SUBMERGED AERATED FILTERS WITHIN PACKAGE TREATMENT PLANTS
SUBMERGED AERATED FILTERS WITHIN PACKAGE TREATMENT PLANTS.
23 September 2003
Package Treatment Plants are largely known as a solution for single households, or a few houses, to replace septic tanks in rural communities where alternatives to mains drainage becomes impracticable or uneconomic. However, they are increasingly being used as a solution for larger flows, including municipal wastewaters and industrial effluents.
The Urban Waste Water Treatment Directive demands that water companies must improve small and medium, waste and water treatment plants that supply less than 15,000 population, this also includes increased capacity for ammonia removal.
In contrast to conventional sewage works designed on a ‘bespoke’ basis using concrete, package sewage treatment plants are prefabricated systems incorporating single or multiple treatment stages. With the latter, this is usually primary sedimentation, secondary biological treatment and final settlement incorporated into a single, compartmentalised tank. As a complete package, these systems offer a low cost alternative in terms of design, capital cost and installation for small to medium-sized sites.
Generally, the design philosophy for package treatment plants is dependant on the flow to be treated. Plants above 100 PE are designed on a modular basis whereby each treatment process is contained within its own tank. This allows a greater flexibility in the design to suit the raw sewage characteristics and the final effluent requirements of the project. Newer designs that incorporate submerged aerated media mean that this size range is likely to be increased.
While most systems use the sedimentation process for primary treatment as well as final clarification, they can be differentiated in their choice of biological unit operations for secondary treatment, which are usually one of three options:
a)Enclosed Trickling Filters
b)Rotating Biological Contactors
c)Submerged Aerated Filters (SAF)
In three recent projects Haith Industrial have provided a package solution utilising SAF units. There proven experience in the supply of SAF units has found that they provide high performance due to their robustness and tolerance to flow and load variances.
For this reason Haith Industrial select an attached growth system carried out in an SAF Unit. Attached growth, which is where the micro-organisms are attached to random packed media as well as the effluent, percolates through the media undergoing purification during its passage. The micro-organisms grow in flocs; these flocs are responsible for the transformation of the organic material into new bacteria, carbon dioxide and water.
Haith have found that such systems are simple to operate and have low operating costs and have the ability to withstand shock and toxic loads owing to the relatively short contact time of the effluent with the slime layer. However, what is seen as the SAF units great attraction is its low maintenance and management requirements.
Haith say that being high rate, the main advantages of the SAF Unit are reduced power and maintenance costs, stability against hydraulic shock loads, and a capability of achieving a high degree of carbonaceous and nitrogenous BOD removal.
Additionally, ponding of the bed and clogging of filter nozzles is eliminated and odours and fly nuisance are also eradicated.
Project One – SAF unit alongside existing plant.
This project required the supply of two SAF modules operating in parallel with existing mineral filter beds in the location of a demolished unit.
Each of the units was manufactured from 5mm stainless steel plate and incorporated sealed roof with closed cell gasket, air blower unit including pressure relief valve, inlet silencer and air flow meter. Additionally a diffused air distribution system mounted on quick access cassettes, random media, media retaining decks and media supporting decks was included.
Both units were sized to treat 32.5% of the flow to the works; in order to establish the specification and size, Haith modelled the process by applying each of the determinants of the consent to highlight the drivers for the system. The calculations revealed that the drivers are a combination of the retention time to achieve nitrification and the BOD/media loading rate to ensure compliance with the Generic SAF Plant Specification. In this project each SAF unit was fed by gravity from a distribution chamber, within this vessel, adjustable weir plates provide an even distribution of settled sewage to each SAF for treatment. Following treatment through the plant the treated effluent will gravity flow to two conical humus tanks.
Built in stainless steel fabricated modules, each factory assembled and tested to allow quick assembly, Haith recommended above ground installation which would require only minimal ground works; access to the aeration equipment would be a simple operation of removing a set of bolts and sliding out the aeration cassette.
Each SAF module has the hydraulic capacity of approximately 4 litres per second, and was designed to the following plant data:
PE 650
Dry weather flow 1.2 litres/sec
Full flow to treatment 2.9 litres/sec
Settled BOD load 26.2 kg/day
Ammonia load 3.6 kg/day
Effluent discharge standard:
20mg/1 BOD (95% ile)
7 mg/1 AmmN (95% ile)
51mg/1 SS (95% ile)
The design of the Haith Aerated Treatment Unit (SAF) is a fixed film reactor for the oxidisation of the BOD and ammonia. As such the micro-organisms are attached to high voidage plastic media, which is random packed to prevent short cutting. Each unit has three compartments with the media selected for each compartment to provide the correct surface area/BOD/oxygen ratio.
In this project, the first compartment was fitted with lower surface area media than the following two; this provided a robust process operating at maximum efficiency with a minimum requirement for backwashing.
The unit was modular built with all ancillary equipment, membrane air diffusers, media and internal baffle plates fitted in the factory. This enabled the filter to be transported to site as an operational unit ready to run following connection of supply and discharge pipe work and the air supply equipment.
The filter was the Haith standard multiple cell upward flow design, each cell being of equal size. Stainless steel ambideck panels mounted in a common frame supported the bacteria supporting media in each cell. Additionally a holding down deck was secured to the top of the filter to retain the “swelling” media during the establishing period.
Aerobic conditions were provided in each cell by an aeration system comprising of E.D.P.M. membrane diffusers mounted on a manifold, which in turn was connected to a centrifugal type air blower set. Each manifold comprised of a number of feeder and cross pipes extending through an access door which in turn was bolted to the filter wall. The complete assembly of manifold and aeration diffusers can be drawn clear of the filter by removing the door bolts, allowing inspection and maintenance without the need to remove the media.
A sectional cover was provided, designed to span the width of the filter without the need for immediate supports. The centre cover was equipped with a water curtain to prevent the release of foam; this was fed by a submersible pump housed in the discharge launder.
Project Two – Below ground modular package plant.
In a second project Haith supplied a stainless steel fabricated package plant, comprising Primary Settlement Tank, SAF module and Final Settlement Tank, which were manufactured and delivered to site as separate modules, these were built as a single package plant on site and connected by pipework below ground.
This package plant was designed to the following plant data:
PE 250
Dry weather flow 64m3/d
Full flow to treatment 142.9m3/d
The primary settlement tank was designed in accordance with BS 6297 which ensured an upward flow velocity of no more than 0.9m3 per m2 per hour at full flow to treatment.
The tank was manufactured as an integral part of the module with a hopper bottom of steep valley angles which encouraged the sludge to compact and concentrate, additionally assisting in its movement toward the sludge gulper suction pipe.
The sewage inlet and outlet arrangements in addition to baffle plates contained the scum layer, which was removed along with the co-settled sludge.
Provision was also made for the introduction of humus sludge, which is returned from the final tank at regular intervals to be co-settled with the crude sludge.
The Haith SAF Unit was designed as project one.
The final settlement tank for this project was also designed in accordance with BS 6297 which dictates the surface loading and the retention period of the tank.
This tank also had a hopper bottom with valley angles of 60ยบ which prevents the sludge becoming trapped where it may go septic causing sludge rising problems.
This allows the sludge to be transferred back to be co-settled with the crude sewage by a lift pump which operates either manually or by timer, frequent humus sludge return is necessary to avoid flotation of the solids caused by denitrification of highly oxidised effluents.
Project Three – Package plant as one module.
In a third project Haith supplied a package plant with the three main elements as one module. The module was split into three main sections, Primary Settlement Tank, SAF unit and Humus Tank.
This module was supplied factory assembled and tested which provided quick site installation without the need for extensive civil works.
This system was designed for the following plant load data:
PE 120
Full flow to treatment 63m3/day
Dry weather flow 28m3/day
Effluent discharge standard
BOD (SAF) 40 MG/l
Suspended solids 60 mg/l
The design of the Primary Settlement Tank was designed in accordance with BS 6297 as project two.
Design of the Haith SAF unit was as previous; however on this project the unit was housed in the centre of the package plant being gravity fed from the Primary Settlement Tank via a series of internal baffles to prevent the carryover of solids into the SAF.
Design of the Final Settlement Tank was again to BS 6297 as previous projects.
Operation of the plant in all three projects is automatic with the minimum of maintenance following commissioning, Haith have found that SAF’s that have been in operation in similar applications have proven to work with little to no operator maintenance and intervention.
As the blower runs constantly the standard Haith design houses this in an enclosure which reduces the noise output to 65-67 dBa at one metre.
Haith point out that the only one essential task to be performed by the operator is de-sludging of the primary tank, this will include the removal of the floating scum which if allowed to accumulate could overflow the scum board and carry into the SAF causing blockages resulting in the deterioration of final effluent quality.
Haith, have in all three different projects provided a package solution derived from their proven experience which considered construction optimisation, looked at operating experiences and evaluated cost reductions in relation to traditional solutions. They have provided a more suitable and competitive solution for small site applications than conventional sewage works designed in concrete.
