Treatment Plants for Phosphorus Removal from Wastewater
10.4.2 Phosphorus binding in soil
Frequently, the phopherous adsorption capacity of a soil is characterized by the
maximum capacity, Xr,a,max, possibly supplemented by the capacity at a given
phosphorus concentration in the soil water, for example 5 g P /m 3 .
There is a big difference in the ability of the different soil types to adsorb phosphorus. The maximum capacity may vary from 20 mg P /kg in sandy soil to 500 mg
P /kg in clay.
In Table 10.7, average figures for 35 soil samples (top soils) in the state of New York,
USA are shown. At the same time, the CEC value for the soil samples in question is
stated. CEC means "Cation Exchange Capacity" and expresses the ability of the soil
to bind cations, for example NI-4 +, K+, Ca ++,Fe+++, AI+++.
By solely considering the phospherous adsorption/ion exchange in the soil and not
the associated slow chemical binding of adsorbed/ion exchanged phosphorus, we
get a conservative estimate of the phosphorus binding capacity of the soil. In /8/ a
phospherous transport model is shown which includes the chemical binding. The
model is based on the expressions for phosphorus binding in the soil as shown in
Section 10.2.4. It should be noted, however, that the experimental basis for estimating the precipitation velocity is extremely vague.
Depth
Soil density
Xr,a,max.
Xr,aat5gP/m 3
Cation Exchange
Capacity, CEC
em
g/cm 3
mgP/kg
mgP/kg
meqv/100g
15
1.3
457
305
18.7
70
1.6
429
334
8.8
94
1.7
189
122
6.5
Table 10.7
Max. phosphorus adsorption in soil, Xr,a,max. and phosphorus adsorption at 5 g P /m 3 in soil water, Xr,a. The
average for 35 top soils in the state of New York, USA, !14/.
Example 10.7
A root zone process for the treatment of wastewater holds 180 m 3 of soil with a density of 1. 7 kg/1. The maximum adsorption capacity, XP,a,max .• is 110 mg P/kg soil.
The plant is loaded with 10 m 3 of wastewater per day with a phosphorus content of 11
g P/m3.
How long does it take before the maximum adsorption capacity for phosphorus is used
up?
It is assumed that the effluent contains 2 g P/m 3 . Adsorption per day is
10 m 3 /d · (11- 2) g P/m 3 = 90 g P/d.
The maximum adsorption capacity is
180 m 3 soil · 1 ,700 kg/m 3 soil · 0.110 g P/kg = 33,660 g P
corresponding to
(33,660 g P)/(90 g P/d) = 374 d.
345
10.4.2 Phosphorus binding in soil
Frequently, the phopherous adsorption capacity of a soil is characterized by the
maximum capacity, Xr,a,max, possibly supplemented by the capacity at a given
phosphorus concentration in the soil water, for example 5 g P /m 3 .
There is a big difference in the ability of the different soil types to adsorb phosphorus. The maximum capacity may vary from 20 mg P /kg in sandy soil to 500 mg
P /kg in clay.
In Table 10.7, average figures for 35 soil samples (top soils) in the state of New York,
USA are shown. At the same time, the CEC value for the soil samples in question is
stated. CEC means "Cation Exchange Capacity" and expresses the ability of the soil
to bind cations, for example NI-4 +, K+, Ca ++,Fe+++, AI+++.
By solely considering the phospherous adsorption/ion exchange in the soil and not
the associated slow chemical binding of adsorbed/ion exchanged phosphorus, we
get a conservative estimate of the phosphorus binding capacity of the soil. In /8/ a
phospherous transport model is shown which includes the chemical binding. The
model is based on the expressions for phosphorus binding in the soil as shown in
Section 10.2.4. It should be noted, however, that the experimental basis for estimating the precipitation velocity is extremely vague.
Depth
Soil density
Xr,a,max.
Xr,aat5gP/m 3
Cation Exchange
Capacity, CEC
em
g/cm 3
mgP/kg
mgP/kg
meqv/100g
15
1.3
457
305
18.7
70
1.6
429
334
8.8
94
1.7
189
122
6.5
Table 10.7
Max. phosphorus adsorption in soil, Xr,a,max. and phosphorus adsorption at 5 g P /m 3 in soil water, Xr,a. The
average for 35 top soils in the state of New York, USA, !14/.
Example 10.7
A root zone process for the treatment of wastewater holds 180 m 3 of soil with a density of 1. 7 kg/1. The maximum adsorption capacity, XP,a,max .• is 110 mg P/kg soil.
The plant is loaded with 10 m 3 of wastewater per day with a phosphorus content of 11
g P/m3.
How long does it take before the maximum adsorption capacity for phosphorus is used
up?
It is assumed that the effluent contains 2 g P/m 3 . Adsorption per day is
10 m 3 /d · (11- 2) g P/m 3 = 90 g P/d.
The maximum adsorption capacity is
180 m 3 soil · 1 ,700 kg/m 3 soil · 0.110 g P/kg = 33,660 g P
corresponding to
(33,660 g P)/(90 g P/d) = 374 d.
345