High-rate Pond Research Articles

The harvest of microalgae from the effluent of a sewage fed high rate stabilization pond by Tilapia nilotica

Abstract The project studied a sewage driven, combined waste treatment—recycling system consisting of three stages: a 200 m 2 high rate stabilization pond, a series of 4 m 3 concrete fish ponds, and a maize plot. A preliminary experiment was conducted in 200 m 2 earth ponds. Since the sewage was weak (mean 45 mg BOD 5 /l, mean surface organic loading 75 kg BOD 5 /ha/day) the pond was nutrient and not light limited. At a detention time of 3 days, the mean phytoplankton concentration was 94 mg/l, and the mean phytoplankton yield 15.7 g/m 2 /day, or an extrapolated yield of 57.3 tons/ha/year. A yield at least double the above yield should be attainable in Thailand without nutrient limitation in a high rate stabilization pond. The variety of dominant algal genera in the high rate pond may have been due to the weak sewage. The phytoplankton community in the high rate pond was generally stable, but collapsed on two occasions. No seasonal variations in composition of the phytoplankton community were discernible.

Algae/Bacteria Ratio in High-Rate Ponds Used for Waste Treatment

Algae, bacteria, and zooplankton were counted in samples drawn from 120- and 150-m high-rate algae ponds (those used for wastewater treatment). The fraction of nondegraded organic matter was estimated by comparing the ratio of biological and chemical oxygen demands and the bacterial, algal, and zooplankton counts to volatile suspended solids. With pond effluent quality at an acceptable level (around 18 mg of dissolved biological oxygen demand), the algae/bacteria ratio was around 1:100 or even higher, the zooplankton count was negligible, and the bacterial concentration was approximately 10 cells per liter by direct count. The data for bacteria exceeded those of earlier studies by one to three orders of magnitude.

Algae production and harvesting from animal wastewaters

High-rate ponds have been studied for many years as a means of treating wastewater to grow algal biomass which can then be harvested to produce a high protein feedstuff. Photosynthetic oxygenation by algae using solar energy is increasingly attractive as an energy-conserving alternative to conventional treatment processes such as activated sludge or aerated lagoons. Previous pond-harvesting investigations have, in most cases, employed sewage as the substrate but the use of wastewater from intensive animal production units provides greater opportunity for incorporation into an integrated system for utilising the harvested product. Earlier investigations are briefly reviewed and a pond-harvesting system for animal wastewater is described. Process variables in high-rate pond operation, experimental work in Australia with a 0·28 ha high-rate pond and a paper-precoated belt filter method of harvesting are also discussed.

Engineering aspects of wastewater treatment in aerated ring-shaped channels

Ring-shaped aerated channels may be used for wastewater treatment as oxidation ditches, aerated lagoons, or high rate ponds. The aerators utilized in these systems provide the necessary oxygen, and generate a flow velocity that will keep particulate matter in suspension. When mechanical surface aerators are used, the flow velocity and the oxygen supply rate depend on channel geometry, and the aerator rotation speed and submergence. A design procedure that will simultaneously satisfy oxygen and flow requirements is presented. It involves process kinetics, open channel flow hydraulics, and aerator performance aspects. Experiments in a pilot aeration channel, operated as a high rate pond, indicate that the minimum flow velocity for such systems is between 3·5 and 6·5 cm sec −1 when the suspended solids concentration is bout 500 mg 1 −1. The pumping efficiency of a 70-cm cage rotor aerator was found to be on the order of 4 per cent when the submergence depth is 12–17 cm. Calculations show that atmospheric reaeration, induced by high flow velocity, is not economical. On the other hand, a combination of a pump and an aerator may prove feasible in cases where flow velocity rather than oxygen supply is the limiting factor.