The new process has already been adopted by a number of wastewater treatment plants (WWTPs), such as the Zurich Werdhölzli plant and the plants in Niederglatt (Canton Zurich) and St Gallen. Other plants – in Thun, Bilten (Canton Glarus) and Dietikon – are employing a very similar method promoted by the German-owned company Cyklar-Stulz GmbH. The plant operators' experience has been positive, and Switzerland is regarded as a leader in this field – thanks not least to the support and consulting services provided by Eawag.

Key factors: anammox bacteria and sludge flocs

The key to the new method is a bacterial process discovered around 15 years ago by Eawag and Dutch scientists – anaerobic ammonium oxidation, or anammox. Anammox bacteria can convert ammonium (NH4+) to harmless nitrogen gas without the need for an external carbon source. To initiate the process, part of the ammonium first has to be oxidized with oxygen to nitrite. With this nitrite, the bacteria then oxidize the remaining ammonium to molecular nitrogen. Previously, the two phases of this process have been segregated. However, two-step systems require more space, and process regulation is highly complex. The breakthrough came when it was realized that the conversion process can also be implemented in a single step – i.e. when, as a result of flocculation, both aerated and anaerobic zones are present at the micro-scale in an anammox reactor: aerobic ammonium oxidation takes place on the outer layer of sludge flocs (barely a millimetre in diameter) consisting of bacterial colonies, while the anaerobic anammox reaction takes place in the anoxic inner part of the flocs.

Numerous advantages over the conventional method

Compared to conventional nitrification/denitrification, nitrogen removal from digester liquid with anammox bacteria offers major advantages: in particular, the aeration requirement is more than halved, and an external carbon source (e.g. methanol or acetate) is no longer needed. This reduces energy consumption and operating expenses, leading to costs of around CHF 2 instead of CHF 4 per kilogram of nitrogen removed. In the case of the Zurich Werdhölzli plant, this amounts to annual savings of approx. CHF 500,000. In addition, the process is highly efficient: more than 90% of the ammonium in the process water is converted to nitrogen, relieving pressure on the plant. Consequently, more organic substances (e.g. waste from food production) can be added to the digestion mixture; the WWTP then produces more biogas. This moves us closer towards one of the long-term objectives of all wastewater professionals – that WWTPs should become net producers rather than consumers of energy, or at least operate without an external power supply.

 
 

Benefits of knowledge and technology transfer

 Process engineer Adriano Joss emphasizes: "We at Eawag are not the only ones who have worked on the anammox method." But in contrast to companies that have sought to patent this process – which also occurs in nature – Eawag is committed to disseminating its knowledge and experience in this area. After all, the developers and researchers involved agree that the anammox process must be widely adopted if wastewater treatment is to become more sustainable. However, as Joss admits, "We still don't know all the details about these processes." But, in his view, effective solutions can be more rapidly identified via open research and broad sharing of experience than via commercial secrecy and patents, and he adds: "Ultimately, good plant performance and confidence in the new method are also beneficial for private sector players."

Reducing wastewater treatment costs and energy consumption