Biofiltration of methane from cow barns: Effects of climatic conditions and packing bed media acclimatization.

The performance of biofiltration to mitigate CH4 emissions from cow barns was investigated in the laboratory using two flow-through columns constructed with an acclimatized packed bed media composed of inexpensive materials and readily available in an agricultural context. The biofilters were fed with artificial exhaust gas at a constant rate of 0.036 m3 h-1 and low inlet CH4 concentration (0.22 g m-3 = 300 ppm). The empty-bed residence time (EBRT) was equal to 0.21 h. Additionally, in order to simulate temperature changes under natural conditions and determine the influence of such cycles on CH4 removal efficiency, the upper part of the biofilters were submitted to temperature oscillations over time. The maximum oxidation rate (1.68 µg CH4 gdw-1h-1) was obtained with the commercial compost mixed with straw. Accordingly, it was considered as packing bed media for the biofilters. The CH4 removal efficiency was affected by the temperature prevailing within the biofilters, by the way in which the cooling-warming cycles were applied and by the acclimatization process. The shorter the cooling-warming cycles, the more oxidation rates varied. With longer cycles, CH4 removal rates stabilized and CH4 removal efficiencies attained nearly 100% in both biofilters, and remained at this level for more than 100 days, irrespective of the temperature at the top of the biofilter, which was - at times - adverse for microbiological activity. The first order rate constant for CH4 oxidation kinetics of the entire system was estimated at 15 h-1. If such rate could be transposed to real field conditions in Canada, home to nearly 945,000 dairy cows, biofiltration may be applied to efficiently abate between 2 × 106 and 3 × 106 t yr-1 of CO2 equivalent (depending on how estimates are performed) from bovine enteric fermentation alone.

Effects of preconditioning the rhizosphere of different plant species on biotic methane oxidation kinetics.

The rhizosphere is known as the most active biogeochemical layer of the soil. Therefore, it could be a beneficial environment for biotic methane oxidation. The aim of this study was to document - by means of batch incubation tests - the kinetics of CH4 oxidation in rhizosphere soils that were previously exposed to methane. Soils from three pre-exposure to CH4 zones were sampled: the never-before pre-exposed (NEX), the moderately pre-exposed (MEX) and the very pre-exposed (VEX). For each pre-exposure zone, the rhizosphere of several plant species was collected, pre-incubated, placed in glass vials and submitted to CH4 concentrations varying from 0.5% to 10%. The time to the beginning of CH4 consumption and the CH4 oxidation rate were recorded. The results showed that the fastest CH4 consumption occurred for the very pre-exposed rhizosphere. Specifically, a statistically significant difference in CH4 oxidation half-life was found between the rhizosphere of the VEX vegetated with a mixture of different plants and the NEX vegetated with ryegrass. This difference was attributed to the combined effect of the preconditioning level and plant species as well as to the organic matter content. Regardless of the preconditioning level, the oxidation rate values obtained in this study were comparable to those reported in the reviewed literature for mature compost.

Does vegetation affect the methane oxidation efficiency of passive biosystems?

It is often reported in the technical literature that the presence of vegetation improves the methane oxidation efficiency of biosystems; however, the phenomena involved and biosystem performance results are still poorly documented, particularly in the field. This triggered a study to assess the importance of vegetation in methane oxidation efficiency (MOE). In this study, 4 large scale columns, each filled with sand, topsoil and a mixture of compost and topsoil were tested under controlled conditions in the laboratory and partially controlled conditions in the field. Four series of laboratory tests and two series of field tests were performed. 4 different plant covers were tested for each series: Trifolium repens L. (White clover), Phleum pratense L. (Timothy grass), a mixture of both, and bare soil as the control biosystem. The study results indicated that up to a loading equal to 100 g CH4/m(2)/d, the type of plant cover did not influence the oxidation rates, and the MOE was quite high (⩾ 95%) in all columns. Beyond this point, the oxidation rate continued to increase, reaching 253 and 179 g CH4/m(2)/d in laboratory and field tests respectively. In the end, the bare soil achieved as high or higher MOEs than vegetated biosystems. Despite the fact that the findings of this study cannot be generalized to other types of biosystems and plants and that the vegetation types tested were not fully grown, it was shown that for the short-term tests performed and the types of substrates and plants used herein, vegetation does not seem to be a key factor for enhancing biosystem performance. This key conclusion does not corroborate the conclusion of the relatively few studies published in the technical literature assessing the importance of vegetation in MOE.