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

In this analysis it can be seen that the SRT is related to the integral of the sludge production XP: in an
ABR, XP depends on the rate of solids retention through settling and the rate of solids hydrolysis
through biological activity. If the rate of generation of micro-organisms is low micro-organisms may
washout since the inflow of micro-organisms to the system is negligible. It is therefore quite possible
that high solids accumulation rates XP are due to low rates of biological activity, despite poor biomass
retention, but only if the net rate of accumulation (in – out + generation – degradation) of
biodegradable solids is larger than the rate of accumulation of the micro-organisms that should
degrade them.
Thus design guides that indicate that methanogenesis will establish at SRT values above some critical
value are empirical and are only valid for the type of systems from which they were determined.
It is further concluded that neither the classically defined SRT nor the SRT defined in this analysis are
appropriate design parameters for an accumulating system.
6.4 CALCULATION OF RANGE OF OUTFLOW CHARACTERISTICS
The outlet stream pH value was consistently below that of the inflow to the ABR, despite increases in
measured alkalinity values between the inlet and outlet. Initially, it was thought that this indicated
process instability as a result of inhibition of acid-removing micro-organisms. However, similar
studies on anaerobic digestion also found that pH values decreased without apparently affecting
process stability (Behling et al., 1997).
Sötemann et al. (2005) demonstrated that for an anaerobic process operated at steady state where
hydrolysis was the rate limiting step, the effluent characteristics could be accurately predicted if the
feed characteristics were sufficiently well understood. It was proposed that the stoichiometry of
Sötemann et al. (2005) could be used to predict the range of effluent characteristics that could be
expected from an anaerobic system treating domestic wastewater, and that this information would
assist in understanding the condition of the outflow (and digestion as a whole) in this study.
6.4.1 Sötemann et al. (2005) model of steady-state anaerobic digestion
Steady-state models are based on the principle of the rate-determining step: in a steady-state system,
the overall rate of treatment will depend on the slowest process that occurs in the system. Provided the
conditions of the system do not change such that another process becomes rate-limiting, a calibrated
steady-state model will give a reasonably quick basis for designing a system and determining
operating parameters, or estimating system performance under slightly different conditions.
Sötemann et al. (2005) developed a steady-state model for anaerobic digestion of sewage sludges,
based on the assumption that hydrolysis of macromolecules is the rate-limiting step in this process.
This is a three step model consisting of (i) a kinetic part for determining COD removal and gas
production, (ii) a stoichiometric part that calculates free and saline ammonia, alkalinity production and
digester gas composition and (iii) a weak acid-base section that calculates the digester pH from the gas
composition and alkalinity.
The COD of the feed in the steady-state model is assumed to be a combination of particulate
biodegradable organic materials (SBCOD, Section 2.4.1) with a known average elemental
composition CXHYOZNA, VFA (represented by acetic acid) and a fraction of unbiodegradable material.
145