VOC monitoring keep things on track!

Award-winning geotechnical company, BAMRitchies Limited, is using an Ion Science handheld Tiger photoionisation detector (PID) for nightly monitoring of volatile organic compound (VOCs) concentrations during on-site headspace testing of contaminated soil samples on railway contracts.

Supplied through Ion Science’s British distributor, Shawcity, as a replacement for one of the company’s older models, BAMRitchies chose the well-proven Tiger for its portability and long battery life between charging. As it is being used in all weather conditions and environments, the instrument’s market-leading humidity and contaminant resistant PID technology was also a key factor.

Ion Science’s Tiger is independently verified as being the best performing PID, providing the most stable, repeatable readings, when tested against competing instruments in humid and contaminated conditions.

BAMRitchies provides fully integrated ground engineering services, including ‘design and construct, for government organisations, local authorities, main contractors, utilities and public / private companies. The company’s worldwide reputation is based on innovative solutions to complex geotechnical problems with reliable delivery by a large, highly skilled and well-equipped workforce.

Stuart McQuade, Senior Geotechnical Engineer at BAMRitchies comments: “Our consultant engineers specify prompt information on contamination levels on a very regular basis making it essential that we quickly found a replacement for our old instrument which had started to fail. As we’ve used Ion Science PIDs before and found them to be good quality and reliable, we were content to go with Shawcity’s recommendation of the Tiger PID.

“Consistency of performance was a key requirement as it is being used to test approximately five to ten soil samples per night. The Tiger is in use during the most severe weather and in the harshest environments so a robust design together with humidity and contamination resistance was also very important to us. Like other Ion Science instruments, the Tiger is extremely easy to use and has proved very reliable so far.”

Providing a dynamic detection range of 1 parts per billion (ppb) to 20,000 parts per million (ppm), the Tiger offers the widest measurement range of any other VOC instrument on the market.

Ready to use, straight out of the box, the instrument requires no complex set up procedures via a PC to perform basic functions and provides the best available VOC detection and software features available.

Ion Science’s Tiger also has the fastest response time on the market of just two seconds and can be connected directly to a PC via the USB offering extremely fast data download capabilities.

It has been designed for the safe replacement of batteries in hazardous environments and is intrinsically safe (IS) – meeting ATEX, IECEx, UL and CSA standards.

ABB Process instrumentation, analytical technology and gas detection in Ireland

Hanley Measurement & Control has built a reputation for the supply of specialist solutions and expertise in process instrumentation, process analytical technology and gas detection. Founded in 1981 it has long been considered as a leading automation in Ireland. The company has recently been appointed as channel partner in Ireland by ABB, to expand its instrument and analyser offering into the Irish process market

Left to Right: Chris Kennedy, Gavin O’Driscoll & Eoin O’Neill of Hanley Measurement & Control together with Aidan Edwards of ABB stand next to a representation of a 2.4 meter magnetic flowmeter (the largest every supplied!) during a recent visit to the ABB flow meter manufacturing facility in Stonehouse, GB.

The partnership will see the company acting as the official sales agent for ABB’s complete portfolio of instrumentation and analyser products for applications in the pharmaceutical, chemical, food and beverage and other related industries.

Chris Kennedy, Managing Director of Hanley Measurement & Control commented that “partnering with ABB enables the company to provide its customers with an enhance product range specifically in relation to flow measurement and analytical solutions.”

Commenting on the partnership, Tim Door, General Manager for ABB’s Measurement and Analytics business in the Britain and Ireland says: “The partnership with Hanley Measurement and Control marks a positive move forward that underlines our intent to grow our presence in the Irish process market. The company is a great fit for our growing range of measurement and control products for improving process performance and efficiency.”

“Utilising a well-known and respected partner such as Hanley Measurement & Control will allow our customers in Ireland to get full access to support and service going forward into 2015 and beyond.”

• Following the completion of a management buyout Hanley Measurement & Control is no longer part of the Hanley group of companies. Hanley Measurement & Control is now a subsidiary of Eolas Scientific which also has an operating company in the UK called Eolas Technology. The management team of Chris Kennedy, Gavin O’Driscoll and Eoin O’Neill are committed to ensuring our customers receive exceptional service and a world class range of products.

A fascinating story: Trash to gas project to help life on Mars!

If you are travelling to Mars on a journey that will last for several months, you need to maintain good breathing air quality and you need to manage your resources very carefully. This article describes research on the off-gases from astronaut waste; checking that they are not harmful and figuring out if they can be converted into water, oxygen and rocket propellant.

As part of a project to measure the effects of long-term isolation on astronauts, small groups of individuals have been selected to live in a tiny ‘Habitat’ perched on the upper slope of a volcano in Hawaii. In doing so, the project team has contributed to the understanding of issues that would confront a manned mission to Mars.

NASA’s Anne Caraccio analyzing waste gases during simulated Mars mission

For example NASA’s Anne Caraccio studied off-gases from the crew’s trash with a portable Gasmet FTIR gas analyzer. “Waste from the crew’s everyday activities are routinely sorted and stored, but we need to know the composition of the off-gases from these materials for health and safety reasons, and also to determine whether these gases could be utilised beneficially,” Anne reports.

The work was undertaken during the second of four HI-SEAS (Hawaiʻi Space Exploration Analog and Simulation) missions which involved living with 5 other crew members for a period of 120 days in a two-story solar powered dome just 11 metres in diameter with a small attached workshop the size of a shipping container. In addition to the completion of a range of tasks that were set by the project, each crew member conducted their own research, which in Anne’s case was known as ‘Trash to Gas’, a programme working on the development of a reactor to convert waste from long-duration missions into useful commodities such as water, life-support oxygen and rocket propellant.

The main objective of the second HI-SEAS mission was to evaluate the performance and the social and psychological status of the crew members whilst they lived in cramped isolated conditions in a lava rock environment that resembled Mars.

Crew members were allowed outside of the Habitat, but in order to do so they had to wear simulated spacesuits and undergo a 5 minute mock compression/decompression. Since the FTIR gas analyser is portable (14Kg), Anne was able to conduct additional monitoring both inside and outside the Habitat in order to compare data with the waste off-gas measurements. “Size, weight and portability are obviously of major importance on a project such as this, but the main advantage of this technology was its ability to measure a large number of compounds simultaneously; I measured 24 VOCs such as acetaldehyde, methane and ethylene, but the instrument also stores spectra for the measurements so it is possible to retrospectively analyze data if it becomes necessary to look for a particular compound at a later stage.”

Anne’s monitoring provided a clear view of the most important gases within the Habitat. For example, stored waste had the highest relative levels of ethanol (due to crew members’ hygiene wipes and cleaning products) and water vapor (due to residual water from food and plant waste). The laboratory where plants were growing had the lowest relative level of methane. The waste bins had higher relative levels of nitrous oxide and pentane, and the bathroom had the highest levels of acetaldehyde.

The FTIR gas analyser, a DX4040, was supplied by the company Gasmet Technologies. “We were very pleased to be able to help with this project,” says Gasmet’s Jim Cornish. “The simultaneous monitoring of multiple compounds is a common application for our FTIR analyzers, however, they are usually employed measuring gases in stack emissions, industrial processes, greenhouse gas research and in hazmat scenarios. We usually tell prospective customers that advanced FTIR technology is simple to use; ‘it’s not rocket science’ we tell them, so I guess we will have to rephrase that now.”

The waste produced during the HI-SEAS mission was measured during the entire mission, although this was for a shorter period than would be expected of an actual long duration mission. The Trash-to-Gas reactor processed HI-SEAS waste simulant at the Kennedy Space Center with results demonstrating that a future reactor would be most efficient with specific material processing cycles to maximize the desired output. Automation will also be needed in the future Trash-to-Gas reactor because the current technology would require too much of a crew member’s logistical time. The Trash-to-Gas reactor first converts waste into carbon dioxide, which is then mixed with hydrogen in a Sabatier reaction to produce methane and water.

The Kennedy Space Center Trash-to-Gas reactor processed three waste types and produced 9% of the power that would have been needed during the HI-SEAS mission. As part of the psychological assessment, each member of the crew completed regular surveys and kept diaries. They also wore ‘sociometric’ badges that recorded conversation patterns and voice tone.

Commenting on the psychological results of the project, Anne says “The crew were essentially strangers when they entered the Habitat, which is unlike a typical space mission in which the crew would have worked and trained together for a number of months or even years. Nevertheless, the crew coped extremely well with living and working in such close proximity, and there were no significant periods of stress in my opinion.”

The third Hi-SEAS mission began on October 15, 2014. Again, a 6 member crew will conduct a similar mission, with the exception that it will last for 8 months. Anne says: “Participation in these missions requires a real passion for science, technology and space travel. The application process includes a class 2 flight medical, a personal research project proposal, essays, interviews and educational requirements, all of which is similar to the NASA astronaut application procedure.” Looking forward, she says: “The technology to travel to Mars has not yet been fully developed, but it is anticipated that a human mission could be possible in the future. The journey to Mars would take around one year, so I hope that our Trash-to-Gas research will contribute to the science that could make such a mission possible.”

Continuous Mercury monitoring benefits cement plants.

Antti Heikkilä from Gasmet Technologies highlights the challenges faced by mercury monitoring in cement kilns, and explains how a new continuous mercury monitoring system addresses these issues and provides process operators with an opportunity to improve environmental performance and demonstrate compliance with forthcoming legislation.

Background
The production of cement klinker and lime in rotary kilns is responsible for 10.7% of mercury emissions to air (3,337 kg) according to a recent study. Most of the mercury and mercury compounds pass through the kiln and preheater; they are only partly adsorbed by the raw gas dust, depending on the temperature of the waste gas. For these reasons, monitoring and controlling emissions of mercury to air is important and steps are being taken in several countries to impose emission limits. In the European Union BREF guidance for Cement kilns (CLM BREF), mercury has a BAT-associated emission level of 0.05 mg/Nm3 (50 µg/Nm3) for the half-hour average.

New monitoring technology

Figure 1

Gasmet Technologies has launched a new continuous mercury emission monitoring system (CMM) based on the cold vapour atomic fluorescence (CVAF) measurement principle. The analyser is integrated in an air conditioned cabinet together with a vacuum pump, an automatic calibrator and a nitrogen gas generator. The sample gas is extracted from the process duct with a dilution probe and heated sample line specially designed for sampling mercury from harsh process conditions (see figure 1 right). The analyser has a detection limit of 0.02 µg/Nm3 and the lowest measuring range for total mercury concentration is 0 – 10 µg/Nm3 when a dilution rate of 1:50 is used in the sample extraction probe.

Since the CMM analyser employs a CVAF spectrometer, the sensitivity of the instrument is excellent and the main source of measurement uncertainty that needs to be addressed by the analyser and the system design is the quenching effect; where other gases present in the sample, such as O2 and H2O, lower the fluorescence signal due to mercury atoms. In order to avoid these adverse effects, a dilution sampling approach is used and the dilution gas is synthetic nitrogen formed in a nitrogen generator inside the analyser cabinet. As the detection limit of the analyser is much lower than would be needed to monitor mercury in low µg/Nm3 ranges, dilution does not compromise the sensitivity of the instrument. On the other hand, dilution lowers the quenching effect by lowering the concentration of interfering gases by a factor of 50. Measuring mercury in a gas consisting of 98% nitrogen guarantees consistent measurement regardless of the fuel or emission abatement techniques used in the plant.

The CVAF spectrometer measures atomic mercury vapour (Hg0) and in order to measure total mercury including oxidized forms, a thermal catalytic converter is used to convert all forms of mercury such as Mercury Chloride into atomic mercury. The converter is close-coupled with the fluorescence cell to minimise the risk of recombination reactions where the atomic mercury converts back to oxidised forms between the converter and spectrometer.

The system has been field tested on various types of industrial plants (coal fired power plant, hazardous waste incinerator, sulphuric acid plant and a cement plant) to characterise the suitability and long-term stability of the sample probe and dilution system in various processes. Given the reactive nature of mercury, special care has been taken to ensure that mercury in the flue gas is not absorbed into dust accumulating in the sample probe filters. Mercury reacts readily with limestone dust, resulting in analyte loss and increased response time of the analyser. The Gasmet CMM solution includes a smaller filter element, which minimises the amount of dust deposition on the filter, and a two-stage blowback mechanism which first removes dust from the filter element and then in the second stage expels the dust from the probe tube back into the process.

Field test at Finnish Cement Plant

Figure 2

The CMM was installed on the emission stack of a rotary kiln cement plant with an Electrostatic Precipitator (ESP) for particulate emission control (see figure 2 above). The test period lasted 30 days. The fuels used during the test included coal, petroleum coke and recovered fuels. The flue gas composition at the measurement point is summarised in table 1. During the field trial, the raw mill was periodically stopped and the variation in mercury levels was monitored together with changes in other process parameters. Average mercury concentration when the raw mill was running was 6 to 8 µg/Nm3 and when the raw mill was stopped, the concentrations could increase to 20 – 40 µg/Nm3. The plant had an emission limit value of 50 µg/Nm3 for total mercury.

Figure 3

Figure 3 (above) shows a typical 24-hour period of emissions including raw mill on and raw mill off conditions. In addition to Hg0 concentration, the dust loading and raw mill state are shown because these are the main parameters expected to have an impact on the mercury analyser.

Results
The main goal of the test was to ensure the stability and repeatability of mercury measurement in demanding process conditions and to determine whether cement dust causes analyte loss and increased response time in the sample extraction probe.

The only process variable which clearly correlates with mercury concentration is the raw mill on/off state. When the raw mill is on, the variation in dust loading or other gas concentrations (O2, H2O, acid gases such as SO2 and HCl) does not correlate with variation observed in mercury concentration. When the raw mill is switched off, all gases including mercury undergo a change in concentration but this is clearly brought about by the raw mill state.

In order to estimate the repeatability of the Hg measurement at zero and span levels, the CMM analyser was configured to perform zero tests with synthetic nitrogen and span tests with Hg0 test gas generated by the mercury calibrator in the CMM system at 4 hour intervals. The normal test interval required by the analyser is 24 hours, but in the interest of creating more test data, the interval was shortened in this test. All test gases are injected into the probe upstream of particle filters so that the test gas has to pass through the potentially contaminated filters.

Figure 4

The results from six repeated span/zero test cycles are shown in figure 4 (above). The target level for the span check was 6.5 µg/Nm3 and the average span level was 6.60±0.036 µg/Nm3. The average result for the zero check was -0.006 ± 0.036964 µg/Nm3. If the dust accumulating in the sample extraction probe were to cause analyte loss during span tests, the later tests would show a decrease from the span check target value, but this was not observed. If the dust in the probe were to make the response time longer (memory effect), the later tests would show a slower response than the first tests. Again, there was no systematic change in the test results and the tests 1-6 exhibited very consistent results.

The span and zero checks also provided an opportunity to characterise the response time of the analyser when the span test at a known concentration is followed by a zero check with a zero concentration. The data from all six tests in figure 3 were combined together into one dataset in figure 4 by synchronising the moment when the span/zero check cycle was started. A Boltzmann sigmoidal curve (eqn 1) was fitted to the experimental data using GRG nonlinear fitting routine in the Microsoft Excel Solver package. The parameters of the response curve are summarised in table 2. The response time was evaluated as T90-10, the time interval between a reading representing 90% of the span check value and a reading representing 10% of the span check value.  The response time from this calculation was 10.15 minutes or just over two measurement cycles (measurement data is obtained as 5 minute rolling averages of the mercury concentration). The live data from the emissions shows peaks of comparable sharpness, but these were not subjected to the same analysis as the span/zero check data.

Summary
The requirements of a Continuous Mercury Monitoring system in a Cement plant are as follows:

• capable of measuring a low baseline level with high sensitivity when the raw mill is on and the fuel feed contains low levels of metals
• capable of measuring excursions to higher concentrations when the raw mill is off
• low cross-interference from gases e.g. SO2
• no analyte loss or other sampling issues in high dust loading
• stable calibration and simplified calibration check routine with built-in calibration gas generator.

Since the main application areas for continuous Mercury monitoring systems have been in hazardous and municipal waste incineration, and coal fired power stations with conditions that are different to Cement plants; care must be taken to ensure that the monitoring system, and especially its sample extraction probe, is suitable for the process conditions. This study demonstrates that a CVAF spectrometer and dilution sampling approach can be successfully used in this application.

Germany lowers biogas formaldehyde emissions

Power generation from Germany’s enormous biogas industry produces emissions to air that are regulated by the Technical Instructions on Air Quality Control (TA Luft). As part of the approval process, the emissions from each plant have to be tested every three years. Formaldehyde is one of the pollutants of greatest concern because of its carcinogenicity and the TA Luft emission limit is 60 mg/m³. However, the German Government has also created a financial incentive scheme to encourage process managers to lower their formaldehyde emissions to below 40 mg/m³. To be eligible for the EEG (Erneuerbare Energien Gesetz) scheme, plants must be tested every year.

Formaldehyde (HCHO) can be difficult to measure in hot, wet emissions, not least because it would dissolve in condensate if the sample gas is allowed to cool. Test engineers in Germany have therefore deployed portable (DX 4000 and CX4000 from Gasmet) FTIR analyzers to measure formaldehyde, and a number of systems are currently in use across Germany.

Background
The biogas industry in Germany has grown enormously in recent years; in 1992 there were 139 biogas plants in the country, but by the end of 2013 there will be almost 8,000 with an electrical capacity of about 3,400 MW – sufficient for the energy needs of around 6.5 million households. Initially, biogas plants were built to handle the by-products of human and animal food production as well as agricultural waste, but with government incentives to generate renewable energy, farmers are now growing crops such as maize specifically for energy production.

Biogas is produced by anaerobic digestion with anaerobic bacteria or fermentation of biodegradable materials. The main constituent gases are methane and carbon dioxide, with small amounts of hydrogen sulphide and water. The products of biogas combustion are mostly carbon dioxide and water, but the combustion of biogas also produces formaldehyde.

Biogas-fuelled combined heat and power (CHP) plants are becoming a very popular source of renewable energy in many countries because they provide a reliable, consistent source of energy in comparison with wind and solar power. In addition to the renewable energy that these plants produce; the fermentation residue is a valuable product that can be used as a fertiliser and soil conditioner for agricultural, horticultural and landscaping purposes.

Exhaust gas tests
The exhaust emissions of each biogas plant are tested every three years for substances hazardous to air quality, such as particulates, carbon monoxide, nitrogen oxides, sulphur dioxide and formaldehyde. Most of these parameters can be measured on-site with portable equipment. However, in the early years and still to this day, the complexity of formaldehyde analysis has necessitated sampling and laboratory analysis – a time-consuming and costly activity.

FTIR_DX4000

In 2007 Wolfgang Schreier from the environmental analysis company RUK GmbH (now part of the SGS Group) started working on the use of portable FTIR gas analysers for formaldehyde analysis. The FTIR analysers are manufactured by Gasmet (Finland) and supplied in Germany by Ansyco GmbH, a Gasmet group company.

FTIR analysers are able to qualitatively and quantitatively analyse an almost endless number of gas species. However, Wolfgang Schreier says: “The Gasmet units are primarily employed for the measurement of formaldehyde, and whilst they are able to measure other parameters of interest such as CO, NOx and Methane, they are not yet certified for doing so in the emissions of biogas plants, unless an internal validation has been undertaken.

“The DX4000 proved to be the ideal instrument for this application because it samples at high temperatures (above 180 Deg C) so formaldehyde cannot dissolve in condensate, and the instrument provides sensitive, accurate, reliable real-time formaldehyde measurements – no other portable analyser is able to achieve this.

“Importantly, the DX4000 is also robust and weighing just 14kg, it is easy to transport from site to site. In addition to a heated sample line, the only other accessory is a laptop running Gasmet’s Calcmet™ software.”

In contrast with the portable FTIR, it is typical for the results of laboratory gas analysis to become available around 2 weeks after sampling. This highlights a further benefit of the direct-reading instrument; real-time results enable plant managers to adjust their process in order to improve efficiency and minimise the emissions of formaldehyde and other gases.

Ansyco’s Gerhard Zwick says: “We hope that the other measurements that are possible with the Gasmet FTIR will also soon be accepted. A new VDI method (VDI 3862-8) for the measurement of formaldehyde by FTIR is being established and this is likely to be published at the beginning of 2014.

“The preparation of this standard involved rigorous field tests with 5 Gasmet FTIR analysers at a live biogas plant. During testing, samples were taken for analysis according to the existing standard laboratory methods and the results showed that portable FTIR produced even better results than lab analysis.”

Formaldehyde reduction incentive
The bonus is paid to the operators of biogas plants which are subject to approval by the Federal Immission Control Act if certain conditions are met. Measurements to demonstrate the effectiveness of emission reduction have be taken each year by an organisation which is approved according to § 26 of the Act.

While the emission limit for formaldehyde is 60 mg/m3, according to the EEG legislative, the plant operator receives a bonus of 1 cent per kW when formaldehyde emission levels are below 40 mg/m3, with simultaneous fulfilment of the emission limits for nitrogen monoxide and nitrogen dioxide (combined), and for carbon monoxide.

With the benefit of real-time readings from the FTIR, process operators are able to employ process control measures to alter formaldehyde emissions. However, this may also affect the efficiency of the combustion process or the concentrations of other limited gases. In addition, it is now commonplace for modern plants to use a catalyst for formaldehyde emission reduction.

Summarising Gerhard Zwick says: “The standard formaldehyde emissions monitoring package consists of a Gasmet DX4000 analyser and a heated sampling system, so no adaptations were necessary for the measurement of biogas emissions.

“We have now supplied instruments to most of the key testing organisations as well as motor and system manufacturers in Germany. Happily, the feedback has been extremely positive because, as a portable analyser, the Gasmet FTIR systems are able to test more plants, more quickly, and this lowers costs.”

Wastewater treatment optimisation provides cost savings

Dr Michael Haeck, Hach Lange

Background
The operators of wastewater treatment plants constantly seek new opportunities to improve plant efficiency and environmental performance. In order to achieve this they need to be able to maintain the effectiveness of the treatment process, producing a consistent discharge within consent limits, whilst minimising inputs such as energy, labour and raw materials.

Real-time control (RTC) has become very reliable.

As technology advances new opportunities materialise and this article will outline the considerable benefits that can be obtained from the latest sensors coupled with a new breed of real-time controllers. Improvements in the accuracy and reliability of sensors, coupled with a new facility providing  information about the sensors’ performance, in addition to the measurement itself, means that real-time control (RTC) has become very reliable which means that it has become an attractive option in a large number of applications.

Hach Lange has developed a set of standardised control modules, enabling the application of processes improvements and optimisation strategies without the need for complex programming and expensive customisation.

In combination with Hach Lange sensors, Nutrient Removal and Sludge Treatment Processes can now be easily optimised in order to achieve savings in aeration energy and chemical consumption, even on small waste water treatment facilities.

RTC opportunities
Stand-alone wastewater treatment optimisation solutions (WTOS) control modules are now available to optimise individual treatment processes at treatment plants. These can be easily integrated into an existing plant structure and currently include (1) the chemical elimination of phosphorus and (2) dissolved oxygen adjustment according to the actual NH₄-N load in an aeration tank.  Control modules for sludge management as sludge retention time controller or desludging controller will be added in the near future.

In addition to the stand-alone modules mentioned above, it is also possible to combine different RTC modules to optimise an entire plant, as outlined in the trial below. Termed an ‘enterprise solution’ this activity involves a review of the plant as a whole and the creation of customised specifications for the application of different control modules for nitrification, sludge retention time, methanol dosing, and/or chemical phosphate removal to achieve the best overall performance.

Sensor technology
In recent years, improvements in sensor technology have focused on greater resolution and accuracy in combination with longer intervals between calibration or service. However, in order for an RTC system to operate effectively it is also necessary for sensors and analysers to be able to provide information on the quality of the signal and the service status.

Hach Lange has filed a patent application for this facility under the brand name ‘PROGNOSYS’. This provides the RTC control modules with a continuous indication of a sensor’s status so that if pre-determined conditions occur (sensor failure, outside calibration, service overdue, drift etc) the RTC automatically adopts an alternative control strategy, which might be a typical weekly and diurnal flow profile that has been stored in the system’s memory.

Stand-alone RTC example: chemical Phosphate removal
As outlined above, the measurement technology for phosphate has advanced considerably in recent years in tandem with a reduction in capital and operational costs. As a result, an easy to integrate RTC module in the phosphate removal process can deliver pay back periods of less than one year.

The measurement of phosphate levels in combination with an RTC system can be utilised to manage the dosing of precipitant salts. This precipitates the phosphate and facilitates sedimentation and removal. Accurate continuous monitoring is necessary to ensure that (a) sufficient dosing is applied to remove the phosphate and (b) excessive dosing does not take place. Over-dosing would be undesirable on three counts; firstly, from an environmental perspective the objective is to minimise the amount of iron being added that could remain in the effluent; secondly, ferric sulphate is expensive and excessive dosing would be costly; thirdly the amount of precipitation sludge should be kept to a minimum because sludge disposal can represent a significant cost.

A unique feature of the RTC system is the continuous automatic calculation of the ‘ß’ value (overdosing rate), which is required to calculate the right amount of precipitant dosing for open loop control. The calculated ß-value takes into account the percentage of phosphate which has to be removed. The less phosphate there is; the more difficult removal becomes and the more precipitant is required to eliminate the same amount. For example, more precipitant is required to lower phosphate concentrations from 4 to 2 mg/l than from 6 to 4 mg/l.

Wastewater treatment plants operating an open loop real time control system for phosphate removal have demonstrated considerable savings – a UK works has saved approximately 37% of the ferric sulphate cost and 57% of caustic chemical costs and a plant in Italy has shown 50% cost savings in comparison with a constant dosing system, which represents a 7 month payback.

If closed loop control is applied, the RTC system requires a measurement of phosphate levels immediately after dosing. As a result, the Phosphate concentration can be held at a fixed desired level and the control performance is monitored as indicated in figure 1.

Figure 1: Example for Stand Alone P-RTC performance

UK RTC Trial – activated sludge process control
The results of a trial investigating the benefits of an RTC system on the management of the activated sludge process (ASP) have been published by Thornton, Sunner and Haeck[i].

Managed by MWH UK Ltd and employing monitoring instruments from Hach Lange, the trial employed online sensors and control algorithms to optimise the operation of the ASP, leading to greater efficiency and sustainability. Undertaken at full scale, the trial assessed the benefits of RTC at a 250,000 population equivalent (PE) works in the UK and consisted of two identical ASPs (each with four lanes) configured as a 4-stage Bardenpho plant with methanol addition in the secondary anoxic zone.

Standard aeration lanes (fixed DO set-points with fluctuating NH₄ effluent concentration) were compared with lanes running an RTC system operating variable DO set-points based on actual load. The RTC lanes deployed extra sensors for dissolved oxygen, ammonium and nitrate.

The trial demonstrated that the RTC system was able to respond quickly to ammonium influent spikes and to maintain a stable effluent ammonium level. The trial also demonstrated that the RTC system was able to reduce methanol consumption by 50% and energy (measured as air flow) by 20% (figure 2). The system has now operated successfully for more than one year

Figure 2: RTC savings

Summary
The Hach Lange optimisation system combines process measurement technology with advanced RTC control modules to provide substantial savings in operational costs at wastewater treatment plants, whilst maintaining compliance with consent values.

Recent advances in sensors, analysers and controllers mean that wastewater treatment no longer has to be managed on a ‘worst case scenario’ basis. Processes can now be monitored and adjusted instantaneously to maximise efficiency and improve process stability. Cost reduction is obviously a key benefit, but the ability to reduce energy consumption is becoming an important objective in many countries.

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[i] Thornton, Sunner and Haeck, 2010. Real time control for reduced aeration and chemical consumption: a full scale study. Water Sci. Technol.61, 2169–2175