Process optimisation by Real-Time Control!

23/08/2014
Major installation at English sewage treatment works.

Wessex Water, an English water authority,  is investing around £20m at its Taunton sewage treatment works to improve the facilities for wastewater and sludge treatment in a project that is due for completion by the end of March 2015. The upgrade to the works under the DWF (Dry Weather Flow) Improvements Scheme will increase the site’s treatment capacity whilst also improving the efficiency and quality of the treatment process, lowering energy costs and reducing the site’s carbon footprint.

Prior to the implementation of the DWF Scheme, the STW was comprised of an inlet pumping station and balance tank, coarse and fine screens, grit removal (detritor), primary settlement tanks, a conventional ASP & biological filter beds, final & humus tanks and final effluent lagoons. The construction work involves the creation of a new four-lane ASP to replace the existing 16 biological filters. To facilitate this, one of the lagoons and four of the filters are being taken out of service to create space for the new works, and this has allowed all development to remain within the existing site boundaries enabling most works to be constructed under permitted development rights.

tauntonProcess optimisation of the new ASP stage will be achieved through implementation of Hach Lange’s Real-Time Control (RTC) system, which monitors influent ammonium concentration and dissolved oxygen concentrations along the aeration lanes, providing more efficient control of the fine bubble diffused aeration. The measurement of other quality parameters in the process train provides feedback to the RTC. A reduction of up to 15% energy usage is anticipated as a result.

Balfour Beatty has provided the civil works and Nomenca Ltd is responsible for the supply, installation, commissioning, and performance testing of the mechanical and electrical components of the new works. Contracts Manager Trevor Farrow says, “Nomenca’s reputation is built on a track record of successfully delivered projects, and the relationships that we develop with both clients and suppliers are key to this success. We have already worked with Hach Lange’s instrumentation on a wide variety of projects, so we are confident that this project will be a further success.”

As Project Manager for Wessex Water, Garry Orford says: “The drivers for this works upgrade include an increased treatment capacity requirement and a tightening of the consent, taking in to account longer-term requirements that may be implemented in AMP6. We have already implemented Hach Lange’s RTC process optimisation systems at our Holdenhurst plant – 175,000 PE – near Bournemouth, and this has delivered energy savings of around 25% so we are confident that we can repeat this success at Taunton – 85,000 PE.”

taunton2Following completion of the new works, the site will meet the following consent conditions:

  • Dry Weather Flow (DWF)   30,595 m3/d
  • Sanitary parameters BOD:SS:AmmN 15:30:3 mg/l

In addition to the upgrade of the sewage treatment facilities, a third anaerobic digester (AD) is also being built at the Taunton works. “This will increase our capacity to generate renewable energy and further reduce our electricity bill,” according to Garry Orford. “The power generation of the AD plants is fairly stable, but the energy demand of the treatment plant varies according to the load, so there will be occasions where we can sell energy back to the grid, and others where we will continue to have a power requirement. It is essential therefore that we use this power as efficiently as possible.” 

Real-Time Control in industrial processes is commonplace. However, wastewater monitoring represents a greater challenge because of its physical and chemical variability. Historically, wastewater monitoring technology was prone to drift (especially galvanic dissolved oxygen monitors) and required a high level of maintenance, so RTC was not feasible. However, the latest sensors offer much higher levels of reliability than was possible in the past, with substantially lower levels of maintenance and recalibration. This has been a major factor in enabling the development of RTC in wastewater treatment. In addition, many of the latest sensors provide a ‘health status’ output in addition to the readings. As a result, if any problems arise they can be quickly remedied, and control systems can ignore data from sensors that are not performing to their target specification.

Monitoring technology
The capital outlay for the addition of RTC to a treatment plant is relatively small; the most significant extra cost is the requirement for extra sensors plus the RTC unit. The Taunton build includes the installation of the latest sensors for dissolved oxygen, ammonium and turbidity, controlled by an sc1000 network, providing reliable data on the influent, and from within the treatment process.

IMG_0056The LDO sc dissolved oxygen sensor employs an optical luminescence method for calibration-free and drift-free measurements. Once the construction work is complete there will be four new lanes, each with three zones, so a total of 12 LDO probes will monitor dissolved oxygen.

In addition, two SOLITAX ts line dip probes will measure Mixed Liquor Suspended Solids (MLSS) content in the aeration lanes and the solids content of the Returned Activated Sludge. The RTC at Taunton will also control sludge retention time, which enhances plant efficiency. The suspended solids probes employ a patented dual scattered light method with a built-in wiper, to provide colour-independent measurement of solids without a requirement for calibration. Ammonium measurements will be undertaken at both the entrance and exit of the aeration lanes with two AMTAX sc instruments; high-precision analysers that continuously collect samples via an air-bubble cleaned filter probe. The ammonium analysers will be mounted directly over the filters to minimise the distance travelled by samples.

Real-Time Control
The Hach Lange RTC is implemented on an industrial PC which communicates with an sc controller network and the local PLC. The RTC system determines the most efficient aeration level and continuously feeds DO set points to the PLC which controls the blowers. This means that under RTC, DO set points are no longer ‘fixed’, instead they ‘float’ according to the load. The RTC modules continuously deliver set points to the PLC, which applies them to the process. This ensures that response to changing conditions is immediate. The algorithms employed by the N-RTC (Nitrification Real Time Controller) are mainly based on the Activated Sludge Models of the International Water Association.

The N-RTC also constantly reads the NH4-N concentration at the outlet of the aeration lane. This value provides a feedback control loop and ensures that the DO concentration is fine tuned to achieve the desired ammonium set point at the end of the ASP. In this way, the N-RTC control module combines the advantages of feed forward and feedback control, which are (1) rapid response, (2) set point accuracy and (3) robust compliance.

Aeration to achieve the biological oxidation of ammoniacal compounds to nitrate is the most energy intensive process at activated sludge plants because blower power consumption can represent over 50% of total costs at some plants. However, in addition to the advantages of the process optimisation system, four new Sulzer high speed HST-20 turbo-compressors are being installed by Nomenca, following trials on similar units by Wessex Water. These machines employ a control system that manages both the number of blowers to run, and the speed of the blowers, which will further improve energy efficiency.

Summarising, Garry Orford says: “Wessex Water has an ambitious long-term objective of carbon neutrality, and these improvement works projects provide us with useful opportunities to make a significant contribution to that goal.”


Instruments down the drain!

18/11/2013
CCTV hire supports excellence in drain services

With more than 40 regional drain cleaning service centres across Britain, Metro Rod provides a 24 hour service to anyone with blocked or damaged drains, pipes, toilets, sinks etc. To support this capability, the company and its franchisees maintain a fleet of the latest equipment such as tankers, high pressure jets, excavators, pipe liners and CCTV surveying equipment.

InpsectionFamilyTo ensure a fast and effective response to all customer requests during periods of peak demand, many of the franchisees take advantage of Ashtead Technology’s equipment rental fleet. For example, Ryan Davis is the owner and manager of the Metro Rod franchise for London East Central. With many years of experience managing Metro Rod in this area, Ryan has established a highly trained team of engineers supported by heavy investment in equipment such as a 3,000 gallon tanker and the latest CCTV drain inspection equipment. However, Ryan says: “Occasionally we need to undertake drain surveys in different locations at the same time, and we also receive enquiries that necessitate more specialised inspection cameras, so if our own equipment is unavailable, it is very useful to be able to hire the best instrument for the job.

“Ashtead Technology maintains a comprehensive fleet of the latest inspection tools including robotic camera crawlers and push-rod cameras and videoprobes, so we are able to quickly supplement our own equipment as and when we need to. This helps to ensure that our customers receive the best service possible.”

Metro Rod’s customers range from domestic customers with a blocked pipe or drain, to large water utilities that rely on a rapid response to any problems in the water and wastewater distribution network. In addition to emergency response, Metro Rod also provides routine preventative maintenance services to a wide range of organisations. For example, the south London franchisee provides ongoing service to the Wimbledon Lawn Tennis Club, including a major service in June, immediately prior to the Championships.

The ongoing operation of many industrial processes relies on the effective discharge of effluent, and any problem in this waste stream can limit or even halt production. Metro Rod’s national network of local teams means that the company is able to provide a fast and effective solution to blockages and related problems in pipes, drains and culverts.

“This ability to respond quickly to customer requests is a key feature of our business,” says Metro Rod’s Marketing Manager Ieuan Nicholls. “Our teams are highly trained and experienced, and they are equipped with the latest equipment to ensure prompt resolution of any problems.

“The facility to rent inspection equipment during periods of high demand means that we can ensure that we have the right kit on the ground for every job.”

Ashtead Technology’s Jay Neermul agrees: “It would not make financial sense for any company to purchase the volume of inspection equipment required to meet the highest level of need, because, by definition, a significant proportion of this equipment would lie unused, depreciating for much of the time.

“We have invested in our wastewater inspection fleet to meet increased demand, and have recently ordered more of the latest crawler units and pushrod cameras. We are therefore delighted to be able to partner with Metro Rod’s teams to ensure that they have exactly the right kit, wherever and whenever they need it.”


Wastewater treatment down-time ‘not acceptable’

07/11/2011
Why monitoring plays such an important role at a manufacturing business in the North East of England.

Reliable 24/7 monitoring and control of a pharmaceutical company’s wastewater facility is fundamentally important to the successful operation of the entire business. This is because failures or down-time in the wastewater treatment process would quickly result in waste stream back up. It follows therefore that monitoring equipment should be extremely reliable and this is why Shasun Pharma Solutions employs Hach Lange monitoring equipment at many locations around their manufacturing facility just north of Newcastle upon Tyne (GB).

Shasun Pharma Solutions provides research and contract manufacture services to the pharmaceutical industry, including small scale manufacture for clinical trials and full scale commercial manufacture of advanced intermediates and active pharmaceutical ingredients. With both large and small customers spread across Europe, North America, Latin America and Asia, the business has to be able to demonstrate a high level of environmental management.

Craig Goodman manages Shasun’s wastewater treatment plant which employs three 2,500m3 tanks to treat industrial wastewater by an activated sludge process that utilises oxygen and a biological floc to break down the waste materials. Craig says he uses liquid oxygen rather than mechanical aeration “because pure oxygen enables the plant to respond much more quickly and requires around one fifth of the volume for aeration.”

The LDO!

The liquid oxygen is stored onsite and provides Craig with almost instantaneous control of dissolved oxygen in the plant. He says: “This is made possible as a result of the new breed of dissolved oxygen sensors that we use – the ‘LDO’.

“In the past, we relied on traditional membrane based DO sensors but these required a high level of maintenance and tended to drift; it was usually necessary to recalibrate every week. However, the new LDO sensors last for over a year without recalibration and we then simply replace the sensor cap, so our monitoring activity is now significantly easier and more accurate and reliable.”

The liquid oxygen is vapourised and fed into the tanks via a single entry point Venturi at around 7bar and the objective is to maintain DO at 3mg/l +/- 0.2. In addition to online sensors for pH and ammonium, the LDO sensors are connected to an SC1000 controller which also monitors the ‘health’ of the sensors and interfaces with the plant’s control systems.

Emphasising the importance of accurate DO control, Craig says: “In addition to the waste stream from our own plant, we also treat waste from third parties so we do not always know what is coming down the line and that is why we need to be able to respond quickly; overdosing oxygen would kill the bugs and underdosing may cause other problems such as bulking.”

Shasun’s wastewater treatment facility effectively removes 95% of COD (Chemical Oxygen Demand) which is a common method for the determination of organic pollution. COD testing is therefore conducted onsite and Craig’s team employ a Hach Lange spectrophotometer for this purpose. The pre-filled, bar-coded Hach Lange COD tubes ensure that every test is conducted in exactly the same way, with exactly the same reagents. Bar-coding ensures that the spectrophotometer recognises each sample and ensures traceability of results.

Craig’s team conducts between 30 and 100 COD tests every week and this data helps in the efficient running of the plant and helps ensure compliance with the discharge consent. It also provides Craig with useful information for the calculation of waste treatment charges for third parties. “This is because 1 tonne of COD requires approximately one tonne of liquid oxygen for treatment,” he explains.

In recent years, all of the laboratory, portable and online wastewater testing equipment has been supplied by Hach Lange. Craig says: “Down-time in our wastewater treatment plant would not be acceptable, so we have to use the most robust and reliable instruments available and in our experience that means choosing Hach Lange.”