Process optimisation by Real-Time Control!

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.

Process 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.”

Following 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.

The 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.”

Final effluent monitors protect wastewater treatment efficiency

Richard Reeves, Principal Process Scientist, Southern Water, and David Ballinger, Optimisation and R & D Manager, Southern Water discuss this application. Southern Water supplies water and wastewater services for Kent, Sussex, Hampshire and the Isle of Wight in the south of England.

Authors Richard Reeves David Ballinger

Southern Water operates 370 wastewater treatment plants (WWTP), many of which are unmanned for most of the time and most have numeric environmental permits, so a network of online monitors has been established to improve treatment and protect discharge compliance. This has involved the installation of final effluent monitors at over 300 sites in a programme that has lasted for more than ten years.

The online monitors are comprised of a Hach Lange turbidity probe with sensors for temperature and level (to show when the turbidity probe is out of the water) and mounted on a plastic ‘spade’ which holds the sensors in position at the final effluent monitoring point.

Using turbidity to estimate BOD & TSS
As a measure of clarity, turbidity provides extremely useful data; a cloudy final effluent suggests poor treatment and possible discharge permit failure.

In 1993 Southern Water conducted an extensive research project to demonstrate that effluent clarity, as measured by turbidity, can be related on a site by site basis to permitted BOD (Biological Oxidation Demand) and TSS (Total Suspended Solids).

The constituents of final effluent are such that biological slimes and algae are prone to develop on optical surfaces, and the trials therefore concentrated on the most efficient method of probe cleaning. Probes with no automatic cleaning were therefore eliminated. The technically preferred monitor was the Hach Lange Solitax turbidity probe, which incorporates a silicon rubber wiper, which sweeps over both optical surfaces at a programmable frequency. Southern Water also found that the cleaning efficiency was improved by the addition of an air purge which blows away the loosened solids.

The Solitax probes use a single LED light source with three detectors, one for light intensity and two for light scatter. Hach Lange’s Clive Murren says “This provides reliable colour-independent readings with a low maintenance requirement. However, we have a contract to routinely visit each site to service and calibrate the turbidity sensors.”

Hach Lange has confirmed that their SC200 and SC1000 controllers and the SOLITAX sc turbidity probe were awarded MCERTS certification on 1^st November 2012. MCERTS is the Environment Agency’s monitoring certification scheme and currently only a small number of analytical instruments have achieved this award. However, an MCERTS certificate demonstrates that equipment has met or exceeded the stringent performance requirements of the scheme.

Solitax Wiper

The monitor control unit is mounted in a separate cabinet which also houses the air compressor. When the plc calls for a clean, the wiper operates followed by two air purges which release the algae/biofilm. A further wipe then removes any remaining material.

Cleaning is initiated every hour and, when a clean is called for, the last recorded turbidity reading is held in the monitor for 5 minutes, to avoid recording the false turbidity generated by the loosened material.

Design and Installation
Linton Electrical Contractors (Kent) Limited has a longstanding relationship with Southern Water and was responsible for the final design and installation of the final effluent monitoring systems. Subject to prior approval by Southern Water, the installations incorporate the latest Hach Lange instrumentation as it becomes available.

Linton Electrical is now installing as standard the 110V MKV SC200 controller and occasionally the MKVI SC1000. Each system is complete with conductivity sensor, level sensor, temperature probe and air cleaning system; all of which is mounted on a PVC spade.

Linton’s Mark Pendry says “The project has been very successful because we have established the skills, tools and spares to ensure that every installation is conducted quickly and efficiently and the quality of the HACH LANGE instruments ensures reliability and accuracy.”

One of the advantages of the SC controllers is the ability to add additional instruments and Clive Murren says: “The facility to add the Nitratax nitrate probe saves time and money when using this platform, because it utilises the same spade design as the turbidity sensor and several of the latest installations have also incorporated the nitrate probe.”

Processing of Data
The WWTP telemetry outstation scans all connected monitors every second. The outstation calculates 15 minute averages (or in the case of temperature takes a 15 min spot reading) and relays the data by a phone line to a central processing unit known as SCOPE. The SCOPE data is available to the Regional Control Centre and local PCs.

The System serves as a single source of process data with automatically generated performance indicators that allow exception notification and strategic analysis.

An important functionality of the System is Exception Reporting. This function compares the recently archived value with a limit value and generates an Exception Report if the value is out of range. The Exception Report, as an email, is sent to selected recipients.

An Operational Database has been designed to hold asset dimensions and other site details (current operational units and trigger (limit) values) which are used in the calculations.

Reporting of Data
A relationship has been established between the sum of the spot sample TSS + BOD and archived turbidity (see Figure 1).

Figure 1. The relationship between final effluent Turbidity and TSS+BOD – WWTP with standard Percolating Filters

The turbidity relationship is used to determine a turbidity level equivalent to the sum of the permitted BOD + TSS. This turbidity ‘permit’ equivalent is used by Southern Water in three ways:

1. 80% of the ‘permitted’ turbidity is used in the outstation to generate an alarm in SCOPE if the 15 minute mean turbidity exceeds the ‘permitted’ turbidity for more than a chosen time span. This may be instantaneous or up to 2 hours depending on the environmental significance of the discharge. A high priority alarm is then issued to the site/standby Operator who will visit the site and take appropriate action.
2. The Process Management System generates an Exception Report by email to selected recipients if the daily average turbidity held in the derived values archive exceeds 80% of the ‘permitted’ turbidity.
3. Each Exception Report generated is investigated by the Process Scientist and an ‘Exception Reason’ is chosen from an agreed list of 43 operational and environmental causes of high turbidity – ranging from storm conditions and mechanical failure to equipment malfunction and vandalism. These reasons are then summarised and used for Business Intelligence purposes. In this way Southern Water is monitoring performance of all WWTPs against a continuously monitored parameter in addition to the compliance statistics generated by spot sampling.

Benefits of on-line monitoring

The installation of on-line monitors has encouraged a proactive response to system deterioration, which has resulted in a significant reduction in staff time associated with operational management of the wastewater assets. Non-routine site visits by Operational Staff have been reduced, and Wastewater Support Staff can target their site visits to WWTPs with poor performance as identified by on-line monitors.

The plant performance data has been improved with the use of 35,040 readings at fifteen minute intervals per monitor per year. The consequences have been protection of compliance and the identification of optimisation opportunities.

Routine final effluent sampling has been substantially reduced, resulting in considerable savings in sampling and analytical costs.

Real-time access to turbidity data also helps troubleshooting. For example, high turbidity values can indicate filter rotation problems, hydraulic/organic overload, final tank scraper failure, secondary treatment bypass or tertiary treatment failure.

In addition to the benefits from early warnings of poor quality effluent and the ability to show deteriorating trends, compliance levels have been maintained at around 98% since installation of the monitors began.

Maximum uptime realised through in-line turbidity at Franklaw WTW

Dr Patsy Rigby, Hach Lange UK discusses a water treatment application at a Preston in North West England

A key United Utilities water treatment facility has significantly reduced the risk of failure to comply with the regulatory requirements of the Badenoch Bouchier report and reduced the number of maintenance hours spent by installation of a continuous in-line turbidity probe. As an additional benefit advanced warning of de-sludging issues can be immediately identified enabling the plant operations to rectify the situation in minimal time.

United Utilities Group PLC forms the largest listed water business in Britain, controlling through its subsidiary United Utilities Water (UUW), the licence to provide water and wastewater services to 7 million customers in the North West region. Between the period 2010-2015, UUW is investing £300m improving treatment at 45 of their facilities promising the development of innovative operational solutions to optimise water treatment efficiency beyond the scope of existing processes.

Franklaw WTW based just outside Preston is a key treatment plant in the North West catchment area drawing raw water from various sources including the rivers Wyre and Lune, Fylde borehole, Barnacre reservoir and Thirlmere aqueduct. As a key facility, the plant supplies treated water to a population of 650,000 within Blackpool, Preston and the surrounding districts, providing up to 220 million litres of water per day (equating to a staggering 93,000 glasses of water per second). The years 2001-2004 saw £35 million investment by United Utilities to upgrade and develop the washwater treatment processing at the plant providing lamella based clarifiers for efficient washwater treatment.

North Lancashire Water Technical Officer James Bowsher (see photograph) explains the current treatment process at Franklaw WTW: “Treatment begins with water drawn from 4 inlet sources which are mixed and course-screened to remove large debris. The coloured raw water is dosed and mixed with Aluminium Sulphate and Polyelectrolyte. The coagulated waters then enter the upward flow super-pulsator units which hold the floc within baffle plates while the cleaner water flows upwards and on to the next processing stage. From here, clarified water enters one of 12 rapid gravity filters to trap any remaining particulates. Periodically these filters are automatically backwashed with air and then water to prevent blockage, and the backwash water passed to dirty backwash water tanks for blending and balancing. These waters then pass through a series of lamella clarifiers for further treatment.

Franklaw WTW Site Manager Michael Tillery and North Lancashire Water Technical Officer James Bowsher

“The lamella plate settlers maximise settling (owing to significantly enhanced surface area of multiple stacked angled plates compared to the surface area of a flat horizontal surface) while demanding only 10% of the space taken up by traditional settling units. Deposited sludge from the plates is drawn off and dewatered via sludge press units into 22% solids cake further downstream and ultimately sent out for landfill. The treated supernatant water from the lamella units is returned to the inlet tower for re-treatment. A maximum 10% proportion of the total plant inlet volume may be recycled by return to the inlet of the treatment works. However, regulation demands that recycled backwash water must contain a turbidity of ≤10NTU in accordance with the Badenoch and Bouchier reports.”

Continuous recycling of a small percentage of the total works throughput has dual benefits: 1. there is no need to discharge excess volumes to the water course – eliminating the imposition of discharge consent limits by the Environment Agency and 2. internal recycling reduces the water draw from local resources with river and water stocks sustainably preserved.

Backwash water heavily loaded with sticky organic residues (which deposit over the lamella plates during settlement) must be carefully monitored to ensure that turbidity complies with the 10NTU limit imposed by the Badenoch and Bouchier reports. As James Bowsher continues “In the past we used off-line turbidity analysers with the sample pump fed from the supernatant holding tank post the 6 lamella clarifiers. This holding tank combines a strategic sampling point with an inbuilt overflow unit to carry excess volumes back to the lamella for recycling.

With previous systems, water from the tank was pumped into a separately housed kiosk to the inlet of the turbidity analyser and this gave a delayed turbidity reading. Off-line instruments give good accuracy by enabling a controlled sample environment at optimum temperatures, with bubble traps to reduce interference from air pockets which can scatter the light path and cause false readings. However the nature of the process water in the lamella application was found to cause organic deposits to build up in the sample lines causing serious blockage in the joints and tubing of the flow path. As a result, turbidity readings were found to frequently flat-line.

“To strip the system down, hose all joints and tubing, then re-calibrate the system before a final flush was demanding several hours of maintenance every couple of days. The demand on operator time and the risk of sending high turbidity water back to the inlet with non-continuous measurement far outweighed the reliability of results. Without regular assessment by the operational team the flat-lining could theoretically be un-noticed for several days.”

United Utilities challenged Hach Lange to provide a reliable plug and play, low maintenance in-situ system to avoid the issues arising with off-line systems. A self-cleaning Solitax sc turbidity probe was recommended, mounted on a sliding pole for simple probe removal and maintenance. Based on a dual infrared light scattering technique reliable, real-time monitoring in very low to highly turbid (up to 15% solids) and coloured water samples is afforded. With an automatic cleaning unit, drift free operation is assured despite continuous operation in the wastewater stream and with maintenance limited to an annual change of the wiper blade maintenance is almost entirely eliminated. The system is factory calibrated and requires just a simple factor against a single laboratory result before use eliminating the need for the commonplace dilution based calibration of contemporary systems.

The Solitax sc probe was pole-mounted approximately 1m below the water surface in the combined holding tank of the backwash water (alongside the overflow pipe). The system operates such that if the turbidity approaches the user-set threshold a high range alarm is triggered to stop the supernatant pump returning back to the inlet and diverting the water back to the lamella for re-treatment. Should the water level drop below the height of the probe, an alarm is raised by localised level sensors and the subsequent rise in turbidity from exposure to excess light provides a secondary alarm of high turbidity providing a dual safety approach.

Initial trial data was verified alongside a portable turbidity analyser and proved that when the lamella were overloaded, causing excessive valve head pressure (which increases the sludge retention and reduces the transfer of water) the set alarms promoted immediate intervention, saving many hours of maintenance and re-treatment costs.

The installation in the combined lamellae outlet

Site Manager Michael Tillery describes the immediate benefits of the system: “The in-line Solitax probe gives instant, accurate turbidity results and much quicker than we could achieve with pump fed off-line systems. The particular advantage in the sampling set up is that it is virtually maintenance free, with automatic self-cleaning of the optical surface every 12-24 hours eliminating the maintenance we previously faced to hose down and remove organic deposits. A precautious wiping frequency keeps the turbidity reading cycling below the target threshold which is much better than letting the value creep towards the consent limit.

“With strategic location of the Solitax on the combined outlet of the lamella clarifiers we now get an instant alarm should levels rise ensuring we are always within compliance and with no pump stoppage for maintenance. With operators freed up to undertake other critical jobs around the site it has greatly improved the plant efficiency. On this basis, the system has not only been purchased by this site but is being rolled out across other sites.”

Thrilling results for nutrient monitors!

Commenting on the results of a 4 month trial of nutrient monitors, Hach Lange’s John Moroney says he is “absolutely thrilled” with the report on his company’s instruments which outperformed the competition in almost every measure.

Hach Lange Ammonia & Phosphate Monitors

Background
The levels of nutrients, such as ammonium and phosphates, entering natural water resources is of great concern because these nutrients can either remove vital oxygen or lead to excessive plant growth and algal blooms, which harm wildlife through eutrophication. In addition, high levels of phosphate or nitrate in abstracted water significantly add to the cost of drinking water treatment. The management of nutrient levels is therefore dependent on the ability to monitor accurately and reliably, and as a result, a group of British water companies organised a joint monitoring trial to determine the best instruments.

Trial results
The trial involved the installation of turbidity, phosphate and ammonium monitors from the market’s leading manufacturers at two designated final effluent plants within Britain. Hach Lange provided an AMTAX sc ammonium analyser, a PHOSPHAX sc phosphate analyser and a SOLITAX sc turbidity analyser for the trial.

The SOLITAX sc performed better than any of its competitors and as a result, Severn Trent Water has adopted the instrument in a framework agreement.

Summarising the report on the AMTAX sc and PHOSPHAX sc, John Moroney says “We are delighted to report that these analysers came first in almost all of the key performance measures, which included correlation to diurnal data, variance to laboratory data, and maintenance requirements. However, the panel decided to take the List Price as opposed to current framework prices for the commercial part of the assessment. However, the overall results are extremely encouraging and I know that our customers are very impressed with the technical performance of these units coupled with such low levels of maintenance. The manufacturers did not have access to their equipment during the trial, but over the 4 month period, the AMTAX required 2 hours of maintenance and the PHOSPHAX just one hour!”

Monitoring nutrients at a reduced cost from inlet to effluent
The whole life costs of the Amtax sc for Ammonium and Phosphax sc for orthophosphate have been drastically reduced as a result of the chemistries employed. With the Amtax sc, a gas sensing electrode is utilised which means that reagent consumption is halved in comparison with traditional colorimetric analysers. For the Phosphax sc, the stability of the vanado-molybdate method, means that calibration is not necessary and therefore no calibration standards are required.

The results of this trial will be of great interest to process managers who have to comply with tighter discharge consents as a result of the Water Framework Directive. Couple this with the fact that Hach Lange can now offer sample preparation systems that deliver continuous samples from the inlet all the way through to final effluent, John Moroney firmly believes these analysers lead the way in reliability, accuracy, stability and the lowest whole-life costs.

Water quality is top priority at Brighton aquarium

The SEA LIFE Centre next to the pier in Brighton (GB) is the world’s oldest operating aquarium. Originally designed by Eugenius Birch in 1872, the popular attraction has recently benefited from substantial renovation and now offers a range of new attractions including a Jellyfish Discovery & a Behind the Scenes tour.

With hundreds of highly valuable marine creatures to protect, water quality is a key issue and routine monitoring is now undertaken with Hach Lange instruments. Displays Curator Carey Duckhouse says “The recent building work presented a series of challenges, but we have been able to protect water quality throughout the project with a monitoring regime that was designed to quickly detect any deterioration in water quality and to provide the highest level of vigilance for the most sensitive species.”

In order to protect water quality, each tank at the Brighton attraction has its own filtration system, including a pressurised sand filter, a biological filter and a carbon filter where appropriate. Some tanks, containing particularly sensitive species such as seahorses, octopus and jellyfish, also feature an ultraviolet treatment system.

Many marine organisms will die quickly if the dissolved oxygen (DO), temperature or salinity levels move outside of acceptable boundaries, so temperature measurements are taken daily on all tanks, DO is measured three times/day in the main ocean tank, and salinity and DO are measured twice per week in all tanks. A hand-held ‘HQD’ water quality meter is employed for this purpose, utilising the latest sensor technology such as an optical LDO™ sensor which substantially improves the reliability of oxygen measurements. However, as Carey explains “Even subtle changes in water quality can stress marine organisms, which makes them more sensitive to disease, so a range of other parameters such as ammonia, nitrate, nitrite, phosphate, copper and iron, are also measured with a Hach Lange DR 2800 spectrophotometer.”

Reagents for the spectrophotometer tests are supplied in small pre-filled powder pillows containing extremely accurate amounts of reagents. This ensures that the tests are conducted in the same way every time and avoids potential errors whilst also saving time and chemical wastage.

The spectrophotometer has an internal memory containing the calibration data for a large number of parameters so that Carey and her colleagues simply choose the pillow reagents for the tests they need.

The test procedure is very simple; the contents of a powered pillow are simply added to a small sample and a coloured solution is allowed to develop for a specific time. The sample tube is then inserted into the spectrophotometer which provides a highly accurate and repeatable reading.

The water quality monitoring equipment is also used in research conducted in collaboration with Sussex University. Much of this work is with Cephalopods such as cuttle fish and addresses a range of issues including feeding behaviour, camouflage and nutrition. Accurate water quality monitoring is necessary in all of this work to ensure that observed effects are not the result of water quality changes. Visitors to the aquarium can view research work during the ‘Behind the Scenes’ tour, in addition to the nursery area, the laboratory and the food preparation section.

Clearly, water quality is key to the success of an aquarium and Carey says “If Eugenius Birch was alive today I am sure he would be delighted to see that the aquarium has continued to thrive, and as an engineer he would be fascinated by the water quality monitoring technology that we are now able to employ.”

Pictures: Julia Claxton 2012

Latest analytical technology ensures biogas efficiency

Anaerobic Digestion (AD) relies on the ability of specific micro-organisms to convert organic material into a gas that can be used to generate electricity. However, these bacteria require specific conditions if they are to function effectively and instrumentation specialist company Hach Lange has developed a range of online, portable and laboratory instruments that have enabled a large number of AD plants to maximise efficiency and prevent the risk of failure.

Introduction
In 2009, renewable energy accounted for just 3% of  Great Britain’s energy supply. However, the Government there  has a target to raise this contribution to 15% by 2020 as part of its strategy to fight climate change. Along with wind, solar and various other sources of renewable energy, AD has an important role to perform in helping to achieve the renewable energy target whilst also helping with the management of organic waste.

Biogas is generated in large anaerobic digesters; air tight tanks in which bacterial digestion takes place in the absence of oxygen. Biogas is a combination of Methane, Carbon Dioxide and many other gases in trace amounts, which can be burnt to produce electricity, and then transported to the National Grid. Alternatively it can be further processed and refined to around 100% methane and injected into the national gas grid.

The remnant digestate can be used for a variety of purposes such as a nutritional additive to crops on arable land, much in the way manure is used, or as a landfill restoration material.

There are two types of biogas plants, determined by the substrate they use; co-fermentation plants and renewable raw material fermentation plants. In co-fermentation plants, substrates of non-renewable raw materials are used, such as residues from fat separators, food residues, flotation oil, industrial waste products (glycerol or oil sludge) and domestic organic waste. Renewable raw material fermentation plants utilise materials such as maize, grass, complete cereal plants and grains, sometimes together with manure slurry.

The need for testing and monitoring
Efficiency is vital to the success of a biogas production plant; bacteria require optimum conditions to effectively produce biogas from the digestion of organic matter. Plant operators therefore have a strong interest in the efficiency of their biogas plant and the activity of the bacteria. Consequently these production plants require reliable, on-site analysis in combination with continuously operating process instruments. Loading excessive levels of biomass into a digester may have severe economic consequences and could potentially lead to biomass inactivation and necessitate a cost-intensive restart. Conversely, under-loading a biomass digester could also have financial implications, because less electricity is produced and potential revenue is lost. Substrate amounts must be tailored to achieve the optimum rate of bacterial digestion.

The degradation process which occurs within the biogas plant digesters does so in a highly sensitive microbial environment. The digesting, methane-producing bacteria, for example, are highly temperature sensitive and most active within the temperature ranges of around 35 to 40 DegC and between 54 to approximately 57 DegC. The specific nature of the microbial environment inside the digesters must be maintained throughout fermentation to increase production and avoid inactivation of the highly responsive bacteria.

Monitoring equipment
Hach Lange provides portable, laboratory and online monitoring systems that facilitate examination at key points within the fermentation process, including eluate analysis, where the substrate is fed into the digester, but also within the digester itself. Online process analysis instrumentation can be employed to continuously maintain optimum conditions within the biogas plant and/or samples can be collected regularly for analysis.

Different analytical instruments are required for different stages of the fermentation process: at the substrate entry point; within the main digester; in post-fermentation tanks and to continuously monitor biogas production.

Process monitoring instruments used across the fermentation cycle allow operators to constantly supervise the anaerobic digestion rate and biogas production.

Hach Lange TIM 840 Titrator

One of the most important measurements for assessing fermentation progress is known as the FOS/TAC ratio. This is determined by their TIM 840 Titrator, and the values generated enable the system supervisor to identify potential process problems such as the imminent inversion of digester biology, so that countermeasures can be initiated. The FOS stands for Flüchtige Organische Säuren, i.e. volatile organic acids while TAC stands for Totales Anorganisches Carbonat, i.e. total inorganic carbonate (alkaline buffer capacity).

To measure the FOS/TAC ratio with the TIM 840 titrator, 5ml of sample is added to a titration beaker containing a follower bar. 50ml of distilled water is then added and the measurement is started. The addition of reagents is then conducted automatically by the titrator which saves operator time and reduces the potential for human error. After about 5 minutes the TAC and FOS values are calculated automatically using a pre-programmed formula.

All measured values can be stored in the autotitrator and/or sent to a printer or PC.

The FOS/TAC ratio provides an indication of the acidification of the fermenter, which is an important measurement because a low acid content demonstrates that the rate of bacterial digestion is not high enough. Conversely, too high an acid content means bacterial digestion is exceeding required levels, due to an overloading of substrate.

Case Study:

Viridor’s Bredbury facility

Viridor’s Resource Recovery Facilities in Reliance Street, Newton Heath, Manchester and Bredbury, Stockport. (GB)

At the Resource Recovery facilities which incorporate AD plants the feedstock is derived from domestic waste collections – the ‘black bag’ portion that would otherwise be destined for landfill. Pre-sorting removes plastics, metals and glass, after which the waste is pulverised to produce a slurry that is passed to the AD plant. This slurry contains the organic fraction that is processed to produce biogas.

Steve Ivanec is responsible for ensuring that the pant operates to optimal efficiency. He says “Monitoring is extremely important at this plant because of the variability of the feedstock – the organic content can fluctuate from one day to another, so we have to be able to respond very quickly.”

Steve’s team uses Hach Lange instruments to closely monitor the entire process and to ensure that the plant’s bacteria are provided with optimal conditions. These tests include chloride, pH, alkalinity and volatile fatty acids; the ratio of the latter two being the same as the FOS/TAC ratio, which is determined by a TIM Biogas titrator. In addition, samples are taken from the feed, the digesters and the effluent to monitor ammonia and COD with a Hach Lange spectrophotometer. This data is essential to ensure compliance with the plant’s discharge consent.

The Reliance Street plant utilises biogas to generate electricity and the residue from the AD process can be defined as a product rather than a waste because it complies with the BSI PAS110 Quality Protocol for Anaerobic Digestate (partly as a result of the monitoring that is undertaken). This product is termed ‘compost-like output’ (CLO) and can be landfilled, used as a landfill cover, or spread on previously developed land to improve that land. However, CLO cannot currently be applied to agricultural land used for growing food or fodder crops.

Summary
The Hach Lange test and monitoring equipment enables the operators of AD plants to ensure that the bacteria are provided with optimum conditions so that biogas production is as efficient as possible. As a result, less waste is sent for landfill and renewable energy is generated efficiently. This ensures the best possible return on investment and by reducing the use of fossil fuels for power generation, helps in the fight against climate change.

Wastewater treatment down-time ‘not acceptable’

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.”

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