How to prevent chemical contamination of waterways when preparing a bridge for coating.
How to prevent contamination of waterways when coating a bridge
Bridge structures over waterways are very common in New Zealand – we have over 15,000 of them crossing our 425,000 kilometres of rivers and streams. The fact that maintenance work has the potential to allow chemicals into the waterways hasn’t been understood historically as the major issue that it is.
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In this article we discuss ways to prepare a bridge for re-coating maintenance whilst minimising any impact on the environment.
First up, what’s the problem?
Steel and timber bridges that have had no surface treatment for the past 6+ months will most likely have lichen, moss or similar firmly attached to the surface – even if it’s not apparent to the naked eye. A paint coating can never be applied over these live spores without jeopardising the life of the coating – so the lichen must be killed. Treating the lichen requires a chemical of some sort to be applied to the bridge structure – meaning there is always a risk of getting this chemical in the waterway beneath.
How does the chemical actually find its way into the water?
There are two common ways the chemical can enter the waterways, so let’s look at each of these problems in turn, along with methods you can use to control them:
Problem 1: Spills. Whilst the chemical will usually be applied in a diluted state, this doesn’t water down the problem. Whenever there is liquid chemical present, there is the chance of spill. There are two ways to minimise the chance and effect of chemical spills:
Using Bunding to establish a designated isolation zone away from waterway but near the work site is the all-important first step. All pouring/diluting must be done in this protected zone.
Pre-dilution is an additional step, where the chemical is diluted to the correct application ratio off site. Removing the concentrated chemical from the work site eliminates the dilution/decanting process which lessens the amount of handling and hence reduces the chance of a spill. In the event of a spill, the diluted chemical has a far smaller effect.
Problem 2: Chemical run-off. Often after the chemical is applied it is water blasted off the surface. This can result in the chemical being washing into the waterway, albeit in a far more diluted state. There are two methods to prevent chemical run-off contamination.
The Apply-and-dry method requires the surfaces to be water blasted prior to applying the chemical. Apply-and-dry moss killer is formulated to allow paint coatings to be applied once the chemical has dried on the surface, preventing the chemical ever entering the waterway. Note the chemical must be specifically formulated for this purpose.
The second option, Containment, uses an aluminium frame covered in fibre mesh (Fibre Containment System or similar) which typically hangs off the structure. A tray in the bottom of the frame catches the water/chemical which is then removed with suction pump.With both methods above, using a Brush/Roll application method, rather than spray application, will prevent airborne aerosol from reaching the waterway.
Why not encapsulate the whole bridge?
When it is possible, fully scaffolding, encapsulating, and draining the zone into filtered tank system - which is taken away from site for disposal - is the very best option for containment of chemical and debris. However in many cases the full encapsulation costs are prohibitively expensive, often dwarfing the cost of the proposed coating maintenance works. Structural weight limitations on the structure, as well as river levels and increased wind loading, can also prevent the use of this option. Typically it is reserved for full sandblast situations or projects with significant structural work.
Final note
The most important aspect for engineers, consultants, council teams and related parties to consider is ensuring the contractor has a clear methodology around containment. This could be following 2 or more of the methods outlined above, or something else.Also note each Regional Council has varying rules and guidelines around containment, and its always advisable to seek guidance from them prior to proceeding with project planning.
