2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 47 -
Optimal Nitrogen Supplementation to Enhance
Anaerobic Digestion of Cassava Pulp
N. Paepatung*, P. Panichnumsin** and A. Nopharatana*
*
Pilot Plant Development and Training Institute,King Mongkut’s University of Technology Thonburi, Bangkhunt
ien,
(Thailand)
** Excellent Center of Waste Utilization and Management, National Center for Genetic Engineering and,
Biotechnology, Bangkhuntien,
(Thailand)
Abstract
-- Cassava pulp has very high C/N ratio
which is regarded as an important imitating factor
for anaerobic digestion. This study was carried out
to determine nitrogen adding strategy in order to
enhance cassava pulp digestion. In batch
experiments, nitrogen was added to obtain C/N
ratio of 20:1, 25:1, 30:1 and 40:1, resulting in an
improvement of cassava pulp digestion by 2.82 2.65
2.35 and 1.7 times comparing to the control,
respectively. However, adding nitrogen had no
significant effect on methane yield. At C/N ratio of
40, VFA productivity decreased drastically. The
results of semi-continuously fed experiment showed
that the reactor with nitrogen concentration of 10
mg L-1 achieved the same biogas production and
VS reduction efficiencies as that with excess
nitrogen addition and nitrogen controlled at 40 mg
L-1. The average nitrogen utilization rates at 10 and
40 mg L-1 were 11.41 and 11.42 mg g-1TS,
respectively.
Index Terms-- Anaerobic digestion, Cassava pulp,
C:N ratio, Nitrogen supplementation
I.
I
NTRODUCTION
Cassava pulp (CP) is considered as a main solid
waste from the extraction process in cassava starch
factories. Approximately 70 cassava starch factories in
Thailand produced 6-7 million tons of CP annually. [1]
CP contains 50
–
60% of starch in dry matter and 60-
70% moisture content [2]. It has potential as a
carbohydrate source for biogas production. However,
CP is a limitation in the conversion to methane because
of the high cellulosic content, low buffering capacity,
and high carbon to nitrogen ratio (C/N).
The C/N ratio
is the amount of nitrogen that must be available in the
digester.
The C/N ratio of CP is approximately 210/1,
which is too high for the anaerobic digestion. The high
C/N ratio feedstock could result in deficient of nitrogen
for microbial growth, which may result in system
failure due to the accumulation of volatile fatty acids
(VFA) [3]. A decrease in the C/N ratio from 40 to
about 27 by the addition of urea and diammonium
phosphate (DAP) showed a slight improvement (8
–
11%) in anaerobic digestion of cattle waste [4].
The
optimal C/N ratio for optimal methane production is
approximately 25:1
[5,6].
Speece [7] stated that an
excess of 40
–
70 mg NH
4
–
N L
-1
must be maintained in
the reactor to prevent a reduction of microbial activity
and the acetate utilization rate was only 54% of
maximum potential at 12 mg NH
4
–
N L
-1
in the reactor.
Consequently,
NH
4
-N must be added in form of urea,
which the price is around
Baht 15,000-20,000 per ton,
if N source is insufficient
. Anaerobic co-digestion
with
manure
provides a higher C/N ratio [3].
However,
waste transportation cost is the key factor that may
restrict the use of certain manure in a CP anaerobic
system.
This study was carried out to determine
optimal nitrogen supplementation enhance anaerobic
digestion of cassava pulp.
II. M
ATERIAL AND
M
ETHOD
A. Feedstock and inoculum
Fresh cassava pulps were collected directly from a
pulp pressing machine of a cassava starch factory in
Chonburi province, Thailand. It was stored at 20
0
C.
The characteristics of the feedstock are presented in
Table 1. The inoculation sludge was taken from an
anaerobic plug flow reactor of pig manure and
contained 65 g L
-1
of volatile suspended solid (VSS).
Prior to use, the inoculum was washed three times with
0.1 M phosphate buffer (pH 7.0) to eliminate the
remaining nitrogen organic compound.
B. Anaerobic biodegradability batch assays
Anaerobic biodegradability batch assays were
performed in glass flasks with a total volume of 800 ml.
The ratio of inoculum to substrate was 1:1. The C/N
ratios were varied to be 20:1, 25:1, 30:1, and 40:1.
Nitrogen source was added in the form of ammonium
chloride (NH
3
Cl), with its concentrations varying
according to the C/N ratio. A basal medium (without
yeast extract) and sodium bicarbonate were supplied
for stable digestion. Temperature was controlled at
37
o
C. The glass flasks vials were flushed with a gas
mixture of 70% N
2
and 30% CO
2
before sealing. The
experiments were conducted for 15-20 days with an
assumption of maximum specific methane production
rate. The control set was not supplied nitrogen source,
which was used as a baseline comparison of methane
production.
