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Sludge management solutions for water and wastewater treatment works

Home Engineering Engineers, Consulting Engineers & Project Management Sludge management solutions for water and wastewater treatment works

IN a recent project, the Bosch Projects and Bosch Capital team investigated sustainable sludge management methods at different wastewater treatment works in KwaZulu-Natal and the Eastern Cape.

“Disposal of sludges at water and wastewater treatment works is a challenge facing water boards and municipalities across South Africa. Current practices include the stockpiling of sludge on site or disposal at a landfill site,” said Jason Holder, Bosch Projects.

“Although disposal of sludge on site may provide the lowest cost option, this method is not seen to be sustainable due to space constraints, environmental risks and non-compliance with the relevant regulations,” he said.

“Our investigations show that reusing and recycling of sludge as compost, bricks or as a fuel source are sustainable management options, but must adhere to key regulatory requirements and require the identification of a customer for these products. Research also indicates that the recovery of energy from sludge can reduce electricity costs on site and surrounds, but may require a relatively larger capital outlay.”

The project methodology incorporated the collection of sludge samples from various water and wastewater treatment works, which were analysed by South African National Accreditation System (SANAS) accredited laboratories.

Holder said not all laboratories in SA are SANAS accredited and able to undertake the full suite of tests and, therefore, samples were exported to the UK for testing. Samples were assessed and classified in terms of the National Gazetted Regulations, South African National Standards and the Sludge Utilisation Guidelines.

A ‘Sludge Disposal/Utilisation Options Analysis Model’, which highlighted the financial and strategic implications and the risks to be mitigated for each option, was developed to enable comparison of the available management options.

The project outcomes also indicated that, although water and wastewater sludge is an inherent part of the treatment process, a critical part of any sludge disposal strategy should be the reduction in the volume of sludge to be disposed, which can be achieved by employing various dewatering and drying options.

The re-use option of applying sludge to agricultural land has advantages, but the identification of a suitable off-taker is necessary, and this process must adhere to key regulatory requirements. Sludge can also be recycled into other products, including compost and bricks and this process may require the services of a specialist contractor. As another option, energy from sludge can be recovered for use at the wastewater treatment works and surrounds through biogas generation.

As a ‘last resort’-option in the waste management hierarchy, Bosch Projects recommends that sludge be disposed of at a registered landfill site and that preferably, sustainable management options be pursued.

In terms of the Waste Act, sludge with a moisture content of greater than 40%, that liberates moisture under pressure in landfill conditions and which has not been stabilised by treatment, is not allowed to be disposed of at a landfill. This imposes regulatory pressure on the water service authorities and water boards and in turn will impact the authorities both in terms of CAPEX and OPEX.

These options however, are not seen to be viable as they do not meet the minimum legislated solids content for the disposal of sludge to landfill in terms of the Waste Act. Note that current legislation deals with disposal of sludge to landfills only, unless there is adequate storage on site in terms of the regulations (i.e lining of disposal area and licenses).

Sludge drying beds are seen to be the most compliant and cost-effective option, as the sludge quality meets the minimum requirements, and the overall costs are lower than other dewatering technologies. The diagram below provides an indication of costs, if the distance to the landfill is increased to 100 km.

It can thus be concluded that the volume and distance of the sludge being transported, coupled with the landfill disposal costs, have a material impact on the sludge disposal strategies being implemented by wastewater treatment works. The development of a sludge disposal strategy should be considered in holistic manner for each water service authority and water board.

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