Mechanical pre-treatment for membrane plants

The introduction of membrane technology solutions for the treatment of municipal wastewater fundamentally changed the requirements on mechanical wastewater treatment systems. Conventional screening systems with bar spacings or perforations from 6 mm were not sufficient to guarantee the stable and low-maintenance operation of downstream membrane plants.This extract article describes the history of fine screens as pre-treatment systems for membrane plants, with particular emphasis on the importance and influence of operational experience.

The efficiency of conventional screens with 6 to 10 mm bar spacing or perforation is insufficient for the membrane plants currently available on the market. Screens with higher separation efficiencies are necessary to ensure the reliable operation of membrane plants without excessive maintenance requirements. The separation efficiency necessary depends on the specific requirements of the type of membrane system used (whether hollow fibre modules fixed on one or both sides, or plate modules).

Hollow fibre modules need a very fine preliminary screening as practical experience has shown that hairs and fibres strongly influence operating stability. Due to their plane surface, plate modules have less tendency to clog.

Fine screening

As more and more membrane plants were put into operation, it turned out that great importance needs to be attached to the reliable removal of hairs and fibres from the inlet to membrane bioreactors. Already from 2002 to 2004, the first membrane plants were equipped with preceding fine screens (WWTP Schilde, Monheim, Nordkanal, etc). The majority of these fine screens were wedge wire screens with apertures from 0.5 to 1.5 mm. It was experienced, however, in the following years that hairs and fibres caused problems with the membrane plants even if functionally efficient fine screens were used as pre-treatment units.

The efficiency of one-dimensional screens, such as wedge wire screens, turned out to be insufficient to guarantee the safe operation of membrane plants. It became necessary to develop a new type of two-dimensional mesh or perforated plate screen to improve the retention of fibres and hairs. In 2004, on WWTP Schilde, the first fine screen was changed from wedge wire to mesh. As the apertures of the screen’s mesh or perforated plate are defined in two dimensions, the screen is able to ensure the maximum retention of hairs and fibres.

Therefore, conventional mechanical preliminary screening systems used upstream of membrane plants today are generally completed with a fine screen equipped with a two-dimensional mesh or perforated plate to remove hairs and fibres. Two-dimensional screens have proven that they achieve a solids removal rate that is by the factor 2 to 4 higher.

Due to the much finer apertures and the operating experience, sealing and cleaning systems have become increasingly important. Sealing systems have been further developed to ensure no particles bigger than screen aperture pass through the screen. In addition, machine cleaning systems have been adapted to the very fine apertures. Adequately designed fine screens are equipped with an additional intervallic high-pressure cleaning system to ensure the fine screen apertures are continuously and reliably kept free even with high contents of grease and oil in the wastewater.

Furthermore, when selecting downstream treatment systems, it is also important to consider the specific properties of the separated screenings and the high amount of screenings. As fine screenings contain much more sludge and fine particles (silt) compared to coarse screenings, the requirements on downstream treatment systems are higher. As an option, the fine screenings can be passed to the sludge treatment system where they reduce specific polymer consumption and lead to increased sludge dewatering degrees being achieved. In this case, the fine screen design would be such that the mix of screenings and spray water is pumped to the sludge treatment system and the need for additional screenings treatment is eliminated.

Useful planning information

Fine screening is the central treatment step of preliminary wastewater treatment. The function of the fine screening system, however, greatly depends on its interaction with the preliminary screen (two-stage pre-treatment) and the design and operation mode of the combined grit and grease trap.

Sewer system

Washing and load peaks should be avoided to ensure the reliable operation of the fine screen. Sudden load peaks lead to increased blinding of the fine screen with the result of bigger machines being required. Also machine running times and wash water demand increase. Details about the sewer system are of great importance for the layout of the preliminary screen. Especially in combined sewer systems, if the sewer network is long and shallow, sudden peaks are likely to occur. Preliminary screens, therefore, must be designed to ensure that big loads of coarse screenings are quickly and reliably removed from the sewer.

Special attention must be paid to sewer systems with stormwater tanks, stormwater overflows and sewers with storage capacity and overflow. When such structures are emptied, very high dry substance amounts arrive in the pre-treatment system. Due to their sludgy consistency they pass through the pre-treatment system. The fine screen must therefore be able to handle such amounts and within a very short time separate a very high amount of screenings and remove it reliably. Specific conditions must be clarified in advance and taken into account in the layout of the mechanical wastewater treatment system.

Preliminary screen

Preliminary screens installed upstream of membrane plants must separate as many as possible of the coarse screenings contained within the wastewater in order to reduce as much as possible the solids load in the fine screening system. Sturdy and efficient machines should be selected that achieve convincing results, such as machines with stationary bars with spacings from 3 mm or step screens with spacings from 3 mm.

Intensive washing of the separated screenings is required to ensure the organic material contained can be returned to the wastewater in liquid form. Screenings free of faeces are very easy to dewater and odour development in the screen room is avoided.

Grit and grease trap

Aerated grit traps are normally designed for 0.2 mm grain diameter. Many grit traps are overdimensioned today, ie, they have the function of a sludge separation plant due to the great variation between dry and stormwater conditions with the result of problems with grit removal.

Tests carried out at Huber SE have shown that huge amounts of fine material (160 μm fine grain, silty fine material) are flushed out of the grit trap along with the fibres. This material is then retained and removed by the fine screen. However, if grit traps were designed for smaller grain fractions, still bigger grit traps with still longer retention times would be needed, but this actually makes no sense. Instead, this knowledge is of great importance for the material selection and design of the downstream fine screening unit and even more for the screenings treatment stage.

The separation of floating grease and oil prevents these materials from later accumulating on the free surfaces and leading to odour problems. A well-functioning grease trap is important. Otherwise, there is the risk that lipophilic material is retained on the fine mesh or perforated plate of the downstream fine screen from where it is difficult to remove. Fine screening elements blinded with grease reduce the free cross-sectional surface, increase hydraulic loss and affect hydraulic plant efficiency.

Fine screen

In contrast to previously known wastewater screens (> 3 mm), fine screens are much more sensitive. Much more separation surface must be provided to handle comparable wastewater flows so the machines must be dimensioned bigger.

Load peaks, as described above, must be considered in the screen layout which must be load and volume related.

The integration of a fine screening system into the overall plant hydraulics can be described with two approaches:

• To reduce the costs for preliminary mechanical treatment, a fine screen with gravity flow should be selected so that additional pumps are not required.
• The inlet and outlet channel to/from the fine screen should be dimensioned to ensure the flow velocity in the channel does not fall below 0.5 m/s. The fine screen itself should be installed inside a chamber.

The screenings separated by the fine screen can either be dewatered and discharged or passed on to the sludge treatment system. Fine screenings are much more difficult to dewater than coarse screenings due to the high amount of sludge contained within fine screenings. In most cases, DR results between only 20 and 30% are therefore achieved. If fine screenings are passed into the sludge treatment system, they increase the content of structural material such as fibres and hairs with the result of reduced specific polymer consumption. As structural material also improves dewaterability, higher DR values of the dewatered sludge can be achieved.

Irrespective of how the fine screenings are treated, it must be considered that the volume of screenings generated is significantly higher than with conventional 6 mm screens. With load peaks the amount of screenings generated by a fine screen can increase to even five times the amount generated by conventional systems.

Screen aperture (mm)	Specific amount of screenings [l/(PE*year)], 25% DR
	Wedge wire screen	Mesh or perforated plate screen
1.0 mm	11	20
0.5 mm	16	25

Table 1.1: Amount of screenings generated by a fine screen.

Emergency operation and incident scenarios

Previous experience has clearly shown that a redundant fine screen is required to ensure maximum operating reliability of a membrane plant. A simple emergency bypass is not state-of-the-art and must strictly be avoided. In the event of a failure, a standby screen must be available to ensure 100% hydraulic screening of the wastewater flow prior to being passed to the MBR plant.