The relationship between mono, di and trichloramine is a function of the pH of the water and the concentration of ammonium and chlorine in solution.

From German DIN standards 19643 Conditioning Swimming Pool Water chloramine production is a function primarily of the pH of the water. The graph opposite  shows the relationship between chloramine formation and pH.

By way of example,  if we take a pH of 7.4, then there will just be free chlorine and  mono chloramine in the water, technically there should be zero or very little dichloramine and trichloramine. However it is trichloramine that has the characteristic smell, and it is predominantly trichloramine that  stings your eyes in swimming pools. This means that there must be a low pH environment at some point in the system in contact with the water. The production of trichloramine is not a reversible reaction, so if there is a low pH at some location,  then trichloramine level will continue to increase

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.  However even if you record a zero phosphate reading you must continue with the NoPhos because phosphate will continue to increase due to the bathers, influent water supply and from other chemical products used in pools.

If the above is accomplished and you still have trichloramine present, 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 chlorine and dissolved proteins and chemicals such as urea (NH₂)₂CO. The following chlorine reactions which are taken from German DIN standards 19643

NH₄⁺ + H₂O  ======>   H₃O⁺ + NH₃  (bacterial mineralisation decomposition of urea from urine, sweat, etc.) 

NH₃+Cl₂    <<======>>     NH₂Cl     monochloramine............................ pH 6 to pH 8

NH₂Cl + Cl₂    ======>>    NHCl₂     dichloramine .............................. pH5 to pH6

NHCl₂ + Cl₂     ======>>  NCl₃        trichloramine ................................pH <5

NH₂R + Cl₂     ======>>          NHRCl    alkylmonochloramine

(  <<======>> denotes a reversible (equilibrium) reaction; =====> denotes a forward one-way reaction.)

DPD 3 reagents for the measurement of total combined chloramines will measure both organo-chloramines as well as inorganic trichloramine.   For a toxicity perspective it is important to know what is actually happening in the water. Unfortunately the water analytical techniques available to the swimming pool industry can not achieve this task. However we can make an approximation by using DPD2 for mono-chloramine analysis and apply an application factor.  The application factor relates to the organo-chloramine intereferance.

Total combined inorganic chlorine equates with approximately 1.4 x (DPD2 - DPD1) =  1.4 x (mono-chloramine) for a sand filtration system  for AFM systems the relationship is closer to 1.05 x mono-chloramine concentration.  The application factor for sand is therefore 1.4 x DPD2 results  and for AFM it is 1.05 x DPD2 results to give you a closer approximation of the total inorganic combined chlorine concentration in the water. If the pH of the water is above 7.2 and there are no biofilms in the system then technically there should be no di-chloramine or trichloramine and the mono-chloramine measurement ( DPD2 - DPD1) will equal the total combined chlorine level.  This is the situation when running pools with AFM with APF, NoPhos and ZPM units under the right conditions.

In Germany, Austria and Switzerland,  the pH is often as low at pH6.8,  from the opposite graph  you will note that there will  be scope for dichloramine to be produced, but not trichloramine.  However dichloramine is a none reversible reaction, and even if a small amount forms becuase of a pH of 6.8, there is an opportunity for it to accumulate.  Fode this reasone we recommend that pools are not operated below pH7.0.  In the UK and other countries the minimum pH is 7.2.

DPD3 - (DPD2 +DPD1 ) = dichloramine and trichloramine,  if the pH is above 7.2 and you are using AFM and NoPhos, and you obtain a high DPD3 combined chlorine level,  but there is no smell of trichloramine, then it is likely that all of the measurement is organo-chloramine  (dissolved organics such as protein and amino acids) and not trichloramine.  If the pH is less than 7.2, then a  proportion of  the DPD3 difference will be dichloramine as well as  organo-chloramines.

AFM filtration systems using APF, NoPhos & ZPM will have a very low bacterial level,  in sand filtration system there is a high bacterial cell biomass in the filters.  The bacteria will actually use  the dissolved organics in the swimming pool water as a food source and will reduce or even eliminate all of the organo-chloramines, however because of the bacterial layer, there is a now a low pH micro environment that produces trichloramine. In a sand filtration system the DPD3 - DPD1 ( or 1.4 x ( DPD2-DPD1) analysis gives a close approximation to the total combined chlorine level. However in an AFM NoPhos system, there is no biological activity and no bacterial assimilation of proteins and amino acids, so there may be a higher organo-chloramine concentrations yet a low combined inorganic chlorine concentration.  AFM system may therefore give a  high combined chlorine level but there will be no smell of trichloramine, and the eyes of the bathers will not be affected. DPD3 reacts with organo-chloramines, so it gives the wrong reading in pools systems.  A more accurate measurement of combined inorganic chlorine is apply the 1.4 for sand or 1.05 for AFM application factor to DPD2 analysis.  .

Low concentrations of dissolved organics are much less harmful than chloramines, especially trichloramine  because they are non toxic, non volatile and are not absorbed through lung tissue into your blood stream.  Dissolved organics still cause an issue when the authorities use DPD3  as a means of monitoring water quality in pools.  However we have known about the organo-chloramine interference of DPD3 since the 1940`s,  it has just never been an issue in the past because sand filters are working as biofilters to remove the organics. The deficiencies of DPD3 analysis has been amplified when monitoring AFM systems because the potential for higher dissolved organic ratio of the water.  However most of the dissolved organics can be removed through coagulation and flocculation with APF, NoPhos and ZPM followed by AFM filtration.    Indeed with the development of APF active polyfloc and ZPM units we can now remove essential all the dissolved organics and ammonium through coagulation and flocculation and achieve  combined chlorine levels consistently below 0.05 mg/l and often below 0.01mg/l  even with DPD3 analysis

While DPD3 is subject to serious interference from organo-chloramines and surfactants etc,  we now have public pools operating in Scotland in which the DPD2 and DPD3 are giving the same reading and  the value is less than 0.01mg/l.  When DPD2 & DPD3 tests give the same reading you know that there is zero  dichloramine, trichloramine, organo-chloramines of surfactants in the water, also when DPD2 - DPD1 is less than 0.01 mg/l you also know there is no ammonium or mono-chloramine in the water.  In effect for the first time in the swimming pool industry we have a  simple means of identifying an end point in terms of the best possible pool water quality that you can achieve.

Best possible swimming pool water is as follows;

DPD2 - DPD1  is less than 0.05 mg/l

 DPD3 = DPD 2 + /- 0.02 mg/l

If you do not have the best possible water then the main reasons are as follows;

 

Key points that may cause high organo-chloramine levels  as  measured by DPD3
Problem	Solution
UVc ( high or medium pressure)	This equipment should not be used in chlorinated or brominated systems,  it will reduced combined chloramines, but increases lipid soluble volatile gas such as THM`s and cyanogen chloride that are extremely toxic.
High water flow rates through the filters	Reduce water flow through the filters to less than 15 m/hr
No coagulation or flocculation	Start APF coagulation at 1 ml of APF per cubic metre of water filtered per hour
Incorrect injection point for APF	Inject between the pumps and the filters, directly into a ZPM unit
Incorrect injection point for chlorine	Inject chlorine after the filter (never before the filters)
Slow turn over rate	For a typical public pool the turn over rate is 4 hours through the filters, for a private pool this may be 6 hours.  If the turn over rate is too slow water quality will deteriorate in the pool.   Turn over rate through the filters should be appropriate for the installation, however it is also a good  idea to recycle the water around the filters and through the pool and a ZPM unit at a higher rate.  The simple action of moving the water actually improves water quality and insures a more stable chemistry.
Back-wash filters at correct flow rate	What goes into a filter must come out again or the organics retained by the filter will react with the chlorine and be digested by bacteria. It is recommended that you air purge for 2 to 5 minutes at 95 m/hr prior to a back-wash. Back-wash frequency should be at least once a week at 45 m/hr for 2 to 5 minutes.  For private filters the air purge is not necessary.
The bathers	all bathers must shower before entering the pool, oils soap, sun tan  lotion etc etc must be  removed for hair and skin, this is very important
Surfactants and cleaning chemicals	Do not use surfactants, and especially anionic surfactant anywhere near the pool.   5ml of anionic surfactant in a 50m pool will throw the DPD3 readings off the scale and will seriously affect the performance of the coagulation reactions.  Best cleaning agent is just the pool water and sodium bicarbonate, use a dilute acid on occasions such as  hydrochloric acid to remove scale. Do not use chemical thermal blankets that contain surfactants
dead or dirty areas in the system	a. contamination below filter bed, best to use a DIN standard filter with nozzle distribution plate. b. dirty scum channels, keep them clean c. dirty balance tank, clean on a regular basis or fit in place cleaning d. filter back-wash pipe work should go down to the floor immediately after the filter, not up to the roof e. poor pipe work installation that leaves a gap in between joins, dirt and bacteria collect in the gaps and cracks f. pipe work that leaves a dead end containing static water g. in correct hydraulics, filter flows not balanced h.  poor grout between tiles, allows bacteria and organics to collect i. contaminated plastic toys, or pool blanket in contact with the water j. a condensing environment and drips of contaminated water into the pool
High free chlorine level	A  high free chlorine level will tend to produce more chlorine reaction product, operate the pool with as low a chlorine concentration as possible. AFM pools with NoPhos, APF & ZPM will generally use up to 90% less chlorine that regular pool system. The chlorine concentration will also be more stable, so it is easier to reduce the levels. For public pools we recommend a minimum concentration of 0.5 mg/l,  but are testing levels as low as 0.1mg/l on system with a high recycle by pass.
mains water supply	check the mains water supply  it may be contaminated with organic matter, or ammonium. Water companies often add ammonium to municipal supplies to form mono chloramines to help prevent the formation of THM`s