in what ways in sludge obtained from wwtp used

Coarse primary solids and secondarybiosolidsaccumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. This material may be inadvertently contaminated with toxic organic and inorganic compounds (e.g.heavy metals).^{[citation needed]}

Many sludges are treated using a variety of digestion techniques, the purpose of which is to reduce the amount oforganic matterand the number of disease-causingmicroorganismspresent in the solids. The most common treatment options includeanaerobic digestion, aerobic digestion, andcomposting.

Anaerobic digestionis a bacterial process that is carried out in the absence of oxygen. The process can either bethermophilicdigestion in which sludge isfermentedin tanks at a temperature of 55C ormesophilic, at a temperature of around 36C. Though allowing shorter retention time, thus smaller tanks, thermophilic digestion is more expensive in terms of energy consumption for heating the sludge.

Anaerobic digestion generatesbiogaswith a high proportion ofmethanethat may be used to both heat the tank and runenginesormicroturbinesfor other on-site processes. In large treatment plants sufficient energy can be generated in this way to produce moreelectricitythan the machines require. The methane generation is a key advantage of the anaerobic process. Its key disadvantage is the long time required for the process (up to 30 days) and the high capital cost.

Under laboratory conditions it is possible to directly generate useful amounts of electricity from organic sludge using naturally occurring electrochemically active bacteria^{[citation needed]}. Potentially, this technique could lead to an ecologically positive form of power generation, but in order to be effective such amicrobial fuel cellmust maximize the contact area between the effluent and the bacteria-coated anode surface, which could severely hamper throughput.

For the Upflow Anaerobic Sludge Blanket treatment seeUASB.

Aerobicdigestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it intocarbon dioxide. Once there is a lack of organic matter, bacteria die and are used as food by other bacteria. This stage of the process is known asendogenous respiration. Solids reduction occurs in this phase. Because the aerobic digestion occurs much faster than anaerobic digestion, the capital costs of aerobic digestion are lower. However, the operating costs are characteristically much greater for aerobic digestion because of energy costs foraerationneeded to add oxygen to the process.

Composting is also an aerobic process that involves mixing the wastewater solids with sources ofcarbonsuch as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat.

Both anaerobic and aerobic digestion processes can result in the destruction of disease-causing microorganisms andparasitesto a sufficient level to allow the resulting digested solids to be safely applied to land used as a soil amendment material (with similar benefits topeat) or used for agriculture as a fertilizer provided that levels of toxic constituents are sufficiently low.

The largest composting site in the world that also processes sewage is theEdmonton Composting Facility, inEdmonton, Canada.

Thermal depolymerizationuseshydrous pyrolysisto convert reduced complex organics to oil. The premacerated, grit-reduced sludge is heated to 250C and compressed to 40 MPa. The hydrogen in the water inserts itself between chemical bonds in natural polymers such as fats, proteins and cellulose. The oxygen of the water combines with carbon, hydrogen and metals. The result is oil, light combustible gases such as methane, propane and butane, water with soluble salts, carbon dioxide, and a small residue of inert insoluble material that resembles powdered rock and char. All organisms and many organic toxins are destroyed. Inorganic salts such as nitrates and phosphates remain in the water after treatment at sufficiently high levels that further treatment is required.

The energy from decompressing the material is recovered, and the process heat and pressure is usually powered from the light combustible gases. The oil is usually treated further to make a refined useful light grade of oil, such as no. 2 diesel and no. 4 heating oil, and then sold.

The choice of a wastewater solid treatment method depends on the amount of solids generated and other site-specific conditions. However, in general, composting is most often applied to smaller-scale applications followed by aerobic digestion and then lastly anaerobic digestion for the larger-scale municipal applications.

When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Typically, sludges are thickened (dewatered) to reduce the volumes transported off-site for disposal. Processes for reducing water content include lagooning in drying beds to produce a cake that can be applied to land or incinerated;pressing, where sludge is mechanically filtered, often through cloth screens to produce a firm cake; and centrifugation where the sludge is thickened by centrifugally separating the solid and liquid. Sludges can be disposed of by liquid injection to land or by disposal in a landfill. There are concerns about sludgeincinerationbecause of air pollutants in the emissions, along with the high cost of supplemental fuel, making this a less attractive and less commonly constructed means of sludge treatment and disposal. There is no process in the commercial marketplace today which completely eliminates the requirements for disposal of biosolids. However California-based Gate 5 Energy Partners Inc. expects to begin construction on 2014 of an innovative sludge to energy plant that will eliminate sewage sludge (and other natural organic waste materials). The Gate 5 Energy System transforms this waste into biofuel that is combusted to produce heat used to produce the biofuel from sludge, run the process and produce surplus heat or energy for use by others.

In South Australia, after centrifugation, the sludge is then completely dried by sunlight. The nutrient rich biosolids are then provided to farmers free-of-charge to use as a natural fertiliser. This method has reduced the amount of landfill generated by the process each year.

In the very large metropolitan areas of southernCaliforniainland communities return sewage sludge to the sewer system of communities at lower elevations to be reprocessed at a few very large treatment plants on the Pacific coast. This reduces the required size of interceptor sewers and allows local recycling of treated waste-water while retaining the economy of a single sludge processing facility