Sampling high salt effluent

The process produces two types of effluent: highly organic wastewaters, removed by tankers for treatment; and less organic effluent with a very high salts content which can be discharged to the main sewer under consent from North West Water which requires Octel to monitor effluent oxygen demand levels. To sample the effluent round the clock, Octel installed an Ionics Model 7800 TOD (Total Oxygen Demand) analyser, which employs high temperature catalytic oxidation.

It was originally envisaged that the effluent from the plant would contain 2% salts by weight – a high level, but one with which the standard Ionics TOD could cope. During pilot trials however, it was found that the actual process would produce nearly ten times this amount – about 9% NaCl plus 9% NaBr by weight – which would adversely affect standard analyser components.

Octel and Ionics worked closely together to develop a unit to cope with the high salt levels, resulting, over time, in several steps. Firstly, the catalyst temperature was reduced to a level below the melting point of the salt, yet sufficiently high to provide effective catalytic oxidation; a 10:1 dilution was maintained, achieved by a specially designed automated dilution system with hastelloy valves and special sample chambers; a three stage rinse-cycle was introduced to flush out any salt remaining from the previous sample; the catalyst’s silica wool plug, prone to rapid deterioration by the salts, was replaced by a flat mesh plate of aluminium; a more robust sample valve with a spring-loaded syringe was provided, which forcibly injected the sample and provided consistent results; finally, a ceramic reaction tube was substituted for the original hastelloy one.

Over an eight week ‘concentrated’ trial (equivalent to four months in practice), the modified analyser produced a continuous run of ‘sensible’ results, and since its installation in April 1996, the analyser has continuously monitored the high salts effluent stream with a downtime of under two weeks. Servicing is carried out every three months.

Further amendments have been made to the analyser recently during a period when the effluent changed to include some solids. Octel replaced the standard stack filter system with a backflush filter which it is planned to retain.