Waste-to-Energy : Closing the loop on incinerator bottom ash

For decades, the residues produced by waste incineration — commonly known as incinerator bottom ash (IBA), bottom ash, or slag — have been treated largely as a nuisance by-product of thermal treatment. At best, processed grate ash has found use as a substitute base course material in road construction. At worst, it has ended up in landfill. Now, a patented technology called IRRT (Innovative Resource Recovery Technology), developed by proCLIR GmbH, is challenging that assumption with striking results.

As Dr Robert Eggersmann, Head of Digitalisation at Eggersmann and one of three Managing Directors at proCLIR, explains: "The IRRT process marks the dawn of a new era in the circular economy. Although the current use of processed grate ash as a base course material is already substituting building materials, it is not yet recycling in the sense of recovering raw materials. This will change with the new process."

The technology can process up to 80 per cent of the dry mass of incinerator bottom ash into fully-fledged raw materials and products in accordance with the REACH Regulation and European chemicals and product legislation (ECHA). Eggersmann Anlagenbau GmbH has been appointed the exclusive plant manufacturer for the patented process.

The IRRT process targets several distinct material streams within IBA. Chief among these are the mineral content and glass fraction, which have largely been overlooked by conventional processing approaches. With IRRT, up to 95 per cent of the aggregate contained in the ash can be extracted, with the company also reporting a recovery rate of 85 per cent for glass.

The aggregate produced using IRRT — marketed under the name UltraLit® — meets the requirements of EU 305/2011 and DIN EN 12620, qualifying it for use in concrete production as a substitute for natural sand and gravel, both of which are increasingly scarce. The glass fraction, marketed as UltraSilit®, is pure enough to be used directly in the manufacture of foam glass, an insulating material with an important role in energy-efficient building refurbishment.

Claus Gronholz, Head of Research and Development at proCLIR and the process's principal inventor, explains the significance: "The aggregate produced with our process fulfils the strict requirements of EU 305/2011 and DIN EN 12620, which means it can be used in concrete production for high-quality building materials and replaces natural products such as sand and gravel, which are already in limited supply. At the same time, this significantly reduces the amount of waste material that has to be landfilled." On the glass: "The glass produced with IRRT is so pure that it can be used for the production of foam glass, a building insulation material, without further processing. In other words, the very insulating material that plays an important role in the energy-efficient refurbishment of buildings due to its diverse properties."

Gronholz has been researching solutions for the treatment of bottom ash since 2009, working at his own research centre in Bispingen through H.U.R. Hamburg GmbH (Hamburger Umwelt Recyclingtechnologien).

Metal recovery from IBA is already standard practice at most processing facilities, particularly for magnetic iron, given its straightforward extraction. However, IRRT claims a significant step change in performance across all metal types. For ferrous metals and stainless steel, the process achieves recovery rates of up to 95 per cent, compared with the 80 per cent typical of existing approaches.

The improvement is even more pronounced for non-ferrous and precious metals. Whereas conventional processes recover up to 64 per cent of these materials, IRRT achieves up to 90 per cent. This is partly attributable to the process’s ability to recover non-ferrous and precious metals in high-purity quality from particle sizes as small as 0.5 mm — a threshold that many existing systems cannot reach.

One critical advantage of IRRT over conventional processes is its handling of aluminium. In most installations, fresh IBA is stored for several weeks to allow carbonation — a process that causes aluminium to oxidise and decompose, rendering it largely unrecoverable. IRRT replaces static storage with a dynamic carbonation system (DyCAR), eliminating the need for lengthy stockpiling and preserving the aluminium’s material value.

André Gronholz, son of Claus Gronholz, Managing Director at proCLIR, and manager of the pilot plant in Bispingen, highlights the process's operational efficiency: "Our water consumption in particular is extremely low. Many processes work with a water-to-solids ratio of 5 to 1 – we manage with just 0.3 to 1 and don't even need fresh water for the treatment. This significantly reduces costs, simplifies authorisation procedures and makes an IRRT plant wastewater-free."

On the financial case, he adds: "The recovery of the metals alone makes the process highly economical and requires no additional payment from the waste incineration plant operator. Together with the marketing of the aggregate and the glass, amortisation can be expected within just 4 to 5 years."

The three proprietary systems at the heart of IRRT are dynamic carbonation (DyCAR), high-speed decomposition (RoTAC), and high-frequency cleaning (ReUST). Together, they are credited with both the high purity of the recovered materials and the process’s overall efficiency.

Because IBA is cooled in a water bath following thermal treatment, the fresh material arrives at the processing stage with a high water content. The first step of the IRRT process removes this moisture through a drying stage. From the resulting dry mass, up to 80 per cent can be converted into directly marketable raw materials. In total, only approximately 20 per cent of the original material requires landfilling — a reduction of up to 70 per cent compared with conventional approaches.

The residual fraction, consisting of fines and process water sludge, is not without merit: it exhibits pozzolanic properties and self-solidifies when mixed, automatically forming a pollutant sink compliant with DK-1 specifications.

From a carbon perspective, the recovered materials — metals, glass, and aggregate — each carry significant CO2 savings potential by displacing primary extraction. Iron, aluminium, copper, stainless steel, zinc, lead, gold, silver, foam glass feedstock, and concrete aggregate are all produced without resort to virgin raw material extraction.

The development of IRRT comes against a backdrop of growing global pressure on WtE operators to do more with the residues they generate. Jörg Eckardt, the third Managing Director at proCLIR and a 30-year industry veteran with experience in waste incineration plant construction, frames the challenge plainly:

"Waste incineration is unavoidable by today's standards. What can be avoided, however, is the loss of raw materials that end up in the IBM during the thermal treatment of waste. As the thermal treatment of waste performs indispensable tasks in waste management and the idea of endless waste recycling is still pure theory for the time being, we have to get to grips with it. More and more countries are facing massive environmental problems as a result of landfilling. Incineration reduces the amount of landfill by around 75 %, thus minimising the environmental impact and at the same time utilising the energy potential in household waste, which is comparable to that of lignite, for example, but has far less environmental impact thanks to the technologies available today. Objectively speaking, waste incineration plants will therefore continue to have their raison d'être."

Claus Gronholz echoes this view: "This was precisely the driving force behind the development of the process. It had to be a practical solution that focuses on the entire recycling potential of IBM and can be implemented directly in waste incineration plant operation. This is where the IRRT process can realise its full potential and offer a way to make thermal treatment significantly more sustainable."

With Eggersmann Anlagenbau now confirmed as the exclusive manufacturer of IRRT plant installations, the technology is poised to move from the pilot facility in Bispingen into broader commercial deployment. For WtE operators facing growing scrutiny over residue management, IRRT represents a compelling case that incinerator bottom ash need not be an environmental liability — but a significant source of secondary raw materials in its own right.