Lindsay Strachan, at Bisasar Road landfill, Durban Solid Waste, Springfield, for providing data and information on Metro landfills. The emission rates of benzene were found to be higher than those of vinyl chloride in the active landfills of Bisasar Road and Shongweni.
INTRODUCTION
BACKGROUND
An estimate of LFG emissions can be determined using two common approaches (Cenuschi and Giugliano, 1996). Some studies have already been done in South Africa to understand aspects of LFG emissions.
AIM OF THE STUDY
To determine LFG emissions from 3 selected landfills in the DMA using the LANDGEM model. To compare the results of the LANDGEM model with the methods currently used by Durban Solid Waste (DSW) to estimate LFG emissions.
BACKGROUNDS AND LOCATION OF STUDY AREAS
In terms of the minimum requirements for waste disposal via landfills (see section 2.3.2), Bisasar Road is a Class G:L:B+ landfill, meaning that the site is a large landfill that accepts general waste and has the potential to generate leachate. . The duration of the dumping operation can be extended to 15 years if a transfer station is installed.
WASTE BIODEGRADATION AND LFG GENERATION
WASTE DEFINITION AND CLASSIFICATION
This waste does not pose a significant threat to public health or the environment if properly managed (DWAF, 1998a). Other wastes such as commercial wastes, garden wastes and certain industrial wastes with similar characteristics to household wastes can also have a negative impact on the environment (Lombard, 1992). iii) Special waste (hazardous waste).
WASTE DISPOSAL BY LANDFILL
In the UK, the term 'controlled disposal' is often used instead of sanitary landfill (Cope et al, 1983). In Scotland they are called 'coups', while elsewhere in the world the term 'dumps' is often used (Grawford and Smith, 1985).
LANDFILL CLASSIFICATION
A hazardous substance is not a hazardous waste until it is no longer, or has been, usable. A toxic chemical (for example, benzene) is not a hazardous waste until it becomes part of the waste stream.
BIODEGRADATION OF REFUSE
This phase of the overall process can only be maintained if hydrogen-utilizing organisms such as the sulfate-reducing bacteria and methane-forming bacteria are active. In the final stage of the process, as the degradable components become depleted, progressive re-establishment of aerobic conditions may occur (DoE, 1995).
COMMON POLLUTANTS EMITTED FROM LANDFILL SITES
In particular, the benzene content of landfill gas should be carefully monitored due to its proven carcinogenic effect (Rettenberger and Stegmenn, 1996). However, in terms of environmental impact due to their persistence (i.e. chemical stability), these substances are the most important trace components found in landfill gas (Rettenberger and Stegmenn, 1996).
FACTORS INFLUENCING THE GAS PRODUCTION
Some of the rapidly and slowly degradable components of organic constituents in solid waste are listed in Table 2.3. The absence of free oxygen is essential for the anaerobic bacteria to grow and carry out the conversion of the fixed carbon into methane and carbon dioxide (Christensenet al., 1996).
LANDFILL GAS CHARACTERISTICS AND HAZARDS
- Toxicity
- Corrosive properties
Most VCCs are toxic, characterized by lipophilic properties and relatively high vapor volatility (Deisper et al., 1996). The C02 and volatile fatty acid (VFA) components of LFG are aggressive to concrete, brick mortar, and mild steel (Lombard et al., 1998).
GAS MIGRATION
- Gas barriers
- Free/Passive venting
- Encapsulation
- Gas extraction system and flaring
It must be kept and ventilated within the landfill premises, except in special circumstances when it. Some of the main elements that make up gas management systems are described below. The part or side of the channel near the boundary of the zone should be closed with a barrier of low permeability.
The gas extraction system is usually a series of wells extending at or near the bottom of the landfill (Ham, 1988).
LANDFILL GAS UTILISATION
- Utilisation options or approaches
Direct use of the LFG is often the simplest and most cost-effective approach. Lower concentrations are the result of the over-pumping of the landfill, the entrainment of outside air into the landfill and the dilution of the LFG (Stegmann, 1996). Natural gas pipelines typically transport high quality gas containing more than 95% methane and the LFG must be processed to remove carbon dioxide and other impurities (Wetherill et al., 1999).
Waste gas processing increases production costs and is usually not economically viable unless natural gas costs are high.
CLASSIFICATION OF MODELS
There are numerous factors that affect either the amount of landfill gas that is ultimately formed, or the speed at which it is produced. Waste composition, which determines the amount of degradable organic carbon in the waste, which is the raw material for landfill gas;. Waste treatment, which involves mechanical pretreatment, homogenization, reduction of particle size and baling, the degree of compaction, the disposal method, the addition of water which all have significant effects on the formation of landfill gas;.
As explained in section 2.5, LFG is formed as a result of the breakdown of organic carbon in the waste.
MODELLING LFG GENERATION
- LFG generation rates
In this model, it is assumed that the generation of landfill gas in a certain amount of waste is constant over time. The zero-order model shows that the rate of methane formation is not affected by the amount of remaining substrate or the amount of biogas already produced (Cossu et al., 1996). Most LFG models follow first-order kinetics (Cossu et al., 1996), meaning that the limiting factor is the amount of substrate remaining or the amount of biogas produced.
This is followed by using equation (3.5) to calculate the amount of gas that can be.
DATA AND METHODOLOGY
LANDFILL GAS EMISSION MODEL (LANDGEM)
- Input data
- Output from model
The design capacity of the landfill, which indicates the total amount of waste that can be disposed of in the landfill;. To estimate landfill gas emissions, the model can be used using site-specific data, or in the absence of site-specific data, two different sets of defaults can be used. The other set of default values, the AP-42 default values, are provided with the model to estimate emissions in the absence of site-specific data (Thomeloeet aI., 1999).
L = potential for methane production, cubic meter per Mg of waste Mi = mass of solid waste in the ith section, Mg.
SENSITIVITY ANALYSIS
Furthermore, gases such as carbon monoxide, which indicate danger (i.e. fire indicator) at the landfill, will also be included.
CHAPTERS
MODELLED RESULTS OF LANDGEM MODEL
LANDGEM MODEL RESULTS
The magnitude of emissions from the Bisasar Road and Buffels Draai landfills are fairly comparable (Table 5.1), which may be due to the fact that they are both (sanitary) landfills without co-disposal. The LANDGEM model shows that the LFG increases as waste deposition increases over time, reaching a peak during the year of closure (Figures 5.1 and 5.2). In the post-closure phase, gas emissions decrease over a period of 200 years according to the LANDGEM model, and become negligible 100 to 150 years after the closure.
Figures 5.1 and 5.2 show that CH4 and CO2 emissions from the Bisasar Road landfill after the year 2160 are negligible.
Projected Methane Emissions
It follows that large amounts of CH4 are oxidized microbiologically to C02 in the landfill cover soil before it reaches the atmospheric environment (Bogneret al., 1997). Methanotrophs are a class of methylotrophs which are able to obtain energy from the oxidation of reduced carbon compounds containing one or more C atoms (Bogner et al., 1997). An increase in moisture content at the top of the landfill would bring methanotrophs to the surface and may be an alternative means of achieving maximum oxidation rates (Visvanathanet al., 1999).
It was recently reported that the Illinois landfill in the USA served as a CH4 sink due to very high levels of CH4 oxidation in the cap soil and a CH4 recycling system that lowers the overall pressure of CH4 in the landfill ( Bogner et al., 1997). .
Projected Hydrogen Sulfide Emissions
Projected Ethyl Mercaptan (VaC) Emissions
However, taking into account the results of the LANDGEM model for other odor emissions, it is expected that these odor components will be at their maximum at the time of landfill closure and that emissions will continue for a long period thereafter. At the joint landfill in Shongweni, a maximum emission of approximately 2.8 x 10-1 Mg/yr was observed (Table 5.3 and Appendix 1). Besides benzene and methyl mercaptan, vinyl chloride (CH2CHCl) is the most hazardous compound emitted from landfills and is a known carcinogen found in significant quantities (Young and Parker, 1983; Rettenberger and Stegmann, 1996).
Vinyl chloride and benzene are generally considered to be the most critical compounds found in LFG (Eikrnann, 1996).
Projected Benzene (HAPNOC) Emissions
MODEL SENSITIVITY ANALYSIS
- NMOC concentration in a landfill
Simulations of CH4, C02 and NMOC emissions were used to evaluate the sensitivity of the model. Using the CAA standards produces higher emission estimates, almost twice as much as those predicted by the AP-42 standard values. It is possible that the CAA standards are more suitable for Durban in light of its climate and that the use of the AP-42 standards underestimates emissions.
From Table 5.7 it is observed that the emissions based on the CAA dryland parameters are similar to those of the default AP-42 parameters, further indicating that using the AP-42 settings may underestimate emissions.
SUMMARY
It was considered very important to look at the limitations of the model as the results of the three case studies were comprehensively defined in the previous chapter. The chapter concludes by comparing the LANDGEM results against the results of the Hofstetter model used by DSW.
CRITIQUE OF THE LANDGEM MODEL
The model does not estimate LFG emissions from typical hazardous landfills such as H:H class in the South African context. To quantify the potential LFG emissions from the landfill, EPA developed the LANDGEM model as a method to estimate LFG emissions. The model helps in the design of LFG recovery systems, as the idea of the trend and amount of potential biogas production can be easily predicted.
This automated tool can be used in the decision making of landfill construction and utilization.
HOFSTETTER GAS YIELD MODEL RESULTS FOR BISASAR ROAD LANDFILL
From Figure 6.1 it can be seen that the gas falls for a period of 50 years after the peak. Ultimately, methane production decreases and the landfill becomes an inert soil-like mass (Micales and Skog, 1997). Studies have shown that the landfill can generate LFG for more than 30 years (see Section 2.5.1) and after its completion.
These two figures indicate that approx. 80% of the original gas generated can be extracted from the landfill (see Appendix 4).
COMPARISON OF LANDGEM AND HOFSTETTER MODEL RESULTS
Based on the current annual waste collection rate at the Bisasar Road Landfill (Table 4.1), the highest emission rate is estimated to be 3.285 x 107 m3i 1. However, the LANDGEM model does not estimate gas production per ton of waste, but rather as a mass emission rate per year (i.e. Mg/ year and m3i 1). Other studies such as Willumsen (1996) have shown that typical CH4 production rates are about 2.5 m3 CH4 per ton of waste per year. According to Gregory (2000), landfill gas production rates typically range between 5-10 m3 per tonne per year during the first decade of a landfill's active life and decline thereafter.
A cumulative yield of 100 m3 per ton per year of MSW can be anticipated over the lifetime of the site (Gregory, 2000).
COMPARISON BETWEEN LFG EMISSIONS FROM HAZARDOUS AND SANITARY (MSW) LANDFILLS
Modeling Landfill Gas Production, Landfilling Waste: Biogas (eds Christensen, T.H., Cossu, R, Stegmann, R), E & FN Spon, London. Effects of Landfill Gas on Groundwater, In Waste Disposal: Biogas (ed. Christensen, T.H., Cossu, R., Stegmann, R.), E& FN Spon, London. Landfill Gas Components, In Depositing of Waste: Biogas (ed. Christensen, T.H., Cossu, R, Stegmann, R), E & FN Spon, London.
Influence of Landfill Gas on Global Climate, In Landfilling of Waste: Biogas (eds. Christensen, T.R., Cossu, R, Stegmann, R), E & FN Spon, Londër.
LANDGEM RESULTS SHOWING AIR POLLUTANT EMISSIONS AS A FUNCITON OF TIME FOR THE SHONGWENI AND
BUFFELSDRAAI LANDFILLS
Projected Carbon Dioxide Emissions
Projected Carbon Monoxide Emissions
Projected Vinyl Chloride (HAPNOC) Emissions
CHCl emissions from Shongweni landfill
Projected Dichlorodifluoromethane (VOC) Emissions
Projected Methyl Mercaptan (VOC) Emissions
SH emissions at Buffelsdraai landfill
Projected Dichlorodifluoromethane (VQC) Emissions
BISASAR ROAD LANDFILL Model Parameters
SHONGWENI LANDFILL Model Parameters
MODEL SENSITIVITY ANALYSIS (TEST STUDY)
HOFSTETTER GAS YIELD MODEL
