Treahnent Plants for Phosphorus Removal from Wastewater
In practice we can almost always leave out the sludge production, 0 3 , in connection
with the water balance - but not in sludge mass balances!
By substitution we find CrP.3
CrP.3 = (11 ,000 m 3 /d· 12 g P/m 3 - 10,980 m 3 /d . 1 g P/m 3 )/20 m 3 /d
CrP.3 = 6,051 g P/m 3
If we assume that the content of dissolved phosphorus in the surplus sludge, SrP,3,
corresponds to the total phosphorus content in the effluent (it is definitely wrong), the
content of phosphorus in the suspended solids of the surplus sludge is calculated:
XrP.3/Xss,3 = (CrP.3- SrP.3)/Xss,3 = (6,051 g P/m 3 -1 g P/m 3 )/200 kg SS/m 3 = 30 g
P/kg SS or 3%.
We see that the content of dissolved phosphorus in the surplus sludge (1 g/m 3 ) is very
low compared with the total content (6,051 g/m 3 ), and that the assumption above that
the content of dissolved phosphorus was 1 g/m 3 is of no practical importance for the
end result.
10.2. Mechanisms for chemical/ physical
phosphorus removal
For chemical and physical processes for phosphorus removal it applies that the rate
of both precipitation/ adsorption and ion exchange is so high that this has no
importance for the achieved treahnent efficiency. What happens is that equilibrium
is reached in a few seconds or minutes whereas the hydraulic retention time is
nevertheless much higher. The only exception is phosphate crystallization where
the formation of apatite crystals is a relatively slow process. Hence the design will
in this case be based on the actual precipitation rate.
Chemical precipitation of phosphate takes place in 4 stages (see Fig 10.3)
- precipitation
coagulation
- flocculation
- separation
The precipitation and coagulation stages take place simultaneously as they both
occur quickly.
In connection with the precipitation, the dissolved phosphorus, primarily orthophosphates, is converted into solids of a low solubility by the addition of metal salts,
typically of iron, aluminium or calcium. The precipitated substance is readily
available after the formation of very small particles, that is, colloids which are
particles of less than 1 J!m.
314
In practice we can almost always leave out the sludge production, 0 3 , in connection
with the water balance - but not in sludge mass balances!
By substitution we find CrP.3
CrP.3 = (11 ,000 m 3 /d· 12 g P/m 3 - 10,980 m 3 /d . 1 g P/m 3 )/20 m 3 /d
CrP.3 = 6,051 g P/m 3
If we assume that the content of dissolved phosphorus in the surplus sludge, SrP,3,
corresponds to the total phosphorus content in the effluent (it is definitely wrong), the
content of phosphorus in the suspended solids of the surplus sludge is calculated:
XrP.3/Xss,3 = (CrP.3- SrP.3)/Xss,3 = (6,051 g P/m 3 -1 g P/m 3 )/200 kg SS/m 3 = 30 g
P/kg SS or 3%.
We see that the content of dissolved phosphorus in the surplus sludge (1 g/m 3 ) is very
low compared with the total content (6,051 g/m 3 ), and that the assumption above that
the content of dissolved phosphorus was 1 g/m 3 is of no practical importance for the
end result.
10.2. Mechanisms for chemical/ physical
phosphorus removal
For chemical and physical processes for phosphorus removal it applies that the rate
of both precipitation/ adsorption and ion exchange is so high that this has no
importance for the achieved treahnent efficiency. What happens is that equilibrium
is reached in a few seconds or minutes whereas the hydraulic retention time is
nevertheless much higher. The only exception is phosphate crystallization where
the formation of apatite crystals is a relatively slow process. Hence the design will
in this case be based on the actual precipitation rate.
Chemical precipitation of phosphate takes place in 4 stages (see Fig 10.3)
- precipitation
coagulation
- flocculation
- separation
The precipitation and coagulation stages take place simultaneously as they both
occur quickly.
In connection with the precipitation, the dissolved phosphorus, primarily orthophosphates, is converted into solids of a low solubility by the addition of metal salts,
typically of iron, aluminium or calcium. The precipitated substance is readily
available after the formation of very small particles, that is, colloids which are
particles of less than 1 J!m.
314