You are a landfill operator and your intention is to do your very best to be a good neighbor. However, from time to time odors originating from your activities are perceived beyond your fenceline. This situation appears unavoidable, however it can be minimized through the application of a systematic odor management approach. Odor monitoring and odor assessment will provide you with better understanding of the problems, provide the data required to optimize your investiment in odor reduction efforts and result in minimized odor episodes.
One of the typical question first faced by an operator is if odors or odorants are to be quantified first to better understand the extent of the odor impact ? Odors in terms of the odor concentration or Odorants in terms of the chemical compounds perceived by the human nose? Another question is : how can the overall cost of the quantification be reduced while still providing the required information?
Since 2000, we have done numerous emission assessments (odors, odorants, methane and toxics) on municipal solid waste landfills (MSWL). We undertook a large scale study to examine the impacts on the air quality of several MSWL in the province of Quebec in terms of odors and potentially toxics compounds related to biogas emissions.
The project was performed on six MSWL. Assessments of odors and potentially toxic emissions followed by dispersion modeling at each landfill site were performed to estimate the human exposure to potentially toxic compounds and odors. The emissions of selected pollutants, such as volatil organic compounds (VOC), terpenes and sulfur compounds, were estimated by using the landfill gas emission LandGEM 2.01 software and based on field measurements.
The present posting presents some interesting findings specific to odors that we would like to share. But first let’s explain little more about this project:
Experimental procedure overview
In order to evaluate the impacts on air quality of the six landfills, which were selected as representative of the typical landfills in the province of Quebec, 5 stages were proposed:
- Scientific planning,
- Experimental measurements,
- Evaluation of the impacts,
- Possibility of identifying some(s) tracer(s),
- Conclusions and recommendations on the methods to monitor and evaluate the impacts on air quality.
The six MSWL represented the three categories defined according to the landfilling capacity.
- A - > 600,000 t/y
- B – 50,000–600,000 t/y
- C - < 50,000 t/y
Extensive landfill gas characterization and sampling was done at each landfill in the gas collection system and on the surfaces of different working area. The analyzed parameters were selected according to their impact on air quality (potential toxicity, imposed standards, potential olfactive nuisance). The odorous landfill gas compounds are mainly terpenes (limonene, careens, camphene, pinene, phellandrene, etc.), sulfur compounds (mercaptans, sulfides) and some VOCs (ethylbenzene, styrene, toluene, benzene, etc).
In this project, odor measurements were performed by the two usual approaches, sensory analyses and analytical techniques. Sensory odor tests concern the perceived effect of the odorants mixture as detected and interpreted by the human olfactory system (olfactometer). Analytical methods or physicochemical analyses relate to the properties of the odorants (GC-MS, GC-PFPD, etc.).
The odor concentration will take into account the complexity brought by the combination of odourus compounds in a mixture. The determination of the odor concentration is performed by olfactometric analysis. Olfactometry consists in measuring the odors according to a standardized and recognized procedure.
For the purpose of understanding the contribution of each chemical to the odor concentration, the following ratio was used to define the dilution to threshold (eq odor units) of each odorant.
(ODT = Odor Threshold values)
The following table presents the calculated % for each measured compound, for each compound using the AP-42 factors, the calculated odor concentrations for H2S and the actual olfactometry results.
- The dominating odorous compound was H2S for all sites according to the odor threshold values retained for calculations.
- The analytical measurement of odorants consistently underestimated (by two orders of magnitude) the odor concentration compared to olfactometric measurements
- Poor correlation between odors and odorants concentrations
- Field values and emission factors of the AP-42 did not identify the same compounds as priority odorants.
- Priority odorants in LFG:
- AP-42: 3 sulfur compounds H2S (25%), methylmercaptan (30%), ethylmercaptan (43%)
- Field measurements: H2S, 55% to 99%
- The results demonstrate that the odor emissions should be established with the odor concentration determined by dynamic dilution olfactometry and not with the odorous compounds, whether the odorants concentrations are determined by the values of the AP-42 or measured in situ.
Generally, it is difficult to properly characterize odor using standard analytical measurements for atmospheric pollutants. Analytical measurements characterize odors in terms of their chemical composition and the quantification of the odorants by their perception threshold. The number of odorants is very large in landfill gas as in most other environmental odors, and the odorants present were at concentrations lower than or close to the detection limits of the measurement methodologies.
The relationship between the concentration of odorants and the perceived odor is difficult to establish, as synergistic and antagonistic effects are expected between odorants. This is especially true for complex mixtures of odorants. Also, the differences in values of odor thresholds in the literature are very considerable, typically showing a range of several orders of magnitude, relying on these can bring large uncertainties in the calculated concentration.
- The evaluation of MSWL air quality impact should rely on actual on-site characterization and quantification of the emissions at the source combined with atmospheric dispersion modeling.
- Since the main impact identified in this research was odor, it is thus recommended to carry out a regular or continuous monitoring of odor emissions in terms of odor concentration.