Read this article to learn about the treatment and disposal processes of sludge and solid wastes.
Sewage sludge is a semi-liquid mass that is produced during the course of sewage/waste water treatment processes. Sludge may be regarded as a semisolid liquid that contains solids in the range of 0.5 to 12 per cent (by weight).
The composition of the sludge is highly variable and depends on the source of raw sewage and its treatment processes. Some authors use the term organic slurries for sewage sludge.
Sources and Characteristics of Sludge:
The following unit operations are the major sources of sludge:
ii. Grit removal
iii. Primary sedimentation
iv. Secondary sedimentation
v. Sludge-processing units.
The important sludge’s and their characteristics are given in Table 58.1. The characters of sludge are variable and depend on the origin of the sludge, and the aging after it is produced.
Chemical composition of sludge:
The chemical composition of sludge is variable. The major constituents of sludge are proteins and other nitrogen containing compounds, grease and fats, cellulose, hydrocarbons, phosphorus, iron, silica, organic acids, heavy metals, pathogens and pesticides. The various methods of sludge treatment and disposal are given in Table 58.2. The important aspects of selected processes are briefly described.
The preliminary operations are carried out to provide a constant and homogeneous sludge for further processing.
Grinding of sludge is carried out to cut a large mass of sludge into small pieces. For this purpose, hardened steel cutters and overload sensors are used.
Sometimes, removal of grit (de-gritting) is necessary for the effective treatment of sludge. De-gritting can be carried out by applying centrifugal force to the flowing sludge. Cyclone degritters are usually employed for this purpose to separate grit particles from the organic sludge.
Sludge produced in primary (settleable solids), secondary (settleable solids and biological solids) and tertiary (biological solids and chemical solids) treatments is mixed (blended) to produce a uniform mixture.
Storage of sludge becomes necessary when the processing units are not in operations (night shifts, weekends). Short-term storage of sludge is accomplished in settling tanks while for long- term Storage, specially designed tanks have to be used.
Sludge Thickening (Concentration):
The solid content of the sludge is variable and may range from 0.5 to 12%. It is necessary to concentrate the sludge (particularly with low solid content) by removing some amount of liquid. Physical methods are employed for sludge thickening.
Gravity thickening is the most commonly used method for the thickening of primary sludge. It can be carried out in the conventional sedimentation tank (circular tank is preferred). As the sludge gets concentrated by gravity, the supernatant can be returned to the treatment plant (i.e. primary settling tank).
Flotation thickening is the technique of choice for the treatment of sludge’s from suspended-growth biological treatment processes. There are three types of flotation thickening processes — dissolved air flotation, dispersed-air flotation, and vacuum flotation. Among these, dissolved-air flotation is widely used.
In this technique, air is passed through the sludge that is held at high pressure. The air raises to the top of the sludge in the form of bubbles along with attached particles to form a sludge blanket. This can be skimmed off at regular intervals. The efficiency of flotation thickening depends on the air to solids ratio.
Centrifugal thickening is based on the principle of settlement of sludge particles under the influence of centrifugal forces. Two types of centrifuges — solid bowl centrifuge and basket centrifuge are used for this purpose. Although concentration of sludge by centrifugal thickening is efficient, it involves high cost of maintenance and power consumption.
Rotary drum thickening:
Thickening of the sludge can be carried out by use of rotary drums. As the sludge is passed over the rotating screen drums, the solids separate from the water and the thickened sludge rolls out at the drum ends.
Sludge requires stabilization to achieve the following objectives:
i. To reduce the load of disease-causing organisms (pathogens).
ii. To inhibit the potential for putrefaction.
iii. To eliminate offensive odours.
Sludge stabilization can be carried out by appropriate biological and chemical means. The techniques used for sludge stabilization are — lime stabilization, heat treatment, anaerobic digestion, aerobic digestion and composting. They are briefly described.
When lime, in the form of hydrated lime [Ca(OH)2] or quick lime (CaO) is added to the sludge, the pH raises to 12 or higher. At high pH, the microorganisms cannot grow. Consequently, the sludge will not putrify, create unpleasant odours and does not pose health hazards.
In the lime pretreatment, addition of lime is done prior to dewatering, while in lime post- treatment, it is done after dewatering.
Heat treatment involves heating of sludge for a short period (250°C for about 30 minutes) under pressure. By this process, sludge is dewatered and sterilized. Heat treatment helps in stabilization and conditioning of the sludge. However, due to high cost, it is less frequently used.
Anaerobic sludge digestion:
Anaerobic digestion of sewage is one of the oldest forms of biological treatment processes. The same operating unit is equally useful for the stabilization of sludge. There are different ways of carrying out anaerobic sludge digestion — standard rate digestion, single-stage high rate digestion and two-stage digestion.
Standard rate digestion:
This is a single-stage digestion process. The sludge is added and heated by external heat exchangers. As the sludge gets digested, the gas released comes out to the surfaces. Along with the gas, certain sludge particles (fats, oils, grease) also raise to the top to form a scum which can be removed.
Single-stage high rate digestion:
This is characterized by high rate of sludge loading, and its digestion under anaerobic conditions. The sludge is continuously mixed by gas, heated and recirculated for optimal digestion.
There are two digestion tanks used in this process of high-rate digestion. The first tank is used for digestion proper (with mixing and heating facilities) while the second digester is employed for storage and concentration.
Thermophilic anaerobic digestion:
Sometimes, sludge digestion can be carried out at relatively higher temperature (45-60°C) by thermophilic bacteria. In fact, the digestion process is faster with increased bacterial destruction and improved dewatering at higher temperature. But the major limitation is the requirement of high energy for continuous heating. For this reason, thermophilic anaerobic digestion process is not widely used.
Aerobic sludge digestion:
Aerobic digestion of sludge’s from various sources (primary, secondary and tertiary treatments) is becoming popular in recent years for the following reasons:
i. The digestion of volatile solids is as effective as in anaerobic digestion.
ii. Fertilizer value of the sludge is much higher.
iii. Lower capital and operational costs.
However, the major limitation in aerobic digestion is the continuous supply of O2 which is a costly affair. The process, aerobic digestion of sludge is comparable to activated sludge process
Principle of aerobic digestion:
When the nutrient (substrates) supply to the microorganisms is depleted, they start oxidizing their own cellular (protoplasmic) materials for energy supply and maintenance. About 70-80% of the cellular organic matter can be oxidized aerobically to produce carbon dioxide, nitrate (NO–3) and water. Approximately 20-30% of the cellular organic matter cannot be oxidized, as it is not biodegradable.
A diagrammatic representation of an aerobic digester is depicted in Fig. 58.1. The process can be carried out by batch or continuous flow system of operation. With regard to the aeration process, aerobic digestion is of two types—conventional aerobic digestion and high purity oxygen aerobic digestion.
Conventional aerobic digestion:
Atmospheric air is used in the conventional digester for the process of aerobic digestion.
High-purity oxygen aerobic digestion:
In this case, a pure grade of oxygen instead of air is used. The digestion process is usually carried out in closed tanks.
Thermophilic aerobic digestion:
This is refinement of the above two techniques, and can effectively digest up to 20% of the biodegradable organics. The process is carried out by thermophilic bacteria at a temperature around 45°C. There are certain advantages of thermophilic aerobic digestion. These include killing of more pathogenic organisms and digestion of more solids, with minimal unpleasant odours in stabilized sludge.
Composting basically involves the process of biological degradation of solid organic waste material to stable end products. Although composting can be carried out under aerobic and anaerobic conditions, aerobic composting is more frequently used. Thus, composting may be considered as an aerobic microbiological process for converting solid organic wastes into stable sanitary, nuisance free, humus like material that can be safely disposed into the environment.
Composting is a cost-effective and environment friendly process for stabilization and ultimate disposal of sludge. The product of composting is useful for soil improvement and the production of mushrooms. Thus, composting is ultimately helpful for reuse and recycling of organic waste materials from domestic, agriculture and industry.
Organisms involved in composting:
A wide variety of organisms (both unicellular and multicellular) are involved in the process composting. The bacteria make up 80-90% of the microorganisms found in the compost. These bacteria possess a broad range of enzymes to degrade a wide range of organic compounds. The other organisms actively involved in composting are actinomycetes (a filamentous type of bacteria), fungi (molds, yeasts), and protozoa, besides earthworms, insects, mites and ants.
Mechanism of Composting:
Composting is a very complex process involving the participation of several microorganisms— bacteria, actinomycetes and fungi. The bacteria bring out the decomposition of macromolecules namely proteins and lipids, beside generating energy (heat). Fungi and actinomycetes degrade cellulose and other complex organic compounds. Composting may be divided into three stages with reference to changes in temperature— mesophilic, thermophilic and cooling.
The fungi and acid-producing bacteria are active in this stage, and the temperature increases from ambient to about 40°C.
As the composting proceeds, the temperature raises from 40°C to 70°C. Thermophilic bacteria, thermophilic fungi and actinomycetes are active in this stage. Thermophilic stage is associated with high rate and maximum degradation of organic materials.
The microbial degradative activity slows down and the thermophilic organisms are replaced by mesophilic bacteria and fungi. Cooling stage is associated with formation of water, pH stabilization and completion of humeic acid formation.
Methods of Composting:
The operation of composting involves the following steps:
i. Mixing of dewatered sludge with a bulking agent (saw dust, rice hulls, straw or recycle compost). The bulking agent improves the porosity of the mixture for good aeration.
ii. Creating aerobic conditions (aeration) by mechanical or other means. This is needed for the supply of oxygen, to control temperature, and for the removal of water (moisture).
iii. Removal of the bulking agent, if possible.
iv. Storage and disposal of the compost.
There are three major methods of composting — aerated static pile, windrow, and in-vessel systems. As per a recent survey, the approximate distribution of different composting methods is given.
Aerated static pile – 55%
Windrow – 30%
In-vessel – 15%
Aerated static pile system:
The dewatered sludge is mixed with a bulking agent (wood chips) and placed over a grid of aeration or exhaust piping. Air is supplied through blowers for efficient aeration. A layer of compost is kept over the top of the aerated static pile for insulation and good aeration (Fig. 58.2). It takes about 3-4 weeks for composting, and another 4-5 weeks for curing. The cured compost is screened to reduce the quantity, besides the recovery of the bulking agent.
Windrows are a type of static piles with periodical turning and mixing of sludge during the composting period (3-4 weeks). The mixing is usually carried out at weekly intervals and is associated with release of unpleasant and offensive odours. The windrows may be open or covered and the aeration is carried out by mechanical means.
In the in-vessel composting system, the process is carried out in a closed vessel or container. This is an advanced method and is designed to control the environmental conditions (temperature, air flow and O2 supply), besides minimizing the release of offensive odours. The advantages of in-vessel system include higher efficiency of composting, with lower labour costs, and smaller area for the plant.
There are two major systems of in-vessel composting — plug flow and dynamic.
Plug-flow in-vessel composting reactors:
There are two types of plug-flow in-vessel reactors— cylindrical tower and tunnel (Fig. 58.3A and 58.3B). In both the cases, the relationship between the particles in the composting mass is maintained the same throughout the process. As the sewage is fed, the composting occurs on the principle of first in and first out.
Dynamic in-vessel composting reactors:
These are also known as agitated bed in-vessel composting reactors. In these units, the composting material is mechanically mixed during the period of processing. Dynamic circular reactors and dynamic rectangular reactors (Fig. 58.3C and 58.3D) are in common use.
In circular reactors, the augurs rotate around the centre of the reaction vessel and mix the composting material. As regards the rectangular reactors, extraction conveyor is responsible for mixing the compost and for the discharge of the compost to the outlet conveyor.
Factors affecting aerobic composting:
There are several factors that influence the sludge-composting process.
The important ones are briefly discussed:
1. Type of sludge:
Both untreated and digested sludge’s can be composted. However, untreated sludge requires more oxygen, and emits unpleasant odours.
2. Carbon nitrogen ratio:
For efficient composting, the carbon nitrogen ratio should be in the range of 25: 1 to 35: 1. It is desirable to periodically check and maintain this ratio.
3. Bulking agents:
Cheap and readily available bulking agents (saw dust, wood chips) are used. Their particle size and moisture content influence composting.
4. Moisture content:
A moisture content of sludge less than 60% is ideal for composting.
It is desirable that the oxygen supply is constantly maintained, and it properly reaches the composting material.
6. Temperature and pH:
Best results of composting are seen when the temperature is between 45-55°C. If the temperature goes beyond 60°C, the process almost gets halted. The optimal pH is between 6-9.
7. Mixing and turning:
For appropriate composting, mixing and turning are required. This prevents drying and caking of the compost.
Vermicomposting refers to the process of compost formation by earthworms. In fact, earthworms are known to play a significant role in the natural cycling of soil organic matter and maintenance of porosity of the soil.
Earthworms are very efficient in nutrient recycling. They can consume organic matter, approximately 10-20% of their own biomass per day. Earthworms can utilize organic matter with variable carbon nitrogen ratio (C: N ratio) and convert to lower C: N ratio. In other words, vermicomposting involves the conversion of carbon-rich organic compounds to nitrogen-rich organic compounds. This is highly advantageous for soil enrichment.
In recent years, vermicomposting of cow and buffalo dung has become a profitable, low (bio) technology industry. The earthworm namely Drawidia nepalensis is most commonly used for this purpose. Vermicomposts are commercially available now. Introduction of earthworms into the soils to bring out a natural process of vermicomposting (for soil enrichment) is also advocated.
Conditioning of Sludge:
Conditioning of sludge is necessary to improve its dewatering characteristics. Chemical and heat treatment methods are most commonly used for this purpose. The other conditioning methods include irradiation, freezing and solvent extraction, which are less frequently used.
By use of chemicals, certain solids in the sludge can be coagulated with a release of absorbed water. Chemical conditioning can reduce the moisture content of the sludge by about 15-30% (i.e. from about 95% to about 65%). The most commonly used chemicals are alum, lime, ferric chloride and organic polymers.
Heat treatment method:
Heat treatment can stabilize and condition the sludge. This process is carried out for a short period under pressure. Heat treatment results in the coagulation of solids, besides reducing the water affinity of sludge solids.
Disinfection of Sludge:
In recent years, disinfection of sludge is becoming significant due to its reuse or its application on the land. This is because the pathogenic organisms of the sludge should be destroyed to protect the health of the inhabitants who are exposed to it.
There are a large number of disinfection methods to choose:
ii. Heat drying
iv. High pH treatment
v. Addition of chlorine
vi. Long-term storage of digested sludge
vii. Complete composting.
Dewatering basically involves the process of reducing the moisture content of sludge. Dewatering can be carried out by vacuum filters, centrifuges and sludge drying beds.
Dewatering by vacuum filtration is rather old, but has been discontinued in recent years due to high operating and maintenance costs, besides the complexicity of the process. In fact, improved and more efficient alternative methods have been developed during the past ten years.
Centrifugation is commonly used for the dewatering of sludge’s obtained from industries. By the process of centrifugation, it is possible to remove liquids from solids. Different types of centrifuges (solid bowel centrifuge, basket centrifuge) are commercially available for sludge dewatering.
Sludge drying beds:
Drying beds are widely used in developed countries (USA, Britain) to dewater the digested sludge. This method produces high solid containing dried product which can be easily disposed off (in a landfill or as soil conditioner). Sludge drying beds are cost-effective.
When the sludge is subjected to mechanical heat drying, the water content can be substantially reduced. The ultimate purpose of heat drying is to prepare a sludge, free from moisture that can be incinerated efficiently. Mechanical heat drying can be carried out by flash dryers, spray dryers, rotary dryers and multiple hearth dryers.
Thermal Reduction of Sludge:
Thermal reduction basically involves the total or partial conversion of organic solids to oxidized end products such as carbon dioxide and water. This may be carried out by incineration or by wet-air oxidation. Thermal reduction of sludge is associated with destruction of pathogenic organisms, detoxification of toxic compounds and reducing the volume of disposable sludge. The major processes employed for thermal reduction of sludge are multiple-hearth incineration, fluidized-bed incineration and wet-air oxidation.
Ultimate Disposal of Sludge:
While considering the final disposal of sludge, its beneficial uses are first taken into account. Sludge is useful for the supply of nutrients to the soil, besides possessing the properties of soil conditioning. Thus, attempts are made to dispose the sludge in a beneficial manner. If this is not possible, alternates are considered.
Land applications of sludge (as a fertilizer):
The spreading of sludge on or just below the soil surface is considered as land applications of sludge. Sludge may be used in the agricultural lands, forest lands, and dedicated land disposal sites. The pathogens and toxic organic compounds present in the sludge can be respectively destroyed by sunlight and soil microorganisms. Sludge applied to land is thus useful as a soil conditioner to improve the characteristics of land-nutrient transport facilitation and increased water retention. Thus, sludge can replace the expensive fertilizers.
The quantity of organic materials and the pathogens must be reduced before the sludge is applied on the land. A high content of organic matter will result in offensive odours while the pathogens spread diseases. There are in fact regulatory requirements to control pathogens of sludge by various means.
Distribution and Marketing:
Distribution and marketing of sludge for beneficial purposes is gaining importance in recent years. It is estimated that about 10-20% of the total sludge produced is utilized in this fashion. The marketed sludge is used as substitution for topsoil and peat on parks, lawns, golf courses and in ornamental and vegetable gardens. There are regulatory requirements to reduce the pathogenic organisms for distribution and marketing of sludge.
Landfilling is a method for the final disposal of sludge that is not useful any more. The sanitary landfill method is most suitable for the disposal of solid domestic wastes. This involves a low-cost anaerobic technology. In this method, the sludge or solid wastes are deposited in low-lying and low value sites. The deposition is done almost daily and the deposits are covered with a layer of soil (Fig. 58.4). With the coverage of the new waste deposits, nuisance conditions such as bad odours and flies are minimized.
It is desirable that the sludge is dewatered so that its transportation becomes easy. Further, the generation of leachate (liquid that percolates out due to leaching) is minimal from a dewatered sludge. At least two impermeable layers are built below the landfill to prevent the leakage of leachate to the surrounding lands. The accumulated leachate can be taken out and treated by appropriate methods.
The complete filling of landfills may take several months or even years, depending on the size of the site and the quantity of waste being deposited. Landfills can be used for the generation of methane gas for commercial use.
However, methane production usually commences several months after the landfill is completely filled. In some countries, there are strict regulations to use landfills for sewage disposal. These include the air- and water tight sites to protect the environment.
A lagoon is a shallow lake (or earth basin) usually located near a river or a sea. Lagooning (disposal of sludge into lagoons) is a convenient method of sludge disposal if the treatment plant is located at a remote place.
In lagooning, the sludge is stabilized by anaerobic and aerobic decomposition which is accompanied by release of objectionable odours. For this reason, lagoons should be located away from dwelling areas and high ways to avoid nuisance conditions. The stabilized solids of the sludge settle to the bottom of lagoon and accumulate. The sludge’s can be stored indefinitely in lagoons or may be removed periodically.
Septage and Septage Disposal:
Septage is a combination of sludge, scum and liquid coming out from a septic tank. It must be disposed under controlled conditions to avoid environmental pollution.
The most commonly used methods of septage disposal are listed below:
i. Land application (surface or subsurface).
ii. Co-treatment with waste water (biological or chemical treatment processes).
iii. Co-disposal with solid wastes (composting and landfilling).
iv. Independent processing facilities (composting, biological treatment, chemical oxidation, lime stabilization).
Treatment and Disposal of Solid Wastes:
Solid wastes are mostly being treated by incineration and landfilling, although these methods have certain limitations.
This requires costly equipment and high power consumption. Incinerators do not allow recovery of any useful materials. Further, it is frequently associated with environmental pollution.
The various aspects of landfilling have been described . The major limitation of landfills is the problem of leachates and gas emissions which pollute the environment. Further, they are not efficient producers of biogas. It is not possible to recycle the reusable products (paper, plastic, construction materials etc.) in landfilling.
Separation and Composting Plants:
The industrial and municipal solid wastes can be effectively treated by separation, followed by composting. In fact, huge plants are constructed to serve the dual purposes.
It is possible to recover several useful materials from the solid wastes by employing physical processes:
i. Plastic materials for reuse.
ii. Sand and gravel for construction purposes.
iii. Paper and cardboard for use in paper industry.
iv. Iron and aluminium for metallurgy.
After the separation of the important reusable materials, the left out in the solid wastes is mainly the biodegradable organic matter.
The solid wastes are mostly composted by anaerobic process and to a lesser extent by aerobic means.
Dry anaerobic composting (DRANCO) process:
In recent years, some companies have developed specific anaerobic digesters for composting solid wastes. The designing of digester is mainly based on the solid content of the waste, the temperature (from 35°C to 55°C) and the number of stages (1 or 2).
Dry anaerobic composting process is a commercially designed anaerobic digester. It is employed for composting of high solid (200-400 g/l) wastes at thermophilic temperature (around 55°C) by using a single stage reactor. The main advantage of DRANCO process is the high rate of composting under controlled conditions. This is evident from the fact that the composting of solids can be completed in about two weeks’ time. This is in contrast to a landfill which takes several years, sometimes even decades (10-30 years)!