Methane production from a capped feedlot pond

Summary

​This study was commissioned in part, as a consequence of Australian government legislation to target a net reduction in greenhouse gas (GHG) emissions from the rural sector, and in part due to the lack of data to support the development of methodologies for generating offset credits under the Carbon Farming Initiative for the feedlot industry.

Despite research investment into characterising rumen- and manure-based GHG emissions, little was known about the contribution to the total GHG profile of a cattle feedlot as a consequence of the environmentally sound practice of collecting rainfall run-off from feedlot pads and storing it in ponds. These, generally anaerobic ponds, are natural biodigesters that could provide options to minimise GHG emissions by using the methane-rich biogas as an alternative source to fossil fuels for energy generation. Moreover, the cost effectiveness to the producer of such a strategy has not been investigated previously in Australian beef cattle enterprises.

To address the shortfall in knowledge, a feedlot pond was monitored for methane emission over a single annual cycle. This enabled the building of the capacity to monitor, transmit (via the 3G network) and store real-time methane emission data. This was done simultaneously to the transmission and storage of key climatic data and water column temperature data. The site selected was a deep sedimentation pond at a feedlot on the Darling Downs in Queensland. The system, consisted of a floating chamber, low cost sensors, a transmitting weather station and automated control of methane concentration-based capture-purge cycles, and was operated according to audited safety requirements and built-in fail-safe systems.

The study was conducted over a 12-month period (27th April 2012 –26th April 2013) and accumulated 650 individual capture-purge cycles resulting in 161 records of daily average methane emission. Although the pond emitted methane all year round, the rate of emission was influenced significantly by rainfall events and seasonal effects on pond water temperatures. In the warmer, wetter summer months between October and March, mid and lower level water temperature medians in the water column, plus accumulated rainfall, were the primary environmental factors driving emission. In contrast, during the cooler months between April and September, the water surface temperature maxima and minima were the primary factors driving a significantly weaker emission response. During the cooler months the average daily rate of methane emission was 18 g/m2 surface area/d compared to 165 g/m2 surface area/d during the warmer months. The composition of biogas produced from the pond was predominantly methane (74%) with smaller amounts of carbon dioxide (24%) hydrogen sulphide (0.006%) and hydrogen (0.001%).

Phylogenetic profiling of the pond sludge indicated that it contained significant microbial biodiversity. Approximately 85% of the sequences were bacterial in origin, while the remaining 15% were archaeal of which members of the Methanosaeta genus were predominant (10.6%). These are acetoclastic methanogens that only make methane from acetate and are common in anaerobic digesters but are rarely found in the gastrointestinal tract of ruminants.

An economic assessment to determine the feasibility of the feedlot to capture and use the methane as an alternative energy resource was performed by Feedlot Services Australia Pty Ltd (FSA) as a contracted adjunct to the project and their report accompanies this report.

The FSA report provide a synthesis of the methane and climatic data modelled against theoretical and estimated methane yield data to determine the volatile solids (VS) required to provide the yield of methane observed. These data suggested that ~17% of the excreted VS at the feedlot would need to have entered the pond to yield the amount of methane emitted. This was a significantly higher VS than the 2% estimated as a typical amount entrained in feedlot pad runoff. The apparent over-estimation of the methane yield was explained by several caveats:

1. that the prior historical VS in the pond was unknown,
2. that there was a higher methane potential in the runoff leachate than was modelled,
3. that there could be an under prediction of the maximum methane yield and methane conversion factor (MCF) used, and
4. that the average emitting surface area of the sedimentation pond was overestimated.

For the purpose of approximating an annual yield of methane from the pond, the emitting surface was arbitrarily set at 1000 m2 although it varied from ~500 m2 to >1500 m2 during the year. Nevertheless, further research is essential in these areas before yields can be accurately assessed and verified.

An economic evaluation was undertaken using scenarios consisting of a covered anaerobic pond (CAP) supplying gas to either a boiler system or a combined heat and power system. Neither scenario was economically feasible for use at the feedlot due to the large variability in methane production, as a result of the variability in feedstock supply to the pond and seasonal effects on air and water temperatures. However, the report also considered a constant feedstock supply scenario where the manure was applied directly to the pond. Under these circumstances, but within defined limits of manure handling costs ($5-14/tonne), both systems were considered economically feasible options. More detail of this assessment is provided in the accompanying FSA report.