Advances In Wastewater Treatment - Reducing Energy Use While Improving Effluent Quality

In recent years there has been significant research into reducing total nitrogen to meet licenced effluent quality from STP’s, with tightened limits progressively being applied. Water authorities are also investigating opportunities to reduce costs by reducing aeration and external carbon requirements, and where possible, redirect carbon to energy generating processes. These strategies are inline and/or directly relevant to industry trends in Asia Pacific, and in particular where tight nutrient criteria are applied, such as the east coast of Australia.

This paper outlines conventional and short-cut nitrogen removal processes, demonstrating the potential reductions in energy use, increases in capacity and improvements in effluent quality that can be achieved. The paper provides an overview of the following nitrogen removal pathways:

Full Nitrification – Denitrification

Simultaneous Nitrification Denitrification (SND)

Nitritation – Denitritation “Nitrite Shunt”

De-ammonification

As Nitrite is an intermediate step for both Nitrification and Denitrification, if the Nitrification process is stopped at the first Nitrite step, and anoxic conditions presented, the nitrite can be reduced to nitrogen gas. This process is known as Nitritation – Denitritation or Nitrite Shunt, and is considered a ‘short cut BNR’ process. Simultaneous nitrification / denitrification (SND) may be considered as the partial condition between the conventional full pathway and the Nitrite Shunt pathway. The short cut BNR processes can lead to significant savings for both oxygen and carbon requirements. Recent advancement in the improved understanding of Nitrite Shunt control strategies may allow for reduced energy consumption through advanced aeration control and improved carbon utilisation for denitrification over carbonaceous oxidation.

A further short-cut in the nitrogen removal pathway is available with the aid of Anaerobic Ammonia Oxidising Bacteria (Anammox). Anammox reduce Ammonia and Nitrite to Nitrogen gas, without the requirement for carbon. With only the requirement for oxygen to partially oxidize ammonia to nitrite, there are significant reductions in aeration and carbon requirements, with 60% less oxygen and no carbon required. Anammox bacteria are slow growing, limiting their application in mainstream wastewater treatment until recently to side stream processes only.

These short cut BNR processes provide the opportunity to redirect wastewater carbon to the generation of biogas and energy generation, and minimise aeration energy consumption and costs. The overall outcome is a facility energy balance that is moving toward energy neutrality or surplus. This is possible whilst achieving enhanced nitrogen removal to maintaining high quality effluent. Mainstream de-ammonification is currently under development by several research groups around the world.

An overview of aeration control strategies to promote SND and Nitrite Shunt, and mainstream deammonification processes is provided.