Low strength wastewater anammox start-up by stepwise decrement in influent nitrogen: Biofilm formation mechanism and mathematical modelling

•A mainstream anammox MBBR was started in 120 days by stepwise decreasing influent N.•4-stage biofilm formation involved distinct morphology and community composition.•Low-affinity Ca. Jettenia and filamentous Anaerolineaceae were initial colonizers.•Mature biofilm was dominated by Ca. Kuenenia and Ignavibacteriaceae.•Inoculation strategy was optimized based on an anammox biofilm development model.