Sludge match - handling and disposal costs

23 May 2016



W2O Environment’s Wolfram Scholz details the challenges of reducing sludge handling efforts and disposal costs.


Tanneries are facing rising environmental and increasing sludge disposal costs. The most common disposal route is sludge thickening, dewatering and conditioning followed by disposal on secure landfill sites. Landfilling of sludge is becoming increasingly difficult for two reasons: the availability of landfills is diminishing and recent environmental legislation, such as the 'Landfill Directive', which aims to prevent or reduce negative effects on the environment from landfilling waste, came into force. Consequently, alternative disposal routes and pre-treatment options need to be developed for larger quantities of sludge.

In general, tanneries generate a solids load during primary treatment equivalent to a sludge volume of about 5-10% of the total volume of effluent being discharged. The settled sludge resulting from these operations is normally in the form of a liquid with a solids content of typically 3-5% dry solids (DS). The operation of dissolved air flotation generates sludge of a higher solids content of 8-10%, which reduces the volumes of the following sludge dewatering.

When employing biological treatment on site, the total sludge generation may be 50-100% greater than the sludge generated during primary treatment. Most tanneries will dewater their sludge to reduce the volume of it for disposal. Sludge can be dewatered by means of filter presses, belt presses, centrifuges and thermal treatment. In most cases, flocculation agents have to be added. Filter presses are capable of producing a sludge cake with up to 40% dry solids, whereas belt presses produce a sludge cake with up to 20-25 % DS. Centrifuges are useful in attaining sludge with up to 25-45% DS.

Typically, dewatered sludge can amount to 10-20t a day of matter (filtercake) for a medium-sized tannery. The costs of transport and safe disposal vary via local regulations and conditions, but can cost $150-250/t. To improve the competitiveness of tanneries, it has become increasingly important to find solutions to reduce sludge generation and disposable volumes. A cost-effective option is further drying with a steam-heated paddle sludge dryer, which increases sludge dryness by to up to 90%.

The sludge dryer is now in operation in China, Argentina and the UK, and has shown to significantly reduce sludge volumes and disposal costs by two thirds.

Sludge dryers are now in operation in China, Argentina and the UK, and have shown to significantly reduce sludge volumes and disposal costs by two thirds. The implementation of a sludge dryer considerably reduces the overall operation, discharge and disposal costs of a wastewater treatment plant and therefore achieves a return on investment within a short pay-back period.

Low-cost paddle sludge dryer

The paddle sludge dryer is made of stainless steel and is manufactured in China. Capital costs are lower than similar products offered by international manufacturers, and operation costs and energy demand are comparable to other sludge dryers. The dewatered sludge with a DS of 25-40% falls through a hopper, and is conveyed at adjustable speed and fed into the stainless-steel chamber of the dryer, where the sludge is macerated and dried with steam-heated paddles.

With the rotation of the paddles, the sludge is slowly moved through the dryer to the outlet, where the dried sludge is cooled down in a conveyor and collected in a skip. During the drying process, the temperature of the filter-cake raises from ambient temperature to about 150°C, resulting in the generation of exhaust steam. A heat exchanger provides heat recycling from the outlet conveyor back to the inlet conveyor, where the sludge is pre-dried. This allows the dried sludge to cool down to about 50°C to avoid self-ignition. The sludge dryer is fully automatic in continuous operation and does not require additional supervision.

About 1.8m3 of condensate is collected per hour in steam traps and discharged back into the effluent treatment plant. The sludge dryer works at negative pressure, with the operation of a draft fan that sucks out the exhaust steam. This eliminates the possibility of emissions getting into the working environment and enables safe operation. A complex exhaust steam-cleaning system consisting of a dust cyclone and a gas-washing tower is foreseen, to avoid any emissions, dust or smells during operation of the sludge dryer.

The implementation of low-cost sludge drying reduces disposal costs by two thirds, and can achieve sludge disposal savings and a payback, depending on sludge disposal, of costs in half a year to a year.

The operation of a paddle sludge dryer with a drying capacity of up to 50t consumes 68kWh, 1.8t/hr steam at 0.5MPa and generates 1.8m3/hr wastewater. The exhaust steam treatment with a cyclone and gas-washing tower ensures that the sludge dryer reaches full compliance with air emissions. ?

Economic considerations

The implementation of low-cost sludge drying reduces sludge disposal costs by two thirds. A tannery effluent treatment plant with 1,000m3/day capacity generates about 20t of dewatered sludge cake a day. By using this low-cost sludge-drying technology, disposal costs can be reduced from $1.5 million a year by two thirds down to $0.5 million a year. The operation of a paddle sludge dryer with a drying capacity of 20-50t consumes 68kWh, 1.8t steam an hour at 0.5MPa and 1.8m3 an hour of water. The Investment costs of $325,000, including operation costs in terms of steam and electricity, have shown to achieve a payback of the sludge dryer in less than half a year.

Sludge handling: before treatment (left) and after treatment (right).
Low-cost sludge dryer made in China.


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