Agricultural Use or Combustion – Life Cycle Assessment of Pulp and Paper Industry Sludge Treatment Practices

Abstract

The pulp and paper industry generates a substantial amount of sludge from effluent treatment processes. The common practice is to utilize the sludge by incinerating it with other biobased side streams like bark. In line with the premises of a circular bioeconomy, alternative applications for sludge have been sought from the agricultural sector.In this study we have assessed how environmentally sound it is to switch from combustion of pulp and paper mill sludge to alternative application in agriculture. Life cycle assessment (LCA) is used to evaluate climate, eutrophication and acidification impacts of composted and lime-stabilized sludge from cradle-to-grave. These impacts are considered in relation to avoided impacts when changing practise from combustion of sludge and bark to pure bark instead.The results show that it is more sustainable to change the common practice of combusting sludge to an alternative use in agriculture. Reduction in climate emissions is achieved by avoiding natural gas use as a supplementary fuel in combustion process. Nutrient leaching is reduced when sludge replaces mineral fertilisers and P leaching through soil erosion is reduced. When sludge is replaced with bark in the combustion process, acidifying SO2 emissions from the biomass boiler are significantly reduced.Further studies are recommended to evaluate formation of N2O and NH3 emissions from organic N contained in the sludge when applied together with mineral fertilizers in soil. N deposition via complex biogeochemical interactions from N2O and NH3 emissions to soil and aquatic systems are difficult to assess and subject to many uncertainties.

Keywords: SludgeLCAPulp&paperAgricultureCombustion