Process Design Considerations

It is important to appreciate that a media system, because it is a chemical process, will remove all of the hydrocarbons in a waste water stream that it sees, depending upon the residence time, or flux rate, through the bed. These contaminants include both dispersed oil and dissolved hydrocarbons such as BTEX, Phenols and other PAHs. For this reason it is important to have a clear understanding of the levels of total hydrocarbons to be expected in the stream. Whilst it might appear from infra-red analysis that the OIW content is down to, say, 30 ppm at the inlet to the bed, it may well be the case that there is a further 50 or 60 ppm of dissolved hydrocarbons present. These must be accounted for in sizing a unit as the media may well absorb these fractions preferentially to the dispersed hydrocarbons. Indeed, there is a body of opinion, particularly in Norway, that it is these lighter fractions that are more carcinogenic and represent a greater threat to the food chain.

Cartridge or bulk media system?

Media can be provided in either cartridges (canisters) or as free flowing bulk material in large sacks. OPS offers cartridge systems for small flow rate applications, generally at temporary installation sites, but has standardised on bulk bed systems for larger flow rates.

Some of the considerations are as follows:

In most cartridge systems the vessel contains the contaminated water which then flows radially into the cartridge and out of the vessel through some form of header assembly. When cartridges are changed the interior of the vessel and the exterior of the cartridge are coated with hydrocarbons. Personnel who change out the cartridges by hand are, therefore exposed to potential health risks.

Radial cartridge systems rely on an initial period of soak to allow the media to expand (the cartridges are not filled to the top). If this procedure is not properly followed, or if the volume/swell characteristics are not very accurate, there is the possibility for flow to bypass the media completely with consequent reduction in cleaning performance.

All cartridge systems rely on an arrangement of seals between the cartridge exteriors and interiors when they are inserted. Any disturbance of any one of these many seals will lead to a system by-pass. There may also be temperature limitations on the types of plastic typically used for the construction of cartridges - softening can also lead to loss of seal.

Bulk systems can be designed to accommodate a wide variety of flow, pressure, temperature and oil removal conditions and generally are sized for convenience of scheduling change-out. They lend themselves to greater automation and process control if desired.

In short, canister/cartridge systems can be suitable and economic for small flows where the quantity of oil to be removed is not great.

Bulk bed vessel design. The media bed is generally used in an upflow configuration to allow the fluid to come into contact with as much of the surface area of the media as possible. This is NOT a filter system in the conventional sense of a sand filter used to remove solids. This configuration is also more tolerant of any solids entering the bed and is less likely to compress or pack than a downflow system. The vessels should be designed with sufficient headroom above the bed to allow the media to swell as it absorbs oil and grease – typically this could be as much as 20 to 30%.

Absorption factor. This is the measure of the media’s ability to absorb oil expressed as a ratio to the weight of clean, dry media. For RM25 media the absorption factor varies from as low as 80% to as high as 150%. The absorption factor is a function of the residence time, temperature and the manner in which hydrocarbons are found in the water: free, emulsified or dissolved. The stronger the emulsion or the higher the dissolved hydrocarbon content, the greater will be the residence time required to achieve acceptable absorption levels.

For design purposes OPS assumes that the media is unlikely to absorb more than 100% of its own weight, particularly if the residence time is relatively short.

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