2.2. Experimental setup and operation
The methane production from straws and animal manures were determined
using a batch method based on the techniques presented by Moller et al.26. Each material was
tested in duplicate using erlenmeyer flasks with a working volume of 250
mL, then kept at a mesophilic temperature 37oC. The
initial ratio of VS of the substrate to inoculum (S/I) was set to 2:1.
After mixing the inoculum with the materials in the erlenmeyer flasks,
distilled water was added to an effective liquid volume of 200 mL. All
the erlenmeyer flasks were tightly closed with rubber septa and foil
after being flushed with nitrogen. In ensuring uniform mixing, the
reactor contents were shaken every day with continued fermentation for a
period of 35 days until no gas production was further observed. The
compositions in the different batch set-ups were as follows: The initial
VS loading included 15 g VS/L of grapes, millet, maize, grass, and dried
straws. The pH values of the different digester were about 7. The
concentrations of Mg, Na and Ca were determined by inductively coupled
plasma-optical emission spectroscopy (ICPOES). The biogas volume (V1)
was measured by its displacement of water. The concentration of methane
was determined using a gas chromatography with the TDX-01 carbon
molecular sieve packing flotation column and high-purity argon as a
carrier gas with a flow rate of 35 ml/min. The peak signal was detected
by Thermal Conductivity Detector (TCD). The parameters for the
instrument were controlled as follows: The temperature of the injection
port, the column, the hot wire and the detector was 210, 135, 175 and
155 oC, respectively. Meanwhile, the gas injection
volume was 100 µL. The TS and volatile (VS)/TS of the inoculum were
3.41% and 47.91%, respectively.
2.3. Data for analysis
The necessary data needed for the analysis was obtained from the papers27-29, dissertation30 and from the current
conducted experiment in this work (Table S3). The results on methane
production for the batch system as well as the amount of Mg, Na and Ca
in the examined materials, were retrieved from literatures26,31.
For conducting analysis in these two works, the following materials were
used: chicken manure CHM, cattle manure CAM, swine manure SM1 and SM2,
corn straw CS and rice straw RS (Table S1). Note that, only mixture of
substrates was taken into consideration in a situation where two
substrates were used in a co-digestion fermentation. Then, the amount of
every element was divided by the mass of the mixture. All the elemental
contents were first converted to milligram of element per 1 gram of the
mixture. The Mg, Na and Ca content in the mixture was established on the
basis of the simple equations 1 – 3.
\(Mg=\frac{\sum_{j=1}^{r}\left(I\text{TS}_{j}\bullet\text{Mg}_{j}\bullet M_{j}\right)}{\sum_{j=1}^{r}\left(M_{j}\right)}\left[\frac{\text{mg}}{g}\right]\)(1)
\(Na=\frac{\sum_{j=1}^{r}\left(I\text{TS}_{j}\bullet\text{Na}_{j}\bullet M_{j}\right)}{\sum_{j=1}^{r}\left(M_{j}\right)}\left[\frac{\text{mg}}{g}\right]\)(2)
\(Ca=\frac{\sum_{j=1}^{r}\left(\text{ITS}_{j}\bullet\text{Ca}_{j}\bullet M_{j}\right)}{\sum_{j=1}^{r}\left(M_{j}\right)}\left[\frac{\text{mg}}{g}\right]\)(3)
Where M is the mass of influent material [g]; ITSrepresents fraction of influent dry matter [%]; Na, Mg and Ca
represents the fraction of sodium, magnesium and calcium content in the
dry mass of material, respectively [%]; j is a number of
materials, r is a maximum number of materials.
2.4 Model assumption
- It was assumed that hydrogen content in the fermenter chamber is
sufficient.
- The mixing has no influence.
- The dissociation rate was not taken into account because biogas plant
is a large object and a huge amount of material cause very small
concentration of the acetic acid. Therefore the dissociation rate is
close to 1 and the made mistake is very small too.
- The same difference between the effluent and influent material in
fermentation chamber. The influent and effluent of the material is
equal.
2.5 Model description and setup
The below chemical reactions (reaction 4 and 5), after simplified to the
reaction 6, were used as the based equation in the presenting
mathematical model. The production of methane in previous stages were
omitted because methane is mainly produced in the last stage
(methanogenesis). These equations refer to the hydrolysis, acidogenesis
and acetogenesis stages but of course it is a big simplification.
\({\text{\ \ \ \ \ }C}_{6}H_{12}O_{6}\rightarrow 2\text{CH}_{3}\text{CH}_{2}\text{OH}+{2\text{CO}}_{2}\)(4)
\(2\text{CH}_{3}\text{CH}_{2}\text{OH}+\text{CO}_{2}\rightarrow{2\text{CH}}_{3}\text{COOH}+\text{CH}_{4}\)(5)
\({6C\rightarrow 2\text{CH}}_{3}\text{COOH}\) (6)
Then the two below chemical reactions (reaction 7 and reaction 8) are
used to calculate total methane production.
\(\text{CO}_{2}+4H_{2}\rightarrow\text{CH}_{4}+2H_{2}O\) (7)
\(\text{CH}_{3}\text{COOH}\rightarrow\text{CH}_{4}+\text{CO}_{2}\)(8)
The previous model based on three phases (log period, exponential phase,
stationary phase 32 )
and it took into account the factors such as: temperature, pH, hydraulic
retention time, mass of the substrate, humidity of the mixture, mass of
available archaea, specific growth rate, volume of the fermenter,
initial carbon content in the dry mass of the substrate, dry mass
content in the substrate, content of organic dry matter in the dry mass,
initial nitrogen content in the dry mass of the substrate and content of
degradable compounds in the dry organic mass. Almost all of them
mentioned factors have strong influence on the fermentation process;
e.g. humidity of the mixture influence strongly the final methane
production. For example, low humidity of the mixture inhibits archaea
growth. New model for continuous technology based on exponential phase
and additionally includes the above mentioned factors and also
considered the influence of the followed factors: biogas plant working
days, influent of new substrate, chamber volume and sodium, magnesium
and calcium content.