Short Retention Time Effluent Management

Managing wastewater ponds to achieve a short hydraulic retention time (HRT) is a potential greenhouse gas abatement tool that farmers can employ to reduce methane emissions from manure management systems. With research data from other livestock sectors suggesting significant reductions are possible, this Action on the Ground project aimed to demonstrate the application of short‐HRT wastewater management on a commercial dairy farm in southern Australia where wastewater storage is considered best practice to avoid nutrient runoff or leaching during wet periods.

This trial demonstrated that while the theory may be sound, there are practical impediments to achieving short‐HRT and that ancillary management practices such as frequent agitation to recover settled solids are likely to have significant influence on methane emissions. Methane emissions during the trial’s short‐HRT mode were 87% higher (on the basis of methane generated per mass of volatile solids added to the storage tank) than the emissions recorded during the baseline phase. Analysis of project data suggests that intermittent mixing caused an increase in methane emissions larger than could be explained by temperature alone and, as agitation for sludge removal is a necessary part of maintaining a functional storage, needs further investigation. However, it must also be noted that methane emissions during the baseline phase were lower than expected.

Other practical challenges identified by the project include; i. reducing HRT’s promptly, and to sufficiently low retention times, in the drier part of the year to capture the potential abatement offered by short‐HRT management, ii. managing multiple pond systems where the bulk of methane emissions are from a primary pond not the managed storage, and iii. generating data that would enable a manager to document if the potential abatement offered by short‐HRT has been achieved.

Data from the Biochemical Methane Potential testing showed that the wastewater at this site had the potential for higher methane emissions than is currently estimated by the default value for ultimate degradability (Bo) within the National Inventory Report and appropriate industry stakeholders should look for opportunities to investigate this further.