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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.
Date Published:01/02/2013
Document Type:Research Report
Document Subjects:Wastewater Management - Sewers
Document Keywords:Municipality
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2005/1/12
ISBN No:978-1-4312-0309-3
Authors:Pocock G; Joubert H
Project No:K5/2005
Originator:WRC
Organizations:Africa Remediation Technologies (Pty) Ltd
Document Size:9 345 KB
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