Decomposition of Sewage Sludge in Drying Beds and the Potential Role of the Earthworm, Eisenia foetida

Abstract

AbstractDecomposition of sewage sludge in drying beds at Ley Creek (LC) and Meadowbrook‐Limestone (MB) Wastewater Treatment facilities in metropolitan Syracuse, N.Y. was monitored. LC and MB beds were derived from an anaerobically digested, primary sludge and an aerobically digested, activated sludge, respectively. Fluxes of O2, CH4, and CO2, as determined by in situ incubation of cylinders and analysis by gas chromatography, showed that at LC, aerobic and anaerobic, decomposition and photosynthesis occurred concomitantly due to high moisture. Similar analyses for two separate trials at MB found that both total decomposition rate and percentage of anaerobic decomposition (1 to 90%), as indexed by CH4 evolution vs. O2 consumption, decreased with time and were inversely related to sludge moisture content. O2 consumption rates, when standardized to 15°C, ranged from 10 to 80 µliter/g dry wt. For both LC and MB sludge, aerobic and anaerobic bacteria were abundant (3 × 107 to 6 × 108 colony‐forming units/g dry wt) and dominant bacteria were not enteric. Nematode densities ranged from 7 to 814 individuals/g dry wt.A computer simulation model on the role of macroinvertebrates in decomposition was used for analyzing the effects of the earthworm, Eisenia foetida. This oligochaete was introduced into one‐half of the MB cylinders and was shown to accelerate decomposition and decrease the proportion of anaerobic decomposition if the sludge was below 375% moisture dry weight and 40% anaerobic decomposition. This stimulation was also reflected in increased nematode density and elevated Eh. Flooding of drying beds caused mortality to E. foetida.