110 Mt/y sewage sludge generated

Inevitable product of wastewater treatment processes, this is the annual production of sewage sludge assuming that the sewage water from the world urban population in 2016 were to be collected and treated.

More than 70% goes back to nature

In major EU countries, most of the sewage sludge still goes to landfill or is used in agriculture. Because of these ways of disposal, land and water bodies can be contaminated by the pollutants contained in sewage sludge.

Severe impact on human health

Lead, mercury and cadmium can be found in sewage sludge and are the most harmful heavy metals for human health causing well-known troubles such as troubles of vital functions but also anxiety and depression.

Our vision

The Clean WOx® technology is implemented for the treatment of urban and industrial liquid effluents with a high content of non-biodegradable organic matter. It is based on the principle of wet oxidation, a process where non bio-degradable organics are degraded by oxidation in liquid phase in a high pressure reactor. This technology is offered via Granit Technologies and Engineering (GRT) SA, in which GRT GROUP participates with 47%.


Typical applications

  • Municipalities or delegate of public service: treatment of sludge generated from waste water treatment plants (WWTP)
  • Industrials: treatment of specific non-biodegradable waste streams, including in some cases, the possibility of recovering valuable mineral compounds contained in these streams

Typical waste

The Clean WOx unit can process effluents containing a COD level between 70 and 120 g/l. Out of this range, the effluent must be diluted or concentrated. The maximal dry solids content is 15%.

  • Sewage sludge after anaerobic digestion.
  • Industrial effluents loaded with concentrated non biodegradable or toxic compounds (e.g. hard COD, cyanides, sulphides, phenols, pesticides).
  • Pulp and paper effluent (black liquors).
  • Spent caustic from the petrochemical industry.
  • Spent ion resins from Nuclear Power Plant decommissioning.
  • Industrial effluents containing valuable recoverable matters.

Process description

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A Clean WOx® unit degrades the non-biodegradable organic matter into smaller molecules by oxidation. It also transforms the inorganic matter into its oxidized form, for recovering these compounds or to neutralize them from a chemical point of view. This reaction is performed at high temperature and pressure in a bubble column reactor, using oxygen or air as oxidation gas. It generates heat, which can be used for electricity production by cogeneration or steam/hot water production depending on local needs. The outputs of the process are carbon dioxide (CO2), water and in some cases, a mineral residue (depending on the treated inlet). The system does not generate any toxic emission of gas.

  1. The inlet to treat is sent to the Clean WOx®; unit through a high pressure pump.
  2. The oxidant gas is mixed to the effluent.
  3. The inlet mix enters a first heat exchanger system for pre-heating thanks to the heat from a dedicated stream of the Clean WOx®; outlet liquid flow.
  4. The wet oxidation occurs under sub-critical conditions in the Clean WOx®; reactor.
  5. Wet oxidation being an exothermal reaction, the temperature of the medium increases in the reactor. If the organic content is high, there is excess heat available for electricity production by cogeneration.
  6. There is remaining available heat for further valorization at lower temperatures.
  7. After cooling, the liquid/gas mixture goes through a phase separator. The liquid phase is then decompressed.
  8. The gas phase is condensed to allow CO2 recovery under its liquid form.
  9. The final outlet gas may contain traces of CO and VOC. To insure clean rejections, the gas passes through a catalyzer, and then it is released into the atmosphere.
  10. Depending on the treatment targets, the liquids can either be recycled into a biological treatment plant or released to the receiving environment. If the operational parameters are chosen accordingly, the resulting liquids after Clean WOx®; treatment contain easily biodegradable compounds that can be used as carbon source.

In the case of sludge treatment, for which an important content of mineral in the effluent entering the Clean WOx® unit, the mineral fraction is separated from the liquid phase and dewatered in a filter press or a centrifuge.

The Clean WOx® process is unique and fully protected by patents.

Key figures for plant design

  • Typical size:
    Municipal sludge application - 3 500 to 30 000 t/y of dry solids
    This treatment capacity is equivalent to the digested sludge production from one or several WWTP,
    with a total capacity between 100 000 and 1 500 000 equivalent inhabitants
    Industrial and hazardous effluent application - 0.5 - 30 m3/h
  • Operating parameters:
    Theses parameters are defined depending on the type of effluent to treat and the treatment targets.
    Temperature - 200 -310 °C
    Pressure: 30 - 200 bar
    Fully-automated plant with off-site control
  • Performances:
    COD reduction between 70 and 99.5 %
    Recovery of specific compounds: characterization on demand

Environmental benefits

  • No toxic gas generation (SOx, NOx, CO, dioxins, furan)
  • No pollution of land or use of space for landfilling
  • No fly ashes generation
  • Small footprint and possibilities to build small installations on operation sites (no transport of waste)
  • Favorable carbon footprint by capturing the CO2 for further valorization
  • Reduction in volume and weight of final waste
  • Possibility of valuable material recovery


  • Project development: Feasibility study, risk analysis
  • Financing
  • Site development and permitting
  • Project management
  • Engineering
  • Procurement
  • Construction, installation and commissioning
  • Operation and Maintenance
  • Process sublicensing


Semi-industrial plant for sludge treatment

(Orbe, Switzerland)

Industrial plant for sludge treatment

(Rovereto, Italy)

Pulp and paper effluent

(Thonon-les-Bains, France)

Spent ion resins


Effluent from industrial production