Tertiary treatment processes have been used for many years to polish effluent, reducing concentrations of suspended solids (TSS), BOD and ammonia in wastewater and effluent treatment plants. Changes in UK legislation including the introduction of the Water Framework Directive and Shellfish Directive has led to increased interest in nutrient removal (P+N) technology and increasing demand for tertiary treatment in the wastewater treatment process. Tertiary treatment is essential to maintain future compliance with ever tightening Environment Agency standards. Tertiary treatment options available for the municipal market include; cloth filtration, sand filtration, membrane bioreactors and UV disinfection. Using the MITA microfibre cloth filtration process it is possible to achieve TSS concentration as low as 0.5mg/l and consequentially achieve ultra-low total phosphorus (TP) concentration. Compared with other technology the high hydraulic capacity of the units makes it an economical option with a small footprint and low operating and maintenance costs.

The most used tertiary filtration techniques for municipal and industrial wastewater treatment plants are cloth filtration and sand filtration. Both offer similar end results, but cloth filters have a number of advantages including a small footprint and online backwash during operation without stopping the process.

MITA Water Technologies pile cloth disc filters allow efficient TSS removal in tertiary phases of wastewater treatment works, which is an efficient way to remove phosphorus as particulate. The filters can be used with or without upstream chemical dosing.

The MITA Water Technologies cloth filtration system provides solid separation using a deep pile cloth which combines the characteristics and advantages of surface filtration with that of deep bed filtration. The MITA cloth filter is made up of a series of filter discs mounted on an inner tube making it a very compact design. MITA cloth filters are sold under license in the UK and Ireland by Evergreen Water Solutions.

• Drum type: The filtration cloth is fitted on a perforated stainless-steel drum operating in a horizontal orientation.
• Disc type: A central drum supports up to a maximum of 32 (No.) discs; each disc comprises 6 (No.) sectors made of reinforced plastic and each covered with an individual deep pile cloth. Horizontal and vertical cloth filters are available for hire.

Energy consumption (W/m3 of effluent treated) for the MITA filter is extremely low, whch is a fundamental driver in process selection. On a recent plant this was <1W/m3. Additionally a head loss of <250ml is considered for normal operation at all flows and load.

Wastewater flows to each filter either though a piped manifold or an inlet channel. The liquid level in the filter chamber is typically just above the cloth covered discs. The level is controlled by the outlet weir. The entire area over the filter is covered by open grid removable GRP decking. The decking panels are specifically designed to facilitate easy maintenance. The influent (post-secondary clarification) passes through the filter cloth whilst the solids are retained on the cloth pile/fibres. Cloth filtration occurs under gravity flow, with all discs completely submerged. The discs will be stationary unless the solids collected on the cloth has resulted in a head loss sufficient to generate a backwash cycle. As only a small percentage of the total filter area is cleaned at any instant, the remainder or the filter cloth can continue to filter the wastewater stream as normal.

The material coating the filter drums and discs is of the ‘pile’ or ‘free-fibre’ type. During the filtration phase the direction of flow causes the fibres to lie flat against the support panels. This results in a fine filtration matrix of overlapping piles. This creates an extremely efficient layer for the separation and retention of suspended solids in a similar manner to sand filters.

Each segment of the cloth filter disc has its own individual cloth membrane that can be changed periodically (up to 8 years). The MITA deep pile cloth is supplied in two sizes; standard pile cloth (10 micron) and microfibre cloth (5 micron). The microfibre pile cloth is made of thinner fibres, but more densely installed on the lower woven polyester supporting fabric, than the standard pile cloth. The footprint of a deep pile cloth filter compared with that of a deep bed sand filter is significantly reduced.

The online backwashing of each filter means that all filters are permanently operating and the use of the filtered water from within the filter itself eliminates the need for a backwash water reservoir. For example; a sand filter surface area of 100m² would require a filter cloth tank of approximately 20m² plan area.

During normal operation, solids are deposited on the cloth, causing the water level in the filter basin to rise, compared to the height of the exit weir. When a preset differential level is reached, the cloth backwash sequence is activated: a unique pumping system, connected to a series of suction nozzles, removes the solids retained on the cloths, resulting in lower operating head loss.

During the backwash process the fibres are raised inside the backwash nozzle, in such a manner that the solids, previously captured, can be readily removed by the counter-current water flow. The backwash water is supplied by the filtered water contained within the filter disc drum. The backwash water (including retained solids) are returned upstream. Any solids settling in the bottom of the filter tank are removed by a pump controlled by a timer.

This filtration technique with free-fibres filter cloth allows the use of very fine fibres and the attainment of optimum separation efficiencies, even in cases of high hydraulic loading or of load peaks and with minimum backwash water flow rates.

• Tertiary treatment: Cloth filters are used with great success for tertiary filtration downstream of secondary settling tanks in biological wastewater treatment plants, both municipal and industrial. The rate of removal of suspended solids is very high and the concentration downstream of the filter can be as low as 1mg/l.
• Phosphorus reduction: Reduction of the phosphorous content when discharged into bodies of water from existing or new build wastewater treatment plant.
• Pre-filtration upstream of UV disinfection: The cloth filter can be included upstream of the UV treatment, ensuring the quality of the water typically specified by UV suppliers and required for good operational efficiency of the disinfection system.
• Alternative to traditional secondary clarifiers: Separation of excess sludge downstream of biological wastewater treatment with RBC, trickling filters or other fixed growth processes.

Crewe is a railway town in Cheshire, with a history of heavy engineering; manufacturing railway locomotives, Rolls Royce cars (1946-2002) and currently Bentley cars. The existing Crewe STW comprised an inlet works, detritor, primary settlement tanks, activated sludge plant and a clarification stage followed by nitrification stage. The consent levels for the works are:

• Consented dry weather flow: 23,300m³/day (269.7 l/s)
• Flow to full treatment: 70,200m³/day (812.5 l/s)
• Average flow: 30,957m³/day (358 l/s)
• TSS: 30mg/l (95%ile)
• Total phosphorus: Phase 1: < 2.0 mg/l. (annual average)
• Total phosphorus: Phase 2: < 0.5 mg/l. (annual average)
• Total iron: 4 mg/l (95%ile)
• BOD: 20 mg/l (95%ile)
• Ammonia: 4 mg/l (95%ile)

The design envelope of the new tertiary treatment project was to meet the existing discharge consent of 2mg/l of total phosphorus (TP) for October 2018 and the new discharge consent of 0.5mg/l TP for March 2020. The design basis of the tertiary solids removal plant also had to achieve future total iron consent of 4mg/l 95%ile (UTL 8). Evergreen Water Solutions (EWS) was the selected bidder by LiMA (Laing O’Rourke/Atkins) with a complete commercial package of design, installation, commissioning and the aftercare package for the mechanical and electrical works including the filters, pipework, panels and access steelwork.

By December 2019, the tertiary solids removal plant will be optimized by EWS process engineers to remove sufficient solids (TSS) in the final effluent such that the new total phosphorus (TP) consent of 0.5mg/l average and iron of 4mg/l (95%ile) can be achieved (Phase 2).

EWS, along with its manufacturing partners, adopted for off-site construction techniques in a bid to improve quality, reduce health and safety risks during installation, reduce capital costs, reduce waste and speed up delivery of project. The solution required 4 (No.) of MITA MSF 24/120 filter made up of 24 (No.) cloth filter discs with a total filtration area of 450m²/ filter.

Mita’s manufacturing experience and product design allowed for the back wash pumps and sludge pumps to be skid mounted and installed outside the Mita cloth filter tanks. The unique cloth filter design was agreed at a HAZOP with the client to improve access and maintenance for operations engineers on site.

The Mita MSF24/120 filter was supplied in a stainless steel tank with an integrated removable access GRP platform and GMS handrail. All process equipment including the GMS access platforms, stainless steel manifold (inlet, outlet, backwash pipework) was manufactured off site resulting in an accelerated build program. Evergreen’s experienced project management team worked closely with LiMA to safely deliver the Crewe Tertiary Treatment Project, with minimal disruption to LiMA’s infrastructure works on site.

Tertiary treatment processes have been used for many years to polish effluent, reducing concentrations of suspended solids (TSS), BOD and ammonia in WwTW effluent plants. Changes in UK legislation including the introduction of the Water Framework Directive and Shellfish Directive has led to increased interest in nutrient removal (P+N) technology and increasing demand for tertiary treatment in the wastewater treatment process. Tertiary treatment is essential to maintain future compliance with ever tightening Environment Agency standards. Tertiary treatment options available for the municipal market include; cloth filtration, sand filtration, membrane bioreactors and UV disinfection. Using the MITA Microfibre Cloth Filtration Process it is possible to achieve TSS concentration as low as 1mg/l and consequentially achieve ultra-low TP concentration. Compared with other technology the high hydraulic capacity of the units makes it an economical option with a small footprint and low operating and maintenance costs.

The most used tertiary filtration techniques for municipal and industrial wastewater treatment plants are cloth filtration and sand filtration. Both offer similar end results, but cloth filters have a number of advantages including a small footprint and online backwash during operation without stopping the process.

MITA Water Technologies pile cloth disc filters allow efficient TSS removal in tertiary phases of wastewater treatment works, which is an efficient way to remove phosphorus as particulate after upstream chemical dosing.

The MITA Water Technologies cloth filtration system provides solids separation using a deep pile cloth which combines the characteristics and advantages of surface filtration with that of deep bed filtration. The MITA cloth filter is made up of a series of filter discs mounted on an inner tube making it a very compact design.

MITA cloth filters are sold under license in the UK and Ireland from Evergreen Water Solutions.

• Drum type: The filtration cloth is fitted on a perforated stainless-steel drum operating in a horizontal position.
• Disc type: A central drum supports up to a maximum of 32 discs, each disc comprises of six sectors made of reinforced plastic and each covered with an individual deep pile cloth.

Energy consumption (measured as W/m³ of effluent treated) for the MITA filter is extremely low. This is a fundamental driver in process selection. On a recent plant this was <1W/m³. Additionally a head loss of <250ml is considered for normal operation at all flows and load.

Wastewater flows to each filter either though a piped manifold or an inlet channel. The liquid level in the filter chamber is typically just above the cloth covered discs. The level is controlled by the outlet weir. The entire area over the filter is covered by open grid removable GRP decking. The decking panels are specifically designed to facilitate easy maintenance.

The influent (post-secondary clarification) passes through the filter cloth whilst the solids are retained on the cloth pile/fibres. Cloth filtration occurs under gravity flow, with all discs completely submerged. The discs will be stationary unless the solids collected on the cloth has resulted in a head loss sufficient to generate a backwash cycle.

The material coating the filter drums and discs is of the ‘pile’ or ‘free-fibre’ type. During the filtration phase the direction of flow causes the fibres to lie flat against the support panels. This results in a fine filtration matrix of overlapping piles. This creates an extremely efficient layer for the separation and retention of suspended solids in a similar manner to sand filters.

Each segment of the cloth filter disc has its own individual cloth membrane that can be changed periodically (up to 8 years). The MITA deep pile cloth is supplied in two sizes; standard pile cloth (10 micron) and microfibre cloth (5 micron). The microfibre pile cloth is made of thinner fibres, but more densely installed on the lower woven polyester supporting fabric, than the standard pile cloth. The footprint of a deep pile cloth filter compared with that of a deep bed sand filter is significantly reduced.

The online backwashing of each filter means that all filters are permanently operating and the use of the filtered water from within the filter itself eliminates the need for a backwash water reservoir. For example; a sand filter surface area of 100m² would require a filter cloth tank of approximately 20m² plan area.

During normal operation, solids are deposited on the cloth, causing the water level in the filter basin to rise, compared to the height of the exit weir. When a preset differential level is reached, the cloth backwash sequence is activated: a unique pumping system, connected to a series of suction nozzles, removes the solids retained on the cloths, resulting in lower operating head loss. During the backwash process the fibres are raised inside the backwash nozzle, in such a manner that the solids, previously captured, can be readily removed by the counter-current water flow. The backwash water is supplied by the filtered water contained within the filter disc drum. The backwash water (including retained solids) are returned upstream. Any solids settling in the bottom of the filter tank are removed by a pump controlled by a timer.

This filtration technique with free-fibres filter cloth allows the use of very fine fibres and the attainment of optimum separation efficiencies, even in cases of high hydraulic loading or of load peaks and with minimum backwash water flow rates.

• Tertiary treatment: Cloth filters are used with great success for tertiary filtration downstream of secondary settling tanks in biological wastewater treatment plants, both municipal and industrial. The rate of removal of suspended solids is very high and the concentration downstream of the filter can be as low as 1mg/l.
• Phosphorus reduction: Reduction of the phosphorous content when discharged into bodies of water from existing or new build wastewater treatment plant (could be downstream of an after-precipitation process).
• Pre-filtration upstream of UV disinfection: The cloth filter can be included in the upstream UV treatment ensuring the quality of the water typically specified by UV suppliers and required for good operational efficiency of the disinfection system.
• Alternative to traditional secondary clarifiers: Separation of excess sludge downstream of biological wastewater treatment with RBC, trickling filters or other fixed growth processes.

Winsford Sewage Treatment Works opened in the mid-1960s and serves a population equivalent of 30,481 (ref: 2011 Census). The existing works comprised an inlet works, detritor, primary settlement tanks, trickling filter biological plant and humus tanks settlement tanks. The purpose of the tertiary project was to meet the new discharge consents.

The design basis of the tertiary solids removal plant is to achieve a future Total ‘P’ consent of <1 mg/l average and future total iron consent of 4mg/l 95%ile post ferric dosing. All flows up to full flow treatment (FFT) plus all return liquors will be pumped through the tertiary plant.

• Average flow: 12,500 (m³/day)
• Peak flow: 28,400 (m³/day)
• TSS: 45mg/L (95%ile)
• Total phosphorus: < 1.0mg/l. (Ave)
• Total iron: 4mg/l (95%ile) / 8mg/l (upper tier limit)
• BOD: 25mg/l (95%ile)
• Ammonia: 7mg/l (95%ile)

In southern Italy, MITA are currently constructing one of the largest tertiary treatment plants worldwide with 18 (No.) MSF 32/120. The MSF 32/120 is made up of 32 cloth filter discs with a total filtration area 120m² per/machine. The design flow rate is 24.000m³/hours of wastewater and the installation will be constructed in concrete tanks. Flows will be gravity fed through the system and will be delivered in December 2018.

With 20+ years of design and production of cloth filters, offering a wide range of designs including cloth drum filters, vertical and horizontal cloth filters, MITA has become a market leader in tertiary filtration. Thanks to consistent technical improvements and product development, added to the wide range of filters (up to 32 discs per machine), and vertical axis configuration, over 320 machines have been successfully installed.

Kelda Water Services Grampian (KWS Grampian) has a 28 year design, build, finance and operate contract with Scottish Water to operate and maintain four wastewater treatment plants serving the north east of Scotland; two sites in Aberdeen (Nigg and Persley) and two sites to the north (Peterhead and Fraserburgh). In addition to treating wastewater, KWS Grampian operate sludge treatment facilities able to receive imported sludge and liquid wastes. Nigg WwTW located to the south east of the City of Aberdeen on the shore of Nigg Bay serves a PE of 250,000. The works was built to provide full treatment to a flow of 1600 l/s and achieve the requirements of the Urban Waste Water Treatment Directive (BOD: 25mg/l and COD: 125mg/l).

A pumping station lifts the flow up from the preliminary treatment stages whence it gravitates through the primary and secondary treatment stages prior to discharge into the North Sea. Due to the compact nature of the site primary treatment is provided by lamella settlement. Lamella settlement uses inclined plates or tubes to provide high settling area in a small plan area; flow travels up these plates or tubes with the solids ‘falling’ onto the inclined surface and falling into the hopper below. These solids are subsequently removed from the hopper and at Nigg WwTW pumped to the sludge treatment centre (STC).

Secondary treatment is provided by a two-stage biological aerated flooded filter (BAFF). The BAFF utilises a down-flow plastic open-structured media, followed by an up-flow through a bed of granular clay media of 3 to 6mm in diameter. This media is regularly backwashed with the resulting dirty water pumped to the lamella stage for treatment.

In addition to the sludge generated on-site, the Nigg STC receives sludge (in either cake or liquid form) from the surrounding area of an additional 200,000 people. After reception and blending, sludges are dewatered and thickened, with the assistance of polymer addition, and pre-treated by thermal hydrolysis before entering 2 (No.) anaerobic digesters with a combined heat and power system. Digested sludge is then dewatered, assisted by further polymer addition, with the resulting product applied to land.

As built, the dewatering and thickening stages deployed 7 (No.) belt presses, but by 2014 these were requiring intensive maintenance and considered to be at the end of their asset life and thus in need of replacement. Consequently, KWS Grampian commenced an evaluation of options which included the Amcon Volute disc press.

The evaluation concluded through a highly structured business case which considered odour generation potential, health and safety, the ability to cope with rags and the total and whole-life cost, that the belt presses used to dewater and thicken the sludges fed to the thermal hydrolysis process should be replaced by Volute technology; calculations, based on pilot-scale trials in Aberdeen, clearly demonstrated that the whole life cost of the Volute technology was around two-thirds that of any competition.

Based on this appraisal KWS Grampian placed an order for the provision of a 2 (No.) Amcon Europe ES354 Volute disc press units to replace five belt presses. These were to represent the first ever full-scale installation in the UK.

The Volute ES Series thickening and dewatering press was introduced by Amcon, a Japanese company in 1992. To date, there are over 3,000 dewatering machines sold in around 50 countries. It is a cost effective, energy efficient, versatile, robust and revolutionary dewatering and thickening technology. Evergreen Water Solutions introduced the Volute in to the UK municipal market in 2011.

Volute is a relatively simple process utilising an integrated flocculation tank, a special spiral tapered screw sitting inside a disc drum of alternate fixed and moveable stainless steel rings.

Performance is enhanced by the addition of polymer to the liquid sludge and the action of the screw transports sludge up the inside of the disc drum rings which induce a squeezing and shearing action on the sludge resulting in the release of liquid. This filtrate is then allowed to gravitate out between the fixed and moving rings and collected in a filtrate pan for discharge to the head of the works. Once the thickened sludge reaches the top of the disc screw press it is further dewatered by a permanent end plate where more pressure is applied to the sludge to improve dewatering.

The dry sludge cake is allowed to fall down a chute where it can be collected and forwarded for further treatment.

Volute achieves three process steps in a single process stage; flocculation, thickening and dewatering. The low power required to operate the Volute disc screw makes it 90% more energy efficient than a typical centrifuge and as its washwater requirement is minimal (just a spray-bar system for odour control) consumption of water is significantly less than that of a belt press.

Volute also operates with very little noise (63dB), little vibration, is self-cleaning and resistant to clogging, which means very little maintenance is required (typically 12,000 hours operation between services) all of which results in a very low whole life operating cost.

The order placed with Evergreen Water Solutions was for the design, manufacture, testing, inspection, delivery, commissioning, training and handover of 2 (No.) Amcon Europe ES354 Volute dewatering presses. This included the provision of mixing and flocculation tanks and any necessary modifications to both the feed and reject pipework and the cake discharge arrangement to suit the existing cake pumps. Evergreen Water Solutions was also responsible for ensuring that their PLC system was compatible with that of KWS Grampian and providing a control philosophy for maintaining the required dry solids concentration of the sludge cake product as part of the PLC control.

As it was absolutely essential that such a major asset should have minimal downtime during the install, Evergreen Water Solutions collaborated, and worked very closely, with the client throughout the project, with just two weeks allowed for the mechanical installation by Evergreen, two weeks for the electrical installation by KWS Grampian and one week for commissioning.

During the ten week lead time for delivery of the units, 3D plans were produced, the units manufactured and FAT tested in the Czech Republic and subsequently delivered to the STC at Nigg.

This identified that the Volute units needed to be raised up 700mm on stainless steel stands to ensure that the existing positive displacement cake pumps feeding the sludge to the THP would not need modification.

During this lead in period KWS Grampian was responsible for removal and disposal of the existing belt presses from the STC building, and this was completed in August 2015. KWS Grampian also assisted with the off-loading of the Volute equipment when it arrived on site and hired in a centrifuge to provide sludge thickening and dewatering, to keep the thermal hydrolysis plant (THP) operational during the project.

Installation of the Volute equipment, into the area previously occupied by the belt presses commenced in September 2015. This included modifications to existing pipework e.g. sludge feed pipework, filtrate drain, washwater, and the improvements to the polymer dosing system (provision of Solitax probes) and was complete by mid-October 2015.

The washwater requirement, to supply the integrated spray bars, was for the provision of 15l/s every 15 minutes per drum (the two Volute units present a total of eight (No.) disc drums), equivalent to 0.5m3/hour. Access platforms and walkways were also provided and installed during this period for inspection of the disc drums and flocculation tanks.

KWS Grampian subsequently undertook the electrical cabling, made all the electrical connections and installed some additional sensors to improve control and set-up interfaces between the PLCs and SCADA to allow control and the exchange of data between the LCP and the main motor control centre (MCC).

This allowed Evergreen Water Solutions to undertake electrical testing and subsequent commissioning, which was completed in December 2015. However, although the units were in use from this time and feeding the THP some subsequent on-site electrical issues had to be resolved and the polymer dosing system set up to suit the requirements of the Volute. These were soon resolved and the process considered to be fully functional during March 2016, allowing full training of KWS Grampian staff to ensue.

Approximately 95% of the Volute plant and equipment was constructed off-site.

Since introduction to service at the Nigg STC, the twin Volute machine has exceeded the Kelda Water Services Management and operations team expectations for being reliable and robust process, achieving high utilisation and delivering a consistently dry sludge cake.

Although feed quality can be quite variable (in the range 2.1 to 3.3% dry solids) the Volute units have demonstrated an ability to produce a sludge cake with a dry solids content in excess of 30%. However, this is not ideal and so the target is to actually achieve a cake of around 25% dry solids which can be fed directly to the thermal hydrolysis unit.

This is being achieved consistently with very little variation (+/- 2%) and is one of the attractions of Volute; its operation can easily be ‘tuned’ to deliver the desired quality of product.

The 2 (No.) Volute units operate at a range of 35-70m3/hour (duty/standby or duty/assist). The Nigg Sludge Treatment Centre treats the equivalent of 300,000m³ and 400,000m³ of sludge per annum, equivalent to the processing of between 8,000 and 10,000T of dry solids. As expected solids capture is excellent at around 98%, which usually results in a filtrate containing around 600mg/l of suspended solids.

Over the first year or so of operation KWS Grampian embarked on a programme of optimisation and this has resulted in a further reduction in operating cost as their good work has allowed a polymer dose around 3kg per tonne of dry solids to be utilised.

With 95% of the build off-site, the Volute installation for KWS Grampian at Nigg WwTW has proved very successful and is consistently delivering the target dry solids sludge cake they require on site, 24 hours operation a day 7 days a week, suitable for feeding directly into the a thermal hydrolysis process at 16.5%.

By utilising a polymer dose of just 3kg per tonne of dry solids, using much less energy than a centrifuge and much less water than a belt press, operating costs are low.

Having become the first user of Volute technology in the UK KWS Grampian is now treating their sludge for a significantly lower cost than previously. As a result of this success Evergreen Water Solutions has subsequently been invited to provide Volute technology at the KWS Grampian Persley site and this is expected to occur in 2017.