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

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 (similar to 30 : 1 and similar to 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.