Continuous effluent analysis replaces batch testing

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Model HSA Indicator

McNab Model HSA color monitoring system was installed in the effluent stream from ion exchange columns, shutting down the process when corrective actions are necessary, and provides a record to prove regulatory compliance.

Problem

Manual batch-testing wash water for dichromate ions prior to sewer discharging was time consuming, high in labor costs and provided no control of the ion exchange process.

In the production of its zinc yellow, a pigment manufacturing company needed to remove the dichromate ions from its waste water prior to transferring the water to the sewers. To accomplish this pollution control, the company employed a series of ion exchange columns to provide the water treatment. To monitor the treatment process, manual batch sampling was performed six times per day.

When the batch sampling indicated an unacceptably high ion content, the entire batch of wash water had to either be recycled through the ion exchange columns or chemically treated to remove the dichromates.

Solution

A color monitoring system was installed in the effluent stream from the ion exchange columns. This system performs continuous monitoring, shuts down the process when corrective actions are necessary, and provides a record to prove regulatory compliance.

The monitoring system consists of a flow cell through which the effluent passes, a light source, a photoelectric scanner mounted directly opposite the light source and an indicator/control cabinet.

The light source is equipped with a blue filter to allow the photo-electric scanner to detect dichromate ions as they escape the exchange column. The electronic signal generated by the scanner is sent to the monitor/indicator box, where a continuous strip recorder logs the results. Also within the cabinet is a relay with two set-points; the lower set-point engages at I part-per-million (ppm) and the upper set-point at 5-10 ppm.

When ppm of the effluent stream reaches the initial set-point setting of 1 ppm, the first relay activates a valve to divert the stream through another ion exchange column to remove the excess dichromate. If the effluent reaches the 5-10 ppm set-point, the relay triggers an alarm and automatically shuts down the treatment process so that corrective action can be taken by plant personnel.

The implementation of this monitoring system has allowed the company to continuously monitor the effluent stream and eliminated the need for batch sampling, and the high costs associated therewith. The strip chart recorder also allows the company to provide proof that the discharge of dichromate ions into the sewer system is within acceptable levels.

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