Effects of C-to-N ratio on vermicomposting of biosolids

The role of organic carbon and inorganic nitrogen for cell synthesis, growth, and metabolism is important in all living organisms. To provide proper nutrition for earthworms during vermicomposting, carbon and nitrogen must be present in the substrates at the correct ratio. The usual practice is to arbitrarily add either a rich nitrogenous material, or a rich carbonaceous material to the feed substrate, depending on the situation, to correct for C-to-N imbalance. In addition, the conventional determination of C-to-N ratio is not always based on the proportion of each nutrient that is available for these processes, but on their absolute content in the substrate. More so, different earthworm species are impacted differently by C-to-N ratio and feed mixture type. Therefore, pilot studies are necessary to establish optimal C-to-N ratio for a specific earthworm species and a specific feed mixture. Specifically, the focus of this study was to investigate and establish a suitable C-to-N ratio for vermicomposting of fresh biosolids (activated sewage sludge) amended with paper mulch, using Eisenia fetida. An optimal stocking density of 1.60 kg-worms/m2 and an optimal feeding rate of 0.75 kg-feed/kg-worm/day (Ndegwa, P.M., Thompson, S.A., Das, K.C., 1999. Biores. Technol. 71 (1), 5–12), were used in this study. A C-to-N ratio of 25 resulted in the highest stability of the product, the best fertilizer-value of the product, and also a product with the lowest potential for environmental pollution.