5 stages

 

The introduction of an anaerobic zone at the head of the process provided the stimulus for the selection and growth  of phosphorus accumulating organisms (PAO’s) which will remove phosphorus from the effluent through wasting of the P rich sludge. Much of the development of these processes occurred in South Africa, and most plants currently in use have evolved from there. Barnard’s pioneering work in the 1970s helped to strictly define the operational requirements for EBPR, although this phenomenon of EBPR was reported much earlier in a number of places.
This process in combination with the 4-stage Bardenpho configuration gave rise to what is known as the 5-stage Bardenpho or Phoredox system, a plant configuration capable of removing both N and P.
Barnard also established that the minimization of the return of nitrate to the anaerobic zone was crucial to efficient P removal and this was assisted by a lower TKN/COD ratio of the incoming wastewater (desirably <0.07-0.08).  Similarly high concentrations of the so-called readily biodegradable (soluble) fraction of the wastewater and adequate anaerobic retention time will promote increased overall P removal in this process.
The use of this process to treat influents with a TKN/COD ratio of > 0.08 is not recommended as the high nitrate levels in the sludge return cycle will reduce the amount of readily biodegradable organic carbon available for anaerobic uptake by the biomass.
The anaerobic zone at the head of the process provides the stimulus for the selection and growth of phosphorus accumulating organisms (PAO’s) which will remove phosphorus from the effluent through wasting of the P rich sludge.
The minimization of the return of nitrate to the anaerobic zone is crucial to efficient P removal and this is assisted by a lower TKN/COD ratio of the incoming wastewater (desirably <0.07-0.08). Similarly high concentrations of the so- called readily biodegradable (soluble) fraction of the wastewater and adequate anaerobic retention time promotes increased overall P removal in this process.
Anaerobic retention needs to be long enough to maximize the uptake/P release that occurs here.  Excessive retention times are not productive, and ranges from 30 min to 3 hours have been commonly reported.
The use of this process to treat influents with a TKN/COD ratio of > 0.08 is not recommended as the high nitrate levels in the sludge return cycle will reduce the amount of readily biodegradable organic carbon available for anaerobic uptake by the biomass.
The low efficiency of nitrogen removal in the secondary anoxic zone led to the ultimate demise of the popularity of this process in favor of the 3 stage process, unless the secondary denitrification zone is enhanced by the addition of external carbon sources like methanol.
This process also commonly uses a flash aeration reactor to clean up ammonia and condition (make aerobic) the mixed liquor by raising its redox potential prior to secondary clarification. Another function of flash aeration is to remove any bubbles of nitrogen associated with the floc – an issue often caused by the rapid denitrification arising with methanol dosing.
Both this plant and the 3 stage favor N removal over P removal by their nature.