Treatment Plants for Phosphorus Removal from Wastewater
Several things are wrong: First of all, Expression (10.17) only applies in a narrow concentration interval (1-10 mg P/1), and secondly it is unlikely in practice to have a phosphorus concentration in soil water of 1 00 mg P/1.
10.3. Treatment plants for phosphorus
removal
10.3.1 Precipitants
In practice, relatively few types of precipitants are used, that is, salts of aluminium,
iron or calcium, for example:
Al+++ : Ab(S04)3 · 16 H20
Fe+++ : FeCb · H20
Fe++ : FeS04 · 7 H20
Ca ++ : Ca(OH)2/Ca0
For financial reasons, technical grade chemicals are always used implying a certain
content of impurities, soluble as well as insoluble.
In many cases combinations of the mentioned chemicals, especially Ca ++ + Fe++, are
used.
Furthermore there are polymerized iron and aluminium salts which should have
the advantage of reducing the amount of metal ions which are "wasted" through
side reactions in the form of hydroxides and to enhance the removal of organic
matter.
Through the precipitation we obtain a conversion of dissolved phosphates into
solid phosphates as well as a flocculation. The latter sub-process can in certain cases
be significantly enhanced by supplementing with organic polymers. Normally,
cationic (positively charged) or non-ionic (neutral) polymers are used.
Example 10.5
332
In an activated sludge plant, the simultaneous precipitation is carried out with ferrous
sulphate. 28 g Fe++/m 3 is added whereby 7.75 g P/m 3
is precipitated.
How much oxygen is used for oxidation of ferrous iron into ferric iron, and what is the
change in alkalinity?
From Expression (10.8) we find the oxygen consumption:
(10.8)
Several things are wrong: First of all, Expression (10.17) only applies in a narrow concentration interval (1-10 mg P/1), and secondly it is unlikely in practice to have a phosphorus concentration in soil water of 1 00 mg P/1.
10.3. Treatment plants for phosphorus
removal
10.3.1 Precipitants
In practice, relatively few types of precipitants are used, that is, salts of aluminium,
iron or calcium, for example:
Al+++ : Ab(S04)3 · 16 H20
Fe+++ : FeCb · H20
Fe++ : FeS04 · 7 H20
Ca ++ : Ca(OH)2/Ca0
For financial reasons, technical grade chemicals are always used implying a certain
content of impurities, soluble as well as insoluble.
In many cases combinations of the mentioned chemicals, especially Ca ++ + Fe++, are
used.
Furthermore there are polymerized iron and aluminium salts which should have
the advantage of reducing the amount of metal ions which are "wasted" through
side reactions in the form of hydroxides and to enhance the removal of organic
matter.
Through the precipitation we obtain a conversion of dissolved phosphates into
solid phosphates as well as a flocculation. The latter sub-process can in certain cases
be significantly enhanced by supplementing with organic polymers. Normally,
cationic (positively charged) or non-ionic (neutral) polymers are used.
Example 10.5
332
In an activated sludge plant, the simultaneous precipitation is carried out with ferrous
sulphate. 28 g Fe++/m 3 is added whereby 7.75 g P/m 3
is precipitated.
How much oxygen is used for oxidation of ferrous iron into ferric iron, and what is the
change in alkalinity?
From Expression (10.8) we find the oxygen consumption:
(10.8)