Functional Relationships of Macroinvertebrates in Heterotrophic Systems with Emphasis on Sewage Sludge Decomposition

The general functional roles of macroinvertebrates in heterotrophic systems are outlined with special reference to sewage sludge decomposition. Specific energetic parameters of an isopod, Oniscus asellus, and an earthworm, Eisenia foetida, and four sludges were determined under a variety of conditions. Oxygen consumption rates for O. asellus, E. foetida, and sludge were measured from 5° to 30°C and found to fit an exponential formulation expressed as Q₁ ₀. The earthworm had a 39% lower respiration rate when food was present. The respiration rate of sludge was directly proportional to its moisture content although the history of the moisture regime also affected oxygen consumption with recently wetted sludge having the highest rate. The aging of sludge brought about an exponential decline in respiration rate. Egestion rates for O. asellus and E. foetida were determined gravimetrically from 5° to 37°C. Egestion was maximal at 20°C for O. asellus and 15°C for E. foetida. The effect of temperature on egestion was formulated as a modified Krogh curve. Moisture above a threshold level was found to have no effect on egestion. Sludge type had a market effect on egestion rate with one sludge even being toxic, although egestion rates were generally similar to those found for other substrates. Sludge egested from the macroinvertebrates had a respiration rate 2.82 times greater than non—eaten sludge. Using the derived functional relationships and constant, along with published values of assimilation efficiency, a FORTRAN simulation model INDEC was constructed for predicting the effect of macroinvertebrate feeding on decomposition. Model predictions were compared with values obtained from sludge—invertebrate microcosms, and a high correlation (r = .916, N = 60) was found. The importance of macrocoinvertebrates in stimulating biological activity in heterotrophic systems was discussed and related to specific parameters of the INDEC model. The roles of specific energetic conversions, such as secondary production, in affecting decomposition processes were demonstrated.