. The efficiency of floatation units is increased by the addition of chemicals like alum and polyelectrolytes. The addition of polyelectrolytes does not increase the solids concentration but improves the solids capture from 90 to 98%.
(iii) Centrifugation:
Centrifugation or centrifugal thickening employs centrifuges which cause settling of sludge particles under the influence of centrifugal forces. Centrifuges are used both to thicken and to dewater sludges. Their application in thickening is normally limited to waste activated sludge.
The three basic types of centrifuges currently available for sludge thickening are:
(i) Nozzle-disc centrifuge;
(ii) Solid-bowl centrifuge- and
(iii) Basket centrifuge.
(i) Nozzle-Disc Centrifuge:
The operation of the nozzle-disc centrifuge is continuous. The centrifuge consists of a vertically mounted unit containing a number of stacked conical discs. Each disc acts as a separate low-capacity centrifuge. The liquid flows upward between the discs toward the centre shaft, become gradually clarified.
The solids are concentrated in the periphery of the bowl and are discharged through nozzles. Because of the small nozzle openings, these units must be preceded by sludge grinding and screening equipment to prevent clogging.
(ii) Solid-Bowl Centrifuge:
The operation of the solid-bowl centrifuge is also continuous. It consists of a long bowl, normally mounted horizontally and tapered at one end. Sludge is introduced into the unit continuously, and the solids concentrate on the periphery. A helical scroll, spinning at a slightly different speed, moves the accumulated sludge toward the tapered end where additional solids concentration occurs. The sludge is then discharged.
(iii) Basket Centrifuge:
The basket centrifuge operates on a batch basis. The liquid sludge is introduced into a vertically mounted spinning bowl. The solids accumulate against the wall of the bowl. The cent rate is decanted. When the solids-holding capacity of the machine has been achieved, the bowl decelerates and a scraper is positioned in the bowl to help remove the accumulated solids.
The method of centrifugal thickening involves high maintenance and power costs. Therefore this method is usually adopted at large sewage treatment, plants (over about 20 Mld) when space is limited and skilled operators are available, or for sludges that are difficult to thicken by more conventional means.
2. Sludge Digestion or Stabilization:
Sludge digestion is a biological process in which the organic matter present in the sludge is decomposed by micro-organisms and is thus stabilized. It is accomplished in sludge-digestion tanks or digesters, which are usually closed cylindrical tanks. The principal purposes of sludge digestion are to reduce its putrescibility or offensive odour, pathogenic contents and to improve it dewatering characteristics.
This can be achieved through any of the following biological processes:
(i) Anaerobic digestion
(ii) Aerobic digestion
(i) Anaerobic Digestion:
Anaerobic digestion is the biological degradation (i.e., conversion into simpler compounds) of organic matter in the absence of free oxygen. During this process, much of the organic matter is converted to methane (CH4), carbon dioxide (CO2) and water and therefore the anaerobic digestion is a net energy producer. Further since little carbon and energy remain available, to sustain further biological activity, the remaining solids in the sludge are rendered stable.
Microbiology of the Process:
Anaerobic digestion involves several successive biochemical reactions carried out by a mixed culture of micro-organisms. There are three degradation stages viz., hydrolysis, acid formation and methane formation. Fig. 16.5 shows, in simplified form, the reactions involved in anaerobic digestion.
In the first stage of digestion known as hydrolysis, the complex organic matter like proteins, cellulose, lipids are converted by extra cellular enzymes into simple soluble organic matter.
In the second stage known as acid fermentation, the soluble organic matter is converted by acetogenic bacteria also called ‘acid formers’, into acetic acid, hydrogen, carbon dioxide and other low molecular weight organic acids.
In the third stage known as methane fermentation, organic acids are converted into methane. In this stage two groups of methanogenic bacteria also called ‘methane formers’, which are strictly anaerobic, are active. While one group converts acetate into methane and bicarbonate, the other group converts hydrogen and carbon dioxide into methane.
For satisfactory performance of an anaerobic digester, the second and third stages of degradation should be in dynamic equilibrium, i.e., the volatile organic acids should be converted into methane at the same rate as they are produced.
However, methanogenic micro-organisms are inherently slow growing compared to the volatile acid formers and they are adversely affected by fluctuations in pH, concentration of substrates and temperature. Hence, the anaerobic process is essentially controlled by the methanogenic micro-organisms.
Advantages and Disadvantages of Anaerobic Digestion Process:
The various advantages of anaerobic digestion process are as indicated below:
1. Recovery of methane, a useful source of energy, as a by-product. The process is a net energy producer, since the energy content of the digester gas is more than the energy demand for mixing and heating of the digester contents.
2. Anaerobically digested sludge contains nutrients and organic matter that can improve the fertility and texture of soils.
3. Pathogens in the sludge die off during the relatively long detention periods used in anaerobic digestion.
The various disadvantages of anaerobic digestion process are as indicated below:
1. Relatively large closed tanks are required, resulting in high capital investment costs.
2. Micro-organisms involved in anaerobic digestion are sensitive to small changes in the environment. Close process control and performance monitoring are required to prevent upsets in the process.
3. Supernatant from anaerobic digestion often have a high oxygen demand and high concentration of nitrogen and suspended solids.
(ii) Aerobic Digestion:
Aerobic digestion is the biological degradation of organic matter in the presence of free oxygen provided by long-term aeration. The aerobic digestion is similar to the activated sludge process. As the supply of available substrate (food) is depleted, the micro-organisms begin to consume their own protoplasm to obtain energy for cell-maintenance reactions.
When this occurs, the micro-organisms are said to be in the endogenous phase. As such aerobic digestion may also be defined as a process in which microorganisms obtain energy by endogenous or auto-oxidation of their cellular protoplasm. The biologically degradable constituents of the cellular material are slowly oxidized to carbon dioxide, water and ammonia.
The ammonia from this oxidation is subsequently oxidized to nitrate as digestion proceeds. A pH drop can occur when ammonia is oxidized to nitrate if the alkalinity of the sewage is insufficient to buffer the solution. Theoretically, about 7.1 kg of alkalinity, expressed as CaCO3, are destroyed per kilogram of ammonia oxidized. In situations where the buffer capacity is insufficient, it may be necessary to install chemical feed equipment to maintain the desired pH.
Aerobic digestion produces volatile solids reductions comparable to those in anaerobic digestion, has low BOD in the supernatant, fewer operational problems and lower initial cost. Aerobic digestion is accomplished in one or more open tanks provided with diffused aeration system for supplying air necessary for sludge digestion and also for causing thorough mixing of the sludge.
Since aerating solids have a low rate of oxygen demand, the need for effective mixing rather than microbial metabolism usually governs the air supply required. The aerobic digesters are generally more economical to construct than the covered, insulated and heated anaerobic digesters.
However, long-term aeration of the waste-activated sludge creates a bulking material difficult to thicken. Aerobic digestion can be used for secondary tank humus or for a mixture of primary and secondary sludges but not for primary sludge alone.
The factors that should be considered in designing an aerobic digester include detention time, loading criteria, oxygen requirements, energy requirements for mixing, environmental conditions and process operation. Typical values of the various parameters adopted in the design of aerobic digesters are given in Table 16.6. The volatile solids destroyed in aerobic digestion in about 10 to 12 days’ time, at a temperature of 20°C would be 35 to 45%.
Higher temperature will result in reduction in the period of digestion. The oxygen requirement for the complete oxidation of cell tissue is 2 kg/kg of cell and that for the complete oxidation of the BOD5 contained in primary sludge varies from about 1.7 to 1.9 kg/kg of volatile solids destroyed. It is also desirable to maintain the dissolved oxygen between 1 and 2 mg/l in the system.
Advantages and Disadvantages of Aerobic Digestion Process:
The various advantages of aerobic digestion process are as indicated below:
1. Lower BOD concentration in digester supernatant.
2. Production of odourless and easily de-waterable biologically stable digested sludge.
3. Recovery of more basic fertilizer value in the digested sludge.
4. Lower capital cost.
5. Fewer operational problems.
The various disadvantages of aerobic digestion process are as indicated below:
1. Higher operating costs.
2. Gravity thickening processes following aerobic digestion tend to generate high solids concentration in the supernatant.
3. Some aerobically digested sludges do not dewater easily in vacuum Alteration.
4. No methane gas is produced for recovery as a by-product.
3. Sludge Conditioning:
Sludge is conditioned mainly to improve its dewatering characteristics. Prior conditioning of sludge before application of dewatering methods renders it more amenable to dewatering.
The two methods most commonly used for sludge conditioning are:
(i) Chemical conditioning, and
(ii) Heat treatment.
(i) Chemical Conditioning:
Chemical conditioning is the process of adding certain chemicals to the sludge which results in coagulation of the solids and release of the absorbed water, thereby facilitating easy extraction of water. The chemicals used are ferric chloride, lime, alum and organic polymers, the more common being ferric chloride with or without lime.
Chemical conditioning may be applied to a digested sludge or a raw sludge. However, digested sludge, because of its high alkalinity exerts a huge demand of chemicals and therefore the alkalinity has to be reduced to effect a saving on the chemicals. This can be accomplished by elutriation.
The choice of chemical depends on pH, ash content of sludge, temperature and other factors. Optimum pH values and chemical dosage for different sludges has to be based on standard laboratory tests. The dosage of ferric chloride and alum for elutriated digested sludge are of the order of 1.0 kg/m3 of sludge.
Alum when vigorously mixed with the sludge, reacts with the carbonate salts and release CO2 which causes the sludge to separate and water drains out more easily. Hence for effective results, alum must be mixed quickly and thoroughly. The alum floe is, however, very fragile and its usefulness has to be evaluated vis-avis ferric chloride. Polymers or polyelectrolytes are found to be quite useful for sludges with finely dispersed solids.
Feeding devices are necessary for applying chemicals. Mixing of chemicals with sludge should be gentle but thorough, taking not more than 20 to 30 seconds. Mixing tanks are generally of the vertical type for the small plants and of the horizontal type for large plants. They are provided with mechanical agitators rotated at 20 to 60 r.p.m.
Elutriation which literally means washing is a unit operation in which a solid or a solid-liquid mixture is intimately mixed with a liquid for the purpose of transferring certain components to the liquid. A typical example is the washing of digested sludge before chemical conditioning to remove certain soluble organic and inorganic components that would consume large amounts of chemicals.
In a digested sludge since alkalinity is present in high concentrations, the conditioning of the sludge would exert a huge demand of chemicals, which may be reduced by diluting the sludge with water of lower alkalinity (usually plant effluent) followed by sedimentation and decantation. Some end products of digestion such as ammonium bicarbonate which exert increased demand of chemicals in conditioning are removed in the process.
In the elutriation process sludge and water are mixed in a tank with mechanical mixing arrangement, the detention period being about 20 seconds. The sludge is then settled in settling tanks and excess water decanted. A maximum surface loading on settling tank of about 40 m3/m2/day and a detention period of about 4 hours are adopted.
Mixing and separating can be carried out either in the same tank or in separate tanks. Further each combination of mixing and washing is called a stage. For each stage usually separate tanks are used.
There are three methods of elutriation viz., single stage, multi-stage and countercurrent washing. The water requirement is 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. Hence single stage washing is used only in small plants. Countercurrent washing, although higher in initial cost, is adopted in all large plants. Water requirement also depends on alkalinity of dilution water, alkalinity of sludge and desired alkalinity of elutriated sludge.
Countercurrent elutriation is generally carried out in twin tanks similar to sedimentation tanks, in which sludge and wash water enter at opposite ends. Piping and channels are so arranged that wash water entering the second stage tank comes first in contact with sludge already washed in the first stage tank. The volume of wash water required is roughly 2 to 3 times the volume of sludge elutriated.
(ii) Heat Treatment:
Heat treatment is both a stabilization and a conditioning process in which sludge is heated for short periods of time under pressure. The treatment coagulates the solids, breaks down the gel structure, and reduces the water affinity of sludge solids. As a result the sludge is sterilized, practically deodorized, and is dewatered readily on vacuum filters or filter presses without the addition of chemicals.
The heat-treatment process is most applicable to biological sludges that may be difficult to stabilize or condition by other means. The high capital costs of equipment generally limit its use to large plants (more than 0.2 m3/s) or facilities where space may be limited.
The following two processes are used for heat treatment:
(i) Porteus process; and
(ii) Low-pressure Zimpro process.
(i) Porteus Process:
In the Porteus process, sludge is preheated in a heat exchanger before it enters the reactor vessel. Steam is injected into the vessel to bring the temperature to within the range of 145 to 200°C under pressures of 10 to 15 kg/cm2 (100 to 150 N/cm2). After a 30 minute detention in the reactor, sludge is discharged through the heat exchanger to a sludge separation tank. The sludge can be filtered through a vacuum filter to a solids content of 40 to 50% with filter yields of 100 kg/m2/hr.
(ii) Low-Pressure Zimpro Process:
In the low-pressure Zimpro process, the sludge is treated as in the Porteus process except that air is injected into the reactor vessel with the sludge. The reactor vessel is heated by steam to temperatures in the range of 150 to 200°C under pressures varying from 10 to 20 kg/cm2 (100 to 200 N/cm2). Heat released during oxidation increases the operating temperature to a range of 180 to 315°C.
The partially oxidized sludge may be dewatered by filtration, centrifugation, or drainage on beds. The solids content of the dewatered sludge can range from 30 to 50%, depending on the degree of oxidation desired. Essentially complete oxidation of volatile solids (approximately 90% reduction) can be accomplished with higher pressures and temperatures.
4. Sludge Dewatering:
Sludge dewatering is a physical (mechanical) unit operation used to reduce the moisture content of sludge.
It is carried out for one or more of the following reasons:
1. The costs for trucking sludge to the ultimate disposal site become substantially lower when sludge volume is reduced by dewatering.
2. Dewatered sludge is generally easier to handle than thickened or liquid sludge. In most cases, dewatered sludge may be shoveled, moved about with tractors fitted with buckets and blades, and transported by belt conveyors.
3. Dewatering is normally required prior to the incineration of the sludge to increase the calorific value by removal of excess moisture.
4. In some cases, removal of the excess moisture may be required to render the sludge totally odourless and non-putrescible. This is especially true for sludges stabilized by processes that create high strength recycle flow.
5. Sludge dewatering is commonly required prior to land filling to reduce leachate production at the landfill site.
Most of the digested primary or mixed sludge can be compacted to a water content of about 90% in the digester itself by gravity, but for further reduction of the water content, methods of dewatering of sludge are required to be adopted. The dewatering of digested sludge is accomplished either by air drying on open sludge drying beds or by mechanical methods such as vacuum filtration, centrifugation, pressure filtration, etc.
(i) Sludge Drying Beds:
This method of dewatering of sludge can be used in all places where adequate land is available and dried sludge can be used for soil conditioning. The method consists of applying the sludge in a 20 to 30 cm thick layer on specially prepared open beds of sand and gravel and allowed to dry. As shown in Fig. 16.12 the sludge drying bed usually consists of a bottom layer of gravel of uniform size over which is laid a bed of clean sand.
Open jointed tile underdrains are laid in the gravel layer to provide positive drainage as the liquid passes through the sand and gravel. Underdrains are made of vitrified clay pipes or tiles of at least 10 cm diameter laid with open joints. Underdrains are placed not more than 6 m apart.
Around the underdrains graded gravel is placed in layers upto 30 cm with a minimum of 15 cm above the top of the underdrains. At least 3 cm of the top layer should consist of gravel of 3 to 6 mm size. Clean sand of effective size 0.5 to 0.75 mm and uniformity coefficient not greater than 4.0 is placed over the gravel. The depth of sand may vary from 20 to 30 cm. The finished sand surface should be level.
The drying beds are commonly 6 to 8 m wide and 30 to 45 m long. A length of 30 m away from the inlet should not be exceeded with a single point of wet sludge discharge, when the bed slope is about 0.5%. Multiple discharge points should be used with large sludge beds to reduce the length of wet sludge travel. In order to have flexibility in operation, beds should be atleast two in number.
The area needed for dewatering and drying the sludge depends on the volume of the sludge, cycle time required to retain sludge for dewatering, drying and removal of sludge and making the sand bed ready for next cycle of application and depth of application of sludge on drying bed.
The cycle time between two dryings of sludge on drying beds primarily depends on the characteristics of sludge including factors affecting its ability to allow drainage and evaporation of water, the climatic parameters that influence evaporation of water from sludges and the moisture content allowed in dried sludge.
The cycle time may vary widely, lesser time required for aerobically stabilized sludges than for anaerobically digested sludge and for hot and dry weather conditions than for cold and/or wet weather conditions.
Areas required for drying beds range from 0.1 to 0.15 m2 per capita with dry solids loading of 80 to 120 kg/m2 of bed per year for digested primary sludge, and from 0.175 to 0.25 m2 per capita with dry solids loading of 60 to 120 kg/m2 of bed per year for digested mixed sludge. The average cycle time for drying may range from a few days to 2 weeks in warmer climates to 3 to 6 weeks or even more in un-favourable ones.
The sludge inlets to the drying bed are so arranged to easily drain and have a minimum diameter of 20 cm terminating at least 30 cm above the sand surface. Splash plates should be provided at discharge points to spread the sludge uniformly over the bed and to prevent erosion of the sand. Drainage from beds should be returned to the primary settling tanks if it cannot be satisfactorily disposed of otherwise.
When digested sludge is deposited on well drained bed of sand and gravel, the dissolved gases tend to buoy up and float the solids leaving a clear liquid at the bottom which drains through the sand rapidly. The major portion of the liquid drains off in a few hours after which drying commences by evaporation.
The sludge cake shrinks producing cracks which accelerates evaporation from the sludge surface. The areas having greater sunshine, lower rainfall and lesser relative humidity, may have drying time of about two weeks, while in other areas it could be four weeks or more.
Dried sludge cake can be removed by shovel or forks when the moisture content is less than 70%. When the moisture content reaches 40% the cake becomes lighter and suitable for grinding. Some sand always clings to the bottom of the sludge cake and results in loss of sand thus reducing the depth of the bed. When the depth of the bed is reduced to 10 cm, clean coarse sand which matches the original sand, should be used for replenishment tg the original depth of the bed.
Wheel barrows or pickup trucks are used for hauling of sludge cakes. In large plants mechanical loaders and conveyors may be required to handle large quantities of dried sludge. Sludge removed from the bed may be disposed of directly or stored to make it friable, thereby improving its suitability for application to soil.
(ii) Mechanical Methods:
i. Vacuum Filtration:
It is the most common mechanical method of dewatering, centrifugation and filter presses being the other methods. Chemical conditioning is normally required prior to mechanical methods of dewatering. Mechanical methods may be used to dewater raw or digested sludges preparatory to heat treatment or before burial or landfill.
Raw sludge is more amenable to dewatering by vacuum filtration because the coarse solids are rendered fine during digestion. Hence filtration of raw primary or mixture of primary and secondary sludges permits slightly better yields, lower chemical requirement and lower cake moisture contents than filtration of digested sludges.
When the ratio of secondary to primary sludges increases, it becomes more and more difficult to dewater in the filter. The feed solids concentration has a great influence, the optimum being 8 to 10%. Beyond 10%, the sludge becomes too difficult to pump and lower solids concentration would demand unduly large filter surface. In this method, conditioned sludge is spread out in a thin layer in the filtering medium, the water portion being separated due to the vacuum and the moisture content is reduced quickly.
ii. Vacuum Filters:
Vacuum filter consists of a cylindrical drum over which is laid a filtering medium of wool, cloth or felt, synthetic fibre or plastic or stainless steel mesh or coil springs. The drum is suspended horizontally so that one quarter of its diameter is submerged in a tank containing sludge. Valves and piping are arranged to apply a vacuum on the inner side of the filter medium as the drum rotates slowly in the sludge.
The vacuum holds the sludge against the drum as it continues to be applied as the drum rotates out of the sludge tank. This pulls water away from the sludge leaving a moist cake mat on the outer surface. The sludge cake on the filter medium is scraped from the drum just before it enters the sludge tank again.
The filtration rate is expressed in kg of dry solids per square metre of medium per hour. It varies from 10 kg/m2/hr for activated sludge alone to 50 kg/m2/hr for primary sludges. Filter drums are rotated at a speed of 7 to 40 revolutions per hour with a vacuum range of 500 to 650 mm of mercury. The filter run does not exceed 30 hours per week in small plants to allow time for conditioning, clean up and delays.
At large plants it may work for 20 hours a day. The moisture of the filtered cake varies normally from 80% in the case of raw activated sludge to 70% for digested primary sludges. Filters should be operated to produce a cake of 60 to 70% moisture if it is to be heat dried or incinerated. At the end of each filter run, the filter fabric is cleaned to remove sticking sludge. A jet of water is used to clean the filter medium.
The operating costs of vacuum filters are usually higher than for sludge drying beds. However, they require less area since dewatering is rapid. The operation is independent of weather conditions and it can be used for dewatering even raw or partially digested sludges requiring drying or incineration.
iii. Centrifugation:
The centrifugal devices viz., nozzle-disc, solid-bowl and basket centrifuges used for sludge thickening. Sludge dewatering may be accomplished by solid-bowl and basket centrifuges.
In a solid-bowl centrifuge sludge is fed at a constant flow rate into the rotating bowl where it separates into a dense cake containing the solids and a dilute stream called centrate. The centrate contains fine, low- density solids and is returned to the untreated-sludge thickener or primary settling tank.
The sludge cake, which contains approximately 75 to 80 percent moisture, is discharged from the bowl by a screw feeder into a hopper or onto a conveyer belt. Depending on the type of sludge, solids concentration in the cake varies from 10 to 40%, but reductions below 25% are not usually feasible economically. The cake can then be disposed of by incineration or by hauling to the landfill site.
Solid-bowl centrifuges are generally suitable in the same applications as vacuum filters. Their performance is governed by the same factors that affect vacuum filters: type and age of sludge, prior sludge processing, etc. They can be used to dewater sludges with no prior chemical conditioning, but the solids capture and centrate quality are considerably improved when sludges are conditioned with polyelectrolytes.
For chemically conditioned sludges the solids capture rates may be 90% or more. Chemicals for conditioning are added to the sludge within the bowl of the centrifuge. Dosage rates for conditioning with polyelectrolytes vary from 1.0 to 7.5 kg per 103 kg of dry solids.
Basket centrifuges have been used for partial dewatering at small plants. They can be used to concentrate and dewater waste activated sludge, with no chemical conditioning, at solids capture rates upto 90%. However, basket centrifuges are not commonly used for dewatering of sludges.
The operation of centrifuges is simple, clean, relatively inexpensive, and normally does not require chemical conditioning. However, the major difficulty encountered in the operation of centrifuges is the disposal of the centrate, which is relatively high in suspended, non-settling solids. The return of the centrate to the sewage treatment plant may result in large recirculating load of these fine solids through the sludge and primary settling system and in reduced effluent quality.
iv. Filter Presses:
In a filter press, dewatering is achieved by forcing the water from the sludge under high pressure. Various types of filter presses have been used to dewater sludge. One type of filter press consists of a series of rectangular plates, recessed on both sides that are supported face to face in a vertical position on a frame with a fixed and movable head.
A filter cloth is hung or fitted over each plate. The plates are held together with sufficient force to seal them to withstand the pressure applied during the filtration process. Hydraulic ram or powered screws are used to hold the plates together.
In operation, chemically conditioned sludge is pumped into the space between the plates, and pressure of 4 to 15 kg/cm2 (40 to 150 N/cm2) is applied and maintained for 1 to 3 hours, forcing the liquid through the filter cloth and plate outlet ports. The plates are then separated and the sludge is removed. The filtrate is normally returned to the head works of the treatment plant.
The sludge cake thickness varies from about 2.5 to 3.8 cm, and the moisture content varies from 55 to 70%. The filtration cycle time varies from 2 to 5 hours and includes the time required to fill the press, the time the press is under pressure, the time to open the press, the time required to wash and discharge the cake and the time required to close the press.
The advantages of filter press include:
(i) High concentrations of cake solids,
(ii) Filtrate clarity,
(iii) Solids capture, and
(iv) Chemical consumption.
The disadvantages of filter press include:
(i) High labour costs,
(ii) Life of filter cloth being limited needs frequent replacement involving a high cost, and (iii) Prior to filtration chemical conditioning of sludge is necessary resulting in higher operational cost.
5. Drying:
In sludge drying beds, the sludge is dried by evaporation to such an extent that the volume as well as weight of the sludge are reduced considerably and the dried sludge cake can be processed into fertilizer by grinding or it can be incinerated efficiently. However, in mechanical methods of dewatering the sludge does not get dried to the same extent and hence these methods are usually followed by heat drying and/or incineration.
Heat Drying:
The purpose of heat drying is to reduce further the moisture content and volume as well as weight of the dewatered sludge, so that it can be used after drying without causing offensive odours or risk to public health, or it can be incinerated efficiently. Several methods such as sludge drying under controlled heat, flash drying, rotary kiln, multiple hearth furnaces, 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 minimize air pollution. The dried sludge removed from the kilns is granular and clinker-like which may be pulverized before use as soil conditioner.
6. Incineration:
The purpose of incineration is to destroy the organic material, the residual ash being generally used as landfill. During the process all the gases released from the sludge are burnt off and all the organisms are destroyed. Dewatered or digested sludge is subjected to temperatures between 650°C and 750°C. Cyclone or multiple hearth and flash type furnaces are used with proper heating arrangements with temperature control and drying mechanisms. Dust, fly ash and soot are collected for use as landfill.
Incineration has the advantages of economy, freedom from odours and a great reduction in volume and weight of materials to be disposed of finally. The process, however, requires high capital and recurring costs, installation of machinery and skilled operation. Controlled drying and partial incineration have also been employed for dewatering of sludges before being put on conventional drying beds.
7. Final Disposal of Sludge:
Sludge (either wet, dry or incinerated) can be finally disposed of by the following methods. 1. Spreading on Farm Land 2. Dumping 3. Land Fill4. Sludge Lagooning 5. Disposal in Water or Sea.
1. Spreading on Farm Land:
Sludge may be disposed of on land as a fertilizer for raising crops. However, the use of raw sludge as a fertilizer directly on land for raising crops is not desirable since it is fraught with health hazards.
Application of sludges to soils should take into consideration the following guiding principles:
(i) Sludge from open air drying beds should not be used on soils where it is likely to come in direct contact with the vegetables and fruits grown.
(ii) Sludge from drying beds should be ploughed into the soil before raising crops. Top dressing of soil with sludge should be prohibited.
(iii) Dried sludge may be used for lawns and for growing deep rooted cash crops and fodder grasses where direct contact with edible part is minimum.
(iv) Heat dried sludge is the safest from public health point of view. Though deficient in humus, it is convenient for handling and distribution. It should be used along with farmyard manure.
(v) Liquid sludge either raw or digested is unsafe to use. It is unsatisfactory as fertilizer or soil conditioner. However, if used, it must be thoroughly incorporated into the soil and 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.
The liquid sludge is usually introduced through shallow trenches 0.5 to 0.9 m wide and 0.3 to 0.4 m deep provided on the farm land about 1.0 to 1.5 m apart. When water gets evaporated a sludge cake is formed which is covered with a thin layer of dry soil. After about a month the land is ploughed and used for cultivation.
In general digested sludges are of moderate value as a source of slowly available nitrogen and some phosphate. They are comparable to farmyard manure except for deficiency in potash. They also contain many elements essential to plant life and minor nutrients in the form of trace metals. The sludge humus also increases the water holding capacity of soil and reduces soil erosion making an excellent soil conditioner specially in arid regions by making available needed humus content which results in greater fertility.
2. Dumping:
The sludge may be disposed of in an abandoned mine/quarry. This method may be adopted only for sludges that have been stabilized so that no decomposition or nuisance conditions will result. Thus this method can be safely adopted for digested sludge, clean grit and incinerator residue.
3. Land Fill:
If a suitable site is available a sanitary land fill can be used for disposal of both stabilized or unstabilized sludges. However, dewatering is recommended before such disposal so that the cost of hauling the sludge to the site of disposal is reduced.
The following points should be considered when land fill is adopted for disposal of sludge:
(i) When organic solids are placed in a land fill, decomposition may result in odour if sufficient cover is not available. Decomposition may also result in soil settlement resulting in surface water ponding above the fill.
Besides this surface water contamination and leaching of sludge components to the groundwater must be considered. Typical depths of cover of clean soil which should be provided over the fill area are 0.2 m after each daily deposit and 0.6 m over an area that has been filled completely.
(ii) Surface topography should be finished to allow rainfall to drain away and not allow it to infiltrate into the solid land fill.
(iii) Land fill leachate requires long term monitoring and should satisfy water pollution standards. Vegetation must be established quickly on completed areas to provide for erosion control. It is general practice not to crop the land fill area for a number of years after completion.
(iv) Ash from incinerated sludge is generally disposed of as land fill.
4. Sludge Lagooning:
A lagoon is a shallow earth basin which may be used for storage, digestion, dewatering and final disposal of dried sludge. This method may be adopted for untreated or digested sludge. The organic matter present in untreated sludge is stabilized in lagoons by anaerobic and aerobic decomposition which may give rise to objectionable odours. Hence the lagoons are usually located away from towns where the soil is fairly porous and there is no chance of groundwater contamination.
Fig. 16.13 shows a section of a typical sludge lagoon. Drainage water should not be allowed to enter the lagoon and hence earth bank is constructed along the periphery of lagoon. The depth of lagoon may range from 0.5 to 1.5 m. The depth of lagoon and its area should be about twice that required for sand drying under comparable conditions.
Tile drains of about 10 cm diameter are placed at 3 m centres at the bottom of the lagoon, and a 15 cm thick layer of ash or clinker is placed over the drains to facilitate draining of water from the wet sludge. Lagoons may be used for regular drying of sludge on a fill and draw basis or allowed to fill, dry and then levelled out and used as lawns.
In the former case the detention time may vary from 1 to 2 months and after the sludge has been stabilized and the water is drained/evaporated, the contents of the lagoon are dug out and used as manure or disposed of otherwise, and the lagoon is again used. Lagoons have also been employed as emergency storage when digesters have to be emptied for repairs.
As lagoons are less expensive to build and operate, they have been resorted to, particularly for digested sludge in areas where large open land suitably located is available. However, the use of lagoons is not generally desirable as they present an ugly sight and cause odour and mosquito breeding.
5. Disposal in Water or Sea:
This is not a common method of disposal of sludge because it is contingent on the availability of a large body of water adequate to permit dilution. At some sea coast sites the sludge, either raw or digested, may be barged to sea far enough to make available the required dilution and dispersion. The method requires careful consideration of all factors for design and siting of outfall to prevent any coastal pollution or interference with navigation.
