Dual Digestion is a two stage system that combines autotherm·a1 thermophilic aerobic
pre-treatment with conventional anaerobic digestion. The practicability of the system
using pure oxygen is well proven. Disadvantages are the high cost of the pure oxygen
and the absence of a detailed evaluation of anaerobic digester performance. This report
discusses the results of a full-scale investigation into the dual digestion system (184m3
aerobic reactor and 1800m3 anaerobic digester), carried out in two phases: In the first
using air alone for oxygenating the aerobic reactor and in the second using a combination
of air and pure oxygen. During both phases the performance of the anaerobic digester
was also monitored, but in greater detail in the second phase as far as the final sludge
product is concerned.
In phase I, with air, it was possible to maintain thermophilic temperatures in the aerobic
reactor throughout the year. However the required retention times were relatively long
(3-6 days) in comparison with the pure oxygen reactor (-1 day) due to the high vapour
heat losses. At long retention times, the volatile solids (VS) destruction was appreciable
(-25%) and the reactor tended towards an autothermal thermophilic digester. Foaming,
although unpredictable in its occurrence, significantly improved aerobic reactor
performance by doubling the oxygen transfer efficiency. From liquid and gas mass and
heat balances it was found that the specific biological heat yield and respiration quotient
were approximately constant at 12.8 MJ/kg(02) and 0. 70 mol(C02)/mo1(02) respectively
over a wide range of operating conditions and consistent relationships between VS
removal, heat generation, and oxygen utilisation could be established. Based on
information collected, it was concluded that increased treatment capacity and greater
temperature control of the aerobic reactor could be provided by supplementing air
oxygenation with pure oxygen.
In phase II, using a combination of air and pure oxygen, much higher loading rates on the
aerobic reactor were possible. Thermophilic temperatures could be maintained at short
retention times (1-2 days). Unfortunately no foaming occurred during this period.
Consequently the benefit of improved oxygen transfer efficiency of the air oxygenation
system, produced by the foam, could not be exploited. Liquid and gas mass and heat
balances confirmed the specific heat yield and respiration quotient values and the
relationship between oxygen utilisation, VS destruction and biological heating. During
phase II, the anaerobic digester operated at a retention time of -1 O days. The sensible
heat content of the hot sludge from the aerobic reactor was sufficient to force the digester
into the thermophilic temperature range. The stability of the anaerobic process and final
sludge product at this short retention time was monitored with % VS removal and residual
specific oxygen utilisation rate tests and found to be similar to that of conventional
mesophilic anaerobic digestion at 20 days retention time. Dewaterability as reflected by
the specific resistance to filtration (SRF) was found to be poor, but 11ot much worse than
for conventional mesophilic digestion.
Sufficient information was obtained during phases I and II to allow a mathematical model
to be compiled, which could reasonably reliably simulate all the main operating
parameters of the dual digestion system. The model provided a means for assessing
different system configurations with mesophilic or thermophilic digestion, with and without
heat exchange or gas engine external heat sources, allowing technical and economical
(capital and operating) feasibility to be evaluated and compared with that for conventional
From both the experimental and modelled results, all the claimed benefits of the dual
digestion system were verified with the exception of the claim that aerobic reactor heat
pre-treatment of the sludge allows the anaerobic digester to operate at short retention
times (-1 O days). However, the digester can be operated at 10 days retention provided
its temperature is in the thermophilic range, in which case a sufficiently stable sludge is
produced; at mesophilic temperatures, a retention time of 15 days or longer is required
to produce a sludge of equivalent stability to that from conventional mesophilic digestion.
Consequently it is not the stability of the anaerobic process per se that governs the
minimum retention time but the quality required for the final sludge product. The aerobic
reactor is an appropriate pre-treatment stage for the thermophilic digester because it
provides the necessary temperature and pH buffering to allow stable operation in the
It is concluded that where application of conventional anaerobic digestion is contemplated,
whether for new installations or for upgrading existing plants, the dual digestion system
should be seriously considered as a possible option. It competes favourably both
technically and economically with conventional mesophilic digestion and produces a
superior sludge product which can be beneficially utilised in agriculture.
SSA, R (2021). The Dual Digestion Of Sewage Sludge Using Air And Pure Oxygen. Afribary.com: Retrieved May 13, 2021, from https://afribary.com/works/the-dual-digestion-of-sewage-sludge-using-air-and-pure-oxygen
Research, SSA. "The Dual Digestion Of Sewage Sludge Using Air And Pure Oxygen" Afribary.com. Afribary.com, 24 Apr. 2021, https://afribary.com/works/the-dual-digestion-of-sewage-sludge-using-air-and-pure-oxygen . Accessed 13 May. 2021.
Research, SSA. "The Dual Digestion Of Sewage Sludge Using Air And Pure Oxygen". Afribary.com, Afribary.com, 24 Apr. 2021. Web. 13 May. 2021. < https://afribary.com/works/the-dual-digestion-of-sewage-sludge-using-air-and-pure-oxygen >.
Research, SSA. "The Dual Digestion Of Sewage Sludge Using Air And Pure Oxygen" Afribary.com (2021). Accessed May 13, 2021. https://afribary.com/works/the-dual-digestion-of-sewage-sludge-using-air-and-pure-oxygen