Biogas recovery from wastewater
Exploring opportunities for recovering biogas from farm effluent has produced a promising low-tech solution – covered anaerobic ponds. Not only is the technology an economical way of generating energy, it also has the dual benefit of providing farmers with a solution to their wastewater management issues while helping reduce their carbon footprint.
Wastewater is a by-product of farming that has traditionally been considered a nuisance to manage and dispose of. The usual practice has been to treat it in open ponds, but this produces odour, emits greenhouse gases and can create nutrient run-off. If farmers are to alleviate these problems and comply with ever-tightening environmental rules, they need realistic alternatives to treating effluent in ponds.
NIWA identified that anaerobic digestion in covered ponds holds significant potential to reduce odour and greenhouse gas releases, while the biogas recovered in the process could, depending on the size of the farm, generate enough energy for electricity and heating. However, for biogas-recovery technology to be attractive to farmers, it needs to be a wastewater solution that is cost-effective and easy to operate and maintain.
We wanted to learn more about the amount of biogas that wastewater ponds are capable of producing, so our team monitored several dairy, piggery and municipal sites in Waikato. Monitoring involved:
- measuring total and seasonal levels of biogas (methane) that wastewater ponds produce
- determining the link between biogas productivity and the input of waste solids into a pond
- evaluating improvements in effluent quality that can be achieved with anaerobic digestion.
Each pond was equipped with a 5 x 5m experimental pond cover. We took three types of measurements:
- continuous gas flow meter and temperature logging at different depths
- assessing biogas composition (e.g. the presence of chemicals such as methane, carbon dioxide and and hydrogen sulphide)
- wastewater samples of pond influent and effluent quality, measuring levels of total solids/volatile solids, nitrogen and ammonia.
Biogas recovery technology
Anaerobic pond cover
We developed a purpose-built covered anaerobic pond on a 400-sow pig farm in Taranaki. The cover was tested for cost reductions and operational simplicity. It was designed to contain biogas so that it could be extracted through pipes for later use as a fuel. A rainwater management system was also installed.
The pond cover incorporates methane-extracting pipes running underneath it around the edge, plus a rainwater collection system.
Generating power with biogas
Biogas can be used as a fuel in combined heat and power (CHP) units, which produce both electricity and heating. The pond's biogas storage capacity allowed the separation of electrical and heat generation, providing a great deal of operational flexibility. CHP units require a small amount of routine maintenance, and basic biogas conditioning (e.g. drying) should be carried out before it can be used as a generator fuel.
At our Taranaki site, a 48 kW biogas generator working simultaneously with the grid provided more than half the farm’s electricity needs. Waste heat recovered from the coolant water was stored in a tank so that the generator could be operated independently of heat demand.
The results of our wastewater monitoring were in line with seasonal variations in pond water and sludge temperature. Contrary to our expectations, daily biogas production during the coldest winter period was only 50% below the annual daily average; daily biogas production during the hottest period of summer was 50% above the annual daily average. Biogas production continued even with pond water temperatures as low as 10°C.
These results were comparable with productivity for heated and mixed farm waste digesters in Europe and North America. This suggests that simple, unheated ponds operating under New Zealand climate conditions can be as effective in turning wastewater solids into biogas as more complicated and expensive systems used overseas.
The pond cover reduced odour emissions significantly, and the rainwater management system we installed prevented water buildup and flooding.
The pond's construction costs were only a fraction of the investment cost required for overseas digester designs, especially where existing wastewater treatment ponds could be retrofitted with a cover. Additionally, the pond was biologically more stable, required less maintenance and had lower operational costs than heated and mixed digesters. (However, these advantages over heated and mixed digester designs have to be balanced against seasonal variability in biogas production.)
Where anaerobic pond technology is used to reduce odour emissions, a simple, low maintenance method for the disposal of recovered biogas is desirable. For these types of installations, our trial showed that solar powered, self-contained gas flares work effectively. However, although using a flare is a reliable way to control odour and greenhouse gases, the method wastes a potentially valuable energy resource.
The volumes of waste heat from the biogas generator were larger than the farm’s heating requirements, which were being met by electric heat lamps. Consequently, the farm’s heating system will be modified so that a larger proportion of electric heating will be replaced with waste heat from the biogas generator.
Because the generator allowed the farm to continue operating during power outages, it has wider applications to enhance energy security.
For most New Zealand farms, the advantages of recovering biogas with covered anaerobic ponds outweigh the costs. Preliminary figures indicate that costs for the entire system - custom-designed covered anaerobic pond, biogas conditioning and grid-integrated CHP unit - can be repaid within 3 years. The technology has the potential to play a major role in reducing agricultural odour and greenhouse gas emissions, and the farming sector could become more self-sufficient as a result.
The anaerobic ponds are well accepted within the pork industry, with the New Zealand Pork Industry Board supporting uptake of the technology. The next phase of our research is to adapt biogas recovery for the dairy sector.
Beyond the dairy sector, we intend to explore opportunities to convert biogas into transport fuel. As part of the ‘Biogas Transport Fuel Cluster’, NIWA belongs to an industry group that is actively demonstrating, testing and evaluating the feasibility of biogas-to-fuel projects.
Radio New Zealand:
Heubeck, S. and Craggs, R.J. (2009) “Biogas recovery from a temperate climate covered anaerobic pond”. Water Science & Technology, Vol. 61 (4) 1019-1026.
Craggs, R.; Park, J.; Heubeck, S. (2008). “Methane emissions from anaerobic ponds on a piggery and a dairy farm in New Zealand”. Australian Journal of Experimental Agriculture, 48: 142–146.