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Relationship between Mono Di and trichloramine
The relationship between mono, di and trichloramine is a
function of the pH of the water.
From German DIN standards 19643 “Conditioning
Swimming Pool Water” chloramine production is a function of the pH of the
water. The graph opposite give a graphical representation of the chloramine
form against pH.
By way of
example, if we take a pH of 7.4, then there will approximately twice
as much mono chloramine as opposed to dicloramine in the water. There
should be no trichloramines present in the system. However it is the
trichloramines that have the characteristic smell, and it is predominantly
trichloramines that sting your eyes. This means that there must be a
low pH environment at some point in contact with the water.
The low
pH is caused by the biofilm which can develop on every surface in
contact with the water. By changing the sand to AFM you reduce the bacterial
load by around one million times. However there will still be bacteria
growing on the pipe work and on the tiles. NoPhos will remove or reduce the
growth of these bacteria and should be dosed continually into the water.
The NoPhos dose rate should be adjusted to give a zero phosphate reading.
If the
above is accomplished and you still have trichloramines present in the
system, it will be a function of the operating procedure, the equipment used
or there is a dead area in part of the water treatment circuit. Analysis of
the water needs to be conducted at different points in the system to
determine the source of the contamination.
Equations
Chloramines are formed in
water by reaction between nitrogen hydrocarbons , more especially ammonia
and compounds like such as urea (NH2)2CO. The
following chlorine reactions which are pH ( taken from
German DIN standards 19643 )
NH4+
+ H2O Û H3O+ + NH3
(bacterial mineralisation decomposition of urea from urine, sweat, etc.)
NH3+Cl2 Þ
NH2Cl monochloramine............................
pH 6 to pH 8
NH2Cl
+ Cl2 Þ NHCl2 dichloramine
.............................. pH5 to pH6
NHCl2
+ Cl2 Þ NCl3 trichloramine
................................pH <5
NH2R
+ Cl2 Þ
NHRCl alkylmonochloramine
(Û
denotes a reversible (equilibrium) reaction; Þ
denotes a forward reaction.)
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Total combined chlorine equates
with approximately 1.4 times DPD2 (monochloramine). If the calculation
gives an under estimate of combined chlorine as measure by DPD3, and there
are no smells of Trichloramine or sore eyes among the bathers. This means
that there are high levels of organics such as surfactants in the water.
Alternatively if high levels of free chlorine are used, organics removed by
the media bed filters will start to oxidize, which again leads to high DPD3
combined chlorine results. The solution is to reduce the chlorine levels and
improve filter back-wash frequency or performance.
If there are high levels of
dichloramine & trichloramine, and the public have eye irritation, then there
is a problem in the pool water treatment circuit. There will be a dead area
with a high level of bacteria in the system and a low pH, or generally a
high level of bacteria in the form of biofilm in contact with the water. The
most likely source of the bacteria and low pH location will be the sand in
the sand filters.
According to the equations trichloramine production occurs predominantly
below a pH5. However swimming pool water will normally have a pH between
6.8 and 7.6, so how can trichloramine be produced? Certainly trichloramine
cannot be produced in the water because the pH is too high, however every
surface in contact with pool water will have a thin biofilm, and within the
biofilm the pH of the water will be acidic. Trichloramine production
therefore takes place on every surface in contact with the water that has a
biofilm and the thicker the biofilm the greater the production. |
