Real-time TOC tightens effluent control

A new continuous Total Organic Carbon (TOC) monitor has increased the capability of a food manufacturer to refine and improve its wastewater treatment process. As a result the plant is better able to quickly respond to changes in the influent.

The Greencore Foods site is operated on a continuous improvement basis with its own dedicated wastewater treatment plant to monitor and control effluent quality and minimise costly discharge fees. Its effluent can contain an array of inorganic salts and organic components which enter the waste stream in part from the intense wash down of processing tanks and lines during clean in place procedures for product changeover. Overload of organics on the effluent treatment plant will adversely affect the efficiency of the treatment process. If the waste is not treated to the required standard, the site would be unable to achieve the river discharge parameters set by the Environment Agency.

The effluent treatment plant at the facility is managed by a specialist team from Veolia Water Industrial Outsourcing. Raw feed water entering the facility, is analysed for TOC to control the strength of the feed transferred to the site effluent treatment plant.

Environment manager at Greencore, David Murtagh, takes up the story: “In the past samples were manually collected from the site drains pit and transported to the laboratory at a second location. Given the geographical distance between sites this led to significant waste, both in terms of time and resources. We had a small team in the laboratory who performed a number of analyses on the delivered samples, which was labour intensive and delayed results. Even state of the art laboratory techniques still took too long for pragmatic purposes, preventing prompt feedback on the process. It provided us with no direct information to control the effluent plant or provide an early warning of process issues. An alternative needed to be sought.

“A further key driver was the impending tightening of the consent to discharge to river by the Environment Agency. We needed more frequent, detailed, prompt, accurate and reliable influent TOC data to help manage the effluent treatment plant more efficiently and provide feedback on the process which is the source of the TOC. We were wary of on-line TOC instruments due to poor experiences in the past; while they worked well on cleaner, particulate-free effluent samples, the high solids, fats, oils and greases, and variable TOC loads coming from raw influent was a major issue for delicate fine capillary tubing and valves, which typically caused instrument seizure.”

Traditional methods for analysis of TOC/COD/BOD were developed as laboratory systems based on UV exposure, wet chemistry or more aggressive digestion by catalysed combustion. While these provide a good basis for spot sampling within the laboratory where sample pre-treatment is controllable, the systems do not often translate well in a real process environment.

Raw effluent presents a series of challenges. With UV based systems, high salt loads can cause scaling of the UV light column creating problems or organic recovery. In high temperature combustion systems, salt loads as low as 0.5% can inhibit the persulphate digestion mechanism and deteriorate the lifetime of the catalyst, demanding excessive maintenance by replacement of the column. Sample volume is commonly restricted to less than 10ul and sample pre-filtering is necessary to avoid particulate blockage of tubing and multi-port valves such that particle size is restricted to <200um. In a process environment, clogging of tubes with crystallised salts and particulates can demand instrument re-calibration every 2-3 days.