Mass balances, biological phosphorus removal plants with activated sludge
in
in
hydrolysis/fermentation
Q · SHAc,l + Qs · SHAc,S + Vs,HAc · kh · S2 -Vz -
out
= Qz.2 · 5HAc,2.2
bio-P HAc uptake
lY,HAc · Vz
(8.1)
The process is so far carried out in activated sludge systems as full-scale biofilm
plants have not yet been developed for biological phosphorus removal.
Normally, Expression (8.1) can be simplified by assuming that:
SHAc,S= 0
(all easily degradable organic matter has been removed in the
treated water I recycled sludge)
Based on Expression (3.36), the removal of easily degradable organic matter, rv,HAo
can be written as:
rv,HAc= kHAc(SHAcf(SHAc+ Ks,HAc)) · Xs,PAO
(3.36)
This can be simplified into:
rv,HAc= kp (SHAcf(SHAc+ Ks,HAd)
(8.2)
where kp
is 1-2 kg COD(S)/(m 3 - d) at 20°C,
Ks,HAc is 2-6 g COD(S)/m 3 .
A combination of Expressions (8.1) and (8.2), and an assumption that SHAc,s = 0,
gives for an ideally mixed anaerobic tank Vz:
(kp · V 2 · SHAc,2)
Ql · SHAc,l + VS,HAc· kh · S2 · V2) = Qz · SHAc,2
(SHAc,2 + Ks,HAc)
(8.3)
SHAc,2can be found from Expression (8.3) if the influent (Ql" SHAc,I), tank volume
(V 2) and the hydrolysis (kp, Ks,HAc) are known.
The removal of easily degradable organic matter in the anaerobic tank, rv,HAo is
closely related to the biological phosphorus uptake, see Example 3.11 and Expression (3.37). In a simplified form it can be written:
rv,p04 = VHAc,P04' rv,HAc
(8.4)
where rv,p04
is a fictitious biological phosphorus uptake rate in the anaerobic
tank (in reality the uptake occurs in the aerobic tank),
VHAc,P04 is the stoichiometric coefficient between easily degradable
organic matter (HAc) and phosphorus (P04). According to
Example 3.11, VHAc,P04- 0.1 g P /g COD.
The mass balance for phosphorus over the whole plant looks as follows:
in
out
Ql · CP,t = Q4 · CP,4 +
274
surplus sludge
Q6 · CP,6
(8.5)