Design of biological phosphorus removal
At 8°C we have:
SP,4 = 0.005- 0.05 · 0.020 = 0.004 kg P/m 3
If detergents without phosphate are used in the catchment area, the influent concentration might be 6 g P/m 3 .1n such a case, the effluent concentration of dissolved
phosphorus will be close to zero at 8°C as well as at 20°C.
Calculation of phosphorus content in surplus sludge
By using the phosphorus balance, Expression (8.5), the phosphorus amount in the
sludge can be calculated:
(8.5)
The phosphorus amount in the sludge is:
(8.9)
The overall sludge production is found from Expression (4.13) as it can be calculated
for a normal aerobic process:
(4.13)
where C 1 and C3 are the influent and effluent concentrations of organic matter.
The combination of Expressions (8.8), (8.9) and (4.13) give the phosphorus content
in the sludge, fx,r:
282
fx,r = (Ql · Cr,I- Q4 · Cr,4l /Fsr
(8.10)
Example 8.4
Calculation of the phosphorus content in the sludge from Example 8.3 at 8°C.
The total phosphorus in the influent is 10 g P/m 3 . Total COD, C1, in the influent is 300
g/m 3 and in the effluent, C4, 50 g/m 3 . The volume of wastewater is 14,000 m 3 /d.
From Example 8.3 we have: CP.4 = SP.4 + XP,4 = 4 + 1 = 5 g P/m 3 .
It is assumed that 0 4 = 0 1.
It is also assumed that Y obs = 0.7 g SS/g COD.
By substitution into Expression (8.1 0) we find:
fx,P = (0, · CP,1 -04 · CP,4)/Fsp
fx.P = (CP,l- CP,4)/(Yobs (C,- C3))
Substituting known values give:
fx,P = (1 0- 5 g P/m 3 )/(0.7 g SS/g COD(300- 50) g COD/m 3 )
fx,P = 0.029 g P/g SS
that is, a phosphorus content of 2.9% of SS.
(8.10)