THE WORLD’S LARGEST LANDFILL
I
ESTIGATR
he aphorism “out of s i g h t , o u t of mind” generally reflects the human attitude toward solid wastes. Once trash is buried in landfills, we tend to ignore i t a n d croorganisms gradually presumeconvert that mithe
disposal option and accounts for more than 70% of U S . municipal solid wastes (1, 2).This amounts to a burden of more than 150 million tons of refuse buried annually. Unlike hazardous or toxic waste spills, which affect relatively few individuals, all of us contribute to the solid waste stream and the attendant difficulties. Landfilling has been practiced in
refuse to innocuous materials. Landfilling-one of the oldest and perhaps the simplest form of biotechnology-is the most popular
all parts of the country with design modifications based on the intended length of operation, local regulations, and site-specific condi-
U
1486 Envimn. Sci. Technol., Vol. 26, NO.8, 1992
JOSEPH M. SUFLITA Deportment of Botony ond Microbiology University of Oklohomo Norman, OK 73019
CHARLES P. GERBA Deportment of Microbiology and Immunology University of Arizono Tucson, AZ 85721
ROBERT K. HAM Deportment of Civil Engineering University of Wisconsin Modison, WZ 53706
ANNA C. PALMISANO Ofice of Novo1 Research Arlington, VA 22217
W I L L I A M 1. R A T H J E Dept. of Anthropology University of Arizono Tucson, AZ 85721
J O S E P H A. ROBINSON The Upjohn Company Kolomozoo, MI 49001 tions (3).Such practices have occasionally led to serious environmental consequences: most notably groundwater pollution (46).As a nation, we now face the so called “garbage crisis.’’ Although opinions differ as to whether the crisis is real or reactionary, it is well established that more solid waste is generated each year, we are running out of convenient places to dispose of it, and tipping fees continue to increase steadily ( I ) . Despite our history of reliance on the more than 100,000 active and
0013-936W92~0926-1486$03.00/0 0 1992 American Chemical Society
closed US.landfills, relatively little is known about how well this disposal technology actually functions. Landfills are associated with a variety of concerns, most of which center on what is actually buried in them, their potential to threaten environmental or human health, and the extent to which microbial decomposition processes occur. To address such issues, a multidisciplinary team of investigators was assembled to study the Fresh Kills Landfill (Staten Island, NY). Sampling Fresh Kills The Fresh Kills Landfill was chosen for study because it is one of the oldest in the US.and it has the dubious distinction of being the largest in the world (2).The landfill was established on Staten Island in 1948 by the city of New York as a means of reclaiming land in a tidal swamp area. It currently receives 17,000 tons of refuse daily and will likely reach capacity by about the year 2000. It now covers more than 1200 hectares and contains 6.7 x lo7 cubic meters of refuse (2). The landfill is unlined, reflecting past disposal practices, and generates on the order of 3 x 10' liters of leachate each day (2). Due to the enormity of the landfill and the exuense involved, a random sampling kffort could not be conducted. Rather, a systematic sampling scheme was employed which took advantage of preliminary re-
connaissance information that included a study of refuse deposition records, testimonials from landfill operators, and preliminary excavation efforts. Our objective was to obtain refuse that had been buried in the landfill for various lengths of time. To that end, a bucket auger was used to drill 14 boreholes at various locations (2,7).The locations were sited in every major section of the landfill, and the dates of recovered newspaper fragments spanned the full age of the landfill. Several hundred kilograms of refuse were collected at about 3-m depth intervals within each borehole. A total of 47 samples containing more than 4500 kg of refuse were obtained. To facilitate the comparison of information, a shared subsampling protocol was employed. When each sample was retrieved from a borehole, it was deposited on a plywood board at the landfill surface. Approximately SO-70% of the refuse collected in this manner was taken for archaeological analysis by methods previously described (7-9). The remainder was used for chemical and microbiological analysis. The former collection techniques have also been described (10): the latter included the field sieving of the refuse (5 x 5 cm)to remove large debris. An exception to this protocol was used to sample disposable diapers from the landfill. Diapers were collected whenever they were ob-
served in either the drilling spoils or the samples. Leachate was also sampled on site when it was available in wetter locations. After collection, the bulk of each sample was transported overland in plastic bags to various laboratories for subsequent analyses. Samples for anaerobic microbiological experiments were shipped under nitrogen in plastic buckets equipped w i t h O-ring sealing closures. Freshly collected diapers and about a 100-g portion of each sieved refuse sample were iced and sent for microbial enumeration analysis. However, extracellular enzyme activities were assayed in the field on sieved refuse material within a few hours of collection.
Sample analyses To determine the contents of the refuse samples, an archaeological analysis was performed which included weight and volume determinations on each sample collected (7, 1 1 , 12). In addition, the samples were dated and sorted, and refuse components were classified based on their visual and tactile characteristics (9). This included an analysis of coarse debris as well as material that passed through 1.27-, 0.63-, and 0.31-cm sieves. This method allowed characterization of the refuse into 32 categories as previously described (7,11. 12). A detailed chemical analysis of the refuse samples was performed
Environ. Sci. Technol., VoI. 26. No. 8, 1992 1487
(IO), including monitoring biogas formation in lysimeters packed in the field with 7-9 kg of selected refuse samples (131. Soluble refuse components were assayed on the liquid portion of those samples that were clearly water saturated. However, for those that were not, a field extraction of the samples with water was done by hand packing refuse in distilled and deionized water, shaking, decanting, and centrifuging the extract (33). Chemical analysis of the solids was done with 23 kg of each refuse sample, which were first shredded, divided into five replicate subsamples, a n d ground to pass through a 1-mm sieve (adapted from Reference 10). The distribution and abundance of microbial enzymes believed to be involved in the initial stages of refuse decomposition were also determined. Biochemical activity assays were conducted for extracellular cellulases, proteases, esterases, and amylases (see boxl. Esterase, amylase, and protease activity were reported as nmoles fluoroscein generated, mmoles p-nitrophenol made, and units of Azocoll hydrolyzed per hour per gram dry weight of refuse, respectively. A unit of Azocoll was equal to 85 wg. The microbial degradation of organic matter was also evaluated by measuring the endogenous rate of methane production in replicate subsamples of the refuse material collected in the plastic buckets. Methane production was measured by placing 200-300 g of the refuse samples i n s i d e specially constructed PVC incubation vessels which were sealed to maintain anaerobic conditions. All sample processing was conducted under a nitrogen atmosphere inside a n anaerobic glovebag. After the incubation vessels were closed, the headspace was exchanged with 0,free N,. Methane that was formed in the headspace of the incubation vessels was monitored by gas chromatography. Microbiological analysis included the enumeration of both nonpathogenic as well as a number of human pathogenic microorganisms. The latter included a variety of viruses and protozoa (see box]. Composition and characteristics Some of the physical and chemical variables that influence biodegradation processes in the landfill are seen in Table 1. Notably, the pH, temperature, and moisture content of the Fresh Kills
r
Lendflflmicrobiology and blochemlotry
Unfortunately, there is no generally accepted "standard method for the microbiological and biochemical analysis of landfills.The salient experimental details of the methodology employed in this investigation are given below. Extracellular enzymes were extracted from 10-g samples of excavated refuse with a detergent sobtion (O.% Triton X-100) amended with magnesium sulfate (14). Following centrifugation, the supernatant was incubated with enzyme substrates at 37 "C and analyzed spectrophotometrically or fluorometrically for activity of esterases ( 1 5 ) , amylase (Amylase 3, Sigma Chemical Co.), and proteases and cellulases (74). Enterlc viruses and protozoa were extracted from the diapers by a modification of the method of J. S. Glass et al. (16). Each diaper was placed in a plastic Ziploc bag and one liter of 1.5% beef extract adjusted to pH 9.5 was added. The diaper was then kneaded for five minutes and the eluent poured into a plastic beaker and the pH adjusted to 7.2.Any Giardia cysts or Crypiosporidiumoocysts were removed by low-speed centrifugation. Viruses in the sample were concentrated to
samples varied from 5.8-8.1, 10.562.8 "C, and 10-75%, respectively. Some of the samples were difficult to characterize visually in the field as municipal refuse. These samples were generally the wetter ones and could probably be best described as sludge-like. Therefore, the moisture content of the refuse samples was an obvious parameter to monitor. This determination was made by measuring the weight loss of the coarse-sieved (5 x 5 cml samples upon drying at 110 OC for 48 h. An analysis of the moisture content of newsprint revealed that the refuse could be subdivided into two major groupings; either wet (250% moisture content) or dry (40% moisture content). The analysis included a quantilquantile evaluation of the moisture content in the newspaper fraction of the samples (Figure 1). This analysis is likely conservative because it did not include the water-saturated samples that were identified as sludge-like. Newspaper was chosen as the component to subdivide refuse into moisture levels because it was found in most
1488 Envimn. ki.Tednol.. Vol. 26. NO.8, 1992
20-30 ml before assay by the method of J. S. Glass et al. (76). Giardia cysts and Cryptosporidium oocysts were detected using the method of J. B. Rose et al. (77). Samples were assayed for enteroviruses using Buffalo Green Monkey cells by obsewation for cytopathogenic effects a s described by J. B. Rose et al. (78) for polio virus, rotavirus, and hepatitis A virus with a gene probe as previously described (79-27). Positive controls were done by contaminating both fresh and landfill-recovered diapers with the pathogens which were to be examined; recoveries after concentration averaged 51%. Aerobic bacterla were cultured on Standard Methods Agar (Difco). Five refuse samples were selected for culture experiments in an attempt to represent a range in chronology from 1965 to 1988. Total bacterlal numbers were determined by direct microscopic counts of acridine orange stained cells (n).This method may overestimate viable cells because nonviable cells may be inadvertently counted. Methanogens and sulfatereducing bacteria were cultured according to Beeman and Suflita (29and Tanner (24), respectively.
samples in enough quantity to make reliable measurements and its physical characteristics remained relatively consistent throughout the deposition history of the landfill. A composite analysis of the refuse in the wet and dry Fresh Kills samples relative to the overall composition typically found in other landfills is seen in Figure 2. The composition of municipal refuse in the dry Fresh Kills samples was comparable to that typically observed in other landfills. That is, the major component of landfills by weight (about 40%) is paper. However, the volume of paper in the wet Fresh Kills samples was less than 15%. The difference in the volume of paper between the wet and dry portions of the landfill might be due to an increase in biodegradation activity in wetter portions of the landfill. However, this possibility needs to be critically explored. The fate of solid wastes Previous investigations showed that refuse materials believed to be
A comparison of environmentalvariables associated with the Fresh Kllls Landfill' N
PH
Temperature ("C) Moisture (% wt.rwt.) Esterase (nmles/h/g) Amylase (mnoles/h/g) Protease (units/h/g) Methane rate (pmdeskg/day) Sulfate (mmoleskg) Volatile solids PA Cellulosefiigin ;ai0 Degradable organic matter (8)
34 46 34
Usan
7.0 29.4 36
Median 7.0 26.1 33
Maximum
8.1 62.8 75
Wlntmun 5.6 10.5 IO
(K)
7.2 24 41
56
3.6 0.029
3
700
3 3
80
31 47
I.7
25
1.2 7.0
76 3.5 610
'All data are normalized on a dry weight basis. N = number of samples: CV I d i c i e n t of "allation
readily biodegradable can persist in landfills for rather long periods of time (2).Items such as paper, food, and grass clippings can be recovered relatively intact following decades of burial. Further, measured m e t h a n e y i e l d s b a s e d on t h e amount of biodegradable refuse in landfills are typically only about 1-50% of that theoretically possible (25).We questioned whether such observations were exceptional and the reasons for the apparent recalcitrance of such materials. When garbage gets buried in landfills, it is generally believed that anaerobic conditions rapidly develop. Aerobic heterotrophic respiration occurs with the more biodegradable forms of organic matter in fresh refuse (25),effectively depleting 0, at a rate exceeding the rate of replenishment of this electron acceptor to the interior of landfills. Aerobic bacteria were cultured from selected Fresh Kills refuse samples at densities of 104-108 colony-forming unitwg dry wt-', whereas the total bacterial numbers were found at densities up to IO1" colony-forming unitsg dry wt-' [see box). The cells were typically associated with refuse surfaces, as shown in the electron microscope photograph. Direct microscopic and cultivation attempts revealed a wide range of cell and colony morphologies. The anaerobic biotransformation of solid wastes in landfills is fimdamentally different from aerobic metabolism, but essentially similar to the fermentation of organic matter in other anoxic ecosystems (26). That is, a consortium of interacting
lot of moisture content of newspapers
microorganisms represents the catalytic entity. Fermentative bacteria convert biological polymers such as cellulose, starch, protein, fats, lignin, and other materials eventually to CO,, H,, and a variety of organic acids and alcohols. Between io5 and 10' fermentative colony-forming units.g dry wt-' were isolated on enriched anaerobic medium (Medium 10) (27).Colonies of anaerobic polymer-degrading microorganisms were identified by differential staining or clearing of polymeric substrates in agar (28). Proteolytic degraders ranged between 2-15% and 0 4 % of the total fermentative bacteria when incubated at 2 2 O and 37 OC, respectively.
Starch degraders ranged between and 2 4 % of the fermenter population when incubated at the same temperatures. Anaerobic cellulose degraders were not isolated. To date, there are only two reports of anaerobic cellulolytic bacteria isolated from refuse (29, 30). The organic acids and alcohols generated by fermentation are often components of leachate and are themselves substrates for the proton-reducing syntrophic bacteria that convert them to suitable methanogenic precursors such as acetate, CO,, and H,. The acetogenic bacteria can convert H, and CO, or simple polyhydroxylated or methoxylated aromatic compounds directly 0.4-12%
Environ. Sd. Technol., Vol. 26, No. 8. 1992 1-
arlson of the typical composition of refuse (by percent weight) in a landfill (A) with that measured in the dry (E)and wet (C) portions of the Fresh Kills Landfill'
I
'1
C
I
( 5
85
I
Paper
3
11 8
c]Yard and food wastes [7 Metal
Plastic
7Other and 20 samples, respe~f~vely
to acetate. The nutritionally limited methanogenic bacteria are able to make CH, from the small molecular weight products of the initial bioconversions, like methanol, acetate, or H, and CO,. The hasic reason for this interaction in anaerobic metabolism is that many bioconversions tend to be thermodynamically unfavorable unless the concentration of H, is maintained very low atm). It is helieved that the methanogens largely fulfill this role in landfills, and th, fermentations are effectively cou pled. Without such coupling, metabolic endproducts accumulate and inhibit the degradation of the starting materials. Because polymeric substrates in landfills are generally solid, high molecular weight, and difficult to transport across microbial membranes, extracellular hydrolyzing enzymes are elaborated by refusefermenting organisms. Structural characteristics of the polymers affecting the potential for attack by extracellular hvdrolases include the degree of cryscallinity, surface area, hydrophilicity characteristics, and overall nhvsical accessihilitv of the polymer to the enzymes. The initial hydrolysis of polymers to lower molecular weight substances is the first step in their biodegradation in landfills. The activity of several hydrolytic enzymes was measured as a general indication of the decomposition process. Table 1gives the distribution and relative activity of three enzymes in 28 refuse samples dating from 1965 to 1988. Esterases, amylases, and proteases were present in all samI
,
1490 Environ. Sci. Technol., Vol. 26,NO.8,1992
Scunninx rlrctrori micrograph showing morphologicolly dii.er(P hoctnrio o$socioted ivifh the curfncc of(1 grass OIudc N : C O V C N ? ~ ~Fresh ~ U ~Kills ~ ~ Lundfrll.
ples and showed a fairly wide range ol activity throughout the 23-year
period. Samples collected from the late l96ns and earlv 1970s showed a range in activity similar to those collected from the late 1980s. Therefore. the leneth of burial time does not appear to he a major variable affecting the distribution and activity of these three enzymes. These enzyme activities correlated weakly with the other measured variables [ A B S (r)2 0.51 (Figure 31. The cellulase activity assay indicated an extremely patchy distribution for this enzyme. Only two of the refuse samples tested positive in this determination. This finding may attest to the difficulty in extracting these enzymes (32)or the
degree of hetenigmeity in the sample size (10 g) examined. l h e endogenous rate of methane oroduction in the Fresh Kill samples varied considerably and ranged f r o m u n d e t e c t a b l e to >700 umoles,ke drv wt,d-' [Table 11. kven thouugh methanogenic bacteria could he cultured from all samples (see box). This rate of methane production was found to correlate reasonably well with the moisture content of the samples (r = 0.69; P = 0.0001: Figure 3), but did not exhibit a strong relationship with other factors such as pH, depth, age of the sample, or temperature [ABS (r)S 0.41. However, refuse methanogenesis was largely limited to samples having a circumneutral pH
(Figure 3). About a third of the refuse samples placed in lysimeters also produced biogas (CH, and CO,) within minutes of collection. This rate of gas praduction in lysimeters was also correlated with the moisture content of the samples (r = 0.73). Clearly the moisture content had a significant influence on landfill methanogenesis, an observation also noted by other investigators (33-36). However, it is conceivable that the anaerobic biodegradation of refuse organic matter could be linked to the reduction of other terminal electron acceptors like nitrate, sulfate, or ferric iron in addition to carbon dioxide. T h e
importance of refuse decomposition under denitrifying, sulfate-reducing, iron-reducing, or other anaerobic conditions in general is largely unappreciated. An extraction and assay of the refuse samples for other potential electron acceptors revealed that sulfate was present in all samples at concentrations ranging from 1.2 to 80 mmolekg dry wt (Table 1). This analysis included extracting the refuse samples with an equivalent weight of water for 1 h, centrifuging the resulting slurries at 20,000 x g for 25 min, and analyzing the supernatant using anion exchange high-pressure liquid chromatography as previously described (23).Moreover, sulfate-
reducing bacteria were also detected in each sample. Such high levels of sulfate in some of the samples suggested that methanogenesis might be adversely influenced (23,37-40). Of course, the presence of sulfate or other electron acceptors in landfills does not preclude the biotransformation of refuse materials. Although methanogenesis may be negatively affected, biodegradation could still occur over time. What then is the evidence for the biodegradation of refuse in Fresh Kills? Figure 4[a) depicts the volatile solids content of the refuse samples as a function of how long the material was buried in the landfill. Envimn. Sci. Technol., VoI. 26. No. 8, 1992 1491
Generally this relationship tends to decrease with increasing time in a landfill (r = 0.68; P = 0.0001). The volatile solids content was also highly correlated with the quantity of degradable organic matter and the cellulose content (Figure 3). We have arbitrarily defined degradable organic matter as the sum of all paper items (corrugated, packaging, nonpackaging, newsprint, glossy, mail, and phone books) as well as yard and food wastes. Of course, there simply was not as much paper or as many paper products discarded 25 years ago. It is estimated that the percentage by weight of municipal solid waste identified as paper and paperboard was 37.4% in 1965 and 42.8% in 1990 (41). Therefore, an additional comparison is needed. Figure 4(b) compares the cellulose-to-lignin ratio as a function of sample age. Cellulose and lignin occur together in wood a n d wood products. Cellulose would be expected to degrade in the anaerobic environment of a landfill, but the transformation of lignin
I he variation in volatile sol in samples of differentage
1492 Environ. Sci. Technol., Vol. 26. No. 8,1992
would be much slower. The lignin in the sample is in effect being used as an internal standard, and the ratio is being used to normalize for differences in refuse composition and deposition rates over time. Fresh refuse has a cellulose-tolignin ratio of about 4;as biodegradation proceeds, that ratio has been observed to fall to 0.2 (IO). This analysis of the Fresh Kills samples indicates that the cellulose-to-lignin ratio generally decreases with increasing burial time in a landfill (r = 0.70; P = 0.0002; Figure 4b, Table 1). The rate of change in this ratio was 0.071 f 0.016 year-'. Thus, biodegradation has occurred or is currently occurring in the Fresh Kills landfill, and some of that degradation need not be linked with methanogenesis. The correlation between the celluloseto-lignin ratio and moisture content, though not as strong, was negative (r = -0.41; P = 0.040; Figure 31, suggesting that moisture is not strictly related to the cumulative amount of refuse decomposition.
cellulose-to-lignin ratlo (b) from the Fresh Kills Landfill
Diapers and public health One of the public health concerns regarding solid wastes in landfills relates to the potential for contamination of surrounding environments with pathogenic microorganisms from buried materials. Sources of pathogen entry to landfills include sewage sludge, septage, and pet and human excreta; the latter is often in disposable diapers (42). Another objective of this project was to quantify the occurrence of enteric viruses and parasites in disposable diapers that had been buried in a landfill for various lengths of time. Previous studies have focused attention on landfill leachates or material being deposited in landfills (43,441,but have not attempted to determine how long pathogens survive in a landfill (4547). We examined diapers recovered from the landfill for the presence of enterovirus, hepatitis A virus, rotavirus, Giardia, and Cryptosporidium (see box). The incidence of enteric viruses that may enter landfills via diapers is difficult to estimate, yet this value must be approximated because not every diaper disposed in a landfill would be expected to harbor pathogens. To date, there is only a single study of the incidence of enterovirus in diapers prior to their deposition in landfills (48)and it was found that approximately 9% of the diapers were contaminated. Other estimates of the incidence of disease can be made based on the percentage of the population infected with the specific organism. For instance, about 10% of children under two years of age are infected with rotavirus at any given time. The incidence increases to 16% for hepatitis A virus infections among children in daycare centers, but it should be noted that such infections are usually asymptomatic in children. There is really no good estimate of the incidence of Cryptosporidium infections in the diaperwearing population, but the 1 4 % incidence of infection of the general population with this organism can be used as a guide. The incidence of children under two years of age excreting Giardia usually ranges from 8 to 26% in daycare centers and from 1 to 3% in children not in daycare centers (49-51).The incidence of Cryptosporidium in children averages 4% of those with gastroenteritis, but the incidence of asymptomatic children in daycare centers has been reported as high as 27-
30% (52-54). The average annual rate of rotavirus gastroenteritis is 10.4% in the United States ( 5 5 ) .The incidence of clinical hepatitis in the United States is O . O l % , but it is believed that this represents only 15% of the actual number of cases. This would make the actual incidence about 0.1%. Hepatitis A is excreted in lower concentrations in the feces than are the other enteric pathogens studied in this project, making it less likely to be detected. However, because it is more resistant to thermal inactivation than the other pathogens, it is more likely that it would survive longer. Thus, any findings of pathogens in diapers recovered from a landfill must be interpreted relative to the expected rates of infection. For the purposes of this study, we assumed a 10% incidence of infection. We collected 70 diapers from the Fresh Kills Landfill of which 54 were soiled. As in other investigations (7, 11, 12), disposable diapers made up only 1.2% of the volume of refuse in Fresh Kills. The oldest diaper was buried in 1965; the most recent was deposited in 1988. Most diapers were deposited from 1980 to 1988. No viable pathogenic viruses or protozoa were detected in any of the diapers recovered from the Fresh Kills Landfill. We questioned the statistical probability of finding such a result. Assuming only the soiled diapers could harbor pathogens, and a 50% pathogen recovery efficiency, the unknown true proportion of diapers potentially contaminated could be as high as 11% ( P = 0.05). This analysis employs the binomial theorem (56). The estimated value is roughly equivalent to our presumed incidence of infection. Therefore, our results argue that pathogens probably do not survive for extended periods of time in diapers in landfills, but there is a low probability that our findings could be due to chance alone.
Conclusions and inferences The Fresh Kills investigation has led to several insights on the functioning of landfills and on the environmental factors regulating landfill decomposition processes. First, it appears that moisture is an important parameter influencing the biodegradation of municipal refuse. Unexpected differences were observed in the relative composition of refuse in wet and dry areas of the landfill relative to other landfills that were similarly analyzed. The
wet areas contained only a fiaction of the amount of paper found in dry areas of the Fresh Kills Landfill, whereas the latter areas exhibited a “typical” refuse composition (Figure 2). Moisture levels also correlated with the endogenous rate of methane formation in landfill samples and biogas production in lysimeters. We also found high levels of sulfate in several samples of the landfill. This terminal electron acceptor could adversely influence the potential generation of methane from landfills. Such findings lead to questions of the origin of the sulfate. In this respect it is important to note that archaeological analysis of the Fresh Kills Landfill revealed that up to 14% of the volume of refuse could be accounted for by construction and building debris. This fraction of refuse undoubtedly contains gypsum wall board and could represent a significant source of sulfate to the landfill. The source of sulfate in the Fresh Kills Landfill and a determination of to what extent such observations can be generalized is being actively investigated. Prospects for the efficient microbial destruction of solid wastes in landfills as a practical disposal goal has been questioned (2). This is understandable considering the substantial variability in biodegradation activity both between and within landfills. However, perhaps the most compelling evidence for biodegradation in landfills is the cellulose-to-lignin ratios as a function of sample age. We found that this ratio decreased with time of refuse burial in the Fresh Kills Landfill; it should be noted that this determination is independent of the nature of the dominant terminal electron accepting process that may be occurring in landfills. Such an analysis forces us to be more realistic in assessing the rates of refuse decomposition in a landfill. Based on the changes in the cellulose-to-lignin ratios depicted in Figure 4(b), it can be predicted that a decade or more may be required to realize a 50% alteration in this measure of biodegradation. Such an interpretation must be made relative to the life span of a typical landfill, which is generally 10-20 years (57). Moisture content seemed to be an important factor associated with key variables that are indicative of refuse biodegradation (Figures 2 and 3). This observation notwithstanding, it is important to emphasize that many interacting factors can also influence biodegradation in
landfills, including the physiology of the requisite microorganisms, the existing environmental conditions, and the physical-chemical nature of the substrates being metabolized (5860). Further, the skewness of many of the variables measured in this study (Table 1)attest to the heterogeneity of the landfill environment. Consequently, it should not be surprising to find refuse artifacts in landfills persisting for long periods of time. It seems clear that our expectations of microbial decomposition processes are unrealistic. In this context, it should be noted that there are many examples of readily degradable forms of organic material that persist for long periods of time when incubated under adverse ecological conditions (58-60). Lastly, our analysis of pathogenic viruses and protozoa in landfills indicates that these organisms probably do not survive for long periods of time. This is also perhaps not surprising because there are many examples of the die-off of nonindigenous microorganisms once they are placed in a foreign environment (61). Mechanisms of this die-off could include both biotic and abiotic factors. In this context the heat lability of many of the pathogenic viruses needs to noted. The average temperature in the Fresh Kills Landfill (29.4 “C) would probably rapidly inactivate these microorganisms (62). We have tried to summarize our most important findings, fully cognizant that any single investigation cannot explore every intricacy associated with a complex environment like a landfill. However, we believe our effort integrates a number of disciplines in a fashion that has not been done before. It is not our attempt to comment here on the important management, societal, regulatory, or economic aspects associated with solid waste landfills. Rather, it is our desire to contribute to the technical foundations upon which decisions on such matters can be reliably based. An understanding of the fate of solid wastes buried in landfills is important to societies across the globe. Our investigation of the largest landfill in the world suggests that the moisture content of refuse may significantly limit the methanogenic fermentation of solid wastes. This and several other environmental variables may help explain why the actual recovery of methane from landfills is often much lower than predicted based
Environ. Sci. Technol., Vol. 26, No. 8,1992 1493
on the amount of degradable organic matter deposited. However. biodegradation per se does occur i n the Fresh Kills Landfill, as evidenced by a decrease in the cellulose-to-lignin ratio from 3.5 to 0.74 in samples of increasing burial age. The rate of change in this ratio was found to be 0.071 k 0.016 per year. This suggests that refuse biotransformations are slow, relative to the length of operation of a typical landfill. Lastly, our assessment of the fate of pathogenic viruses a n d protozoa that may be deposited i n diapers indicates that these organisms probably do not s u r v i v e for extended periods of time in a landfill.
Joseph M. Suflita is a professor of microbiology in the Department of Botany and Microbiology at the University of Oklahoma. A microbial ecologist, his research focuses on the metabolic fate of organic contaminants in anaerobic environments.
Acknowledgments The Fresh Kills Landfill research project was funded in its entirety by a consortium of private foundations and industries, including the Council for Solid Waste Solutions, E. I. Dupont de Nemours & Co., Exxon Chemical Co., Hercules, Inc., Hoechst Celanese. Jefferson SmurfittIContainer Corporation of America, the National Council of the Pa~~, ner on Air and Stream lmr - lndustrv ~ provement. The Perscco C o . . T h e Procterand GambleCo.. Scott Paper Co.. and Sonoco Products Co. Special thanks are extended to members of the research teams without whom this project could not have been conducted. They include W. W. Hughes, M. Tani. D. C. Wilson. G. Archer, M. Huber, and S. Bradford of the University of Arizona: P. R. Fritschel and M. R. Norman of the University of Wisconsin: D. A. Maruscik, B. S. Schwab, L. W. King, and B. E. Jones of The Procter and Gamble Co.: R. E. Barnes of The Upjohn Co.: J. Rose of the University of South Florida; and K. R. Gurijala. M. R. Mormile, and F. Concannon of the University of Oklahoma. ~~~
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Anna C. Palmisono is (I microbiologist at the OfJim of Nova1 Research. The work detailed in this article was performed while she was with the Environmental Safety Department of Procter and Gamble Company. Cincinnati. OH.
A
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Charles P. Gerba is a professor in the departments of Soil and Water Science, and Microbiology and Immunology at the University of Arizona. His research interests are the detection, fate, and tmnsport of pathogenic microorganisms and their removal by water treatment processes
William L. Rofhje is a professor of anthropology al The University ofArizona. He was trained as a Maya archeologist. He founded the Garbage Project which, since 1987, has conducted excavations at 14 landfills across North America.
Robert K. Ham is o profrssor of civil and environmental engineering at The University of Wisconsin-Madison. His teoching and research center on solid and hazardous wastes engineering and management. Recent projects relate to degradation of solid waste in landfills and setting up and monitoring recycling at ogencies.
Joseph A. Robinson is ~i research scientist in the Agricultural Division of The Upjohn Company. His biological research focuses on the identification of compounds or biological agents for prevention of ruminant acidosis in the bovine. His mathematical interests include the modeling of microbially catolyzed processes in ecosystems.
Rinehart and Winston: New York.
Solid Waste Management and Moterials Policy Proceedings; NY Legislative Committee on Solid Waste Management: Albany, NY, 1989:p. 111. 1131 . . Harlz. K. E: Ham. R. K. Consew. Re-
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