Determination of design parameters for macrophyte-based and algal-based ponds in Ghana

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Waste stabilisation ponds (algal systems) have been identified as an appropriate technology for wastewater treatment in developing countries. However, final effluent quality in terms of nutrients, unfiltered BOD5 and suspended algal mass makes the effluent from these ponds difficult in meeting the increasingly stringent effluent standards. In recent years ponds with aquatic plants known as macrophyte ponds have been used either to polish effluents from facultative ponds or to treat raw wastewater. This system has the advantage of reducing the suspended algal mass, nutrients and the unfiltered BOD5 as well as using the macrophytes produced for other economic ventures especially in aquaculture. Ghana as developing country could use such a treatment system for waste water recovery. However, before such a system can be introduced, performance under local climatic conditions has to be studied, confirmed and design parameters determined before adopting the technology. Pistia (water lettuce) and duckweed have been identified as macrophytes that could be used for wastewater treatment in Ghana This thesis has determined the design parameters; detention time, BOD5 reaction rate constant, and the die-off coefficient for algal, duckweed and Pistia wastewater treatment systems. These parameters were determined under batch conditions and give a first indication of how these systems may perform under local climatic conditions. Using Environmental Protection Agency guidelines for discharge of wastewater into natural waters, effluent quality from the three systems at the end of 29 days were acceptable. A detention time of at least 28 days or more has thus, been recommended for macrophyte based systems in Ghana. BOD5 reaction rate constant determined for the three systems were greater than the 0.26d1 recommended for stabilisation ponds. Die- off constants at an average wastewater temperature of 29°C has also been determined. It has been recommended at the end of the study that, the treatment systems should be set-up as Continuous flow systems on a bench scale and the experience gained transferred to a pilot scale. Also presented in this thesis is a, preliminary design of a proposed pilot plant to be constructed at a site near the KNUST wastewater treatment plant.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Science in Water Supply and Environmental Sanitation, 1999