Filtra Systems

Using a Walnut Shell Filter as an RO Protection Filter

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Courtesy of Filtra Systems

The purity of boiler feed water can be a critical part of the manufacturing process. This water needs to be free from contaminants in order for the boiler to operate correctly, without frequent shutdown for maintenance. For this reason, a membrane filtration train is frequently used. Unfortunately, the exposure of plant water to the manufacturing process means that the feed water can be contaminated with both high levels of oils and solid material. Frequently, the membrane filtration will need an upstream filter, to extend the life of the filters and protect membranes.

The STiR walnut shell filter was selected by a global tire manufacturer to solve issues related to its’ membrane filtration train. This train includes reverse osmosis and nanofiltration, and was made unviable by high levels of oil in its’ boiler feed water. Oil levels were high enough that the train was taken offline until a solution could be found, and the recirculated water could not be used in the boiler.

Walnut shell media was selected because of its’ ability to remove free oils and greases without causing damage to the media bed. Sand filters and carbon filters may blind prematurely when exposed to free oils, necessitating frequent bed changes. Walnut shells catch oil droplets on their surface, which are easily removed during backwash cycles. This means that a walnut shell media bed does not need to be replaced when exposed to oil, and has a lifespan of approximately 20-25 years.

The walnut shell filter was able to protect the RO system and nanofiltration membranes, significantly reducing the oil load to them. The free oil concentration were reduced from as high as 85 ppm, to less than 1 ppm. This reduced frequency of membrane fouling, reducing the total purchasing cost. It also reduced the maintenance requirement of the membrane filters, while protecting the RO system from oil slugs which had caused it to rapidly foul. Additionally, water heating costs were reduced, as water could now be sent directly back to the boiler.

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