RP18 as a Filter case study
More than just an adsorbent fabric, RP18's special design qualities make it a good filter too. The construction of RP18 is such that there is a large amount of free space which allows water to pass through whilst retaining the oil.
This has been illustrated also in 'A method for Cleaning Foreshores' case study.
This fabric is useful where water flows are high and the amount of oil retained is low. If the fabric is used as a full flow filter care must be taken in the design of unit to ensure that flow through the fabric is not enough to cause captured oil to migrate through the filter element. A large area with very low flow is ideal and the thickness of the element will determine the time between renewals. Because the fabric is white the captured oil causes a brown stain as it percolates through the fabric, it is therefore easy to see when the element is fully loaded with oil requires replacement.
This picture shows RP18 as a filter in a drainage outlet alongside a locomotive works. Tests showed that the oil discharged into the freshwater streams was reduced from 50 ppm to less than 10 ppm using this method of oil pollution prevention. The RP18 fabric was changed every 10 days depending on flow rates. Fabric on final weir was replaced every 4 weeks - used purely as a final polish.
- At 1 cm head - 0.141cc/sq.cm/sec
- At 10cm head - 0.550cc/sq.cm/sec
Fabric loaded with oil at 0.55cc/sq.cm
- At 1 cm head - 0.027cc/sq.cm/sec
- At 10cm head - 0.186cc/sq.cm/sec
In order to prevent carry over of retained oil it is best not to exceed a flow density with a head of 10 cm of 330 litres/min/sq.m.
When the quantity of oil retained reaches 0.8cc/sq.cm the flow rate should not exceed 100 litres/min/sq.m.
The above parameters refer to a single thickness of 9mm nominal, the number of layers do not significantly affect the overall performance.