Odor emissions control in yard trimmings composting

University of Gent researchers evaluate effects of turning frequency, shredding and screening at three facilities.

IN the Flanders region of Belgium, about 270,000 tons/year of yard trimmings are collected separately and composted in windrows at 15 outdoor facilities. Although the method is widely used, few systematic scientific studies have been done on factors that are controllable by the compost operator. These factors include turning frequency and its impact on odor generation.

A key question is: "To minimize odor emissions during composting, is it better to turn frequently or infrequently?" Unfortunately, there isn't a simple answer. On the one hand, turning releases odorous compounds trapped within the windrow. On the other, lack of turning allows anaerobic conditions and the associated odorous compounds to proliferate within the compost windrow. This increases the severity of odors that are released when the pile is eventually disturbed.

To get a clearer view on turning frequency and odor production, a monitoring campaign was set up at three Flemish yard trimmings composting plants with three different turning frequencies. General information about turning frequencies, duration of the composting process and capacity for the different plants are given in Table 1.

Two different odor measurements were applied. To evaluate the total impact of the composting plants, sniffing team measurements were carried out. Secondly, at each plant one pile was evaluated from the beginning to the end of the composting process by olfactometric measurements.

Sniffing And Olfactometric Measurements

Sniffing team measurements allow evaluation of the total (including diffusive sources) odorous impact on the neighborhood, but it does not distinguish the relative importance of different emission points of a source. Although it is not as standardized as olfactometric measurements, the sniffing approach gives reasonable results. Over the years, we developed our own standard operational procedures, which result in intrapanel standard deviations of 20 to 30 percent.

During sniffing team measurement, one walks around the composting plant starting downwind and afterwards going to the composting plant perpendicular to the wind direction. Every time odor is noticed, it is noted on a topographical map. Afterwards, the central axis of the odor spot can be drawn. The maximum distance of odor observation along this axis is called the maximum distance of odor perception (MDOP) or the sniffing border. With the distance and the meteorogical conditions during the sniffing team measurements, one can calculate the overall odorous emission, which is expressed in sniffing units per second (su/s), using a short term atmospheric dispersion model.

The aim of olfactometric measurements study was to get odor emissions for the different composting plants expressed in ouE/ton compost/h (ouE = European odor unit). Expressing the emission per weight unit of compost allows a comparison between the different composting plants. To achieve this unit, not only the odor concentration was needed but also the flow rate of the air emitted by the compost piles. Compost piles are naturally ventilated surface sources which make it difficult to measure the flow rate of the outcoming air. Furthermore, flow rates are very low. In order to convert the surface source to a guided source, a part of the compost pile was covered with a measuring tent.

For the olfactometric measurements, a method was used to determine odor concentration of a gaseous sample by presenting a panel of selected and screened people with that sample. Varying concentrations were presented by diluting samples with neutral gas, in order to determine the dilution factor at the 50 percent detection threshold. At that dilution factor, the odor concentration is 1 OUE/m3 by definition. The odor concentration of the examined sample is then expressed as a multiple (equal to the dilution factor) of one European Odor Unit per cubic meter (ouE/m3) at standard conditions for olfactometry.

Determining Odor Cause And Effect

At a yard trimmings composting plant, different activities take place and they all are considered to cause a different odor emission. When the yard trimmings arrive at the plant, size of material is reduced with a chipper or grinder. From time to time, material is turned to speed up the process. At the end of the process, composted material is screened to two or three fractions. During the different activities, sniffing team measurements were carried out to see if there was a difference between the plants.

At each plant, one compost pile was followed up from the start for three months. Every week, an olfactometric sample was taken and the measurements were done more frequently during periods of turning of the compost piles.

For the different activities, we could say that the highest odor emission occurs during size reduction and turning. When the odor emissions of the three composting plants were compared, it became clear that all three composting plants had nearly the same odor emissions during the first ten days. Every time the compost was turned, there was a peak emission of odor that dedined to the level of odor emission before turning within three to four days. These peak emissions were the lowest for composting plant B. Afterwards odor emission level was in the same range for all three plants (100-700 ouE.ton-l.h-1). It can be concluded that the composting plant with the highest turning frequency had the lowest overall odor emission.

[Author Affiliation]

N. Defoer and H. Van Langenhove are in the Department of Organic Chemistry and Applied Biological Sciences at the University of Gent, Belgium. The authors acknowledge the financial support of Vlaco (the Flemish organization for promotion of biological and vegetation compost) based in Mechelen, Belgium. The report is excerpted from a paper presented at the Microbiology of Composting Conference in Innsbruck, Austria in October, 2000.