Phycoremediation is a well-known tertiary treatment process used to remove nutrients from
swine wastewaters. The produced microalgae can provide a valuable source of renewable
feedstock for biofuels and/ or food production. The objectives of this work were: (1) verify the
effects of biogas from the anaerobic degradation of raw swine manure on the growth rate of
Chlorella sp., (2) characterize cellular composition of microalgae, and (3) determine the effects
of centrifugation (EVODOS) and flocculation (tannin) harvesting methods on cellular
composition.
Lab scale photobioreactor (PBR) was maintained at room temperature (23oC), continuously
stirred under mixotrophic conditions and exposed to red LED (148.5 µmol m-2 s-1). Biogas was
purged in the headspace. A 400L tank was also used to grow microalgae, which was placed in
a greenhouse under natural sun light and stirred continuously with vertical rotor paddle.
Digestate from field scale UASB was used as nutrient (6% v/v dilution). 30% v/v (≈10 g L-1 dry
weight) of a stock microalgae consortium dominated by Chlorella vulgaris was used as
inoculum.
Microalgae specific growth rate was 3× higher when exposed to atmospheric biogas. The
increase in biomass coincided with the reduction in CO2 (6.4% uptake) and reached a plateau
once CO2 was no longer available. H2S present in the biogas was rapidly removed from the
PBR headspace. Methane concentrations remained constant throughout the experimental time
frame.
Microalgae were harvested from the tank after 11 days when ammonia and phosphorus were
completely removed (> 99.4%). Cells harvested by flocculation showed 50,3% proteins, 41%
carbohydrates and 1,3% lipid. Stress-induced mechanical centrifugation increased lipid by 3-
fold.
Despite the superior microalgae growth using biogas, unwanted CO2 and H2S are removed
which enhances the value of biomethane. The high nutritional composition of the biomass
suggests it could be recycled as processed food minimizing our dependence on crop
production.