with Hansen et al. [10]. Even though, the ammonia concentration
threshold inhibition varies widely, some authors set the concentration
in the range of 100–150 mg N–NH3 L1 [18,19], some others
increased it up to 1100 mg N–NH3 L1 in batch culture at pH 8.0
with proper sludge adaptation [10]. Since ammonia levels were
in the lower limit for R1 (130 and 122 mg N–NH3 L1 for an HRT
of 25 and 15 d, respectively), no ammonia-mediated inhibition
was expected (Table 3). These results indicated that the main benefit
of the addition of VPW may be related to the increase in biodegradability
which may also keep relation with the modifications in
C/N ratio.
Average weekly TVFA concentration in reactors is presented in
Fig. 1B. At the beginning of the experiment, an accumulation of
TVFA was registered for R2. Callaghan et al. [13] found that codigestion
with 30% (wet weight) or more of fruit and vegetables
wastes resulted in high concentrations of TVFA and performance
of the reactor became instable. In the present study, after a period
of adaptation of around 30 d, TVFA were almost completely consumed
by methanogenic microorganisms (Fig. 1B) indicating that
no inhibition was taking place.