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
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Date
2013
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Massey University
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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.
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Keywords
Biomass energy, High rate algal ponds, Wastewater treatment, Biofuel, Pediastrum boryanum