The novel KiteMix system for anaerobic wastewater treatment ponds tested at the pilot-scale under varied substrate viscosity and mixing velocity.

Pond systems represent the simplest and most widely used technology for treating high-strength wastewater containing biodegradable suspended solids. When covered, they offer advantages such as odour control, intensified organics degradation, and biomethane capture. However, their efficiency is often limited by unmixed zones and the formation of floating or sinking layers, which reduce residence times and treatment performance. Here, we developed a novel mixing concept for anaerobic pond systems and systematically tested its mixing efficiency. The novel mixing concept avoids permanently installed mechanical components and instead relies on a planar, kite-like mixing tool that is moved horizontally through the pond by an external rope-guided system. This design enables flexible, low-maintenance operation with minimal energy input and is particularly suitable for shallow, large-scale ponds where conventional submerged mixers are impractical. Three different mixing tool designs were evaluated using dye and conductivity tracer experiments with model substates in a 330 L pilot-scale pond. All tools were based on perforated planar plates with identical open area ratio (44 %), but differed in hole geometry. The effect of substrate viscosity was assessed at two distinct velocities. Results showed that increasing viscosity significantly prolonged the mixing time, while doubling the mixing velocity reduced it by a factor of four. The mixing tool design strongly impacted flow patterns and therewith the mixing efficiency. Findings were integrated into an operation scheme for full-scale anaerobic pond systems equipped with planar mixing tools that accounts both for the mixing performance and the economic efficiency.