Water Innovate’s N-Tox^® nitrification toxicity monitor gives early plant failure warnings

Dr Steve Callister

In this article, Steve Callister, Water Innovate Limited’s Managing Director, describes N-Tox^®, a new patented nitrification toxicity monitor that provides an early warning of wastewater treatment works breakdown and helps prevent ammonia pollution of the aqueous environment.

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 technology from the laboratory into the international water market.

The Company’s current focus is on marketing N-Tox^®, ODOURsim^®, (a software package that predicts formation and emission of H₂S from sewage treatment works) and a novel high performance chemical additive for water and wastewater treatment. Water Innovate is also developing three advanced tertiary treatment process technologies.

Developed at the School of Water Sciences, the N-Tox^® monitor avoids problems associated with probe fouling because it relies upon gas-phase detection of dinitrogen oxide (N₂O) gas rather than detection of a chemical in the aqueous phase.

Research undertaken at Cranfield has demonstrated that N₂O is rapidly detected when nitrification starts to fail. The rate of N₂O production has been shown to be linked to oxygen depletion and ammonia shock loading. Increases in N₂O levels are directly related to nitrification failure and hence measurement of the N₂O off-gas level allows the monitoring of nitrification performance to prevent the release of ammonia into receiving waters.

Using N-Tox^® to detect an increase of N₂O above the start of an activated sludge aeration lane or trickling filter bed means that at least one hydraulic retention time of the bioreactor tank and final clarifier would pass before failure of nitrification. The example shown in the graph shows the time lag between detection of increased N₂O at the inlet of the activated sludge process, and the appearance of ammonia in the final effluent. The retention time, in this case seven hours, allows the operator to investigate potential nitrification failure.

Once a potential failure is detected, a number of process options can be followed to restore nitrification. These may include increasing rates of aeration, bypassing influent to storage tanks before trickle feeding back to the process at a later stage, or reducing ammonia loading by optimising the return flow of high ammonia liquors to the works inlet.

This graph shows what happens when a short aeration failure occurs in an activated sludge plant. The dissolved oxygen rapidly falls and 7 hours later ammonia rises in the effluent. The resulting N-Tox^® alarm allows time for
remedial action to be taken.

The key markets for N-Tox^® are municipal sewage and industrial effluent treatment. Many treatment works that discharge direct to the aqueous environment are consented for ammonia at 5 mg/l or less from influent concentrations of at least 30 mg/l. These plants rely on biological processes for removal of ammonia through bacterial conversion to nitrate.

Inhibition by aeration failure, toxic chemicals or high ammonia concentrations can result in a wastewater treatment works not meeting its required ammonia discharge consent. Some industrial effluents, such as landfill leachate and pharmaceutical wastewaters, are very high in ammonia. The requirement for an N-Tox^® alarm in these circumstances can be critical as the consequences of nitrification failure are more serious.

Other methods of detecting nitrification failure include generalised toxicity tests, on-line ammonia probes and on-line respirometry systems. However, generalised toxicity kits do not target nitrification, and on-line ammonia 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.

Water Innovate’s N-Tox^® monitoring system provides an effective solution to alleviate these problems by providing an earlier warning than other systems as N₂O gas is produced and detected rapidly. A proven, standard detector for N₂O is used and a simple sample hood and tubing is all that is required for delivery of gas to the detector.

N-Tox^® introduces a significant advantage for plant operators by alerting them to problems early so that remedial action can be taken. Also, it is the only nitrification toxicity sensor that relies upon gas-phase detection rather than having to sample the sewage or mixed liquor.

Professor Tom Stephenson, Water Innovate’s Technical Director, said: “the patent for this technology was developed by understanding the nitrification process –
N-Tox^® should be easier for operators to use compared to current alternatives.”

Authors’ Note

Steve Callister is Managing Director at Water Innovate Limited. Visit www.waterinnovate.co.uk or telephone 01234 756014 for further details.