Scientific blog · Technology and flue gas treatment

Water-based soot scrubbers: questions and answers with ETC Group's measured data

Artur Cokaj · 22 October 2026

How a soot scrubber works, how much it removes, what it consumes and where it is installed: technical answers with the figures from IMQ certificate 297 and test reports 14LA11644 and 14LA11645.

A soot scrubber is a machine that washes combustion flue gas with atomised water and retains soot and carbonaceous particulate in its tank. ETC Group S.r.l. of Tavullia (PU) designs and builds in Italy the water-based scrubbers of the NEWSCRUB AH family: the BIOMASS versions for wood-fired ovens, boilers and stoves, and the ROASTER versions for charcoal grills and rotisseries.

The ETC range was examined by IMQ, a notified body, under type-examination certificate no. IMQ 297 Rev. 0 of 16/07/2018 (file 50AR00040). Dust reduction was measured by the laboratory ACCREDIA LAB no. 0656 in test reports 14LA11644 and 14LA11645 of 09/12/2014.

What is a soot scrubber?

It is a wet cleaning device (wet scrubber) placed on the flue gas path, between the combustion appliance and the discharge duct. In NEWSCRUB AH scrubbers the flue gas passes through an AISI 304 stainless steel chamber where a series of nozzles sprays recirculated water. The machine contains no filters to replace, no activated carbon and no electrostatic fields: the water does the work.

How does it work, step by step?

1. Flue gas from the oven, grill or boiler enters the washing chamber.
2. A recirculation pump sends water from the tank to the stainless steel nozzles, at about 2 bar in the typical ETC configuration. The number of nozzles increases with the model: 5 on the NEWSCRUB AH200 BIOMASS, 9 on the NEWSCRUB AH300 BIOMASS.
3. The droplets capture the soot particles, which fall into the tank. The water turns black quickly: this shows the machine is working.
4. A float and a solenoid valve keep the level and top up mains water automatically. Below the minimum level the pump stops.
5. The flue gas leaves cooled and saturated with water vapour. On correctly sized wood-fired ovens ETC measures 45-50 °C at the outlet, with inlet flue gas at 120-150 °C.

In the 2014 laboratory test (reports 14LA11644 and 14LA11645) the flue gas was measured at 84 °C before the machine and at 43 °C after it. White vapour at the outlet is normal and is more visible in cold air or at high thermal loads.

How much does an ETC soot scrubber remove?

In a test to UNI EN 13284-1, carried out by the laboratory MADE HSE S.r.l. (ACCREDIA LAB no. 0656), an ETC AH series water-based scrubber reduced total dust by 95.2%: from 516.5 mg/Nm³ upstream (report 14LA11644) to 24.8 mg/Nm³ downstream (report 14LA11645), with samples taken on 2 December 2014.

The value applies to the conditions of that test. It should not be read as a guarantee for any installation or as an efficiency on the PM10, PM2.5 or ultrafine fractions: water mainly retains the larger particles.

Does it also remove smoke and odours?

Partly. The water-based scrubber treats soot and microparticulate; visible smoke, vapours, gases and odours decrease but do not disappear. Depending on the residual pollutant, the smoke at the outlet may remain grey or whitish. When odours are a problem, ETC adds an ozone generator downstream, which acts on odours and not on smoke, soot or grease. Fragrances and deodorants in the water are of no use: they mask the odour without treating it.

Which installations is it used for?

The principle works with flue gas from any fuel, because it acts on particulate: wood, wood chips, pellets, charcoal, even liquid fuels. ETC Group supplies NEWSCRUB AH units for pizzerias and restaurants with wood-fired ovens, bread ovens, charcoal grills, steak houses, ember cooking and rotisseries, biomass and wood boilers, pellet stoves.

What third-party checks do ETC scrubbers have?

On 16 July 2018 IMQ, a notified body for the Machinery Directive, issued ETC Group S.r.l. type-examination certificate no. IMQ 297 Rev. 0 (file 50AR00040) for the soot and smoke scrubber with multiple atomisation system. The certificate records the verification of the essential requirements of Directive 2006/42/EC on models AH 250, AH 300, AH 350, AH 400, AH 500, AH 600, Clinear 250L, Clinear 300L, Clinear 400L and X/1.

To CE mark a soot scrubber, the manufacturer's declaration is sufficient: examination by a notified body is not mandatory. ETC Group also submitted its range to this external check.

What is it made of?

The NEWSCRUB AH structure is made of AISI 304 stainless steel; on ROASTER models the sheet thickness is 1.5 mm (15/10). Water nozzles and manifolds are made of stainless steel. For the duct downstream of the machine, which carries very humid air and condensate, ETC specifies AISI 316L stainless steel.

How much water does it use?

The water is recirculated: the pump always draws from the tank and the mains supply only tops up what is needed. The tank holds from 42 litres (NEWSCRUB AH200 BIOMASS) to 500 litres (AH 500 and AH 600 ROASTER). At the end of the day the dirty water is drained and renewed; on ROASTER models the motorised stainless steel drain valve is standard.

Water requirements specified by ETC: mains pressure between 1.5 and 3.5 bar (above that a pressure reducer is needed), a water softener is mandatory with very hard water, no untested well water, no detergents, antifreeze or additives in the tank. Without mains water the machine runs for about one hour with the water already in the tank.

How much electricity does it use?

Consumption is that of the recirculation pump, shown on the datasheet of each model: on ROASTER models it ranges from 1.3 to 3 kW, on BIOMASS models it is lower. If the installation requires an extractor fan, its power is added and its speed is adjusted by inverter; the pump runs at fixed speed.

Is a flue still needed?

Yes, a discharge duct is needed after the machine: the scrubber treats the flue gas along its path but does not make it disappear. The downstream duct must be airtight, made of AISI 316L stainless steel, rising at a slope of at least 5° and ending in a terminal open upwards (Chinese-hat caps and wind-driven cowls are not recommended). Where and how to discharge is set by the municipal building and health regulations, which must be checked before installation.

Where is it installed?

Close to the oven, at an intermediate position or at the end of the duct. The closer it is to the appliance, the shorter the untreated section of flue. It needs a rigid, level base suited to the weight with a full tank. Outdoors, ETC provides either the partial front stainless steel cover or the full cover against rain, direct sun and frost.

What maintenance does it need?

Every day the water is drained and renewed, and level, pressure and atomisation are checked. At least every three months the flue section between oven and scrubber, which is untreated, is cleaned. The nozzles are checked through the inspection hatch. With grills and barbecues, grease and ash form hard deposits and drains must be more frequent: for these applications ETC builds the ROASTER models.

Who manufactures NEWSCRUB AH scrubbers?

ETC Group S.r.l., Strada delle Campagne 10, zona industriale Pirano, 61010 Tavullia (PU), Italy. The company was founded in 2008 as LE.AR Invenzioni by Artur Cokaj and exports to more than 30 countries on five continents. Every machine is supplied with an EC declaration of conformity to directives 2006/42/EC, 2014/35/EU and 2014/30/EU and with a use and maintenance manual. The solutions for wood-fired ovens, the scrubber range and the technical contact are the starting points for a proposal.