Environmental Pollution Control Microbiology - Chapter 13 ppt

Construction and Demolition SW — all solid wastes generatedduring the normal construction of houses, apartments, commercialestablishments, and industrial factories or from the destructio

Chapter 13

SOLID WASTES

Solid wastes are one of the more interesting environmental contaminants to dealwith The primary characteristic of importance for solid wastes lies in the factthat they are solid Unlike gaseous wastes that flow into the vast atmospherearound us or liquid wastes that flow downhill until they ultimately reach theocean, solid wastes tend to stay put until a major effort is made to move them toanother spot Solid wastes tend to remain as such until someone finds a use forthe solid wastes and converts them into something of value Suddenly, the solidwastes disappear until the new product loses its value and becomes solid wastesonce again The lack of value of solid wastes has resulted in limited information

on their interaction with various microorganisms Few people bothered to studysolid wastes until the Federal government provided the funds for studies It isnot surprising that most of our understanding of solid wastes has come in the last

40 years

SOLID WASTE CHARACTERISTICS

One of the frustrating problems confronting environmental scientists andenvironmental engineers lies in trying to determine the characteristics of solidwastes Solid wastes are simply solid materials that have lost value for their

owner and are discarded It does not mean that the solid wastes being discardedhave no value It simply means that the solid wastes have no value for thecurrent owner It may well have value for another owner If the solid wasteshave value for a new owner, these materials are no longer solid wastes, butrather raw materials for further use with renewed value until the new ownerdecides to discard them as solid wastes All material goods that society makesand uses will become solid wastes in time.

Solid wastes cannot be characterized by their chemical composition or their size

or their weight alone Solid wastes are characterized by many parameters.Chemical composition is an important parameter, along with size and weight.Bio-stability is also an important parameter for solid wastes that has largely beenignored Bio-stability is the parameter that defines how the solid waste materialsreact to microorganisms and the rate of that reaction Most solid wastescurrently being produced are largely bio-stable, showing little to no reactionwith microorganisms Food wastes are the least bio-stable solid wastes,undergoing rapid reaction with microorganisms Grass clippings and leaves areseasonal solid wastes that are not very bio-stable From a practical point of viewenvironmental microbiologists are only interested in the solid wastes having theleast bio-stability

There are many ways to characterize solid wastes No one classification isperfect Classifications tend to start with the major sources of solid wastes

1 Residential SW - all solid wastes produced by people living inresidences within the classification area Residences includesingle-family houses, duplexes, and apartments with multiplefamily units

2 Commercial SW - all solid wastes produced by commercialestablishments within the classification area

3 Industrial SW - all non-hazardous solid wastes produced byindustrial manufacturing plants within the classification area

4 Construction and Demolition SW — all solid wastes generatedduring the normal construction of houses, apartments, commercialestablishments, and industrial factories or from the destruction ofhouses, apartments, commercial establishments, or industrialfactories

5 Street Sweepings SW - all solid wastes collected by streetsweepers operating in urban communities

6 Water & Wastewater Treatment Plant Sludge SW - all sludge solidwastes produced by water and wastewater treatment plants withinthe classification area

7 Automotive SW - all solid wastes generated when automobiles

and trucks are junked.

8 Bulky SW - all large solid waste items from residences andcommercial establishments that require special collection andhandling Bulky solid wastes include washing machines,refrigerators, stoves, sofas, etc

9 Trees - large trees that die or trees that are cleared for constructionprojects require special handling

10 Agricultural SW - all solid wastes produced from farmingoperations

11 Mining SW — all residues remaining from mining and mineralprocessing

These broad classifications of solid wastes have value for looking at the totalsolid waste problem at the national level Regional classifications normally look

at municipal SW, rural SW, agricultural SW, and mining SW Prior to 1965 data

on solid waste characteristics were quite limited Few people were concernedenough about solid wastes to determine their characteristics The major problemwas simply that it was too difficult to take representative samples of solid wastesand make complete analyses Although every person was concerned with thegeneration and disposal of solid wastes, no one cared enough to characterizethose solid wastes No one knew what to do with the limited data that appeared

in the literature When Congress passed the first solid waste legislation at thefederal level, it became immediately apparent that no one had any real data onthe magnitude of the solid waste problem All available data was largelyextrapolation of a limited database that dealt with the weight and volume ofsolid wastes generated by municipalities Much of the early efforts of the newfederal Office of Solid Wastes dealt with gathering more reliable data Contractswere given to various engineering firms and research organizations to determinedata on solid waste characteristics across the country The data generated fromthese studies is the basis for most of the solid waste characterization

MUNICIPAL SOLID WASTES

Municipal solid wastes include the residential SW, the commercial SW, andlimited industrial S W generated within the jurisdiction of the municipality Thelatest EPA published data on municipal solid wastes indicated that the UnitedStates produced 232 million tons in 2002 The per capita SW generationaveraged 4.5 Ibs/d The composition of municipal solid wastes in the UnitedStates and the percentage of each group by weight are given in Table 13-1

Examination of the different groups of municipal solid wastes indicates thatglass and plastics are the most biostable with metals being relatively biostable.Paper, wood, rubber, leather and textiles are slowly biodegradable under specificconditions Food wastes and yard wastes are biodegradable One of the key

Table 13-1 COMPOSITION OF MUNICIPAL SOLID WASTES IN 2000

PaperYard WastePlasticsFood WastesMetalsRubber, Leather,

& TextilesGlass

WoodOther

INDUSTRIAL SOLID WASTES

Industrial solid wastes consist of hazardous solid wastes and non-hazardoussolid wastes The hazardous solid wastes are dangerous for the environment andare controlled by separate federal legislation than the non-hazardous solidwastes The characteristics of industrial solid wastes are highly variable,depending upon the specific industrial processes and the workers within specificplants Industrial solid wastes range from inorganic materials to organicmaterials The EPA's estimate of non-hazardous industrial solid wastes in 1996was 7.6 billion tons/yr

CONSTRUCTION AND DEMOLITION SOLID WASTES

The construction of new houses and new commercial buildings results in theproduction of construction solid wastes in direct proportion to the materials

employed in the construction project The characteristics of the constructionsolid wastes will vary with each project Normally, construction wastes willinclude the residual materials that could not be incorporated into the finishedbuilding The construction of houses will include pieces of wood, wallboardfragments, partial shingles, and excess roofing paper If the house has a cinderblock foundation and/or a brick front, broken cinder blocks and broken brickswill also be included The amounts of construction wastes produced are directlyproportional to the number of houses and buildings constructed As a net result,the construction waste quantities and characteristics are highly variable and noteasily estimated.

Eventually, every house and every building will become solid wastes when theirvalue is too low and the maintenance costs are too high Under good economicconditions, old buildings are bulldozed to allow new replacement structures onthe same site Demolition of old structures results in complete destruction of theold structures Demolition debris will consist of all the materials remaining inthe structure These materials are simply bulldozed and placed in a truck forremoval from the site The entire weight of all the materials in the old structurecomprises the demolition wastes Major urban renewal projects create largeamounts of demolition wastes over a specific time period The amount andcharacteristics of demolition wastes require separate evaluation of every projectand cannot be generalized

STREET SWEEPINGS

Many cities in the United States use street sweepers to collect dirt and trash thataccumulates on streets in the major commercial and industrial areas.Occasionally, the residential streets will also be swept The shape of roadsurfaces allows the dirt and trash to accumulate next to the curb and the road.For this reason street sweepers normally clean the area of the street next to thecurb The quantity of street sweepings collected is a function of the size of thecity and the rainfall characteristics Large cities produce more street sweepingsthan small cities Large cities tend to attract dirty industries that can operate with

a minimum of complaints Cities tend to keep the dirty industries together,making it easier to keep the streets reasonably clean Industrial areas tend tohave large impervious surface areas, allowing maximum runoff during and afterevery rain event The rapid runoff of rainwater carries the contamination on thestreets into storm sewers for collection and removal Areas with heavy rains willfind that the streets are cleaned of all but the heaviest solid materials Areaswith light rains, occurring frequently, will find only the lightweight materialsremoved by the runoff water

Examination of street sweepings indicates they are composed of dirt, sand, grit,

paper products of various types, plastic materials, broken glass, rubber particles,and miscellaneous materials too difficult to identify and measure Most of thestreet sweepings are relatively inert material that can be used as fill or placedinto a sanitary landfill One of the problems with street sweepers is theirtendency to suspend fine particles around the street sweeper rather thancapturing the fine particles There are no uniform analyses for street sweepings.Every community must determine its own characteristics of street sweepingwastes, both as to quantities produced and their chemical characteristics.

WATER AND WASTEWATER SLUDGES

Every community that has a water treatment plant and/or a wastewater treatmentplant will have water treatment sludge and/or wastewater treatment sludge.Emphasis on water treatment plants and wastewater treatment plants is on theremoval of pollutants from the water being treated The treatment processesproduce a clean water and waste sludge The water treatment plants generatealum sludge and calcium carbonate sludge, if the treatment plant softens thewater The clean water, produced in the water treatment plant, is distributed toevery user in the community through a complex pipe network Once the cleanwater has been used, it is returned as wastewater The wastewater collectionsystem is also a network of complex pipes that services every house andbuilding in the community The wastewaters are collected and discharged into awastewater treatment plant The pollutants in wastewater are classified assuspended pollutants and soluble pollutants The majority of the suspendedpollutants are removed by gravity sedimentation The remaining suspendedpollutants and the soluble pollutants are treated biologically with the pollutantsconverted into suspended solids that form settleable floe particles that areremoved by gravity sedimentation The two types of wastewater sludge arenormally mixed together and treated in an anaerobic digester The anaerobicdigester destroys the readily biodegradable suspended solids, leaving a relativelyinert residue for disposal on land

The characteristics of water treatment plant sludge are quite similar from plant

to plant as far as chemical characteristics are concerned The quantity of sludgeproduced varies with the magnitude of treatment required to remove thecontaminants Alum treatment of surface water produces an aluminumhydroxide floe that removes the suspended solids in the surface water Riverwater will contain both organic and inorganic particles During periods of heavyrainfall and runoff in rural areas the river water will contain soil particles andany materials washed from the soil surface The net result is for wide variations

in alum use and in sludge production Lake water contains colloidal suspendedsolids and various microorganisms Understanding the source of the water beingtreated is essential to knowing the type and magnitudes of contaminants in the

water plant sludge Water plants that soften the water will produce calciumcarbonate sludge A few plants may produce magnesium hydroxide in addition

to the calcium carbonate Softening sludge is an inorganic sludge Each watertreatment plant must determine its own sludge production rates as they arevariable from plant to plant

Wastewater treatment plants produce screenings and grit in addition to thesludge generated every day Screenings are large solid wastes that are collected

on bar screens at the head of the treatment plant Most plants collect thescreenings and dispose of them with the other municipal solid wastes in sanitarylandfills A few plants grind the screenings and return them to the liquid stream.Logic indicates that it is simpler to handle the screenings as solid wastes oncethey have been collected Grinding the screenings and returning them to theliquid wastes means that the screenings have to be removed twice from thewastewater Grit is largely sand and dense organic particles that could damagemechanical equipment in the units that follow the grit chamber Grit is usuallywashed as it is collected and used as fill material The primary sludge and thebiological sludge are combined and anaerobically digested before beingdewatered and placed on the land Most municipal wastewater treatment plantswill produce about 0.14 kg/m3 (1,200 Ibs TS/MG) wastewater treated If 0.02kg/m3 (200 Ibs TSS/MG) are lost in the treated effluent, 0.12 kg/m3 (1,000 IbsTSS/MG) will be returned to the land environment Wastewater treatment plantshandling large quantities of industrial wastes will produce even more excesssludge

AUTOMOTIVE SOLID WASTES

Every automobile sold in the United States will eventually become solid waste.Old automobiles are processed by automobile scrap dealers With an average life

of 7 to 8 years the number of automobiles scraped each year can be estimated bylooking at the production data 7 to 8 years ago With increased emphasis onwaste recycling the automobile manufacturers have tried to use materials thatcould be processed and reused with a minimum of effort The automobile scrapdealers remove all the parts of value before shipping the residue to a metalgrinder and separator to recover the metal for reuse Automobile tires pose one

of the major processing problems for recycling

BULKY SOLID WASTES

Every community produces bulky solid wastes from time to time Bulky solidwastes are all large household items that are discarded Stoves, refrigerators,washing machines, sofas, large chairs, beds, mattresses, and other large items

require special collection Usually, the person discarding the item is responsiblefor seeing that the bulky waste is collected and delivered to the proper disposalsite Often, when a large item is replaced, the dealer selling the new itemcollects the old item and delivers it to the disposal site Reclamation dealersremove any useful parts before sending the residue for recycling or for burial in

a sanitary landfill

TREES

As communities age, there will be a steady production of dead trees and treesremoved for residential or commercial expansion The dead trees are usuallyground up and used for mulch in the community Living trees removed fromprivate or public property will be used commercially, where possible Largetrees can be cut for lumber; while small trees are mulched for use in paperproduction or in parks and gardens Every effort is made to reuse the woodrather than burying it in sanitary landfills

AGRICULTURAL SOLID WASTES

Farms produce large quantities of solid wastes The quantities of solid wastesproduced are a function of the specific crops grown on the farm Farmers havelong recognized that manure from farm animals and crop residues after harvestmust be returned to the land to help maintain the soil quality for future cropproduction Farmers are among the oldest of the recyclers in our society Themajor problems in recent years have come from the large confined animal farms.The manure from the confined animal buildings is handled as a liquid wasterather than as a solid waste Operators of the large confined animal farms havehad to learn new methods of handling the liquid manure to minimizeenvironmental pollution The basic problem is processing the liquid manure forreturn to the land environment It is not surprising that there are no simplesolutions for processing the liquid manure as previously indicated

MINING SOLID WASTES

The mining industry produces large quantities of solid wastes that must also bereturned to the land environment Mining results in the removal of non-valuablematerials along with the valuable materials The failure of the mining industry toproperly handle the non-valuable solid materials at the same time they handledthe valuable materials has produced localized environmental damages andcreated a negative image for the mining industry Too often mining companiessimply closed down and walked away from the mined area when the miningoperation stopped being profitable Some of the mining residues contain

hazardous chemicals that slowly wash into nearby streams, creating even moreenvironmental damage The net effect has been for the mining industry to havecreated a very negative impression on the public at large It will take decades ofmajor effort by the existing mining industry to overcome the current negativeimage and to correct the environmental damage that has been done over theyears.

PROCESSING SOLID WASTES

Normal processing of solid wastes has always followed the path of leastresistance Solid wastes have always been handled with the least effort and thelowest cost Since our interests are focused on the microbiological side ofenvironmental pollution control, we will only examine those treatment processesthat significantly deal with microorganisms Our focus will be primarily onmunicipal solid wastes and agricultural solid wastes and how microorganismscan be used to stabilize these solid wastes We will briefly examine how bacteriareact with mining solid wastes to extract mineral elements

Biological treatment of solid wastes falls into three general categories: sanitarylandfills, composting, and soil stabilization Although sanitary landfills havebeen extensively used in the United States, the lack of understanding of howmicroorganisms react in sanitary landfills has created problems in both designand operation of sanitary landfills Composting is another biological process thathas not been fully evaluated from a microbiological point of view Soilstabilization of solid wastes has long been used for treating agricultural solidwastes and has potential for treating any readily biodegradable solid waste.Since the production of solid waste will continue to be related to existingpopulations, we can expect to see increased production of solid wastes It isessential that we understand how the microorganisms metabolize thebiodegradable materials in solid wastes and what we can expect from biologicaltreatment systems currently in operation

SANITARY LANDFILLS

By definition sanitary landfills are engineered burial of solid wastes.Unfortunately, sanitary landfills grew up at a time when municipal governmentpaid little attention to solid waste processing Although municipal government isthe primary beneficiary of well-designed and well-operated sanitary landfills,municipal government must accept full responsibility for all the problemsrelated to sanitary landfills Municipal officials put solid waste processing at thebottom of the problem pile until the environmentalists forced the federalgovernment to make municipal officials face up to years of neglect in the area of

solid waste processing.

Sanitary landfills should be carefully designed for maximum operatingefficiency with a minimum of discomfort and objections from the citizens livingaround the landfill Fundamentally, all sanitary landfills should be located withinthe boundaries of the community producing the solid wastes This locationallows the citizens of the community to continuously observe how their electedofficials manage one of the most important municipal functions It is completelypossible to design and operate a sanitary landfill within the municipality withoutcreating problems for the citizens or the municipal officials Unfortunately, fewcommunities are willing to make the effort that is required to construct andoperate a sanitary landfill correctly Most communities choose to establish theirsanitary landfill far enough outside the community boundaries that few citizensknow where the sanitary landfill is located or how it is being operated

Basic Concepts

A sanitary landfill should be constructed in dry soil on flat terrain capable ofholding at least 20 to 30 years production of solid waste from the contributingpopulation The natural water drainage should be away from the landfill site orshould be diverted around the landfill site to prevent excess runoff from crossingthe landfill and eroding the cover soil Because of concerns about leachateformation and its effect on groundwater below the landfill, the bottom of thelandfill should be sealed with clay or an impermeable membrane A series ofperforated pipes should be placed at intervals across the bottom of the landfill tocollect the leachate and to convey it to a concrete sump where the leachate cancollect and be pumped to a leachate storage facility prior to treatment A layer ofcoarse sand is placed over and between the leachate pipes to prevent possibledamage to the collection pipes by the tractor or tractors used for compacting thesolid wastes in the landfill The solid wastes should be placed first at the far end

of the landfill Bulldozers push the solid wastes into a corner and compact thesolid wastes to their maximum concentration By moving back and forth overthe solid wastes the bulldozer is able to compact the solid wastes to aconcentration close to 593 kg/m3 (1000 Ibs/cy) Figure 13-1 shows a bulldozercompacting solid waste in a sanitary landfill At the end of each day thecompacted solid waste is covered with about 0.15 m (6 inches) of topsoil to act

as a temporary cover As more solid wastes are added to the landfill, the landfillslowly fills to the design level As the landfill fills with solid waste, perforatedgas collection pipes are placed in the upper part of the landfill to collect the gasproduced in the landfill The landfill is covered with a layer of clay or animpermeable membrane to minimize infiltration of water The top of theimpermeable layer is then covered with at least 0.61 m (2 ft) of topsoil Thetopsoil is sloped to allow the natural storm water runoff to flow away from the

Figure 13-1 BULLDOZER COMPACTING SOLID WASTES AT THELAWRENCE, KS SANITARY LANDFILL

landfill into catch basins and a storm water collection sewerage system tominimize the infiltration of rainwater into the landfill Sanitary landfills can beconstructed as a series of trenches or as areas, depending on the specific terrain.Trench landfills are built into the soil surface while area landfills are builtagainst a hill The compaction and burial process for the solid wastes continuesuntil the landfill has been filled and the landfill site has reached its designcapacity for solid wastes A new site must be designed and developed before theold landfill site is filled The production and processing of solid wastes is anever-ending process that must be continuously handled

Biological Activity

The municipal solid wastes placed into the sanitary landfill contain materialsthat are readily biodegradable In fact, biological activity is well underway bythe time the solid wastes are collected and transported to the sanitary landfill.The food wastes that contain over 60% moisture will allow bacteria tometabolize the organic matter in the food waste materials Fungi will be able tometabolize organic matter with only 40% moisture content, provided oxygenand other required nutrient elements are available for aerobic metabolism Eventhough solid wastes have been compacted to a density of 593 kg/mj (1,000

Ibs/cy), there are still plenty of void spaces in the compacted landfill Bothbacteria and fungi are able to grow on the moist food wastes initially Themicrobes grow directly on the surface of the food materials being metabolized.The metabolic reactions are aerobic with carbon dioxide, water, new cell mass,and heat as the end products As the initial microbial layer spreads across theorganic surface, some microbes are forced to grow on top of the existingmicrobial layer Soon a second layer of microbes becomes a third layer and then

a fourth layer Metabolism of the food material shifts from being aerobic tobeing oxygen limiting The microbes at the surface of the food material areunable to obtain sufficient oxygen for aerobic metabolism The bacteria and thefungi continue to metabolize to the best of their ability As oxygen becomeslimiting, the microbes slow their rate of metabolism and organic end productsincrease in solution Soon oxygen becomes exhausted and only the facultativebacteria are able to continue metabolism at the food surface Fungi and strictaerobic bacteria on the food surfaces cease growing The bacterial layer next tothe food surfaces slow their growth as metabolism shifts to the production oforganic acids and other partially oxidized end products rather than carbondioxide and water The pH in the liquid around the bacteria decreases, as theorganic acids accumulate Further metabolism causes the pH to drop sufficientlylow that metabolism stops and the microbes die

If the food waste is in contact with a metal can, the organic acids will cause themetal to dissolve and neutralize some of the organic acids The presence ofalkaline materials in the vicinity of the food wastes will also neutralize theorganic acids, raise the pH, and allow metabolism to continue As theneutralized organic acids accumulate around the organic matter beingmetabolized, methane bacteria will begin to metabolize the neutralized organicacids, producing methane gas and carbon dioxide as primary end products.Metabolism of the neutralized organic acids releases the cations, allowing thecations to form bicarbonate alkalinity in the water that can neutralize moreorganic acids Slowly, the methane bacteria metabolize the neutralized organicacids, producing methane, carbon dioxide, water, new cell mass, and heat.Sulfate reducing bacteria will also grow under anaerobic conditions where thereare sulfates in the solid wastes The sulfate reducing bacteria can compete withthe methane bacteria for organic nutrients in the right environment and mayreduce the methane gas production The metabolic process is a slow processsince there is no mixing of the organic matter and the microorganisms in thesanitary landfill The localized environment determines what microbes can growand to what extent they grow Temperature, moisture, and alkaline materials arethe three major factors that determine the rate of biodegradability of the organicmatter in the landfill A rising temperature will increase the rate of metabolism.Moisture within the landfill will determine how far the microorganism will beable to move to find new food for metabolism The presence of alkaline

materials will determine the pH of the water around the bacteria and the foodwastes being metabolized The production of water forms a thin liquid film forthe microorganisms to move to a new location for metabolism For the mostpart, microbial movement inside the sanitary landfill is restricted to theimmediate environment around the rapidly biodegradable organic matter Thegas movement in the sanitary landfill depends upon pressure differentials andavailable void spaces The heat released by metabolism is quickly absorbed bythe materials in the sanitary landfill Overall, microbial metabolism within asanitary landfill is limited, the environment within the sanitary landfill is simplynot conducive to rapid microbial metabolism

There are few organic compounds in municipal solid wastes that bacteria canmetabolize under the best of circumstances Most paper in solid wastes containslignin as well as cellulose Yard wastes also have a considerable amount oflignin with the cellulose The lignin prevents bacteria from metabolizing thecellulose, allowing the waste paper and much of the yard wastes to remainuntouched once the oxygen in the void spaces has been removed Fungi have theability to metabolize lignin as well as cellulose; but the fungi must havedissolved oxygen available for metabolism Bacteria are able to metabolize thecellulose materials that do not have the cellulose combined with the lignin.Completely sealing the solid wastes in the sanitary landfill, as currentlyrecommended, creates an environment that limits the overall metabolism to 10%

to 15% of the municipal solid wastes no matter how long the solid wastes arecontained in the sealed sanitary landfill It also means that 85% to 90% of themunicipal solid wastes placed into a completely sealed sanitary landfill will beavailable for reuse whenever conditions favor recycling municipal solid wastes

Stimulating Bacterial Activity

It is important for everyone to recognize that completely sealing a sanitarylandfill, as currently recommended to minimize groundwater contamination,limits biodegradation within the landfill The simplest method to stimulatebiodegradation inside the sanitary landfill is to pass water through the landfill atperiodic intervals to allow the bacteria to migrate throughout the landfill andfind all the readily biodegradable materials Metabolism in the landfill willcontinue to be anaerobic with the production of methane gas Recirculation ofliquid around the sanitary landfill on a continuous basis has been recommended

to speed the stabilization of the biodegradable organic matter The problem withrecycling the leachate from the bottom of the landfill around the landfill is theaccumulation of soluble, non-biodegradable contaminants in the leachate Thesoluble non-biodegradable materials accumulate in the recycled leachate on eachpass through the solid wastes in the landfill Eventually, the accumulated non-biodegradable materials may adversely affect metabolism of the biodegradable

organic compounds.

Under the anaerobic environment inside the sanitary landfill iron and othermetallic ions, released by reaction with organic acids, tend to remain partlysoluble Some of the iron and metallic ions may form insoluble sulfide salts thatmove with the leachate as colloidal precipitates When the leachate is exposed toair, oxygen in the air reacts with the ferrous iron to form insoluble ferric oxide.Some of the other metallic elements also form insoluble metallic oxides Theorganic compounds are unaffected by the changes in the metallic elements andremain in solution Leachate tends to be on the acidic side and may needadditional alkalinity to provide a suitable pH, pH 7 to 8, when the leachate isrecycled back to the landfill surface It is important to maintain a suitableenvironment within the landfill for maximum stabilization of the organic matter.Recycling of leachate will also remove any salts that are soluble in water,increasing the mineral content of the leachate with time Maintaining a goodenvironment within the sanitary landfill is not an easy task The solid wastematerials are fixed within the landfill and cannot move to produce a betteropportunity for biodegradation The recycled leachate always takes the path ofleast resistance and flows through the void spaces between the solid particles bygravity to the bottom of the landfill for collection

Continuous recirculation of leachate should result in moving the bacteriathroughout the entire volume of the sanitary landfill and the maintenance ofanaerobic conditions within the sanitary landfill Care must be taken that therecycled leachate is not applied by spraying in the air Spaying the leachate willcreate small droplets and allow oxygen in the air to enter the leachate before itreenters the sanitary landfill A high rate of continuous recirculation of leachatewill be a waste of energy will produce a limited increase in treatment efficiency.The basic problems in the biological degradation of solid wastes lie in the largemass/surface area ratio and the complexity of the chemical composition of thedifferent solid waste components

Periodic application of leachate to the landfill surface, followed by sufficienttime to allow the free water to drain through the sanitary landfill should produceadequate dispersion of the microbes throughout the sanitary landfill withoutslowing the metabolic reactions with the solid waste As the upper layers oforganic material are stabilized, the upper environment will shift from anaerobic

to aerobic, allowing the fungi and the actinomycetes to metabolize the complexpaper products in proportion to the availability of oxygen to the microorganisms

on the surface of the solid wastes Slowly, aerobic conditions will be establishedthroughout the entire volume of the sanitary landfill

The lack of nitrogen, phosphorus, and trace elements can also limit both the rate

of biodegradation and the total amount of biodegradation Remember that themetabolism of organic matter by microorganisms results in the production ofnew cell mass Without the synthesis of new cell mass, the microorganisms willproduce very limited metabolism, only endogenous respiration and the synthesis

of new cells from the nutrients released Adding treated domestic wastewater tothe sanitary landfill on a periodic basis is one method that can be used to addwater and to increase the nitrogen, phosphorus, and trace metals that themicroorganisms can use in their metabolism of the biodegradable materials inthe sanitary landfill Storm water runoff can be collected and stored for use asneeded during the dry season in water limited regions There is no single methodfor stimulating microbial growth in a sanitary landfill It is possible to design thetop of the landfill to allow natural rainfall to enter the landfill as the primarywater source while still capturing the gas and the leachate In every sanitarylandfill there is a fixed amount of biodegradable materials that can bemetabolized Normally, there will be a slow increase in biological activity to amaximum, followed by a slowing rate of increase to a plateau and a slowing rate

of metabolism that eventually ceases to be significant It normally takes severalyears for the biodegradable materials in a sanitary landfill to be metabolized

Gas Production

In recent years efforts have been made to collect the gas from sanitary landfillsfor power generation The concept of gas production and energy generation hasattracted considerable attention from landfill owners and landfill operators as it

is a method for recovering some of the operating costs of the sanitary landfill.While sanitary landfills handling biodegradable solid wastes will producemethane gas that can be collected and burned for energy, there is a fixed amount

of potential energy available that can be produced The actual energy recoverywill be something less than the potential energy available from the solid wasteanalyses The value of the landfill gas must be sufficient to defray the cost of theextraction equipment and the energy conversion equipment along with theoperating costs for the energy system The landfill gas will contain methane,carbon dioxide, and low concentrations of hydrogen sulfide, the same ascontained hi gas from an anaerobic wastewater sludge digester The methanefraction is important in determining the extent of biodegradation and in energyrecovery The amount of methane gas that can potentially be produced is related

to the biodegradable COD (BCOD) of the organic matter metabolized If theBCOD of solid wastes in the sanitary landfill averages 1.2 times the weight ofthe biodegradable organic fraction of solid wastes, the maximum amount ofmethane that could be expected from a municipal solid waste sanitary landfillwould be about 40 m3 CH4/m3 (1,080 cf CH4/cy) solid waste The theoreticalenergy yield from the biodegradable organic compounds in the solid wastescontained in the sanitary landfill should be about 1.5 x 106 kJ/m3 (1.1 x 106