The techniques used to treat and dispose of the sewage sludge generated in wastewater treatment are referred to as sewage sludge treatment. What is that? To cut down on transportation and disposal expenses as well as potential health risks associated with disposal alternatives, sludge treatment focuses on reducing sludge weight and volume. Weight and volume reduction are primarily accomplished by removing water, but pathogen eradication is frequently accomplished through heating during endothermic digestion, composting, or cremation. The volume of sludge produced and a comparison of the prices of the various disposal choices determine the best sludge treatment procedure. While restricted land availability favors aerobic digestion and mechanical drainage for cities, and economies of scale favor energy recovery solutions in urban areas, air drying and composting may be appealing to rural communities. With some particulates removed from liquid wastewater, sludge is primarily made up of water. Primary clarifiers remove settle able materials as part of the pre-treatment process to create primary sludge. Sludge separated in secondary clarifiers is referred to as secondary sludge and is employed in bioreactors for post-treatment or in inorganic oxidation processes. The resulting sludge from pressured wastewater treatment operations must be continuously removed from the liquid line since the tanks' volume is insufficient to hold the sludge. This is done to maintain the treatment procedures' balance and compactness (sludge production roughly equals sludge removal). 
The liquid line's extracted sludge is routed to the sludge treatment line. Anaerobic processes tend to create less sludge than aerobic processes, such as activated sludge processes. On the other hand, when using extensive (natural) sewage treatment methods like pond systems and wetlands, the resulting sludge builds up in the sewage treatment facilities (liquid line) and isn't cleared for a while. Options for sludge treatment rely on the production of solids and other site-specific factors. Small-scale plants frequently use composting, whereas medium- and large-scale plants use aerobic digestion and anaerobic digestion, respectively. Sometimes the sludge is dewatered by passing it through a pre-thickener. Pre-thickeners come in a variety of forms, such as belt press filters, rotary drum sludge thickeners, and centrifugal sludge thickeners. Dewatered sludge can be utilised as an agricultural soil additive, for landfill disposal, or for incineration. However, the energy yield is frequently insufficient to evaporate the water content of the sludge or to power the fans, pumps, or centrifuges required for dewatering. Energy can be recovered from the sludge through methane gas generation from anaerobic digestion or dry sludge incineration. It is possible for coarse primary solids and secondary sludge to include hazardous substances that are drawn out of the liquid effluent by adsorption onto the solid sludge particles. The quantity of certain of these harmful compounds in the sludge may rise when the volume of the sludge is reduced. 
What Is Wastewater Treatment
Municipal water agencies often refer to the reuse of sewage sludge after the sludge has gone through the proper wastewater treatment processes as "bio solids" in wastewater engineering literature and public relations. What is it? In actuality, bio solids are defined as wastewater organic materials that can be composted or anaerobically digested before being used again. The Water Environment Federation in the United States coined the term "bio solids" in 1998. Some claim that the word is intentionally misleading because sewage sludge may contain elements like heavy metals and persistent pharmaceutical contaminants that might harm the environment when applied to land.
- Therapeutic methods
Sludge generated during the wastewater treatment process needs to be handled carefully and disposed of properly. Digestion reduces the volume of raw sludge in many major operations. Sludge thickener for sewage. Often, the initial phase in the sludge treatment process is thickening. Larger aggregates can be stirred up in the sludge from primary or secondary clarifiers to help it settle more quickly (typically after the addition of clarifiers). In contrast to secondary sludge, which can be thickened to around 4% solids, primary sludge can be thickened to roughly 8 or 10% solids. Thickeners frequently resemble clarifiers with an additional agitator. Liquid thickener overflow is returned to the wastewater treatment process whereas thickened sludge with less than ten percent particles may undergo extra sludge treatment. 
- Drainage
Diagram of a belt press for dewatering sewage sludge. Gravity is used to extract the filter first, followed by rollers forcing the fabric through. German sewage treatment facility's sludge treatment. Mechanical dewatering (centrifuge) in a big wastewater treatment plant (Arudas treatment plant, Belo Horizonte, Brazil) (Arudas treatment plant, Belo Horizonte, Brazil). To lower transport costs for disposal or recovery suited for composting, the water content of sludge can be lowered using centrifugation, filtration, and/or evaporation. Centrifuging can be used as a first step to decrease the volume of sludge prior to filtration or evaporation. Filtration can be carried out mechanically in a belt filter press or by draining in a sand dryer bed. Concentrates and filtrates are typically recycled back into the clarifying procedure. After dewatering, the sludge can be regarded as a solid with a water content of 50–75%. Higher moisture content dewatered sludges are typically regarded as liquids.
- Digestion
Different digesting techniques are used to treat a variety of sludge with the goal of lowering the levels of organic matter and harmful bacteria in the solids. Composting, aerobic digestion, and anaerobic digestion are the most popular forms of therapy. By lowering the volume of sludge by around 50% and producing biogas as a useful energy source, sludge digestion offers major cost advantages. The reduction of harmful bacteria and organic materials in solids is the goal of digestion. Methane gas, which can be utilized as a fuel to run a power plant or sold, is frequently produced using this process when it is optimized to do so. 
Fermenting without oxygen Overview of anaerobic digestion Brazilian wastewater treatment facility Arrudas uses anaerobic digesters to treat sewage sludge. Bacterial activity that takes place without the presence of oxygen is known as anaerobic fermentation. This can be mesophilic digestion, which takes place at about 36°C, or thermophilic digestion, where sludge is fermented in tanks at 55°C. While thermophilic digestion allows for shorter residence times (and hence smaller tanks), the cost of heating the sludge is higher. Another typical method for treating the sewage sludge created in sewage treatment plants is mesophilic anaerobic digestion (MAD). To go through the four-stage digestion process needed for sludge digestion, sludge is transferred to big tanks and held there for at least 12 days. These include methanogens, acid genesis, hydrolysis, and acetogenesis. Proteins and complex carbohydrates are converted into water and other less complicated substances during this process. High methane biogas produced by anaerobic digestion can be utilized to power motors or micro turbines for other on-site procedures as well as to heat the storage tank. The anaerobic process produces methane, which is one of its main advantages. Its primary drawbacks are the lengthy process time (up to 30 days) and expensive capital expense. The temperature of the digester is maintained at 35 3 °C at many larger plants that use biogas for cogeneration. This allows for the production of more electricity than what is required by machines. The Sludge Treatment Plant ("T-PARK") is able to generate electricity for both its own operations and even Hong Kong's public power system using the heat produced during the combustion of sludge. 
- Aeration of the food
A continuation of the activated sludge process, aerobic digestion is a bacterial process that occurs in the presence of oxygen. Bacteria quickly devour organic stuff and turn it into carbon dioxide when the environment is aerobic. When there isn't enough organic matter, bacteria perish and become food for other bacteria. Endogenous respiration is the name of this step in the process. A solids reduction occurs in this step. The capital expense of aerobic digestion is cheaper since it is significantly faster than anaerobic digestion. However, because the energy needed to run the fans, pumps, and motors needed to deliver oxygen to the process are used up, the running expenses for aerobic digestion are often substantially greater. Non-electric ventilation filtration systems, on the other hand, employ natural air currents to ventilate instead of electric devices, thanks to recent technological advancements. By oxidizing the sludge with spreader systems or jet aerators, aerobic digestion can be accomplished. The less expensive diffusion method is often fine bubble diffusers, although clogging is frequently an issue because silt collects in the tiny air holes. The majority of the time, activated sludge tanks or flocculation procedures use coarse bubblers. Making ensuring the diffuser delivers the requisite oxygen transfer rate is a crucial consideration in diffuser type selection. 
- Complementary medical techniques
The thickened or dewatered sludge and a liquid fraction known as sludge treatment liquors, sludge dewatering streams, liquors, concentrates (if they come from a centrifuge), filtrate (if from a belt press filter), or something similar are the two byproducts of sludge treatment technologies. This liquid needs additional processing since it contains a lot of nitrogen and phosphorus, particularly if the sludge underwent anaerobic digestion. The treatment may occur separately from the treatment process or within the treatment plant itself (with liquid recirculation up until the start of the treatment process).
- Phosphorous restoration
Utilizing a technique for phosphorus recovery is one way to treat sludge drainage streams. Reduced struvite deposits in pipes, pumps, and valves is another advantage for wastewater treatment plant operators when treating sludge drainage streams for phosphorus recovery. Such obstructions may cause maintenance issues, particularly in biological denutrient systems where the phosphorus content of sewage sludge is considerable. For instance, the Canadian company Ostara Nutrient Recovery Technologies sells a method that recovers struvite in the form of crystalline pellets from sludge dewatering streams by the controlled chemical precipitation of phosphorus in fluidized bed reactor. Under the trade name "Crystal Green," the resulting crystal product is offered as fertilizer to the agricultural, grass, and decorative industries. 
- Composting
In the aerobic process of composting, agricultural waste products from carbon sources like sawdust, straw, or wood chips are combined with sewage sludge. When there is oxygen present, bacteria that break down both sludge and plant matter produce heat that kills parasites and harmful microbes. Fillers are necessary to maintain aerobic conditions at 10-15% oxygen by allowing air to pass through the fine mud particles. Corn cobs, nut shells, wood chips, or bark from lumber or paper mills are examples of hard materials that separate the sludge for greater aeration than softer leaves and grass clippings. Small, soft plant materials can be the main source of carbon in a framework made of light, biologically inert fillers like tyre shreds. By spreading an insulation layer of pre-composted sludge over aerated compost piles, it is possible to help ensure a uniform distribution of pathogen-killing temperatures. Compost mixture should start out with a moisture content of about 50%. However, temperatures could not be high enough to kill microorganisms when wet mud raises the compost's moisture content to beyond 60%. On concrete slabs with internal air ducts, compost mixtures can be used to add a layer of unmixed filler. Odors can be reduced by sucking previously composted sludge through filter stacks, which are replenished when humidity reaches 70%, and by using a vacuum aeration fans from inside compost piles through ducts below. You can feed the sewage treatment facility with the liquid that builds up in the floor drain. Additionally, compost pads can be improved for better moisture management. Compost piles can be screened to collect undigested fillers for reuse after a composting period long enough to kill microorganisms. Additionally, composted materials that get through the filter can be employed as soil supplements and have similar advantages to peat. The ideal initial carbon-to-nitrogen ratio for a compost mixture is between 26 and 30, however the amount of agricultural byproducts that must be composted to reduce the amount of hazardous compounds in the sludge to a safe level can be calculated. Compost that will be diluted before usage. Although the toxicity of the majority of agricultural wastes is modest, pesticide residues that are detrimental for some agricultural uses may be present in suburban grasses. Phytotoxins can also be present in recently composted wood wastes, which hinder seedlings from germination until they are detoxicated by soil fungus. 
- Combustion
However, combustion is utilized considerably less frequently. The need to release additional air and fuel (typically natural gas or fuel oil) in order to burn the low calorific value sludge and evaporate the leftover water makes the incineration of sewage sludge less common. On a dry solids basis, sewage sludge has a calorific value of about 9,500 calorific value per pound (980 cal/g), while primary sludge that has been digested has a calorific value of 2,500 calorific value per lb (260 cal/g). The most often utilized systems for burning of sewage sludge are fluidized bed incinerators and long residence time incinerators. In municipal incinerators, co-incineration is occasionally used, albeit this is a less expensive option if solid waste facilities are already in place and supplemental fuels are not needed. The heavy metal content in the solid ash that is left after incineration, which must be disposed of, tends to be maximized. When wet scrubber effluent is capable of being returned to the septic system, the amount of dissolved salts in the effluent from the WWTP is increased, which can minimize air emissions.