Effects of temperature gradients on AOB/NOB competition in MABR biofilms

The membrane aerated biofilm reactor (MABR) is an emerging wastewater treatment technology that can greatly decrease energy requirements for wastewater treatment. It consists of cassettes of air-supplying, hollow-fiber membranes that can retrofit existing activated sludge processes. MABR behavior differs from conventional biofilm processes due to the counter-diffusion of the electron donor (ammonia) and acceptor (oxygen).

 

Partial nitrification (PN), or partial nitrification Anammox (PNA), can further improve MABR energy efficiency and cost effectiveness.  To achieve this, ammonia oxidizing bacteria (AOB) must outcompete nitrite-oxidizing bacteria (NOB).  High temperatures favor AOB, but it is not feasible to heat the wastewater influent.  However, high-temperature compressed air can be supplied to the membrane lumen, increasing temperatures inside the biofilm without increasing the bulk temperatures. No previous research has addressed temperature gradients in biofilms, which can lead to gradients in  biodegradation kinetics, diffusivities, and O₂ solubility.

 

The objective of this research was to explore the effect of temperature gradients in MABR biofilms, especially with respect to PN. We used a one-dimensional multi-species biofilm model, which considers the MABR physical and biochemical behavior, especially with respect to temperature. The model was implemented using COMSOL Multiphysics. We also used bench-scale experiments to explore the effect of biofilm temperature gradients on MABR nitrification and PN performances and microbial community structure.

 

Model simulations showed that MABR biofilms exposed to a temperature gradient from 20 ºC (biofilm interior) to 10 ºC (bulk liquid) had a 60% increase in nitrification rates compared with biofilms at 10 ºC. More importantly, the model predicted a complete out competition of NOBs within the biofilm.

 

Preliminary experimental results confirm a significant (105%) increase in nitrification fluxes with a temperature of 30ºC compared to ambient temperatures (20ºC). Future experiments will validate the model predicted effects of biofilm temperature gradients on nitrification fluxes and microbial community structure.