Landfill Bio-Stabilization through Aerobic Aeration

by Hilary Haskell

Landfill aeration serves as an economically and technologically viable option for decreasing the duration of landfill after-care, through the bio-stabilization of landfill contents. Ritzkowski and Stegmann (2013) based their analysis on the determination of carbon balances through waste characterizations and online monitoring. The biodegradable organic carbon (BOC) content of a landfill is the main determinant affecting landfill bio-stabilization rates, while landfill settlements and temperatures are also important indicators of stabilization. Using BOC, the authors determined the total and remaining stabilization periods for aerated landfills. This study used a landfill in Northern Germany that required 6.2 years to stabilize after a volume of 63 million m3 of air had been applied to increase the rate of bio-stabilization by a factor of six. These findings suggest that landfill aeration may substantially decrease time and costs associated with landfill after-care by reducing cumulative leachate and landfill gas emissions.

According to Ritzkowski and Stegmann (2013), more stringent municipal solid waste landfill regulations in Germany have resulted in the closure of many landfills as they reach their final capacity. These landfills continue to emit both landfill gas emissions and leachate, due to anaerobic biodegradation and insufficient protective barrier linings or caps. The German Landfill Ordinance states that landfill emissions must be controlled before and after closure for both leachate and landfill gas. This pollution control is costly and must be utilized over long time periods. Landfill aeration serves to avoid or minimize current emissions as well as accelerate reduction of the remaining emission potential. Aeration reduces landfill gas formation potential, improves leachate quality, accelerates landfill settlement, and bio-stabilizes the landfill in order to complete landfill aftercare more rapidly.

Landfill emission intensity and duration are linked to the amount of organic carbon in deposited landfill wastes. Although top covers can reduce emission potential, many landfills do not have pollution control technology. Aeration requires controlled reduction of biodegradable organic carbon (BOC) to a level of 90% remaining organic carbon. At 90% BOC, emissions are limited, and there is little residual emission potential. It is unreasonable to assume 100% reduction of BOC could be achieved over a reasonable time frame; therefore the authors use 90% as a target level.

With sufficient moisture and no inhibition, conversion of biodegradable organic carbon through aeration depends on the amount of supplied oxygen. Therefore, monitoring air supply and carbon discharge (the reduction in BOC) is essential. An accurate BOC assessment requires a set-up of a complete carbon balance, which determines how much biodegradable organic carbon will be emitted as leachate and how much will be emitted as landfill off-gas. In addition, supplied and bio-converted oxygen must be considered to conclude when bio-stabilization will occur for a particular landfill. The steps required in this process include determining BOC levels through lysimeter tests, assessing the oxygen required to bio-stabilize the BOC, and estimating the aeration period based on supplied air volume.

Landfill simulation reactors (LSR) may be used to determine how much organic carbon will be biodegradable under aerated conditions. The authors recommended using LSRs under parallel aerated conditions in the same mesophilic temperature range, and stress the importance of monitoring carbon discharge rates for leachate and gas frequently. When the carbon load from landfill off-gas equals approximately 2.5% (2.6 g carbon per cubic meter) of the maximum carbon load from theoretical complete oxidation of C-6-carbohydrates, then the supplied oxygen has been completely bio-converted and the aeration period is complete. This assumes that 97.5% of the maximal stoichiometric carbon discharge has been reached. The authors set up a carbon balance using infrared spectroscopy after thermal oxidation, then calculated the difference between total organic carbon content and leachate carbon content. This calculation is equivalent to the difference between total carbon and total inorganic carbon. The carbon dioxide and methane concentrations in the gas phase samples were analyzed using gas chromatography.

For this study, the authors studied a landfill aeration project in Germany, where they filled six LSRs with waste from an old landfill that was about to be aerated. After a short anaerobic period, the LSR waste was aerated, and carbon discharge from leachate and gas were monitored for 19 months until more than 97.5% of the stoichiometric carbon had been discharged. From this experiment, the authors found that the calculated reduction in total solid organic carbon corresponds to the measured carbon discharge from aeration. Most carbon is discharged as carbon dioxide (84%), while the remaining emissions are methane and leachate. The carbon load from gas and leachate is about 25.3% of the initial total solid organic carbon content, which is the biodegradable fraction of the waste from the landfill requiring aeration. In a full scale landfill operation, the target value for full scale aeration is 17.5 g of carbon per kg of dry matter, which amounts to 22.8% of the total solid organic carbon content of the landfill waste prior to beginning aeration.

Most landfill after-care duration predictions are based on anaerobic landfill conditions, with slow microbial metabolism and water infiltration. Therefore, these duration estimates are based on liquid to solid waste mass ratios (L/S-ratio). Aerated conditions rely instead on aerobic environments, which result in more rapid microbial metabolism, and render the L/S-ratio inaccurate. The authors remedied this discrepancy by using a reference value that describes the proportion of bio-converted oxygen to a specific solid waste mass (O2, con./S-ratio). Then, carbon discharge was based on this O2, con./S-ratio, the aeration rate, and the oxygen conversion rate. Some landfill wastes have preferential flow paths (macro-porous spaces) and differences in moisture distribution that make the aeration flow rate an inadequate reference parameter. In these cases, the actual conversion rate of supplied oxygen is used as a reference parameter.

For the case study landfill, an average oxygen conversion quota of about 30%–14% was found during aeration, but declined from 60% to 20% during the six year monitoring period. Oxygen conversion quota reduction is due to preferential flow path formation, reduced moisture content, and a gradual decrease in the biodegradability of the residual organic substances. Based on the LSR tests, the average oxygen conversion quota with aeration was 80%, which would indicate a notable reduction in aeration time periods. There is a high correlation between the carbon discharge from aerated LSRs and the calculated stoichiometric carbon discharge. But, the measured data for the aerated landfill falls slightly below the stoichiometric values due to the additional oxygen consumption from the oxidation of reduced leachate compounds. The target value for the completion of aeration is 32.6 (m3O2/MgDM), based on the O2, con./S-ratio.

The required aeration period is determined by the O2, con./S-ratio in relation to the supplied air volume. This represents a non-linear estimation despite constant supplied air volume. The case study landfill required an estimated 5.4–8.2 years to reach the required O2, con./S-ratio, using an air supply of 55–81 million m3. To bio-stabilize the landfill, the total aeration period required 6.2 years and a supplied air volume of 63 million m3.

Stabilization criteria for aerated landfills define an endpoint for aeration periods, through online criteria and laboratory analysis of waste samples. Online criteria are crucial, cost-effective determinants of the bio-stabilization process. The BOC level depends on the quantity and quality of emissions, and is determined by LSR solid waste samples taken prior to the start of aeration. Labs provide optimized conditions in comparison to full scale landfills, thus the BOC conversion rate was reduced by 10% from 100%, BOC90, indicating a realistic biodegradable organic carbon reduction rate for a given landfill.

Landfill off-gas volume and composition are monitored during aeration so that carbon discharge can be calculated. After reaching the BOC90 level, the residual landfill gas and leachate emissions are assumed to be emitted at a negligible level. Bio-stabilization progress can be traced through landfill off-gas analysis. If the amount of BOC found in a given landfill is known, it serves as a target value for complete bio-stabilization of a landfill. Thus, measuring landfill off-gas is an indicator of stabilization progress.

At the case study landfill site, approximately 2,272 tons of BOC exists based on solid waste samples in LSRs. Therefore, the BOC90 value of 2,045 tons could be reached after aeration of about 6 years. Carbon discharged under anaerobic landfill conditions amounts to only about 32–53% of the BOC90 value, meaning that without aeration, bio-stabilization would not be achieved for 15–17 years. The shorter duration under aerated conditions speeds up bio–stabilization by a factor of 6. The authors indicate that the 15–37 year projection seems overly optimistic, and suggest that actual durations under anaerobic conditions may require more time. The highest annual discharge rates occurred in the first three years of aeration while they decreased continuously after this period. The standardized carbon discharge rate (carbon load per volume of extracted off-gas) is a better indicator for performance of an aeration system. The decrease in carbon loads is due to reductions in methane generation during aeration and the increase in ambient air supplied during the bio-stabilization process. This finding ascertains that after the BOC has been reduced by 90%, a residual amount of biodegradable organic carbon remains. This remaining carbon is not degradable under anaerobic conditions, and thus will not result in landfill gas production after aeration is complete.

The authors suggest conducting landfill gas extraction tests near the end of aeration for comparison with initial landfill gas production rates in order to verify progress in bio-stabilization. Landfill aeration criteria may be designated as a methane generation rate of <0.51 CH4/m2h per landfill gas production rate of 1.01/m2h. These low values ensure that a complete biological methane oxidation will occur in a qualified top cover, regardless of climatic influences.

Aerated landfill bio-stabilization progress can be followed universally while BOC90 levels must be determined on a landfill-specific basis. Landfill aeration involves anaerobic pre-stabilization of BOC. Therefore, gas formation potential is considered to be 101/kg/DM, and a 90% reduction in BOC is the standard for gas formation. Respiration activity under aerobic conditions should also be considered as a completion criteria. This parameter estimates the amount of residual organic material that could potentially decomposed under aerated conditions. At the completion of landfill aeration, 2.5 mgO2/g DM should occur, and will remain once BOC90 levels have been reached.

Landfill settlements and temperatures are also important criteria for determining the conclusion of landfill aeration, although they do not have target values. Most settlements occur during the first 18 months of aeration, and annual settlement is about 5% of landfill height. Landfill temperatures increase during aeration due to heat production from microbial metabolism. Hotter temperatures indicate more intense biological carbon conversion. In the case study landfill, the average landfill temperature increased over the first years of aeration, and then decreased considerably.

Landfill aeration serves as an increasingly common economically and technologically feasible after-care method. The main parameter in assessing biological landfill stabilization through aeration is the biodegradable organic carbon content, which is important in determining anaerobic biodegradation progress as well as necessary aeration duration periods. Solid waste samples and LSR’s are necessary in tracking aeration progress. The O2, con./S-ratio is used to determine total and remaining aeration. In addition, settlements and temperatures within the landfill are also indicators of bio-stabilization.

Ritzkowski, M., Steggman, R., 2013. Landfill aeration within the scope of post-closure care and its completion. Waste Management 33, 2074–2082. http://bit.ly/1ywsqEf

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