Limnological aspects of small sewage ponds

Four small-scale sewage ponds were linked in series and fed settled sewage for 21 months. Inorganic nutrient (NH 3-N) and (PO 4-P) removal was primarily by biological uptake and export as algal material. NH 3-N was reduced by 82% in the pond system, half of this removal according to chlorophyll α and total suspended solids data occurring in pond 1. Up to 50% of the influent PO 4-P concentration was removed in the pond system, removal rates of between 10–20% occurring, respectively, in three of the four ponds. Uptake by and export as algal biomass indicated that between 16–65% of the PO 4-P removed was by these processes in the ponds. Chlorophyll α and total suspended solids concentrations were significantly correlated for ponds 2, 3 and 4, suggesting that the majority of solids exported from the ponds was algal material. Laboratory algal bioassays indicated that ammonia toxicity was present in the settled sewage inflow and pond 1 water. The ponds were highly productive according to productivity values calculated from diurnal oxygen curves. Winter and spring productivity values were on the whole greater than those in summer. Production/respiration ratios of the ponds were close to 1, suggesting that these ponds were relatively robust and stable systems. The flagellated algal genera, Euglena, Lepocinclis and Chlamydomonas were dominant in association with Micractinium. Zooplankton biomass was low in ponds 1, 2 and 3, probably because of an exclusion effect of high pH and NH 3-N concentrations. In pond 4, zooplankton biomass peaked during dominance of flagellated algae.