Biogas production with horse dung in solid-phase digestion systems.

The research article evaluates the potential of horse dung as a substrate for biogas production in solid-phase digestion systems. The study explores the effectiveness of different process technologies and parameters like temperature, percolation process, and pretreatments to optimize energy recovery.

Experiment Design

• The researchers carried out the experiment in laboratory conditions, using batch reactors to assess the dung’s potential for methanogenesis, i.e., the conversion of organic material into methane and carbon dioxide by microorganisms.
• The horse dung used in the tests was sourced from a stable with straw bedding. Such bedding can act as a source of carbon, enhancing the fermentation process.
• The temperature was maintained in the mesophilic range – the ideal temperature for most methanogenic bacteria.

Percolation Process

• In the percolation process, the researchers sprinkled process water over a stack of biomass. Notably, the investigators found that a 10-20% addition of solid inoculum (pre-digested horse dung), was appropriate for this process. The solid inoculum acts as a seed, introducing a population of microorganisms that can accelerate the digestion process.

Comparison between Percolation and Flooding

• In a comparative study between the percolation and flooding process, the flooded process resulted in a higher volume of biogas production because it didn’t require the addition of solid inoculum.
• With both percolation and flooding, the methane yield from fresh material was found to be similar – around 170 L(N) CH(4) per kg VS added was obtained in six-week cycles with untreated material under optimized conditions.

Impact of Pretreatments

• The research revealed that chopping the substrate increased the methane production. Finely divided or ground substrate provides a larger surface area for the microorganisms to work, thereby enhancing the rate of digestion and methane production.
• On the other hand, a pre-aeration treatment led to a decrease in methane production. Pre-aeration introduces oxygen into the system – which could inhibit methanogens that are strictly anaerobic in nature.