UNIT FIVESludge Digestion and Dewatering•Anaerobic Digestion of Sludges:

Anaerobic digestion is the most common process for dealing withwastewater sludge containing primary and secondarysludges. Primary sludge is the solids which settle out of thewastewater in the sedimentation tanks just after the wastewaterpasses through the grit chambers. The settled material represents40% to 60% of the suspended solids and 30% to 35% of the BOD thatexist in the wastewater. Secondary sludges are also sent to thedigester. Secondary sludge is generated in the trickling filter andactivated sludge processes. The process is much economical thanaerobic digestion.

Sludge Digestion Contd.

• An anaerobic sludge digester is designed to encourage the growth of anaerobicbacteria, 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 formvolatile fatty acids and carbon dioxide.

(3) Acetogenesis: Breakdown of volatile acids to acetate, formaldehyde andhydrogen, and

(4) Methanogenesis: Acetate, formaldehyde, hydrogen and carbon dioxide areconverted to methane and water. In this process the volume of sludge getsreduced and sludges become easily dewater-able, because the biochemicalreactions in digester break the complex bond between water and organic matterin sludge.

Sludge Digestion Contd.

• The stability of the anaerobic process is very fragile. The balancebetween several microbial populations must be maintained. Undernormal circumstance pH range is self maintained. However, duringshock loading the acid concentration can increase. When thishappens methane production stops and the acid levels rise to thetolerance level of the methane formers. At this point the system fails.Temperature is also a critical element. Sudden changes intemperature adversely affect the methane producers.

Sludge Digestion and Dewatering Contd.

• When operating properly, the sludge goesinto the digester (closed tank with airtight cover). The digester can be operatedas single or two stages. DT is between 10to 90 days depending on thesystem. Temperature is maintained 37degree Celsius in mesophilic and 55degrees in thermophilic digesters. The,methane, carbon dioxide and traces ofhydrogen sulfide go out the gas outlet.Supernatant, the water generated by theprocess and the water in the sludge, isdrawn off as necessary and sent backthrough the plant, and stabilized sludge ispulled off the bottom to go to the dryingbeds.

Sludge Digestion Contd.

• High rate digesters are more efficientand often require less volume thansingle stage digesters. In the firststage the sludge is mechanically mixedto ensure better contact between theorganics and the bacteria. The unit isheated to increase the metabolic rateof the microorganisms, thus speedingup the digestion process. In thesecond stage the sludge is allowed tostratify and separate into layers. Thegas storages have floating cap forcollection and for gas recovery. Thesupernatant, scum and digestedsludge are drawn out of this unit.

Design Criteria

Other Parameters

• The digesters are cylindrical in shape having dia ranging from 2-12m andheight 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 asit contains 1.7% nitrogen, 1.5% phosphorous and 0.5% potash. Sometimesit 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 forpromoting 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 bedewatered for further use or disposal.

• In India, because of favorable weather dewatering is usually done onopen beds of land. Each bed is 15 x 30m in plan, 45-60 cm deepconsisting of 30-45 cm of thick graded layer of gravel at bottomoverlain by 10-15 cm of coarse sand. Open jointed under-drain pipes(15 cm dia@ 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 distributiontroughs. 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. Thesolid waste generated by domestic and commercial establishments is calledmunicipal solid wastes (MSW) or REFUSE. Animal excreta (semi-solid waste) alsofind 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 findsits way into MSW.

• Solid wastes are also generated in hospitals called – hazardous bio medicalwastes.

• 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 mainlyorganic waste from kitchens, hotels, restaurants, in the form of wastefood articles, vegetable and fruit peelings, cow dung, manure, etc. Itdecomposes quickly. It normally weighs from 450 to 900 kg/m3. Itshould be handled carefully, because flies, insects, rats etc. breed init.

• The other fraction of wastes are non-putrescible matter. These arealso called Rubbish.

• Rubbish may contain combustible materials e.g. paper, rag, sack,plastic and also non combustible matter, e.g. glass, plastics, metalcans, demolition residues of buildings, or dusts, sand and silt comingfrom street sweepings, etc. The density of rubbish varies from 50-400kg/m3

• In addition to above, solid waste are also due to :

• Street refuse : This consists empty packets and bottles, emptymatches and cigarette boxes, fruit peels, tree leaves, branches oftrees, street sweepings etc. and

• Ashes : Ashes are incombustible waste products from houses,industries, hearths and furnaces. With the introduction of keroseneoil and cooking gas, its quantity is now gradually decreasing.

• Safe disposal of solid wastes is necessary otherwise they will pile inheaps and cause foul smell. They will also promote breeding of flies,mosquitoes, rodents (rats) and cause outbreak of plague. Thesewastes also lead to formation of leachate (dark black colored liquid)during rains which may seep down and pollute ground water. Thisspecially happens when wastes contain non biodegradablesubstances, plastics, paints, insecticides, unused medicines etc.

Quantity of MSW

• Quantity of MSW produced by a society depends upon the livingstandards of its residents, the degree of commercialization andurbanization. This figure is as high as 2.8 kg/c/d in US and is about 0.5kg/c/d for India. The quantity of MSW depends upon season too. Insummer the value increases by about 25% because of refuse fromfruits (mangoes and melons etc).

• The average composition of refuse by weight is about - Organicwastes generally 55% while inorganic wastes are 45%. These figurescan 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.

COLLECTION AND REMOVAL OF- REFUSE

• Generally, refuse is collected from individual houses in small containers or cans keptoutside the premises of the house, from where it is removed daily by sweepers inhand driven carts, tricycles and then dumped into masonry chambers constructed bymunicipalities 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 arenot careful and there is a lot of littering that goes alongside collection..

• Public dust-bins are provided by the municipalities/local boards at convenient placesby the sides of roads. Dry refuse fallen on the public streets and roads, along withroad sweepings, are usually collected once or twice a day by the sweepers employedby the local authority. For this purpose, a portable galvanized iron receptacle with-aclosely fitting lid, and having a capacity of 0.02 to 0.1 m3 is generally used. These areagain collected in tucks.

• The frequency of refuse collection should be such that the refuse may not startgiving nuisance by odor and fly breeding. The collection of refuse from the businessareas 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 tokabaris. Often these materials are also recovered by scavengers atmunicipal dump sites. From here the stuff is sent to factories toproduce 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 availablenearby. However, since good scientific disposal practices may or maynot be followed in many towns in India, this method may give rise toodors and other health hazards.

• While disposal by filling the low lying areas is simplest andeconomical because no heavy machinery or plant are required, theprocess has many disadvantages e.g. (i) wind direction may not befavorable (ii) large land areas are required (iii) dumped garbagecontaining carcinogenic non-biodegradable matter (such as plastics,unused medicines, paints, insecticides, etc may cause trouble laterbecause of leachate coming out of the dump during rainy seasonwhich 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) isdivided into cells. Refuse is dumped and compacted in cells with bull dozers inlayers of 0.5 m depths. When the compacted depth becomes 1.5 m, it is coveredby 0.2 m thickness of good earth. After filling the cells in the first lift the secondlift is laid. The process goes on till the top most lift is pilled up, over which thefinal cover of 0.6 m of earth cover is laid. This ensures that there is no scatteringdue to wind blowing or foul smell. In about 2-4 months the filled up refuse getsstabilized 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, greenland or other recreational spots.

• The bottom of the landfill site is made water proof so not to allow any leachate toflow down and pollute the ground water. A leachate collection mechanism isinstalled – which is then separately treated and disposed of. Also an arrangementfor venting of gas is made much like the septic tank system

Composting

• Composting is a biological method in which putrescible organicmatter in the solid waste/refuse is digested. It is a hygienic methodwhich coverts the refuse into nitrogen rich humus through thebacterial agencies.

• Due to composting, the volume of refuse is very much reduced, andthe resulting matter becomes free from pathogenic organisms. InIndia normally, night soil of the conservancy system is also disposedof simultaneously along with refuse, producing valuable manure.

• Basically composting is an aerobic process because it involves piling ofrefuse and regularly turning it to ensure sufficient supply of oxygen.Due to oxidation of organic matter the temperature rises to 60degrees in 3-4 days time. This needs to be maintained for next 4 daysto kill pathogenic microbes. The pile is then turned more frequentlyto slow the reaction down for another 4-5 weeks. The compost isthen allowed to cure for 2-8 weeks within which a total volumereduction of about 50% is ensued, leaving dark brown coloredproduct 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 vegetablewastes and night soil are alternately piled in depths of about 7.5 – 10cm up to a total depth of about 1.5 m in a trench or above the groundto form a mound. Chemicals are added to prevent fly breeding. Thematerial is turned regularly for a period of about 8 to 12 weeks, andthen stored on the ground for 4-6 weeks. In about 6-8 turnings and inabout 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 thanIndore method and hence widely adopted for municipalities throughoutIndia. Earthen trenches of size 10 x 1.5 x 1.5 m deep are filled up inalternate layers of refuse and night soil/cow dung in piles of 15 cm depthsThe material is covered with 15 cm layer of good earth and left fordecomposition. In about 4-5 months, the compost becomes ready. Thehumus is sieved through 12.5 mm sieve to remove broken glass, stones etcand is ready for use.

• Normally, a city produces 200 to 250 kg/capita/year of refuge and 8 to 10kg/capita per year of night soil. Hence a town of 10,000 population willproduce about 2000 tonnes of refuse and 800 tonnes of night soil annually.The composting will produce about 1400 to 1680 (50 to 60%) of compostannually from the above waste.

Incineration

• This consists of burning the refuse in the incinerator plant. This iscommonly used in disposing of garbage from hospitals andindustrial plants. Before incineration, non-combustible and inertmaterial like earth, broken glass, chinaware, metal etc areseparated, so as to reduce the load on the hearth. The by productof this method is ash and clinker which can be easily disposed of byland filling. The heat generated by burning the refuse may beutilized for raising steam power. Emission of air pollutants fromincinerators includes particulates such as fly-ash, and other productof combustion. Permissible level of particulate emissions from largeincinerators is 0.23 g per standard cubic meter of exhaust gas.Smoke is also undesirable product. Smoke can be eliminated bymixing the exhaust with hot air to complete combustion.

• Advantages of incineration : (i) This is most hygienic method, since itensures complete destruction of pathogens (ii) The heat generatedcan be used for raising steam power (iii) Clinker produced can be usedfor road purposes (iv) The disposal site (i.e. incineration site) can belocated at a convenient distance (vi Lesser space is required fordisposal of residues (vii) Adverse weather condition have no effect onthe incinerator's operation.

• Disadvantages : (i) Large initial expenditure (ii) Improper operationresults in air pollution problems and incomplete reduction- of thewaste materials . (iii) Disposal of the remaining residue is required (iv)High stacks needed for natural draft chimneys present safetyproblems.

Pyrolysis

• This is also known a thermal cracking. In this process the waste isheated in closed containers. The reaction is endothermic (consumesheat). After the process is complete we obtain (i) gas stream -hydrogen, methane, CO and CO2 etc, depending upon thecharacteristics of wastes (ii) liquid fraction – consisting of tar and/oroil stream. It is also found to contain acetic acid, acetone etc., and (iii)a solid fraction – the charred solid waste.

• Pyrolysis considerably reduces the waste quantity but is expensiveand hence not used in India.