Water companies today operate in a challenging environment of constant regulatory change. Just as they comply with one set of standards, regulations shift and more stringent discharge levels are put into place for them to meet. Tighter controls on Biological Oxygen Demand (BOD), suspended solids (TSS) and phosphorus are the most common. In an effort to meet new discharge limits, many treatment works which had previously been operating effectively now require upgrades and, possibly, new treatment technology to meet tighter standards.
All water companies in the UK are bound by the Environment Agency (EA) requirements on consent levels for effluent, and the companies undertake careful analysis and discussion when determining the best treatment option to meet these levels. Technical evaluation of different technologies based on existing plant performance, water effluent quality and required consents, in addition to detailed capital and operating cost analysis, is typically undertaken.
TETRA® DeepBed™ Filters have been used by numerous water companies around the world as a means of not only meeting tighter effluent standards but also guaranteeing system performance. For example, in 2003, Severn Trent Services was awarded a contract, as part of a tertiary filtration framework agreement, to supply a TETRA® DeepBed™ Filter to United Utilities’ Fazakerley Wastewater Treatment Works (WwTW) for the removal of suspended solids from municipal wastewater.
The Fazakerley WwTW is susceptible to unpredictable sudden flushes of solids, requiring a treatment technology flexible enough to handle and effectively treat the changing demand. The biologically aerated flooded filter (BAFF) that had been previously installed at the site was successfully removing solids at average flows. However, during high flows the system struggled and flushed solids through, causing the plant to exceed the EA consent.
The EA consent for the Fazakerley WwTW site is 25 mg/l TSS (Suspended Solids) and 10 mg/l BOD at average flow and 10 mg/l at maximum flow. The plant’s average flow is 54,000 m3/day and the maximum flow is 140,000 m3/day net of DBF backwash returns.
Since the installation of the TETRA DeepBed rapid gravity filter at Fazakerley, the site has not failed its solids consents, which has given site operators great confidence in the plant’s ability to meet EA standards. Data from the site has highlighted that the levels have been averaging 6 mg/l TSS and 3 mg/l BOD since the site was commissioned nearly five years ago.
TETRA DeepBed Filters are constructed of layers of gravel and carefully graded media, which are supported on a patented T-Block floor. The T-Block is a plastic-jacketed, concrete-filled block which provides both support for the filter medium and a floor for the backwash distribution system. The floor has no nozzles or small orifices, which could lead to blockage by biofouling, and as such has proved to work exceptionally well in a wastewater environment.
The filter at Fazakerley consists of eight cells, each 22.3 m in length and 3 m wide, providing an area of 535.2 m2. It uses 1410 tonnes of media and 4,280 TETRA T-Blocks.
The plant operates as a single filter consisting of a number of cells into which the influent flow is distributed. The flow passes through the media and suspended solids are removed. Filtered effluent is discharged through a clear well which retains a reservoir of water for cell backwashing. The size of the media used enables solids to penetrate deep into the bed. This use of depth rather than surface filtration gives the filters greater solids-holding capacity and thus longer run times between backwashes.
The filter is ‘regenerated’ by taking one cell offline at a time and employing an air and water backwash to clean the media. The dirty backwash water generated is returned to the treatment works. Whilst a cell is offline for backwashing, the influent flow is redistributed to the other cells, and operation of the filter continues without seriously affecting the quality of the effluent.
The minimisation of operating costs is a subject high on the agendas of many water companies. TETRA DeepBed Filtration is a mature system with a proven record of efficient operation. As a result of extended filter run times, backwashing power costs are lowered and the amount of dirty backwash water produced is reduced, thus lessening the impact on the existing works. For many TETRA DeepBed Filter plants the dirty backwash water generated is as low as 2-3% of the design flow to the filter.
A further benefit of the TETRA system is that the filter is able to accept influent flow ranging from zero to its maximum design with no detrimental effect on performance. Furthermore, it easily can tolerate extended periods of no flow and can be brought back on line simply and practically instantaneously.
This capability is now being increasingly used in other DeepBed Filters, including other United Utilities plants, where the filters are left offline for considerable periods of time and then brought into action only if the suspended solids in the effluent starts to rise towards the set point limit.
This method of operation significantly reduces operating costs for both filter operation (backwashing) and especially for feed pumping whilst still providing the optimum security net to protect the plant effluent quality.
The Fazakerley site was recently updated to employ ferric dosing for phosphorus removal. This was not in the original scope of the DeepBed Filter design and could have negatively impacted on the performance of the filter’s ability to remove suspended solids. However, the site manager has confirmed the filter’s continued excellent performance, particularly given the increased requirements of the plant compared to what was originally planned.
TETRA DeepBed filtration has traditionally been employed on medium/large sewage treatment works like Fazakerley, where the filters have been constructed in a concrete blocks with rectangular cells. As tighter effluent standards have been applied to smaller and smaller works, this has necessitated a re-thinking of the filter designs to enable them to compete with alternative technologies at lower flow rates. Severn Trent Services’ solution was to take the same proven process but package it into different vessels, such as circular cells in stainless steel, GRP or glass coated steel rather rectangular cells in concrete. Utilising the same process internals means the same robust performance can still be achieved, but the smaller cells enable the filters to be self-contained, skid-mounted units. This allows the filters to be fully assembled and factory tested prior to delivery, minimising time on site; all that is required onsite is a concrete slab to act as a base, three pipework connections and a link to a power supply.
To date, the TETRA DeepBed Filter installed for United Utilities at Fazakerley has proven to be successful during trials and flexible in the removal of suspended solids in the face of varied conditions. As other UK water companies seek to comply with new EA consents, many of them also have discovered the cost effectiveness and process security of the DeepBed technology. •
Contact:
Fran House
Severn Trent Services
Email: FHouse@SevernTrentServices.co.uk
www.severntrentservices.com
