A membrane supported biofilm apparatus has a plurality of hollow fiber gas permeable membranes in a tank containing water to be treated. A biofilm supported on the membranes occupies between about 40% and 80% of the volume of water to be treated in a reactor. Wastewater treatment processes are described. A process to denitrify water or treat oxidized contaminants comprises introducing hydrogen into an inner volume of the membranes to grow autotrophic organisms in the biofilm near the membrane and heterotrophic organism near the water. Another process is operated as a biomass concentration of at least 10 g/L and up to about 40 g/L to maintain a biofilm having a surface area of over 1000 square meters per cubic meter of tank volume. A hybrid process has suspended biomass and a membrane supported biofilm.