Sludge inertisation

Sludge inertisation by ultrahigh temperature pyrolysis

Sewage sludge, a product of wastewater treatment, consists to about 50% of organic matter. Untreated it poses a potential risk for the health of people and animals and the environment. Before it is finally disposed off in any form, it has to be treated to minimise above risks. Ultra-high temperature gasification (pyrolysis) is an alternative to sludge incineration, which is nowadays most widely used in countries where sludge use in agriculture is prohibited.

Ultra high temperature pyrolysis exposes the sludge to temperatures above 1200°C in the absence of oxygen. Organic compounds are nearly completely destroyed and the absence of oxygen allows the conversion of it to valuable SYNGAS (more than 90% is a mixture of CO and H₂) with the energy content of around 3.5kWh/m³. In addition, the higher process temperature eliminates the tar in the gas (no expensive gas cleaning).

The key element of technology is the ability to create intense heat through a specially developed heating induction system. To achieve this goal, a special metal mixture was developed which can withstand such elevated temperatures over a long period of time without corrosion..

The pilot plant was constructed and delivered by Pyromex, Switzerland and the first trials for the goal of the EU project Neptune were performed in cooperation with Eawag.

Major conclusions from the experiments done so far are the following:

• The obtained gas was free of tar and consisted of CO (~40%), H₂ (~50%), CH₄ and CO₂;

• Higher temperature and longer residence time reduced the amount of the solid residue coming out of reactor and increased the gas production, but did not reduce the concentration of heavy metals in the residue. Its concentration is below the value set by the EU standards for sludge use in agriculture.

• The content of phosphorus in the solid residue is around 6%, of which 10% is bioavailable (leaching test in the 2% citric acid), compared to only 0.1% of bioavailable phosphorus from the incineration ash. Phosphorus behaves similar to the heavy metals (either staying in the solid residue or in the gas). However, results indicate that the phosphorus in the solid residue is more bioavailable with lower temperature and shorter reaction time. According to the next graph, the obtained solid product could be applied as a fertiliser in some European countries

• Due the high phosphate and low heavy metal content it could also be a suitable as raw product for the fertilizer industry.
  
  

• Heavy metal volatilization can be increased by addition of chlorides to the feeding material due to the formation of volatile metal-chlorides, which makes the solid residue even more interesting as fertilizer.
• The heavy metals water leaching properties of the solid residue obtained after high temperature pyrolysis (gasification) were compared with the ones of other sludge inertisation methods (incineration, wet oxidation process and middle temperature gasification; T<900°C). Based on the absolute amount of heavy metals in the solid residues as well as the percentage leached after five days in the water solution, the conclusion is that the product of the high temperature pyrolysis (gasification) has the least effect on the environment if used in agriculture.