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This is the accepted version of a paper published in Algal Research.
Citation for the original published paper (version of record):
Nordlander, E., Olsson, J., Thorin, E., Yan, J. (2017)
Simulation of energy balance and carbon dioxide emission for microalgae introduction in wastewater treatment plants.
Algal Research, 24(part A): 251-260
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Permanent link to this version:
http://urn.kb.se/resolve?urn=urn:nbn:se:mdh:diva-36513
A model for the simulation of energy balance for microalgae introduction in wastewater treatment plants
Eva Nordlander *, Jesper Olsson , Eva Thorin , Emma Nehrenheim
The School of Business, Society and Engineering, Mälardalen University, Box 883, SE-721 23 Västerås, Sweden
Appendix - Sensitivity analysis
A sensitivity analysis was conducted to test the robustness of the solution. All
parameters related to the microalgae were tested (CO2 absorption by microalgae, NH4 reduced by microalgae, P reduced by microalgae, Minimal quanta required to liberate O2 for sunlight) as well as the observed yield for bacterial biomass (Yobs).
Figure 1 a-b Sensitivity analysis for ability of microalgae to reduce P (a) and N (b). Each parameter is changed by ±50% and the resulting change in the calculation outputs is shown (outputs that did not change were not included in the figure). Outputs whose results are indistinguishable from each other are grouped together
Each parameter was individually changed to +/- 50% of its original value and the effect on calculated CO2 emission, heat and electricity consumption and biomass concentration was examined for each WWTP. A surface factor of 12 was used for the sensitivity
analysis since the impact of the parameter on the result increases with the surface factor. The results are shown in Figures 1-3. These results were insensitive to changes in ability of the microalgae to remove P and N, as demonstrated in Figure 1 a and b. Only the net electricity consumption for each WWTP shows a change of more than 1% as the N reduction capability of the microalgae is varied between -50% to +50%. The
parameters Yobs and CO2 absorption ability of the microalgae have a large impact on
-50 0 50
% -8
-6 -4 -2 0 2 4 6 8
%
10-3 a) P red. microalgae Västerås Net el. consump.
Eskilstuna Net el. consump.
Uppsala Net el. consump.
-50 0 50
% -4
-3 -2 -1 0 1 2 3 4
%
b) N red. microalgae
CO2 emission summer CO2 emission Net el. consump.
some of the calculated outputs. In particular, the CO2 absorption by the microalgae has a large impact on CO2 emissions from the plant. These emissions also depend on operating conditions, as shown in [1] where they varied from 1.5 to 1.9 gCO2 g-1 microalgae
depending on light wavelength.
Figure 2 a-b Sensitivity analysis for changes in the observed yield of bacteria biomass (a) and ability of microalgae to absorb CO2. Each parameter is changed by ±50% and the resulting change in the output of the calculation is shown (outputs that do not change are not included in the figure). Outputs whose results are indistinguishable from each other are grouped together
The observed yield of the bacteria biomass mainly affects the biomass concentration in the basin. The bacteria make up a majority of the biomass and it is therefore not
surprising that changing the Yobs has an almost 1:1 effect on the biomass concentration.
The oxygen yield per microalgae and minimal quanta need to liberate O2 are the two parameters with the largest influence on the calculations. The quanta needed to liberate O2 can be particularly difficult to determine since it is affected by the operating
conditions. If biomass concentration in the reactor becomes too high and/or the stirring is not sufficient the quanta need would be expected to increase since more sunlight will be dissipated on its way to the microalgae. If the quanta need is too high the microalgae population may become too small to have an impact on the system. However, it should
-50 0 50
% -50
-40 -30 -20 -10 0 10 20 30 40 50
%
a) Yobs
Biomass conc.
CO2 emission CO2 emission summer Västerås net heat consump.
Eskilstuna/Uppsala net heat consump.
-50 0 50
% -40
-30 -20 -10 0 10 20 30 40
%
b) CO2 absorped by microalgae CO2 emission CO2 emission summer
be noted that according to the calculations the biomass concentration in the basins is on the same order as those usually found in photobioreactors [2].
Figure 3 a-b Sensitivity analysis for the microalgae yield per O2 (a) and the quanta need to liberate O2 for sunlight. Each parameter is changed by ±50% and the resulting change in the calculation outputs is shown (outputs that did not change are not shown). Outputs whose results are indistinguishable from each other are grouped together
References
[1] T.-H. Kim, Y. Lee, S.-H. Han, S.-J. Hwang, The effects of wavelength and wavelength mixing ratios on microalgae growth and nitrogen, phosphorus removal using
Scenedesmus sp. For wastewater treatment, Bioresource Technol. 130 (2013) 75–80 [2] D. Vandamme, I. Foubert, K. Muylaert, Flocculation as a low-cost method for
harvesting microalgae for bulk biomass production, Trends Biotechnol. 31 (2013) 233- 239.
-50 0 50
% -40
-30 -20 -10 0 10 20 30 40
%
a) Microalgae biomass per O2 Eskilstuna/Västerås biomass conc.
Uppsala biomass conc.
CO2 emission CO2 emission summer
Eskilstuna/Västerås Net heat consump.
Eskilstuna Net el. consump.
Uppsala Net heat consump.
Uppsala/Västerås Net el. consump.
-50 0 50
% -80
-60 -40 -20 0 20 40
%
b) Min. quanta need
Eskilstuna/Västerås biomass cont.
Uppsala biomass conc.
CO2 emission CO2 emission summer Västerås Net el. consump.
Eskilstuna/Västerås Net heat consump.
Eskilstuna Net el. consump.
Uppsala Net heat consump.
Uppsala Net el. consump.
