AmmoRe – Recovery of Ammonium from Wastewater

Nitrogen (N) is one of the nutrients which exists in high concentration in the sludge liquor produced in wastewater treatment plants. Conventional wastewater treatment plants use a high amount of energy to recover nitrogen out of the wastewater – and to release it in its unusable molecular form (N₂) into the air. 

Innovative procedure for elimination and recovery of nitrogen

At the Fraunhofer IGB, we research the process of chemical transmembrane absorption as a novel procedure to not only remove ammonium out of waste water, but to recover it for the use as fertilizer instead.

Our approach consists of recovering nitrogen from wastewater, through a  transmembrane chemical absorption process (TransMembrane ChemiSorption, TMCS), for the use as fertilizer in agriculture. Ammonium is hereby provided as an ammonium sulphate solution.

Function principle of the transmembrane chemical absorption

The recovery procedure functions after the principle of gas absorption with membrane contactors, and is characterised by its high selectivity against nitrogen. For this purpose, the nitrogen in water has to be transformed into gaseous ammonia (NH₃). The higher the pH value and the temperature of the wastewater, the higher the ratio of gaseous ammonia.

The hydrophobic membranes in the membrane contactor module hold back liquid substances, but allow gaseous ammonia to pass through their pores. On the other side of the membrane ammonia is absorbed by an absorbing medium, for example sulphuric acid (H₂SO₄), and gets directly transformed into a fertilizer (NH₄)₂SO_4.

Benefits

• High selectivity of the membrane against nitrogen
• Nitrogen gets recovered as an instantly usable fertilizer for agriculture: ammonium sulphate solution

After research at laboratory-scale, we have designed and built our first pilot plants.

The AmmoRe procedure is currently being demonstrated in two pilot projects, to test its performance under real conditions and to optimise the procedure further:

• RoKKa – Sewage sludge as a source of raw materials and climate protection at wastewater treatment plants
• Cométha – SIAAP Syctom innovation partnership for the treatment of organic waste and sewage sludge in the greater Paris area

Our pilot plant consists of four parallel placed hollow fibre membrane modules, which in the current demonstration, treat around 300 litres of process water per hour.

Pilot run at the waste water treatment plant in Erbach

In the framework of the Baden-Württemberg Ministry for Environment, Climate and Energy Policy and the EU-funded project “RoKKa – Sewage sludge as a source of raw materials and climate protection at wastewater treatment plants”, the process is currently being tested at the waste water treatment plant in Erbach.

The pilot plant works under conditions of pH 10 at 40°C, at which the ammonia percentage is, theoretically, around 95 percent. At these conditions, we were able to reduce the ammonium concentration in the process water from 600 to 65 mg/L. These numbers correspond to an ammonium elimination rate of 90 percent.

After the first few operating months, the water vapour transport within our system amounts to an average of 29,7 L/h (0,15 L/m²/h). Compared to the results observed in other similar plants, this figure is equal or even lower.

Current challenges

We are currently optimising the procedure for its extended use. The main challenges encountered are that the process still uses high amounts of chemicals and energy. Furthermore, the concentration of the resulting product is lower than in commercially available fertilizers.

That is because the hydrophobic membranes let other gases besides ammonia through – including water vapour, which dilutes the product. In order to reduce the dilution of the ammonium sulphate solution as much as possible we are testing the plant under different conditions, e.g. reduced temperature. The goal hereby is to adjust the operating parameters, so that water vapour transport is reduced, but the ammonium recovery rate is still high enough.

The procedure is also suitable for the food industry and the agricultural biogas production, whose waste waters are rich on ammonium and phosphate.

A process module for the recovery of phosphor, our ePhos®-procedure, can be added to the existing procedure, to recover phosphor.