The addition of ferrous iron induced a ‘stable and excellent’ COD conversion rate. As the COD load increased from 2.0 to 10.0 g COD/l day, the settling characteristics of sludge also improved while in the control reactor they did not alter. It became possible to increase liquid upflow velocities
in the iron-fed reactor due to its sludge bed having well-defined, larger, granules capable of resisting high hydraulic pressures.