Prevention, recycling and disposal of wood ash

When wood fuels are used for energy generation, wood ashes are produced in varying quantities and qualities. Depending on the form of wood fuel - pellets, wood chips or lump wood - the ash content is between 0.5 and 8 percent by weight based on the amount of wood burned. Pellets have the lowest percentage of ash. When using wood fuels with a high bark and needle content and many impurities, as well as when using waste wood, the ash content can be as high as 8 percent by weight. Environmentally sound disposal of the ashes to protect waters and soils is central. Wood ash is divided into grate ash, bed ash, fly ash and filter ash (see details below). In 2020, a total of 70,700 t of wood ash was produced in Switzerland.

Fuel definition according to the Clean Air Ordinance

The Ordinance on Air Pollution Control (OAPC) differentiates between wood fuels and non-wood fuels (see Annex 5, item 3). With regard to ash disposal, the fuels used have an impact, among other things, on the way in which fly ash from the combustion of non-wood fuels is treated, from which the heavy metals zinc and lead must be extracted as of January 1, 2026, in accordance with the Ordinance on the Prevention and Disposal of Waste (VVEA).

Problematic wood waste includes wood,

• which is organohalogen coated (e.g. with PVC),
• was treated with pentachlorophenol (PCP),
• has been subjected to a deep treatment with wood preservatives (e.g. pressure impregnation)
• or has lead-containing paint (doors, window frames) (mainly wood that has been used outdoors).

Used or treated wood, which does not belong to the problematic wood waste, is called waste wood. It is mainly wood that was used in the interior.

Residual wood is exclusively mechanically processed wood or wood treated or coated only with non-problematic substances from wood processing companies such as joineries (e.g. chipboard offcuts, offcuts, sanding dust) and disposable pallets made of solid wood.

Wood waste from the processing of clean, untreated or merely mechanically processed solid wood/solid wood, i.e. wood that is neither painted, coated, glued, treated nor otherwise contaminated (e.g. wood waste from sawmills) counts as wood fuel in the same way as untreated forest wood. Untreated fence posts, bean poles and other objects made of solid wood used in the garden or in agriculture also belong to wood fuels.

Wood ash composition

Ash from natural wood consists mainly of minerals, alkali metals and salts. Phosphorus and potassium are contained in certain quantities as nutrients. In addition, there are numerous other value-giving ingredients such as calcium and magnesium, as well as trace elements such as manganese and sulfur.

However, pollutants are also enriched in the ash, with the heavy metals arsenic, lead, cadmium, chromium (as chromium-total or chromium-III and as chromium-VI), copper, nickel, mercury and zinc being of particular importance. These are found in higher concentrations in the filter ash and are highest in the incineration of waste wood. However, grate ash from the combustion of untreated wood also contains heavy metals. These were absorbed by the tree during its life through the roots and are found in concentrated form in the ash at the end. In the case of waste wood, the heavy metals additionally originate from fittings, paints, coatings and foreign substances in the fuel.

The toxic heavy metals include above all chromium-VI. This is taken up by the tree from the soil as chromium-III and oxidized to chromium-VI in the thermal process when the wood is burned. This oxidation occurs largely independently of the wood assortment used and is practically not influenced by combustion technology measures. In contrast to chromium-III, chromium-VI is highly water-soluble, toxic, mutagenic and carcinogenic. For this reason, care must be taken when handling wood ash to avoid dust formation and to use appropriate personal protective equipment. Wood ashes cause skin irritation on contact, severe eye irritation and can irritate the respiratory tract when inhaled. Chromium-VI can be converted back to chromium-III by adding a reducing agent (e.g. ferrous sulfate).

What does the legislator say?

Wood ashes are legally considered waste. Operators of wood-burning furnaces are therefore considered to be waste producers. The Waste Ordinance VVEA, which has been in force since January 1, 2016, specifies how to handle wood ash properly (cf. here Fig_2_Infographic ash colored).

Wood ashes from small plants should be disposed of with the refuse in a refuse incineration plant after complete cooling. Grate, cyclone and filter ashes from the incineration of forest and residual wood in larger plants are to be deposited in type D and E landfills. Grate ashes from the incineration of waste wood are also to be disposed of at these two landfill types D and E. Since slag from waste incineration, which contains large amounts of free iron, is also disposed of at these two landfills, the chromium-VI is reduced to chromium-III again at these landfills if it is well mixed. At the Type D landfill, the ashes must meet a TOC limit of no more than 20,000 mg/kg; at the Type E landfill, the TOC limit is 50,000 mg/kg. Filter ashes from the incineration of waste wood must be treated (heavy metal recovery) in accordance with the Waste Ordinance VVEA from January 1, 2026.

The cantons are responsible for the enforcement of the VVEA. However, the landfills are operated by private companies. Since wood ash does not belong to municipal waste, landfill operators are not obliged to accept wood ash. As a result, wood ash disposers sometimes have to travel long distances to find a customer. An overview of Type D and Type E landfills in Switzerland can be found here: www.holzenergie.ch/ueber-holzenergie/holzaschen.html 

Fly ash from the combustion of non-wood fuels

Fly ash from waste wood incineration must be stored separately and disposed of differently from January 1, 2026. From this date on, this type of ash must be processed before being landfilled. Clarifications are currently underway within the industry regarding the possibilities of this treatment. Depending on the TOC content, the grate ashes can be deposited in landfill type D or E.

In addition to the VVEA, the Ordinance on the Handling of Waste (VeVA) and the Ordinance on Air Pollution Control (LRV) must also be taken into account when disposing of wood ash.

Other provisions

Ash from untreated wood contains potassium, calcium, magnesium, sodium and phosphorus as fertilizer elements. The use of wood ash as fertilizer is only permitted if a quality test for the ash has been carried out at least once a year in accordance with Annex 2.6 Number 2.2 of the Chemicals Risk Reduction Ordinance (ChemRRV). The supply of ash as fertilizer to third parties is subject to approval in accordance with Article 10 of the Fertilizer Ordinance.

According to Annex 2.6 Number 3.3.1 of the ChemRRV, the use of fertilizers in the forest is prohibited. Thus, wood ashes may not be spread in the forest, as is practiced in Germany, for example, as a measure against the acidification of forest soils.

Disposal

Small amounts of ash: Wood ashes from single-room furnaces (such as fireplaces, indoor stoves, tiled stoves and pellet stoves) in private households can be disposed of with an official fee bag of the waste associations via the refuse collection. Care must be taken to ensure that the ash has cooled down completely (fire hazard). Even with smaller quantities of ash, cooling can take several days. The ash must be packed dust-tight so that no ash dust can escape during transport.

Large amounts of ash: Commercial furnaces (larger than 50 kW) can have their ashes extracted and professionally disposed of by specialized companies directly at the furnace. The extraction with special vehicles has proven itself according to the state of the art. Ideally, the wood ashes are temporarily stored at the plant in closed containers, skips or silos. The wood ashes are extracted dust-free from the ash containers by special vehicles via the suction nozzle of the vehicle or via permanently installed lines/hoses into the pressure tank of the truck.

Dust free ash deposition

Dust-free ash disposal can be technically guaranteed if the entire disposal chain from ash storage to and including installation in the landfill body can be carried out continuously and with few interfaces and adapted equipment. Without suitable measures, the unloading of wood ashes at the landfill repeatedly led to undesirable dust formation. A solution to this problem can be found at the Cholwald landfill in Ennetmoos. There, the wood ashes are mixed with water before being spread on the landfill body. This is done in a specially developed apparatus. An auger conveys the wood ashes from the delivery container, spraying them with water at the same time. This creates a moist mixture that can be spread on the landfill without emitting dust. The wood ashes are delivered in a container, which can be connected directly and airtight to the mixing unit. The mixing unit is built on wheels; it can be easily moved within the landfill.

Dust-free unloading of wood ash at the Cholwald landfill. (Image: A. Keel)

Recycling options

As mentioned, ashes are considered waste and are subject to the VVEA and the LRV. However, the ordinance also describes that material recovery is preferable to landfilling in the waste hierarchy. At present, only very few recovery paths are known. One recycling option has been developed and implemented by the company LogBau AG in Maienfeld together with the OST University of Applied Sciences. Wood ashes (grate ashes from forest wood) together with gravel sludge are processed into an earth concrete.

In general, the requirements for possible utilization of wood ashes can be summarized as follows:

• It must be possible to store and dispose of different types of ash separately.
• The ash quality must be continuously ensured. The individual requirements are determined by the recyclers.
• Suitable are usually only grate and bed ashes from the combustion of natural forest wood.
• A TOC content as low as possible is an important prerequisite.
• The ashes must be stored in a dry place.
• Foreign matter and contamination of the ashes must be avoided.
• It should be possible to provide the ashes in as large and uniform quantities as possible.