Tom Stephenson describes N-Tox®, an early-warning nitrification toxicity monitoring system, enabling the prevention of harmful ammonia release to the environment. As an online nitrification monitoring device, N-Tox is suitable for municipal and industrial wastewater treatment works utilising activated sludge treatment. Using a novel non-invasive gas phase monitor, N-Tox is simple, robust and low-cost technology that is now commercially available having undergone successful full-scale field trials.
Maintaining a continuously high quality effluent at municipal and industrial wastewater treatment works (WWTW) is critical under a regime of strict legal discharge consents. Instrumentation providing early warning of treatment process deterioration or failure is not only important for ongoing environmental protection, but also avoids high penalty fines being imposed by regulators on operators.
The nitrification process at WWTW usually involves the microbially mediated oxidation of ammonia (NH3, predominantly as dissolved NH4+) to nitrate (NO3-) via an intermediate nitrite (NO2-) stage. The efficiency of the microbiological treatment process is dependent upon a variety of factors, including hydraulic retention times (HRT), effluent load, and wastewater composition, all of which can vary significantly. Therefore, it is critical to measure nitrification efficiency. Conventional approaches to this involve measurement of effluent samples using nitrate, nitrite and ammonia probes, along with respirometers and laboratory microbial methodologies. This invasive sampling suffers from probe fouling and can result in time delays as laboratory tests are conducted. N-Tox (see Figure 1) adopts a unique alternative monitoring approach.
Figure1
The N-Tox continuous nitrification monitor. Novel non-invasive gas phase monitoring allows robust, reliable real-time monitoring in-line at low cost
N-Tox provides on-line, real-time, non-invasive infrared (IR) measurement of nitrous oxide (N2O) gas above the treatment process. Research by Cranfield Univerisity and Water Innovate Ltd. identified N2O off-gas as an early event during nitrification inhibition and lead to the development of N-Tox. N2O off-gas rapidly increases as oxygen depletion and other nitrification inhibiting factors allow nitrogen reductase to convert NO2- to N2O.
The N-Tox unit utilises a proven N2O gas detector housed within an IP65 rated enclosure together with an integral sample pump and a gas conditioning device. N-Tox also comprises an auto-calibration and data logging system. There is no need for chemical reagents or other expensive consumables, and power requirements are low (0.015 kW). This simple, robust design requires minimal operator intervention, and being non-invasive avoids the fouling problems and down-time commonly encountered with probe alternatives.
The IR detector measures every second in the range 1-2000 ppm N2O or 2-4000 mg/m3 with a resolution of 1ppm. Depending upon the length of the gas-line from the floating sample hood, the instrument response time is from 8-30 seconds. Measurements are visually displayed on user-friendly operating screens, and an alarm is incorporated. Data from the N-Tox can be accessed remotely via a modem or intergrated into a site’s existing telemetry system using the 4-20 mA output.
Continuous, fast-response measurement of N2O enables corrective engineering at the WWTW prior to elevated ammonia discharges from the plant (see Figure 2). Depending upon specific HRT, normally several hours are available to plant operators to prevent environmental release and restore effective nitrification treatment.
Figure 2
Temporary aeration failure at an activated sludge plant is rapidly detected by N-Tox 7 hours prior to the resultant effluent ammonia increases in breach of discharge consents
This time can be used to increase aeration rates, reduce return-flow high ammonia liquors or bypass influent to storage. Nitrification deterioration alarms from N-Tox are reliable. Furthermore, false-negative readings associated with respirometry systems are avoided, since N2O gas increases are detected with high sensitivity even under circumstances where dissolved oxygen concentrations are not yet reduced below threshold values. For example, chemicals such as ATU inhibit ammonia-nitrite oxidation leading to an increase in N2O, however, dissolved oxygen concentrations will remain stable. Rapid detection of N2O and subsequent control of its emission from WWTW is also desirable as, molecule-for-molecule, nitrous oxide has 310 times the radiative forcing potential of carbon dioxide and hence is an important greenhouse gas. Operators can use N-Tox in plant optimisation in order to minimise their greenhouse gas emissions.
N-Tox has been subjected to extensive testing at pilot-scale and in fully operational municipal and industrial wastewater plants (see Table 1). N-Tox systems have operated successfully and reliably under widely varying real situations, and N-Tox is now being commercially rolled out as a novel, low-cost alternative to conventional nitrification monitoring approaches.
Useful Links
Water Innovate: www.waterinnovate.co.uk
About the Author
Professor Tom Stephenson is Technical Director at Water Innovate. Contact via their website.