From Sea to Silo: Ensilage as a Sustainable Solution for Year-Round Seaweed Biofuel Production

The seasonal nature of seaweed growth presents a significant challenge to its exploitation as a year-round feedstock for biofuel production. Seaweed is often harvested only once a year, meaning that an effective and economic preservation method is needed to supply biofuel facilities continuously. While drying is a traditional preservation method, it is highly energy-intensive, and in many cases, requires more energy than the calorific value of the seaweed itself. This has led to the exploration of alternative low-energy preservation techniques, with ensilage emerging as a promising solution.

The Ancient Art of Ensilage, Reimagined for the Blue Bioeconomy

Ensilage is a preservation method, routinely used for storing forage crops for animal feed, that utilizes anaerobic fermentation to convert water-soluble carbohydrates into organic acids, primarily lactic acid. This process lowers the pH of the biomass, creating an acidic environment that inhibits the growth of spoilage microorganisms like Clostridia and preserves the wet biomass.

For terrestrial crops, ensiling is a highly effective way to preserve the “energy potential” of wet biomass, with some studies showing that it can preserve over 90% of the original energy and has negligible energy losses. Compared to drying with natural gas, ensilage can reduce energy input by 8-10% and greenhouse gas emissions by 20-25%. These benefits make ensiling a “green approach” for preserving biomass without intensive energy input.

The Unique Challenges of Ensiling Seaweed

While ensilage is a proven method for terrestrial crops, seaweed presents a unique set of challenges that can hamper its success:

• High Moisture and Ash Content: Seaweed has a very high moisture content, often with a water activity of 0.974-0.979, which can hinder the ensiling process. It also has a high ash content, which can be correlated with high anion content and lead to high buffering capacity, making it difficult to achieve the rapid drop in pH needed for preservation.
• Varied Composition: The cell wall structure and carbohydrate composition of seaweeds are very different from terrestrial plants. They contain unique carbohydrates like alginates and laminarin, as well as compounds like phloroglucinol and sulfated carbohydrates, which can be challenging for lactic acid bacteria to ferment.
• Low Natural Bacteria: Seaweed inherently has low concentrations of the lactic acid bacteria (LAB) needed to initiate spontaneous fermentation, which can lead to a slow pH drop and the growth of undesirable spoilage bacteria.

Despite these hurdles, research has shown that ensiling can be a viable method for preserving seaweed, particularly brown seaweed species like Laminaria digitata and Saccharina latissima. Most studies, however, have not achieved a pH below 4.3, which is the recommended pH for grass silage.

Optimizing the Ensilage Process for Seaweed

To overcome the challenges of ensiling seaweed, several pretreatments and additives have been investigated:

• Reducing Moisture Content: Wilting the seaweed before ensiling can reduce moisture content and improve ensiling performance. This is a common practice for ensiling fodder crops and has been shown to reduce overall energy losses during seaweed storage. However, washing seaweed with fresh water to remove salt, while beneficial for reducing the salt content that can inhibit anaerobic digestion, can also increase moisture content and lead to considerable losses during storage.
• Size Reduction: Chopping or milling the seaweed before ensiling can increase the surface-area-to-volume ratio, which can accelerate the fermentation rate and lactic acid concentration. Studies have shown that size reduction can reduce leachate losses and may be a valuable pretreatment, though its benefits can vary by species.
• Enzymes and Lactic Acid Bacteria (LAB): Because seaweed carbohydrates can be complex, the addition of enzymes like cellulase can hydrolyze them into simple sugars, providing a more readily available substrate for LAB to produce lactic acid. Similarly, adding Lactobacillus cultures is a routine practice in forage crops to achieve a rapid pH drop and improve silage stability. For some seaweed species like Ulva lactuca, the addition of both cellulase and LAB inoculum has been shown to be necessary to achieve a pH below 4 in a reasonable amount of time.

The Biomethane Potential of Seaweed Silage

The primary application for ensiled seaweed is as a feedstock for anaerobic digestion (AD), a process that converts organic matter into biogas, predominantly methane. Seaweed has a higher biomethane productivity per unit of land than many terrestrial plants, making it an attractive option for biofuel production.

Research has shown that:

• Ensilage preserves biomethane potential (BMP): Ensilage has been found to have no major detrimental effect on biomethane production and can preserve the majority of a substrate’s BMP for up to a year.
• Low energy and dry matter losses: Losses from the ensiling of brown seaweed are generally low, often less than 10%. This is comparable to, or even better than, the losses seen in terrestrial forage crops.
• Seasonal variation matters: The chemical composition of seaweed, including the concentration of fermentable sugars, varies seasonally, which can impact the success of ensilage and the biomethane yield of the final product.

In conclusion, ensiling is an energy-efficient method for preserving seaweed, particularly brown seaweeds, for year-round production of biogas. While more research is needed to optimize the process for a wider range of seaweed species, it is a promising and simple conservation method that can help the seaweed biofuel industry overcome one of its key barriers to commercialization.