Bio sludge is a waste that is produced in large amounts at the pulp and paper industry waste water treatment. It is wet and has no obvious advantages, therefore few recycling methods are in praxis but focus is on disposal. Common disposal methods are incineration and composting. In Sweden and Norway about half of the sludge is incinerated, but in Finland almost all of it is incinerated. Landfilling of the sludge is common in Chile where half the sludge is landfilled and the other half is incinerated.
Efficient use of material and energy is crucial for future more sustainable industry practices. This is true for primary products and commercial available energy but also for material that today is regarded as waste. The pulp and paper industry is subjected to a shift in consumer behavior initiated by modern information technology that results in lower demand for newspapers. In the future there are reasons to expect increased competition from new producers in countries where the productivity of the forests are higher and thereby the production costs is lower. These two aspects forces the pulp and paper industry in the Scandinavian countries to optimize the efficiency of their mills and to seek revenues from new types of products.
Biomass waste in general could be recycled for its energy content or after conversion/extraction as a chemical product or a solid material. Different forces like legislation, market demand and scientific innovations exert a push or pull towards different paths of recycling. They may either cooperate or counteract each other and the sum of their impact may favor one of these paths.
Some possible paths of recycling inspired by industrial symbiosis will be presented and the maturity of these technologies assessed. The sludge’s content of heavy metals have been compared to limit values in Swedish national regulation and standards.
The content of metals in the sludge exceed some of the current limit values and could in a greater extent exceed the future limit values if they will be lower than today. Therefore direct application of the sludge can only be a short term solution.
Most treatment methods produce either a liquid or solid residue and the metals could be concentrated in any of these so the metals in the residue must be assessed regardless of what technology is used. To maximize the usefulness of the sludge it is essential to assume that the problem with the metals can be solved.
Production of bricks and glass stabilizes the metals but the resources are then locked in a matrix which make them to a lesser extent available for use.
The production of SCP and a lead adsorber are two alternatives that is regarded both circular and relative mature. These two should be assessed according to the LCA-method together with incineration combined with improved dewatering.
Global Cleaner Production & Sustainable Consumption Conference 1 - 4 November 2015, Sitges, Barcelona, Spain