density system resulted in reduced costs and enhanced the feasibility
of using microalgae to treat wastewater. The microalgae, as
the value-added by-product of wastewater treatment, can also be
utilized as a fertilizer or as stock feed, which enhances the nutritional
recycling aspect of the wastewater treatment system (Gonzalez
et al., 1997; Hoffmann, 1998). In summary, the RN175 system
had the advantage of attaining the highest level of pollutant removal
from the wastewater and obtaining the highest biomass,
making it the most favorable of the systems investigated in this
study.
Since our research was carried out at a laboratory scale, difference
between lab-scale and real application should not be neglected
when undertaking mass scale applications. The results
also demonstrated that bacteria would reproduce in large scale
at above 340 mg/L COD, which sharply decreased the algal density
for the competence with bacteria. As the discussion mentioned before,
bacteria could not be avoided in real application due to the
high cost for sterilization. Therefore, the initial COD concentration
needs to be controlled under 340 mg/L to obtain desire treatment
effect. The study indicates that the algae–bacteria consortium is
suitable for the treatment of wastewater with high nutrition concentration
and low level of organic matters. In addition, outdoor
temperatures may fluctuate drastically so it will not be possible
to maintain an optimal range for the growth of microalgae (Morweiser
et al., 2010). It can also be expected that in such conditions
the diurnal cycle and low sunlight intensity would have a negative
impact on the growth of microalgae as well as the removal of pollutants
(Chen et al., 2011). Thus, the different conditions associated
with lab-scale culture and real application should be recognized
clearly, as this has an impact on many important factors affecting
the viability of the water treatment system.