Conclusions
In this work, the effects of temperature and photoperiod on
dissolved nitrogen and phosphate removal, microalgal growth and
productivity were studied. A clear understanding of the abiotic and
biotic mechanisms involved is required in order to improve HRAP
performances in terms of biomass productivity and wastewater
treatment. An increase in temperature from 15 to 25 C led to a
slightly higher apparent specific growth rate, an increase in the
microalgae concentration and the biomass productivity. Neither
growth, nor metabolic activity was detected for cultures at 5 C.
The apparent specific growth rate was mainly influenced by
temperature and not by photoperiod duration.
High nitrogen depletion levels (between 72 and 83%) were
measured in our study. The majority (72–81%) of the nitrogen in
the wastewater was found in the form of ammonium. The highest
ammonium removal rate was determined at 25 C. The biomass
nitrogen content obtained under different conditions (temperature
and photoperiod) was similar, suggesting identical nitrogen
assimilation abilities. Nitrogen assimilation by the biomass
contributed to nitrogen removal from the wastewater (50
2%
of the total nitrogen). Ammonia stripping seems one of the
mechanisms responsible of nitrogen elimination, contributing to at
least 17% of N-removal. Nitrification was also a pathway for
ammonium transformation in our experimental system.
All phosphate ions were removed from the wastewater after
approximately 100–150 h. The highest rate of phosphate removal
was measured in cultures at 25 C and 12 h photoperiod together
with 15 C and 18 h photoperiod. The phosphate concentration
profile suggests a peculiar trend: a theoretical adsorption of
phosphate on the cell surface, followed by consumption associated
with microalgal growth and
finally chemical precipitation due to
the high pH values.
Under our conditions the nitrogen and phosphate depletion
rates were high and that the system did not seem to require organic
carbon supply for this process. This could be attractive for the
treatment of secondary effluents where there is little organic
carbon available. Nevertheless, further work must be carried out to
confirm these preliminary results. Studies should assess the effect
of daily and seasonal temperature and light intensity changes on
nutrient removal and microalgal growth. The use of continuous
HRAP and
finally an LCA (Life Cycle Assessment) and cost analysis
must be performed to fully confirm this system’s applicability at
wastewater treatment with an indigenous microalgae-bacteria
consortium. Moreover, mechanisms of adsorption of phosphate on
the cell surface and its assimilation by microalgae should be
further studied. In order to improve the understanding of this
complex microbial system, research is required to characterize the
microbial consortium in batch systems.
As a conclusion, microalgal treatment of wastewater, through
biological and physico-chemical mechanisms, could represent an
attractive addition to existing biological treatments used to purify
wastewaters. The advantages of using microalgae for this purpose
are: an in situ production of oxygen by microalgae for bacterial use
and the possibility of recycling assimilated nitrogen and phosphorus
as a fertilizer.