Comparative performance of algal and macrophytes - based wastewater batch treatment systems

The use of Waste Stabilisation Ponds (WSPs) is favoured option for wastewater treatment in developing country like Ghana. They are cheap to construct, simple and easy to operate and maintain. Aquatic plants have been found to aid the wastewater treatment process. Much research has been conducted on WSP with algae and water hyacinth. The problem with the use of these plants is how to control them from becoming an environmental problem. As a solution to such problem, other plants have been tried elsewhere. These include water lettuce (Pistia stratiotes) and duckweed (Lemna spp). Pistia and duckweed are easy to harvest and have both the added advantage of being useful agriculturally. However, not much information has been documented about Pistia and duckweed. This study was therefore, aimed at finding: • The environmental conditions in pathogen removal efficiencies of algal, Pistia and duckweed-based wastewater treatment systems on a batch scale. • The effect of deferent pH levels on faecal streptococci die-off. • The diurnal variations in the environmental conditions. • The organic matter and nutrient removal efficiencies by the systems. The environmental conditions considered were pH, temperature, dissolved oxygen (DO), total dissolved solids (TDS), conductivity, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), phosphates (P043-), nitrates (N03-), nitrite (NO2-), and ammonia nitrogen (NH3-N). The total plate counts and faecal streptococci (as pathogen indicator organism) populations were also monitored. Under the local tropical conditions, temperatures were similar among the treatment systems. Acidic conditions were created in the Pistia - based system while the algal - based system was alkaline. The duckweed- based system was found to create a neutral condition. The DO was highest in the Algal - based but lowest in the Pistia - based systems. TDS and conductivity values were highest in the algal - based and lowest in the duckweed based system. Pistia - based system produced the highest bacterial die-off and the duckweed - based system the lowest. Faecal streptococci, the removal efficiencies in the Pistia and duckweed -based systems were not significantly different (P<0.05). There was strong negative correlation between the bacteria population and pH, DO, TDS, and conductivity. The environmental conditions were found to have the highest values during the afternoons than in the mornings and evenings hence lower bacterial populations in the afternoons. The Pistia - based system was the most efficient in organic matter (BOD and COD) removal followed by the duckweed - based. On the other hand, the duckweed - based system was the most efficient in the removal of nutrients. The study of the effect of selected pH levels revealed that the pathogen die off rate constants was higher in sunlight than in the darkness (1.6401 and 1.4505 in light and dark respectively for pH 7). Similarly they Do levels in light were higher than in dark. Thus sunlight enhances pathogen die off and oxygen dissolution. The pH4 value was found to be a critical point in the survival of faecal streptococci.

Source: KNUST