Sludge Treatment and Disposal

The residue that accumulates in sewage treatment plants is called sludge (or biosolids). Sewage sludge is the solid, semisolid, or slurry residual material that is produced as a by-product of wastewater treatment processes. This residue is commonly classified as primary and secondary sludge. Primary sludge is generated from chemical precipitation, sedimentation, and other primary processes, whereas secondary sludge is the activated waste biomass resulting from biological treatments. Some sewage plants also receive septage or septic tank solids from household on-site wastewater treatment systems. Quite often the sludges are combined together for further treatment and disposal.

Treatment and disposal of sewage sludge are major factors in the design and operation of all wastewater treatment plants. Two basic goals of treating sludge before final disposal are to reduce its volume and to stabilize the organic materials. Stabilized sludge does not have an offensive odour and can be handled without causing a nuisance or health hazard. Smaller sludge volume reduces the costs of pumping and storage.

Treatment methods

Treatment of sewage sludge may include a combination of thickening, digestion, and dewatering processes.


Thickening is usually the first step in sludge treatment because it is impractical to handle thin sludge, a slurry of solids suspended in water. Thickening is usually accomplished in a tank called a gravity thickener. A thickener can reduce the total volume of sludge to less than half the original volume. An alternative to gravity thickening is dissolved-air flotation. In this method, air bubbles carry the solids to the surface, where a layer of thickened sludge forms.


Sludge digestion is a biological process in which organic solids are decomposed into stable substances. Digestion reduces the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge. Digested sludge is inoffensive, having the appearance and characteristics of a rich potting soil.

Most large sewage treatment plants use a two-stage digestion system in which organics are metabolized by bacteria anaerobically (in the absence of oxygen). In the first stage, the sludge, thickened to a dry solids (DS) content of about 5 percent, is heated and mixed in a closed tank for several days. Acid-forming bacteria hydrolyze large molecules such as proteins and lipids, breaking them into smaller water-soluble molecules, and then ferment those smaller molecules into various fatty acids. The sludge then flows into a second tank, where the dissolved matter is converted by other bacteria into biogas, a mixture of carbon dioxide and methane. Methane is combustible and is used as a fuel to heat the first digestion tank as well as to generate electricity for the plant.

Anaerobic digestion is very sensitive to temperature, acidity, and other factors. It requires careful monitoring and control. In some cases, the sludge is inoculated with extra hydrolytic enzymes at the beginning of the first digestion stage in order to supplement the action of the bacteria. It has been found that this enzymatic treatment can destroy more unwanted pathogens in the sludge and also can result in the generation of more biogas in the second stage of digestion.

Another enhancement of the traditional two-stage anaerobic digestion process is thermal hydrolysis, or the breaking down of the large molecules by heat. This is done in a separate step before digestion. In a typical case, the process begins with a sludge that has been dewatered to a DS content of some 15 percent. The sludge is mixed with steam in a pulper, and this hot homogenized mixture is fed to a reactor, where it is held under pressure at approximately 165 °C (about 330 °F) for about 30 minutes. At that point, with the hydrolytic reactions complete, some of the steam is bled off (to be fed to the pulper), and the sludge, still under some pressure, is released suddenly into a “flash tank,” where the sudded drop in pressure bursts the cell walls of much of the solid matter. The hydrolyzed sludge is cooled, diluted slightly with water, and then sent directly to the second stage of anaerobic digestion.

Sludge digestion may also take place aerobically—that is, in the presence of oxygen. The sludge is vigorously aerated in an open tank for about 20 days. Methane gas is not formed in this process. Although aerobic systems are easier to operate than anaerobic systems, they usually cost more to operate because of the power needed for aeration. Aerobic digestion is often combined with small extended aeration or contact stabilization systems.

Aerobic and conventional anaerobic digestion convert about half of the organic sludge solids to liquids and gases. Thermal hydrolysis followed by anaerobic digestion can convert some 60 to 70 percent of the solid matter to liquids and gases. Not only is the volume of solids produced smaller than in conventional digestion, but the greater production of biogas can make some wastewater treatment plants self-sufficient in energy.


Digested sewage sludge is usually dewatered before disposal. Dewatered sludge still contains a significant amount of water—often as much as 70 percent—but, even with that moisture content, sludge no longer behaves as a liquid and can be handled as a solid material. Sludge-drying beds provide the simplest method of dewatering. A digested sludge slurry is spread on an open bed of sand and allowed to remain until dry. Drying takes place by a combination of evaporation and gravity drainage through the sand. A piping network built under the sand collects the water, which is pumped back to the head of the plant. After about six weeks of drying, the sludge cake, as it is called, may have a solids content of about 40 percent. It can then be removed from the sand with a pitchfork or a front-end loader. In order to reduce drying time in wet or cold weather, a glass enclosure may be built over the sand beds. Since a good deal of land area is needed for drying beds, this method of dewatering is commonly used in rural or suburban towns rather than in densely populated cities.

Alternatives to sludge-drying beds include the rotary drum vacuum filter, the centrifuge, and the belt filter press. These mechanical systems require less space than do sludge-drying beds, and they offer a greater degree of operational control. However, they usually have to be preceded by a step called sludge conditioning, in which chemicals are added to the liquid sludge to coagulate solids and improve drainability.


The final destination of treated sewage sludge usually is the land. Dewatered sludge can be buried underground in a sanitary landfill. It also may be spread on agricultural land in order to make use of its value as a soil conditioner and fertilizer. Since sludge may contain toxic industrial chemicals, it is not spread on land where crops are grown for human consumption.

Where a suitable site for land disposal is not available, as in urban areas, sludge may be incinerated. Incineration completely evaporates the moisture and converts the organic solids into inert ash. The ash must be disposed of, but the reduced volume makes disposal more economical. Air pollution control is a very important consideration when sewage sludge is incinerated. Appropriate air-cleaning devices such as scrubbers and filters must be used.

Dumping sludge in the ocean, once an economical disposal method for many coastal communities, is no longer considered a viable option. It is now prohibited in the United States and many other coastal countries.


Sewerage System

A sewerage system, or wastewater collection system, is a network of pipes, pumping stations, and appurtenances that convey sewage from its points of origin to a point of treatment and disposal.

Combined systems

Systems that carry a mixture of both domestic sewage and storm sewage are called combined sewers. Combined sewers typically consist of large-diameter pipes or tunnels, because of the large volumes of storm water that must be carried during wet-weather periods. They are very common in older cities but are no longer designed and built as part of new sewerage facilities. Because wastewater treatment plants cannot handle large volumes of storm water, sewage must bypass the treatment plants during wet weather and be discharged directly into the receiving water. These combined sewer overflows, containing untreated domestic sewage, cause recurring water pollution problems and are very troublesome sources of pollution.

In some large cities the combined sewer overflow problem has been reduced by diverting the first flush of combined sewage into a large basin or underground tunnel. After temporary storage, it can be treated by settling and disinfection before being discharged into a receiving body of water, or it can be treated in a nearby wastewater treatment plant at a rate that will not overload the facility. Another method for controlling combined sewage involves the use of swirl concentrators. These direct sewage through cylindrically shaped devices that create a vortex, or whirlpool, effect. The vortex helps concentrate impurities in a much smaller volume of water for treatment.

Separate systems

New wastewater collection facilities are designed as separate systems, carrying either domestic sewage or storm sewage but not both. Storm sewers usually carry surface runoff to a point of disposal in a stream or river. Small detention basins may be built as part of the system, storing storm water temporarily and reducing the magnitude of the peak flow rate. Sanitary sewers, on the other hand, carry domestic wastewater to a sewage treatment plant. Pretreated industrial wastewater may be allowed into municipal sanitary sewerage systems, but storm water is excluded.

Storm sewers are usually built with sections of reinforced concrete pipe. Corrugated metal pipes may be used in some cases. Storm water inlets or catch basins are located at suitable intervals in a street right-of-way or in easements across private property. The pipelines are usually located to allow downhill gravity flow to a nearby stream or to a detention basin. Storm water pumping stations are avoided, if possible, because of the very large pump capacities that would be needed to handle the intermittent flows.

A sanitary sewerage system includes laterals, submains, and interceptors. Except for individual house connections, laterals are the smallest sewers in the network. They usually are not less than 200 mm (8 inches) in diameter and carry sewage by gravity into larger submains, or collector sewers. The collector sewers tie in to a main interceptor, or trunk line, which carries the sewage to a treatment plant. Interceptors are usually built with precast sections of reinforced concrete pipe, up to 5 metres (15 feet) in diameter. Other materials used for sanitary sewers include vitrified clay, asbestos cement, plastic, steel, or ductile iron. The use of plastic for laterals is increasing because of its lightness and ease of installation. Iron and steel pipes are used for force mains or in pumping stations. Force mains are pipelines that carry sewage under pressure when it must be pumped.

Alternative Systems

Sometimes the cost of conventional gravity sewers can be prohibitively high because of low population densities or site conditions such as a high water table or bedrock. Three alternative wastewater collection systems that may be used under these circumstances include small-diameter gravity sewers, pressure sewers, and vacuum sewers.

In small-diameter gravity systems, septic tanks are first used to remove settleable and floating solids from the wastewater from each house before it flows into a network of collector mains (typically 100 mm, or 4 inches, in diameter); these systems are most suitable for small rural communities. Because they do not carry grease, grit and sewage solids, the pipes can be of smaller diameter and placed at reduced slopes or gradients to minimize trench excavation costs. Pressure sewers are best used in flat areas or where expensive rock excavation would be required. Grinder pumps discharge wastewater from each home into the main pressure sewer, which can follow the slope of the ground. In a vacuum sewerage system, sewage from one or more buildings flows by gravity into a sump or tank from which it is pulled out by vacuum pumps located at a central vacuum station and then flows into a collection tank. From the vacuum collection tank the sewage is pumped to a treatment plant.


Pumping stations are built when sewage must be raised from a low point to a point of higher elevation or where the topography prevents downhill gravity flow. Special nonclogging pumps are available to handle raw sewage. They are installed in structures called lift stations. There are two basic types of lift stations: dry well and wet well. A wet-well installation has only one chamber or tank to receive and hold the sewage until it is pumped out. Specially designed submersible pumps and motors can be located at the bottom of the chamber, completely below the water level. Dry-well installations have two separate chambers, one to receive the wastewater and one to enclose and protect the pumps and controls. The protective dry chamber allows easy access for inspection and maintenance. All sewage lift stations, whether of the wet-well or dry-well type, should include at least two pumps. One pump can operate while the other is removed for repair.

Flow rates

There is a wide variation in sewage flow rates over the course of a day. A sewerage system must accommodate this variation. In most cities domestic sewage flow rates are highest in the morning and evening hours. They are lowest during the middle of the night. Flow quantities depend upon population density, water consumption, and the extent of commercial or industrial activity in the community. The average sewage flow rate is usually about the same as the average water use in the community. In a lateral sewer, short-term peak flow rates can be roughly four times the average flow rate. In a trunk sewer, peak flow rates may be two-and-a-half times the average.

Although sewage flows depend upon residential, commercial, and industrial connections, sewage flow rates potentially can become higher as a result of inflows and infiltration (I&I) into the sanitary sewer system. Inflows correspond to storm water entering sewers from inappropriate connections, such as roof drains, storm drains, downspouts and sump pumps. High amounts of rainwater runoff can reach the sewer system during precipitation and stormflow events or during seasonal spring flooding of rivers inundated with melting ice. Infiltration refers to the groundwater entering sewers via defective or broken pipes. In both these cases, downstream utilities and treatment plants may experience flows higher than anticipated and can become hydraulically overloaded. During such overloads, utilities may ask residents connected to the system to refrain from using dishwashers and washing machines and may even limit toilet flushing and the use of showers in an attempt to lessen the strain. Such I&I issues can be especially severe in old and aging water infrastructures.


Sewage Characteristics

Types of sewage

There are three types of wastewater, or sewage: domestic sewage, industrial sewage, and storm sewage. Domestic sewage carries used water from houses and apartments; it is also called sanitary sewage. Industrial sewage is used water from manufacturing or chemical processes. Storm sewage, or storm water, is runoff from precipitation that is collected in a system of pipes or open channels.

Domestic sewage is slightly more than 99.9 percent water by weight. The rest, less than 0.1 percent, contains a wide variety of dissolved and suspended impurities. Although amounting to a very small fraction of the sewage by weight, the nature of these impurities and the large volumes of sewage in which they are carried make disposal of domestic wastewater a significant technical problem. The principal impurities are putrescible organic materials and plant nutrients, but domestic sewage is also very likely to contain disease-causing microbes. Industrial wastewater usually contains specific and readily identifiable chemical compounds, depending on the nature of the industrial process. Storm sewage carries organic materials, suspended and dissolved solids, and other substances picked up as it travels over the ground.

Principal Pollutants

Organic material

The amount of putrescible organic material in sewage is indicated by the biochemical oxygen demand, or BOD; the more organic material there is in the sewage, the higher the BOD, which is the amount of oxygen required by microorganisms to decompose the organic substances in sewage. It is among the most important parameters for the design and operation of sewage treatment plants. Industrial sewage may have BOD levels many times that of domestic sewage. The BOD of storm sewage is of particular concern when it is mixed with domestic sewage in combined sewerage systems.

Dissolved oxygen is an important water quality factor for lakes and rivers. The higher the concentration of dissolved oxygen, the better the water quality. When sewage enters a lake or stream, decomposition of the organic materials begins. Oxygen is consumed as microorganisms use it in their metabolism. This can quickly deplete the available oxygen in the water. When the dissolved oxygen levels drop too low, trout and other aquatic species soon perish. In fact, if the oxygen level drops to zero, the water will become septic. Decomposition of organic compounds without oxygen causes the undesirable odours usually associated with septic or putrid conditions.

Suspended solids

Another important characteristic of sewage is suspended solids. The volume of sludge produced in a treatment plant is directly related to the total suspended solids present in the sewage. Industrial and storm sewage may contain higher concentrations of suspended solids than domestic sewage. The extent to which a treatment plant removes suspended solids, as well as BOD, determines the efficiency of the treatment process.

Plant nutrients

Domestic sewage contains compounds of nitrogen and phosphorus, two elements that are basic nutrients essential for the growth of plants. In lakes, excessive amounts of nitrates and phosphates can cause the rapid growth of algae. Algal blooms, often caused by sewage discharges, accelerate the natural aging of lakes in a process called eutrophication.


Domestic sewage contains many millions of microorganisms per gallon. Most are coliform bacteria from the human intestinal tract, and domestic sewage is also likely to carry other microbes. Coliforms are used as indicators of sewage pollution. A high coliform count usually indicates recent sewage pollution.


Rotating Biologycal Contactor (RBC)

RBC or Rotating Biological Contactor is a liquid waste treatment process using a method in which this wastewater treatment unit rotates with a center on an axis or axle which is driven by a motor drive system and/or air blowing (air drive system) from a diffuser immersed in wastewater. , under media. Made of plastic, the media for attachment of microbes is installed in such a way that there is the widest possible contact with waste water and oxygen that occurs alternately. Where the method involves contact with biological elements in rotation or rotation.

RBC is like a collection of discs where on the surface there is a disk media as a place for microorganisms to eat the organic matter content in the waste, it is cultivated that the disk media can be provided as widely as possible so that microorganisms can easily take pollutants in the flowing waste. The RBC operating system uses microorganisms to eat organic matter. Microorganisms need food and O2 to survive. So that the RBC is set like a spinning wheel so that when it is below the microorganism can take food while it can process it by taking oxygen first when it is above. However, it should also be noted that if RBC has been used for a long time on the surface of the disk media, large piles of microorganisms will form due to the growth of MO (microorganisms). if this MO continues to pile up, then the MO in the lowest pile that lives is only an aerobic MO because it is covered by the MO above it. so sometimes it forms like a crust.

RBC Working Principles

The working principle of wastewater treatment with RBC is that wastewater containing organic pollutants is contacted with a layer of micro-organisms (microbial film) attached to the surface of the media in a reactor. The media where the biological film is attached is in the form of a disk (disk) made of lightweight polymer or plastic and arranged in a row on an axis to form a module or package, then the module is rotated slowly in a state partially immersed in flowing wastewater. continuously into the reactor.

In this way micro-organisms such as bacteria, algae, protozoa, fungi, and others grow attached to the surface of the rotating medium forming a layer consisting of micro-organisms called biofilm (biological layer). Micro-organisms will decompose or take organic compounds in water and take oxygen dissolved in water or from the air for their metabolic processes, so that the content of organic compounds in wastewater is reduced.

When the biofilm attached to the medium in the form of a thin disc is immersed in wastewater, micro-organisms absorb organic compounds present in the wastewater flowing on the surface of the biofilm, and when the biofilm is above the water surface, the micro-organisms absorb oxygen from the surface of the biofilm. air or oxygen dissolved in water to decompose organic compounds. The energy resulting from the decomposition of organic compounds is used by micro-organisms for the process of reproduction or metabolism.

Compounds resulting from the metabolic processes of these micro-organisms will come out of the biofilm and be carried away by the flow of water or in the form of gas and will be dispersed into the air through the cavities in the medium, while suspended solids (SS) will be retained on the surface of the biological layer (biofilm). ) and will decompose into water-soluble forms.

The growth of these micro-organisms or biofilms is getting thicker and thicker, until finally due to gravity some of it will peel off from the medium and be carried out by the water flow. Furthermore, micro-organisms on the surface of the medium will grow again by itself until an equilibrium occurs according to the content of organic compounds present in the wastewater.

Aplication of Rotating Biological Contactor (RBC)

The performance of the RBC depends also on the number of compartments. One module can contain four or five compartments. In the first compartment, backflow can be added to the initial settling unit so that the conditions are not too anaerobic so that the bad smell is reduced while helping the dynamics of microbial growth. Likewise, in the final compartment a backflow to the initial settling unit may be installed for the same purpose. Generally, the RBC contact medium is submerged in wastewater as high as 40% of its diameter. The rotation speed is between 1 – 3 revolutions per minute. This rotation provides sufficient energy for the hydraulic force to dislodge the biofilm and the water flow is turbulent so that the solid remains suspended (does not settle). The hydraulic residence time in each module is relatively short, ie 20 minutes at normal load. Each stage or module tends to operate as a completely stirred reactor.

Regarding the microbial adhesive media, there are several materials that can be used. What is often chosen is HDPE (high-density polyethylene) plastic media with a diameter of 2-4 m, with a thickness of up to 10 mm. The form of media can be in the form of plates but can also be in the form of pipes or tubes mounted on an iron shaft with a span of up to 8 m. The media along with the shaft and the motor is called a module that keeps on rotating in the tub. Several modules can be installed in series or parallel according to the discharge requirements of the treated wastewater. Usually the modules are separated by a baffle to avoid short circuiting in the tank. RBC performance is also influenced by wastewater temperature, influent substrate concentration, hydraulic residence time, ratio of tank volume to media surface area, media rotation speed, and dissolved oxygen.

Generally, to treat RBC domestic wastewater, it does not require microbial seeding. This is because the microbes are already available in sufficient quantities to start the process. Approximately a week to two weeks after starting the processing, on the surface of the media will stick biomass with a thickness of 1-4 mm. This thickness depends on the strength of the wastewater and the rotational speed of the adhesive medium. According to Antonie, 1978, the concentration of these microbes reached 50,000 – 100,000 mg/l, a very high amount so that quite a lot of organic pollutants and nitrogen were removed with the help of dissolved oxygen.

Waste Treatment Process with RBC

In general, the wastewater treatment process with the RBC system consists of a sand separator, initial settling basin, flow control tank, rotary biological reactor/contactor (RBC), final settling basin, chlorination tank, and a sludge treatment unit.

Sand Separator Tub. Wastewater flows quietly into a sand separator, so that dirt in the form of sand or coarse silt can be deposited. Meanwhile, floating dirt such as garbage, plastic, cloth waste and others are stuck in the screen installed at the inlet of the sand separator pool.

Early Sediment Tub. From the separator/sand settler tank, wastewater is drained to the initial sealing tank. In this initial settling tank mud or suspended solids mostly settle. The residence time in the initial sealing tank is 2 – 4 hours, and the sediment that has settled is collected and pumped into the sludge deposition tank.

Flow Control Body. If the wastewater flow rate exceeds the planning capacity, the excess wastewater discharge is channeled to a flow control tank for temporary storage. When the flow rate is low, the wastewater in the control tank is pumped into the initial settling basin together with the new wastewater according to the desired discharge.

Biological contactor (reactor) Swivel. In this contactor bath, the medium is a thin disk (disk) of polymer or plastic material in large quantities, which is attached or assembled on a shaft, rotated slowly in a state partially immersed in the wastewater. The residence time in the contactor bath is approximately 2.5 hours. Under such conditions, micro-organisms will grow on the surface of the rotating medium, forming a biological film. The biological film consists of various types/spicies of micro-organisms such as bacteria, protozoa, fungi, and others. Micro-organisms that grow on the surface of this media will decompose organic compounds in the wastewater. The biological layer is getting thicker and thicker because of the gravity it will peel off by itself and the organic mud will be carried away by the water flow out. Furthermore, the biological layer will grow and develop again on the surface of the media by itself.

Final Precipitation Tub. The wastewater that comes out of the contactor (reactor) tank is then channeled to the final settling basin, with a settling time of about 3 hours. Compared to the activated sludge process, the sludge from RBC is easier to settle, because it is larger in size and heavier. The runoff water (over flow) from the final settling basin is relatively clear, then it is flowed into the chlorination tank. Meanwhile, the sludge that settles at the bottom of the tank is pumped to the sludge concentration tank together with the mud from the initial settling basin.

Chlorination tub. Treated water or runoff water from the final settling basin still contains coli bacteria, pathogenic bacteria, or viruses that have the potential to infect the surrounding community. To overcome this, the wastewater that comes out of the final settling basin is channeled into a chlorination tank to kill pathogenic micro-organisms present in the water. In the chlorination tank, the wastewater is spiked with chlorine compounds with a certain dose and contact time so that all pathogenic micro-organisms can be killed. Furthermore, from the chlorination tank, wastewater can be discharged into water bodies.

Mud Concentration Tub. Sludge from the initial settling basin and the final settling basin is collected in a sludge concentration basin. In the tank, the mud is stirred slowly and then concentrated by allowing it to stand for about 25 hours so that the mud settles, then the supernatant water at the top is flowed into the initial settling basin, while the concentrated mud is pumped into the mud dryer or accommodated in the tank. separately and periodically sent to a sludge treatment center elsewhere.

Reaction on RBC

In the RBC process, there are several reactions that occur, namely:
1. Oxidation
2. Nitrification
3. Denitrification

This is described as follows. Organic material contained in the waste then takes oxygen so that there is a reaction between organic matter, O2 and nutrients (usually already contained in the waste) in the metabolic process and then NH3, CO2, C5H7HO2 (new cells) are released into the air.

In addition to the above process, there is endogenous respiration to obtain energy, namely:

C5H7HO2 + O2   –>>    5CO2   +   H2O   +   Energy

in the nitrification of waste has a pollutant containing ammonia NH4 which smells very pungent. with the following reaction:

2NH4   +   O2 (with the help of nitrosomonas)  —>>   2NO2 + 4H + 2H2O