Assessing stimulation and inhibition of anaerobic lagoons: Project 4C-105

Assessing stimulation and inhibition of anaerobic lagoons: Project 4C-105

Covered anaerobic lagoons are critical to achieve low carbon pork, however, potential failure of pond systems is a key issue, especially with a number of potential inhibitors in piggery flush effluent. Despite the importance of quantifying the influence of inhibitory compounds, there is currently no well-defined method to assess inhibition and experimental methods vary widely/are frequently very time-consuming and not workable. As a result, proper inhibition testing rarely forms part of anaerobic digestion studies, despite its usefulness and importance. To promote the widespread use of inhibition testing, the present study develops and validates a simplified and unified inhibition assay. The detailed project approach and results are primarily defined in a draft journal paper.

However, in summary, a simplified and unified inhibition test protocol was successfully defined and validated through extensive experimental test work. The targeted biological step was aceticlastic methanogenesis as it is typically most sensitive to inhibition and provides rapid inhibition testing. In the method development experiments, attention was given to important details of the unified method, including identification of the optimum carbon source and a preferred inoculum concentration. Measured specific methanogenic reaction rates covered a reasonable range of 0.005 (fully inhibited) to 0.16 (native inoculum, no inhibitor added) gCOD methane.gVS-1 inoculum.day-1, which indicated reasonable test outcomes. Results suggested that acetate (added as sodium acetate) was a better carbon source than acetic acid, and that an inoculum-to-acetic acid volatile solids ratio of five was preferred to not limit the availability of carbon and to not have conditions which are too dilute. Further, of relevance to concentrated biomass sources such as covered lagoon settled sludge, the results suggested that dilution of sludge samples down to 10 gVS.L-1 is required before testing. This is to obtain reliable measurements by minimizing mass and gas transfer limitations. The method was tested on two very distinct inoculum sources, one being sludge from a covered piggery lagoon, and the other being conventional municipal wastewater anaerobic digester sludge. Testing was done with two well-known inhibitors, namely ammonia and sodium. The inhibition response was observed to vary slightly with the inoculum under trial and was found to be largely consistent between the simplified (minimum number of sampling events) and the full-test method (multiple sampling events). Significantly, in the simplified version of the test technique, as few as two sampling events over 1.5 days provided a reliable measure of inhibition. The test was also successfully applied to an antibiotic sulfathiazole and gave results in line with expectations. Further testing of the method on other inhibitors/stimulants has been deferred to the larger Pork CRC inhibition/stimulation project 4C-109.

The inhibition test method will form an integral part of the toolbox of analyses to be used in future anaerobic digestion studies including Pork CRC projects 4C-111 Anaerobic treatment for emissions reduction from solid manure residues and expanded inhibition and stimulation of anaerobic lagoons in Project 4C-109 Enhanced methane production from pig manure in covered lagoons and digesters. Further, through the Appended draft journal paper (a Methods paper), the inhibition assay will be more broadly promoted also outside the pork industry. Adoption of the test method by NATA accredited analytical test laboratories across Australia is expected to provide testing capabilities to support producers across the country that are undertaking biogas projects.

Report prepared for the Co-operative Research Centre for High Integrity Australian Pork.