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DEMON (DE-amMONification) is a cost-effective technology for the removal of nitrogen compounds from waste water with a high ammonium concentration using nitrification and autotrophic denitrification.

The process was developed by the University of Innsbruck and can now be seen as proven technology. There are currently six full-scale DEMON installations operational:
in Strass (Austria),
Glarnerland (Switzerland),
Plettenberg (Germany),
Heidelberg (Germany),
The largest installation is located in Apeldoorn at the Veluwe Water Board
and the last installation in Nieuwegein (Netherlands).

The biggest installation was delivered by the combination Grontmij/Logisticon Water Treatment.
The installation in Nieuwegein was build in combination between GMB, Grontmij and Logisticon Water Treatment.
DEMON is used for various purposes, including the treatment of rejection water released when fermented sludge is dewatered. In theory, this technology is also suitable for nitrogen removal from the effluent from solid waste fermentation installations, for example.

Compared with other processes for rejection water treatment such as Sequencing Batch Reactors (SBR), Membrane Bioreactors (MBR), Single reactor system for High activity Ammonium Removal Over Nitrite (SHARON), and non-biological treatment methods such as ammonia stripping, the operational costs for DEMON are substantially lower. The process is highly stable.

Process description

Two process actually take place in a DEMON installation. These processes are outlined here.

1. the oxidation of some of the ammonium fed in to nitrite. This partial nitrification is carried out by nitrifiers such as Nitrosomonas;

2. the reduction of the nitrite produced, in which ammonium is used as electron donor. In this reaction, some of the nitrite is oxidised to nitrate. This reaction is carried out by anammox bacteria, and is called deammonification.
The name of the DEMON process is derived from this term.
 Deammonification (Demon) process 1

Compared with other biological nitrogen removal processes, the DEMON process is characterised by:

• a low energy requirement, because only around 50% of the ammonium supplied has to be oxidised;

• it is not necessary to dose an organic carbon source (such as methanol) because ammonium is used as electron donor instead;

• a low sludge output, because of the low yield from the anammox bacteria.

In theory, oxygen (which is needed for partial nitrification) has an inhibitory effect on the anammox bacteria. Nevertheless, in a DEMON reactor nitrification and deammonification do take place in a single reactor volume. This is possible because a low oxygen set point is maintained. This low oxygen content also prevents nitrite oxidising bacteria (such as Nitrobacter) from surviving in the system.

Because of the relatively low rate of growth of the anammox bacteria, a DEMON reactor must be operated with sludge of a reasonable age. Sludge retention is therefore essential.

Despite this, no growth of nitrite-oxidising bacteria such as Nitrobacter takes place, because of the low oxygen set point. A DEMON reactor is operated as a Sequencing Batch Reactor (SBR) in order to enable sludge retention.

The pH is the primary regulating variable in the DEMON process. The advantage of this method of regulation is that it prevents the nitrite concentration from becoming too high.

For more information, please see Grontmij in De Bilt.