Spiral Screens for Wastewater: How They Work and Sizing

A spiral screen, also called a screw screen, removes solids from wastewater and conveys, washes and compacts them in a single machine. That makes it the standard fine screen for small and medium treatment plants, pumping stations and industrial wastewater. This guide explains how it is built, how it is sized and where its limits are.

How a spiral screen is built

• Screen basket — a half-round basket of perforated plate or wedge wire sits in the channel, usually at an angle of 35°. The wastewater flows through it; the solids stay on the inside.
• Shaftless spiral — a strong spiral without a centre shaft turns in the basket. Brushes on its outer edge sweep the perforation clean and move the screenings upwards.
• Conveying tube — above the water level the spiral runs in a closed tube that carries the screenings to the discharge height.
• Washing zone — spray nozzles wash faecal matter out of the screenings and back into the channel.
• Press zone — in the upper part of the tube the screenings are compacted against a back pressure and dewatered. The water runs back through a drain pipe.
• Discharge — the compacted screenings drop into a container, often through a bagging unit that keeps odour in.

The drive sits at the top end, out of the water. There are no bearings below the water level.

How it operates

The screen does not run continuously. As solids collect on the basket, the water level in front of it rises. A level probe starts the spiral, the basket is swept clean, the level falls, and the spiral stops again after a run-on time. Operating in this way lets a mat of screenings build up on the basket, which holds back finer material than the bare perforation would.

Sizes and typical data

Typical values; the capacity of a given size depends on the perforation and on the permissible water level in front of the screen.

Selection and sizing

• Peak flow — the screen is sized for the highest inflow, including storm water where the sewer is combined.
• Perforation — 6 mm protects pumps and pipework; 3 mm is usual ahead of biological treatment; 2 mm or finer is specified ahead of membrane plants. A finer perforation lowers the capacity of a given size and raises the head loss.
• Channel dimensions — width, depth and the highest permissible water level upstream decide the basket diameter.
• Discharge height — the tube length follows from the channel depth and the height of the container. Very deep channels reach the limit of what a spiral can lift.
• Tank version — where there is no channel, for example after a pumping main or in industry, the basket sits in a closed stainless steel tank with inlet and outlet flanges.
• Frost protection — outdoor machines get insulation and trace heating on the tube and the washing system.
• Emergency bypass — an overflow or a bypass channel with a manual bar screen protects the plant if the screen is blocked or switched off.

Grit and grease are removed in the next step; see combined grit and grease units. For other screen types and larger flows see our mechanical fine screens and the guide Fine screens for wastewater: types and how to choose.

Where a spiral screen fits and where it does not

• Good fit — treatment plants up to a few tens of thousands of population equivalents, pumping stations, small industrial flows, replacement of manual bar screens, and sites where one machine has to do everything.
• Less suitable — very large flows, where step screens or belt screens offer more open area; very deep channels; wastewater with long fibres or rags in large quantities, which can wind around the spiral; and heavy grit loads, which wear the brushes and the basket.

Typical mistakes

• Sizing for the average flow instead of the peak, so that the channel backs up in rain.
• Choosing a very fine perforation without checking the head loss.
• No wash water connection: unwashed screenings smell and cost more to dispose of.
• No frost protection outdoors.
• No bypass for maintenance.

Maintenance

The wearing part is the brush on the spiral. It is checked a few times a year and replaced when the basket is no longer swept clean, typically after one to three years. The spray nozzles are kept clear, and the gearbox oil is changed at the interval given by the manufacturer. Because the spiral has no bearing in the water, there is little else to maintain.

Frequently asked questions

What is the difference between a spiral screen and a step screen?

A spiral screen is a compact machine with integrated washing and pressing for small and medium flows. A step screen lifts the screenings with moving lamellae, handles larger flows and deeper channels, and needs a separate wash press for the screenings.

How dry are the screenings?

With an integrated press zone, typically 30–40 % dry solids. Washing before pressing lowers the organic content and with it the odour and the disposal cost.

Can a spiral screen be installed in an existing channel?

Usually, yes. The machine is supplied to the width and depth of the channel and is fixed with a pivot support, so that it can be swung out of the channel for maintenance.

Which perforation should be chosen ahead of an MBR?

Membrane plants need screening to 1–2 mm with perforated plate, not slots, because hair and fibres pass through slots. A coarser screen is often installed first to protect the fine one.

How much water does the washing need?

The washing runs only while the spiral turns and can use treated effluent. The consumption is small compared with the flow through the plant.

Send us the peak flow, the channel dimensions and the required perforation via our contact page. We will select the size and prepare a proposal with screenings handling. Screenings and other solids are moved on with our conveyors.