Evaluating The Waste Water Treatment Processes Environmental Sciences Essay

Evaluating The Waste Water Treatment Processes env crusademental Sciences EssayDomestic photocopy body of piddle discussion or cloaca discussion, is the wreak of removing contaminants from fellate urine and household sewer, both runoff (effluents) and domestic. It onlyow ins physical, chemical, and biological processes to ask come on physical, chemical and biological contaminants. Its objective is to produce an environmentally-safe fluid waste stream (or enured effluent) and a solid waste (or enured goo) suitable for disposal or reuse (usually as farm fertilizer). Using advanced technology it is to day sequence possible to re-use sewer effluent for drinking peeing, although Singapore is the only soil to implement such(prenominal) technology on a production subdue in its production of NEWater.1.2 ORIGIN OF WASTE WATER sewerage is created by residential, institutional, and commercial and industrial establishments and let ins household waste quiet from toilets, bath s, showers, kitchens, sinks and so forwards that is accustomed of via sewers. In many an early(a)(prenominal) beas, sewage withal complicates liquid waste from industry and commerce. The separation and draining of household waste into grey wet and blackwater is becoming to a greater extent(prenominal) harsh in the developed world, with greywater being permitted to be utilise for tearing lay come forths or recycled for flushing toilets.Sewage whitethorn include stormwater runoff. Sewerage dodges capable of discourse stormwater argon known as combined systems. Combined sewer systems ar usually avoided now because precipitation causes widely varying flows reducing sewage give-and-take plant efficiency. Combined sewers require much divulgestandingr, much(prenominal) pricy, discourse facilities than sanitary sewers. Heavy storm runoff may overwhelm the sewage word system, ca use a spill or overflow. Sanitary sewers atomic number 18 typicly much smaller than comb ined sewers, and they atomic number 18 not designed to transport stormwater. Backups of raw sewage support occur if excessive infiltration/Inflow is exited into a sanitary sewer system.Modern sewered developments tend to be submitd with sepa say storm drain systems for rainwater. As rainfall travels over roofs and the ground, it may pick up various contaminants including soil routineicles and different sediment, heavy metals, radical compounds, animal waste, and oil and grease. (See urban runoff.) more or less jurisdictions require stormwater to receive approximately level of manipulation before being issued directly into waterways. Examples of manipulation processes apply for stormwater include retention drainage areas, wetlands, buried vaults with various kinds of media permeates, and vortex separators (to remove coarse solids).CHAPTER TWO2.1 OVERVIEW OF WASTE WATER TREATMENT PROCESSESSewage hind end be look at close to where it is created, a de centralized sys tem, (in septic tanks, biofilters or aerophilic sermon systems), or be stack away and transported via a lucre of pipes and pump stations to a municipal treatment plant, a centralised system, (see sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, secernate and federal regulations and standards. Industrial sources of effluent much require peculiar(a)ized treatment processes as shown in the draw belowProcess Flow diagram for a typical treatment plant via Sub get on Flow Constructed Wetlands (SFCW)Sewage treatment generally involves three stages, called primary, substitute(prenominal) and 3rd treatment. simple treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can prove to the bottom while oil, grease and twinkle solids float to the out-of-doors. The settled and float poppycocks are removed and the remaining liquid may be accomplish or subjected to secondary treatment. indir ect treatment removes dissolve and suspended biological upshot. Secondary treatment is typically per masterminded by indigenous, water-borne micro-organisms in a wind habitat. Secondary treatment may require a separation process to remove the micro-organisms from the do by water prior to discharge or tertiary treatment.Tertiary treatment is some successions defined as anything more than primary and secondary treatment in order to give rejection into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs etc.). treat water is one-time(prenominal)s disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be utilize for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or countrified social functions.2.2 PRE-TREATMENTPre-treatment removes bodilys that can be easily collected from the raw waste water before they ill-treat or clog the pumps and skimmers of primary treatment clarifiers (trash, direct limbs, leaves, etc.).SCREENINGThe influent sewage water is screened to remove all queen-size objects like cans, rags, sticks, plastic packets etc. carried in the sewage stream. This is virtually(prenominal)(prenominal) unremarkably done with an automated mechanically raked bar screen in ultra innovative plants serving rotund populations, whilst in smaller or slight modern plants a manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The solids are collected and later given in a landfill or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids remotion. If gross solids are not removed they become entrained in pipes and moving part of the treatment plant and can cause substantial damage and inefficiency in the process.GRIT remotionPre-treatment may include a sand or grit contri savee or chamber where the velocity of the incoming wastewater is adjusted to allow the small town of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other equipment. For small sanitary sewer systems, the grit chambers may not be necessary, but grit removal is desirable at vauntinglyr plants.FAT AND GREASE removalIn some larger plants, plunk down and grease is removed by passing the sewage through and through a small tank where skimmers collect the fat floating on the surface. striving blowers in the house of the tank may also be used to help recover the fat as a froth. In most plants however, fat and grease removal takes orient in the primary colonisation tank using mechanical surface skimmers.2.3 PRIMARY TREATMENTIn the primary sedimentation stage, sewage flows through large tanks, habitually called primary clarifiers or primary sedimentatio n tanks. The tanks are used to settle gook while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected gook towards a hopper in the base of the tank where it is pumped to droppings treatment facilities. Grease and oil from the floating worldly can sometimes be recovered for saponification.The dimensions of the tank should be designed to effect removal of a high voice of the floatables and scoop. A typical sedimentation tank may remove from 60 to 65 part of suspended solids, and from 30 to 35 percent of biochemical group O demand (BOD) from the sewage.2.4 SECONDARY TREATMENTSecondary treatment is designed to advantageously degrade the biological content of the sewage which are derived from human waste, aliment waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobiotic biological processes. To be effective, the biota require both type O and sustenance to live. The bacteria and protozoa consume biodegradable soluble essential contaminants (e.g. sugars, fats, ingrained short-chain atomic number 6 molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed-film or suspended-growth systems.Fixed-film or link up growth systems include trickling filters and rotating biological gatherors, where the biomass grows on media and the sewage passes over its surface.Suspended-growth systems include set off sludge, where the biomass is mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the kindred hail of water. However, fixed-film systems are more able to cope with drastic changes in the standard of biological existent and can post high removal rates for constitutive(a) material and suspended solids than suspended growth systems.611-13Roughing filters are intended to treat curi ously strong or variable organic loads, typically industrial, to allow them to and thence be treated by conventional secondary treatment processes. Characteristics include filters filled with media to which wastewater is applied. They are designed to allow high hydraulic loading and a high level of aeration. On larger installations, dividing line is forced through the media using blowers. The resultant wastewater is usually inside the normal range for conventional treatment processes.A generalized, schematic diagram of an activated sludge process.A filter removes a small percentage of the suspended organic matter, while the majority of the organic matter undergoes a change of character, only due to the biological oxidization and nitrification taking place in the filter. With this aerobic oxidisation and nitrification, the organic solids are converted into turn suspended mass, which is heavier and bulkier, and can settle to the bottom of a tank. The effluent of the filter is the refore passed through a sedimentation tank, called a secondary clarifier, secondary settling tank or humus tank.ACTIVATED SLUDGEIn general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen to promote the growth of biological floc that substantially removes organic material.The process traps particulate material and can, under ideal conditions, convert ammonium hydroxide to nitrite and nitrate and ultimately to northward gas.SURFACE-AERATED BASINS (LAGOONS)Many small municipal sewage systems in the United States (1 million gal./day or less) use expose lagoons.Most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or cable) and microbial action. Surface-aerated basins achieve 80 to 90 percent removal of BOD with retention times of 1 to 10 days. The basins may range in depth from 1.5 to 5.0 metres and use motor-driven aerators floating on the surface of the wastewater.In an aerated b asin system, the aerators provide two functions they transfer ambience into the basins involve by the biological oxidation reactions, and they provide the conflate required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7kg O2/kWh. However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units.Biological oxidation processes are sensitive to temperature and, between 0 C and 40 C, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between 4 C and 32 C.CONSTRUCTED WETLANDSConstructed wetlands (can either be surface flow or subsurface flow, horizontal or vertical flow), include engineered reedbeds and belong to the family of phytorestoration and ecotechn ologies they provide a high degree of biological expediency and depending on design, act as a primary, secondary and sometimes tertiary treatment, also see phytoremediation. unmatched example is a small reedbed used to clean the drainage from the elephants enclosure at Chester Zoo in England legion(predicate) CWs are used to recycle the water of the city of Honfleur in France and numerous other towns in Europe, the US, Asia and Australia. They are known to be highly amentaceous systems as they copy natural wetlands, called the Kidneys of the earth for their fundamental recycling efficacy of the hydrological cycle in the biosphere. Robust and reliable, their treatment capacities improve as time go by, at the opposite of conventional treatment plants whose machinery age with time. They are being increasingly used, although adequate and experienced design are more fundamental than for other systems and space limitation may impede their use. perk up BEDS (OXIDIZING BEDS)In older pl ants and those receiving variable loadings, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a bed do up of coke ( ampere-secondized coal), limestone chips or specially fabricated plastic media. such media must have large surface areas to support the biofilms that form. The liquor is typically distributed through perforated spray arms. The distributed liquor trickles through the bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the medias surfaces and eat or otherwise wither the organic content. This biofilm is often grazed by insect larvae, snails, and worms which help maintain an optimal heaviness. Overloading of beds increases the thickness of the film raceing to clogging of the filter media and ponding on the surface.SOIL BIO-TECHNOLOGYA new process called Soil Bio-Technology (SBT) de veloped at IIT Bombay has shown tremendous improvements in process efficiency enabling total water reuse, due to super low operating power requirements of less than 50 joules per kg of treated water. Typically SBT systems can achieve chemical oxygen demand (COD) levels less than 10mg/L from sewage input of COD cdmg/L. SBT plants exhibit high reductions in COD values and bacterial counts as a result of the very high microbial densities operational in the media. Unlike conventional treatment plants, SBT plants produce insignificant amounts of sludge, precluding the fate for sludge disposal areas that are required by other technologies.biological AERATED FILTERSBiological Aerated (or Anoxic) Filter (BAF) or Biofilters combine filtration with biological carbon reduction, nitrification or denitrification. BAF usually includes a reactor filled with a filter media. The media is either in suspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highly active biomass that is attached to it and to filter suspended solids. snow reduction and ammonia variation occurs in aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is operated either in upflow or downflow configuration depending on design specified by manufacturer.Schematic diagram of a typical rotating biological contactor (RBC). The treated effluent clarifier/settler is not included in the diagram.ROTATING BIOLOGICAL CONTACTORSRotating biological contactors (RBCs) are mechanical secondary treatment systems, which are robust and capable of withstanding surges in organic load. RBCs were first installed in Germany in 1960 and have since been developed and refined into a reliable operating unit. The rotating disks support the growth of bacteria and micro-organisms present in the sewage, which break down and brace organic pollutants. To be successful, micro-organisms need both oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the micro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage. wastewater from the RBC is then passed through final exam clarifiers where the micro-organisms in suspension settle as a sludge. The sludge is withdrawn from the clarifier for further treatment.A functionally similar biological filtering system has become popular as part of home aquarium filtration and purification. The aquarium water is drawn up out of the tank and then cascaded over a freely spinning fold up fiber-mesh wheel before passing through a media filter and backside into the aquarium. The spinning mesh wheel develops a biofilm coating of microorganisms that feed on the suspended wastes in the aquarium water and are also exposed to the atmosphere as the wheel rotates. This is especially good at removing waste .MEMBRANE BIOREACTORSMembrane bioreactors (MBR) combine activated sludge treatment with a membrane liquid-solid separation process. The membrane component uses low pressure microfiltration or ultra filtration membranes and eliminates the need for clarification and tertiary filtration. The membranes are typically immersed in the aeration tank however, some applications utilize a separate membrane tank. One of the key benefits of an MBR system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes. The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) preoccupancy than CAS systems, which are limited by sludge settling. The process is typically operated at MLSS in the range of 8,000-12,000mg/L, while CAS are operated in the range of 2,000-3,000mg/L. The elevated biomass concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable materials at high er loading rates. Thus increase sludge retention times, usually exceeding 15 days, ensure assoil nitrification even in extremely cold weather.SECONDARY SEDIMENTATIONThe final step in the secondary treatment stage is to settle out the biological floc or filter material through a secondary clarifier and to produce sewage water involveing low levels of organic material and suspended matter.TERTIARY TREATMENTThe purpose of tertiary treatment is to provide a final treatment stage to raise the effluent whole step before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called effluent polishing.FILTRATION mainstay filtration removes much of the balance suspended matter. Filtration over activated carbon, also called carbon adsorption, removes residual toxins.LAGOONINGLagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often move ond. Small filter feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates.NUTRIENT remotionWastewater may contain high levels of the nutritives nitrogen and friction match. Excessive release to the environment can leave behind to a build up of nutrients, called eutrophication, which can in turn embolden the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a quick growth in the population of algae. The algae numbers are unsustainable and at long last most of them die. The decomposition of the algae by bacteria uses up so much of oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causation deoxygenation, s ome algal species produce toxins that contaminate drinking water supplies. divers(prenominal) treatment processes are required to remove nitrogen and phosphorus.NITROGEN REMOVALThe removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. normality gas is released to the atmosphere and indeed removed from the water.Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonia (NH3) to nitrite (NO2) is most often facilitated by Nitrosomonas spp. (nitroso referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp.Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process (if designed well) can do the barter the most easily. Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, organic matter (from faeces), sulfide, or an added donor like methanol.PHOSPHORUS REMOVALPhosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water systems. (For a description of the negative effects of algae, see Nutrient removal). It is also oddly important for water reuse systems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverse osmosis.Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass). When the biomass enriched in these bacteria is uninvolved from the treated water, these biosolids have a high fertilizer value.Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminum (e.g. alum), or lime. This may lead to excessive sludge production as hydroxides precipitates and the added chemicals can be expensive. Chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is easier to operate and is often more reliable than biological phosphorus removal. Another method for phosphorus removal is to use granular laterite.Once removed, phosphorus, in the form of a phosphate-rich sludge, may be stored in a land fill or resold for use in fertilizer.DISINFECTIONThe purpose of disinfection in the tr eatment of waste water is to substantially reduce the number of microorganisms in the water to be discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. ambiguous water give be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate once against effective disinfection. Common methods of disinfection include ozone, centiliter, ultraviolet light, or sodium hypochlorite. Chloramine, which is used for drinking water, is not used in waste water treatment because of its persistence.Chlorination remains the most common form of waste water disinfection in North America due to its low cost and semipermanent history of effectivenes s. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. quietus chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is nephrotoxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the ta rget microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the United Kingdom, UV light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Some sewage treatment systems in Canada and the US also use UV light for their effluent water disinfection.Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a trinity oxygen atom becoming attached and forming O3. Ozone is very unstable and unstable and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated onsite as neede d. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators.ODOUR CONTROLOdours emitted by sewage treatment are typically an indication of an anaerobic or septic condition. Early stages of processing will tend to produce smelly gases, with hydrogen sulfide being most common in generating complaints. Large process plants in urban areas will often treat the flavours with carbon reactors, a contact media with bio-slimes, small doses of chlorine, or circulating fluids to biologically capture and metabolize the obnoxious gases. Other methods of odour control exist, including addition of iron salts, hydrogen peroxide, calcium nitrate, etc. to manage hydrogen sulfide levels.PACKAGE PLANTS AND BATCH REACTORSTo use less space, treat difficult waste and intermittent flows, a number of designs of hybrid treatment plants have been produced. Such pl ants often combine at least two stages of the three main treatment stages into one combined stage. In the UK, where a large number of wastewater treatment plants serve small populations, parcel of land plants are a viable alternative to building a large structure for each process stage. In the US, package plants are typically used in rural areas, highway rest stops and monotone parks. One type of system that combines secondary treatment and settlement is the sequencing tummy reactor (SBR). Typically, activated sludge is mixed with raw incoming sewage, and then mixed and aerated. The settled sludge is run off and re-aerated before a proportion is returned to the headworks. SBR plants are now being deployed in many parts of the world.The disadvantage of the SBR process is that it requires a precise control of timing, mixing and aeration. This preciseness is typically achieved with computer controls linked to sensors. Such a complex, fragile system is unsuited to places where contro ls may be unreliable, poorly maintained, or where the power egress may be intermittent. Extended aeration package plants use separate basins for aeration and settling, and are somewhat larger than SBR plants with reduced timing sensitivity.Package plants may be referred to as high charged or low charged. This refers to the way the biological load is processed. In high charged systems, the biological stage is presented with a high organic load and the combined floc and organic material is then oxygenated for a few hours before being charged again with a new load. In the low charged system the biological stage contains a low organic load and is combined with change for longer times.SLUDGE TREATMENT AND governing bodyThe sludges accumulated in a wastewater treatment process must be treated and be givend of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The mos t common treatment options include anaerobic digestion, aerobic digestion, and composting. Incineration is also used albeit to a much lesser degree.Sludge treatment depends on the amount of solids generated and other site-specific conditions. Composting is most often applied to small-scale plants with aerobic digestion for mid coat operations, and anaerobic digestion for the larger-scale operations.ANAEROBIC DIGESTIONAnaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55C, or mesophilic, at a temperature of around 36C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy phthisis for heating the sludge.Anaerobic digestion is the most common (mesophilic) treatment of domestic sewage in septic tanks, which normally retain the sewage from one day to two days, reducing the BOD by about 35 to 40 percent. This reduction can be increased with a combination of anaerobic and aerobic treatment by installing aerophilous Treatment Units (ATUs) in the septic tank.One major feature of anaerobic digestion is the production of biogas (with the most useful component being methane), which can be used in generators for electricity production and/or in boilers for heating purposes.AEROBIC DIGESTIONAerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. The operating costs used to be characteristically much greater for aerobic digestion because of the energy used by the blowers, pumps and motors needed to add oxygen to the process.Aerobic digestion can also be achieved by using diffusor systems or jet aerators to oxidize the sludge.COMPOSTINGComposting is also an aerobic process that involves mixing the sludge with sources of carbon such as sawd ust, 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.INCINERATIONIncineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically natural gases or fuel oil) required to burn the low calorific value sludge and vaporize residual water. Stepped multiple hearth incinerators with high residence time and fluidized bed incinerators are the most common systems used to combust wastewater sludge. Co-firing in municipal waste-to-energy plants is occasionally done, this option being less expensive assuming the facilities already exist for solid waste and there is no need for auxiliary fuel.CHAPTER THREETERTIARY TREATMENT3.1 SLUDGE DISPOSALWhen 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 f or disposal. There is no process which completely eliminates the need to dispose of biosolids. There is, however, an additional step some cities are taking to superheat sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials. In N