This section presents biomass productivity (and harvestable biomass productivity) data from 2 years of the pilot-scale HRAP studies.
3.3.3.1. Biomass productivity in Year 1 (from Jul 09-Jun 10)
In Year 1 (Figure 3.4 and data summarized in Table 3.3), recycling in HRAPr improved
biomass productivity by 20% compared with HRAPc without recycling (HRAPc: 9.2 g/m2/d;
HRAPr: 10.9 g/m2/d). However, as previously discussed, harvestable biomass productivity is
a more important parameter since it takes into account both biomass productivity and harvest efficiency. As reported in Chapter 2, since recycling also improved biomass harvest efficiency from 63% to 85%, the harvestable biomass productivity increased from 5.8 g/m2/d
(HRAPc)to 9.2 g/m2/d (HRAPr) as noted in Figure 3.4.
The benefits of recycling on harvestable biomass production are illustrated by comparison of our results with a previous study (García et al 2006) in which an outdoor experimental HRAP received domestic wastewater in Barcelona, Spain for one year (pond surface area: 1.54 m2;
water depth: 0.3 m; HRT of the HRAP: 7 - 10 days; no CO2 addition). They reported annual
average biomass productivity (as TSS) of 12 g/m2/d in the pond and TSS removal efficiency
of 77% in a subsequent gravity settler (HRT of the settler: 1-3 d). Assuming a VSS to TSS ratio of 80% (as we typically measured), approximately 10 g VSS/m2/d of biomass
productivity was achieved. This value is similar to the biomass productivity measured in our HRAPs (HRAPr: 10.9 g VSS/m2/d; HRAPc: 9.2 g VSS/m2/d) with similar solar radiation
(Barcelona: 15 MJ/m2/d (García et al. 2006); Hamilton: 14.5 MJ/m2/d). However, we had 16%
higher harvestable biomass productivity (HRAPr: 9.2 g/m2/d) than García et al (2006)
reported (7.7 g VSS/m2/d) as a result of the improved harvestability of HRAP
r effluent
(annual average of 85% in ASCr compared with 77% of García et al (2006)), despite our
ASCr having a 4-6 times shorter HRT (6-12 h depending on season) than their gravity
Figure 3.4: Biomass productivity in the HRAPs (HRAPr with recycling; HRAPc: without
recycling) and harvestable biomass productivity in ASCs in Year 1 (July 2009 to June 2010) (note: Both HRAPs were susceptible to grazing by zooplankton (e.g. rotifers, Moina sp. or Daphnia sp.), which reduced biomass concentrations when a population of Moina sp. increased to ~560 individuals/L).
Chapter 3: Enhancing biomass energy yield from pilot-scale high rate algal ponds with recycling
Table 3.3: Year 1 (July 2009 – June 2010) biomass and harvestable biomass productivities in the pilot-scale HRAPs (HRAPr:
with recycling; HRAPc: without recycling).
Winter in 2009 Spring Summer Autumn Winter in 2010
67 days 76 days 113 days 70 days 34 days
(July 1-Sept 6, 09) (Sept 7-Nov 22, 09) (Nov 23, 09-Mar 16, 10) (Mar 17-May 25, 10) (May 26-Jun 30, 10) HRAPr P. boryanum (~90%) P. boryanum (~92%) P. boryanum (~98%) P. boryanum (~80%) P. boryanum (~90%)
HRAPc P. boryanum (~70%) Micractinium sp. (>70%) P. boryanum (~90%) Unicellular algae (~45%)
Dictyosphaerium sp. (~40%) Dictyosphaerium sp. (~80%) HRAPr 6.0±1.8 8.9±3.3 13.5±3.9 6.4±3.4 2.6±1.1 HRAPc 7.2±2.8 8.4±4.5 10.6±2.6 7.3±1.9 2.3±1.1 HRAPr 4.9±1.4 7.3±2.7 12.2±3.5 4.8±2.6 2.2±0.9 HRAPc 4.9±1.9 5.3±2.8 8.1±2.0 3.3±0.8 1.3±0.6 Biomass productivity (g VSS/m2/d) (2) Dominant algae (1)
Note: (1) Algal dominance in the HRAPs was previously shown in Chapter 2
(2) Weather compensated (daily precipitation and evaporation) productivity calculated using Equation 3.2. Parameter
Days Experimental period
Harvestable biomass productivity (g VSS/m2/d)
If the HRAP is being operated at a low hydraulic retention time (HRT), the algal population (concentration) may have been insufficient to fully utilize the incident light energy and available nutrients, resulting in sub-optimal biomass production. Implementing biomass recycling separates the mean cell retention time (MCRT) from the HRT. In Year 1, recycling in HRAPr extended the MCRT so that it was longer than the HRT (by 0.5 d in summer and
3.4 d in winter, Chapter 2). Therefore, it is feasible that the increases in the MCRT and thus concentration contributed to improved biomass production in HRAPr by enabling more light
and nutrients to be utilized.
Gravity settling in ASCr selected for larger P. boryanum colonies from the HRAPr effluent
(Figure 3.5), which were then recycled back to the pond. Approximately 55% of the colonies in the harvested biomass had a diameter of >35 µm compared with only 25% in the mixed pond water (Figure 3.5). Not only does this have clear benefits in terms of improved harvest efficiency (and thus the overall harvestable biomass productivity) as discussed above, but, it may also explain the improvement in the biomass productivity in the pond itself. Research undertaken by Tukaj et al. (2003) on Scenedesmus armatus, noted that the maximum algal growth rate occurred at when the alga had reached about 80% of their full size (i.e. reproductive colonies). This indicates that there is a variation in the efficiency at which the alga can convert light energy to biomass throughout different stages of their life-cycle. If P.
boryanum exhibits similar behaviour it therefore offers a possible explanation as to why
recycling was observed to improve biomass productivity, because recycling increases the proportion of large colonies with higher net growth rates.
Chapter 3: Enhancing biomass energy yield from pilot-scale high rate algal ponds with recycling
Figure 3.5: Size distribution of P. boryanum colonies in gravity settled biomass that collected in ASCr and the HRAPr water and microscopic photos of a. the HRAPr water and
b. the settled biomass (1/100 diluted) which were recycled back to HRAPr.
3.3.3.2. Biomass productivity in Year 2 (from Jul 10 - Jun 11)
Over the seasons, biomass productivity in the HRAPs varied from 3.3 to 12.1 g/m2/d in
HRAPc and from 4.3 to 12.7 g/m2/d in HRAPr, as shown in Table 3.2.
For the five month period (Period 1) before P. boryanum was established as the dominant species in HRAPc, this pond had significantly lower biomass productivity than that in HRAPr
where P. boryanum had been established by recycling (Figure 3.6; Table 3.2). However, when both HRAPs had similar P. boryanum dominance (Period 2), similar biomass productivities (summer: 12 g/m2/d; autumn: 4 g/m2/d; winter in 2011: 5 g/m2/d, Table 3.2)
and harvestable biomass productivities (summer: 11 g/m2/d; autumn: 4 g/m2/d; winter in
Figure 3.6: Biomass productivity in the HRAPs (both with recycling) and harvestable biomass productivity in ASCs in Year 2 (July 2010 to June 2011).