Intermittent agitation of liquid manure: effects on methane, microbial activity, and temperature in a farm-scale study.

Liquid manure storages are a significant source of methane (CH4) emissions. Farmers commonly agitate (stir) liquid manure prior to field application to homogenize nutrients and solids. During agitation, manure undergoes mechanical stress and is exposed to the air, disrupting anaerobic conditions. This on-farm study aimed to better understand the effects of agitation on CH4 emissions, and explore the potential for intentional agitation (three times) to disrupt the exponential increase of CH4 emissions in spring and summer. Results showed that agitation substantially increased manure temperature in the study year compared to the previous year, particularly at upper- and mid-depths of the stored manure. The temporal pattern of CH4 emissions was altered by reduced emissions over the subsequent week, followed by an increase during the second week. Microbial analysis indicated that the activity of archaea and methanogens increased after each agitation event, but there was little change in the populations of methanogens, archaea, and bacteria. Overall, CH4 emissions were higher than any of the previous three years, likely due to warmer manure temperatures that were higher than the previous years (despite similar air temperatures). Therefore, intermittent manure agitation with the frequency, duration, and intensity used in this study is not recommended as a CH4 emission mitigation practice. Implications: The potential to mitigate methane emissions from liquid manure storages by strategically timed agitation was evaluated in a detailed farm-scale study. Agitation was conducted with readily-available farm equipment, and targeted at the early summer to disrupt methanogenic communities when CH4 emissions increase exponentially. Methane emissions were reduced for about one week after agitation. However, agitation led to increased manure temperature, and was associated with increased activity of methanogens. Overall, agitation was associated with similar or higher methane emissions. Therefore, agitation is not recommended as a mitigation strategy.

Characterizing the Performance of Gas-Permeable Membranes as an Ammonia Recovery Strategy from Anaerobically Digested Dairy Manure.

Capturing ammonia from anaerobically digested manure could simultaneously decrease the adverse effects of ammonia inhibition on biogas production, reduce reactive nitrogen (N) loss to the environment, and produce mineral N fertilizer as a by-product. In this study, gas permeable membranes (GPM) were used to capture ammonia from dairy manure and digestate by the diffusion of gaseous ammonia across the membrane where ammonia is captured by diluted acid, forming an aqueous ammonium salt. A lab-scale prototype using tubular expanded polytetrafluoroethylene (ePTFE) GPM was used to (1) characterize the effect of total ammonium nitrogen (TAN) concentration, temperature, and pH on the ammonia capture rate using GPM, and (2) to evaluate the performance of a GPM system in conditions similar to a mesophilic anaerobic digester. The GPM captured ammonia at a rate between 2.2 to 6.3% of gaseous ammonia in the donor solution per day. Capture rate was faster in anaerobic digestate than raw manure. The ammonia capture rate could be predicted using non-linear regression based on the factors of total ammonium nitrogen concentration, temperature, and pH. This use of membranes shows promise in reducing the deleterious impacts of ammonia on both the efficiency of biogas production and the release of reactive N to the environment.

Does Fall Removal of the Dairy Manure Sludge in a Storage Tank Reduce Subsequent Methane Emissions?

When liquid manure is removed from storages for land application, "sludge" generally remains at the bottom of the tank. This may serve as an inoculum when fresh manure is subsequently added, thereby increasing methane (CH) emissions. Previous pilot-scale studies have shown that completely emptying storages can decrease CH emissions; however, no farm-scale studies have been conducted to quantify the effect of sludge removal. In this study, a commercial dairy farm removed as much manure and sludge from their concrete storage as possible in the fall (∼2% by volume remained). Emissions of CH were measured during the following winter, spring, and summer, and compared with emissions measured the preceding 2 yr when most of the sludge had not been removed (∼14% of tank volume remained). Emissions were measured using a micrometeorological technique, utilizing open-path CH lasers. Contrary to what was hypothesized, removing the majority of sludge in fall did not delay the onset of CH emissions and did not decrease emissions the following summer. In fact, annual CH emissions were ∼16% higher. It is possible that fall removal provided sufficient time for microbial dynamics to be restored before the following summer when emissions were high. Future farm-scale research should examine the effect of spring (rather than fall) emptying for on-farm CH mitigation in both concrete tanks and earthen storages.

Methane emissions from digestate at an agricultural biogas plant.

Methane (CH4) emissions were measured over two years at an earthen storage containing digestate from a mesophilic biodigester in Ontario, Canada. The digester processed dairy manure and co-substrates from the food industry, and destroyed 62% of the influent volatile solids (VS). Annual average emissions were 19gCH4m(-3)d(-1) and 0.27gCH4kg(-1)VSd(-1). About 76% of annual emissions occurred from June to October. Annual cumulative emissions from digestate corresponded to 12% of the CH4 produced within the digester. A key contributor to CH4 emissions was the sludge layer in storage, which contained as much VS as the annual discharge from the digester. These findings suggest that digestate management provides an opportunity to further enhance the benefits of biogas (i.e. reducing CH4 emissions compared to undigested liquid manure, and producing renewable energy). Potential best practices for future study include complete storage emptying, solid-liquid separation, and storage covering.