A comparative study to produce the correct influent for Anammox process from anaerobic sludge reject water (700-800 mg NH4+-N L-1) was considered here. The influent for the Anammox process must be composed of NH4+-N and NO2--N in a ratio 1: 1 and therefore only a partial nitrification of ammonium to nitrite is required. The modifications of parameters (temperature, ammonium concentration, pH and solid retention time) allows to achieve this partial nitrification with a final effluent only composed by NH4+-N and NO2--N at the right stoichiometric ratio. The equal ratio of HCO3-/NH4+ in reject water results in a natural pH decrease when approximately 50% of NH4+ is oxidised. A Sequencing batch reactor (SBR) and a chemostat type of reactor (single-reactor high activity ammonia removal over nitrite (SHARON) process) were studied to obtain the required Anammox influent. At steady state conditions, both systems had a specific conversion rate around 40 mg NH4+-N g(-1) volatile suspended solids (VSS) h(-1). but in terms of absolute nitrogen removal the SBR conversion was 1.1 kg N day(-1) m(-3), whereas in the SHARON chemostat was 0.35 kg N day(-1) m(-3) due to the different hydraulic retention time (HRT) used. Both systems are compared from operational (including starvation experiments) and kinetic point of view and their advantages/disadvantages are discussed. (c) 2006 Elsevier Ltd. All rights reserved.