How to monitor nitrous oxide generation

Nitrogen removal is likely to become required at increasing numbers of wastewater treatment plants. A nitrous oxide monitor by Water Innovate can optimise treatment processes. Dr James Ellis reports.

The wastewater treatment industry is regularly called upon to improve treatment processes or increase treatment capacity.

Higher-quality treatment is increasingly required by regulators, for example through the recent reviews of discharge consents during IPPC implementation. And further improvements are often demanded to treat contaminants previously not removed by existing processes.

Toxic ammonia has long been treated in wastewater through chemical or microbiological oxidative processes, making nitrification an important feature at many wastewater treatment works (WwTW).

As the adverse impacts of aqueous nitrogenous emission to the environment have become clearer, reductive denitrification of wastewater is increasingly required where discharges are made to areas prone to eutrophication.

Implementation of the Water Framework Directive (WFD) in the UK, whereby emissions to rivers will be viewed catchment-wide, is likely to see nitrogen removal required at more wastewater treatment sites.

Denitrification typically relies on the reduction of nitrates by means of controlled de-oxygenation, having fully nitrified in the influent in preceding treatment steps.

Although the nitrification/denitrification treatment is effective, it can be expensive in terms of energy and process control. The environmental benefits of reliable wastewater treatment are clear. But problems arise when treatment processes deteriorate and fail, and it is incumbent on operators to minimise this risk.

The effectiveness of nitrification treatment can deteriorate without necessarily being observed in a timely way through conventional monitoring systems. This can result in unacceptably high ammonia concentrations in the final effluent.

Furthermore, both nitrification and denitrification process failure results in the emission to the atmosphere of oxidised nitrogenous gases, including the powerful greenhouse gas nitrous oxide (N2O), posing a potential environmental impact currently of high concern to policy makers and regulators alike. As reliable and high-quality wastewater treatment becomes more widespread, operators must look to ways to ensure that treatment reliability is improved to minimise emissions to water and the wider environment, while optimising their processes to reduce the increased energy requirements and its associated environmental (carbon footprint) and financial costs.

The nitrification process, converting ammonia to nitrate through intermediate chemical stages, is normally achieved microbiologically at WwTWs. And, as a result, it is potentially subject to many inhibiting impacts. The causes of nitrification inhibition can broadly be summarised as:

· Lost or insufficient aeration (normally temporary), resulting from plant failure or variable influent load

· Shock ammonia loads, particularly for influent in receipt of industrial, pharmaceutical or landfill leachate wastes

· Shock toxin loads, including a wide variety of substances including metals, phenols and sodium azide (a common inorganic fungicide)

Conventionally, nitrification is monitored at WwTWs by monitoring the effluent for ammonia, nitrate and/or nitrite using probe technology, or by respirometry (in-line,

or laboratory based), or by using a raft of microbiological techniques.

These monitoring methods either are insufficiently sensitive to detect ammonia until concentrations have built-up, or do not provide an instantaneous response, with significant time delays involved particularly for the laboratory-based methods.

This can mean that nitrification failure, though eventually detected, can result in effluent high in ammonia passing much further along the treatment process, in the worst case being discharged breaching consent limits.

The intermittent nature and unpredictability of nitrification inhibitor introduction to the influent remains a problem for WwTW operators.

A more responsive technology to rapidly identify nitrification failure is desirable. Aqueous nitrite (NO2-) is an intermediate ion formed during both nitrification and denitrification; nitrite is converted to gaseous N2O by the enzyme nitrite reductase; as the nitrification process fails and more nitrite is produced, N2O is rapidly produced (Fig 1).

Monitoring N2O generation provides a way of monitoring nitrification efficiency.

Significant increases in N2O generation identify nitrification efficiency problems, while toxins will ultimately affect nitrifying bacteria as confirmation of inhibition.

Conventional dissolved oxygen (DO) monitoring can give false positives for nitrification monitoring, as it is possible to observe almost consistent DO should non-nitrifying bacteria continue to grow throughout nitrification inhibition.

Global warming

N-Tox, from Water Innovate, is a nitrous oxide gas monitor designed for use as a nitrification failure alarm. It also allows operators to optimise their treatment process in order to minimise energy and consumable usage while maintaining treatment standards.

Nitrous oxide emissions themselves should ideally be minimised, since N2O has 310 times the global warming potential (GWP) of carbon dioxide, comprising a potentially significant contribution to the carbon footprint of WwTWs.

Water Innovate is a company derived from the School of Water Sciences at Cranfield University. It aims to bridge the gap between research and business innovation through the development and production of new technology in the water and waste treatment sector.

The N-Tox nitrification monitor is based on robust, simple, existing technology and has proven success in field applications. The gas detector uses non-dispersive infrared absorption, a proven technology for detection of a range of gases.

Detection relies on the specific absorption spectra of target gases, and is thus highly selective. It eliminates false-positive results. N2O is detected in the range 2-2,000ppm (4-4,000mg/m3 N2O), adopting a measurement interval of one second and an overall measurement response time of between eight and 30 seconds depending upon sample-line length between the effluent and the instrument.

An analogue signal (4-20mA) proportional to the N2O concentration is output from the detector. Thus, the highly sensitive detector monitors N2O changes essentially in real-time.

The sample is collected immediately above the effluent using a floating hood connected to the instrument. Commonly, samples are collected above the activated sludge tank, although multiple sampling points across the treatment process can be arranged with particular utility for process optimisation.

The sampled gas is conditioned through a gas filter and semi-permeable membrane drier prior to measurement, although the detector has high humidity (0-100%) and temperature (0-50°C) tolerances. Non-intrusive gas detection offers a key advantage over direct effluent measurement, since the latter is prone to probe fouling and associated measurement quality and maintenance problems.

Data is stored on a programmable automatic data-logger, integrating user-controlled N2O alarm thresholds. Instrumental digital readouts can also be completely viewed via USB on a computer interface, or via a modem to a remote control system.

The data can be graphically presented to facilitate user process monitoring. The detector is auto-calibrating (adjusting for temperature, pressure, infrared source ageing, and instrumental drift).

Overall, N-Tox comprises an automated sampling and measurement system that requires no expensive consumables. The detector and logger come housed in a space-efficient IP65 enclosure, which has low power requirements.

Emission control

One important application of N-Tox is as an early warning alarm for failed nitrification.

By sampling above an activated sludge tank, for example, operators have up to one complete hydraulic retention time to intervene in the event of treatment failure before discharge of ammonia-contaminated effluent. (This is typically around seven hours.)

Depending upon specific circumstances, this might allow remedial measures to be initiated, such as increasing aeration, implementing the recirculation of final effluent, or bypassing influent to storage tanks.

This robust real-time nitrification monitoring allows toxic inhibition to be identified well before the discharge of contaminated effluent, protecting the receiving environment and reducing the risk of operator fines for discharge consent failure.

Another application of N-Tox technology is in process optimisation and gas emission evaluation for the purposes of attaining best energy efficiency and minimising the carbon footprint of WwTWs.

This aspect of emission control is of increasing interest to operators (in efforts to minimise energy use at a time of high prices) and regulators (focusing strongly on greenhouse gas emissions and overall installation carbon footprints) alike.

Using the capability to measure N2O generation across complete treatment processes (during aeration, nitrification and, where appropriate, denitrification stages), designers and operators can optimise oxygenation even where influent loads are variable. This ensures complete nitrification and minimises atmospheric emissions of N2O, with its significant GWP.

Water Innovate tested N-Tox prior to its commercial launch in pilot studies and field applications. The technology operated at municipal WwTWs across the UK, including Severn Trent Water, Anglian Water and Wessex Water sites.

A nitrification inhibition event at such plants can be detected. Operator intervention can then ensure that a consent failure is avoided.

Water Innovate says the estimated cost savings from successful treatment interventions are between £20,000 and £60,000 derived from savings in effluent removal by tanker, and activated sludge re-seeding costs, in addition to any regulatory fines avoided.

N-Tox has also been successful as a nitrification monitor at industrial WwTW (GlaxoSmithKline) and landfill leachate WwTW (Waste Recycling Group), the company says. Across these municipal and industrial WwTWs, aeration and nitrification is achieved by a variety of means, including aeration lanes, trickling filters and oxidation ditches. All of these were monitored by the N-Tox gas monitor.

Effective monitoring of wastewater nitrification is essential to prevent harmful discharges of ammonia to the environment in the event of inhibition of the nitrification process.

In most municipal and industrial applications, nitrification is microbiologically mediated. But nitrifying bacteria are sensitive to inhibition for a variety of reasons.

Conventional approaches to nitrification monitoring are subject to fouling by the effluent (eg probe technology) or significant time delays in detection (eg respirometry).

The N-Tox nitrification monitor is a new technology. It monitors the evolution of nitrous oxide gas immediately above the effluent; N2O being an excellent indicator of nitrification failure. It uses a real-time, sensitive and selective gas monitor. This non-invasive technique avoids many of the problems associated with conventional nitrification monitoring.

According to Water Innovate, N-Tox is robust, easily installed (plug-and-play), auto-calibrating, low maintenance and capable of routing data to remote control centres.

A key role for N-Tox is to act as an early warning alarm for nitrification failure. Real-time nitrification monitoring allows operators to implement contingency measures in the event of toxic inhibition, preventing harmful ammonia releases in the final effluent.

The N-Tox monitor can also be used to accurately assess atmospheric emissions of N2O from WwTWs. This allows designers and operators to optimise their treatment processes to minimise such emissions.

Dr James Ellis is commercial manager at Water Innovate.

T: 01243 758054

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