Re: Water treatment - phosphates and nitrates

> From: Paul Lad <Paul_Lad at mail_dnr.state.ga.us>
> Date: Wed, 14 Feb 1996 13:11:29 -0500
> Subject: Water Treatment - CHEMISTRY -Phosphates
>   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.  
	Since phosphate and nitrate are the most oxidised forms of
phosphorous and nitrogen you can get (formal state +5 in both
cases), you _cannot_ oxidise either out of the water.  What oxidation
will do is to convert lower oxidation state forms to phosphate and
nitrate in solution.  It is possible to remove the phosphate by
precipitation in the flocculation stage, but the nitrate will go
right through the system unless very specialised processes are used
to remove it.  This is very expensive, so one doesn't use water with
much nitrogen in it if at all possible.  Fortunately, most water supplies
in N. America are not (yet) seriously contaminated by nitrates.  The 
situation in Europe is not so favourable, and a lot of groundwater there
_is_ seriously contaminated by nitrates.   We could end up in the same
situation here eventually as agricultural fertilizer and septic system
leachate enter the groundwater - it has happened already in some areas.

>   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.

	This makes free phosphate - it doesn't remove it.
It may be removed in the flocculation stage.

>  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.

	Orthophosphate is _not_ a long chain phosphate, it is the
usual phosphate, PO4---, HPO4--, H2PO4- or H3PO4, depending on the pH.
At pH of interest to us, it will be almost entirely in the middle two
forms.  Hexametaphosphate _is_ polymeric. 

> Fertilization:
>   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.  

	Nitrate is the most _oxidised_ form.

Paul Sears       Ottawa, Canada.