Carefully screened

Frank Rogalla, Jim Welp and John Keller of Black & Veach discuss developments in wastewater screening

Wastewater screening is one of the most fundamental pre-treatment processes, often the first step of a treatment plant.

Over the years, screen openings have tended to decrease in size. While coarse

19-25mm openings were common in the 1970s, during the 1980s and 1990s the market moved towards medium screen openings of 10-15mm. In recent years even finer 3-10mm screens have become the norm in parts of Europe. Some specific advanced processes, such as some membrane bioreactors or biofilm systems, require even finer, often 2mm, protection, to prevent accumulation of material.

Smaller openings remove much of the material that can cause scum blanket accumulations in digesters, can plug sludge pumps, and results in the accumulation of floatable materials in clarifiers and channels. The move to smaller screens also is driven by the need to remove objectionable material from the sludge to facilitate land application or other beneficial reuse.

The move to fine screens, while providing many benefits for the operation of a sewage treatment plant, also presents some key issues that must be considered when retrofitting fine screens.

This is especially challenging in wet weather situations, as more and more storm overflows are being required to increase their screening efficiency. Screens are categorised by their screen opening. This is the effective area through which flow will pass. As shown in Figure 1, there are three different types of screens, which are broken down by their screening opening.

Screening is normally implemented at the head of the WwTW. Some designers, especially if a vortex type grit removal system is employed, favour installing screens prior to grit removal to ensure that all large material (such as plastics or rags) is removed. This prevents the accumulation of floatables and minimises the amount of putrescible material collected in the grit process, as well as protecting the grit pumps from plugging.

Others designers favour screening downstream of grit removal to reduce wear of the screen equipment itself, and prevent the low approach velocities which can result in grit sedimentation in the screen channels causing potential for odours and a need for increased maintenance.

What’s my size?

Fine screens, by definition, have less free area for water to flow through than medium screens. So, for any given hydraulic capacity, a fine screen will require more total surface area.

Table 1 illustrates the available openning of some perforated fine screens, showing that with a halving of screen openings from 3-6mm, the available surface area is cut by 30%. This increase in area required is somewhat dependent on the type of fine screen employed (bar, perforated, step or oscillating) but changing the screen opening will almost certainly require expanding the existing channels or constructing additional channels. For example, for a given existing channel, a supplier quoted a “12mm climber screen” with a capacity of 220ML/d and a “6mm step screen” with a capacity of 140 ML/d.

Speed bumps

Because of more area being covered by structural elements, fine screens result in a substantially higher headloss for any given capacity than a coarse or medium screen. Many fine screens, such as the step screen, are actually designed to operate with a built up matt of material that results in a normal operating headloss in the order of 200mm, versus, 50mm for a typical bar screen.

Under peak loading, due to the rapid

accumulation of material, in addition of

higher flows, a doubling of headloss to 300mm to 400mm can be expected. Careful consideration must be given to the additional head created to ensure that flooding does not occur within the collection system.

The installation of fine screens at an

intermediate point in the treatment train (for example upstream of an IFAS or membrane system) is almost certain to affect upstream processes since it is unusual to design for a reserve head of more than 300mm between processes. In these cases some modification of the operating hydraulic elevation in one of the processes will have to be implemented.

As discussed above, fine screens require a wider channel for a given capacity and operate at a higher head. These two factors combined result in generally slower approach velocities. If much lower than the recommended 1m3/s, significant deposition of grit and oversized material can occur in the screen channels.

One way to reduce the impact on channel velocity at low flow conditions is to construct the screen channels with a chamber upstream and downstream of the screen itself. This reduces the area and consequently increases the velocity. Channel mixing

– aeration – can also be considered, though this could have an impact on the sizing of any odour control system.

That much?

Traditional bar screens typically capture only 15%-40% of the solids in the raw wastewater stream. Fine screens have capture rates about twice as high in the range of 50-80%.

The results of independent testing by UKWIR are summarised in table 2, expressed as solids capture rate (SCR). Some manufacturers have improved their designs as a result of this testing, for instance the Andritz values were improved to 80% capture.

In the design of the fine screen installations, a minimum of two fine screens should be provided. Each screen unit should then be capable of independent operation while meeting the peak hour wastewater flow, providing total redundancy. This allows a fine screen to be taken out of service without adversely affecting the operation of the facility.

In the past, screens have been provided with carbon steel components that corroded after a few months of operation. As a result of a very humid atmosphere with high hydrogen sulphide levels, all components of a screen should be stainless steel.

Retrofitting from a bar screen to a fine screen can be expected to result in the doubling, or greater, of the solids collected. Typical expected solids production for larger screen openings are given in old reference material, as illustrated in the figure below.

The solids-handling system must be designed to take into consideration this increased solids collection rate (conveyors, bin size, disposal contract).

Screening disposal costs can also be expected to increase, though savings elsewhere in the process should more than offset these costs in the long run.

Stormy weather

The nature of the collection system can have a significant impact on the implementation of fine screening. Combined collections systems are much more likely to transport oversized material (such as rocks or wood) that can damage fine screens, therefore coarse bar racks upstream of the fine screens should be considered for protection.

Another consideration is the impact of “surge” loading during a storm event from a combined system. The quantity of solids in the influent stream can result in overloading of the fine screens and significantly reduce the hydraulic capacity of the screens, causing a by-pass or flooding within the collection system.

Come clean

Many facilities with coarse or medium screens have the screenings directly deposited into a bin for disposal. Fine screens result in the removal of significant quantities of organic material in addition to the conventional solids.

This material, if deposited directly in a bin, would result in significant odours during storage and possibly result in disposal problems. To overcome these issues it is necessary, as a minimum, to provide a screenings washing system to remove the organic material (returning it to the influent stream for treatment).

To help further reduce faecal matter in the screenings and reduce odours, some manufacturers have developed screenings cleaner systems which incorporate a grinder. These systems direct screenings to a grinder-based washing unit, improving the release of attached faecal matter to be returned to the liquid flow for treatment. These systems visibly reduce the amount of organics in the dewatered screenings and reduce odours compared with conventional washer compactor systems. The above picture shows the results of pilot testing using a grinder-based washer compactor. The results of that test demonstrated the ability to achieve extremely clean, compacted screenings with minimal odours.

The headworks of a WwTW represent one of the most significant sources of odours due to the influent itself and the collection or storage of grit and screenings material. Fine screens themselves, by design, are generally easily amenable to enclosure of the entire screen, facilitating odour control and minimising the volume of air to be treated, and proportionally reducing building ventilation and odour control costs.

The design of the new screenings facilities must take into account the need for maintenance, which in general is not any more complicated or difficult than for a conventional screen. Some screens have submerged parts (such as bearings, chain or sprockets) and the ability to pivot out of the channel for maintenance and to service or replace elements without draining the channel. In larger systems with screenings conveyance systems, the ability to pivot the screen may be impacted by the installation of screw conveyors.

One solution does not fit all. Each facility must review the specific implications and make a decision based on the most appropriate technology for their application. The decision to implement fine screening can have cost implications far greater than the cost of the screen itself.

The cost of a fine screen may be similar or marginally higher than the cost of a similar sized medium screen, however:

  • The hydraulic capacity can be significantly lower, resulting in the need for more screens and channels
  • The cost of the ancillary devices (such as washing or conveyors) may exceed the costs of the screens themselves
  • There can be significant building costs to house new screens and the ancillary devices
  • There can be significant electrical costs since the number of drives will increase
  • The cost for adding odour control may be lower due to the low profile and enclosed nature of most fine screens.

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