Environmental Engineering Reference
In-Depth Information
Biogas
Effluent-to
further
treatment
Settling
Excess
suspended
solids
Wa stewater
preparation
-buffer tank
Excess
microorganisms
biomass
Influent
FIGure 32.8
UASB process.
solid substrate (total solids concentration 20-30% by mass), the digested substrate is mechanically
dewatered first (by belt press or centrifuge) and then liquid and solid parts are used separately. Solid
remains after dewatering can be used as a fertilizer directly, or they can be material for composting
(see further section). The liquid part can be used in the substrate preparation as dilution water;
however, great caution must be given to nutrient build-up and consequently possible inhibition in the
anaerobic digestion. Usually only a portion of that liquid is used in the substrate preparation. The
rest must be further aerobically treated as a wastewater.
32.3.3.7 Biogas Production, storage, treatment, and use
When operating a biogas plant, biogas is the main product and considerable attention must be given
to its production, storage treatment, and use. Biogas production completely depends on the efficiency
of the anaerobic digestion and its microorganisms. Previous sections have shown what conditions
must be met to successfully operate anaerobic digestion. There are two distinct parameters that
describe the biogas production:
1.
Specific biogas productivity—SBP
(also called biogas yield) is defined as volume of biogas
produced per mass of substrate inserted into digester (m
3
/kg). There are variations; SBP
can be expressed in m
3
per kg of substrate mass, total solids, volatile-organic solids or
COD. SBP tells us how much biogas we have gained from the substrate. Maximum pos-
sible SBP for a certain substrate is called biogas potential. Biogas potential can be deter-
mined by the standard method (ISO 1998).
2.
Biogas production rate—BPR
is defined as volume of biogas produced per volume of the
digester per day (m
3
m
-3
/day). BPR tells us how much biogas we can gain from the active
volume of a digester in a day.
Typically, SBP values of an optimally operating digester are 80-90% of the biogas potential. Typical
values of SBP for farm waste and liquid manure are shown in Table 32.7 and Table 32.8. Typical values
of BPR for mesophilic digesters are from 0.9 to 1.3 m
3
m
-3
/day. Lower values indicate the digester is
oversized; higher values are rare or impossible, because of anaerobic process failure. For thermophilic
or two-stage digesters, the typical BPR values are from 1.3 to 2.1 m
3
m
−3
/day, respectively. UASB
reactors are much less volume demanding and can achieve a BPR of up to 10 m
3
m
−3
/d ay.
Biogas production is rarely constant and fluctuates (because of different loading rates, possible
inhibitions, etc.). Therefore, a buffer is required for the biogas storage. This enables the biogas user
