Center Parcs opened its first holiday village in Nottinghamshire in 1987. More than 25 years later the company has five villages across the UK with leisure facilities and activities which include swimming pools, restaurants and retail outlets as well as a spa. With over two million visitors each year, sustainable, behind the scenes waste water treatment (WWT) is an essential part of the support infrastructure, providing 24/7 operation, 365 days a year. The focus at all times is threefold: reliable, high quality treatment and sustainability.
So Center Parcs turned to international water and wastewater solutions provider, ACWA Services, and now treats up to 34 million litres of wastewater per month from accommodation, restaurants, water attractions, the spa and retail outlets.
Center Parcs' leisure facilities need sustainable wastewater treatment.
At the heart of the system is ACWA's MEMTREAT® membrane bio-reactor (MBR), an advanced, compact treatment solution that provides a high quality effluent without requiring any additional primary or secondary clarification or tertiary treatment. It also produces less sludge biomass than conventional secondary treatment processes.
Core components of the solution included influent screening to ≤3 mm with gravity discharge into the raw water sump; a balancing tank with venturi mixing to maintain the waste water in suspension, followed by an anoxic tank and an aeration tank utilising a fine bubble aeration system, both constructed in concrete. From here the flow discharges into two membrane bio-reactor (MBR) tanks each with seven single deck submerged flat sheet MBR modules. Clean water passes to an effluent storage tank and from there gravity discharges it to the local watercourse via a water quality monitoring station. The quality of the effluent at this point is: BOD <10 mg/l, Suspended Solids <20 mg/l and Ammonia <2 mg/l.
A proportion of the treated effluent is further treated to an extremely high standard: BOD <5 mg/l, Suspended Solids < 10 mg/l, Ammonia < 1 mg/l and phosphorus < 0.5 mg/l by passing it through a reverse osmosis plant. This water is used to top up levels of the on-site lakes.
Membrane bio-reactor technology
ACWA's approach, using membrane bio-reactor technology rather than conventional activated sludge processing, brings a number of benefits. Firstly the aeration tank operates at a higher Mixed Liquor Suspended Solids (MLSS) level compared to activated sludge processes. Typically a conventional system would operate at 3–3–5000 mg/l MLSS whilst the Memtreat system operates at 10–10–12,000 mg/l MLSS. This allows for a smaller process volume and results in a significantly reduced aeration tank footprint. In addition the increased MLSS helps to optimise the biological process within the bio-reactor, helping to minimise the volume of surplus sludge and any consequent disposal issues.
At the same time the submerged membrane units replace the need for secondary settlement tanks. These normally require a large footprint to maintain a low up-flow velocity, to allow the biomass (solids) to settle naturally to the bottom of the tank for removal. Removing the requirement for these delivers further significant site space savings.
In place of settling tanks the process separates the treated wastewater from the purifying bacteria through membrane filtration. The submerged membranes effectively act as a physical barrier with a nominal pore size of 0.08µm. This produces high quality effluent and prevents the biomass within the liquor from passing forward. Instead the sludge is drawn off from the bottom of the MBR tanks by the Return Activated Sludge (RAS) and Surplus Activated Sludge (SAS) pumps for recycle or removal.
The membrane system consists of a membrane panel held on a support frame. A number of these panels are mounted together in a membrane cassette. The membrane plates are based on the principle of cross-flow filtration ensuring that all solids, including micro-organisms and viruses, are retained within the activated sludge. The system is a simple to operate, as it uses gravity. No suction pumps are required to remove the treated wastewater from the membrane units.
Coarse bubble diffused air continuously operates underneath the membrane units. This has the dual function of providing oxygen for continual degradation of incoming organic matter and also providing a scouring action against the surface of the membrane to prevent the membrane pores becoming blocked with biomass.
The treated effluent that passes through the membranes is of a quality that is suitable for direct discharge without the need for further treatment. Normally sand filtration or similar processes would be required. Alternatively the resulting crystal clear water can be re-used, for example in irrigation, or other processes. ACWA has designed, supplied and installed in excess of 40 MBR systems, including one of the world's largest MBR treatment plants at AL ANSAB, in Oman.
Second project underway
Center Parcs Woburn Forest opened to guests in June 2014, and ACWA Services were about to start work on a second project for Center Parcs involving the complete modernisation and upgrade of the WWT facilities at the company's first UK site – Center Parcs Sherwood Forest in Nottinghamshire, which first opened back in 1987.
Similar in many ways to the solution at Woburn Forest and again based around ACWA's Memtreat membrane bio-reactor, the project was designed to upgrade waste water infrastructure that had reached the end of its working life, the existing system was a combination of activated sludge, biological filtration and tertiary sand filtration, replacing it with an up-to-date energy efficient solution in line with the company's continuing environmental focus.
A major difference at Sherwood Forest was the fact that the facility would remain open to guests during construction, requiring the existing plant to remain operational at all times. ACWA, which was responsible for all aspects of the project, came up with a phased approach, dividing the work into three stages, each involving the demolition and environmental disposal of redundant systems, construction of the replacement solutions and their integration into the Center Parcs network, followed by testing and commissioning.
The lake at Center Parcs' Sherwood Forest holiday village.
Like Woburn Forest, the finished installation includes flow balancing, anoxic, aeration and membrane bioreactor (MBR) tanks, together with all associated ancillary systems. The set up differs slightly in that the majority of the process tanks at Sherwood Forest are constructed in glass lined steel whereas the ones at Woburn Forest are constructed in concrete. Total process capacities for both sites are very similar – in the region of 1200 m3 per day. Work on the Sherwood Forest site is scheduled to be completed in October 2016.
Aside from the compact footprint and efficiency of the system compared to conventional processes, the submerged membrane bio-reactor is easy to use and simple to maintain. Both sites include an MBR clean in place (CIP) system to provide a periodic chemical clean of the membrane elements. A relatively simple process, this is carried out typically every six months and can be completed in a few hours. Importantly there is no need to drain the tanks, remove the biomass or the membranes, and it doesn't require a system shut down.
Following commissioning and testing, AWCA arranges operator training to ensure Center Parcs' personnel get the most out of the equipment with uninterrupted operation. This is backed up with office based assistance to operators on site as required, together with periodic site visits to help maintain a healthy treatment system and to offer advice as required.
Supporting Center Parcs' vision for sustainable infrastructure across all its sites has underpinned ACWA's project teams at Woburn Forest and Sherwood Forest. The Memtreat membrane bio-reactor, at the heart of both wastewater projects, is helping to deliver a sensitive, low carbon solution, with the least environmental impact on the woodland environment in which Center Parcs operates.