Organic Waste Recycling

Biofuels production 153

and neutralize the pH of the digester slurry by converting the volatile fatty acids
into CH 4 and other gases. The conversion of H 2 into CH 4 by the methanogens
helps reduce the partial pressure of H 2 in the digester slurry which is beneficial
to the activity of the acetogenic bacteria. If the methanogenic bacteria fail to
function effectively there will be little or no CH 4 production from that digester
and waste stabilization is not achieved because the organic compounds will be
converted to only volatile fatty acids, which can cause further pollution if
discharged into a water course or on land. Since the methanogenic bacteria are
obligate anaerobes, their growth is inhibited even by small amounts of oxygen
and it is essential that a highly reducing environment be maintained to promote
their growth. The methane bacteria are also sensitive to other environmental
factors which are discussed in detail in section 4.3.
The current understanding of the microbiology of anaerobic digestion is
illustrated in Figure 4.3. There are four main groups of bacteria involved in the
process namely (Brown and Tata 1985):

1. Acid-forming (hydrolytic and fermentative) bacteria,


2. Acetogenic (acetate and H 2 -producing) bacteria,


3. Acetoclastic (methane-forming) bacteria, and


4. Hydrogen-utilizing methane bacteria

Acid-forming and acetogenic bacteria are collectively called non-
methanogenic bacteria in which the major bacterial species are given in Table
4.3. The acetoclastic and hydrogen-utilizing methane bacteria are collectively
called methanogenic bacteria as shown in Table 4.4.
The acid-forming bacteria are involved in the hydrolysis and break down of
complex organic compounds into simple products such as CO 2 , H 2 and other
volatile fatty acids via two main pathways (Gunnerson and Stuckey 1986):

Substrate CO 2 + H 2 + acetate (4.2)

Substrate propionate + butyrate + ethanol (4.3)

The products from Equation 4.2 can be utilized directly by the acetoclastic
bacteria (Equation 4.4) and the hydrogen-utilizing methane bacteria (Equation
4.5) to produce CH 4.

CH 3 COO

-

+ H 2 O CH 4 + H CO 3 - + energy (4.4)

(acetate)

4H 2 + H CO 3 - + H+ CH 4 + CO 2 + energy (4.5)