Water Treatment - CHEMISTRY -Phosphates

Dirk ,
  WOW!  You have asked some good questions.  I think all
aquarists have wondered at one time where does that tap
water come from and how did it get this way.  I will attempt
an answer.
   Let me say that an aquarium with live plants is like trying
to create a piece of the real world in a confined space.  It is
not an easy task to imitate nature!  We must think in terms
of ecosystems.  Everything is linked together.  The tropical
stream is poor in dissolved nutrients and is very soft.  It is
the biomass that has the nutrients.  The system is in
constant flux.  This situation is fantastic and very hard to
reproduce in a tank.
  Ok, in a nut shell - Water Treatment
  If the water you get from the faucet comes from a surface
water plant the following takes place. (A surface water plant
takes its raw water from a stream or  lake or reservoir).
  The water is pre-disinfected - chlorine or chlorine dioxide or
potassium permanganate is used at this stage.  All the
phosphates and nitrates and bacteria are oxidized out of the
raw water.  Water plants down stream of large cities have
more problems and expense 'cleaning' the water for drinking
  If algae is a problem in the raw water copper sulfate is
added to the reservoir or lake.
  Next, we add coagulant chemicals - coagulation in water
treatment refers to the bringing together of all the bacteria,
viruses and suspended colloidal particles in the raw water. 
Alum(aluminum sulfate) is the most commonly used
coagulant.  The pH of the raw water is adjusted to be
between 6.0 to 7.0 to make the most productive flocculation. 
The alkalinity must also be adjusted prior to the addition of
the alum.  Lime, soda ash, sodium hydroxide are a few of the
chemicals used. 
   Large particles called floc form in the water.  The floc tends
to stick together and grow into settleable masses.
   The coagulated water enters the settling basin at a very
slow rate of speed.  The floc falls to the bottom.  The clean
water runs off the top.  
   The water is now filtered thru a sand and gravel filter. 
There are many ways of constructing a filter; most are simple
sand and gravel filters, the filter has a specific purpose in
water treatment.  That is to remove any particles that may
not have settled from the water in the settling basins.  This is
where you stop cryptosporidium which is a parasite that is
not affected by chlorine.  
   Finally, chlorine is applied to the water to establish a
chlorine residual of at least 1 ppm (mg/l).  Some cities use
chloramines but luckily not here in Atlanta.   This residual
continues to disinfect the water after it leaves the plant and is
a safety measure used to protect the public's health. 
   Corrosion Control  - if necessary the water system has to
add lime to adjust pH or a phosphate material to protect the
piping system.  Sometimes a phosphate is used to
sequester iron in the water (hold the iron in solution).  
   Fluoride is also added to a large number of water systems.
I hope I didn't bore you with the above water lesson.

  Lets get back to the questions:
  The phosphates in wastewater and farm runoff are organic
forms. These are oxidized at the beginning of the water plant.
 For corrosion control we add orthophosphate and
hexametaphosphate, etc.  These are long chain phosphates
in the form of salts. So there is no other phosphate in the
water other than that added by the water plant.
  Plants require N, P, K  as macro-nutrients. N=nitrogen,
P=phosphorous, K =Potassium.  All other nutrients are
micro-nutrients.  The nitrogen can be in all kinds of forms
some are urea, ammonia, ammonium nitrate, ammonium
sulfate.  Nitrate is the most reduced form and it is why we do
water changes.  Unless you have a denitrator on your tank.
(expensive).  The Higher plants use ammonia and nitrate as
a nutrient.  Some higher plants only use ammonia.  Well,
algae can out compete the higher plants because they live in
poorer quality conditions. 
   Phosphate fertilizers are in the form of phosphoric acid,
ammonium phosphate, normal superphosphate.  Also, the
fish food and fish waste contribute inorganic phosphate as
the result of metabolic breakdown of proteins and of the
urine.  However, the phosphates are a nutrient for the algae. 
Do not add any phosphate to your tank.  The fish waste and
the food are doing this enough.  Yes, the plants are using the
phosphate for growth.  
   I have used Seachem Phosgard to remove phosphates
from my tap water.  It works. but it is expensive and I don't
know what else it is removing!  It is an ion exchange media
and it is taking out many different ions I assume!
  The real hard part with keeping aquatic plants is getting
good information.  And from what I have read the big question
asked is: "Where do the 'aquatic' plants get there nutrients -
the substrate thru the roots or from the water thru the
leaves".  Well, in my opinion it is the substrate.  Because
this is where you have colloids (clay, organic matter) which
can hold a positively charged cation.  These colloids can
hold huge quantities of cations.  This is where the real
exchange of nutrients takes place in terrestrial plants and in
plants grown under hydroponic conditions.  A great book for
all aquatic plant lovers is: "The Optimum Aquarium" by
Kaspar Horst & Horst E. Kipper.
 I hope I have answered your questions. And I hope I have
kept it simple enough to understand.
Paul L.  In Atlanta the Olympic City.