An Observational Method for the Assessment of Biogas Production from an Anaerobic Waste Stabilisation Pond treating Farm Dairy Wastewater

A biogas production assessment method based on the visual monitoring of biogas evolution events in an anaerobic waste stabilisation pond was developed and applied to an anaerobic pond treating farm dairy wastewater in New Zealand. Major biogas-induced perturbations at the pond surface were classified as either type 1 or 2 events and other observed biogas activities as small bubble events. Mean counts of types 1 and 2 events varied from 7·3 to 30·0 per hour and 4·3–34·0 per hour, respectively, over the pond surface and the frequency of events decreased as both organic loading and temperature increased. Preliminary estimates of areal gas production rates, obtained using the observational method, ranged from 0·002 to 0·015 m³ m⁻² day⁻¹ for major eruptions and 0·0004–0·024 m³ m⁻² day⁻¹ for small bubble events, giving a total range of 0·002–0·039 m³ m⁻² day⁻¹. Pond temperatures at 2·75 m depth showed relatively minor fluctuations on a diurnal basis and ranged between 13 and 15°C from days 1–60, reaching a maximum of 24°C at day 190. Refinements proposed for future method development include an increased number and range of event categories, the automatic recording of events and the use of an improved cover. Further work is required to assess the general applicability of the method to anaerobic ponds.