Baffled primary facultative ponds with inlets and outlets set at different levels treating domestic wastewater in northeast Brazil

This study evaluates the performance of four 2.3 m deep pilot-scale, independently loaded, primary facultative ponds treating predominantly domestic sewage in northeast Brazil. The ponds contained longitudinal baffles giving different length to width ratios from 3.55 to 32.4. The ponds had mean hydraulic retention times of ~15 days, and mean surface organic loadings of 330 kg BOD₅.ha.d⁻¹ during the first experimental phase and 375 kg BOD₅.ha.d⁻¹ in the second. The vertical inlets and outlets pipes were positioned at 1.8 m and 5 cm respectively below the pond surface in the first phase and at 50 cm and 1.8 m respectively in the second. All the ponds functioned as efficient primary facultative ponds but statistical analysis demonstrated no differences in effluent quality for most of the parameters measured for the various configurations of baffles and inlet and outlet depths. All behaved similarly to the unbaffled pond. The only exceptions were suspended solids and chlorophyll a concentrations which tended to be lower for all combinations of baffles with the outlets set 1.8 m below the surface. This study suggested that the longitudinal baffling of primary facultative ponds when using vertical inlets and outlets may well not significantly improve pond performance.