|dc.description.abstract||With the exceptions of mixing and heating mechanisms, and the recycling of settled solids, no
radical changes or improvements have been made to conventional anaerobic digesters treating
municipal sewage. These digesters usually function with a hydraulic retention time of 30 to 60
days and at a total solids concentration of 2.6 %(m/v). Volumetric loading is limited since high
loadings effect the displacement of the slow growing methanogens. Thus, the hydraulic retention
time is coupled to the solids retention time.
A crossflow microfiltration unit has been constructed at Northern Waste Water Treatment
Works, Durban, to concentrate sludge from a conventional anaerobic digester and, thus,
facilitate operation with a higher solids concentration. In addition, this process should result in
the retention of the active biomass which would otherwise be lost as a waste product of the
treatment process. The solids retention time is, thus, decoupled from the hydraulic retention time.
The net result could be higher volumetric loadings, increased microbial activity and increased
volatile solids destruction and, hence, improvement in the efficiency of anaerobic digestion of
sewage sludge. To test these, different experiments were conducted to specifically determine the
effect of higher solids loads.
Preliminary experiments were undertaken to determine the biodegradability of primary sludge
from the Northern Waste Water Treatment Works. Results showed that primary sludge of 76%
VS could be reduced to approximately 48 to 50% VS during an experimental period of 85 days.
Reduction of the first 20% VS was rapid if conditions were optimum but subsequent reduction
from 55 to 50% VS was slow. It was calculated that approximately 0.88 l gas was produced for
every g volatile solids catabolised.
Further experiments were conducted to investigate the effects of different solids concentrations
on microbial activity. The results showed that the volume of gas produced increased as the solids
concentration increased from 2 to 6%(m/v). Digesters with solids concentrations of 6 to
13%(m/v) produced similar volumes of gas. Digesters with solids concentrations of 6 to
13%(m/v) TS produced approximately 300 ml more gas than the control during the 20 days
experimental period. The rate of gas production also increased as the solids concentration
increased. However, digesters containing 11%(m/v) and 13%(m/v) TS produced similar rates.
These results indicate that the introduction of concentrated sludge into the digester improves
Finally, a semi-continuous digester was operated at a 30 days retention time and at optimum
temperature to investigate the efficacy of digesters with increased solids concentrations. The
results showed that the rate of gas production increased as the solids concentration increased
from 2%(m/v)(control) to 3.8%(m/v). However, the digester operated with 4.7%(m/v) TS
produced gas at a rate lower that the digester with 3.8%(m/v) TS. The volatile solids
concentrations of all four digesters were similar, indicating neither favourable nor unfavourable
effects from increased solids concentrations. The digesters operated with 3.8%(m/v) and
4.7%(m/v) TS produced higher concentrations of volatile acids than the control. The alkalinity
concentrations (>_4000 mg t-1 ) were similar for all four digesters.||en