Composting sewage sludge amended with different sawdust proportions and textures and organic waste of food industry – assessment of quality

Abstract

The quality of compost made from dewatered sewage sludge, sawdust (SD) and organic wastes of a potato-processing industry (OW), in terms of chemical and biological properties, was assessed. Mixtures of the sludge, SD and OW were composted for 57 days in insulated containers at two C:N ratios (∼ 30:1 and ∼ 20:1) and SD textures (coarse- and fine-textured SD). The parameters monitored over this period were pH, electrical conductivity (EC), C:N ratio, CO2 evolution and two spectrophotometric ratios (Q 2/6 and Q 2/4). All the studied parameters were in general similarly influenced by initial C:N ratio and texture of SD except for EC, Q 2/6 and Q 2/4. At high C:N ratio of both textures, the EC of the final products increased but were less than those of low C:N ratio of both textures. Thus, final product can be used alone as growth medium without the need for grinding or blending with other materials. The spectrophotometric ratios (Q 2/6 and Q 2/4) dramatically decreased two weeks after composting and then slightly increased at the end of composting process. However, coarse-textured SD at the low C:N ratio and fine-textured SD at both C:N ratios resulted in lower Q 2/6 and Q 2/4 ratios, reflecting a better degree of aromatic condensation and organic matter humification. Considering these parameters, co-composting sludge with fine-textured SD and OW at high initial C:N ratio would represent the best compromise.