Limnological aspects of small sewage ponds

Four small-scale sewage ponds were linked in series and fed settled sewage for 21 months. Inorganic nutrient (NH3-N) and (PO4-P) removal was primarily by biological uptake and export as algal material. NH3-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 PO4-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 PO4-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 NH3-N concentrations. In pond 4, zooplankton biomass peaked during dominance of flagellated alga

Fish production in small oxidation ponds

Four 10 m3 pilot scale, oxidation ponds linked in series were stocked with Clarias gariepinus (first three ponds) and on various occasions, combinations of Oreochromis mossambicus; Hypopthamichthys molitrix, Cyprinus carpio and Labeo umbratus were stocked in the fourth and last pond. During summer, O. mossambicus and C. carpio had average relative daily growth rates of 1.4 and 2.1%, respectively, while in winter both H. molitrix and L. umbratus lost weight and C. carpio grew slower. Small C. gariepinus ( 300 g) grew during summer at 0.5% body wt d-1. Extrapolated yields of 3-4 tonnes ha-1 of C. gariepinus and 2-3 tonnes ha-1 of O. mossambicus and C. carpio could be harvested from large scale oxidation pond systems. Low winter temperatures and low dissolved oxygen concentrations during spring restrict production. Mass mortality of fish occurred during summer probably due to oxygen deficiency and high unionized ammonia concentrations. Large reductions in Total Suspended Solids, Chlorophyll a and Chemical Oxygen Demand were achieved in the fourth pond though what effect fish had on these reductions was not conclusively determined. The possible role of fish in eutrophic water bodies is discussed

Recirculation in a small sewage pond system

Recirculation in a small scale waste stabilization pond system operated over a period of 12 months. Effluent from the third pond in a series of four ponds was added to the primary pond at a ratio of 1:1 (recirculated effluent:settled sewage effluent). Recirculation was stopped during winter for 35 days and during summer for 21 days to ascertain the seasonal benefits of recirculation. Dissolved oxygen concentration in pond 1 increased by up to 234% during recirculation in summer, reducing the potential of algal photosynthesis inhibition by high concentrations of ammonia-nitrogen. Algal cell numbers did not increase significantly in pond 1 during recirculation. Recirculation was unable to reduce Total Suspended Solids (TSS) and Chemical Oxygen Demand (COD) concentrations of the final effluent to meet the Government effluent standards of 25 and 75 mg 1-1, respectively. An increased recirculated effluent:settled sewage ratio is recommended