Environmental Engineering Reference
In-Depth Information
Box 8.1
Worked example
A farmer wants to build a biogas plant. Amounts and characteristics of the
two available substrate types are
• slurry 9000 t/year, 8% DM, 85% VDM (density:
1 t/m
3
)
• food waste from restaurants 500 t/year, 16% DM, 82% VDM (density:
0.75 t/m
3
)
Calculate the effective digester volume, when ORL is fixed at maximum
3 kg VDM/m
3
day and HRT at minimum 37 days. Which is the determining
parameter, HRT or OLR?
Solution
Total organic matter: 678 t VDM/year
Total substrate volume: 9667m
3
/year
Volume calculation in regard to OLR: V = 1856 kgVDM/day/3 kg VDM/m
3
day
= 619m
3
= 26.5m
3
/day * 37 days = 980m
3
HRT is the determining parameter. The effective digester volume must be
minimum 980m
3
The organic loading rate is 1.9 kgVDM/m
3
day.
Volume calculation in regard to HRT:
V
gas should be protected by a plastic layer in order to resist small
deformations or hairline cracks in the material. It must also be resistant
to temperature variations, humidity and to aggressive substances that may
be resident within reactors.
Bitumen, emulsion paints (for underwater use), polyurethane, polystyrene
or epoxy are used for coatings. They are applied by painting, spraying or by
spatula and must form a tight and completely covering layer. The
application of a primer can be helpful, especially in the case of rough
surfaces. Lining sheets are another protection option. Polyethylene tiles or
films can be set up in cast form and sealed with concrete.
8.2.7 Reactor insulation and heating
A constant temperature in the digester is essential for a stable digestion
process; digesters are therefore insulated and heated in order to reduce and
compensate heat losses. Feedstock may also be heated before entering the
digester, which helps to avoid temperature fluctuations.
Reactor insulation can be placed inside or outside the digester wall, the
advantage of the latter being full availability of the digester volume. Mineral
wools (glass, rock and slag wools) and foamed plastics are the most
commonly used and approved materials (Eder and Schulz, 2006), but
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