effects of Waste Disposal on Groundwater and Surface Water (Proceedings of the Exeter Symposium, July 1982). IAHS Publ. no. 1 39.
The pollution potential of sanitary landfill
R. Stegman Institut fiir Stadtbauwesen, Technische Universitat Braunschweig, Postfach 3329, 3300 Braunschweig, West Germany
ABSTRACT Dependent upon its nature, soil beneath sani tary landfill is contaminated with organic and inorganic compounds from leachate. In addition gas generated from the landfill can migrate through the soil causing addi tional problems. This is exacerbated if landfill sites are not sealed, but it should be taken into consideration that liners are rarely completely impervious. Leachate production rates and data on leachate quality data are presented. Gas emission data are considered and proce dures for minimizing these by optimizing landfill tech niques are discussed.
INTRODUCTION
By far, most of the municipal solid waste (MSW) today is disposed of in sanitary lanfill. New waste handling technologies such as re cycling as well as well-established technologies such as incinera tion and composting only result in a reduction of the amount of waste that has to be landfilled. For this reason sanitary landfill will also have to be used in the future. Although landfill has been used for a very long time, little has been done to optimize landfill operation techniques in order to minimize emission rates. The defi ciency in knowledge concerning the processes that take place in sanitary landfill is one reason for this situation. It should be the aim of future research and practice to test new landfill con struction and operation techniques in order to reduce leachate pollution emissions into the environment. Landfill gas should be collected and used. Research is also very much needed in the field of long term behaviour of landfill, since only limited data are available con cerning the gas production and leachate quality over time. These activities have to be seen in connection with different operating techniques.
BIOCHEMICAL-, CHEMICAL AND PHYSICAL PROCESSES IN SANITARY LANDFILL
Leachate quality is to a high degree dependent upon the kind of processes that take place in a landfill. For a better understanding of leachate quality data, a short summary of the processes that take place in landfill is given. 125 126 R. Stegman
Aerobic processes take place on the surface of the fill as well as during a short period of time within the fill until the oxygen is consumed. This oxygen had been trapped in the voids of the refuse during landfilling. By far the main degradation of the organics takes place under anaerobic conditions. Under these conditions the organics are broken down subsequently in different phases mainly into CH CO (figure 1). In the acid phase high concentrations of organic acids, hydrogen and CO are produced. Some of these products can be used directly by me tnane-forming bacteria to produce methane and in some cases water. Organic acids that cannot be used directly by methane- forming bacteria will be converted by an inbetween step to methane. This step includes a bioenergy symbiosis of "acitogen" and methane- producing bacteria where the last group uses the hydrogen that provides adequate energy conditions for the "acitogen" bacteria (Rees, 1980, Schobert, 1978). Concerning enhancement of methane production, milieu conditions have to be installed that provide a balance between acid- and hydro gen-, on one hand and methane-producing bacteria on the other. In actual landfill it takes some time for this balance to be reached since the generation rate of methane bacteria is about 10-20 times lower than the generation rate of the acid and hydrogen producers. In addition, high organic acid concentrations inhibit the methane bacteria, which results in highly organic polluted leachate over a long period of time. By changing traditional landfill operation techniques the acid phase should be minimized in order to achieve low organic leachate concentrations and early methane production (Stegman, Ehrig, 1982). Figure 2 shows the results from a batch test in laboratory scale where MSW and shredded composted MSW were mixed and placed in gas- tight containers (room temperature 30°C). The addition of the composted refuse was necessary to avoid the aforementioned inhibi tion of the methane formers due to high organic concentrations. The collected leachate had been recirculated. These tests show the different degradation phases of MSW in a compressed time scale. The high organic leachate concentrations decrease with an increase in methane concentration and gas production. Sulphates are reduced biologically; as a result non-soluble or low solubility metal sul phides are formed in the landfill which reduce metal emission rates. The stable methane production phase also results in high pH-values which in turn affects the solubility of metal ions. The alkalinity is influencd to some extent by the gas production since high CO -concentrations in the leachate result in a high solubility potential of bicarbonate ions. This alkalinity can be leached in a similar manner to the chlorides. In addition to the aforementioned biological processes all the other processes that are known from soil, e.g. complexing, ion exchange, adsorption take place in landfill. Almost nothing is known about the effectiveness of these processes in sanitary land fill.
WATER BALANCE IN SANITARY LANDFILL
A scheme of the water balance in sanitary landfill is presented in Pollution potential of leachate 127
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XI o J3 u c-j n en M •H X! O SJ a» (0 a o 0) e a i—i 128 R. Stegman tn S -a - " '^ Cmg/ i o u o 4-1 C.M. o O \ ! in O I •P L/>^ •H o i> i-H 4-1 o o 3 tn «JM S C3 CS CS S es es es es es es EVAPO EVAPORATION TRANSPIRATION PRECIPITATION SURFACE RUNOFF •or Vy w -<>• y '»».%^l¥Sp»SpïfiS»OISTRIBUTION UPPEH^yoooR LAYER ^^i^-#ycTg=a.ggsià;gVSTORAG; E DRAINPIPE FIG. 3 Scheme of water balance in sanitary landfill. figure 3. Since MSW is relatively dry (20-35% moisture based on wet weight), the rate of water infiltration as well as water distribu tion is essential for the processes that take place in a landfill. Biological processes need a high moisture content (~50%) and leach ing of the waste is also very much moisture dependent. Leachate production starts before the MSW is saturated with water. This observation is based on a monitoring programme of leachate production where 15 full scale landfill sites, sealed either by plastic liner or natural soil, were investigated (Ehrig, 1980). The explanation for the early leachate production is the routing of leachate through the MSW. Leachate production is influenced by the annual precipitation and evaporation rate, the infiltration rate and the moisture uptake of the MSW. When the MSW is at field capacity the annual leachate production rate will correspond with the annual rate of infiltra tion. Based on the aforementioned investigations the following mean annual production rates were measured in West Germany (500-800 mm annual precipitation rate): 1. highly compacted landfill (by means of a compactor) 15-25% of the annual precipitation (~5 m3 ha-1d-1) 130 R. Stegman 2. loosely compacted landfill (by means of a crawler tractor) 25-50% of the annual precipitation (~7.5 m3 ha-1d-1) The density of the landfill surface controls the infiltration rate. The figures are mean values where there is no dilution by surface or groundwater. The fluctuation of leachate production can be very high but cannot be quantified since it is, in general, very much dependent upon the climatic situation. LEACHATE QUALITY Compared with sewage, leachate is an organically and inorganically highly pollution water. High salt but relatively low heavy metal concentrations were detected. Some of the parameters are dependent upon the biological stage (acid or methane phase) in the landfill. Almost all parameters are influenced by the age of the landfill, although there was no indication of the influence of landfill height. TABLE 1 Mean (x) and maximum (max) concentrations of different leachate quality parameters representing the "acid" and "methane" phase (Ehrig, 1982) _"Acid Phase" "Methane Phase" X max X max pH* 6.1 5.5 8.0 8.5 BOD /COD* 0.68 - 0.06 - COD 22000 38100 3000 4340 BOD 13000 30425 180 383 Fe b 925 2120 15 29.3 Ca 1300 1130 80 534 Mg 600 1130 250 1.73 Mn 24 65.5 0.65 1.75 Zn 5.6 68.4 0.64 3.78 Sr 7.2 14.7 0.94 7.25 so„ - 1745 - 884 4 * without dimension; other parameter: [ppm] In order to get a feeling for the order of magnitude of the pollution potential of leachate, mean and maximum numbers are pre sented in Table 1. These values were measured during the investi gation of 15 sanitary landfill in West Germany and correspond to undiluted leachate. The BOD,.- COD-ratio as well as the pH are valuable indicators if a leachate is diluted or not (i.e. low BOD- concentration with a BOD-COD-ratio of 0.5 indicate that the analyzed leachate is diluted). Parameters that are not found to be dependent upon the stage of decomposition of the waste are presented in Table 2. A comprehen sive study of leachate quality was also carried out by Robinson & Maris, 1979). Pollution potential of leachate TABLE 2 Mean (x), maximum (max) and minimum (min) concentrations of different leachate quality parameter where no dependency could be found (Ehrig, 1982) X min max Spec. c:onduct .ivity * 13986 2100 27150 NH.-N 741 26.3 3075 4c NO -N 3.3 0.2 35.0 Org. N 593 8.3 4245 CI 2119 134 4953 K 1085 94 2420 Na 1343 70 3560 ges. P 5.7 0.5 30.2 Alkalinity** 6609 677 11575 As 0.126 0.007 1.56 Pb 0.087 0.008 1.02 Cd 0.0052 0.001 0.00629 Cr 0.275 0.029 1.57 Co 0.05 0.004 0.954 Cu 0.065 0.010 1.39 Ni 0.166 0.0183 2.05 Hg 0.0002 0.061 F - 0.6 Phenol 0.022 75.0 * [us cm-1]; ** [ppm CaCo ]; other parameter [ppm] The change of leachate pollution with time depends upon the different parameters. This could be seen already from the labora tory scale test (figure 2). Data from actual landfill are presented in figure 4 and give more realistic pollution trends with the age of landfill. In actual landfill it takes much more time before the methane production starts and as a consequence the BOD and COD concentrations drop. The decrease in metal concentrations occurs in parallel with a decrease in sulphate concentrations. It is obvious that the bio chemical reduction of sulphate results also in a production of metal sulphide, which precipitates to a high degree in the fill. During a period of 6-7 years no change in NH -N and Cl-concentrations was measured. 132 R. Stegman COO .BOD 5 Cmg/I] BOD S/COD pH [-] >ffl 4- N ,CI ,S0 4 [mg/H SiTTiï 2899 I, /, CI 1508 SO A 1888 4 **'' A'"' 598 LANDFILL AGE [YEARS] FIG. 4 Example of leachate quality data from a high compacted full scale landfill in West Germany relative to time (Ehrig, 1982). The influence of landfill operation techniques on the organic leachate concentrations becomes obvious from figure 5. The enhance ment of gas production can be achieved by compacting MSW in thin layers (~0.5 m height) over large areas with no daily cover. Lea chate recirculation results in higher moisture content of the MSW. This might also cause an early start of methane production. POLLUTION POTENTIAL OF GAS AND LEACHATE The amount of pollutants that leave a landfill can be calculated by multiplying the leachate concentrations with the leachate production rate. As an example the chloride emission load of a 10 ha high density landfill can be approximated: 10 ha x 5 m3/ha d x 1500 ppm CI = 75 kg CI d-1 This kind of calculation can be done for all parameters in order to get a feeling for emission rates. As already mentioned, a long term calculation is not possible now. Pollution potential of leachate 133 COD [mg/1] 40000 2 m—ioyttrs 35000 - 30000 fchîn loyars 25000 jlatlo 20000 - / ' i i N 15000 " i! 1 10000 "U i i '.A \ A 1 5000 s A \ - r' l \ I 0 V* 4 5 6 7 londfiIi age [years] S 6 7 andfilf age [years] FIG. 5 Examples of leachate concentrations from high compacted full scale landfill sites built up in 2m layers, thin layers and in 2 m layers with leachate recirculation (Stegmann, Ehrig, 1982). If landfill sites are not sealed at the bottom, these emissions will enter the soil and, in some cases, the groundwater. If land fill sites are sealed, leachate will be collected at the bottom of the fill and leave it by means of perforated pipes. The leachate has to be aerobically treated resulting in a reduction in organics, salts and metals. The aerobic biological treatment of leachate works if treatment plants are well designed and operated (Stegmann, Ehrig, 1981). After biological treatment there are still considerable amounts of emissions that enter the surface water. Table 3 shows an example of leachate before and after biological treatment. A further treat ment of the leachate is possible by means of physical-chemical methods such as activated carbon, filtering and precipitation. It 134 R. Stegman TABLE 3 Leachate quality data before and after bio chemical treatment (F = filtered) Influent Effluent Influent Effluent pH* 6.3 9.2 Na 1070 1615 CI 1140 1006 Ca 376 54.5 NH. 480 2.8 Fe 62 5.5 4 22.0 0.46 Mn 1.53 0.71 N0BS3B -F 14300 13.6 Ni 0.170 0.251 COD-F 22800 3660 Cu 0.043 0.068 org. N-•F 381 29.4 Zn 1.94 0.23 Alkalinity** 7620 4242 Cd 0.0117 0.0017 Cr 0.128 0.058 should be mentioned that these methods are very costly and when the precipitation process is used secondary pollutants from the coagu lants can be anticipated. If sanitary landfill are operated in pits that are not sealed, migration of landfill gas into the adjacent soil becomes possible. As a consequence the air in the soil is substituted by landfill emissions which kill off existing vegetation. In addition, a cer tain pollution of the water in the soil can take place by the disso lution organic compounds of the gas. CONSEQUENCES The emissions from sanitary landfill should be minimized before being returned to the natural environment. For this reason, sani tary landfill should be built on natural soils with low permeability or they should be artificially sealed. Leachate should be captured and treated. The degree of treatment has to be determined indivi dually for each specific situation. Leachate can be treated sepa rately or together with municipal sewage. Landfill operation technique should be changed in such a way that the biological processes will be enhanced. As a consequence the organic leachate emissions rate will be reduced. Landfill gas consists of about 50% CH. and 50% CO and can be used for energy production. There are several ways enhancement may be achieved (Stegmann, Ehrig, 1982). One method is the recirculation of leachate in order to increase moisture content in the landfill. After the landfill (or part of it) is closed up, leachate recirculation should be practiced in order to maximize evaporation on top of the landfill. The rate of recirculated leachate should be adapted to the maximum evaporation rate; leachate should be spray-irrigated on top of the landfill in order to get a further reduction of leachate volume by spray losses. Leachate production of closed-up landfill sections can also be reduced by supporting vegetation growth on top of the landfill. In addition the shape of the landfill should be designed in such a way that surface runoff is improved. This can be achieved, for example, Pollution potential of leachate 135 when relatively steep slopes are broken by paved throughs that collect the surface water and lead it out of the fill area. Erosion prevention methods should be used, such as growing grass on slopes. No valleys should exist on landfill sites where surface water can be collected. Also, if the landfills are not sealed, the aforemen tioned landfill operation technologies should be taken into account in order to minimize soil pollution. The specific climatic and regional situation has to be taken into account when sanitary landfill sites are planned and operated. The aforementioned relationships should be used as a background on which concepts are developed. Better landfill operation also needs better educated people on the lanfill sites. REFERENCES Ehrig, H.-J. (1980) Beitrag zum quantitativen und qualitativen Wasserhaushalt von Miilldeponien. Veroffentlichungen des Instituts fur Stadtbauwesen, T.U. Braunschweig, Heft 26. Ehrig, H.-J. (1982) Sickerwasser aus Hausmulldeponien Menge und Zusammensetzung. To be published in: Kumpf, Maas, Straub: Miill-und Abfallbeseitigung, Erich Schmidt Verlag. Rees, J. (1980) The fate of carbon compounds in the landfill dis posal of organic matter. J. Chem. Tech. Biotechnol., 30, 161-175. Robinson, H.D., Maris, (1979) Leachate from domestic waste: genera tion, composition and treatment. Technical Report TR 108, Water Research Centre, Stevenage. Schobert, S. (1978) Mikrobielle Methanisierung von Klarschlamm. Expertengesprach 20.6.78, Projekttrager Biotechnologie, Kernforschungsanlage, Jiilich GmbH. Stegmann, R. & Ehrig, H.-J. (1982) Enhancement of gas production in sanitary landfill sites - experiences in West Germany. Confe rence on Resource Recovery from Solid Waste, 10-12 May, Univer sity of Miami, Coral Gables, Florida, USA. es i/ï" tA. es m es w