Uncoupling the solids retention times of flocs and granules in mainstream deammonification: A screen as effective out-selection tool for nitrite oxidizing bacteria

•More than 80% of NOB washout through screen is key for out-selection.•Efficient retention of anammox granules is achievable by screen.•Appropriate screen size and structure needs to be chosen for separation purposes.•High N removal efficiency of 70% is achievable while applying screen selection.•Vibrating screen retains 5 times more anammox activity than vacuum screen.

This study focused on a physical separator in the form of a screen to out-select nitrite oxidizing bacteria (NOB) for mainstream sewage treatment. This separation relied on the principle that the NOB prefer to grow in flocs, while anammox bacteria (AnAOB) reside in granules. Two types of screens (vacuum and vibrating) were tested for separating these fractions. The vibrating screen was preferred due to more moderate normal forces and additional tangential forces, better balancing retention efficiency of AnAOB granules (41% of the AnAOB activity) and washout of NOB (92% activity washout). This operation resulted in increased NOB out-selection (AerAOB/NOB ratio of 2.3) and a total nitrogen removal efficiency of 70% at influent COD/N ratio of 1.4. An effluent total nitrogen concentration <10 mg N/L was achieved using this novel approach combining biological selection with physical separation, opening up the path towards energy positive sewage treatment.

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