Abstract:
The combination of microalgae-based wastewater treatment with biomass production is
currently considered as a viable option for wastewater remediation and biofuel production.
The objective of this study was to evaluate the potential of Scenedesmus sp. for the
treatment of anaerobically digested brewery effluent and carbohydrate accumulation for
bioethanol production. The Scenedesmus sp. was cultivated on unsterilized and sterilized
brewery effluents under maximum light intensity of 5500 lux and a photoperiod of 12:12
light-dark cycle at room temperature to evaluate its growth, biomass production and
nutrient removal. The microalgal biomass obtained from brewery wastewater was used for
bioethanol production after pretreatment and optimization of carbohydrate and reducing
sugar extraction. The concentrations of COD, NH4
+-N, TN, PO4
3--P, and TP in brewery
effluent were found above the permissible discharge limit of brewery effluent set by
Ethiopian Environmental Protection Authority. The Scenedesmus sp. achieved a maximum
biomass production of 1.05 g/L and biomass productivity of 64.33 mg/L/d on unsterilized
effluent. The maximum removal efficiencies obtained were 99.89% NH4
+-N, 96.14% TN,
and 67.77% PO4
3--P on sterilized effluent, and 69.32% TP and 77.78% COD on
unsterilized effluent. The final effluent quality of COD and nitrogen nutrients were below
the permissible discharge limit for Ethiopia brewery effluent standard, but phosphorus
nutrient is above the permissible limit. Results showed that microwave pretreatment with
HCl produced a higher total carbohydrate of 207.7 and 222.6 mg/g and reducing sugar of
146.8 and 159.2 mg/g compared to autoclave and oven pretreatments from the whole and
lipid extracted microalgal biomasses, respectively. Optimizations of four independent
variables (HCl concentration, microwave power, temperature, and extraction time) were
performed by response surface methodology (RSM) for the whole microalgae biomass and
single-factor approach for lipid extracted biomass. The models derived from RSM for both
responses were significant (P<0.05) and fitted (R2 > 0.9) to the experimental value. The
predicted values obtained at optimum conditions were 260.54 mg/g for carbohydrates and
175.49 mg/g for reducing sugar. The experimental values under the optimum conditions
were 259.9 mg/g for carbohydrate and 172.5 mg/g for reducing sugar, showing a good
agreement with the predicted values. The highest carbohydrate and reducing sugar
contents achieved from lipid extracted biomass were 277.24 ± 0.98 and 192.54 ± 1.37
mg/g, respectively, under optimum conditions. The maximum bioethanol yield was
obtained at 24 h fermentation time from both biomasses. However, the bioethanol yield
obtained from lipid extracted microalgal biomass was 0.1 g/g microalgae, which was 20%
higher than that found from the whole microalgal biomass. From this study, it can be
concluded that the use of indigenous microalgae for wastewater treatment couple with
biomass production offer a promising results for wastewater remediation and bioethanol
production.