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. Controls on Biological Oxygen Demand (BOD), total suspended solids (TSS) and even phosphorus are the most commonly tightened. In an effort to meet new discharge limits, many treatment facilities now require upgrades and, possibly, new treatment technology to meet tighter standards.
All water companies in the United Kingdom are bound by the Environment Agency (EA) requirements on consent levels for effluent. Technical evaluation of different technologies based on existing plant performance, water effluent quality and required consents and detailed capital and operating cost analyses typically are made.
TETRA® DeepBed™ Filters from Severn Trent Services have been used by many water companies around the world as a means of not only meeting tighter effluent standards, but also guaranteeing system performance. For example, in 2003 the Fazakerley Wastewater Treatment Works (WwTW), a United Utilities facility in a northern suburb of Liverpool, England, purchased a TETRA DeepBeed Filter for the removal of suspended solids from municipal wastewater as part of a tertiary filtration framework agreement
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 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 fall short of the EA consent.
The EA consent for the Fazakerley WwTW site is 25mg/l TSS and 10 mg/l BOD, and the average flow through the site is 54,000m3/day. The installation of a filter was required to reduce the level of TSS in the effluent down to 10mg/l TSS from the initial level of 45mgl. 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 shown average levels of TSS at 6 mg/l and BOD at 3 mg/l since the equipment was installed nearly five years ago.
The TETRA DeepBed Filter at Fazakerley consists of eight cells, each 22.3m in length and 3m wide, providing an area of 535.2m2. It uses 410 tons of media and 4,280 TETRA TBlocks.
The filter is 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 that can lead to blockage by biofouling and as such has proved to work exceptionally well in a wastewater environment.
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. While 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.
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 two to three percent of the design flow to the filter.
One of the benefits 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 can easily tolerate extended periods of no flow and can be brought back online simply and almost instantaneously. These features are now being utilized in plants, including other United Utilities plants, where the filters are left offline for considerable periods of time and then brought into operation if the suspended solids in the effluent start to rise toward the set point limit. This method of operation can significantly reduce operating costs for both filter operation (backwashing) and especially feed pumping while still protecting 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 had a negative impact on the performance of the filter’s ability to remove suspended solids. However, the site manager has confirmed the filter’s 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 to large sewage treatment works where the filters have been constructed in a concrete block with rectangular cells. As tighter effluent standards have been applied to smaller and smaller facilities, 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 a different vessel, namely circular cells in stainless steel rather than rectangular cells in concrete. Utilizing 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. All that is required onsite is a concrete slab to act as a base, three pipework connections and a link to a power supply.
The TETRA DeepBed Filter installed by United Utilities at Fazakerley has proven to be successful 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, increasing numbers also have discovered the cost effectiveness of the DeepBed technology.
For more information, e-mail info@severntrentservices.com.
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