Water & Waste Treatment 2

Preventing wastewater treatment failure with N-Tox® nitrification toxicity monitoring

Dr Steve Callister and Prof Tom Stephenson
In this article we describe a revolutionary technique for evaluating nitrification efficiency in activated sludge systems that uses direct measurement of the greenhouse gas dinitrogen oxide as an indicator of nitrification failure.
Developed by Water Innovate, the N-Tox® technology helps prevent ammonia pollution of the aqueous environment by providing an early warning of wastewater treatment works breakdown and, by monitoring dinitrogen oxide levels, helps operators to reduce greenhouse gas emissions.


Extensive research carried out at the School of Water Sciences, Cranfield University, has been utilised by Water Innovate to underpin this technology and develop it into a commercially available and patented product. The N-Tox® monitor relies upon gas-phase detection of dinitrogen oxide, rather than detection of a chemical in the aqueous phase. Hence, the non-invasive technique avoids probe fouling problems.
Dinitrogen oxide is rapidly detected when nitrification starts to fail, and the rate of this off-gas production is linked to oxygen depletion, the presence of toxic substances, and ammonia shock loadings. Increases in off-gas levels are directly related to nitrification failure. Hence, measurement of dinitrogen oxide concentration allows the monitoring of nitrification performance to prevent breakthrough of ammonia in final effluent. The detection of nitrification failure by N-Tox® is rapid, allowing plant operators to take remedial action before works breakdown and possible breaching of discharge consents.
The robust N-Tox® monitor design (see Figure 1) comprises an integral sample pump, gas conditioning device, non-dispersive IR gas analyser, auto-calibration system and data logging unit, housed within an IP65 enclosure. The instrument transmits 4-20 mA in proportion to dinitrogen oxide level and has various configurable alarms for setting plant failure and greenhouse gas emission warnings


N-Tox® can be used to reliably monitor any activated sludge system in either municipal sewage or industrial effluent treatment systems. Many treatment works that discharge direct to the aqueous environment are consented for ammonia at < 5 mg/l NH3-N. Some industrial effluents, such as landfill leachate or pharmaceutical wastewaters, have high ammonia levels. The requirement for a nitrification failure alarm here is critical as the consequences of nitrification failure are more serious.
Greenhouse gas emissions from wastewater treatment plants can be quantified using N-Tox®. This is an area of increasing concern for plant operators and regulators as the global warming potential of dinitrogen oxide is almost 300 times that of carbon dioxide.

Figure 1
Water Innovate’s N-Tox® monitor

According to the US Environmental Protection Agency, wastewater treatment accounts for up to 4 % of dinitrogen oxide emissions and tightening of ammonia discharge consents means more nitrification and denitrification processes will be needed. Hence, as a significant increase in dinitrogen oxide release is predicted, Water Innovate’s new monitoring technology can help plant operators control processes effectively to reduce greenhouse gas emissions.
The alarms provided by N-Tox® allow an immediate need to be met – the avoidance of prosecution through pollution of the aqueous environment. Typical fines imposed by the courts for prosecutions brought by the UK Environment Agency are up to £50,000 and are increasing both in number and value.

Nitrification Failure

Biological nitrogen removal by nitrification and denitrification has been applied in many industrial and municipal wastewater treatment plants because of the increasing demand for better effluent quality.
Nitrification, the oxidation of ammonia to nitrite and nitrate, is a sensitive process undertaken by nitrifying bacteria which can be inhibited by high ammonia concentrations and chemical pollutants, resulting in lower treatment efficiency or complete treatment system breakdown. Inhibition of nitrification is not easily predicted but the result is leakage of ammonia into effluent.
The occurrence of dinitrogen oxide under standard stable process conditions is normal in the denitrification part of the activated sludge process, but this occurrence is short lived and at very low concentrations. However, the presence of nitrification inhibiting compounds causes an increased production of dinitrogen oxide in the nitrification component of the cycle. It is measurement by N-Tox® in this part of the activated sludge process that provides the early warning of nitrification failure and helps prevent ammonia release to the aqueous environment.
As Inhibition events often occur for limited periods, the need to stringently monitor for toxic shocks has led to an increase in technologies available for monitoring changes in wastewaters.

Figure 2
Graph showing the period following a short
aeration failure in an activated sludge plant

In addition to N-Tox®, methods for detecting nitrification failure include generalised toxicity tests, on-line ammonia probes and on-line respirometry systems. However, toxicity kits do not target nitrification, and on-line probes are susceptible to fouling, require frequent recalibration, and do not give a rapid failure indication. On-line respirometry requires the operation of a mini biological treatment plant, with all the attendant maintenance problems.


Studies using pilot-scale activated sludge plants have been undertaken in the Pilot Hall facility at Cranfield University. These have demonstrated that the initial detection of increased dinitrogen oxide concentration above the start of an activated sludge aeration lane indicates that at least one hydraulic retention time (HRT) of the aeration tank and final clarifier passes before nitrification failure occurs. Figure 2 demonstrates that, as a result of a short aeration failure, the time lag between detection of increased dinitrogen oxide, and the appearance of ammonia in the effluent, provides typically an advance warning of nitrification failure equal to this HRT.
Once a failure is detected, a number of process options can be followed to restore nitrification. These can include increasing aeration rates, bypassing influent to storage tanks, recycling final effluent to the treatment works inlet, or controlling the return of high ammonia sludge streams to the head of the works.
Severn Trent Water and Cranfield University set out to examine whether an N-Tox® prototype could be applied to monitor dinitrogen oxide off-gas emissions on a full-scale municipal activated sludge plant. A detailed study was carried out at Finham Wastewater Treatment Works near Coventry.

Figure 3
Oxic and anoxic activated sludge zones
in an aeration lane at Finham WWTW

The effluent treatment system at Finham incorporates a single stage nitrification process, with both chemical oxygen demand (COD) and ammonia removal taking place in a single biological process with a total volume of 4500 m3. The aeration lanes are 30 m long, the first 5 m portion relating to the anoxic zone as shown in Figure 3. The activated sludge lanes have a daily average influent flow of 110,000 m3 corresponding to a total HRT of 12 hours. The anoxic component of this HRT is 2 hours.
The N-Tox® prototype was utilised on one of the activated sludge lanes at Finham and a sampling system designed with the purpose of producing a lane profile of dinitrogen oxide levels so that off-gas production could be determined and potential toxic shock upsets monitored.
Each sampling point was analysed for 24 hours and then either moved across the lane or down the lane by 5 m. A floating hood was used, suspended over the aeration lane and connected to sample line tubing, which went to a gas analyser that measured dinitrogen oxide in the range 0-1000 mg/l.
The gas analyser remained separated from the activated sludge at all times, drawing the off-gas via a small pump inside the gas analyser, from the headspace via additional tubing. This avoided the operational problems normally associated with corrosion and sensor fouling.

Figure 4
Graph showing lane profile at Finham WWTW
of average off-gas N2O concentrations at 5 m
intervals, for a 24-hour period at each interval

Daily measurements were taken of the influent and effluent COD, ammonia, nitrite, nitrate and pH levels. Mixed liquor suspended solids (MLSS) were also determined and online pH, temperature and dissolved oxygen measurements were recorded.
During the trial the MLSS was in the range 2034 – 2879 mg/l. Influent ammonia was 35.2 – 78.5 mg/l with COD at 571.5 – 687.5 mg/l. Effluent produced had ammonia levels of 0.1 – 1.2 mg/l, with COD in the range 37 – 67 mg/l.
These results for effluent ammonia show that no toxic shock loadings were observed. If there had been a shock loading then an increase in the ammonia concentration would have been detected.
Dinitrogen oxide off-gas measurements were averaged at each 5 m interval. Results from three monitoring points at the left, middle and right sides of the aeration lane were averaged at each interval down the lane for the corresponding 24 hour period. The lane profile shown in Figure 4 shows an initial increase in off-gas levels followed by a decrease as the sampling system moved down the lane.
At ‘0 m’ a rise in the average off-gas peaked at 7.2 mg/l (7 hours), followed by a decrease and then a second increase at 20 hours. At ‘5 m’ average dinitrogen oxide concentrations declined and were found to be less than 0.9 mg/l up to 46 hours into the trial, with low levels maintained apart from a 2.1 mg/l peak (84 hours), a peak of 1.8 mg/l (107 hours), and a minor increase to 0.9 mg/l (138 hours).
These results suggest that the initial increase of dinitrogen oxide concentration was a consequence of the denitrification process. As the monitoring system was moved down the activated sludge lane there was a decrease in the off-gas level detected to background emission levels.
The pilot scale experiments at Cranfield had used high ammonia loadings of 2.44 g/l NH3-N combined with oxygen deprivation to yield a maximum dinitrogen oxide level of 18.7 mg/l as a result of nitrification failure. A second experiment with an increased shock load of 7.32 g/l NH3-N produced a significantly higher maximum concentration of off-gas. However, even the lower peak at 18.7 mg/l was well in excess of the background dinitrogen oxide levels monitored at Finham.
The pilot scale experiments had shown an initial increase in dinitrogen oxide concentration followed by a time lag before a sharp increase of ammonia in the effluent. It is this period, before ammonia appears in the effluent, which is used to indicate failure of the nitrification process, thus providing an early warning for plant managers and operators.
The monitoring of dinitrogen oxide at Finham detected small variability in the background emissions of 0 – 2 mg/l. Hence, an off-gas peak would be measured in the event of a toxic shock load to the activated sludge plant.
If this was to occur at Finham, then the dinitrogen oxide peak would have been expected from the activated sludge lane followed by a time lag of approximately one HRT (10 hours) before ammonia levels increased in the effluent. Hence, staff at Severn Trent Water would be given an N-Tox® alarm followed by up to 10 hours to take emergency remedial action to rectify the problem and prevent ammonia discharge.


Water Innovate, a spin out from Cranfield University’s School of Water Sciences, is bridging the innovation gap by transferring N-Tox® and other revolutionary technologies from the laboratory into the international water market. The Company brings together a combination of proven management, secured financial backing, expert technical knowledge and experience, and protected intellectual property.
The current focus is on marketing N-Tox®, ODOURsim®, a software package that predicts formation and emission of hydrogen sulphide from sewage treatment works, and ZR-Coag®, a novel high performance chemical additive for water and wastewater treatment. Water Innovate is also developing three advanced tertiary treatment process technologies.


The pilot and full scale experimental work was supervised by Dr Elise Cartmell and carried out by Mr Mark Butler of Cranfield University and funded by Severn Trent Water through an industrial CASE studentship and the UK Engineering and Physical Sciences Research Council. 

Download PDF

Back to top