J. Bontoux

Occurrence and removal of Giardia sp. cysts by Kenyan and French waste stabilisation pond systems

A study was conducted to determine the occurrence and removal of Giardia sp. cysts in municipal wastewater by two waste stabilisation pond systems; one at Eldoret, Kenya and one at Meze, France. A total of 71 and 26 grab raw wastewater samples from Eldoret and Meze, respectively, were examined for the presence of cysts. Cysts were detected in 37% (26) and 100% (26) raw wastewater samples, respectively. Cyst concentrations detected in raw wastewater samples ranged from 1×10 3 to 2.5×10 4 cysts/l at Eldoret and 230 to 2.5×10 4 cysts/l at Meze. At Eldoret, all anaerobic (8/8), secondary facultative (5/5) and primary maturation (5/5) pond effluent samples examined were positive for Giardia . Cyst concentrations ranged from 1.25 to 483 cysts/l (anaerobic), 23.5 to 192.5 cysts/l (secondary facultative) and 17.2 to 118.5 cysts/l (primary maturation), respectively. At Meze, cysts were detected in 12 of 12 primary facultative (range 4.4 to 48 cysts/l) and 9 of 11 (82 %) primary maturation pond effluents (range 0.2 to 16 cysts/l). In final maturation pond effluent, Giardia cysts were detected in 100% of final pond effluents discharged from Eldoret (n=10; range 21 to 90 cysts/l) and 7 of 16 (44%; range 0.1 to 2.5 cysts/l) from Meze. The occurrence of cysts in final pond effluents examined would indicate that cumulative retention periods of up to 25.3 days at Eldoret and 40 days at Meze would be sufficient to remove between 99.1% and 99.7% of cysts, respectively, but that such retention periods are insufficient to ensure the complete removal of Giardia sp. cysts.

Modelling orthophosphate evolution in a high rate algal pond

High rate algal ponds (HRAPs) are an efficient treatment for controlling wastewater pollution by reducing the organic matter and the inorganic nutrient content. An experimental HRAP was set up in Meze (France) and sampled over 24 months. A model simulating orthophosphate (PO4) evolution was constructed using Stella II software. It was thought that deterministic modelling of the temporal evolution of PO4 might provide a rational basis for pond management policies. In hardwater, two processes have to be taken into consideration: PO4 uptake by algae and PO4 precipitation. The model also has to take into account the fact that the latter process is very sensitive to nycthemeral variations in pH. Simulated PO4 concentrations over a two-year period show that about 10% of total input is removed by precipitation whereas about 30% is removed by absorption. These results are of critical importance for the management of this type of wastewater treatment system.

Some difficulties in modelling chlorophyll a evolution in a high rate algal pond ecosystem

Abstract The High Rate Algal Pond (HRAP) is an efficient treatment for controlling wastewater pollution by reducing the organic matter and the inorganic nutrient content. Deterministic modelling of temporal evolution of algae could provide a rational basis for pond management. An experimental HRAP was set up in Meze (France) and sampled weekly over 24 months. Models simulating the evolution of chlorophyll a concentration and nutrients (N and P) were constructed using Stella II software. The seasonal pattern of chlorophyll a concentrations results from the annual cycle of solar irradiance and temperature, whereas shorter trends (1 to 4 weeks) are dependent on the evolution of zooplankton groups. The first difficulty is to determine the functional relationships of the phytoplankton and zooplankton groups. In the model the evolution of the phytoplankton taxa is considered to be dependent on (i) inherent parameters of phytoplankton taxa (mortality rate, growth rate, saturating light intensity, etc.) and on (ii) parameters of zooplankton taxa (filtration rate, size selectivity, etc.). To take all these taxa as state variables, and all the associated parameters, into account is impossible: to solve such a problem, we forced the evolution of the biomass of the phytoplankton and zooplankton taxa. This approach improves the agreement between the simulated and observed chlorophyll a concentration values. The second difficulty concerns the determinism of appearance of the phytoplankton and zooplankton taxa used in the model: up to now we are only able to force these appearances. Thus, even in quite simple ecosystems, using deterministic modelling as a predictive tool requires a full understanding of the exact biological succession and interaction processes.