GPS-X Technical Reference

Attached Growth Models 242
For this purpose the hydraulics of the filter (which now can be thought of as a series of
units) is described in three different ways. A certain fraction of the total number of units
is in filtration mode, i.e. layered with influent flowing through them. Another fraction is
in standby mode, with light aeration and no influent loading. A third fraction is in
backwash mode. The sum of the three fractions adds up to the total number of units. At
times any one of these fractions may be completely missing, i.e. the number of units in
backwash or standby mode can be zero. In GPS-X, the individual fractions are specified
in two ways:
1. As constants – This allows a simple run when the number of units in standby
does not change. When a particular backwash criterion is reached, a certain
number (user-specified) of units in a filtration mode are backwashed. This mode
can be used if an actual operation of a filter plant is recorded and has to be
replayed in GPS-X. The number of units can be read in through the GPS-X input
file facility, and the original loading conditions recreated.
2. As volume fractions which vary according to a target load on the filter
component – This way the ration of the operating filter volume, and the standby
volume changes continuously according to changes in influent load. This
operation mimics a "constant loading" operational policy, although the loading is
completely constant on the filter volume because the limitation created by the
individual bio filter unit volumes are ignored.
During the simulation when volume fractions in different operating modes change, the
model correctly keeps track of mass balances by recalculating the mass of all model
components currently in that element. As an example consider the event of one standby
unit coming on-line due to increasing load on the filter. The other elements in the filter,
which have been in operation, may contain much higher active biomass than one, which
has been on standby for a period of time. When the standby component comes on-line, a
volume weighted average biomass concentration is calculated for the new, increased filter
volume for all horizontal sections and all biofilm layers, and integration continues using
the new conditions.
GPS-X Technical Reference