Size matters – Microalgae production and nutrient removal in wastewater treatment high rate algal ponds of three different sizes

Abstract

High rate algal ponds for coupled wastewater treatment and resource recovery have been the focus of much international research over the last 15 years. Microalgal biomass productivity reported in full-scale studies (1-ha or greater) have often been substantially lower than that reported from smaller scale ponds in similar climates, regardless of the season or the dominant microalgal species used. The disconnect between smaller-scale and full-scale productivity is unclear and uncertainty remains regarding the applicability of smaller scale studies to full-scale systems. In order to better understand the differences in reported productivity, the performance of three different size wastewater treatment high rate algal ponds (5 m2, 330 m2 and 1-ha) were assessed with respect to nutrient removal and microalgal productivity over three seasons. Both daily areal nutrient removal and biomass production were affected by the size of the pond. NH4-N removal via nitrification/denitrification decreased with increasing pond size, with the highest removal rate in the 5 m2 pond and the lowest in the 1-ha. Microalgal areal productivity was maximal in the 330 m2 pond, suggesting that a combination of mixing frequency and higher photosynthetic potential under low light conditions were the main drivers of enhanced productivity in this pond compared to the 5 m2 (mesocosm) and 1-ha (full-scale) ponds. The lowest daily nutrient removal and biomass production occurred in the 1-ha (full-scale) pond. Our results suggest that, based on the current design and operation of high rate algal ponds, the optimum size for maximum productivity is considerably smaller than the current full-scale systems. This has implications for commercial scale systems, with respect to capital and operational costs.