Desalination by Captive-Head Washing (BlueVac)

Addendum 2018

After several years of using the CHW System, test results have proven that the System works well to lower Salt content in porous substrate and is now recommended by reknowned Heritage Architects in Australia and New Zealand. Test resoult can be made available upon request.

See article below 

Fabric Conservation Report

A Simple Trial Result by David Young

Captive-head washing is a system designed for cleaning dirt and grime from building façades in which the dirty wash water is captured by a wet vacuum cleaner (BlueVac), thus minimising clean-up and waste disposal issues. Picture below shows the head in use, cleaning dust and dirt from a brick wall prior to render repairs. The head contains a low pressure spray nozzle which is connected to a water supply. A flexible ‘skirt’ encloses the head and seals the unit against the wall surface so that the attached wet vacuum-cleaner recovers almost all of the wash water (Captive Head Tool).
The system’s potential for reducing salt loads has been recognised for some years (Young, 2008) (ref: Salt Attack by David Young) and anecdotal evidence suggests that it works well enough to justify its on-going use, yet there is little data to supports this. Trials were conducted during the 2014 Longford Academy, a combined training and fieldwork program organised by the Australasian Chapter of the Association for Preservation Technology International (APTI) and located at the Brickendon and Woolmers World Heritage properties at Longford, Tasmania.

Captive-head washing in use 

Results and discussion

The first pass extracted 81.6 g, the second 27.8 g making a total of 109.4 g of salt, extracted from a wall area of 8.8 m2, at an average of 12.4 g/m2. Deriving a weight percent salt extraction depends on two assumptions:

• the depth of effective extraction in mm; and

• the density of the brickwork, which is assumed to be 2.0 g/cc (kg/L).

Assuming that the depth of effective extraction is one mm into the brickwork, the average salt extraction is around 0.6% by weight.

This is a high figure; it is more than the commonly used 0.5% threshold above which salt extraction is warranted. Alternatively, if the effective depth of extraction is two mm, then the salt extracted is 0.3% by weight, and if the extraction depth is three mm, the figure becomes 0.2% by weight of the brickwork.

Any of these results is a good outcome; substantial salt has been removed from the wall.

The amount of salt extracted in the second pass was about one third that of the first pass. However, less water was used in the second pass (15 L instead of 25 L in the first pass), which indicates that the second pass was faster, assuming the water supply rate remains constant. Allowing for the differences in water used, the second pass extracted about 57% of the salt extracted in the first pass for the same rate of passing (i.e. dwell time as measured by water consumption). These results suggest that a third, or even fourth, pass may be effective at salt extraction, and suggest that slower passes extract more salt.

 

Conclusions

Captive-head washing has proven to be effective at desalinating porous masonry walls with high salt loads. Slower passes across the wall surface will extract more salt. Future trials should test:

• three or four passes across the surface;

• whether time should be allowed for deeper salts to come the surface;

• varying speeds of passing across the surface (i.e. dwell times);

• different substrates and salt loads, and

• different nozzle flow rates, in order to optimise the technique.

 

Reference

Young, David. 2008. Salt attack and rising damp: a guide to salt damp in historic and older buildings. Heritage Council of NSW, South Australian Department for Environment and Heritage, Adelaide City Council, Heritage Victoria, Melbourne.

 

About David Young

David Young

David Young OAM is a heritage consultant specialising in building materials conservation. With a background in geology he has been involved in the diagnosis and repair of historic buildings and sites for over thirty-five years, with a particular focus on those of stone. He also undertakes a range of teaching and training activities and has run short courses on building conservation in Australia, New Zealand and South Africa. He was convenor of the Australia ICOMOS working group that undertook major revisions to the Burra Charter in 1995–1999.
David is the author of the technical guide, Salt Attack and Rising Damp: a guide to salt damp in historic and older buildings, and the forthcoming Mortars: materials, mixes and methods — a guide to repointing mortar joints in older buildings which will be published by all State heritage agencies.

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