Executive Summary: Valorisation of paunch grass: bioenergy yield improvement and commercial-scale testing

Summary of Project and Key Findings:
Partially digested feed (paunch grass) from livestock rendering is a significant waste stream from New Zealand’s beef and lamb sector. Beneficial utilisation of this bioresource will provide environmental relief, reduce waste management costs and increase revenue generation from abattoir operations.

The objective of this project was to investigate the techno-economic feasibility of converting paunch grass into biofuel, bioCO2 and biofertilizer via anaerobic digestion and assess the suitability of the rumen fluid content as a pet food ingredient.

The project essentially consisted of four work packages.

1. Characterisation, pretreatment and biomethane potential (BMP) testing
2. Animal trial to assess rumen fluid as a pet food supplement
3. Bench and commercial scale biogas trials to validate quality and compliance of products for commercial use
4. Techno-economics of biogas and biofertilizer production from paunch grass

Ecogas Ltd as the industry partner of this project arranged for paunch grass from Silver Fern Farms for testing. Ecogas also carried out scale up testing at their organic reprocessing facility in Reporoa and applied the digestate to pasture via existing commercial arrangements.

The results showed that the paunch grass is suitable as an anaerobic digestion feedstock, but its high moisture content makes long-distance transportation of the material uneconomic. The dried grass had similar chemical characteristics to other such biomass being rich in carbon but low in nitrogen and phosphorus content. The rumen fluid present in the partially digested material is rich in protein, minerals, vitamins and microbes and significantly aided the digestion process contributing to a higher methane (biofuel) yield. Other pretreatments such as particle size reduction, heating and alkaline hydrolysis did not contribute to increasing the methane yield from the dried paunch grass to an appreciable extent.

The rich soup of vitamins and minerals present in the rumen fluid exceeded the minimum nutrient requirements specified for most of the nutrients for domestic pet foods. An animal study conducted with adult dogs showed that the rumen fluid containing diet was palatable with no reduction in digestibility based on the parameters analysed. The rumen fluid appears to be a viable ingredient for inclusion in pet food formulations from a nutritional and overall pet gut health perspective.

Continuous anaerobic digestion of paunch grass at bench-scale produced biogas at a rate and composition similar to most other lignocellulosic feedstocks; demonstrating stable gas production over a longer period. The nutrients contained in the paunch material ended up in the digestate enriching the nutrient value of this biofertilizer. Co-digestion at the Reporoa organic processing facility further confirmed the feasibility of paunch grass anaerobic digestion with no noticeable change in gas production from the mixed feedstock. However, the digestate produced a distinct malodour and colour attributed to paunch grass and the lignocellulosic material caused some process issues at the Reporoa plant which is primarily designed to pretreat and digest food waste.

The key gate fee determinant for any feedstock for an anerobic digestion plant are the alternative means of disposal. For paunch grass, this is often some form of composting, sometimes even co-located at the producer’s site due to the large volumes of this material typically produced. Given the moderate methane yield from paunch grass, processing challenges of feeding a low-density lignocellulosic feedstock and the risk of handling and processing a malodourous and potentially biohazardous feedstock; a higher gate fee is demanded for to compensate for the risks. However, the “avoided” disposal unit cost and transport cost is below the gate fee required at the anaerobic digestion plant for a viable business case, especially once the operational cost and risks are factored in.
Paunch grass processing via anaerobic digestion may be commercially viable where the only option for disposal is landfill with substantially higher gate fee than for anaerobic digestion or where the anaerobic digestion plant is co-located at the producer’s site to feed energy back into their process.

The study identified processing challenges of lignocellulosic material at the Reporoa facility that could help rectify future risks of using food-organic-garden-organic (FOGO) type feedstocks Ecogas intend to move into. Scion’s is happy to discuss and share its woody biomass pretreatment know-how to mitigate future challenges in this space.