Optimised aeration for precise regulation and minimised energy consumption

The organic compounds removed during the biological treatment of wastewater contain significant amounts of chemical energy that can be utilised and used for optimising the treatment processes. And with the knowledge and the technology currently available, you can actually set up energy neutral – or even energy producing – plants.

Aeration is an important part of wastewater treatment and it is the most energy consuming process in the treatment plant. Optimising this process is therefore an important step on the way to an energy producing plant.

What is aeration?

Aeration is the process of adding air to a liquid or a substance. Water aeration thus means adding oxygen to water or increasing its oxygen saturation, and there are a number of different ways to do this.

The most common methods of water aeration are the water-fall and the air diffusion methods. Water-fall aerators break up the water using spray nozzles to increase its contact with and thereby improve its ability to absorb the oxygen. Air diffusion aerators, on the other hand, are pumping air into the water. This creates tiny air bubbles travelling through the water and this way increases the water’s surface area being exposed to the air [1].

Biodegradation

The process of removing contaminants as phosphor, nitrogen and organic compounds in wastewater treatment plants usually contains the steps of mechanical, biological and chemical treatment.

Most biological wastewater treatment systems include aeration as part of the process. The biological treatment uses naturally occurring microorganisms to degrade contaminants in the wastewater, and by adding air to the water, oxygen is provided for aerobic biodegradation of organic materials. The biodegradation will be an extremely slow process in case the oxygen level is not sufficient. On the other hand, an excess level of oxygen may lead to emission of N₂O – a gas way stronger and more violating for the environment than CO₂. Therefore, it is important with a precise regulation of the aeration [2].

Removal of inorganic compounds

In addition, aeration in wastewater treatment is typically used to reduce the level of carbon dioxide, ammonia and hydrogen sulfide and to remove e.g. iron and manganese by oxidation. Even though sulfur compounds are not necessarily damaging, the potentially very bad smell and taste may encourage removal of these compounds from the water. Saturation of the water with nitrogen and oxygen approaching equilibrium with the surrounding atmosphere almost completely removes CO2, hydrogen sulfide and ammonia. And as an alternative to chlorine as a chemical disinfectant, aeration can efficiently remove smaller amounts of sulfur compounds, and both iron and hydrogen sulfide can effectively be oxidized to convert ferrous iron to ferric iron and to reduce hydrogen sulfide to atomic sulfur. These compounds can then physically be filtered from the water [1].

Precise regulation is important

Aeration is the most energy consuming activity on a wastewater treatment plant and this process typically counts for as much as up to 50% of the total power consumption at the plant [3].

Due to the high energy consumption, aeration is seen as one of the most critical steps of wastewater treatment and a well-designed aeration system directly impacts both the effectiveness and the economy. Controlling the aeration process does not only reduce the energy used for the wastewater treatment, it can also reduce the emission of CO₂. However, as already mentioned the energy saving actions may give rise to N₂O emission if the process is not controlled correctly. Thus, a precise regulation is important – not only for the economy but also for the environment.