Powerstep

Air stripping is a proven technology which utilises ambient air to strip ammonia in a first stripper column, which is then recovered into sulfuric acid in a second column. Air stripping is typically operated at 70°C and pH 10 in columns filled with carrier material to enhance surface area for gas-liquid transfer. Air stripping is most cost-effective at high ammonia concentrations and is limited to a maximum elimination of nitrogen due to the chemical equilibrium of ammonia.

Deammonification process can be configured in one or two reactors. Performing deammonification in two reactors, where aerobic nitritation and anoxic ammonium oxidation are performed separately, gives better possibilities for optimising the growth conditions for AOB and anammox bacteria. On the other hand, deammonification in one single reactor benefits from generally lower investment costs and lower demand for operational maintenance. To allow simultaneous nitritation and anammox process, one stage deammonification is carried out in biofilms, where the biofilm can grow on carriers, or be in granules or flocs. In the biofilm, the AOB are enriched in the outer layer of the biofilm and the anammox bacteria in the anoxic inner layer of the biofilm. Specific operational conditions (pH, temperature, oxygen level) are maintained within the reactor to allow the process specific bacteria to grow in the biofilm.

Deammonification for side-stream treatment is a state of art technology, where high ammonia concentration with low COD concentration are typical characteristics of the side-stream water from dewatering of digested sludge. This reject water is usually sent back to the main treatment line, where the treatment of this ammonia load through conventional nitrification/denitrification can be quite cost-intensive. The use of deammonification process on side-stream water can dramatically reduce the nitrogen load on the existing biological treatment line. It is also a way to treat a part of the nitrogen at a low energy. In addition, since it allows a reduction of the nitrogen load on the main treatment line, it is also a solution to upgrade an overloaded existing wastewater treatment plant at a low cost.

Optimising pH control (e.g. by CO2 stripping combined with some caustic) and temperature conditions minimises the efforts in heat and chemical costs, and allows cost-effective operation of membrane stripping. Pre-treatment of the sludge liquor with coagulation/filtration also has to guarantee particle-free influent to the membrane in order to protect the membrane module from mechanical damage and clogging.

The second step of nitrogen conversion is denitrification, which converts nitrate to gaseous nitrogen. Denitrification can be realised by many groups of bacteria, but it requires anoxic conditions to promote the use of chemically bound oxygen of the nitrate. In addition, denitrification requires a carbon source as electron donor, which is an important aspect in WWTP operation. Organic matter in the wastewater acts as carbon source, so that the ratio of bioavailable organics (COD) and nitrogen should be sufficient to guarantee full denitrification. Different configurations are available for WWTP operation such as intermittent denitrification (variation in oxygen supply over time), pre-denitrification with recycling of nitrate to anoxic tanks upstream of nitrification tanks, or cascade denitrification with multiple serial tanks of anoxic and aerobic conditions. Utilisation of microbial sludge as carbon source (“endogenous denitrification”) can also be an option, although the kinetics of this process are limited by hydrolysis of the sludge and thus require higher tank volumes. The dosing of an external carbon source (e.g. acetate) can be a last measure if residual COD for denitrification is limited.