Heterotrophic anaerobes
and methanogens must have been present in below-surface layers
of the aerobic sludge flocs or granules, as assumed above, that were
able to produce methane under anaerobic conditions similar to
other assays where the anaerobic seed inoculum allowed an immediate
methane production e.g. as in assays B or C (Fig. 3). When
the ratio of DECA was decreased and replaced by increased
amounts of AS the highest cumulative methane production of
947 L/m3 was observed in assay B (Fig. 3), although the COD was
same in all assays except for the control (Table 6, assay E). The initial
dissolved COD for assays AeD was approximately similar, however
the COD removal depended upon the ratio of AS (Tables 1 and 6). In
assay B with a DECA: AS ratio of 5: 13, the highest methane production
ofw28 L CH4/d per unit of CODadded was observed. It seems
that DECA as a co-substrate in an optimum ratio might increase
methane production of digesters that stabilize excess sludge from
aerobic wastewater treatment. The concentration of total solids in
all assays of this batch was w8 g/kg which was reduced to 20e34%
with a corresponding decrease of the VSS to 12e35% from the initial
4.3 g VSS/kg. During the course of the experiment the pH remained
constant at 7.5e7.6 and only a minimal fatty acid production was
observed