Modelling the Risk Assessment of Groundwater Pollution by Leachates

Modelling the risk assessment of groundwater

pollution by leachates and landfill gases

T. C. ~olio~oulos',V. P. ~ollias~& P. S. lCol1ias3 I Centre for Environmental Management Research, Dept. of Civil Engineering, University of Strathclyde, Glasgow, UK. 2 Dept. of History and Philosophy of Sciences, University ofAthens, Greece. 3 Sanitary Civil Engineer, Naxou 21, 112 56Athens, Greece.

Abstract

This paper presents the variation of specific chemical parameters, which represent waste biodegradation. A quantitative risk assessment is presented of probable groundwater and aquifer contamination by landfill emissions. Modelling of biogas production and leachate emissions in landfills is presented.

The mathematical modelling is made in order to assess waste biodegradation processes and their environmental impact based on case studies' field data. Particular characteristics and risks of groundwater pollution load by landfill emissions, like COD, TOC, LFG production, methane, carbon dioxide are

evaluated malung usell conclusions. The examining risk analysis modelling presents the entire life cycle of landfill emissions' characteristics. In the end, is presented an application of earthquake magnitude xisk assessment for a landfill design.

1 Introduction

Solid Waste Management (SWM) covers a complicate chain (collection, transport, recycling, treatment, recovery, and waste disposal technology). The

waste pre-treatment and effective landfill design can be characterized as the most important final stage SWM biodegradation factors, for efficient waste

1 60 Water Pollution \//I: Modclling, Mca~uringand Prediction

management systems. Landfill design compromises the most important public health factor, as landfill remains the final disposal method (for the non-recyclable materials) after composting and incineration one. This is verified due to landfill's environmental pollution impacts (groundwater pollution; air pollution - global warming; LFG migration - explosions, other human impacts) [6,10,12,15,17]. As well as sanitary landfill remains an attractive disposal route fox municipal solid waste, because it is more economical than other alternative solutions, several attempts have been made to enhance and accelerate landfill stabilization. The landfill gas and leachate generation is an inevitable result of the solid waste biodegradation in landfills and their study is necessary for future efficient designs, controlling air and groundwater pollution. Despite several analyses in landfill gas, leachate emissions and waste biodegradation, there is a lack of comprehensive investigations on the long-term conversion processes of organic and inorganic waste components in landfills [g,1 0,l l]. This paper presents, a numerical modelling of risk assessment of groundwater pollution by landfill emissions, as a result of change in waste composition and management. Mid Auchencaroch's landfill experimental variables are evaluated and are presented in relation to associated groundwater contamination. As increasing solid waste amounts are disposed on territorial surfaces, several questions are raised around environmental management and public health. The main landfill emissions are the landfill gas and the leachates. Leachate generation and transport results when precipitated water infiltrates through the disposed waste in landfills. Biogas is produced by anaerobic fermentation of solid waste in landfills or liquid waste in wastewater treatment [3,6,10,12,15,17]. Therefore, there are two stages for the confrontation of the existing problems. The first stage corresponds to the waste management processes, which must be realised before the waste disposal. The second stage includes the waste treatment and management methods, which must be taken into account, during the final waste disposal into the landfill.

2 Landfill emissions

This paper presents the modelling of Mid Auchencarroch's experimental landfill waste biodegradation based on characteristic leachate emissions. Also are presented, landfill emissions of Tagarades (TGRDS) landfill. Controlled leachate recirculation could be useful for quick site biodegradation and stabilization. An important factor in landfills is the moisture, as it presents a great influence on waste biodegradation. Mid Auchencarroch (MACH) experimental landfill, in Scotland, has been capped since 1995. The experimental variables are waste pretreatment, leachate recirculation and CO-disposalwith inert material. The project consists of four cells each of nominal plan dimensions 28m X 30m and 5m deep, giving a nominal volume of 4200 m3. In cells 1 and 3 there is pretreatment by wet pulverisation and in cells 2 and 4 the disposed waste is untreated. In cells and 1,2

3 there is recirculation of leachate and in cell 1 there is addition of inert material around 20% by volume [9,lO, 11,181. The landfill site of Tagarades has been available since 198 1 and it has been used to serve the total of greater Thessaloniki since 1985. Thessaloniki is the second biggest city of Greece with over 1,000,000 inhabitants. It covers a surface of around 300,000 m'. The quantity of Municipal Solid Waste of major Thessaloniki is about 400,000 tonnes per year [13]. A landfill gas recovery installation operates on the site. The Association of Local Authorities of Thessaloniki manages the site.

Table 1: Characteristic leachate parameters and their concentrations in time. 5-10 10-20 yr v Parameter

10,000-- 10,000- 1,000- (mg/l) 60,000 20,000 5,000 Ammonia 100-1,500 300-500 50-200

I Chloride 1 500-3,000 1 500-2,000 1 100-500

(mgll) Sulphate 50-2,000 200-1,000 50-200 (wll)

In table 1, are presented the estimated concentrations' variations of characteristic leachate parameters during the landfill life cycle in time [2,4]. Higher risks exist in short-terms than in long-term of waste biodegradation. Moreover, landfill gas (LFG) emissions and their probable migration

from landfill boundaries are another threat of groundwater pollution. However, elaborated studies have been made for landfill gas migrating in the surrounding unsaturated zone, which is exposed to infiltrating water and the water contamination, LFG carbon dioxide is highly water-soluble with a solubility 2320 rngll at 10 'C and only 30 mgll for methane [7]. Future investigations and technologies have to be made on water contamination by LFG migration taking into account the geological conditions and groundwater levels next to landfill boundaries.

3 Actual release of emissioins

In Table 2 are presented leachate emissions according to the available data for examining case studies. For each parameter are presented its average, maximum

and minimum value (Aver.;Max;Min). The only data, which were continuous in

1 62 Water Pollution \//I: Modclling, Mca~uringand Prediction

Table 2: Leachate emissions of seven case studies

Site MACH cell 1,2,3,4

Parameters

I I COD cell 7O,857;llS,OOO; place 1 1 1 1 44,000 11 cell 1 3,531; 4,920; 1 place 2 2,460 2

cell 2,932; 11,896; place 3 685 3

cell 1 29,720; 80,795; 1 :lace 3. 9.500 cell 1 768; 1,500; 1 place

2 1 375 12 cell ( 609; 2,000; ( place

place 4.88

7.1; 7.3; place 2 6.83 2 cell 7.7; 7.9; 7.3 place

cell 3,102; 10,250; place

1 / 1.400 I l cell 1 835; 1,375; 1 place 2 1312 12 cell 327.4; 720; place 1 1 3 39 3 cell 4 Chloride cell 3,255;10,100; place 1 580 1 cell 3,978; 7,200; place 2 2,480 2

cell 2,872; 5,960; place 3 3 3

532.2; 665; 445 cell 4

Sulphate 79.4; 496.5; 41.5 cell 1,607; 2,500; place 1 400 1 51.6; 164; 27 cell 212; 425; place (mg/l) 2 96 2 54.8;163.5; 38 cell 212; 500; place 3 55 3 51.2; 292; 19.7 cell

time, were the Mid Auchencarroch ones and they selected for further modelling analysis of waste biodegradation. In Table 2, the presented TGRDS leachate data, have been collected periodically from 1990 and once every month from June 1993-May 1994. At TGRDS site, place 1 is referenced to fresh samples (only few days) collected directly from deposition area; place 2 is referenced to middle aged samples (6-7 years) from a 6 m depth sampling well and place 3 to a leachate pond from old

leachate samples (approx.12 years) [16]. The MACH landfill gas and leachate data cover simultaneously the 22-month period from November 1995-August 1997 19,101. Below in figures 1-4, are presented the COD, TOC emissions of MACH'S four case studies (four cells).

B odegadation Time [mci-tw

Figure 1,2: Mid Auchencarroch COD,TOC concentrations vs time for Cell 1 and 2.

1 64 Water Pollution \//I: Modclling, Mca~uringand Prediction

Cell 3

Figure 3,4: Mid Auchencarroch COD,TOC concentrations vs time for Cell 3

and 4.

However, a least-square modelling has been developed and is presented below so as to evaluate Mid Auchencarroch's landfill behavior and its environmental impact assessment. There were selected indicative parameters for the waste biodegradation evaluation and the environmental impact assessment of the examining case study. These parameters include the variation of: Chemical

Oxygen Demand (COD) vs Total Organic Carbon (TOC).

4 Modelling landfill emissions - R.A.

The development of mathematical modelling of leachate emissions' characteristics in landfills needs a wide collection of field data. In this paper some indicative parameters are presented in order to evaluate Mid Auchencarroch's landfill biodegradation and to set up a base for future comparisons of landfill emissions and effective Environmental Impact

Assessment (E.I.A.), Risk Assessment (R.A.). The presented mathematical modelling of leachate emissions is based on the least-square theory [8,14] taking into account the available site measurements. This paper presents Mid-Auchencarroch landfill characteristic leachate emissions. Leachates are generated in landfills through complex physical and physicochemical processes and subsequently are transported to underlying aquifers when a liner does not exist. Leachates can cause dangerous environmental impact on the groundwater pollution due to their high pollution load during the waste biodegradation (high COD and BOD values). Leachate generation and transport results when precipitated water infiltrates through the disposed waste in landfills. Leachates are generated in landfills through complex physical and physicochemical processes. Based on the available measurements of

Mid Auchencaroch landfill are presented the following least-square formulas of MACH'S four case studies (four cells) [9]:

Cell 1:

Cell 2:

Cell 3:

Cell 4:

where CODl,COD2,COD3,COD4 are the respective COD values per cell divided by 103in mgll; TOCl,TOC2,TOC3,TOC4 are the respective TOC values per cell divided by 102 in rngll; SD = Standard Deviation; CV = Coefficient

Variability.

Evaluating the above results it is clear that there was higher depletion of carbon and COD pollutants at cell 1 than the rest. Moreover, cell 4 presents higher max COD concentrations due to the fact that there has been disposed higher waste fraction of biodegradable carbon content in it than at cell 3 and 2. The above least square models are applicable to Mid Auchencarroch landfill conditions. For other landfills should be taken representative monitoring samples so as to evaluate the biodegradation rate and its pollution load in time. Moreover, the Andreottola and Cossu model could be used for the estimation of landfill gas yield production [l,1 l]. The SIMGASRISK (Simulation of Gas Risk) model could be used for the risk assessment quantification, of probable landfill gas migration in lateral distance of landfill boundaries [ll]. For a common landfill site the produced landfill gas composition, approximately, consists of 60% vol. Methane and 40% vol. Carbon dioxide. Stoichiometric calculations could be used for initial estimation of LFG production. Mathematical formulas based on field measurements of LFG production provide high accuracy and robust results. The equations, which describe the landfill gas production in time, are the following [3,1 l]: kl(t1-t) Gt = Gt mm'- (5)

where t time (year) time period peak LFG production (year) t I G,,, peak LFG production at time tl (It) G, LFG production at time t (It) k = - ln(0.5)/to.s(year-1) kl = (lnGtmax-ln0.01)ltl (year1) to.S half time, approaches the time when half of the ultimate gas production is reached (year)

1 66 Water Pollution \//I: Modclling, Mca~uringand Prediction

Generally, the values of k1 and k parameters vary for different waste treatments - landfill conditions, nature of the bacteria in the waste mass and their biodegradation stages (hydrolysis, acidogenesis, acetogenesis, methanogenesis).

Equation (l) describes the increasing gas production in time and equation (2) the decreasing one. According to the above it is clear that higher pollution loads of particular biochemical and biodegradation indexes exist in the short time period just after the waste disposal. Therefore, there is a hgh risk of environmental pollution during that period and any Environmental Impact Assessment (EIA), Risk Assessment (RA), toxicological assessment and remedial treatment of contaminated land have to be focused on that time period. Risk factors should be taken into account during the stage of the RA, llke permeability of the surrounded geological strata, precipitation, pollutant attenuation and magnitude of earthquakes [5,6,10,1 l]. Below is analyzed the associated risk assessment of landfill design in seismic loading. A landfill area, where exists a mean frequency of 3.2 Richter earthquake magnitude per year, is examined. In the examining case, is supposed that the probability of earthquake magnitude M greater than m one, follows an exponential distribution and it is described by equation (7), r =2.35.

Also is taken that the probability of high earthquakes magnitudes in time, follows the Poisson distribution. The examining landfill case study has been designed for

60 years operation and its collapse probability by earthquake should not be higher than 10%. According to the above is demanded to be calculated the earthquake magnitude of the relative landfill design. Therefore, the probability not to collapse the landfill design in 60 years, for a high earthquake magnitude is 0.9. According to the Poisson distribution, with average period T of earthquake design, it yields the following formulas.

1 T = - z 570years a

The probability of low earthquake rnagnitudes M>ml, is given by the following formula

The probability of high earthquake magnitudes M>m2, is given by the following formula

Also it has been taken that the earthquake magnitudes in time follow exponential

distribution. Hence:

p(M > ml) - e-"'lr - l-(l-l/Tl)t -- (13) p(M > m2) e-rn21r I - e-tm

According to the last equation for ml=3.2, T1=l year, T2369.5 years, r ~2.35 and t =60 years it yields m2=8.5. Therefore the earthquake magnitude, which will be taken in the examining landfill design, is 8.6 Richter.

Effective monitoring, measures and controls have to be set up for short-term period since waste disposal and later so as to avoid any environmental impact in groundwater pollution, contamination. Lower concentrations of COD, Nitrogen and BOD there are in long-term behavior. The latter indexes can be used so as to

evaluate that the relative landfill stabilization has been achieved in time. The validity of the presented least-square models could be enhanced greatly by comparison with data obtained from further field investigations and experiments. The application of the above-presented results could be used not only for the

control and management of the presented case studies, but also for efficient EIA's, RA's of other similar cases and efficient contamination control.

5 Conclusions

According to the MACH emissions and the calculated equations is clear that the

MSW CO-disposalwith inert material is sustainable as well as the pretreatment by wet pulverisation since the leachate recirculation expedites the biodegradation. The high CH4 concentrations and the reduced CO2 emissions in short-term period, show that the methanogenesis and quick site stabilization was achieved.

At MACH, neutral pH buffering action stabilized from early time for high ammoniacal nitrogen concentrations except cell 4, which presented light acid environment at initial period but it was stabilized later to neutral one. According to the TOC-COD equations, in cell 3 there was the best

waste biodegradation and in cell 1 there was good organic depletion, minimizing both their emissions in short time. At TGRDS site, leachate characteristics and methanogenesis were influenced mainly by the high disposed putrescible waste fraction. Several acts could be realised between the local authorities and public

1 68 Water Pollution \//I: Modclling, Mca~uringand Prediction

for the confrontation of environmental pollution. Educational and information programs should take place so as to be well informed the public awareness about the risks by landfill emissions. Future risk assessments of contaminated sites have to evaluate the particular risk factors, taking into account any available monitoring samples in time. In ~s way can be studied better the behaviour of a hazardous site, so as to control effectively its pollution.

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