|Title||The removal of faecal coliforms in waste stabilization pond systems and eutrophic lakes|
|Source||Wageningen University. Promotor(en): H.J. Gijzen, co-promotor(en): H.J. Lubberding. - Leiden : CRC/Balkema - ISBN 9789461735362 - 114|
|Publication type||Dissertation, externally prepared|
|Keyword(s)||fecale coliformen - afvalwater - afvalwaterbehandeling - hergebruik van water - algen - stedelijk afvalwater - faecal coliforms - waste water - waste water treatment - water reuse - algae - municipal wastewater|
|Categories||Waste Water Treatment|
The reuse of domestic wastewater presents many challenges including the risk of pathogen infection; hence the removal of pathogens from domestic wastewater is very relevant. It is known that algae play a crucial role in the process of their removal by raising the pH and dissolved oxygen concentration which tend to be injurious to bacteria. It is however not known how algal disinfection ability is affected by biomass changes in sewage of varying strengths and whether algae contribute in sedimenting faecal coliforms (FC) from the water column through attachment to their surfaces. Experiments were conducted to investigate the importance of FC attachment to algae, the effect of varying concentration of algae in sewage of different strengths on FC removal and the effect of algae on FC removal in a tropical eutrophic lake. The effect of reducing algal densities in a pilot-scale hybrid algae-duckweed pond system on FC removal was also investigated with the aim of understanding the importance of FC attachment in such a treatment system in relation to pure algal and duckweed treatment lines. Algae helped in sedimenting FC to the bottom of reactors. It was shown by experimentation under laboratory conditions that in domestic wastewater treatment an optimum algal density exists at which maximum FC removal is achieved. Algae were also important in significantly reducing Escherichia coli contamination in a eutrophic lake through increased oxygenation and pH elevation. At algal density ≤0.08mgL-1 in the Weija Lake, decay rate of E. coli was directly proportional to the chlorophyll a concentration of the lake. The strength of domestic wastewater undergoing treatment may also affect the rate of decay of FC, particularly as algal concentration changes. In darkness, higher algal biomass (or chlorophyll a concentration) resulted in higher inactivation of FC although dissolved oxygen concentration and pH were low suggesting a role by another factor in the inactivation of FC. At algal densities ≥ 13.9mg L-1 higher removal of FC occurred in MSW (medium strength wastewater) compared with LSW (low strength wastewater) whether in light or in darkness. The highest rate of decay in LSW occurred at 3.2mgL-1 chlorophyll a concentration in light while that of the MSW occurred at 20.0mgL-1 in light. Addition of raw wastewater to an ongoing wastewater treatment process lowered the rate of FC removal for a wide range of algal densities (0.6 – 19.6mgL-1 chlorophyll a concentration), under light conditions. The hybrid pond system performed well in FC (4.3 log units) and Biochemical Oxygen Demand (BOD) (89%) removal and these parameters in addition to total phosphorus were not affected by seasonal changes. FC attachment to suspended matter was important only in the first two ponds of the duckweed, algal and hybrid pond systems. Little variation of FC decay with depth was observed. FC decay rates in the mornings were usually lower than in the afternoons in algal ponds but not in the duckweed ponds. High densities of macro-invertebrates belonging to the class Ostracoda were associated mainly with the surface and bottom of duckweed ponds and these were much higher than that of algal ponds at similar locations. FC numbers in duckweed ponds correlated strongly and positively with mean ostracod numbers in ponds. FC numbers also correlated well with Shannon-Wiener diversity index of macro-invertebrates in all the three pond systems. Integrating a hybrid pond system such as this for aquaculture would be a big boost economically and health-wise for communities in developing countries with warm tropical conditions.