McKay, Donald. "Front matter" Multimedia Environmental Models ...

An alternative and more elegant method is to set up the equations in matrix form
as described by Campfens and Mackay (1997) and solve the equations by a routine
such as Gaussian elimination. This permits complex food webs to be treated with
no increase in mathematical difficulty.
A DOS-based BASIC model Foodweb is available that performs these calculations
as described in detail by Campfens and Mackay (1997). It is an expansion of
the Fish model, and it treats any number of aquatic species that consume each other
according to a dietary preference matrix. A steady-state condition is calculated using
matrix algebra. All organism-to-organism fluxes (i.e., food consumption rates) are
given. The model is also useful as a means of testing how concentrations in top
predators respond to changes in food web structure. It is essentially a multibox
model with one-way transfers from box to box.
An obvious extension is to include nonaquatic species such as birds and mammals.
This has been discussed by Clark et al. (1989), who showed that fish-eating
birds could be included in an aquatic food web model. In the long term, it may be
possible to build models containing all relevant biota, including fish, birds, mammals,
insects, and vegetation. A framework for accomplishing this has been described by
Sharpe and Mackay (2000). The primary difficulties are in the development of
species-specific mass balance equations, determining appropriate parameters for the
organisms and obtaining validation data. There is little doubt that comprehensive
food web models will be developed and validated in the future, even models including
humans.
©2001 CRC Press LLC
8.10 SEWAGE TREATMENT PLANTS
Many chemicals are discharged to sewers and are subsequently treated biologically
in sewage treatment plants (STPs), also called publicly owned treatment works
(POTWs). Treatment configurations vary from simple lagoons to more complex
systems in which the concentration and activity of the biomass are optimized by
recycling the biomass or sludge. Such activated sludge STPs essentially consist of
a series of connected vessels, the contents of which are well mixed, having contact
with air either at the surface (as in a lake) or by forced aeration. A typical STP flow
diagram is shown in Figure 8.11 with illustrative chemical fluxes. The influent
sewage is settled by primary sedimentation, followed by secondary treatment under
aeration conditions with subsequent settling and recycling.
The flows of water, solids, and air are defined by the plant operating conditions.
The task is then to deduce the corresponding fluxes of the chemical present in the
influent. Steady-state mass balance equations can be set up for each vessel and solved
for the three fugacities, from which all chemical fluxes can be deduced. This requires
that D values be defined for flows of chemical in water, biomass solids, and air, for
both degradation and surface volatilization.
Clark et al. (1995) have described such a model, and Windows software and a
BASIC program (STP) are available. The model is particularly useful not only for
estimating the overall treatment efficiency but also the fraction of the chemical input
that is volatilized, degraded, left in the sludge, or remains in the effluent.