Page 784 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactors 739

Bulking sludge is a not an uncommon problem and has been 23.3.1.1 Structure
studied extensively since the 1960s by Professor David Jenkins Biofilms develop on virtually all surfaces in an aqueous
at Berkeley and Dr. Michael Richard, Fort Collins, Colorado. environment. They are prone to form on surfaces adjacent to
Rising sludge may be an issue due to gas precipitation. In flowing water, where a steady nutrient supply facilitates organ-
primary clarifiers, methane and carbon dioxide are produced ism growth. Along with cellular growth, extra-cellular organic
in the sludge-thickening zone at high enough concentrations polymers are excreted which are the slime layers on surfaces
to come out of solution as bubbles and attach to sludge
(Prescott et al., 2005, p. 897). Most bacteria excrete these
agglomerations. In the final clarifier, the anoxic conditions polymers, which extend from the cell, forming a tangled matrix
may cause denitrification with the precipitation of nitrogen of fibers giving a structure to the associated biofilm (Characklis
gas, which may cause rising sludge. and Marshall, 1990, p. 4). After initial colonization by a single
species, monolayer, layers of other species may be added; as
23.3 BIOFILM REACTORS the biofilm matures, the structure becomes complex with cell
aggregates, interstitial pores, and channels. The biofilm may
Biofilm reactors include traditional trickling filters, deep-bed
evolve into a ‘‘mat’’ with appreciable thickness.
trickling filters, rotating biological disks, and sand beds designed
as biofilm reactors, also called ‘‘attached growth reactors’’
23.3.1.2 Transport of Nutrients
(Grady et al., 1999). The idea is that the biofilm that develops
The nutrients and dissolved oxygen are transported by advec-
on surfaces is the site for a reaction. The media can be almost any
tion to the vicinity of the biofilm, where the transport mechan-
material. Biofilms are found throughout the natural environment
ism changes to diffusion across a liquid ‘‘film’’ and then into
and occur spontaneously on almost any surface, e.g., rocks in
the complex macro-structure (see Chapter 18). As the biofilm
streams, pipes in water distribution systems, in soils, membranes
develops macro-dimensions, the inner structure becomes
for water treatment, granular activated carbon, etc. Biofilms are
anaerobic and may cause sloughing of discernible pieces.
important also in ‘‘passive’’ kinds of designs for water treatment,
such as the leach field of a septic tank system where the water
treatment occurs. The septic tank merely settles whatever will 23.3.2 BIOFILM REACTORS MODEL
settle; the solids degrade anaerobically. Since about the late
Appendix 23.A shows the rationale for a biofilm reactor
1980s, research has been underway to explore biofilm reactors
model starting with a mass balance equation for an infinitesi-
in the removal of natural organic matter in order to reduce
mal slice of the reactor. From the mathematical development
disinfection by-product precursors.
shown, a ‘‘final’’ equation, Equation 23.A.29, is the result,
The reactor design is a fixed bed in most cases, which means
which has been used in practice since about 1960 when
that the substrate concentration changes with bed depth. The
proposed by Eckenfelder (1961). Table CD23.6 is a spread-
qualification ‘‘most cases’’ is included because at least one
sheet model with finite elements; an excerpt is shown.
proprietary innovation has involved fluidized beads with a
biofilm. For a stationary-bed reactor, which includes virtually 23.3.2.1 Empirical Equation
all cases, the associated reactor analysis must be for a finite
As seen by Equation 23.A the substrate concentration
element (since the bed as a whole is not homogeneous).
decreases exponentially with depth. The decline increases
further with lower HLR. The model is characterized by a
23.3.1 BIOFILMS
coefficient, k, which may be fitted to operating results or
Not much was known about biofilms until about the 1970s. In pilot plant experimental measurements.
recognition that biofilms were important but little understood,

the Center for Biofilm Engineering was established in 1991 at Z
S ¼ S o exp k (23:9 )
0
Montana State University as a National Science Foundation HLR
Center of Excellence. Since then and from other sources, an
understanding of biofilms has evolved along with knowledge where,
on how to utilize them for practical purposes. In many cases S is the substrate concentration at any depth (kg sub-
3
biofilms are a problem and control is the objective. A book strate=m )
edited by Characklis and Marshall (1990) was one of the first S o is the substrate concentration at entrance to the reactor
3
to elucidate some of the characteristics of biofilms. (kg substrate=m )
1
The stoichiometric relations and Monod kinetic equations k is the reactor coefficient (s )
as developed in Chapter 22 are applicable in general to bio- Z is the depth within the reactor (m)
3 2
films. The reaction rate may be limited, however, by the HLR is the hydraulic loading rate, HLR ¼ Q=A (m =m =day)
diffusion-transport rate within the aqueous film and a pseudo
solid biofilm. The biofilm reactors are mostly stationary and A simple stationary-bed model is developed to illustrate,
depend on advective transport to the microorganism (the again, the application of the materials-balance principle, com-
reaction site). This is in contrast to the suspended growth bined with kinetics. The stationary bed could be a traditional
reactors where the reactants come into contact with each trickling filter, about 2 m depth, or any deep-bed nitrification
other by fluid turbulence, with a diffusion-transport step. biofilm reactor. The approach is generally applicable.