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

As with the total volume of sludge calculated with Eq. 5-3, this quantity only takes into account the
solids content of the upflow side of each compartment and does not reflect the total mass of solids in
the reactor and therefore cannot be used for mass balance purposes.
Figure 5.18 shows that total solids accumulated in the upflow side of each compartment at a rate
between 34 and 88 kg dry solids/year (Table 5.3).
5.6.1.4 Rate of sludge accumulation: dependence on organic loading rate
It was surprising that although the OLR in Phase III was double that of Phase IV, the Phase III sludge
accumulation rate appears to be significantly more than double that of Phase IV. The ratio of the
sludge accumulation rate in Phase III to that of Phase IV had a 95% confidence interval of 2.6 to 6.1
(Table 5.3), calculated using Fieller’s theorem, Appendix A2.4) i.e. it can be said with 95%
confidence that the rate of sludge accumulation in Phase III was between 2.6 and 6.1 times greater
than that of Phase IV despite the fact that the OLR was approximately double.
Table 5.4: Calculation of sludge accumulation rates normalised by organic loading rate for
Phase III and Phase IV.
Sludge accumulation rate Organic loading rate Normalised sludge accumulation
rate
Units m 3 sludge/year kg COD applied /
m 3 reactor volume.year
109
ℓ sludge accumulated /
(kg COD applied)
Phase III 3.46 269 4.3
Phase IV 0.901 146 2.1
Phase IV
(kg dry solids)
60.7[kg dry solids/year] 146 0.14 [kg dry solids/kg COD applied]
To explain this observation, the sludge accumulation rate was normalised using OLR 1 . Using average
values only, the normalised sludge accumulation rates (i.e. amount of sludge accumulated per kg COD
applied) were calculated and are presented in Table 5.4.
Initially, it was expected that the normalised sludge accumulation rate would be higher in Phase IV
than in Phase III since the lower feed flow rate resulted in lower upflow velocity in the upflow
compartments, and this was expected to lead to better sludge retention. However,
Table 5.4 shows a significantly lower normalised sludge accumulation rate in Phase IV than in Phase
III.
The most obvious explanation for the difference in normalised accumulation rates is that the additional
solids accumulated in Phase III were undigested biodegradable particulate material originating from
the feed material. The implication is that the resident anaerobic biomass population in the pilot-scale
ABR in Phase III was either not sufficiently concentrated or not sufficiently active to convert
1 The sludge accumulation rate per organic loading quantity was calculated roughly for the purposes of
simplifying the calculations and therefore confidence intervals were not calculated.