Environmental Engineering Reference
In-Depth Information
Table 5.4 Comparision of one- and two-stage digestion of grass silage
SLBR-UASB
CSTR
HRT (days)
30
50
CH
4
content of biogas (%)
71
52
lCH
4
/kg VS
341
451
VS destruction (%)
75
90
DS content of digestate (%)
10
5.6
Source: Nizami et al. (2011b).
5.7.2 Wet continuous digestion of grass silage
The two-step system (two reactors each with an effective volume of 300 l)
when optimised produced 451 l CH
4
/kg VS with a 50-day retention time and
an organic loading rate of 2 kg VS/m
3
/day (Thamsiriroj and Murphy, 2011).
The methane content averaged 52%. The digestate had a solids content of
5.6%. Volatile solids destruction averaged 90%.
5.7.3 Two-phase digestion of grass silage
This system comprised six separate leach beds. These were each loaded
sequentially with 3.5 kg of baled grass silage in a 5-day cycle: day 0, feed bed
1; day 5, feed bed 2; day 10, feed bed 3, and so on till day 30 when leach
bed 1 was emptied and re-fed (Nizami and Murphy, 2011). Liquor was
sprinkled over the leach beds; this was collected in a leachate tank. From the
leachate tank the liquor was either recirculated over the beds or pumped
through the UASB (30 l volume) before recirculation over the beds (Nizami
and Murphy, 2011).
When optimised, 75% of volatiles were destroyed in the leach beds
producing 1.4 kg COD/kg VS. Each kilogramme COD destroyed generates
350 l CH
4
(Sperling and Chernicharo, 2005). The UASB achieved 93%
removal efficiency (Nizami and Murphy, 2011). Thus, methane production
in theory
should be
(0.75 destruction of
volatiles
6
1.4 kg COD/
kg VS
0.93 = ) 342 l CH
4
/kg VS. The
methane, as measured above the UASB, averaged 342 l CH
4
/kg VS. The
digestate had a DS content of about 10%.
6
350 LCH
4
/kg VS
destroyed
6
5.7.4 Comparison of systems
A choice exists for the designer in crop digestion (Table 5.4). A traditional
wet system (in this case a two-step system with recirculation of liquor) or an
innovative system (with associated risk?) employing two phases. The two-
phase system has 60% of the retention time of the wet system, has a
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