Pre-processing

The feed materials for biogas systems are composed of different biomass materials. Some materials, such as starches and sugars digest very easily. Other, such as woody materials, that contain a large amount of lignin, take a long time to break down. The pre-processing of biogas feed materials can be used to begin the process of breakdown, while separating the materials that are easily digested from those that are not.

The Nisargruna process, developed by Bhabha Atomic Research Centre (BARC) in Mumbai, India, uses a solar heated aerobic first stage. This accelerates the break down of the food waste that is added to the system, allowing the second stage, which is a standard mesophyllic (35°C) digester generating biogas to process the feed more quickly and efficiently.

Nisargruna plant at BARC, Mumbai

In the UK, Bioplex Ltd originally developed a thermophyllic (57°C) first stage to separate cattle dung from straw. Its use has been extended to separate easily digestible matter in food wastes from less easily digestible ligninaceous materials.

The solid materials can be composted, once the digestible liquor has been transferred to a biogas

Nisargruna plant, Shatabdi Hospital Site at Govandi, Mumbai

digester. Since the solid materials have been seeded by the right bacteria, the composting occurs twice as quickly. The compost process is also totally smell-free, as the volatile fatty acids that give rise to the smell of dung and rotting food are also the materials that produce biogas.

In the UK, this predigestion process allows regulations for dealing with animal byproducts (PAS 100 and PAS 110) to be meet for category 3 wastes.

Technologies to deal with category 2 animal by-products have been tested and produce a product that is ideal for processing through the Bioplex system and an anaerobic digester.

Materials to be processed through these systems do need to be chopped to a size of less than 5 mm, to allow the organisms to process the food materials.