Sustainable Management of Sewage Sludge via Vermicomposting: Synergistic Effects of Hazelnut Husk and Cow Manure on Microbial Dynamics

This study was conducted to investigate the stabilization and transformational potential of anaerobically digested sewage sludge (SS) through vermicomposting, using hazelnut husk (HH) and cow manure (CM) as bulking agents and carbon sources. The research focused on the performance of the epigeic earthworm Eisenia fetida in managing these organic wastes and monitored the resulting changes in microbial dynamics. An incubation experiment was established with eleven different feed mixtures, ranging from 0% to 100% SS proportions by weight. The mixtures were incubated under controlled laboratory conditions at 20 °C in total darkness for a period of 90 days, with moisture levels maintained at 80% of the water-holding capacity. Microbial activity indicators, specifically basal soil respiration (BSR) and microbial biomass carbon (Cmic), were systematically analyzed at 15-day intervals (days 15, 30, 45, 60, 75, and 90). The experimental results revealed that Eisenia fetida could not maintain viability in mixtures containing 60% or higher doses of sewage sludge, identifying a critical toxicity threshold for the vermicomposting process. Statistical analysis showed significant differences (P<0.001) in microbiological properties across all feed rations. Both BSR and Cmic levels reached their maximum values on the 30th day of incubation, followed by a gradual decline, indicating the onset of organic matter stabilization. The study concluded that mixing sewage sludge with HH and CM at proportions below 50% SS provides an ideal environment for earthworm activity and microbial proliferation. These findings suggest that the produced vermicompost serves as a sustainable and efficient soil conditioner for land restoration and agricultural practices.