Following the emergence of the European Community as a unified trading and economic entity, the various member states are required to follow certain directives governing their social affairs. Many of these directives concern the quality of the air we breathe, and the water that we use for consumption or in industrial processes. In order to comply with these directives a major overhaul of the water industry in the UK was required. For example, in many coastal towns and resorts there has never been any formal sewage treatment since, traditionally, all waste was dumped at sea through outfall pipes. Because this is no longer acceptable, whole new treatment systems are required

Brokenshire, J. L.

Cumming, C. A.

English

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N95- 33281 / / ; o
AN AMMONIA MONITOR FOR THE WATER INDUSTRY
J.L.Brokenshire and C.A.Cumming,
Graseby Dynamics Ltd., Watford, UK.
HISTORICAL BACKGROUND
Following the emergence of the European Community as a unified trading and economic entity, the
various member states are required to follow certain directives governing their social affairs. Many of
these directives concern the quality of the air we breathe, and the water that we use for consumption or in
industrial processes. In order to comply with these directives a major overhaul of the water industry in the
UK was required. For example, in many coastal towns and resorts there has never been any formal
sewage treatment since, traditionally, all waste was dumped at sea through outfall pipes. Because this is
no longer acceptable, whole new treatment systems are required.
CURRENT ORGANIZATION OF THE UK WATER INDUSTRY
For reasons which one can probably surmise, the first part of this overhaul was to privatize the industry.
This consisted of creating ten individual Water Authorities according to geographical location within
England and Wales with separate Authorities for Scotland and Northern Ireland. Each authority is
responsible for the supply of drinking water and water for domestic and industrial consumption, and the
treatment of all wastewaters, to the required standards laid down by both UK legislation and the
appropriate EC directives. Compliance is monitored by the National Rivers Authority which has overall
responsibility for all the freshwater supplies in the UK. The financial side of the industry is monitored on
behalf of the consumer by OFWAT a quasi-government organization or Quango, which has the power to
limit the water rates charged by the individual Authorities.
COST REDUCTION AND REMOTE MONITORING
The ten UK Water Authorities are autonomous companies listed on the UK stock exchange. They
currently face a difficult time in undertaking a huge capital investment programme to update entire
systems, and demonstrate compliance with legislative requirements, while being limited in the price
increases that they can pass on to the consumer. Part of the solution is to increase efficiency and reduce
costs, the latter inevitably entailing manpower reduction. The intention is to reduce manpower by
automating treatment processes and monitoring them remotely from a central system.
In order to automate a process it is necessary to control it; in order to exercise control the process has to
be monitored, and in order to monitor certain fundamental measurements are required. In order to make
these measurements, it is essential that the critical variables are known, and that there exists a reliable
means of measuring each of these variables which is amenable to long term unattended operation, and can
be interrogated remotely. A considerable number of drinking water treatment facilities have been
essentially unmanned for many years, but large sewage treatment works have not, and this is where the
first serious look at demanning is taking place.
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SEWAGE TREATMENT
The first task is to identify the critical parameters in the sewage treatment process. Sewage is the waste
waters and excreta of both domestic and industrial processes, and is an undefined, complex mixture of
chemical and biological components. It is a highly turbid liquid, greyish in color, which normally
putrefies quickly using up any dissolved oxygen. The treatment of sewage must convert this mixture to
relatively clean water prior to returning it to the freshwater system.
The first stage involves removal of large solid objects, and grit derived from road and roof surfaces
during heavy rainfall. After grit separation and screening the sewage is subjected to sedimentation in
which impurities in the form of suspended solids gradually settle and are removed in the form of a liquid
sludge. Secondary treatment involves the use of biological filters in which bacteria, protozoa, fimgi,
worms, larvae and flies convert the impurities in the settled sewage. The final process involves a further
settling or filtering referred to as 'polishing', before the final effluent is returned to the receiving
watercourse.
ESSENTIAL PARAMETERS
In order to monitor and control the entire process for the majority of the time, excluding accidents and
emergencies, there are several parameters which can be used to give guidance on the state of the treated
sewage. These include temperature, flowrate, dissolved oxygen content, pH, and turbidity. The quality of
the final effluent depends upon its biological and chemical activity, and historically these are determined
by the chemical and biological oxygen demand, however there are certain specific eomlxments which
require more detailed measurement, and one of these is ammonia.
THE ROLE OF AMMONIA
Ammonia is present in natural body wastes and is formed by microbial action on the organic nitrogen
containing chemical species present in the sewage. In the time it takes the sewage to reach the treatment
works, virtually all of the nitrogenous species have been converted to ammonia. In addition the rim-off
from farmland introduces ammonia directly or more nitrogenous material to the watercourses. Ammonia
levels are reduced in the sewage treatment process by 'nitrification', the process by which particular
bacteria convert ammonia to nitrate in the presence of oxygen. Ammonia causes specific problems if the
levels are too high in freshwater supplies. Firstly, it is toxic to fish. Fish kills are immediately obvious
and prompt swiR action by the public and the NRA. Secondly, ammonia prevents effective disinfection of
water by chlorination, resulting in drinking water supplies with high bacteriological activity. If a Water
Authority returns treated sewage with a high ammonia content to the river, kills a lot of fish, then
abstracts the water downstream and fails to disinfect it effectively before supplying it as drinking water,
the local populace tends to take a pretty dim view. The concentration of ammonia in water is therefore
regarded as a critical parameter.
AMMONIA IN WATER
Ammonia in water is subject to an acid-base equilibrium:
NH3(g ) + 1-120(1) ¢:_ NH4 + + OH"
for which 1: K b = INn4 +] [OH'] / [NH3]
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Since water is also ionized:
for which 2:
I-t20 ¢, H + + OH-
K w = [H+I [OH-]
For simplicity all activity coefficients have been assumed to be unity.
Equations 1 and 2 can be combined to give:
3: [NH3] / [NH4 +] = K w / K b [H +]
from which 4: log [NH3] / [NH4 +] = log Kw - log Kb + pH
The ratio of free to ionized ammonia is crucial, since it is neutral ammonia which is toxic to fish 1.
If virtually all the ammonia is present in the free form, i.e. at least 99%, then log INn3] / INn4 +] would
be >2. Since K w is typically 3 x 10-15 and Kb is 1.6 x 10 -5 then the pH would be have to be at least 11.5.
By adding KOH to the sample to raise the pH to at least this level, the total ammonia concentration m the
sample can be determined. Knowing the original pH of the sample prior to the addition of KOH, allows
the level of free ammonia in the sample to be calculated.
At the present time the total ammonia level is measured and the neutral ammonia calculated, with varying
degrees of complexity, using the measured temperature, pH and a knowledge of the typical level of total
dissolved solids in the sample. The accuracy of this method is limited primarily by the accuracy of pH
measurement. Tables of data are available 2 which relate the percentage of unionized ammonia to pH,
temperature and total dissolved solids, and also give the total ammonia concentration which would breach
the EC directives for protection offish stocks as a function of these variables. Table 1 gives a selection of
percentages of unionized ammonia as a function of pH and temperature with a total dissolved solid level
of 400mg/l, while Table 2 gives selected concentrations of total ammonia having a free ammonia level of
0.021mg/l ( the current mandatory EC limi0 as a function of pH and temperature for the same level of
total dissolved solids.
Table 1: Percentage unionized ammonia as a function of pH and Temperature
pH Temperature (°C)
5 15 25 35
5.5 0.0036 0.0078 0.0162 0.0321
6.5 0.0355 0.0778 0.1616 0.3196
7.5 0.3542 0.7727 1.5924 3.1063
8.5 3.4330 7.2250 13.928 24.276
9.5 26.226 43.781 61.806 76.224
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Table 2: Concentration of Total Ammonia (mg/i) containing an unionized ammonia level of 0.021
mg/! as a function of pH and temperature with 400mg/I total dissolved solids.
pH Ternperature(°C)
5 15 25 35
5.5 579.1 264.4 127.2 64.24
6.5 57.93 26.46 12.74 6.443
7.5 5.812 2.664 1.293 0.663
8.5 0.600 0.285 0.148 0.085
9.5 0.079 0.047 0.033 0.027
REQUIREMENTS FOR AN AMMONIA ANALYZER
The majority of Ammonia analyzers currently use ion-specific electrodes with pH reference electrodes to
perform the ammonia measurements. The typical life of electrodes in continuous operation is 3 months.
The total ion measurement is performed by raising the pH using caustic solutions until virtually all of the
ammonia is present in the unionized form. The ion-specific electrodes are subject to drift and therefore
require calibration on a continuous basis. This type of analyzer is very expensive to maintain in terms of
the cost of consumables, frequency of replacement and required routine maintenance. In addition the
accuracy of the final result is subject to question.
Any replacement analyzer would need to demomtmte improved perfornmace capabilities in terms of data
reliability, while showing marked improvements in the cost and consumption of consumables, together
with significant reductions in required maintenance. It would also need to be capable of long-term
unattended operation with all data available for transmission to remote locations on demand.
THE NEED FOR SAMPLE PREPARATION
The initial stages of proof of the viability of a new analyzer involve demonstrable performance, under
controlled conditions, using known solutions of ammonia in clean water, at different pH levels over a
range of temperature. These initial studies assess accuracy, precision, stability, consumables and
maintenance with a view to estimating the potential whole life costs of the analyzer.
Having successfully negotiated this phase of the evaluation, the next step is to undertake field trials. It is
at this point that all previous problems pale into insignificance. The actual real world sample is an
unstable liquid of unknown chemical and biological activity, containing a variety of suspended species.
The presence or absence of light encourages bacterial growth as does temperature, and any surface acts
as a growth medium. The only guarantee is that the major component will be water.
The analyzer is required to sample this liquid 24 hours per day for long periods without any maintenance
and without any degradation of analyzer performance. In order to do this some form of continuous sample
preparation is essential.
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SAMPLE TREATMENT
The initial step is to transfer the sample to the analyzer. This has to be done on a continuous basis and, if
a free ammonia measurement is being made, without changing the relevant conditions of the sample. This
usually entails transfer at high velocity through smooth bore, light proof pipework using an appropriate
long life pump.
In order to prevent degradation of the analyzer it is necessary to reduce the amount of suspended material
and to prevent biofouling. Biofouling is caused by the initial coating of surfaces by active organic
materials, followed by the use of these organics as a source of food by bacteria which subsequently
multiply forming films of slime. As these films build up, the bacteria on the original surface wall are
starved of dissolved oxygen in the liquid and die, causing sections of slime to detach from the walls and
flow with the liquid. Biofouling can lead to the loss of ammonia by nitrification resulting in inaccuracies
in the recorded measurements, and also to alteration of flowrates and blockages within small bore tubing
in the analyzer. It is necessary to have a means of continuous filtration together with biocidal and
algaeeidal treatment. All of these processes must be done on a continuous basis, involve minimal low cost
consumables and operate for long periods without maintenance. In addition, none of these processes must
affect the concentration of ammonia in the liquid, nor the ratio of free to ionized ammonia.
Ideally this whole treatment process is continuous and uninterrupted, so that sample is constantly
available for transfer to the analyzer to determine the ammonia concentration.
FREE VERSUS TOTAL AMMONIA
On the assumption that an improved analyzer measures both free and total ammonia, rather than
measuring one and calculating the other; then, provided the concentration of ammonia is effectively
unchanged by the pre treatment process, and that the ratio of free to ionized ammonia is unaltered, a
measurement of the free ammonia content may be made on the treated sample. In order to perform the
total measurement, the pH of the sample must be increased so that all of the ionized ammonia is converted
to free ammonia and the measurement repeated.
Table 3 lists some of the proposed Environmental Quality Standards for ammonia in water in connection
with EC directives 3, where * indicates mandatory limits.
Table 3 WRC recommended EQS values for Total and Unionised Ammonia.
Type Usage Total Unionised
mg/1 mg/l
Fresh Abstraction to Potable Supply
Treatment A 1 0.04
A2 1.17"
A3 3.1"
EC designated Salmonid and Cyprinid waters 0.78' 0.021 *
Non-EC designated waters 0.015
Industrial abstraction for Food Processing 0.04
Salt Protection of saltwater fish and shellfish 0.021
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Therequiredrangesfor detectionof totalammoniare0 - 1 mg/l for drinking water treatment processes
and 0 - 20 mg/l for sewage treatment processes 2. Recorded mean total ammonia levels in UK freshwaters
vary from 0.007 to 8.2 mg/l 1. Published data indicate that in many other EC member States permitted
levels are also exceeded. Similar trends are observed in EC tidal estuaries. Although considerable data are
available there is not always corresponding information on the fishery status at these sites, and the effects
of high ammonia levels on these ecosystems cannot be assessed.
Because of the conservative nature of the Water Industry, the fact that legislation currently requires the
measurement of total ammonia, that the Industry has no experience of IMS as a measurement technique,
and that Graseby is not known as a supplier to the Water Industry; it has been decided that the first
monitor will be completely conventional and measure total ammonia. Once IMS, the technique and
Graseby have been accepted, a free ammonia monitor can then be made available, since there are moves
within the NRA and the WRC to change from measuring total ammonia to free ammonia.
IMS DETECTION OF AMMONIA
As everyone skilled in the art of IMS knows, ammonia is readily detectable, however there have always
been problems associated with obtaining fast response and recovery characteristics. In addition the
ammonia monitor for water is going to be sampling ammonia in liquid water and inevitably high humidity
levels will be present. Although high humidity levels are normally regarded as anathema in IMS, they do
assist in the desorption of ammonia from surfaces. A careful choice of dopant is required to eliminate
interferences and give acceptable ammonia peak resolution and sensitivity, while having a high tolerance
to humidity. Careful attention to the design of the detector front end, the choice of materials, the design of
the water/air interface and the optimization of flow characteristics are all of vital importance in achieving
the required performance characteristics combined with stability and long life.
PERFORMANCE
When we embarked on this project and did a preliminary assessment, we set ourselves a design goal of 6
months for operating periods between maintenance for the complete system, based on what we thought
could be achieved realistically with an IMS system and what we assmned a bought in pre treatment
system would deliver. When we spoke to the WRC and to manufacturers of Ammonia Analyzers and
Filter Systems, the scheduled maintenance for Analyzers ranged from twice daily to two to three weeks at
best, while that for Filter Systems ranged from a few days to a maximum of one month. All of the
required maintenance activities and costs were significant, involving expensive consumables such as
chemicals and filters. The reaction to our proposed design goal was one of incredulity. Our first
experience of a bought-in Filter System enabled us to understand this attitude.
The manufacturer had tailored the system to our requirements and recommended that we use a cross-flow
filter system with a compressed air backflush facility. Two filters were incorporated, one active and one
in standby mode. A daily backflush cycle had was recommended. After installation at our test site and
extensive modification resulting in the backflush cycle being carried out every three minutes, the longest
period of continuous operation achieved was 2 days. At the end of this period the entire filter system had
to be stripped down and manually cleaned.
At the present time after extensive laboratory testing of the analyzer, and approaching a year of field
testing for the analyzer and pre treatment system, we believe that the 6 months maintenance cycle for the
analyzer is achievable, while that for the pre treatment system is currently being pushed towards this
figure.
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ACKNOWLEDGMENTS
The authors would like to thank all their colleagues for their efforts during this development programme.
In addition they would like to express their appreciation of the help given by Dr. Paul Thomas and Dr.
Andy Przybylko of UMIST. The considerable advice, assistance and the use of facilities given by the
Yorkshire Water Authority, is also acknowledged, since without this help the programme would not have
progressed nearly so rapidly. In particular we would like to recognize the efforts of David Best, George
Palfreyman and Harold Naylor of YWA.
REFERENCES
1 Mallett, M.J., Review of the Toxicity of Common Pollutants to Indigenous Species of Freshwater
Fish. I. Ammonia, Arsenic and Cadmium. NRA Report No. 2541, August 1990.
2 Seager, J., Wolff, E.W. and Cooper, V.A. Proposed Environmental Quality Standards for List II
Substances. Ammonia. ESSL TR260, 1988, WRC, UK.
3 Water Industry Specifications, Information and Guidance Notes, No. 7-08-01 Ammoma, 1990,
Water Research Council, UK.
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An ammonia monitor for the water industry

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