Fertiliser Properties of Wastewater Sludge and Sludge Ash - A Case Study from the Finnish Forest Industry

In this case study we compared the fertiliser properties of an industrial wastewater treatment sludge and a sludge ash to the requirements of the Finnish Fertiliser Product Decree. The sludge was obtained from the activated sludge wastewater treatment plant of a Finnish non-integrated pulp mill. The sludge was furthermore incinerated at a laboratory in a muffle furnace (850 °C) to obtain sludge ash. The total Cd (4.9 mg/kg d.m.) concentration in the pulp sludge exceeded the Finnish limit value (1.5 mg/kg d.m.) for fertiliser products used in agriculture. In the sludge ash, the total concentration of Cd (39 mg/kg; d.m.) exceeded the Finnish limit value (25 mg/kg d.m.) for ash fertilisers used in forestry. These results restrict the potential reuse options of these residues. However, from the utilisation point of view, the enrichment of essential plant macro-nutrients was most notable, resulting to the following total concentrations of these elements in the sludge ash: P (26,000 mg/kg d.m.), S (40,000 mg/kg d.m.), K (11,000 mg/kg d.m.), Ca (83,000 mg/kg d.m.) and Mg (10,000 mg/kg d.m.). Therefore, we conclude that, the converting of sludge into ash may promote the reuse of this wastewater treatment residue to a more value-adding fertiliser by-product to be used as a soil improver and growing medium in landscaping or landfills sites or other closed industrial areas, where heavy metal limit values for fertilisers are not applied in Finland.