Walnut Shell Filter Raises Flux for On-shore and Off-shore Produced Water Treatment
By Chris Catalanotto
Walnut shell filters have been used for many years to remove free oil and suspended solids from water in refineries and for onshore produced water processing. Typically, walnut shell filters are used as part of an overall treatment train as tertiary separation, following upstream oil and suspended solids removal technologies. The oil and solids removal treatment can include primary separators such as hydrocyclones, API separators, and plate separators and skimmers that are followed by secondary separators such as dissolved or induced air or gas flotation systems. In most walnut shell filters, water is introduced in a downward flow through a bed of walnut shells that absorb oil and filter suspended solids. Since walnut shellsﾗ especially black walnut shellsﾗ have an equal affinity for oil and water, oil can be captured on the surface of the shells and then scrubbed off during a backwash cycle, allowing them to be re-used. During forward-flow operations where oil and solids are being filtered, the system is typically pressurized, and the water is forced through a bed depth of walnut shells that is sufficient to achieve the desired performance. Once the pressure drop across the filter bed occurs, or a timed sequence is initiated, a backwash cycle physically removes the oil from the surface of the shells and dislodges any trapped suspended solids.
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