Mid Sweden University

Bio sludge is a waste that is produced in large amounts at the waste water treatment plants at the pulp and paper industry. It is wet and has no obvious economic advantages, therefore present industrial focus is not on recycling methods but on disposal. Common disposal methods are incineration and composting. In Sweden and Norway about half of the sludge is incinerated, the other half is composted in Finland almost all is incinerated whereas landfilling of the sludge is common in Chile where half the sludge is landfilled and the other half is incinerated.

Biomass waste in general could be recycled for its energy content or as a resource for chemical products or solid materials. Different forces like legislation, market demand and scientific innovations will exert push or pull towards different potential paths of recycling. To choose a recycling method for a sludge from a specific pulp mill one must consider the properties of the sludge. These properties depends on the type of tree species used (hard wood or soft wood), amount of extractive substances, type of process the pulp mills use (mechanical pulp or Kraft), reject handling and the configuration of the water treatment plant. It could be that one single recycling method is not the optimal one for all pulp mills.

In this study, a comparison of bio-sludge properties between different Swedish pulp mills was investigated and its impact on the recyclability assessed. Ten mills were chosen that were expected to represent the extremes or unique combinations of raw material, type of cooking process, bleaching etc., and at the same time were easy to distinguish from each other. The samples were analysed for their content of possible useful substrates like lignin, fat or protein. Also the content of heavy metals were analysed, they could be problematic when handling rejects from the recycling process. Totally 21 parameters were analysed.

The potential relationship between types of processes, raw material, chemicals used etc. and sludge properties and its recyclability were assessed. Potential recycling methods found in the scientific literature was screened and evaluated for technology readiness level, environmental aspects, the potential market value and profitability for production of sludge derived benefits was estimated.

The two complementing perspectives mentioned above, sludge properties and appropriateness of technology were combined with the goal to suggest suitable recycling methods for the different sludge qualities that are produced today.