The Dual Digestion Of Sewage Sludge Using Air And Pure Oxygen

ABSTRACT

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

digestion.

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

thermophilic range.

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.