Proposal for additional national standard hygienisation methods

To be able to merchandise manure or digestate, the material must be sanitized to ensure infection control. Swedish biogas facilities today have the choice between using one of two standard methods (at least 1 h at 70 °C, or 10 h at 52 °C) or going through a costly validation process if using alternative parameters. The main aim of this project has been to increase the profitability in the handling of biofertilizers by developing additional approved national standard methods for hygienization and thereby simplify the validation process. To achieve this, we have conducted interviews with stakeholders to find out which hygienisation parameters are most interesting for them. We have also compared the situation in other European countries and quantitatively and qualitatively reviewed scientific studies considering hygiene and inactivation data of microorganisms. However, the largest part of the project was laboratory studies where we generated novel inactivation data for the heat-tolerant bacterium Salmonella Senftenberg W775 at different temperatures and time intervals. The requirement for an approved hygienisation is a 5-log reduction of the bacteria. The choice of input parameters in the lab studies was based on literature as well as on the interviews where often preference for hygienization at lower temperature was expressed to save energy and to avoid the need to adapt the equipment to higher temperatures.

Another request that emerged from the interviews was that, in connection with emptying vessels, they want to avoid leaving bottom sediment for destruction, which is a costly procedure. Therefore, we have investigated the inactivation data of the roundworm Ascaris suum using limestome combined with heat treatment.

Based on our results, we propose the establishment of the following methods as new Swedish national methods for hygienisation:

1. Heat treatment of substrate for 3.5 hours at 60 °C
2. Heat treatment of substrate for 1 hour at 65 °C
3. Heat treatment of pure, source-separated food waste for 1 hour at 52 °C
4. Addition of at least 5 mass-% of lime (CaO) to bottom sediment, and ensuring a temperature of at least 65 °C for at least 15 minutes.

Our assessment and recommendation are that the Swedish regulating authority can use the data regarding proposed treatment methods to approve new or existing biogas plants without the need to carry out a complete validation process including in-situ killing trials.