Editorials

Chemistry & Industry

N-Tox® prevents activated sludge failure and monitors greenhouse gas emissions

Dr Steve Callister and Prof Tom Stephenson
A new technique for evaluating nitrification efficiency in industrial and municipal activated sludge systems is described that uses direct measurement of dinitrogen oxide, an important greenhouse gas, as an indicator of nitrification failure. Developed at Water Innovate, N-Tox® helps control ammonia levels in treated effluent by providing an early warning of wastewater treatment failure. The technology also helps plant managers control greenhouse gas emissions.

Development

When nitrification starts to fail, dinitrogen oxide is detected rapidly, and the rate of production is directly related to nitrification failure. Hence, measurement of dinitrogen oxide concentration allows monitoring of nitrification performance to prevent breakthrough of ammonia into effluent. N-Tox® can be used to reliably monitor any activated sludge system in either industrial effluent treatment or municipal systems.
Extensive research at the School of Water Sciences, Cranfield University, enabled Water Innovate to develop this technology into a commercially available and patented product. The N-Tox® monitor relies on non-invasive gas-phase detection of dinitrogen oxide, rather than detection in the aqueous phase, avoiding probe fouling problems. Nitrification failure detection by N-Tox® is rapid, giving plant operators the time to take remedial action before possible release of ammonia.
The N-Tox® monitor is housed within an IP65 enclosure and comprises an integral sample pump, gas conditioning device, non-dispersive IR gas analyser, auto-calibration system and data logging unit, The robust instrument transmits 4-20 mA in proportion to dinitrogen oxide level and has various configurable plant failure and greenhouse gas emission alarm settings.
Industrial effluents, such as landfill leachate or pharmaceutical wastewater, have high ammonia levels. Also, many wastewater treatment works are consented by the Environment Agency for ammonia at < 5 mg/l NH3-N in treated effluent. Hence, the requirement for an alarm is critical as the consequences of nitrification failure can be serious.
An area of increasing concern to both wastewater treatment plant operators and regulators, is the little thought-of global warming potential of dinitrogen oxide, which is almost 300 times that of carbon dioxide. This contribution to greenhouse gas emissions can be quantified using N-Tox®.
A significant increase in dinitrogen oxide release is predicted. According to the US Environmental Protection Agency, wastewater treatment accounts for up to 4 % of dinitrogen oxide emissions and tightening of ammonia discharge regulations means more nitrification and denitrification processes will be needed. Hence, Water Innovate’s new monitoring technology can help plant operators control processes effectively to reduce greenhouse gas emissions.
The alarms provided by N-Tox® can allow the avoidance of prosecution. Typical fines imposed by the courts for prosecutions brought by the Environment Agency are up to £50,000 and are increasing both in number and value.

Nitrification Problems

Nitrification, the oxidation of ammonia to nitrite and nitrate, is a sensitive process undertaken by nitrifying bacteria in part of the activated sludge system. Inhibition by high ammonia concentrations and chemical pollutants results in lower treatment efficiency or complete treatment breakdown. Nitrification Inhibition is not easily predicted but the result is leakage of ammonia into effluent.


Figure 1 - Graph showing the period following a
short aeration failure in an activated sludge plant


The occurrence of dinitrogen oxide is normal in denitrification, but this occurrence is short lived and at very low concentrations. The presence of nitrification inhibiting compounds causes an increased production of dinitrogen oxide in nitrification, and it is measurement by N-Tox® in this part of the activated sludge process that provides the early warning of nitrification failure.
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 wastewater changes. In addition to N-Tox®, methods include generalised toxicity tests, on-line ammonia probes and on-line respirometry systems. However, toxicity kits do not target nitrification, and on-line probes require frequent recalibration, are susceptible to fouling, and do not give a rapid failure indication. On-line respirometry requires the operation of a mini biological treatment plant.

Studies

Research using pilot-scale activated sludge plants at Cranfield University has been undertaken. This demonstrated that initial detection of increased dinitrogen oxide concentrations at 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 - Oxic and anoxic activated sludge
zones in an aeration lane at Finham


As a result of a short aeration failure, Figure 1 demonstrates that the time between detection of increased dinitrogen oxide and the appearance of ammonia provides typically an advance warning of nitrification failure equal to this HRT.
Once failure is detected, a number of solutions can be implemented to restore nitrification. These include increasing aeration rates, bypassing influent to storage tanks, recycling final effluent, or controlling the return of high ammonia streams to works inlet.
Severn Trent Water, Cranfield University and Water Innovate set out to show that an N-Tox® prototype could be applied to monitor dinitrogen oxide off-gas emissions on a full-scale activated sludge plant. A detailed study was carried out at Finham Wastewater Treatment Works near Coventry.
The Finham process incorporates a single stage nitrification process, with both chemical oxygen demand (COD) and ammonia removal taking place in a single biological process. Aeration lanes are 30 m long, the first 5 m portion relating to the anoxic denitrification zone as shown in Figure 2. The activated sludge lanes have an HRT of 12 hours, the anoxic component being 2 hours.
The N-Tox® prototype was tested on one of the activated sludge lanes and a sampling system designed to produce a lane profile of dinitrogen oxide levels so that off-gas production could be determined and 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 connected to sample line tubing, which went to a gas analyser that measured dinitrogen oxide.

Figure 3 - Graph showing lane profile at Finham
of average off-gas N2O concentrations at 5 m
intervals, for a 24-hour period at each interval

The gas analyser remained separated from the activated sludge at all times, drawing off-gas via a small pump inside the gas analyser. This avoided the operational problems normally associated with corrosion and sensor fouling.
Daily measurements were taken of the influent and effluent parameters during the trial. The mixed liquor suspended solids 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, and effluent had ammonia levels of 0.1 – 1.2 mg/l, with COD in the range 37 – 67 mg/l.
The effluent ammonia measurements show that no toxic shock loadings were observed. If there had been a shock loading then an increase in ammonia concentration would have been detected.
Dinitrogen oxide 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 3 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. 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 ammonia shock loadings to yield a maximum dinitrogen oxide level of 18.7 mg/l as a result of nitrification failure. This peak was well in excess of the background dinitrogen oxide levels monitored at Finham, small variations of up to 2 mg/l being detected. 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 followed by a time lag of approximately one HRT (10 hours) before ammonia levels increased in the effluent. Hence, the N-Tox® alarm would be followed by up to 10 hours to take emergency remedial action to rectify the problem and prevent ammonia discharge.

Water Innovate

As a spin out from Cranfield University’s School of Water Sciences, Water Innovate 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 effluent treatment plants, 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.

Acknowledgements

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. 

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