It is only very rarely recognised in literature that anaerobic reactors may contain protozoa
in addition to various bacterial and archeal groups. The role of protozoa in anaerobic
degradation was studied in anaerobic continuous stirred tank reactors (CSTR) and batch
tests. Anaerobic protozoa, especially the ciliated protozoa, have direct influence on the
performance of CSTR at all organic loading rates (1–2 g CODl1 d1) and retention times
(5–10 days). The studies revealed that chemical oxygen demand (COD) removal is strongly
correlated to ciliate density in CSTR fed with oleate (suspended COD) and acetate
(soluble COD). There was no significant difference in COD removal between reactors fed
suspended COD and those fed soluble COD. However, the diversity and number of ciliates is
greater in CSTR fed with particulate feed. The mixed liquor suspended solids (MLSS)
representing biomass was significantly lower (16–34%) in CSTR with protozoa. In batch
tests, increased COD removal and methane production was observed in sludge having
ciliates as compared with sludge without protozoa. Methane production increased linearly
with number of ciliates (R2 ¼ 0.96) in batch tests with protozoa. Direct utilization of COD by
flagellates and ciliates was observed in bacteria-suppressed cultures. The technological
importance of these results is that reactors with protozoa-rich sludge can enhance the rate
of mineralization of complex wastewater, especially wastewater containing particulate
COD.