What's causing the green appearance?
The green colouring is often caused by algae, moss or other organic growth.
These organisms thrive where moisture remains on the surface. A driveway surrounded by trees, fencing or buildings may stay damp for much longer than one exposed to direct sunlight.
Leaves and other organic debris can also provide an ideal environment for growth.
Why does it return after cleaning?
Cleaning removes the existing growth, but it doesn't necessarily remove the conditions that encourage it.
For example, if a driveway is shaded for most of the day, it may remain damp even after it has been thoroughly cleaned.
This is why some driveways naturally require more frequent maintenance than others.
Is pressure washing enough?
Pressure washing can be very effective on suitable driveway surfaces, particularly when there is a significant build-up of dirt and organic contamination.
However, the cleaning process should be appropriate for the type and condition of the driveway.
Block paving, concrete and other materials can all respond differently to pressure.
In some circumstances, a suitable treatment can also form part of the cleaning process rather than relying entirely on mechanical pressure.
What about the joints between block paving?
Block paving can require additional care because cleaning may disturb some of the material between the blocks.
Depending on the condition of the paving, re-sanding may be appropriate after cleaning.
This is one of the reasons it's worth considering the driveway as a whole rather than simply concentrating on getting the surface as clean as possible.