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Optimising wastewater treatment can result in significant cost savings, according to Dr Michael Haeck of Hach Lange.

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 while minimising inputs such as energy, labour and raw materials.

As technology advances, new opportunities materialise and considerable benefits can be obtained from the latest sensors coupled with 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.

This, in turn, means that it has become an attractive option in a number of applications.

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

In combination with the company’s sensors, nutrient removal and sludge treatment processes can now be optimised in order to achieve savings in aeration energy and chemical consumption, even on small wastewater treatment facilities.

Standalone wastewater treatment optimisation solution (WTOS) control modules are now available to optimise individual treatment processes at treatment plants.

These can be integrated into an existing plant structure and currently include the chemical elimination of phosphorus and 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 de-sludging controller will be added in the near future.

In addition to the standalone 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.

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 predetermined conditions (such as sensor failure, outside calibration, service overdue or drift) occur, 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.

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 payback 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 sufficient dosing is applied to remove the phosphate and that excessive dosing does not take place.

Overdosing would be undesirable on three counts: first, from an environmental perspective the objective is to minimise the amount of iron being added that could remain in the effluent; second, ferric sulphate is expensive and excessive dosing would be costly; and third, the amount of precipitation sludge should be kept to a minimum because sludge disposal can represent a significant cost.

The RTC system features 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 that 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 4mg per litre to 2mg per litre than from 6mg per litre to 4mg per litre.

Wastewater treatment plants operating an open-loop real-time control system for phosphate removal have demonstrated considerable savings.

A UK works, for example, has saved approximately 37 per cent of the ferric sulphate cost and 57 per cent of caustic chemical costs, while a plant in Italy has generated 50 per cent cost savings in comparison with a constant dosing system, which represents a seven-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.

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.

Managed by MWH UK 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 four-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 showed that the RTC system was able to reduce methanol consumption by 50 per cent and energy (measured as air flow) by 20 per cent.

The system has now operated successfully for more than a year.

The Hach Lange optimisation system combines process measurement technology with advanced RTC control modules to provide savings in operational costs at wastewater treatment plants, while 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 a key benefit, but the ability to reduce energy consumption is also becoming an important objective in many countries.

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