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

lanket consisting of smaller granules, biomass flocs and dispersed sludge, and (e,) a clear zone
consisting of liquid flow with entrained fine solid particles. Once the compartment became filled with
sludge, the clear zone would be eliminated by the sludge blanket.
The sludge bed and fluidised sludge blanket increased the treatment potential of the system by (i)
creating a filter through which fluid flow had to pass, causing entrapment of less dense particulate
material and colloidal material and (ii) by creating a matrix in which anaerobic micro-organisms could
establish, and through which flow had to pass, ensuring that good contact between biomass and
degradable components of the feed occurred. Soluble components originating from the feed or
generated through hydrolysis of particulate material passed through the sludge bed and fluidised
sludge blanket with the liquid flow. Inert soluble components passed through the reactor and exited
with the outflow from compartment 8. Degradable soluble components may have degraded if they
came into contact with appropriate anaerobic micro-organisms. The probability of this occurring
would have been highest in the sludge bed or blanket since this is where the greatest concentration of
micro-organisms may be found. However, small anaerobic consortia or free suspended anaerobic
micro-organisms may have been entrained in the liquid flow; thus digestion of soluble or colloidal
components may also have occurred in the relatively clear liquid above the sludge bed/blanket.
As sludge beds developed in later compartments, the contact time between biodegradable material and
micro-organisms increased since the probability that a biodegradable molecule would come into
contact with appropriate anaerobic micro-organisms increases with increased amount of time spent in
the sludge bed or blanket. The overflow from each compartment (including the last, and thus the outlet
stream) therefore contained a combination of completely degraded material i.e. the distribution of
degradability of material in the outlet of a compartment depended on the distribution of contact times
of material in the compartment (Section 2.3.1.2).
Sludge retention has two components: firstly, retention of biodegradable particulate material increases
the residence time of the material and therefore the probability that it will be degraded within its
residence time in the reactor; Secondly anaerobic micro-organisms, the active agents of anaerobic
digestion form part of the solid sludge, therefore retention of sludge implies retention of anaerobic
micro-organisms and enhancement of anaerobic digestion.
Two critical questions arise from this analysis:
• What is the relationship between organic load, upflow velocity and sludge accumulation rate
(specifically, micro-organism retention rate)? i.e. how do the upflow velocity and organic load
affect the degree of solids retention, and the rate of microbial growth?
• What is the relationship between contact time and sludge accumulation? i.e. what is the
probability that the actual contact time experienced by a particle of biodegradable organics at a
particular OLR and with the prevailing sludge load exceeds the ultimate contact time required
for complete degradation?
6.1.2.1 Upflow velocity vs. specific sludge accumulation rate
The relationship between the rate of solids carry-over from one compartment to the next, and upflow
velocity was not well known. Technically this information could be inferred by comparing sludge
loads per compartment (such as compartment total solids or compartment sludge height) to solids
concentration in the liquid passing over the standing baffle of each compartment at different upflow
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