Wastewater Glossary: Sludge Incineration

Sludge incineration is a sludge treatment process. Dewatered sludge is heated and dried in an incinerator, and the organic matter in the sludge is then oxidised at high temperature, leaving only a small amount of ash.

It is a volume-reducing, stabilising and harmless post-treatment method for sludge. The process removes water and organic matter almost completely and destroys pathogens. Sludge incineration can be carried out as complete combustion or incomplete combustion.

It removes all the water and organic matter in the sludge, destroys all pathogens and reduces sludge volume to the greatest extent possible.

Sludge incinerators come in several types, such as vertical multi-hearth incinerators, rotary-drum incinerators, fluidised-bed incinerators and spray incinerators. The vertical multi-hearth type is the most widely used today. The furnace is divided vertically into 4-12 hearths, each fitted with a horizontal circular plate to form a multi-hearth bed, and rotating rake arms above the beds turn at 0.5-4 rpm. Sludge is fed at the top, is stirred on the upper hearth and drops successively to the hearth below. The upper hearth is usually held at about 300-550 °C, where the sludge is further dewatered and dried; in the middle section it is incinerated at 750-1000 °C; and the bottom hearth, at about 220-330 °C, is air-cooled. The flue gas passes to a gas scrubber to prevent air pollution. Such incinerators are mostly installed at municipal wastewater treatment plants.

In recent years pyrolysis has been developed. Sludge is heated to 370-870 °C in the absence of oxygen, and organic matter decomposes into gaseous products, oily liquids and residue. The gases include methane, carbon monoxide, carbon dioxide and hydrogen; the liquids include acetic acid compounds and methanol-type substances; and the solid residue finally becomes an ash containing 2-15% carbon. The decomposition takes about 25 minutes.

The advantage of this method is that wet sludge or high-strength organic wastewater can be treated effectively without prior dewatering, and heat consumption is low. The treated residue dewaters well and can generally be vacuum-filtered without coagulant, with a cake moisture content of only about 50%. Because the reaction takes place in a closed vessel, odour, dust and soot are essentially eliminated; pathogens in the residue are destroyed; and the separated water is easy to treat biologically.

Undigested sludge contains a great deal of water and, at atmospheric pressure, can only be heated to 100 °C. Pressurising achieves the temperature needed for oxidation and also lowers the oxidation temperature of organic matter. For example, at about 100 kgf/cm² and 250-300 °C for one hour of wet combustion, the sludge from municipal wastewater treatment reaches a COD removal of 70-80% and an insoluble volatile solids removal of 80-90%.

The process exploits the fact that organic impurities in water can be oxidised under high pressure and temperature, removing organic matter from sludge under suitable conditions inside the unit. It is commonly called wet oxidation or wet combustion, and applies both to sludge with a high organic content and to high-strength organic wastewater.

The ash left after incineration can be reused as a resource. If it still contains toxic substances such as heavy metal ions, it must undergo final disposal by solidification and deep burial.

Incineration requires a large investment and demanding management, but it remains a feasible way to solve the problem of mounting sludge stockpiles.