|Optimisation of Waste Stabilisation Ponds by Combining Duckweed-Based and Algal-Based Systems
|Expanded Title:||Waste water stabilization pond (WSP) technology is one of the most important natural methods for wastewater
treatment, especially in rural areas. It was found that while there was a wealth of information available on the
design considerations for algal pond systems, there was a lack of information on duckweed based systems,
particularly with respect to the optimal growth conditions, expected nutrient uptake rates and recommended
harvesting rates for removal of nutrients from the system. This study therefore focused on duckweed-based
treatment at a pilot scale.
The following conclusions and observations were made from the studies undertaken:
• The surface density of duckweed in the duckweed ponds is important. If too high, the plants will have limited
access to nutrients in the upper layers, and limited light, gas exchange and space to grow, reducing the potential
for nutrient uptake.
• The harvesting rate is important for the maintenance of the correct surface density and to allow for the growth of
the duckweed to reach its full potential. If the frequency of harvesting is too high, young plants will continually be
removed from the system.
• At the concentrations of nutrients tested under the artificial light conditions with low light intensity, higher
concentrations resulted in lower growth rates and wash out of the cultures at the harvesting rates tested, especially
at the lower temperatures of 13 and 18°C. Thus at lower temperatures for full scale duckweed systems it may be
necessary to dilute the influent with either final effluent of the treatment system or of the duckweed ponds
themselves through a recycle.
• At lower nutrient concentrations, where duckweed were expected to be nutrient limited, it was observed under all
temperatures and light intensities that the roots and fronds of the Lemna spp. increased in length and size.
• Duckweed preferentially take up ammonia nitrogen as a nitrogen source, rather than nitrate. Duckweed ponds
must therefore precede algal ponds, rather than vice versa, as ammonia nitrogen will be converted to nitrate
nitrogen through nitrification in the aerobic environment of algal ponds. It is also important that an anaerobic
process precede the duckweed stage, where organic material can be mineralized and ammonia-nitrogen and ortho
-phosphorus released in the bulk liquid.
• The light intensity and temperature applied to a mixed duckweed culture affected the species composition, with
Lemna turionifera being the dominant species under high light intensity in the sun, and Wolffia spp dominating
under medium light intensity in the shade.
• It is important that the duckweed layer not become mass transfer limited, as this will result in low nutrient uptake.
Introduction of turbulence in the duckweed treatment system is therefore a requirement, either by gentle
mechanical mixing or through the use of baffles.
The results of the laboratory study were applied to develop potential conceptual designs for a pilot scale trial proposing several configurations for an integrated WSP system.
|Document Type:||Research Report
|Document Subjects:||Wastewater Management - Sewers
|Document File Type:||pdf
|Research Report Type:||Standard
|WRC Report No:||2005/1/12
|Authors:||Pocock G; Joubert H
|Organizations:||Africa Remediation Technologies (Pty) Ltd
|Document Size:||9 345 KB