Enhancing harvestable algal biomass production in wastewater treatment high rate algal ponds by recycling : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering at Massey University, Palmerston North, New Zealand

Abstract

High Rate Algal Ponds (HRAPs) are an efficient and cost-effective system for wastewater treatment and produce algal biomass which could be converted to biofuels. However, little research has been conducted to improve harvestable biomass production from these ponds. Laboratory and small-scale outdoor research reported in the literature indicates that selective biomass recycling is partially effective at controlling algal species in HRAP. This, therefore, offers the potential to select and maintain a rapidly settleable algal species. To date, algal species control of similarly sized, co-occurring algae has not been demonstrated in wastewater treatment HRAPs. Furthermore, the influence of algal recycling on biomass harvest efficiency, harvestable biomass productivity, net biomass energy yield and the growth of the dominant algal species in the HRAPs have never previously been investigated. The main hypothesis of this Ph.D. was: ‘Recycling a portion of gravity harvested biomass (‘recycling’) back into the HRAP improves harvestable biomass production’. To test this, a series of experiments was conducted using pilot-scale wastewater treatment HRAPs, outdoor mesocosms and laboratory microcosms. Firstly, the influence of recycling on species dominance and biomass harvest efficiency was investigated using two identical pilot-scale HRAPs over two years. This pilot-scale study showed that recycling promoted the dominance of a rapidly settling colonial alga, Pediastrum boryanum, and maintained its dominance over the two year experimental period. Moreover, P. boryanum dominance was relatively fast to establish and was then stable and sustainable between seasons. The higher dominance of P. boryanum in the HRAP with recycling improved biomass harvest efficiency by gravity sedimentation from ~60% in the control HRAP without recycling to 85%. Unexpectedly, recycling also improved the ‘in-pond’ biomass productivity by 20%. The combination of the increased biomass productivity of the HRAP and the increased biomass harvestability with recycling improved the ‘harvestable biomass productivity’ by 58%. Overall, recycling increased the net biomass energy yield by 66% through the combined improvements in biomass productivity, harvest efficiency and a small increase in algal biomass energy content. To determine the reproducibility of these findings and investigate the mechanisms responsible, twelve outdoor mesocosms were studied. This mesocosm research repeatedly confirmed that recycling can establish P. boryanum dominance, and improve biomass productivity and settleability. Settleability was not only found to be improved by recycling the ‘solid’ fraction of the harvested biomass but also by recycling of the ‘liquid’ fraction, potentially indicating the presence of extracellular polymeric substances. Several possible mechanisms to explain the increase in biomass productivity were identified. However, after review all but two were discounted: (i) the mean cell residence time (MCRT) was extended thereby increasing the algal concentration and thus allowing better utilization of incident sunlight; and (ii) the relative proportion of algal growth stages (which may have different net growth rates) was shifted, resulting in an increase in the net growth rate of the algal culture. To investigate these mechanisms further, the life-cycle of P. boryanum was studied in detail and showed, for the first time in the literature, that its net growth rate does indeed vary between the three life-cycle stages (‘growth’ > ‘juvenile’ > ‘reproductive’). Given that the mesocosm studies in Chapter 4 showed that recycling increased the number of growth colonies by ~2-fold and juvenile colonies by ~4-fold then it is proposed that mechanism (ii) does appear to be viable. This Ph.D. work has demonstrated that recycling a portion of gravity harvested biomass could be a simple and practical method to enhance biomass productivity, harvest efficiency and energy content, which contribute to achieve higher ‘harvestable biomass productivity’ and ‘energy yield’ in wastewater treatment high rate algal ponds.