unit five sludge digestion and dewatering

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UNIT FIVE Sludge Digestion and Dewatering Anaerobic Digestion of Sludges: Anaerobic digestion is the most common process for dealing with wastewater sludge containing primary and secondary sludges. Primary sludge is the solids which settle out of the wastewater in the sedimentation tanks just after the wastewater passes through the grit chambers. The settled material represents 40% to 60% of the suspended solids and 30% to 35% of the BOD that exist in the wastewater. Secondary sludges are also sent to the digester. Secondary sludge is generated in the trickling filter and activated sludge processes. The process is much economical than aerobic digestion.

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UNIT FIVE Sludge Digestion and DewateringUNIT FIVE Sludge Digestion and Dewatering •Anaerobic Digestion of Sludges:
Anaerobic digestion is the most common process for dealing with wastewater sludge containing primary and secondary sludges. Primary sludge is the solids which settle out of the wastewater in the sedimentation tanks just after the wastewater passes through the grit chambers. The settled material represents 40% to 60% of the suspended solids and 30% to 35% of the BOD that exist in the wastewater. Secondary sludges are also sent to the digester. Secondary sludge is generated in the trickling filter and activated sludge processes. The process is much economical than aerobic digestion.
Sludge Digestion Contd.
• An anaerobic sludge digester is designed to encourage the growth of anaerobic bacteria, The process undergoes in the following four steps:
(1) Hydrolysis: large polymers are broken down by enzymes to simpler ones.
(2) Fermentation: Simpler organic compounds are then metabolized to form volatile fatty acids and carbon dioxide.
(3) Acetogenesis: Breakdown of volatile acids to acetate, formaldehyde and hydrogen, and
(4) Methanogenesis: Acetate, formaldehyde, hydrogen and carbon dioxide are converted to methane and water. In this process the volume of sludge gets reduced and sludges become easily dewater-able, because the biochemical reactions in digester break the complex bond between water and organic matter in sludge.
Sludge Digestion Contd.
• The stability of the anaerobic process is very fragile. The balance between several microbial populations must be maintained. Under normal circumstance pH range is self maintained. However, during shock loading the acid concentration can increase. When this happens methane production stops and the acid levels rise to the tolerance level of the methane formers. At this point the system fails. Temperature is also a critical element. Sudden changes in temperature adversely affect the methane producers.
Sludge Digestion and Dewatering Contd.
• When operating properly, the sludge goes into the digester (closed tank with air tight cover). The digester can be operated as single or two stages. DT is between 10 to 90 days depending on the system. Temperature is maintained 37 degree Celsius in mesophilic and 55 degrees in thermophilic digesters. The, methane, carbon dioxide and traces of hydrogen sulfide go out the gas outlet. Supernatant, the water generated by the process and the water in the sludge, is drawn off as necessary and sent back through the plant, and stabilized sludge is pulled off the bottom to go to the drying beds.
Sludge Digestion Contd.
• High rate digesters are more efficient and often require less volume than single stage digesters. In the first stage the sludge is mechanically mixed to ensure better contact between the organics and the bacteria. The unit is heated to increase the metabolic rate of the microorganisms, thus speeding up the digestion process. In the second stage the sludge is allowed to stratify and separate into layers. The gas storages have floating cap for collection and for gas recovery. The supernatant, scum and digested sludge are drawn out of this unit.
Design Criteria
Other Parameters
• The digesters are cylindrical in shape having dia ranging from 2-12m and height greater than 6m.
• They have a bottom sloping 1:1 to 1:3The gas produce ranges between 11- 21 liters per capita per day.
• The digested sludge can be used for filling low lying areas or as manure as it contains 1.7% nitrogen, 1.5% phosphorous and 0.5% potash. Sometimes it is also disposed by burning.
• The gas collected has 70% methane, has a fuel value of 5800 k cal/cub m.
• Gas may be utilized for operating gas engines, for heating sludges for promoting digestion or in case of larger plants (more than 50000 persons) for supplying domestic power.
Sludge Dewatering
• The digested sludge has a lot of water (85-90%) and it is to be dewatered for further use or disposal.
• In India, because of favorable weather dewatering is usually done on open beds of land. Each bed is 15 x 30m in plan, 45-60 cm deep consisting of 30-45 cm of thick graded layer of gravel at bottom overlain by 10-15 cm of coarse sand. Open jointed under-drain pipes (15 cm [email protected] 5-7 m spacing are laid below to collect the water.
Sludge Dewatering Contd.
• Sludge is spread on top in depths of 20-30cm through distribution troughs. A portion of moisture also is lost through evaporation. Sludge is removed manually after a period of 7-10 days.
• Other methods of dewatering include vacuum filtration, centrifugation and filter press.
Unit Seven – Solid Wastes
Solid wastes:
• These include all solid and semi-solid materials discarded by community. The solid waste generated by domestic and commercial establishments is called municipal solid wastes (MSW) or REFUSE. Animal excreta (semi-solid waste) also find its way in to municipal systems
• Solid wasted generated by industries is called Industrial solid wastes.
• Solid wastes are also generated in agricultural activities, large part of which finds its way into MSW.
• Solid wastes are also generated in hospitals called – hazardous bio medical wastes.
• For the purpose of this course our discussions are limited to MSW.
• MSW is a complex mixture of various kinds of unwanted materials as follows:
Garbage :
• This consists of all sorts of putrescible matter. These are mainly organic waste from kitchens, hotels, restaurants, in the form of waste food articles, vegetable and fruit peelings, cow dung, manure, etc. It decomposes quickly. It normally weighs from 450 to 900 kg/m3. It should be handled carefully, because flies, insects, rats etc. breed in it.
• The other fraction of wastes are non-putrescible matter. These are also called Rubbish.
• Rubbish may contain combustible materials e.g. paper, rag, sack, plastic and also non combustible matter, e.g. glass, plastics, metal cans, demolition residues of buildings, or dusts, sand and silt coming from street sweepings, etc. The density of rubbish varies from 50-400 kg/m3
• In addition to above, solid waste are also due to :
• Street refuse : This consists empty packets and bottles, empty matches and cigarette boxes, fruit peels, tree leaves, branches of trees, street sweepings etc. and
• Ashes : Ashes are incombustible waste products from houses, industries, hearths and furnaces. With the introduction of kerosene oil and cooking gas, its quantity is now gradually decreasing.
• Safe disposal of solid wastes is necessary otherwise they will pile in heaps and cause foul smell. They will also promote breeding of flies, mosquitoes, rodents (rats) and cause outbreak of plague. These wastes also lead to formation of leachate (dark black colored liquid) during rains which may seep down and pollute ground water. This specially happens when wastes contain non biodegradable substances, plastics, paints, insecticides, unused medicines etc.
Quantity of MSW
• Quantity of MSW produced by a society depends upon the living standards of its residents, the degree of commercialization and urbanization. This figure is as high as 2.8 kg/c/d in US and is about 0.5 kg/c/d for India. The quantity of MSW depends upon season too. In summer the value increases by about 25% because of refuse from fruits (mangoes and melons etc).
• The average composition of refuse by weight is about - Organic wastes generally 55% while inorganic wastes are 45%. These figures can also be represented as: 25% cinder, 27% fine dust, 15% ashes, 4% empty tins and cans, 14% putrescible matter, 2% glass and crockery, 2% rags, 1% bone and 10% miscellaneous matter.
• Generally, refuse is collected from individual houses in small containers or cans kept outside the premises of the house, from where it is removed daily by sweepers in hand driven carts, tricycles and then dumped into masonry chambers constructed by municipalities along the roadside. The refuse from there is carted away by trailers (2- 3 tonnes) or trucks (5-10 tonnes). Generally persons responsible for collection are not careful and there is a lot of littering that goes alongside collection..
• Public dust-bins are provided by the municipalities/local boards at convenient places by the sides of roads. Dry refuse fallen on the public streets and roads, along with road sweepings, are usually collected once or twice a day by the sweepers employed by the local authority. For this purpose, a portable galvanized iron receptacle with-a closely fitting lid, and having a capacity of 0.02 to 0.1 m3 is generally used. These are again collected in tucks.
• The frequency of refuse collection should be such that the refuse may not start giving nuisance by odor and fly breeding. The collection of refuse from the business areas should be done during non-working hours.
Recycling and reuse
• In India old newspapers, magazines, books, empty bottles, glass, plastics, metal cans and discarded appliances are salvaged and sold to kabaris. Often these materials are also recovered by scavengers at municipal dump sites. From here the stuff is sent to factories to produce low cost products e.g. chappals, mugs, utensils, toys, recycled paper, glass containers and hollow wares.
Disposal of Solid Wastes
Refuse or solid waste in India is disposed of by the following methods.
• Filling of low lying areas.
• Controlled tipping – Sanitary land fill.
• Composting.
• Incineration and thermal Pyrolysis
• Filling of low lying area (land-filling) : This method is quite common. The garbage is dumped into low lying areas or depressions available nearby. However, since good scientific disposal practices may or may not be followed in many towns in India, this method may give rise to odors and other health hazards.
• While disposal by filling the low lying areas is simplest and economical because no heavy machinery or plant are required, the process has many disadvantages e.g. (i) wind direction may not be favorable (ii) large land areas are required (iii) dumped garbage containing carcinogenic non-biodegradable matter (such as plastics, unused medicines, paints, insecticides, etc may cause trouble later because of leachate coming out of the dump during rainy season which may pollute surface water as well as ground.
Controlled tipping – Sanitary land fill
• In this method the entire landfill area (enough to dump waste for 5-10 years) is divided into cells. Refuse is dumped and compacted in cells with bull dozers in layers of 0.5 m depths. When the compacted depth becomes 1.5 m, it is covered by 0.2 m thickness of good earth. After filling the cells in the first lift the second lift is laid. The process goes on till the top most lift is pilled up, over which the final cover of 0.6 m of earth cover is laid. This ensures that there is no scattering due to wind blowing or foul smell. In about 2-4 months the filled up refuse gets stabilized due to decomposition of organic matter and settles down by about 20- 40% of its original depth. This land is then used for developing some parks, green land or other recreational spots.
• The bottom of the landfill site is made water proof so not to allow any leachate to flow down and pollute the ground water. A leachate collection mechanism is installed – which is then separately treated and disposed of. Also an arrangement for venting of gas is made much like the septic tank system
• Composting is a biological method in which putrescible organic matter in the solid waste/refuse is digested. It is a hygienic method which coverts the refuse into nitrogen rich humus through the bacterial agencies.
• Due to composting, the volume of refuse is very much reduced, and the resulting matter becomes free from pathogenic organisms. In India normally, night soil of the conservancy system is also disposed of simultaneously along with refuse, producing valuable manure.
• Basically composting is an aerobic process because it involves piling of refuse and regularly turning it to ensure sufficient supply of oxygen. Due to oxidation of organic matter the temperature rises to 60 degrees in 3-4 days time. This needs to be maintained for next 4 days to kill pathogenic microbes. The pile is then turned more frequently to slow the reaction down for another 4-5 weeks. The compost is then allowed to cure for 2-8 weeks within which a total volume reduction of about 50% is ensued, leaving dark brown colored product with earthy smell.
In India, there are two methods of mechanical composting :
• (i) Indore method and (ii) Bangalore method
• Indore method uses aerobic pathway. In this, layers of vegetable wastes and night soil are alternately piled in depths of about 7.5 – 10 cm up to a total depth of about 1.5 m in a trench or above the ground to form a mound. Chemicals are added to prevent fly breeding. The material is turned regularly for a period of about 8 to 12 weeks, and then stored on the ground for 4-6 weeks. In about 6-8 turnings and in about 4 months time, the compost becomes ready for use as manure.
• Bangalore method: In this method the refuse is stabilized anaerobically. This does not require turning or handling of mass so it is cleaner than Indore method and hence widely adopted for municipalities throughout India. Earthen trenches of size 10 x 1.5 x 1.5 m deep are filled up in alternate layers of refuse and night soil/cow dung in piles of 15 cm depths The material is covered with 15 cm layer of good earth and left for decomposition. In about 4-5 months, the compost becomes ready. The humus is sieved through 12.5 mm sieve to remove broken glass, stones etc and is ready for use.
• Normally, a city produces 200 to 250 kg/capita/year of refuge and 8 to 10 kg/capita per year of night soil. Hence a town of 10,000 population will produce about 2000 tonnes of refuse and 800 tonnes of night soil annually. The composting will produce about 1400 to 1680 (50 to 60%) of compost annually from the above waste.
• This consists of burning the refuse in the incinerator plant. This is commonly used in disposing of garbage from hospitals and industrial plants. Before incineration, non-combustible and inert material like earth, broken glass, chinaware, metal etc are separated, so as to reduce the load on the hearth. The by product of this method is ash and clinker which can be easily disposed of by land filling. The heat generated by burning the refuse may be utilized for raising steam power. Emission of air pollutants from incinerators includes particulates such as fly-ash, and other product of combustion. Permissible level of particulate emissions from large incinerators is 0.23 g per standard cubic meter of exhaust gas. Smoke is also undesirable product. Smoke can be eliminated by mixing the exhaust with hot air to complete combustion.
• Advantages of incineration : (i) This is most hygienic method, since it ensures complete destruction of pathogens (ii) The heat generated can be used for raising steam power (iii) Clinker produced can be used for road purposes (iv) The disposal site (i.e. incineration site) can be located at a convenient distance (vi Lesser space is required for disposal of residues (vii) Adverse weather condition have no effect on the incinerator's operation.
• Disadvantages : (i) Large initial expenditure (ii) Improper operation results in air pollution problems and incomplete reduction- of the waste materials . (iii) Disposal of the remaining residue is required (iv) High stacks needed for natural draft chimneys present safety problems.
• This is also known a thermal cracking. In this process the waste is heated in closed containers. The reaction is endothermic (consumes heat). After the process is complete we obtain (i) gas stream - hydrogen, methane, CO and CO2 etc, depending upon the characteristics of wastes (ii) liquid fraction – consisting of tar and/or oil stream. It is also found to contain acetic acid, acetone etc., and (iii) a solid fraction – the charred solid waste.