Environmental Engineering Reference
In-Depth Information
5.3 (a) One-step and (b) two-step continuous digestion with
recirculation of liquid digestate (adapted from Nizami and Murphy,
2010).
achieve a relatively homogenous production of gas (Nizami and Murphy,
2010).
5.5.3 Continuously stirred tank reactors
Commonly known as CSTRs, engineers have more experience with this type
than any other reactor. The reactor is a continuous wet process, treating
feedstock with between 2 and 12% DS. There are numerous such facilities
throughout the world treating sewage sludge, agricultural slurries and crops.
The mixing systems are a crucial design element of this process. Various
systems are employed, for example, in recycling biogas - paddle stirrers that
rotate vertically in a circular motion about a horizontal axis, paddles that
rotate horizontally about a vertical axis and inclined paddle stirrers.
Typically, the hydraulic retention time (HRT) is the same as the solid
retention time (SRT). Due to the slow growth rate of aceticlastic
methanogenic bacteria and the syntropic nature of all trophic levels of
bacteria in the anaerobic process, failure would occur if the retention time
were lower than the doubling time of these bacteria, due to washout.
Organic loading rates tend to be in the range 1-4 kg VS/m
3
reactor/day
(Murphy et al., 2011).
The system may be operated as a single-step system but more often a two-
step system is used with all bacterial groups (acidogenic, acetogenic and
methanogenic) in each vessel or step (Fig. 5.3). Liquid digestate may be
recycled from the second vessel to the first step. This recycle is a useful tool
in the system as it allows for dilution of feedstock; it can also be used to
balance the system (Thamsiriroj and Murphy, 2010). Generally the majority
of the gas is produced in the first step of the two-step system. Thamsiriroj
and Murphy (2010) found about 80% production of biogas in the first vessel
when digesting grass silage.
Search WWH ::
Custom Search