This thesis covers work carried out on algae bioreactors as a tertiary treatment process for
wastewater treatment. The process was primarily assessed by the removal of Phosphorus
and Nitrogen as an alternative to chemical and bacterial removal.
Algal bioreactors would
have the added advantage of carbon sequestration and a by-product in the energy rich algal
biomass that should be exploited in the existing AD capacity.
Laboratory scale bioreactors were run (4.5-30L) using the secondary treated final effluent
from the local Loughborough sewage works. In a preliminary series of experiments several
different bioreactor designs were tested. These included both batch feed and continuous
flow feed configurations.
The bioreactors were all agitated to keep the algal cells in suspension. The results
demonstrated that the most effective and easy to operate was the batch feed process with
the algal biomass by-product harvested by simple gravitational settling. Experiments also
compared an artificial light source with natural light in outdoor experiments. Outdoor
summer light produced greater growth rates but growth could not be sustained in natural
UK winter light.
Light intensity is proportional to productivity and algae require a minimum of around
97W/m2 to grow, an overcast winter day (the worst case scenario) was typically around
78W/m2, however this was only available for a few hours per day during Nov-Jan. The
process would be better suited to areas of the world that receive year round sunlight.
It was shown that phosphorus could be totally removed from wastewater by the algae in
less than 24 hours depending on other operating variables. With optimisation and addition
of more carbon, a HRT of 10-12 hours was predicted to achieve the EU WFD / UWWTD
standard. It was further predicted that the process could be economically and sustainably
more attractive than the alternatives for small to medium sized works. Biomass
concentrations of between 1-2g/L were found to best achieve these removals and produce
the fastest average growth rates of between 125-150mg/L/d. The uptake rates of
phosphorus and nitrogen were shown to be dependent on the type of algae present in the
bioreactor. Nitrogen removal was shown to be less effective when using filamentous bluegreen
algae whilst phosphorus removal was almost completely stopped compared to
unicellular green algae that achieved a nitrogen uptake of 5.3mg/L/d and phosphorus
uptake of 8mg/L/d. Soluble concentrations of Fe, Ni and Zn were also reduced by 60% in
the standard 10 hours HRT.
The predominant algae were shown to depend largely on these concentrations of
phosphorus and nitrogen, and the strain most suited to that specific nutrient or
temperature environment dominated.
Nutrient uptake rates were linked to algal growth rates which correlated with the
availability of Carbon as CO2. CO2 was shown to be the limiting factor for growth; becoming
exhausted within 10 hours and causing the pH to rise to above 10.5. The literature showed
this was a common result and the use of CO2 sparging would more than double
performance making this process a good candidate for waste CO2 sequestration. Heat
generated from combustion or generators with exhaust CO2 would also be ideal to maintain
a year round constant temperature of between 20-25°C within the bioreactors. A number of
possible uses for the algal biomass generated were examined but currently the most
feasible option is wet anaerobic co-digestion.
Further economic analysis was recommended on the balance between land area and
complementary biomass generation for AD. It was also suggested given the interest as algae
as a future fuel source, the process could also be adapted for large scale treatment and
algal biomass production in areas of the world where land was available.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.