urea-formaldehyde a landfill cover

UREA-FORMALDEHYDE FOAM AS A LANDFILL COVERMATERIAL: SIMULATED LANDFILL INVESTIGATIONS

Johannes T. Graven *t and Frederick G. Pohland *

* School of Civil Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.&dagger; Present address: Directorate of Combat Developments, Academy of Health Sciences, U.S. Army (HSHA-

CDS), Ft Sam Houston, Texas 78234-6100, U.S.A.

(Received 27 June 1986)

A study was conducted, utilizing simulated landfill cells, to assess the relative suit-ability of urea-formaldehyde foam as an alternative landfill cover material. Researchefforts were directed toward assessment of the extent and impact of formaldehydeleaching within the landfill environment when combined with natural processes ofstabilization, and determination of the relative contribution of formaldehyde leachedfrom the foam as compared with the other constituents present in the solid waste.The formaldehyde detected in the leachate samples (3-4.0 mg 1-1), some of whichwas attributable to the solid waste itself, was determined not to adversely affect themicrobially mediated processes of stabilization within the landfill, even though theamount of foam used during this study (2.8 g kg-1 dry solid waste) was consideredto be greater than that which may be typically used at a landfill (0.5-1.5 g kg-1).Furthermore, as stabilization progressed, formaldehyde was converted and reducedin concentration. Consequently, adverse environmental impacts attributable to therelease of formaldehyde from urea-formaldehyde foam within a landfill environmentwere considered unlikely to occur.

Key Words—Anaerobic processes, formaldehyde, gas production, leachate recycle,leaching, municipal wastes, sanitary landfill, solid wastes, stabilization,urea-formaldehyde foam.

1. Introduction

In the overall planning, design and operation of sanitary landfills, requirements fordaily, intermediate and final cover may impose economic and regulatory constraints.Although soil is the most frequently used material for intermediate and final cover, theuse of alternative cover materials may be more advantageous in many applications. Urea-formaldehyde (UF) foam has received recent attention as an alternative cover materialbecause of its ease and homogeneity of application, its ability to increase landfill

capacity and extend service life, and its frequent economic advantage over alternativecover methods. Although efforts to determine the general applicability of UF foam tolandfilling operations appear promising (Allen 1980; Firstman & Pohland 1981), theyhave not adequately addressed the relatively long-term issues of the impact of the foamand its leachable constituents on the landfill environment and vice versa.

Because of some evidence of carcinogenicity of formaldehyde in experimental ani-mals, as well as numerous health-related complaints concerning the release of formal-dehyde from consumer products (National Academy of Science 1980; National Re-search Council 1981), public and regulatory concern over releases of formaldehyde tothe environment have increased (Hileman 1982). Although most of the concerns arerelated to indoor atmospheric releases, an increased focus on potential releases to the

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aqueous environment, including the leaching of formaldehyde from landfills, has re-sulted. Furthermore, because of its toxicity to lower animals and microorganisms(Environmental Protection Agency 1976), formaldehyde leached from sanitary landfillsmay have an adverse impact on the microbial processes of stabilization within thelandfill. Hence, the leaching of formaldehyde during and after landfilling operations,particularly in the case of the acceptability of foam applications as cover, was in needof resolution within a process assessment as well as environmental health and regulatoryperspective.

In order to determine the efficacy of UF foam as an alternative cover material, astudy was initiated to assess its relative suitability and/or potential applicability duringlandfill disposal of solid wastes, with special emphasis on the fate and impact of leachedformaldehyde on landfill stabilization processes and the environment. To accomplishthis objective, research efforts were directed toward assessment of the extent and po-tential impact of formaldehyde leaching within the landfill environment when combinedwith natural processes of stabilization and gas production, and the determination ofthe relative contribution of formaldehyde leached from the UF foam as compared withthe other constituents already present in the solid waste.

2. Preliminary considerations

Most sanitary landfills proceed through a series of relatively predictable microbiallymediated events. A landfill functions throughout much of its active life as an anaerobicmicrobial process, analogous in concept to a batch anaerobic digester (Pohland et al.1983).Normally, these processes occur over extended periods of time which makes analysis

and interpretation of conditions difficult and often impractical. By collecting andrecycling leachate through the waste mass, the microbiological processes can be madeto occur in a more predictable and manageable time frame. Therefore leachate recyclewas selected as an experimental technique to accelerate stabilization and concomitantexposure of the UF foam to various environmental conditions within the landfill andto enable assessment of the potential impact of formaldehyde and possible other con-stituents leached from the foam during the overall progress of landfill stabilization.

3. Experimental procedures

3.1. Simulated landfill construction and operation

Two simulated landfill cells with the necessary appurtenances to permit leachate andgas collection for a single-pass cell, as well as leachate recycle for a recycle cell, wereconstructed as shown in Fig. 1. The single-pass cell was intended to simulate the impactof rainfall-induced leaching events during conventional landfill operations; the recyclecell was intended to simulate conditions under which leachate is formed, contained,collected and recycled. Thus, the recycle cell was used as an in situ leachate treatmentsystem as well as to accelerate microbially mediated stabilization processes. In bothcells, a 5-cm intermediate layer of UF foam was placed between two layers of shreddedresidential-type solid waste. A total of 60 kg (39 kg dry weight) of solid waste andapproximately 2.8 g of UF foam per dry kilogram of solid waste were placed in eachcell. In an actual landfill, the amount of foam could typically range from 0.5 to 1.5 g of



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Fig. 1. Schematic representation of simulated landfill cells. 1, thermocouple-temperature recorder; 2, to gasmeter; 3, water addition port; 4, flow distribution plate; 5, leachate drain pipe/sampling port; 6, leachatereservoir; 7, leachate reservoir gas line; 8, leachate recycle line; 9, pH-ORP measuring loop; 10, leachate

recycle pump; 11, urea-formaldehyde foam layer; 12, solid waste.

Fig. 2. Cumulative moisture addition to simulated landfill cells. 0, Recycle cell; 0, single-pass cell.

UF foam per dry kilogram of solid waste depending on the operational and manage-ment techniques employed, i.e. cell depth and thickness of foam.

After the cells were sealed from the atmosphere, moisture was added to initiallyestablish apparent field capacity and to subsequently simulate rainfall events (Fig. 2).



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TABLE 1Characterization of solid waste added to simulated

landfill cells, single-pass and recycle cells

Accumulated leachate was collected and recycled in the recycle cell and collected andremoved to storage in the single-pass cell. In the recycle cell, leachate was recycled on adaily basis between Days 42 and 194; only intermittent recycle of small quantities ofleachate was performed between Days 195 to 358. Anaerobic digester sludge was addedon Day 358 to both cells to establish an active methanogenic microbial population.Following sludge addition the cell was again operated with daily recycle. Leachate andgas samples were collected from both cells at periodic intervals and analysed to deter-mine the progress of leachate stabilization within the cells.

3.2. Analytical proceduresAs the solid waste was being placed in the landfill cells, samples were collected andanalysed for moisture, total volatile solids (TVS), and carbon, hydrogen and nitrogencontent. In addition, the as-placed density of the solid waste was determined. Collectionand analysis of leachate samples was initiated when sufficient quantities of leachatewere generated by the cells and continued at frequencies dictated by the evaluation ofthe progression of stabilization events within the landfill. Leachate samples from bothcells were analysed for pH, oxidation-reduction potential (ORP), conductivity, totalalkalinity, volatile organic acids, chemical oxygen demand (COD), biochemical oxygendemand (BOD,), total organic carbon (TOC), chlorides, sulphides, nutrients (nitrogenand phosphorus), selected metals and formaldehyde.Measurement of formaldehyde in the leachate was performed using the derivatization

of 2,4-dinitrophenylhydrazine to 2,4-dinitrophenylhydrazone (2,4-DNPH) (Graven etal. 1984).

4. Presentation and discussion of results

Selected data from the studies are presented in Table 1 and Figures 3 to 12. These datarecord the most pertinent results of analyses of the solid waste as well as leachate and



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gas samples during the study period. Results are reported in relation to the time indays since leachate generation began which was 7 days after the solid waste was initiallyplaced in the cells.

4.1. Solid waste characteristics

The characteristics of the shredded residential-type solid waste added to each of thesimulated landfill cells are presented in Table 1. The solid wastes placed in each of thecells were similar in composition and compacted densities. In addition, these char-acteristics were typical of landfilled residential solid waste (Tchobanoglous et al. 1977).Although the ratio of UF foam to solid waste may vary in practice, the ratio usedduring this study (2.8 g of UF foam per kilogram dry weight or 8% by volume for the0.6-m lifts in the simulated landfill cells) was intentionally chosen to be greater thanthe 1.7% by volume for a 3.0-m lift commonly used (0.56 g of UF foam per kilogram).

4.2. Leachate and gas characteristics

4.2.1. Organic strength (COD, BOD,, TOC)The leachate COD, BOD 5 and TOC analyses were used to reflect the changes in organicstrength throughout the experimental period. All three parameters indicated essentiallythe same pattern for each cell. Figure 3 shows the COD. The initial high concentrationsof COD, BODS and TOC were the result of solubilization and extraction of organicspresent in the solid waste. The concentration decreases during the first 60-80 dayswere attributable to the dilution effect of moisture additions in the recycle cell and thewashout and removal of these constituents from the single-pass cell. Subsequent to thisperiod and up to about Day 400, these parameters also indicated a prolonged acid

Fig. 3. Chemical oxygen demand of leachate. ·, Recycle cell; 0, single-pass cell. at PENNSYLVANIA STATE UNIV on March 4, 2016wmr.sagepub.comDownloaded from


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formation phase (Pohland et al. 1983). The rapid declines in the concentrations ofthese parameters observed after Day 400 in the recycle cell were considered indicativeof the accelerated biological conversion of the readily available organic constituents asstabilization proceeded into the methane fermentation phase.

4.2.2. Volatile acids and pHThe initial appearance of volatile acids in the leachate from both cells and the enhanced

utilization of these acids in the recycle cell corresponded to the trends observed forCOD, BOD, and TOC. Furthermore, this sequential behaviour of acid formation andsubsequent methane fermentation in the recycle cell could be contrasted with the con-tinuation of the acid formation phase in the single-pass cell.As could be anticipated, accumulations of volatile acids caused an initial decrease in

the leachate pH for both cells (Fig. 4). Moreover, decreased pH conditions were sus-tained in the single-pass cell as leachate volatile acid concentrations remained high andrelatively unchanged. In contrast, a rapid increase in pH toward neutral was observedfor the recycle cell with the microbial conversion of the volatile acids. This change inpH corresponded to a buffer shift from that established by the volatile acids to thatcharacteristic of the bicarbonate system. Such a shift of pH and volatile acids is con-sidered typical of viable methane production.

Fig. 4. pH of leachate. 0, Recycle cell; O, single-pass cell.

4.2.3. Gas production and compositionThe majority of gas produced within landfills occurs during the methanogenic phase ofstabilization when the volatile acids are converted to CH~ and C02. Hence, gas pro-duction could be coupled to the disappearance of organic pollutants from the leachate.As indicated in Fig. 5, no significant gas was produced during the first 350 days ineither landfill system, thereby indicating that acid fermentation was dominant and theconversion of organic acids to CH~ and C02 had not yet been achieved. However,subsequent to the innoculation of both cells with digested sludge in order to establish aviable methanogenic population, an increase in gas production from the recycle cellwas observed.

Gas production in the recycle cell increased commensurate with the reduction ofvolatile acids (and COD, BOD and TOC) and increase in pH. To emphasize thisrelationship, daily gas production for the recycle cell (Fig. 6) indicates that the majorityof gas production occurred during a relatively short period of approximately three



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Fig. 5. Cumulative gas production from simulated landfill cells. 0, Recycle cell; 0, single-pass cell.

Fig. 6. Daily gas production from recycle cell after sludge addition. _

months and coincided with the period of most rapid stabilization. Total gas productionat standard pressure and temperature (STP) was approximately 72 1 per dry kilogram(72 m3 tonne-I of solid waste, dry basis; total methane production was approximately40 1 per dry kilogram (40 m3 tonne-I).The composition of the gas produced by the recycle cell (Fig. 7) showed a continuous

gradual increase in CH~ from 26 to 56% which is characteristic of anaerobic stabili-zation by methane fermentation. In contrast, the composition of the gas from the single-pass cell (Fig. 8) only showed an increase in CH~ content from approximately 17 to38%, with a corresponding decrease in N2 content from 44 to 20%. This indicated



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Fig. 7. Composition of gas from recycle cell after sludge addition. V, CH4; 0, C02; 0, N2’

Fig. 8. Composition of gas from single-pass cell after sludge addition. V, CH4; 0, CO2; 0, N2.

that, although anaerobic stabilization did occur in this cell, it apparently occurred at avery low rate since insufficient gas was produced to dilute out the nitrogen present inthe cell. Furthermore, corresponding decreases in volatile acids, COD, TOC and BODS,and characteristic changes in other indicator parameters (pH, ORP, alkalinity) reflec-tive of more active stabilization were not observed. Hence, it is unlikely that anymeasurable gas production occurred in this cell, although gas production data are notavailable for this period.

4.3. Fate and impact of.forYVCaldehydeThe detection and quantification of formaldehyde in leachate samples from the simu-lated landfill cells was provided at periodic intervals and the results of these analysesare presented in Fig. 9.



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Fig. 9. Concentration of formaldehyde in leachate. 0, Recycle cell; 0, single-pass cell.

4.3.1. Recycle cellConcentrations of formaldehyde measured in leachate samples from the recycle cellranged from 30.4 to 16.8 mg 1-1 through Day 360, increased to approximately 42mg 1-’ during the following 40 days, and then decreased rapidly to approximately 3mg 1-1 at the end of the study period. The increase in the concentration of formaldehydebetween Days 360 and 400 was attributable to changes in the operation of the recyclecell previously discussed.The subsequent rapid decrease in the concentration of formaldehyde was attributable

to the conversion of formaldehyde within the landfill cell. Biologically mediated con-version would be anticipated as formaldehyde has been shown to be anaerobicallydegraded (Wolfe 1971) and was present in the leachate in concentrations much lessthan those that have been reported as being inhibitory to anaerobic degradation pro-cesses (Parkin et al. 1983). Indeed, such inhibition was not observed once a viablemethanogenic population was established. Furthermore, the removal of formaldehydecoincided with corresponding decreases in organic strength (COD, BODS, TOC) andincreased gas production.

4.3.2. Single-pass cellFormaldehyde concentrations in the single-pass cell ranged from 9.5 to 28.6 mg 1-1prior to decreasing to 6 mg 1-1 on Day 491. The gradual increase in formaldehydebetween Days 285 and 410 could be attributed to some transformation of the UF foamwithin the cell as a result of physical-chemical or microbially mediated processes andassociated release of entrapped formaldehyde to the leachate. The subsequent decreasebetween Days 410 and 490 was likely due to the continuing washout of the residualamounts of formaldehyde remaining within the cell, since there was little evidence thatany significant stabilization occurred (see Fig. 3).

4.3.3 Amount of Formaldehyde Removed.from CellsTo permit further interpretation of these data, the estimated cumulative quantity offormaldehyde removed during the study period from both the recycle and single-passcells was determined (Fig. 10). Because no leachate was removed from the recycle cell,removal of formaldehyde could only be attributed to either the physical removal ofleachate samples from the cell for analysis or to conversion of the formaldehyde during



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Fig. 10. (a) Estimated cululative amount of formaldehyde removed from recycle cell: V, total amountremoved; 0, removed as sample; 0, removed by in situ conversion. (b) Cumulative amount of formaldehyde

removed from single-pass cell.

landfill stabilization. Volatilization of formaldehyde from the leachate during recyclewas not considered to be significant as formaldehyde is extremely soluble in water (0.4g 1-1 of water at 20 °C; Walker 1964). These results indicated that a total of 1811 mg(16.5 mg g-1 of UF foam) were removed from the cell during the study period ofwhich 753 mg (6.8 mg g-1 of UF foam) were attributable to conversion of formal-dehyde within the simulated landfill cell.Washout was the major means by which the highly soluble formaldehyde was re-

moved from the single-pass cell. A similar estimation of the cumulative quantity offormaldehyde removed from the cell indicates that a total of 2025 mg (18.4 mg g-1 ofUF foam) of formaldehyde was removed from this cell during the study period [Fig.10(b)]. Hence, similar quantities of formaldehyde were removed from both cells, withonly a slightly lesser quantity removed from the recycle cell. This difference may beattributed to the possible differences in the actual quantities of UF foam placed ineach cell as well as possible sampling and analytical uncertainties.

4.3.4. Presence of formaldehyde in solid wasteTo determine if the formaldehyde detected in landfill leachate was attributable only tothe UF foam or also to other constituents in the solid waste, a supplementary studyusing two additional bench-scale simulated landfill cells was conducted. In one cell, a2.5-cm layer of UF foam was placed between two layers of shredded solid waste; onlyshredded solid waste was placed in the control cell. A total of 5.5 kg (3.9 kg dryweight) of solid waste was placed and compacted in each cell. Subsequent to sealingthe cells and initial moisture addition to bring the cells to apparent field capacity,moisture was added to ensure that sufficient volumes of leachate would be generatedfor formaldehyde determinations at periodic intervals. Additional analyses indicative



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Fig. 11. Concentration of formaldehyde removed with leachate from bench-scale simulated landfill cells with(0) and without (0) formaldehyde.

of landfill stabilization processes were performed on selected samples. These resultsindicated that the characteristics of the leachate generated by these cells were not onlysimilar and indicative of the presence of similar waste constituents and environmentalconditions within both cells, but also typical of leachates generated by sanitary landfills.

Results of formaldehyde analyses performed on leachate samples from both bench-scale cells are presented in Fig. 11. These results indicated that low concentrations (1-2mg 1-1) of formaldehyde were present in the leachate from the cell containing onlyshredded municipal waste. Therefore, some formaldehyde originated from the variousconstituents in the solid waste. This was not considered to be particularly unexpected,since there is widespread use of products containing formaldehyde-based resins, i.e.

pressed wood products, insulation, paper, fabric and carpet (National Research Council1981; Environmental Protection Agency 1976; Pickrell et al. 1983), many of which mayeventually be disposed of in sanitary landfills. Moreover, since formaldehyde is ex-tremely soluble in water, it could be readily extracted from these materials as fieldcapacity is reached and leachate is generated within a landfill.

Formaldehyde concentrations in the leachate from the cell containing a 2.5-cm layerof foam between the layers of solid waste ranged from 1.5 to 3.0 mg-I during theinitial phase of the test. This was slightly greater than the concentration measured inthe cell containing only solid waste and could be attributed to the leaching of additionalformaldehyde from the foam. Subsequent leachate samples indicated an increase in theconcentration of formaldehyde present in the leachate removed from the cell. (A similartrend was demonstrated previously for the single-pass simulated landfill cell where thegradual degradation of the UF foam within the cell led to the subsequent release offormaldehyde.) Over the duration of the bench-scale study, approximately 12 mg (3.1 Img kg-I of solid waste) were removed from the cell containing only solid waste, whereasapproximately 96 mg (24.6 mg kg-1 of solid waste) were removed from the cell thatalso contained UF foam. Consequently, although the solid waste contained constituentscapable or releasing formaldehyde to the leachate, additional amounts of formaldehydewere also extracted from the foam.

4.3.5. Impact of urea-formaldehyde foam on landfillsIn an actual landfill using UF foam as a daily cover, the amount of formaldehyderemoved from the foam would be dependent on several factors including depths of the



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lifts, thickness of the foam layer, moisture addition to the landfill, and opportunity forcontact between the leachate and the foam. All of these might vary significantlythroughout the landfill and from site to site. Since the design and operation of thesimulated landfill cells were conceived to ensure the presence of moisture within thecells and to maximize the opportunity for contact between the leachate and the foam,the results obtained from these studies might be considered to reflect the maximumamount of formaldehyde that could be expected to be removed from the foam withinthe landfill environment. A much smaller ratio of foam to solid waste prevailing in anactual landfill setting (i.e. about 1.7% as compared with 8% for the simulated landfillcells) would correspondingly decrease the quantities of formaldehyde contributed bythe foam.As previously mentioned, the concentrations of formaldehyde measured during the

study were much less than those that have been reported as being inhibitory to an-aerobic degradation processes (Parkin et al. 1983). Indeed, inhibition of micro-

bially mediated degradation processes was not observed once a viable methanogenicpopulation was established. Consequently, the potential for inhibition as a result offormaldehyde leaching from UF foam in an actual landfill setting is unlikely. Further-more, adverse health and environmental impacts as a result of the release of for-maldehyde containing leachate to the environment are similarly remote provided thatthe amounts of UF foam used are not excessive and leachate management is employed.If such a leachate was permitted to enter a groundwater system, the potential adverseimpacts of other leachate constituents would be far more important than the relativelysmall quantity of formaldehyde present.

5. Summary and conclusions

In additicn to verifying the previously demonstrated advantages of leachate contain-ment, collection and recycle as a landfill management option for accelerated landfillstabilization within a more manageable and predictable time frame, several observa-tions with respect to the potential impact of using UF foam as an alternative landfillcover material can be made. The total mass of formaldehyde removed from the cells(16.5 and 18.4 mg g-I of foam for the recycle and single-pass cells, respectively) mightbe considered to be the maximum amount of formaldehyde that could be removedfrom UF foam under actual landfill conditions, because the design and operation ofthe simulated landfill cells were conceived to maximize the opportunity for removal offormaldehyde from the UF foam. In addition, since some of the formaldehyde detectedin the leachate originated from the solid waste itself, and a much smaller ratio of foamto solid waste would exist in an actual landfill setting (i.e. about 1.7% versus 8.0% forthe simulated landfill cells), the relative quantities of formaldehyde contributed by thefoam as compared with the solid waste would be correspondingly less in an actuallandfill. Furthermore, the formaldehyde and other possible constituents leached fromthe foam did not preclude anaerobic microbial production of methane during landfillstabilization. In fact, conversion of formaldehyde appeared to occur during the methanefermentation phase. Hence, the potential for inhibition as a result of formaldehydeleaching from UF foam in an actual landfill setting is unlikely. Likewise, adverse healthand environmental impacts attributable to formaldehyde as a result of its release to theenvironment with the leachate is also remote, particularly if the potential for furtherdegradation and dilution is considered within the hydrogeologic setting and ground-



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water system. Consequently, it may be concluded that UF foam may be used as an

alternative daily landfill cover without posing adverse environmental impacts attribu-table to the release of formaldehyde from the foam during landfill stabilization.

Acknowledgements

This research project was sponsored in part by a research grant to Georgia Institute ofTechnology from SaniFoam, Inc., Costa Mesa, Calif.

References

Allan, G. G. (1980), Environmental Impact of the Use of Plastic Foam as a Daily and InterimLandfill Cover. College of Forest Resources, Physical Science Division, University ofWashington, Seattle, Washington.

Environmental Protection Agency (1976), Investigation of Selected Potential EnvironmentalContaminants: Formaldehyde. EPA 560/2-76-009. Washington, D.C.

Firstman, S. I. & Pohland, F. G. (1981), Operational Test of Sani-Blanket Daily and IntermediatePlastic Foam Landfill Cover, Georgia Institute of Technology, Atlanta, Georgia.

Graven, J. T., Giabbai, M. F. & Pohland, F. G. (1984), Method development for the determi-nation of trace levels of formaldehyde in polluted waters. In Advances in Chemistry, Series210, vol. 4, pp. 43-55. American Chemical Society, Washington, D.C., U.S.A.

Hileman, B. (1982), Formaldehyde—how did EPA develop it’s formaldehyde policy? Environ-mental Science and Technology, 16, 543A-547A.

National Academy of Sciences (1980), Formaldehyde and it’s Health Effects. Government PrintingOffice, Washington, DC.

National Resource Council (1981), Formaldehyde and Aldehydes. National Academy Press,Washington, DC.

Parkin, G. F., Speece, R. E., Yang, C. H. J. & Kocher, W. M. (1983), Response of methanefermentation systems to industrial toxicants, Journal of the Water Pollution Control

Federation, 55, 44-53.Pickrell, A., Mocker, R. V., Griffis, L. C. & Hobbs, C. H. (1983), Formaldehyde release coef

ficients from selected consumer products, Environmental Science and Technology, 17, 753-757.

Pohland, F. G. (1980), Leachate recycle as landfill management option, Journal of the Environ-mental Engineering Division, ASCE, 10b, 1057-1069.

Pohland, F. G., Dertien, J. T. & Ghosh, S. B. (1983), Leachate and gas quality changes duringlandfill stabilization of municipal refuse. In Third International Symposium on AnaerobicDigestion, Boston, Massachusetts, pp. 185-201.

Tchobanoglous, G., Theisen, H. & Eliassen, R. (1977), Solid Waste-Engineering Principles andManagement Issues. McGraw-Hill, New York.

Walker, J. F., (1964), Formaldehyde, 3rd Ed. Reinhold, New York.Wolfe, R. (1971), Microbial formation of methane, Advances in Microbial Physiology, 6, 107-146.



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