A new £19M 4,200Kg/d COD capacity effluent treatment plant at a leading pharmaceutical manufacturer’s works on the Severn Estuary is already paying dividends for its owners. It is enabling them to meet the required environmental legislation, the company’s social responsibilities, safeguard the company’s asset and protect the coastal waters of the estuary.

The project construction phase began in September, 2003, and the plant began to process effluent flows on 12 September, 2005, giving AstraZeneca beneficial use of the plant from that date.

The AstraZeneca plant at Avlon Works, Avonmouth, has been designed by process engineering company Purac to meet the current load and the likely future expansion of the effluent load from the plant. And it will enable treatment on site of effluent that was previously being taken out in tankers for treatment elsewhere – in particular phosphate effluent.

In addition, the advanced control system incorporated at the site allows Purac personnel to remotely administrate the SCADA system, allowing them to retain historical data so they can check how efficiently the plant has been running, and to actually mimic the system at their headquarters in Kidderminster – as if they were in the control room on site. Particularly in the early days of the new system, this offered a significant benefit to AstraZeneca, which is able to tap into remote expertise to help ensure the new plant is operating as it should and to facilitate training of the company’s own team.

The work undertaken for AstraZeneca far exceeds the requirements of the Environment Agency and current legislation. The site had a number of quite acceptable discharge consents from the Environment Agency that it could happily have continued to work with for the short-term future.

However, the company decided that it wanted to aim higher, achieve better treatment of its effluent and to position itself as a plant capable of taking on a wider range of projects – but projects where it could still happily deal with the effluent produced as part of the site’s manufacturing processes. It also

recognised its responsibilities to the communities in which it is located and to the coastline along which it is situated, with its flora and fauna, and wanted to minimise its impact.

Improvements in pollution control

The site’s manufacturing processes are regulated under the IPC regime and, as part of this, the Environment Agency was seeking improvements in pollution control. These improvements included the elimination of discharges of substances proscribed for release to water and a reduction in the COD of the main site, together with reductions in the release of volatile substances.

The Avlon Works was producing four effluent streams, the first two – strong effluent and weak effluent – were blended together to form a waste stream containing COD concentrations of up to 3,000mg/l with a varying flow rate from 500-2,500m3/day. Stream 3 – high-strength effluent – containing COD concentrations up to 90,000mg/l, was normally loaded into road tankers for off-site disposal by incineration or treatment. But it is now fed into the treatment plant. Stream 4 – foul effluent – was a combination of sanitary effluent, effluent from the site restaurant and laboratories. The weak effluent and foul flows make up the majority of the treatment volume while the strong and particularly the high-strength effluent contribute the bulk of the COD load.

A best practicable environmental option (BPEO) study was undertaken to assess the present and longer-term requirements for treatment of liquid waste at the works. The report indicated an aerobic oxidation process as the preferred treatment technology. Initial laboratory and pilot trials utilising SBR technology found that the process was not robust enough to cope with drastic changes in influent conditions and would retain toxic components.

AstraZeneca and Purac moved towards the Anox moving bed bio-reactor process, which provides a more robust treatment route, followed by chemical phosphate removal and a dissolved air flotation (DAF) plant. This was considered to be more suitable for treating different waste streams from pharmaceutical and fine chemical processes. Purac is a licensee of the Anox technology. The Anox process has also been successfully used on another AstraZeneca plant in Sweden.

In conjunction with further pilot trials at AstraZeneca, Purac developed a front-end engineering design producing a robust, flexible and fully automated plant capable of handling a wide range of effluent flows and loads, while maintaining performance.

The effluent treatment plant is designed to receive all four streams at a battery limit. In addition, a new weak effluent collection and pumping pit discharges to an existing interception facility upstream of the plant. All utility services required by the plant are also piped to a termination point at the battery limits.

The first stage of the plant provides coarse solids separation for the weak effluent and foul arisings and balancing for all streams to provide a sufficient hold-up volume for toxic-shock control. Strong effluent is pumped via a Lamella separator to the blending tank to reduce activated carbon particulates. Weak effluent is pumped via a heat exchanger to the blending tank to raise the mixed fluid temperature following process upsets, while the high-strength stream is pumped via a balance tank into the blending tank which provides residence time for thorough mixing, nutrient addition and pH control prior to pumped discharge to the biological second stage. High-strength effluent has to be maintained at 25ºC by a shell and tube heat exchanger to prevent crystallisation at lower temperatures.

The biological process is driven by gravity flow and consists of two reactor streams, each of three reactors is fabricated from high molybdenum 316L stainless steel, arranged in parallel with cross connections providing operational flexibility.

The first reactors operate at pH4 to promote fungal growth and adjusted to pH7 in the second and third reactors to enable bacteria to proliferate. The robust fungal stage removes approximately 60% of the COD load, while the bacterial stage further improves water quality prior to discharge to the DAF process.

In all reactor cases, the biology is sustained by means of aeration and mixing. The organic substrate adheres to suspended Anox bio-carriers that have extended surface area to maximise the efficiency of aeration and mixing. Foaming is suppressed by spray nozzles mounted in the tank roof and is controlled by means of foam level probes. Additionally, antifoam can be dosed into the spray water to enhance foam collapse if required. Reactor tank headspaces and all other internal vessels are forcefully extracted to atmosphere via the vent stack to control. The Anox bio-carriers are retained within each reactor by means of screens.

The fluid then gravitates into a final stage of treatment where polymer, ferric sulphate and Kalic (calcium hydroxide suspension) is dosed to flocculate the dislodged/sloughed biomass and precipitated phosphate salts. The sludge is then separated via dissolved air flotation (DAF).

Floated and settled, material is removed via a skimmer and hopper bottom while effluent discharges under gravity into a final effluent balance tank. The final effluent is pumped via the existing effluent discharge pipeline to the River Severn.

Sludge removed from the primary lamella and DAF units is stored and dewatered using centrifuges on an intermittent basis. Sludge is exported from site as sludge cake for final disposal elsewhere by landfill in agreement with the Environment Agency.

COD to the treatment plant has been extremely variable during the first three months of operation with peaks of 2,600kg COD/d down to almost zero load. However, the final effluent quality has consistently achieved consent levels, which are set at 1,000kg COD/week with a daily maximum of 500kg COD/d.

The robustness of the design has been demonstrated during the period 8 November, 2005, to 14 November, 2005, when the plant was fed almost zero load. When higher COD loads were applied, the fungal and biological processes had survived to the extent that removal performance was maintained. Phosphate load has also been variable, but again, the plant has coped well. Process optimisation has further improved phosphate removal performance.

The £19M design and build contract was awarded to Purac with a remit to design, supply, erect and commission the plant, including the process, mechanical and electrical systems engineering, to be operational by the summer of 2005. As main contractor, Purac also undertook the role of CDM planning supervisor and principal contractor.

The team included Alfred McAlpine Capital Projects as the preferred civil partner who selected Pell Frischmann Water to undertake architectural, civil and structural design as well as the design for the administration building. Pell Frischmann appointed Stride Treglown as architect. Thermal Transfer was the nominated sub-contractor to undertake design and installation of building services to the administration building.

Power is provided by a new HV sub-station and switchgear room. The contract also saw the construction of a 390m3 two-storey administration building, comprising offices, laboratory, plant room, welfare facilities, conference rooms, MCC room, interface room and control room. A 1,200m3 reactor hall housing the six Anox reactor tanks, a 700m3 multi-storey equipment hall for sludge holding tank, DAF units, centrifuges, inlet screens, odour extraction equipment and air blowers and a 170m3 chemical storage building was also provided.

Purac installed the latest control technology utilising Profibus distributed field-bus networks to control and monitor motor drives, actuators and field instruments. Siemens S7 PLC controls the process, with data acquisition by WINCC SCADA system using servers with in-built redundancy. Purac Systems Department has designed, tested and installed software to meet the exacting standards required by AstraZeneca. The in-house design and integration of the software was a key factor in AstraZeneca selecting Purac for the project.

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