The cycle begins with simultaneous influent feeding and
effluent discharge (Fig. 2 and Fig. 6). Influent is pumped into the
reactor from the bottom with an upward velocity of 3e3.3 m h1
without further mixing or aeration. Simultaneously, effluent is
pushed over the overflow weirs at the top of the reactor. In these
anaerobic feeding conditions, easy biodegradable organic carbon
will be converted into storage polymers by glycogen (GAO) and
phosphorus (PAO) accumulating organisms in the granule bed. The
uptake of easy biodegradable substrates by PAOs (Fig. 7h) is
accompanied by the release of ortho-phosphate into the bulk liquid
as can be seen from the relative high phosphate peak at the
beginning of the aeration period. The volume exchange ratio in
each cycle is limited by dispersion of the influent and thus the
imperfection in the plug flow pattern during feeding. Since the
sampling points for nitrogen and phosphorus are located 0.5 m
under the liquid surface (just below the effluent weirs), the influent
feeding can and needs to be stopped as soon as the phosphate
concentration increases at this sampling point, since this indicates
that a small fraction of the influent is reaching the effluent weir.
This occurs when the effective volume exchange ratio is 65%, while
during dry weather flow in accordance with the design this effective
volume exchange ratio is typically between 30 and 40 %.
Ammonium present in wastewater is partially adsorbed by the
biomass during the feeding period (Bassin et al., 2011b).