Optimization of partial nitritation in a continuous flow internal loop airlift reactor.

Highlights: [•] A central composite design (CCD) was applied. [•] The factors influencing the -Neff/ -Neff ratio and NAR were investigated. [•] Optimal conditions regarding DO, Alk/ -N and -Ninf were determined. [•] The validity of the response surface models was confirmed in verification trials. [•] Different types of alkali were compared for use in the PN process. [Copyright &y& Elsevier]