The design volume of the three identical batch flow anaerobic
digesters was sized according to the amount of volatile solids that
must be treated and the period of time the material will remain in
each of the digester (retention time). The design of the digesters
was based on Ajoy Karki’s Biogas model (Karki, 2002) incorporating
the separate floating gas holder system for ease of daily measurement of gas volume. The cylindrical shape was adopted to enhance
better mixing. The digester is a separate component, with the gas
holder in a separate water jacket.
The theory behind the design is simply ‘‘downward delivery and
upward displacement’’ following the example ofUludag-Demirer
et al. (2008). The slurry on fermenting in the digester produces
gas. This gas is delivered to the bottom of the water jacket via a
pipe; the pipe extends above the surface of the water level (water
seal) in the water jacket. The gas displaces the gas holder (upward)
and gets trapped between the gas holder and the water seal. The
displacement of the gas holder is dependent on the pressure and
volume of the gas produced. The setup is as shown inFig. 1.
The Lemon grass (C. citratus) was harvested from gardens
around some houses within Area BZ Staff Quarters, Ahmadu Bello
University, Zaria and crushed to smaller particles using the Hammer mill before they were transported to the research field for further pre-treatment. Cow dung, on the other hand, was collected in
sacks fresh and free from impurities from the Zango abattoir and
transported to the research ground while Poultry droppings were
obtained (fresh and free from impurities such as wood filings) from
the Poultry Department (Deep Litter section) of the National Animal Production Research Institute, Shika-Zaria and transported to
the research site.
Partly decomposed slaughter house waste was used as seed
material for the substrates digested in this study. The Lemon grass
(C. citratus) was pre-fermented for a period of 40 days while Cow
dung and Poultry droppings were pre-fermented for a period of
15 days each in respective plastic drums (Karki et al., 2005). The
longer period of pre-fermentation for the Lemon grass was as a
I.M. Alfa et al. / Bioresource Technology 157 (2014) 270–277 271
result of the slow rate of decomposition of succulent plants which
had not undergone any prior digestion unlike the Cow dung and
Poultry droppings that had gone through the digestive systems of
the respective animals.
The digestion was a batch process and 6 kg each of pre-fermented Lemon grass, Cow dung and Poultry droppings was respectively mixed with water to form slurry in the ratio 1:1 by volume
and introduced into Reactor A, B and C respectively through an inlet pipe of 50 mm at the top of each reactor. The slurry was allowed
to occupy three quarter of the digester space leaving a clear height
of about 0.0625 m as space for the gas production. Before feeding
the reactors, the flexible hose connecting the gas outlet from the
reactor to the gas holder was disconnected, such that the gas outlets from the reactors were left open. This was done to prevent
negative pressure build up in the reactors. The gas was collected
from the digesters through a 10 mm diameter flexible hose connected from the digesters to the bottom of the gas collection systems. The collected gas was allowed to pass through water and
slaked lime respectively as scrubbers.
Slaked lime (Ca(OH)2) is known to be used for carbon dioxide
(CO2) removal from gas according to earlier finding byChen et al.
(2004) and that there is evidence that the CO2/Ca(OH)2reaction
also requires the uptake of water to have reaction. The overall reaction is expressed by the Eq.(1)as follows