Treatment Plants for Phosphorus Removal from Wastewater
Added aluminium= (0.08 · 250 g/m 3 )/(27 gAl/mol)= 0.74 mol Al/m 3
Phosphate content= (14 g P/m 3 )/(31 g P/mol) = 0.45 mol P/m 3
The molar ratio AVP = 0. 7 4/0.45 = 1.64 mol Al/mol P
2.
The flocculation takes place in 2 identical flocculation chambers in series. A degree of flocculation of 30 is required.
Find the necessary mean velocity gradient and the tank volume.
From Fig 10.17 we can read that a degree of flocculation of 30 can be obtained at a hydraulic retention time of 55 min. and with a mean velocity gradient, G, of 20 s- 1 . This could
be found by using Expression (10.14).
The total flocculation chamber volume is
V = 9 · Q =55 min.· (180 m 3 /h)/(60 min./h)= 165m 3
Each of the two chambers must therefore have a volume of 82.5 m 3 .
10.2.4 Phosphorus binding in soil
There are many points of similarities between the phosphorus binding in a chemical
precipitation plant and in the soil medium. Large contents of iron, aluminium and
calcium in the soil give a relatively strong phosphorus fixation. However, the
binding does not occur as quickly in the soil as when adding chemicals to the
chemical treatment plant.
The sketch below illustrates the principle in a model which reasonably well explains
the phosphorus binding in the soil.
p
in soil water~
p
p
adsorbed/ion exchanged -
precipitated
in the soil
Phosphorus adsorption/ion exchange to the soil matrix occurs so quickly that
equilibrium can be assumed to exist at the usual infiltration velocities. Subsequently
the adsorbed/ion exchanged phosphorus will slowly be converted to a precipitated
form of relatively low solubility.
The precipitation process can be described as a first order reaction, /8/:
- dXp,a
~ = k1·Xr,a
where XP,a is the concentration of adsorbed phosphorus in the soil, for example mg P /kg soil,
k1
is a first order rate constant.
(10.15)
The concentration of adsorbed/ion exchanged phosphorus in the equilibrium state
can be expressed using an adsorption isotherm, I 8 I:
Langmuir isotherm:
330
Added aluminium= (0.08 · 250 g/m 3 )/(27 gAl/mol)= 0.74 mol Al/m 3
Phosphate content= (14 g P/m 3 )/(31 g P/mol) = 0.45 mol P/m 3
The molar ratio AVP = 0. 7 4/0.45 = 1.64 mol Al/mol P
2.
The flocculation takes place in 2 identical flocculation chambers in series. A degree of flocculation of 30 is required.
Find the necessary mean velocity gradient and the tank volume.
From Fig 10.17 we can read that a degree of flocculation of 30 can be obtained at a hydraulic retention time of 55 min. and with a mean velocity gradient, G, of 20 s- 1 . This could
be found by using Expression (10.14).
The total flocculation chamber volume is
V = 9 · Q =55 min.· (180 m 3 /h)/(60 min./h)= 165m 3
Each of the two chambers must therefore have a volume of 82.5 m 3 .
10.2.4 Phosphorus binding in soil
There are many points of similarities between the phosphorus binding in a chemical
precipitation plant and in the soil medium. Large contents of iron, aluminium and
calcium in the soil give a relatively strong phosphorus fixation. However, the
binding does not occur as quickly in the soil as when adding chemicals to the
chemical treatment plant.
The sketch below illustrates the principle in a model which reasonably well explains
the phosphorus binding in the soil.
p
in soil water~
p
p
adsorbed/ion exchanged -
precipitated
in the soil
Phosphorus adsorption/ion exchange to the soil matrix occurs so quickly that
equilibrium can be assumed to exist at the usual infiltration velocities. Subsequently
the adsorbed/ion exchanged phosphorus will slowly be converted to a precipitated
form of relatively low solubility.
The precipitation process can be described as a first order reaction, /8/:
- dXp,a
~ = k1·Xr,a
where XP,a is the concentration of adsorbed phosphorus in the soil, for example mg P /kg soil,
k1
is a first order rate constant.
(10.15)
The concentration of adsorbed/ion exchanged phosphorus in the equilibrium state
can be expressed using an adsorption isotherm, I 8 I:
Langmuir isotherm:
330