Sludge Wastes Treatment - Methods, Types, Factors, Uses

What is Sludge?

During sewage/waste water treatment procedures, a semi-liquid substance known as sludge is generated.
Generally speaking, sludge is a semisolid liquid containing between 0.5 and 12 percent solids (by weight).

The composition of sludge is highly diverse, depending on the origin of the raw sewage and the treatment techniques involved. Some authors refer to sewage sludge as organic slurries.
Variable chemical composition characterises sludge. Proteins and other nitrogen-containing molecules, grease and lipids, cellulose, hydrocarbons, phosphate, iron, silica, organic acids, heavy metals, pathogens, and pesticides are the primary components of sludge.

Objectives of Treating Sludge

1. Reducing its Volume

Especially if it is untreated or has a high water content, sludge can be found in vast quantities. Therefore, it must be treated to minimise its volume prior to disposal.

In addition, lesser sludge quantities reduce pumping and storage expenses.

2. Stabilizing its Organic Materials

Sludge from chemical treatment or wastewater treatment plants may contain substances that will continue to react if disposed of in the open air.
This is why it is treated to stabilise these chemicals so that they do not negatively impact the health of the plants, animals, and humans in the area, as well as the environment as a whole.

Stabilized sludge emits less odour, contains plant- and soil-beneficial nutrients and microbes, and is more predictable.

3. Kill off the odor

Stabilized sludge lacks an objectionable odour, unlike untreated sludge, which is a nuisance and can pose health risks. It is essential to remediate sludge prior to releasing it into the environment.

4. To Ensure it Can be Safely Used

Once sludge has been processed, it can be used safely as fertiliser or to prevent soil erosion, among other applications. It can be hazardous not only to persons who come into contact with it, but also to the ground on which it is discarded and any crops grown there.

5. Pathogen Inactivation

Numerous pathogens, including disease-causing helminths, bacteria, protozoa, and viruses, are present in sludge. In multiple methods, treating sludge, particularly faecal sludge, deactivates these bacteria.
If the sludge is to be put to crops, more treatment is required to eliminate pathogens than if it is to be buried.

6. Dewatering

Sludge contains a great deal of water. Dewatering is distinct from drying, as it reduces the volume and weight of sludge, making it safer, easier, and less expensive to manage.

Fewer rats, flies, and other vectors are attracted to dewatered sludge, and it rarely smells. Wet sludge pathogens will also permeate the earth more quickly than dry sludge pathogens, meaning they will leave the surface very easily and quickly, minimising the potential of groundwater pollution.

7. The alternative (incineration and landfill disposal) are Not Environmentally Friendly

Additionally, sludge can be disposed of at a landfill or by incineration. However, incineration is not eco-friendly because it creates a great deal of energy.
1 kilogramme of dried sludge contains 3,300 kcal of energy, while 1 kilogramme of automobile tyres yields 8,300 kcal. Even though it produces far less energy, it is still not an environmentally friendly alternative.

Sources of Sludge Wastes

The following unit operations are the primary sludge generators:

Screening
Grit removal

Primary sedimentation
Secondary sedimentation
Sludge-processing units.

Types of Sludge

1. Drinking Water Sludge

This is the sludge obtained from water treatment facilities or storage tanks. Sludge is typically disposed of in landfills as nonhazardous waste and does not require costly treatment techniques.
Additionally, it has virtually no germs and can therefore be safely discarded. However, the waste is controversial, particularly in Spain, because it is uncertain if drinking water sludge may be transferred to inert landfills or if it can be used for other reasons.

2. Faecal Sludge

It consists of sludge collected from pit latrines, on-site sanitation systems, and septic tanks. It includes human excreta, solid wastes, urine, and water, as well as any other item that can be disposed of in pits, vaults, or tanks of sanitation systems.

Typically, vacuum trucks are used to transport faeces to specialised faeces treatment factories. The treated sludge can be utilised for irrigation, as a soil conditioner, or in the manufacture of biogas, biodiesel, charcoal, powdered industrial fuel, and electricity.

3. Industrial Wastewater Sludge

This is the sludge collected from warehouses, manufacturing facilities, and other commercial establishments.
It contains significant concentrations of heavy metals, microorganisms, and other pollutants, which can leach out if not treated properly.

The toxins in the sludge can have negative impacts on persons and the environment. Before it may be disposed of on land, careful and meticulous treatment is required.

4. Sewage Sludge

This is the sludge generated as a byproduct of industrial wastewater sludge treatment or sewage treatment.
It comprises both organic and inorganic substances, vast quantities of plant nutrients, pathogens, and trace levels of elements. Additionally, it may contain human waste and other consumer goods.

As a result, after treatment, sludge can be utilised as a fertiliser in landscaping and horticulture. Its anaerobic treatment could also produce methane, which is utilised for cooking and home heating.
As it eliminates pathogens, decreases the volume of the sludge, and decomposes most organic compounds, sewage sludge can also be incinerated.
However, it also emits greenhouse gases and contaminants into the atmosphere. Additionally, it results in the generation of inert and inorganic ash, which has few applications or benefits.

Steps of Sludge Treatment

Sludge Treatment involve the following steps;

Preliminary Operations of sludge Treatment
Thickening of Sludge (Concentration)

Stabilization of Sludge
Composting
Vermicomposting

Conditioning of Sludge
Disinfection of Sludge
Dewatering

Heat Drying
Thermal Reduction of Sludge
Disposal of Sludge

Landfilling
Lagooning
Septage and Septage Disposal

Treatment and Disposal of Solid Wastes
Separation and Composting Plants

1. Preliminary Operations of sludge Treatment

The preliminary procedures are performed in order to generate a steady and homogenous sludge for subsequent treatment.

1. Sludge grinding

Sludge is ground to fragment a big quantity of sludge into small fragments. For this reason, cutters made of hardened steel and overload sensors are utilised.

2. Sludge de-gritting

Sometimes, removal of grit (de-gritting) is required for effective sludge treatment. The sludge can be de-gritted by applying centrifugal force to its flow.
Typically, cyclone degritters are used to separate the grit particles from the organic sludge.

3. Sludge blending

The sludge from the primary (settleable solids), secondary (settleable solids and biological solids), and tertiary (biological and chemical solids) treatment stages is blended (combined) to generate a homogenous mixture.

4. Sludge storage

When the processing machines are not in operation, storage of sludge becomes necessary (night shifts, weekends). For short-term storage of sludge, settling tanks are utilised, however specifically built tanks are required for long-term storage.

2. Thickening of Sludge (Concentration)

The solid percentage of the sludge is varied, ranging between 0.5% and 11%. Liquid must be extracted from the sludge in order to concentrate it (especially if it has a low solids content).

Utilizing physical processes, sludge is thickened.

5. Gravity thickening

Gravity thickening is the most prevalent technique for thickening primary sludge.
It can be performed in the standard sedimentation tank (circular tank is preferred).

Gravity concentrates the sludge, allowing the supernatant to be returned to the treatment facility (i.e. primary settling tank).

6. Flotation thickening

Flotation thickening is the technique of choice for the treatment of sludges generated by biological processes involving suspended growth.
There are three different kinds of flotation thickening processes: dissolved air flotation, dispersed-air flotation, and vacuum flotation. Dissolved-air flotation is extensively employed among these methods.

In this method, air is circulated through the sludge while it is under high pressure.
The air rises to the surface of the sludge in the form of bubbles and particles to form a blanket of sludge.
This can be removed at periodic times. The ratio of air to solids affects the efficacy of flotation thickening.

7. Centrifugal thickening

Under the action of centrifugal forces, sludge particles settle to the bottom during centrifugal thickening.
There are two types of centrifuges used for this purpose: solid bowl centrifuges and basket centrifuges.
Although centrifugal thickening is an effective method for concentrating sludge, it has high maintenance and energy costs.

8. Rotary drum thickening

Utilizing rotational drums allows for the thickening of sludge. The solids separate from the water as the sludge passes over the revolving screen drums, and the thickened sludge rolls out of the drum ends.

3. Stabilization of Sludge

Sludge must be stabilised to achieve the following goals:

To decrease the number of pathogens (pathogens).

To reduce the likelihood of putrefaction.
To remove disagreeable smells.

The stabilisation of sludge can be accomplished by biological and chemical techniques. Lime stabilisation, heat treatment, anaerobic digestion, aerobic digestion, and composting are employed to stabilise sludge. They are detailed briefly.

1. Lime stabilization

When lime is added to sludge in the form of hydrated lime [Ca(OH)2] or quick lime (CaO), the pH rises to 12 or higher.
At a high pH, microorganism growth is inhibited. Consequently, the sludge will not putrify, create unpleasant odours and does not pose health hazards.
In lime pretreatment, lime is added before to dewatering, whereas in lime posttreatment, lime is added after dewatering.

2. Heat treatment

Heat treatment involves heating sludge under pressure for a brief period of time (250°C for approximately 30 minutes). This method dehydrates and sterilises sludge.
The application of heat aids in the stabilisation and conditioning of sludge. However, it is utilised less frequently due to its high price.

3. Anaerobic sludge digestion

The anaerobic digestion of sewage is one of the first biological treatment methods.

The same operational unit is equally effective for sludge stabilisation. There are various methods for anaerobic sludge digestion, including standard-rate digestion, single-stage high-rate digestion, and two-stage digestion.

4. Standard rate digestion

This is a single-stage process of digestion. External heat exchangers are used to warm the additional sludge.
As sludge is digested, the resulting gas rises to the surface.
Along with the gas, certain sludge particles (fats, oils, and grease) rise to the surface to form a removable scum.

5. Single-stage high rate digestion

This is distinguished by a high rate of sludge loading and anaerobic digestion.
For optimal digestion, the sludge is continuously mixed by gas, heated, and recirculated.

6. Two-stage digestion

Two digestion tanks are utilised in this high-rate digesting procedure.
The first digester is used for actual digestion (with mixing and heating capabilities), while the second is utilised for storage and concentration.

7. Thermophilic anaerobic digestion

Occasionally, thermophilic bacteria can digest sludge at a significantly higher temperature (45-60°C).
At a greater temperature, the digestion process is accelerated due to increased bacterial killing and enhanced dehydration.
However, the greatest drawback is the high energy need for continuous heating. Due to this, thermophilic anaerobic digestion is not commonly utilised.

8. Aerobic sludge digestion:

Aerobic digestion of sludges from different sources (primary, secondary, and tertiary treatments) has gained popularity in recent years due to the following factors:

As effective as anaerobic digestion is for the digestion of volatile substances.
The sludge’s fertiliser value is significantly higher.
Reduce capital and operational expenditures.

However, the most significant constraint of aerobic digestion is the expensive requirement for continual O2 delivery. Similar to the activated sludge process, aerobic digestion of sludge is a comparable process.

Principle of aerobic digestion

When the microbes’ nutrition (substrates) supply is reduced, they begin oxidising their own protoplasmic components for energy supply and maintenance.
Seventy to eighty percent of cellular organic matter can be converted aerobically to carbon dioxide, nitrate (NO–3), and water.
Twenty to thirty percent of cellular organic matter cannot be oxidised because it is not biodegradable.
The procedure can be carried out in either batch or continuous flow mode.
Regarding the aeration process, there are two types of aerobic digestion: traditional aerobic digestion and high purity oxygen aerobic digestion.

Conventional aerobic digestion

In the typical digester, atmospheric air is utilised for aerobic digestion.

High-purity oxygen aerobic digestion:

In this instance, oxygen is utilised instead of air. Typically, the digesting process takes place in enclosed tanks.

Thermophilic aerobic digestion

This technique is a development of the previous two, and it can effectively digest up to 20% of biodegradable organics.
Around 45 degrees Celsius, thermophilic bacteria carry out the activity. There are a number of benefits associated with thermophilic aerobic digestion.
These include the destruction of more harmful organisms and the digestion of more solids, while stabilising sludge’s disagreeable odours are reduced.

4. Composting

Essentially, composting is the process of biologically degrading solid organic waste into stable end products.
Composting can occur under both aerobic and anaerobic conditions, but aerobic composting is more common.
Thus, composting can be viewed as an aerobic microbiological process for transforming solid organic wastes into humus-like material that can be safely disposed of in the environment.
Composting is a cost-effective and environmentally beneficial method for sludge stabilisation and disposal.
The output of composting is beneficial for soil improvement and mushroom cultivation.
Thus, composting is eventually beneficial for the reuse and recycling of organic household, agricultural, and industrial waste materials.

Organisms involved in composting

Numerous species (both unicellular and multicellular) are engaged in the composting process.
80-90% of the microorganisms detected in the compost are bacteria. These bacteria contain a vast variety of enzymes capable of degrading a large variety of chemical molecules.
In addition to earthworms, insects, mites, and ants, the other creatures actively involved in composting are actinomycetes (a type of filamentous bacteria), fungi (moulds, yeasts), and protozoa.

Mechanism of Composting

Composting is a highly complex process that involves the participation of multiple microorganisms, including bacteria, actinomycetes, and fungi.
In addition to producing energy, bacteria decompose macromolecules, especially proteins and lipids (heat).
Fungi and actinomycetes decompose cellulose and other organic substances with complicated structures. According to changes in temperature, composting can be separated into three stages: mesophilic, thermophilic, and cooling.

Mesophilic stage

At this stage, fungus and acid-producing bacteria are active, and the temperature rises from ambient to approximately 40 degrees Celsius.

Thermophilic stage

As the composting process continues, the temperature increases from 40 to 70 degrees Celsius. At this stage, thermophilic bacteria, thermophilic fungi, and actinomycetes are active.
The thermophilic phase is characterised by a high rate and maximal decomposition of organic substances.

Cooling stage

The rate of microbial decomposition decreases, and thermophilic species are replaced by mesophilic bacteria and fungi.
The cooling phase is accompanied by the creation of water, the stabilisation of the pH, and the completion of humeic acid generation.

Methods of Composting

The following procedures comprise the composting process:

The combination of dehydrated sludge with a bulking agent (saw dust, rice hulls, straw or recycle compost). The bulking ingredient enhances the mixture’s porosity for optimal aeration.
Mechanically or otherwise producing aerobic conditions (aeration) This is required for oxygen delivery, temperature regulation, and water removal (moisture).
If possible, the bulking agent should be eliminated.
Compost storage and disposal procedures

There are three major composting techniques: aerated static piles, windrows, and vessel-based systems. According to a recent survey, the approximate distribution of various composting techniques is provided.

Aerated static pile – 55%
Windrow – 30%
In-vessel – 15%

Aerated static pile system

The dehydrated sludge is combined with a bulking agent (wood chips) and spread across a grid of aeration or exhaust pipework. Through blowers, air is supplied for optimal aeration.
A layer of compost is maintained atop the aerated static pile to provide insulation and enough ventilation.
Three to four weeks are required for composting, followed by four to five weeks for curing. In addition to reclaiming the bulking agent, the aged compost is filtered to reduce its quantity.

Windrow system

Windrows are a sort of static piles in which sludge is periodically turned and mixed during the composting process (3-4 weeks).
The mixing is often performed weekly and is related with the release of irritating and unpleasant odours.
The windrows may be open or covered, and aeration is accomplished mechanically.

In-vessel system

In an in-vessel composting system, the process takes place inside a closed vessel or container. In addition to controlling the ambient variables (temperature, air movement, and oxygen supply), this procedure is meant to minimise the discharge of unwanted odours.
The benefits of the in-vessel method are greater composting efficiency, fewer labour costs, and a smaller plant footprint.
There are two principal in-vessel composting systems: plug flow and dynamic.

a. Plug-flow in-vessel composting reactors

Two varieties of plug-flow in-vessel reactors exist: cylindrical tower and tunnel. In both instances, the association between particles in the composting mass is preserved throughout the entire process.
Composting occurs according to the first-in, first-out rule as sewage is added.

b. Dynamic in-vessel composting reactors

These are also known as in-vessel agitated bed composting reactors. During processing, the composting material is mechanically combined in these units.
Utilization of dynamic circular and dynamic rectangular reactors is widespread.
In circular reactors, the augurs revolving around the vessel’s centre and mixing the composting material.
Regarding the rectangular reactors, the extraction conveyor is in charge of mixing the compost and discharging it onto the outlet conveyor.

Factors affecting aerobic composting

Composting of sludge is influenced by a variety of variables. The significant ones are mentioned briefly:

1. Type of sludge

Composting is a viable option for both untreated and digested sludge. However, untreated sludge demands additional oxygen and generates foul odours.

2. Carbon nitrogen ratio

For efficient composting, the carbon nitrogen ratio should be in the range of 25: 1 to 35: 1. Periodically checking and maintaining this ratio is desirable.

3. Bulking agents

Utilized are inexpensive and widely available bulking agents (sawdust, wood chips). Their particle size and water content affect the composting process.

4. Moisture content

The optimal moisture content for composting sludge is less than 60 percent.

5. Aeration

It is preferable to maintain a consistent oxygen supply and ensure that it reaches the composting material adequately.

6. Temperature and pH

Temperatures between 45 and 55 degrees Celsius are optimal for composting. If the temperature goes beyond 60°C, the process almost gets halted. Optimal pH range is between 6 and 9

7. Mixing and turning

For effective composting, mixing and rotating are necessary. This keeps the compost from drying out and clumping.

5. Vermicomposting

Vermicomposting is the process through which earthworms create compost. In reality, it is well recognised that earthworms play a crucial role in the natural cycling of soil organic matter and maintenance of soil porosity.
Earthworms are highly efficient at recycling nutrients. Daily, they can eat 10-20% of their own biomass in the form of organic materials.
Utilizing organic materials with a variable carbon nitrogen ratio (C: N ratio) and converting it to a lower C: N ratio is possible for earthworms.
In other words, vermicomposting entails the transformation of organic molecules rich in carbon into organic compounds rich in nitrogen. This is really beneficial for soil enrichment.
Vermicomposting of cow and buffalo manure has become a viable, low-technology industry in recent years.
Drawidia nepalensis is the most common earthworm utilised for this purpose.
Vermicomposts are now available commercially. Also promoted is the incorporation of earthworms into soils to induce a natural process of vermicomposting (for soil enrichment).

6. Conditioning of Sludge

It is required to condition sludge to improve its dewatering properties. Typically, chemical and thermal treatment procedures are employed for this purpose. Less often employed conditioning procedures include irradiation, freezing, and solvent extraction.

Chemical method

Certain solids in the sludge can be coagulated with the discharge of water that has been absorbed by the employment of chemicals.
Chemical conditioning can lower the moisture content of sludge by 15-30%, or from approximately 95% to approximately 65%.
Alum, lime, ferric chloride, and organic polymers are the most widely used compounds.

Heat treatment method

The application of heat can stabilise and condition sludge. This procedure is conducted for a brief time period under pressure.
In addition to decreasing sludge solids’ affinity for water, heat treatment results in the coagulation of solids.

7. 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 sludge’s pathogenic organisms must be eliminated to protect the health of those who are exposed to it.

There are numerous techniques of disinfection from which to choose:

Pasteurization
Heat drying
Irradiation
High pH treatment
Addition of chlorine
Long-term storage of digested sludge
Complete composting.

8. Dewatering

Essentially, dewatering is the process of decreasing the moisture content of sludge. Vacuum filters, centrifuges, and sludge drying beds are all viable options for dewatering.

Vacuum filtration

Vacuum filtration dewatering is an old technique, but it has been stopped in recent years due to high operating and maintenance expenses and the process’s complexity.
During the past decade, improved and more effective alternative procedures have been created.

Centrifugation

Typically, centrifugation is utilised for the dewatering of industrial sludge. By use centrifugation, it is possible to separate liquids from solids.
For sludge dewatering, various types of centrifuges (solid bowel centrifuge, basket centrifuge) are commercially available.

Sludge drying beds

In industrialised nations (U.S., U.K.), drying beds are commonly employed to dewater digested sludge.
This process yields a dry product with a high solids content that is easily discarded (in a landfill or as soil conditioner). Sludge drying beds are economical.

9. Heat Drying

When sludge is treated to mechanical heat drying, the water content can be reduced significantly.
The ultimate goal of heat drying is to produce a sludge that is devoid of moisture and can be efficiently burned.
Flash dryers, spray dryers, rotary dryers, and multiple hearth dryers can do mechanical heat drying.

10. Thermal Reduction of Sludge

Thermal reduction entails the conversion of organic materials into oxidised end products such as carbon dioxide and water. This could be accomplished through cremation or wet-air oxidation.
Thermal reduction of sludge is linked to the elimination of pathogenic organisms, detoxification of harmful chemicals, and volume reduction of sludge for disposal.
Thermal reduction of sludge typically involves multiple-hearth incineration, fluidized-bed incineration, and wet-air oxidation.

11. Ultimate Disposal of Sludge

Prior to addressing the eventual disposal of sludge, its beneficial applications are considered. In addition to containing soil conditioning capabilities, sludge is beneficial for supplying nutrients to the soil. Thus, efforts are made to dispose of the sludge in a manner that is advantageous. If this cannot be accomplished, alternatives are investigated.

Land applications of sludge (as a fertilizer)

Land applications of sludge are the spreading of sludge on or just below the surface of the soil. The use of sludge is permitted on agricultural fields, forest lands, and designated land disposal sites.
Pathogens and harmful organic chemicals in sludge can be eliminated by sunshine and soil microbes, respectively.
As a result, sludge put to land is effective as a soil conditioner for enhancing land-nutrient transport facilitation and water retention. Thus, sewage sludge can substitute costly fertilisers.
Before sludge may be applied to the land, the quantity of organic compounds and pathogens must be minimised.
A large concentration of organic debris will produce foul odours, while infections will spread disease.
There are regulatory obligations to manage sludge pathogens through a variety of techniques.

Distribution and Marketing

In recent years, distribution and marketing of sludge for positive applications has gained prominence.
It is believed that 10 to 20% of all produced sludge is utilised in this manner.
The commercialised sludge is utilised in parks, lawns, golf courses, and ornamental and vegetable gardens as a substitute for topsoil and peat.
For the distribution and marketing of sludge, there are regulatory requirements to decrease pathogenic organisms.

12. Landfilling

Landfilling is a process for the final disposal of no longer useable sludge. Domestic solid wastes can be disposed of most effectively via sanitary landfills. This utilises an inexpensive anaerobic technique.
The sludge or solid wastes are deposited in low-lying, low-value areas. Almost daily deposition occurs, and the deposits are covered with a layer of dirt.
With the new garbage deposits in place, nuisance circumstances such as foul odours and flies are reduced.
It is preferable that the sludge be dehydrated to facilitate its transfer. In addition, less leachate (liquid that percolates out owing to leaching) is produced by dewatered sludge.
At least two impermeable layers are constructed beneath the landfill to prevent leachate from seeping into the surrounding soil. The accumulated leachate can be extracted and appropriately treated.
Depending on the size of the site and the amount of waste being placed, the entire filling of landfills could take many months or even years. Landfills can be utilised to produce methane gas for commercial purposes.
Nevertheless, methane generation often begins several months after the landfill has been entirely filled.
In several nations, it is strictly prohibited to dispose of sewage in landfills. These include airtight and watertight sites for environmental protection.

13. Lagooning

A lagoon is a shallow lake (or earth basin) that is typically found close to a river or the ocean. Lagooning (disposal of sludge into lagoons) is an advantageous method of sludge disposal if the treatment plant is located in a remote location.
Lagooning stabilises sludge through anaerobic and aerobic decomposition, which is accompanied by the production of offensive odours. To avoid nuisance situations, lagoons should be situated away from residential areas and major roads.
The stabilised solids of the sludge sink and deposit at the bottom of the lagoon.
The sludge can be held in lagoons forever or occasionally removed.

14. Septage and Septage Disposal

Septage is the mixture of sludge, scum, and liquid that exits a septic tank. It must be disposed of in a regulated manner to prevent environmental contamination.

The following are the most frequent techniques of septage disposal:

Land administration (surface or subsurface).
combined treatment of waste water (biological or chemical treatment processes).
Co-dumping of solid wastes (composting and landfilling).
Facilities for processing independently (composting, biological treatment, chemical oxidation, lime stabilization).

15. Treatment and Disposal of Solid Wastes

Although both approaches have their limits, incineration and landfill disposal account for the majority of solid waste treatment.

Incineration

This takes expensive equipment and a significant amount of energy. Incinerators prohibit the recovery of any recyclable materials. Additionally, it is commonly linked to environmental pollutants.

Landfilling

Various facets of landfilling have been outlined. The contamination of the environment by leachates and gas emissions is the primary drawback of landfills.
Moreover, they are inefficient biogas producers. In landfilling, recyclable products (paper, plastic, construction materials, etc.) cannot be recycled.

16. Separation and Composting Plants

The separation of industrial and municipal solid wastes, followed by composting, is an effective method for their treatment. In fact, enormous plants are developed to serve both functions.

Separation

Using physical processes, it is possible to recover several useful materials from solid wastes:

Plastic materials for reuse.
Sand and gravel for building purposes.
The use of paper and cardboard in the paper industry.
Iron and aluminium are used in the metalworking industry.

After the separation of valuable reusable materials, the majority of the remaining solid waste consists of biodegradable organic matter.

Composting

Composting of solid wastes is dominated by anaerobic processes and to a lesser extent, aerobic processes.

Dry anaerobic composting (DRANCO) process

Some companies have developed anaerobic digesters for composting solid waste in recent years.
The solid composition of the waste, the temperature (from 35°C to 55°C), and the number of stages govern the design of the digester (1 or 2).
The anaerobic digester used in the dry anaerobic composting process is a commercially manufactured device. Utilizing a single stage reactor, it is used to compost high solid (200-400 g/l) wastes at thermophilic temperature (about 55°C).
The primary benefit of the DRANCO technique is the high rate of controlled composting. This is obvious by the fact that solid waste may be composted in approximately two weeks.
This is in contrast to a landfill, which can take anywhere from ten to thirty years!

Uses of Sludge

1. For Agricultural Purposes

Treated sludge can be utilised in home gardening, forestry, and parks for agricultural purposes. Unfortunately, sewage sludge provides lesser quantities of nitrogen, potassium, and phosphorus than conventional fertilisers.
Additionally, it has been questioned for possessing possibly high quantities of pollutants and metals. Regardless, conventional fertilisers include metals and other pollutants in various concentrations.
Compared to industrial wastewater sludge, faecal sludge is somewhat safer due to the absence of chemical inputs. They are therefore favoured for use as fertilisers. Approximately 80% of the 1.4 million tonnes of sludge produced annually in the United Kingdom is used as manure.

2. Helpful in Controlling Soil Erosion

It has been discovered that sludge contains features such as thickening and water retention. The capacity of sludge to retain more water also contributes to its capacity to contain more soil.
For this reason, it is chosen for reducing soil erosion, particularly in sloppy areas where soil erosion may be prevalent.

3. Landscaping

Eventually, sludge will dry out and become part of the earth. This makes it ideal for landscaping as opposed to importing soil from elsewhere.
It contains nutrients that promote plant and grass growth, and as a result, it can speed their development. It can also hasten reforestation if the area where the sludge is deposited is utilised for this purpose.

4. Used as an Alternative Fuel Source in the Cement Industry

It is an attractive way of sewage sludge disposal. Multiple European countries, including Germany and Switzerland, have followed this technique.
In cement kilns, sewage sludge has a comparatively high net calorific value of 10-20MJ/kg and a lower carbon dioxide emission factor than coal.

5. Domestic Use for Heating and Cooking

For heating and culinary purposes, sludge that has undergone anaerobic digestion can be advantageous to households. This sludge is stored without air at temperatures between 20 and 55 degrees Celsius (68 and 131 degrees Fahrenheit) for 15 to 60 days.
This procedure will result in the creation of methane, among other gases. The gathered methane, carbon monoxide, and hydrogen may be burned or oxidised with oxygen. It means that the biogas produced will be used as cooking fuel and can be transformed in a gas engine to generate electricity and heat.

References

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/sludge-management
https://www.eea.europa.eu/publications/GH-10-97-106-EN-C/file
https://web.deu.edu.tr/atiksu/ana52/sludis.html
https://www.wateronline.com/doc/step-wastewater-sludge-treatment-process-0001
https://www.iwapublishing.com/sites/default/files/ebooks/9781780402130.pdf
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