Planar Mixing Tool for Enhanced Performance of Covered Anaerobic Pond Systems

Pond systems are the simplest and most widespread technology for the treatment of high-strength wastewater containing biodegradable suspended solids. When covered, they offer significant advantages such as odour control, intensification of the decomposition process, and the potential to capture methane as a bioenergy fuel. However, process performance is challenged by occurrence of unmixed (dead) zones, as well as the formation of floating and sinking layers lowering residence times, degradation rates, and biogas yields. Here we aimed at the integration of a novel mixing concept for covered anaerobic pond systems to overcome these problems. A lab-scale pond (V = 330 L) was manufactured from transparent PVC. The effect of the substrate’s apparent viscosity (1, 100 and 1,000 mPa s; at 1 s-1), hoist speed (6 and 12 cm/s) and three alternative mixing tool designs on the mixing process was evaluated in dye and conductivity tracer experiments. Results show that mixing time strongly increases with increasing substrate viscosity and could be reduced (factor 4) by doubling the hoist speed of the mixing system. The design of the mixing tool largely affects the flow conditions and needs to be adjusted to the viscosity of the substrate.