3D characterisation of a large high rate algal pond for wastewater treatment

The Peterborough high rate algal pond (HRAP) is a large scale (5000 m ²) wastewater treatment system using microalgae, located in South Australia. The purpose of the study was to assess the homogeneity of wastewater composition in this meandering 1 km-long, 30 cm-depth, 4 m-wide channel, mixed by paddlewheel. In situ measurements of flow velocity, dissolved oxygen, pH and contemporaneous wastewater sampling were conducted along the length, depth and width of HRAP. The mean flow velocity, 0.2 m.s ⁻¹, was conserved along the channel with deviations observed near the pond bottom and the 180° bends. The chlorophyll a was constant through the channel length. The HRAP wastewater was homogeneously over saturated with photosynthetically derived dissolved oxygen (DO >20 mg.L ⁻¹) at midday. However, solids sedimentation occurred near the bends where flow velocities decreased to <0.05 m.s ⁻¹ on the inner bank. Nitrification was occasionally incomplete and varied along the channel length and through depth. Given alkalinity and DO was sufficient, factors impacting nitrification included the relative proportion of ammonia and nitrite-oxidizing bacteria as well as the stability and/or presence of algal-bacterial flocs affecting substrate access. Nitrification showed less variability when these flocs were absent, and the algal population was unicellular/colonial. The wastewater composition within the 1 km-long HRAP was predominantly homogenous and efficiently mixed by the paddlewheel driven by an electric motor with an energy consumption of 0.85 kWh. Further investigations are needed to clarify associations amongst mixing, flocs size, nitrification and denitrification in micro anaerobic zones within flocs.