Microwave Heating as a Method to Improve Sanitation of Sewage Sludge in Wastewater Plants

For long-term sustainable agriculture, it is critical that we recycle nutrition to the soil that it came from. One important source is sewage sludge, but it must be sanitized from undesired pathogens before it may be spread on arable land. One common method today is deposition in about six months or more. Not only is such a long deposition-time costly due to the required storage-space, in the future usage of the method is likely to be more restricted from a regulatory perspective. To heat up sewage-sludge is a known method to speed up the sanitation process. However, achieving an even guaranteed temperature is not easy with porous sewage sludge. This is mainly due to the limited heat conductivity of the sludge. Microwaves at a frequency of 2.45 GHz have a penetration depth of a few centimeters and therefore has an advantage compared to other heating methods which only heats the surface. In the proposed system, the sewage sludge is continuously processed through a series of microwave cavities. The pathogen removal effectiveness was studied for different exposure settings, e.g., conveyor speed and applied microwave power in each cavity.