Screw presses for sludge dewatering at recycling facilities

By Doug Kvistbo, Dagex Inc.

Doug Kvistbo inspects a Huber screw press just after unloading at the Dufferin facility.

For the past several years the City of Toronto has increasingly been faced with a growing garbage disposal problem. As local waste dumps are filling up, there is more of a prevailing attitude that waste disposal is a local issue and that garbage should not be shipped hundreds of kilometres for disposal.

The search for local solutions includes reducing the amount of garbage that needs to be sent to landfill through recycling, and finding ways to recycle material that previously couldn't be or wasn't being recycled. During the summer of 2000, a plant was opened in Newmarket, Ontario, by Canada Composting Inc. (CCI) to process up to 150,000 tons/year of source separated organics, mixed and high fibre waste which forms the majority of Toronto's waste problem. The plant provides a high level of diversion from landfill.

After separating the ferrous metals using an electro-magnet and the out-of-spec material using a trommel screen, the garbage is sent to a pulper, which creates a liquid suspension from the solids waste. The heavy materials are separated off the bottom and the light materials are separated off the top, allowing the recycling of certain products.

The highly organic slurry is pumped to eight Huber screw presses, which dewater the sludge from a typical concentration of 3-7% to over 40% solids. The liquid phase is then digested anaerobically to produce methane. The methane is used to power two 820 KWe (1,100 hp.) electrical generators, which feed surplus electricity into the local grid as well as powering the plant. The solids phase is treated aerobically to convert it to market grade compost.

Each of the Huber screw presses will dewater 10 m³/hr of primary sludge, for 24 hours per day. The presses have a wedge wire screen surrounding the screw, with openings of 0.2 mm to allow the liquid phase (digester feed) to flow out while retaining the majority of the solids (raw compost). As the screen openings are smaller than the size of paper fibres, the screw presses are particularly effective in applications where cellulose fibre is used.

After the successful start-up of the privately owned Newmarket plant, the City of Toronto decided to invest in a similar operation at their Dufferin Mixed Waste Recycling Facility in north Toronto. Both the Newmarket and Dufferin plants were designed and built by Stone & Webster, using the patented BTA process. The City of Toronto has contracted CCI to operate and maintain the Dufferin facility.

Huber screw presses were also chosen for the Dufferin plant where they will be used to dewater the treated sludge, again with a target of at least 40% cake solids. The screw presses were installed in July and the plant is presently on schedule for a winter start-up.

The screw presses are small and light enough that they can be placed on a steel grid floor on the mezzanine level of the plant. This allows the solid waste to be discharged into a hopper on the floor below, allowing easy removal by truck.

The machines run at a speed of only 5 rpm so that there is little abrasion or noise. The low speed also means that electrical power consumption is minimal with each machine being run by a 4 kW electric motor.

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