The Strongford Sewage Treatment Works recently began operation of the United Kingdom’s largest – and one of the greenest – tertiary wastewater treatment plants.
Located close to the village of Barlaston in the West Midlands region of England, the Severn
Trent Water facility serves a population of 330,000 in the towns of Stoke-on-Trent and Newcastle-under-Lyme by treating a mixture of industrial and domestic wastewaters. The final effluent is discharged into the River Trent, which runs north through the village and eventually to the North Sea via the Humber estuary. In order to meet strict ammonia standards in the River Trent, Black & Veatch constructed the TETRA® NSAF™ (nitrifying submerged aerated filter) from Severn Trent Services, an up-flow, fixed-film biological reactor, to reduce the ammonia in the effluent from 15 mg/l down to 3 mg/l. The new tertiary treatment process will ensure Strongford STW continues to achieve compliance with an Environment Agency discharge consent driven by the EU Fisheries Directive.
Similar in operation to a submerged aerated filter, the TETRA NSAF has a small footprint, and is a proven, robust submerged aerated media process for the biological oxidation of municipal wastewater. The technology has been proven to effectively remove ammonia through the installation of several such plants across the United Kingdom over the last 10 years. The NSAF acts purely as a biological oxidizer; thereby only generating minimal additional solids that require handling and disposal following treatment.
The total plant area for the Strongford NSAF is 2,000 m2, comprising a treatment area of 1,400 m2 that will accommodate a flow of 10,000 m3/hr. The technology itself is extremely flexible and as such is available in a range of sizes from small modular units suitable for above or below ground construction to custom-made configurations for large plants.
The TETRA NSAF uses blast furnace slag media graded to an optimal size for
efficient ammonia removal, offering surface areas up to 240 m2/m3.
The media cannot collapse or be lost by flotation, and with a proven long lifetime,
media replacement costs are minimized. The greater thermal capacity of the
media compared to plastic alternatives helps to maintain high levels of ammonia
removal during winter periods.
The inside of the NSAF cells are based upon the same floor technology as the TETRA® DeepBed™ Filter.
Influent distribution pipes are cast into the concrete floor upon which there
is a header and manifold air distribution system and TETRA® T
Block™ floor, which supports the blast furnace slag process media.
Media depths are typically between 2 m and 4 m. The Strongford facility is
constructed utilizing rectangular concrete cells. The plant has 10 cells, each
6 m wide and 22.75 m in length, with a media depth of 4 m. There are 10,700
T Blocks in the facility, which measure 5½ km when placed end to end.
A standard TETRA NSAF utilizes a simple process in which air is supplied at a fixed rate, thus eliminating the need for a complicated control system. Simple monitoring of the process blower(s) and switching over to the standby unit if necessary are the primary operational requirements. However, due to the sheer size of the facility at Strongford, the process has been developed to a more sophisticated capability allowing the operation and aeration to be tailored to suit the incoming load. The new process, NSAF+, calculates the number of cells required to meet the desired output quality and brings them into service as needed. Assessment of the incoming effluent flow and load allows the plant to operate cell by cell as needed with air flow adjusted accordingly. This reduces energy consumption and provides significant savings in electricity costs compared to a conventional NSAF plant.
The system also features 2 km of stainless steel pipes to distribute the air under the media to oxidize the ammonia to nitrate. The facility employs innovative air blower technology to deliver up to 13,000 m3/hr of free air to the process. The blowers operate up to 25,000 rpm with the impellers rotating within a magnetic field, negating the requirement for conventional bearings.
Now in operation for approximately nine months, the Strongford plant underscores the advantages of biological treatment offered by TETRA nitrifying submerged aerated filters, a technology that has become established for wastewater treatment in the United Kingdom and across the world.
Bruce Ainsworth, executive managing director, Regions, Black & Veatch Water, points out some additional benefits of the technology. “The NSAF+ plant at Strongford shows it is possible to address concerns about energy efficiency while simultaneously meeting increasingly stringent discharge consents, for ammonia in this case. The expertise to achieve greater energy efficiency in addition to attaining higher quality standards is something water companies are increasingly seeking.”
For more information, e-mail info@severntrentservices.com.