analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

The original steady-state model describes a CSTR with no unsteady sludge accumulation.
Consequently, the HRT and sludge age are identical and are calculated from the ratio of the volume of
the reactor V and feed flow rate Q. As with Zeeman and Lettinga’s definition of sludge age
(Section 6.3), this definition is not appropriate for an accumulating system. Thus a new hydrolysis
model had to be developed to describe the effect of sludge retention on apparent yield in an
accumulating system.
6.4.2.2 Hydrolysis model for a solids retention system
The same general methodology as published in Sötemann et al. (2005) was followed, with the
following assumptions and changes:
• A pseudo-steady-state condition was assumed. This implies that the concentrations of reacting
species in the digester did not change with time, i.e. biomass and SBCOD concentration were
approximately constant. (The corollary of this assumption is that only inert solid material
accumulated in the digester. This is in line with observations from the nitrogen balance that
little nitrogen accumulates with solids in the digester, Section 6.2.2)
• The concentration of solids exiting the digester (XADe) was a fixed fraction fX of the total solids
concentration in the digester (XAD):
Using these assumptions, an expression for the apparent system yield, E was derived:
E =
f
Where
X
X =
fX = Ratio solids COD conc. in outflow: average solids conc. in reactor
YAD = Yield co-efficient for acidogenic micro-organisms
V = Volume of digester [m 3 ]
Q = Feed flow rate [m 3 /d]
= Endogenous decay rate [d -1 ]
bAD
ADe
f X ⋅Y
V
+ ⋅ bAD
Q
f
X
AD
X
AD
( 1−
Y )
AD
The complete derivation is presented in Appendix A6.
147
Eq. 6-5
Eq. 6-6
No differentiation was made between unbiodegradable particulate material (UPCOD) that is retained
in the digester, and that which passes out with the effluent. Therefore, it is not possible to accurately
predict outflow COD concentrations (denoted Ste in the model) since these values are inflated by the
concentration of UPCOD that should be retained in the ABR.
The pseudo-steady-state assumption that implies that reacting species concentrations are
approximately constant and that only UPCOD accumulates, seem reasonable when considering data
from Phase IV where outlet species concentrations do not appear to change significantly with time.