Main-stream Partial Nitritation/Anammox process coupled to a MBR-reactor

The practical implementation of this process is currently limited to side-treatment (for example with warm reject water after the dewatering of digested sludge). The concept proposed in this TKI project could be used for main-stream nitrogen removal via Anammox in wastewater treatment, including potential application at lower temperatures.

Goal

The aim of this project was to test the Anammox process for the removal of nitrogen from communal and industrial wastewater in main-stream mode in a small, modular membrane bioreactor (MBR), making use of fine screening, online monitoring and remote control.

Technology

During the project, communal and industrial wastewater was treated with the Anammox process using a small, modular MBR pilot. In broad terms, the plant consists of a partial nitrification reactor (PN) and an Anammox reactor followed by an MBR system (see Figure 1).

By using a selective removal technique based on size (fine screen), the desired Anammox bacteria, which grow in the form of granulates, are separated from unwanted microorganisms originating from the excess sludge. The separated Anammox granulates are then returned to the Anammox reactor. The excess biological sludge with the unwanted microorganisms is transferred to the sludge treatment unit.

Challenge

The Anammox process is already in use, for example to treat reject water at wastewater treatment plants, but at higher temperatures (around 30°C). Application in the communal main stream at lower temperatures (10-20°C) presents a major challenge. The desired biological response is a conversion of over 50% of ammonium to nitrate by means of ammonium oxidising bacteria (AOB), and then the conversion of the remaining ammonium with the formed nitrite by Anammox bacteria. However, there is competition for nitrite between Anammox bacteria and nitrite oxidising bacteria (NOB). The retention of active Anammox bacteria in a reactor is a challenge because they grow very slowly at low temperatures.

Developments

This project fell significantly behind schedule due to the corona crisis, principally because of delays in the construction and the shipping of the pilot plant from China. The pilot plant finally arrived at the site in Germany in the autumn of 2021. Following initial operational tests and start-up, the plant began operating on-site in Simmern in November 2021. It was finally decided to focus the research entirely on the treatment of this communal waste water (with a relatively low temperature). The results were nevertheless assessed by comparing them with those of an existing industrial Anammox plant in the Netherlands.

Summary of results

This project tested the two-stage (PN-Anammox) pilot plant with communal waste water. The aim was to investigate the long-term stability of the PN/A process and the effect of low temperatures on the Anammox process. The pilot plant with suspended biomass was tested at the Simmern WWTP for a period of 214 days with a start-up in winter, and a second period of 80 days with a start-up in the autumn. The concentration of dissolved oxygen was used to control aeration in the PN reactor. During the trial period, COD removal efficiency in the pilot plant was stable at between 90 and 95%. However, nitrogen removal by the pilot plant was unstable due to problems with the influent and air supply problems caused by specific malfunctions.

Throughout the trial period, there were several operational problems with the pilot plant, as a result of which the plant did not achieve stable operation.  The plant operation demonstrated that aeration control alone is not sufficient to achieve the desired conversion of ammonium. It emerged that most of the ammonium from the influent was consumed in the PN reactor, and that most of it was oxidised to nitrate (rather than nitrite), despite low DO concentrations. In general, no nitrite was built up in the PN reactor at any time during the operation of the plant and this may have limited Anammox activity to such an extent that it was ultimately not possible to demonstrate “cold” Anammox conversion.
The Anammox granulate found did demonstrate the feasibility of using fine screening to selectively retain the Anammox biomass. The application of the MBR ensured that the COD concentration in the effluent was stable.

COD pretreatment may still be necessary to reduce COD levels in the influent and the COD:N ratio. The main challenge for the pilot plant continues to be the initiation of nitritation in normal conditions. The final report describes optimisation possibilities from the standpoint of operation, performance and evaluation. Since this pilot plant will operate in Germany and China in the future, suggestions for optimisation, particularly with regard to control strategies, can be used to improve the operation and performance of the process and the pilot plant.

For a full description of the tests conducted, please go to the final report, which can be accessed through the link below. The report is in English.