Page 413 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Agroindustry wastes: biofuels and biomaterials feedstocks for sustainable rural development 373

Acid forming Methane forming
organisms organisms

Methane
Organic Volatile and
waste acids
carbon dioxide
Phase 1 Phase 2
Acid Methane
fermentation fermentation
Figure 16.3 Two-step microbial fermentation process for methane production (Obi et al.,
2016).
bacteria. Then, these organic acids are consumed by methanogenic organisms to
yield gas that has high methane content (Obi et al., 2016). The typical composition
of the produced gas has 50% 70% of methane, 25% 45% of CO 2 , 0.5% 3% of
N 2 ,1% 10% of H 2 with little amount of H 2 S, whose heating value is in the range
3
of 18 25 MJ/m (Timbers and Downing, 1977). The major disadvantage of the
digestion system is high cost. Further, the explosive properties of the methane gas
make working condition difficult for operators. However, the advantages of the
anaerobic digestion are that the process makes the treatment and disposal of large
quantity of wastes such as poultry and dairy easier. Thus it helps to minimize prob-
lem due to foul smell. The process helps to stabilize the waste and the fertilizer
value of the original waste is retained with the digestion sludge.

16.3.3 Adsorption process adsorbents in the elimination
of heavy metals

The agroindustry waste can also be utilized as adsorbents in the elimination of
heavy metals using the adsorption process. The presence of heavy metal ions
(copper, cadmium, mercury, zinc, chromium, and lead ions) in higher concentra-
tions than threshold can cause severe threats to many life forms. As compared to
organic pollutants (i.e., susceptible to biodegradation) the heavy metal ions do not
degrade into harmless end products. Many studies (Garg et al., 2004; Ngah and
Hanafiah, 2008; Mo et al., 2018) have demonstrated the efficiency of adsorption
process for the removal of heavy metal from waste using activated carbon. Many
studies (Mondal et al., 2011; Low et al., 2011) have demonstrated the use of
agroindustry wastes as a low-cost technique for the removal of heavy metals
through the adsorption process. Various agroindustry wastes have been utilized,
such as rice husk (Ayub et al., 2002), coconut husk (Tan et al., 1993), sawdust
(Ajmal et al., 1996), sugarcane bagasse (Mohan and Singh, 2002), oil palm shell
(Khan et al., 2003), neem bark (Ayub et al., 2002), etc., for the removal of heavy
metals from wastewater.