Autotrophic removal of nitrogen from wastewater: future trends

Abstract

Anaerobic ammonium oxidation (Anammox) is considered one of the most promising alternative processes for the conventional nitrification and denitrification. It is an efficient and cost effective process; environmentally sustainable and provides many advantages compared with the conventional process such as lower sludge production, lower oxygen demand, no requirement of an external carbon source and the lower N2O gas emission. It is performed by an autotrophic bacteria of the order of Planctomycetales called Anammox bacteria (characterized with low growth rate). The process can be done either with a single stage, two stages or three stages. In the single stage process there is an interaction between the aerobic ammonium oxidizing bacteria and the anaerobic ammonium oxidizing bacteria in the same reactor. The most common systems used are the granular and the biofilm systems, where the ammonium oxidizing bacteria (AOB) grow in the outer layer. In this process the space is reduced and the maintenance and operational cost are also reduced. In the two-stage process there is firstly a partial nitrification where the nitrite is produced and afterwards there is an Anammox reactor where the nitrite and the remaining ammonium are converted to dinitrogen. This process has more control of the dissolved oxygen and it is ease to optimize. The three-stage process is composed of a pretreatment stage (where the biodegradable organic substances are removed), primary stage (where the partial nitrification occurs) and finally the Anammox process. Compared with sidestream full-scale application, the mainstream is more challenging since the out-selection of nitrite oxidizing bacteria (NOB) and heterotrophic denitrifiers is more difficult. The application of the process in the sidestream of the wastewater treatment plants has been developed positively and is a mature technology, implemented in more than a hundred full-scale plants. Not being the same for the mainstream application, that still has some key factors that concern its performance...