Aquacultural Engineering 22 2000 213 – 224
Biofilm characteristics as affected by sand size and location in fluidized bed vessels
Teresa K. Nam
a
, Michael B. Timmons
a,
, Carlo D. Montemagno
a
, Scott M. Tsukuda
b
a
Department of Agricultural and Biological Engineering, Cornell Uni6ersity, Ithaca, NY
14853
, USA
b
Freshwater Institute, Shepherdstown, WV
25443
, USA Received 5 December 1997; accepted 21 January 2000
Abstract
Biofilm structure was quantitatively analyzed to detect differences in biofilm structure due to sand size effective diameter D
10
of 0.23 and 0.60 mm and location within a fluidized-sand filter vessel. Laser scanning confocal microscopy and 3D image analysis were used to
quantify biofilm volume per unit sand area, biofilm surface area per unit sand area, and biofilm surface area per biofilm volume. Biofilms from reactor vessels using 0.60 mm sand
size had thin, smoother biofilms which was attributed to higher shear environments, in contrast to the thick, rough, porous films that were measured on the 0.23 mm sand samples.
The thicknesses of the biofilms in the small sand reactors were affected by their location within the reactor vessel, i.e. films from the base of the reactor in the more highly turbulent
zone had thinner biofilms than films on sands that were taken from a higher less turbulent location in the reactor vessel. A major finding of this study was that biofilm surface area per
biofilm volume remained fairly constant regardless of sand size and sampling location. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Biofilm structure; Fluidized sand filter; Particle size www.elsevier.nllocateaqua-online
1. Introduction
Fluidized sand beds are rapidly gaining acceptance in commercial large scale aquaculture applications primarily due to their cost advantage. Since the units
Corresponding author. Tel.: + 1-607-2552801; fax: + 1-607-2554080. E-mail address
:
mbt3cornell.edu M.B. Timmons 0144-860900 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 4 4 - 8 6 0 9 0 0 0 0 0 4 0 - 6
consist of low cost reactor vessels, conventional plumbing components, and sands — these units can be fabricated on-site at a substantially lower cost than competing
technologies Summerfelt and Wade, 1997; Timmons and Summerfelt, 1998. Designing fluidized sand beds is very dependent upon choice of sand size as the size
of the sand defines the up-flow velocity required to expand the sand bed to some pre-selected expansion percentage. Generally, Summerfelt and Wade 1997 recom-
mend expansion percentages of 50 in order to ensure that the larger sands are fluidized and do not form static pools of sand at the base of the reactor vessel that
can go anoxic. Other than the mechanics of reactor vessel design and overall nitrification rates achieved by fluidized sand beds, there is minimal if any informa-
tion on actual biofilm structure as impacted by the choice of sand size and the associated fluidization velocities. For example, the upflow velocity required to
expand a bed by 50 for an effective diameter, D
10,
sand size of 0.23 mm is 0.82 cms versus 2.94 cms for a D
10
sand size of 0.60 mm. Clearly, the turbulence and shearing action that the fluidized sand particles are subjected to would be dramat-
ically different. Whether or not this different fluid environment and the resulting mechanical shear forces have any impact on the physical properties of the biofilm
has never been described for aquaculture type waste water which is characterized by extremely low ammonia levels in contrast to municipal waste waters.
Jewell 1990 provides a lengthy discourse on biofilm structure and the impact of sand size on theoretical biomass that can be supported by a solid substrate particle.
In effect, biofilm characteristics are typically reported based upon change in volume of a substrate with and without biofilm and then calculating the ‘average’ thickness
of biofilms based upon changes in volume. This of course assumes that biofilm is uniform in its attachment and growth over the entire surface of the substrate
particle. To our knowledge, there has been no published data that describes biofilm characteristics quantitatively as impacted by sand size and fluidization characteris-
tics for low waste water strength influents such as occur in aquaculture.
We do know from previous studies that the biofilm is actually a complex structure consisting of bacterial clusters, extracellular polymers, water channels, and
voids Lawrence et al., 1991; Keevil and Walker, 1992; Korber et al., 1993; Stewart et al., 1993; de Beer et al., 1994; Wolfaardt et al., 1994; de Beer and Stoodley,
1995. It has been hypothesized that biofilm structure reflects the adaptation of unicellular organisms to physical, chemical, and communal circumstances on
surfaces Costerton et al., 1995. van Loosdrecht et al. 1997 theorized that biofilm structure is largely determined by the substrate concentration gradient at the
biofilm – liquid interface and the detachment forces working on the biofilm. Jewell 1990 assumed a maximum substrate diffusion into a biofilm of 20 mm. This
assumption could be dramatically tempered depending upon the actual structure of the biofilm.
The objective of this study was to quantitatively detect differences in biofilm structure due to sand size and location within a reactor vessel using low strength
waste waters consistent with aquaculture applications. Structural parameters of interest included biofilm volume per unit sand area, biofilm surface area per unit
sand area, and biofilm surface per biofilm volume. Differences detected would
provide insight to how biofilm structure develops and adapts to certain environ- mental conditions within fluidized sand filters.
2. Materials and methods
