Summary of Harper Adams University/Silage Solutions Ltd trial
To evaluate the effects of maize silage treatments on biomethane production.
The process of ensiling forage for ruminant livestock has been commonplace for many years in the UK and around the world. Many scientific trials have been conducted looking at the benefits of different biological and chemical treatments and their effects on forage quality and livestock performance. However, very little has been done to study the specific effects of these treatments on forages destined for biogas production.
Previous trial work (Ververen and Willems 2010) showed that treatment with a biological inoculant widely used in AD feedstock production across Europe improved aerobic stability of ensiled maize and increased the biogas produced per unit of ODM (organic dry matter) entering the digester. The type of fermentation produced by this type of treatment (known as heterofermentative) is, however, responsible for increased DM (dry matter) losses because the production of acetic acid, which aids stability, also produces CO2, water and alcohols (Muck, 1999; Driehuis et al., 1999a; Ranjit & Kung, 2000). The theory behind such treatments is that production of acetic acid in the clamp, as opposed to during the acetogenesis stage of the AD process, is beneficial.
Safesil Pro, a chemical preservative, has been widely documented to improve aerobic stability in the clamp by reducing the activity of yeasts and moulds (Davies IBERS Report 2010). It also destroys unwanted bacteria, allowing the naturally occurring (epiphytic) lactic acid bacteria to reduce the pH more rapidly. This reduces the quantity of undesirable fermentation products such as ethanol, butanediol, acetic acid, butyric acid and CO2 produced (Knický & Spörndly 2009) leading to a reduction in DM loss and more retained energy per hectare harvested.
The objective of this trial was to compare these two treatments to an untreated control to ascertain whether either treatment gave a financial return in terms of bio-methane produced per hectare of maize harvested. In order to replicate on-site conditions, maize silage that had been exposed to air (described as ‘aerobically spoiled’) for three days was used because, once opened, a clamp is continually aerobic and subject to bio-chemical change.
Freshly-harvested wholecrop maize with a chop length of approximately 20mm was collected direct from the forage harvester.
Triplicate aliquots of maize were ensiled untreated, a biological inoculant-treated and Safesil Pro-treated in 20kg mini-silos by Dr Dave Davies in accordance with the trial protocol. After ensiling for 70 days, dry matter loss calculations were completed and samples of freshly opened silage and silage after three days exposure to air (aerobically spoiled) were frozen in 100g packs and transferred to Harper Adams University for gas yield and quality evaluation.
The gas evaluation trial was divided into two parts. Part one evaluated methane production kinetics while part two was a continuously-fed trial to simulate, more closely, an operational AD plant.
Effect of treatment on biogas yield in continuously-fed trial
AD culture pH reflects the removal of available H+ ions as methane. Safesil Pro-treated maize resulted in a higher mean culture pH in AD cultures fed continuously for ~160 days. It should be noted that Safesil Pro silage also had the lowest pH value, significantly lower than control (P<0.001) and numerically lower than the biological inoculant treatment, but resulted in a higher culture pH when fed, suggesting that the acid-loading to microbial culture was less important than the resulting fermentation of the chemical components added. Total biogas (methane and carbon dioxide) yield (m3/tDM) was not found to be affected by treatment. However, the proportion of carbon dioxide in the biogas was found to be reduced by the Safesil Pro silage additive (P<0.001), resulting in an increased CH4 : CO2 ratio (P=0.006).
These results suggest that, although the biological inoculant treatment resulted in a higher gross energy-value (GE) silage, its availability to the microbial population of the AD culture was reduced, or less efficiently used, for the production of methane.
Results from the fermentation kinetics study concurred with those from the continuously-fed experiment. Safesil Pro-treated maize was found to produce the most gas. Both silage additives were found to marginally increase the rate of gas production in comparison to the untreated silage, but rate of fermentation was the same for both additive types.
Aerobic deterioration of the silages reduced the mean biogas production from the continuous fed cultures for both the control and the biological inoculant-treated maize. Safesil Pro-treated maize was not found to have reduced biogas production following aerobic spoilage, which is concurrent with the reduced chemical composition deterioration together with increased WSC (water soluble carbohydrate) content following spoilage, observed for this treatment.
- Treating maize with a silage additive reduces aerobic spoilage.
- Treating maize with Safesil Pro improves CH4 : CO2 ratio.
- Safesil Pro-treated maize maintains biogas production after three days of aerobic deterioration.
- Carbon dioxide production, as a proportion of biogas, is reduced following Safesil Pro treatment of maize.
In practical terms, the improvement in biogas yield, based on a 39.5t/ha maize crop harvested at 32% DM, when Safesil Pro treatment is used, equates to an additional gas yield of up to 760m3/ha due to improved DM retention and digester fermentation dynamics.
What is this worth to your business?
Often, the results of scientific trials are not easy to translate to tangible benefit for your particular situation, to show what really matters – the effect the investment would make to your bottom line. In this case, aside from the direct return on investment per hectare due to increased gas production, other benefits (such as reduced land rent and growing and harvesting charges) all play a significant part in overall profitability. For this reason, we have developed an innovative online calculator.
This useful tool allows you to input many variables, such as land rent, growing and contracting costs, plant efficiency, tariffs and treatment costs for your CHP or gas to grid plants, to show the potential financial benefit of investing in Safesil Pro treatment.
Finally, don’t forget the further benefits to the environment and local community. Fewer vehicle movements, reduced fuel use and cost, together with reduced chemical use, will all contribute to a smaller carbon footprint, to produce the same volume of biogas for your business.