Ammonia treatment for split flows
Preussag Noell Wassertechnik has now built two stripping plants for the recovery of ammoniacal liquor from the wastewater left at the end of sludge dewatering. The plant at Bremen's Seehausen WwTP processes 180m3/d and the one at Cloppenburg 300m3/d. Dr Jörg Rolfs of Preussag explains how the ammonia stripping process works.
The elimination of nitrogen from wastewater is one of the main goals of municipal sewage treatment. Standard nitrogen removal techniques are based on biological processes taking place in large basins. The typical concentration of ammoniacal nitrogen in the feed to such processes is in the range of approximately 50mg/l.
However, ammoniacal nitrogen is not only fed directly from incoming wastewater: there is also an internal circulation in the WwTP, which comes from dewatered digested sludge. Its concentration of ammoniacal nitrogen is in the range of 1000mg/l.
Preussag Noell Wassertechnik of Bremen has developed a technique to treat this split flow using a process which it claims reduces the cost and size of the WwTP. The technique can be used to:
• relieve high-loaded WwTPs
• minimise expansion requirements
• avoid construction of a new WwTP where space is limited
The high concentration of ammoniacal nitrogen in the split flow makes it possible to use a chemical or physico-chemical process for the elimination of nitrogen. There are various ways of doing this. However, when choosing the most suitable technique, it is important that the process used should not produce any product which could cause new problems for the plant operator.
Techniques like precipitation of magnesium-ammonia-phosphate or stripping ammonia by air are to be avoided because it is very difficult to dispose of or market the recovered products. Stripping by steam on the other hand results in a saleable product (ammoniacal liquor), but the process is very energy intensive.
The fact that steam stripping of ammonia generates a saleable product but is not energy efficient because of conventional steam production gave Preussag the idea of developing a 'Horizontal Tube Spray Film Evaporator' with steam compression.
Preussag tested this evaporator on highly polluted industrial wastewater and the experience gained was used to develop the 'Ammoniak Stripping-Verfahren' (ammonia stripping process). The ammonia treatment process not only gives a product with considerable market value but is energy efficient as well. The process can be divided into three main steps.
The first step consists of pre-treatment, where the pH value is raised and insoluble particles are separated out. The pH value has to be raised to move the chemical equilibrium of ammonia from the ionic form to the molecular form, which can then be stripped.
Lime is used to raise the pH value. Thus, calcium carbonate is precipitated because the concentration of carbonate in the wastewater from sludge dewatering is high. It forms a calcium carbonate sludge of high purity, which can be easily separated from the wastewater.
During the process of calcium carbonate precipitation other insoluble particles are also incorporated, leaving the wastewater very clear. Pre-treated wastewater is then heated to the stripping temperature.
Effective heat transfer
(Using the heat content of the condensing steam)
This hot wastewater is then fed to the stripping column in the second stage. In the stripping column, steam is passed in counter-current flow and takes up the ammonia. The ammonia-loaded steam is then evacuated from the stripping column, compressed and condensed in the Horizontal Tube Spray Film Evaporator. Here, the heat of condensation is used to evaporate wastewater and produce fresh steam.
The condensed ammonia/water mixture with an ammonia content of about 1% is passed to the third stage, where it is concentrated in a rectification column* to a product quality of 25%. This product is then stored in a special depot with the necessary safety equipment.
The ammonia reduced outflow at the ammonia treatment plant has a remaining ammonia concentration which is in the range of normal municipal wastewater.
The end-products of this process are ammoniacal liquor and a dewatered calcium carbonate sludge. The ammoniacal liquor has the quality of the standard industrial product and it can easily be sold to large incineration plants or refuse incinerators. The latter need the ammoniacal liquor for flue gas denitrification.
The calcium carbonate meets the requirements of German laws on fertilisers and can therefore be used in agriculture. It can also be used in the cement industry.
Using waste heat
The process can be adapted so that waste heat from digester gas incineration or sludge drying can be used for steam generation - an alternative for WwTPs with sufficient waste heat.
Both processes have now been used commercially, the first in Bremen for 180m3/d of wastewater from sludge dewatering, the second in Cloppenburg for nearly 300m3/d.
* Rectification (purification of a liquid by distillation)