Physical and hydrodynamic characteristics of a dairy shed waste stabilisation pond system

Waste stabilization pond systems are widely used to treat animal wastes under highly variable hydraulic loading regimes. These systems have received limited research attention with regard to their hydrodynamic behaviour and the potential impact of shock hydraulic loading on their performance. In this study a two-stage dairy shed waste stabilisation pond system was topographically surveyed to determine the physical shape and the theoretical hydraulic retention time (HRT) of each pond, as well as the extent of sludge accumulation in the primary pond. The primary pond was then subjected to a series of drogue tracking runs whereby weighted floating survey targets with submerged 'sails' were tracked during their movement through the pond at times of peak flow in order to characterise the hydrodynamic behaviour of the pond. The full capacity volumes of the primary and secondary ponds were calculated to be 1285 m(3) and 2391 m(3), respectively. Sludge had been accumulating in the primary pond at a rate of 0.73 m(3)/d over a period of 2.4 years and this has reduced the active treatment volume of the pond to 657 m(3). Based on mean outflow, the HRTs of the ponds were 40 d and 137 d, respectively. The drogue runs revealed a vortex-like mixing pattern within the pond with higher velocities around the perimeter of the pond between the inlet and outlet, and lower velocities in the centre of the pond. In-pond velocities seemed relatively high in comparison with those from other drogue studies of larger ponds and the surging inflow caused the formation of a flow 'jet' that potentially contributed to significant short-circuiting. The range of influence of this flow jet, however, was limited to within 15 m of the inlet, suggesting that short-circuiting would be likely to occur only under certain high inflow conditions.