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176 Advances in Eco-Fuels for a Sustainable Environment

Fe-ZSM-5 and Fe-NaZSM-5 catalyst, the higher yield of 31.51% and 74% of methane
conversion was obtained, respectively [69]. Moreover, the enzyme catalyst is classi-
fied into two categories such as to synthesize the methanol or else recover the methane
from solution. In these, the first is a complex form of iron present in the cytosol of
metabolizing bacteria, which is called soluble methane monooxygenase (sMMO).
Particulate methane monooxygenase (pMMO) is made up of protein and a complex
of Cu, which is the second type of enzyme. Apart from these two, recently biobased
Methylosinus sporium B-2121 bacteria has been used to convert methane to methanol
through the biochemical process [81].

6.11 Scientific and technological hurdles in solid-state
anaerobic digestion

The growth of the fermentation method of various feedstocks in anaerobic digestion is
limited because of scientific as well as technological hurdles associated with batch or
continuous solid-phase anaerobic digestion. Various hurdles have been identified and
addressed to the extent to which they affect the anaerobic digestion. These hurdles are
associated mainly with monitoring tools, interlaboratory reproducibility and repeat-
ability, inoculum, rheology, pretreatment, dynamics of populations, managing inhibi-
tions, hydrodynamics, and liquid transfers.

6.11.1 Monitoring tools
The observing apparatus used in the solid-phase anaerobic digestion is basically
arranged for the gaseous and liquid states [82]. With some errors, these tools are
employed for solid-phase anaerobic digestion monitoring but have not completely
adjusted for the solid phase. The biodegradability of solid waste is imagined by mea-
suring the chemical oxygen demand through dilutions. Additionally, the requirement
of reactants is huge [83]. In recent events, a new method for measuring the chemical
oxygen demand for solid waste has been developed. It utilizes commercial tools with-
out diluting the solid substrate, giving an advantage over previous methods. This
methodology is quick with high repeatability and has approximately 5% bias. It is
capable of measuring the chemical oxygen demand in a range of 1–8mg COD/L.

6.11.2 Biochemical methane potential

The methane potential test represents the complex heterogeneous sample and the first
hurdle is in this regard. The solid fraction of a few grams from tons of material will
create variation in the methane production test due to highly heterogeneous sampling.
This problem of involving various types of substrates arises due to the major complex-
ities involved during sampling. Because of the sampling procedures, the methane
potential determination is lacking with respect to rules impacting on the level of meth-
ane production obtained. Thus, it results in low-accuracy BMP values. This low accu-
racy creates trouble for researchers to design a proper sized biogas plant. A France