Solving final effluent problem

Final effluent is an attractive alternative to tap water in cleaning sewage treatment equipment. But it can contain solid particles that pose a problem. Cross Manufacturing, however, has a solution.

Water shortage compels companies to improve their sewage treatment performance, and, at the same time, reduce the amount of clean, treated water that is used during the treatment process.

Both these issues focus attention on the quality of the final effluent produced by the WwTW.

In the first case, failure to meet increasingly strict environmental consents can result in the imposition of financial penalties if effluent returned to watercourses is found to be sub-standard. Secondly, final effluent can be a valuable resource as a virtually free-of-charge alternative to the expensive tap water used to clean and maintain the efficient operation of sewage treatment equipment.

In many cases, the main problem with final effluent is the level of solids remaining after conventional treatment. Although as a percentage it is tiny, the very low thresholds imposed by European environmental standards can still be an everyday challenge.

The same solid particles also cause problems if the effluent is recycled and used as an alternative to clean water. Typically, spray nozzles on the cleaning circuits for inlet screens or drum thickeners become blocked and the machines have to be stopped and cleaned, causing disruption to the treatment process.

Cross Manufacturing is working to find a solution to both these challenges, using simple mechanical filtration on long-term site tests.

Engineers at Cross believe that the design of its filter has inherent features that could be particularly effective for this type of duty. The technology had originally been applied to maintaining cleanliness in building services pipework - air-conditioning and heating - and reducing the risk of Legionella in cooling towers.

These and other applications, including machine shop coolants, are still important markets for Cross, while the filter has also proved to be successful in a number of water industry environments.

Zero gravity
Cross filtration technology is based on a stainless-steel coil, designed with raised nodes on the surface that define a precise filtration gap (rating). Selectable in seven ratings from 400 to 12 microns, the design traps any particle above the specified coil rating.

Known as the zero-gravity coil, it is designed to open slightly and evenly along its total length when the flow across it is reversed during backwashing. All the trapped particles are quickly and thoroughly flushed away, with the minimum use of backwash water.

Apart from actuated valves and the coil itself, there are no moving parts used during backwashing, facilitating an efficient long-term backwash performance with minimal risk of solids build-up on the filter coil. A differential switch with a time lapse back-up normally triggers backwashing.

Cross filter systems are tailored to suit any size of duty by adding enough zero gravity coils to meet the required flow capacity. The coils can also be quickly exchanged, enabling different micron ratings to be tried in the same application until the optimum filtration performance is achieved.

As described later, this feature has proved to be an effective way of fine-tuning the pilot plants installed in the water industry effluent programme.

Cross has designed and installed pilot plants for long-term testing at two sites. Both filters are of equal capacity (average flow rate 3l/s). At one site, serving a small rural community, this represents virtually all of the final effluent flow from the works, while at the other site, serving a small town, it is only about 1% of the total final effluent flow.

Both filters operate in the same way. A submersible pump draws the effluent from the site's outlet chamber and delivers it at a pressure of 5 bar to the first stage of filtration, comprising two coils rated at 50 microns.

The flow then passes through a bank of four coils rated at 25 microns. All six coils backwash individually and sequentially when the pressure drop across both stages reaches 1 bar.

Rogue particles
Finally, the effluent flows through two further housings designed to catch any rogue particles which may, for example, be soft enough to squeeze through the zero gravity coils. These were originally fitted with disposable elements demanding regular replacement. But it was soon felt that the level of maintenance required would be too much for a permanent installation, while these types of filters would also be unacceptable due to the environmental disposal and ongoing cost considerations.

The housings were therefore modified to backwashable versions and tested with a series of permanent element designs. The most successful has proved to be of stainless-steel static screen construction, rated at 25 microns. This, although requiring a longer backwash than the zero gravity coil, operates satisfactorily without significant blinding-up. The recycling test programme has been performed on a drum thickener installation in an attempt to replace the use of large quantities of fresh water traditionally used to clean the equipment during operation.

Here, the effluent recycling source is taken from the WwTW's settling tanks and therefore contains a much higher percentage of solids than would be found in final effluent.

Combined with the high flow rate and pressure required for the application, this has resulted in a very severe test environment.

The equipment selected was a standard Cross Phoenix filter, with a maximum rated flow rate of 19l/s and maximum pressure rating of 10 bar. This was equipped with a 7kW centrifugal pump that was soon upgraded to an 11kW version to increase the flow rate. Input pressure to the filter was set at between 7 and 8 bar to ensure a decent backwash performance, which would prove to be essential.

The filter was fitted with 50 micron rated coils - the Phoenix uses seven zero gravity coils - at the start of the programme. But these quickly blinded in spite of multiple backwash attempts, due to the high solids content and fluctuating quality of the effluent supply.

Coils with various different micron ratings were therefore introduced and eventually 200 microns proved to be the most successful choice. By a happy coincidence, this rating was the same as that recommended by the drum thickener manufacturer.

Since the thickeners normally operate on a 24-hour time scale, the recycled effluent circuit will now also operate on this basis. But, occasionally, the quality of the effluent still deteriorates, which causes the flow to decline.

At this point, an alarm is given and the original fresh water circuit resumes its duty. But it has also been established that, by the manual initiation of its backwash programme, the Phoenix filter will clear the blockage after a few attempts. And the recycled circuit will therefore automatically restart after only a short interruption.

With further development, Cross will be able to modify

the Phoenix control system to carry out this process automatically on applications where it is a persistent problem.

These tests have now been in operation for nearly a year, and have proved to be a valuable source of detailed data and knowledge about these specialised applications.

Cross believes that the significant cost saving and efficiency improvement implications of these types of filter installations will be of interest to the water industry in general and is seeking more sites for testing or full-scale projects.

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