D10: Impact of Contaminants - Hydromod

Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas
EUROLAKES
D10: Impact of Contaminants
FP5_Contract No.: EVK1-CT1999-00004
Version: 4.0
Date: 25/07/01
File: D10-vers.4.0.doc
Page 96 of 136
Anaerobic digestion
In an anaerobic digester the organic material is metabolised into approx. 2 /3 CH4, 1 /3
CO2 and biomass. The process is performed in a stirred reactor at a temperature between
30-37 °C. The gas can be used for heating or energy generation. After anaerobic
digestion the stabilised sludge is thickened to the extent required for the final disposal.
Aerobic stabilisation
Excess sludge can also be stabilised aerobically by increasing the aerobic sludge age
to an extent where organic material is aerobically degraded and mineralised. This is
practically done simultaneously in the aeration tank in small plants (up to 10.000 P.E),
or in a separate aerated stabilisation basin. The aerobic stabilisation process is much
more energy consuming than anaerobic fermentation. Also the de-watering of the aerobically
stabilised sludge is more difficult and energy consuming. For this reason aerobic
stabilisation is only applied in small treatment plants. (up to 10.000 – 25.000 PE).
Supernatant liquids and process water
The supernatant liquids from all processes for thickening (static thickeners, mechanical
de-watering) are usually charged with high loads of nutrients and also COD. For that
reason different strategies for adding the waters to the plant or even for separate treatment
are applied.
A general flow sheet showing the mass streams in a typical wastewater treatment plant
designed according to the state of the art is shown in Figure 11-2
Figure 11-2 Mass streams in a typical wastewater treatment plant, according to
[KREUZINGER, 1998]