analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

samples (Paired two-sample Student’s t-test, P = 2.2). The measurement obtained at the outlet was
related to wastewater that entered the reactor approximately one retention time before. Thus it is not
possible to determine the amount of free and saline ammonia produced by anaerobic digestion of the
feed wastewater by considering an inflow and outflow sample drawn simultaneously The coefficient
of variation 1 for inflow free and saline ammonia measurements was high (38 %); thus it is possible
that measurements from inflow samples exceeded those measured in the outflow at a particular point
in time, despite the fact that the outflow sample had increased free and saline ammonia concentration
relative to that which entered the reactor one retention time before. This problem could have been
overcome by analysing many more samples; the mean values calculated from the inflow and the
outflow free and saline ammonia concentrations would have been better estimates of the population
means, and a significant increase between inflow and outflow concentrations may have been observed.
Total Kjeldahl Nitrogen (TKN) measures the sum of organically bound nitrogen and free and saline
ammonia. In a steady-state anaerobic digestion system, (i.e. no net accumulation) the TKN
concentration should not change between the inlet and outlet since no TKN exits in the gas stream.
During Phase IV, 8 ad hoc measurements of TKN were made of inflow and outflow samples, yielding
average TKN concentrations of 44.6 and 37.1 mgN/ℓ respectively. This indicates a statistically
significant reduction of mean TKN between inflow and outflow (Paired two-sample Student’s t-test,
P = 0.02). A considerable proportion of TKN is understood to be associated with organically bound
nitrogen in particulate form (Raunkjear et al., 1994). Thus retention of solids in compartments of the
ABR would account for the reduction in TKN observed between the inflow and outflow. However,
given the large variation seen in most determinands measured, the TKN measurements were probably
too few to provide reliable information about the fate of TKN in the ABR.
5.4.5 Phosphorus
Few phosphate measurements were obtained in Phase II and III. The number of measurements was too
small to be able to infer any kind of statistically verifiable pattern in phosphate consumption or
production. Spectrometric dissolved phosphate measurements were obtained on inflow and outflow
samples in Phase IV. Mean values and 95% confidence intervals are presented in Table 5.6. A small
but significant decrease in phosphate was observed but, since anaerobic digestion has no mechanism
for the removal of significant amounts of phosphate, the apparent removal is expected to be a
sampling phenomenon related to the small number of samples analysed (n=7).
5.4.6 Sulphate
No sulphate measurements were performed in Phase II and III. Spectrometric sulphate measurements
were obtained on inflow and outflow samples during Phase IV. A statistically significant decrease
(Student’s t-test, P = 0.001) from 4.5 mgSO4 2- /ℓ in the inflow to 0.4 mgSO4 2- /ℓ in the outflow was
observed. Mean values and 95% confidence intervals are presented in Table 5.6. It is probable that
active sulphur-reducing micro-organisms were present and active in the ABR converting sulphate to
sulphide according to Eq. 5-1 (Speece, 1996).
1
standard deviation
C of V =
⋅100%
arithmetic mean
98