Impacts of operational conditions on oxygen transfer rate,
mixing characteristics and residence time distribution in a
pilot scale high rate algal pond
L. A. Pham, J. Laurent, P. Bois and A. Wanko
ABSTRACT
Different combinations of operational parameters including water level, paddle rotational speed and
influent flow rate were applied to investigate their impacts on mixing characteristics, residence time
distribution and gas transfer rate in a pilot-scale high rate algal pond. In closed condition, the paddle
rotational speed had a positive correlation with the Bodenstein number (Bo), water velocity and
oxygen volumetric mass transfer coefficient (k
L
a
O2
) while increasing water level generated a negative
impact on these parameters, although the impact of water level on water linear velocity was small.
The amplification effect of water level and paddle rotational speed on the sensitivity of Bo and k
L
a
O2
should be noticed. Moreover, paddle rotational speed had more impact on k
L
a
O2
than on Bo. The
study in open condition indicated that effective volume fraction had a positive correlation with inlet
flow rate and negative correlation with paddle rotation, while the opposite was observed in the case
of Peclet number. The impact of water level variation on these parameters was unclear. Both water
level and paddle rotational speed had negative impacts on the short-circuiting index, while no
correlation was observed when varying inlet flow rate. In this study, the optimal operational
conditions included low water level (0.1 m) and medium paddle rotational speed (11.6 rpm).
L. A. Pham (corresponding author)
J. Laurent
P. Bois
A. Wanko
ICube, UMR 7357, ENGEES, CNRS,
Université de Strasbourg,
2 rue Boussingault, 67000 Strasbourg,
France
E-mail: le-anh.pham@etu.unistra.fr
Key words | high rate algal pond (HRAP), mixing characteristics, oxygen transfer rate, residence time
distribution
INTRODUCTION
Microalgae have received considerable attention due to
their wide range of application. Algal biomass can be used
as a source of protein and other high value molecules for
human consumption. Their application also expands to the
field of agriculture, including fertilizer and animal feed
(Lawton et al. ), and also energy as material for biofuel
production (Voloshin et al. ). Especially, when cultured
in suitable conditions, microalgae showed a potential oil
yield of 58.7 m
3
/ha/year, while a current terrestrial plant
used for producing biofuel only reached 5.4 m
3
/ha/year
(Mata et al. ). Moreover, microalgae can use wastewater
and flue gas as nutrient sources, thus serving also as a treat-
ment unit (Muñoz & Guieysse ). Therefore, in order to
apply microalgae cultivation at large scale, many efforts
have been spent to study the use of photobioreactor systems
to culture microalgae (Muñoz & Guieysse ). Among
them, the high rate algal pond (HRAP) showed strong
advantages including low energy consumption and financial
requirement, ease of maintenance and feasibility in expand-
ing to large scale (Kumar et al. ).
HRAP is a shallow raceway-type pond with a paddle-
wheel as the only source of movement (Park et al. ).
The system was developed as a result of early intensive
studies on photosynthesis in sewage wastewater treatment
(Oswald & Gotaas ). Since then, HRAP has been
applied to treat various types of effluents such as aquacul-
ture (Posadas et al. b), domestic (Posadas et al. a),
piggery (de Godos et al. ) and industrial (Van Den
Hende et al. ) wastewaters. Moreover, the system is
also recognized for its potential as a sustainable solution
for nutrient recovery (Muñoz & Guieysse ). Besides
wastewater treatment application, it was estimated that
HRAP accounted for 95% of large scale microalgae pro-
duction facilities worldwide (Kumar et al. ).
1782 © IWA Publishing 2018 Water Science & Technology | 78.8 | 2018
doi: 10.2166/wst.2018.461
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