Recovery of sludge from urban water treatment plants

The main characteristics of sludge from water treatment plants is that it has a low calorific value and a high degree of organic matter. It can also contain pathogenic substances, components of pharmaceuticals and trace elements of metals. Incineration alongside household refuse is the best solution for treatment such waste.

CNIM has developed joint incineration systems for sludge and household waste in a waste-to-energy conversion plant.
Equipment supplied by CNIM provides the facilities for

  • unloading
  • transfer
  • drying, if required
  • storage
  • operating methods for introducing sludge into the waste-to-energy furnaces

Equipping a waste-to-energy conversion plant for sludge treatment

The sludge from an urban water treatment plant can be incinerated in an existing waste-to-energy conversion center, without any major technical adjustments, as long as certain measures are taken to ensure good combustion conditions.

Increasing the calorific value of sludge

The sludge usually undergoes a mechanical form of dehydration at the water treatment plant, so that the dry matter content is around 20-25%. When delivered to the plant for incineration, it can be treated in its current state, and simply mixed with household waste. However, it may sometimes be necessary to increase the proportion of dry matter – in order to increase its capacity for incineration.

When required, a drying phase can be carried out at the incineration plant, without the requirement for any combustible additives. All waste treatment plants that use heat as part of the process are already equipped with boilers and the extra energy required for drying is provide by the on-site combustion of domestic waste.

Drying sludge at the waste-to-energy conversion plant

When it’s necessary to increase the dry matter content of sludge before incineration alongside domestic waste, CNIM can provide a range of tailored processes. Drying can either be carried out directly, by steam-based fluidized bed combustion (as is the case for a plant in Nice, France), or indirectly.

Sludge storage and preservation of air quality for local residents

Sludge is delivered to the incineration plant and then gravity-poured into a silo. The silo area is made airtight from the exterior, and ventilation is provided – with the ventilated air being directed toward the air intake of the furnaces, in order to prevent any odors from escaping.

Guaranteeing the combustion stability of a sludge/waste mixture

The nature of water treatment plant sludge means that the way it is introduced into the furnace is an essential factor in achieving good combustion. There are different methods for the continuous addition of sludge, and the formation of a homogeneous mixture with household waste. Good conditions are essential for combustion stability, especially when the sludge/waste ratios become significant.

Most furnaces at plants that incinerate these two components together were originally designed to cater only for household waste. However, aside from the system for introducing the sludge, no major modifications have been necessary – either to the furnaces or to the grates. The only changes have been a simple adjustment of certain parameters, such as the air temperature for combustion (from 120° to 200° C), the excess air and the way it is distributed.
Aside from it being a simple and economical approach, joint incineration also provides a means of energy recovery from sludge.
Plants where CNIM has deployed this technology include those at Thiverval-Grignon, Rennes and Saumur in France, and Thumaide in Belgium.

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