Real-time TOC tightens effluent control

17 April 2012

The installation of a Total Organic Carbon (TOC) monitor has improved the wastewater treatment process for Greencore Foods, a UK-based food manufacturer whose effluent can contain an array of inorganic salts and organic components which enter the waste water stream.

The TOC monitor in-situ at Greencore Food
The TOC monitor in-situ at Greencore Food

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 10µl and sample pre-filtering is necessary to avoid particulate blockage of tubing and multi-port valves such that particle size is restricted to <200µm. In a process environment, clogging of tubes with crystallised salts and particulates can demand instrument re-calibration every 2-3 days.

New monitoring technology
To achieve continuous, function in harsh process environments the BioTector from HACH LANGE uses an innovative advanced oxidation process to offer aggressive breakdown of organics without thermal dependence. This is achieved by exposing high pH reagents to ozone. Highly corrosive unstable hydroxyl radicals are generated and by a two stage advanced oxidation this enables the system to handle very large sample volumes without the need for syringe controlled dilution mechanisms. A self cleaning facility with micro-bubbles prevents particles becoming trapped and an inbuilt salt trap allows salt loads as high as 30% and calcium loads of up to 12% without dilution. Without the temperature resistant ceramic components of contemporary systems where salt can easily crystallise and cause blockage, the catalyst within the BioTector is attached to the exterior of the furnace, extending lifetime and facilitating easy replacement.

Unaffected by fat, grease,, salt, and particulates up to 2mm diameter, a representative sample is achieved without pre-filtering allowing in excess of 80,000 continuous measurements per year. Maintenance requirements are minimal. The overall result is a 99+% uptime for the end user.

Veolia’s continuous improvement manager, Marcus Hardiker, explains more about the project: “All previous concerns regarding the use of an on-line TOC meter were addressed with practical solutions. The instrument takes samples from the raw effluent that is fed to the site’s treatment plant. However, we needed the sample to be as representative as possible and therefore no in-line filters were installed in the sample line. The sampling arrangement was carefully engineered by the team to deal with the site’s unique type of effluent, containing high sand and silt content.”

The signals from the BioTector unit have been integrated into both Veolia Water's effluent plant control system and Greencore’s own data acquisition system. Veolia uses the analysis to control the strength of feed transferred to the site effluent treatment plant. Any loads that exceed a pre-determined level are diverted to the site calamity tank. This control has enabled the performance of the effluent treatment plant to be greatly improved.

The signals from the TOC meter are also transmitted to Greencore’s production areas, and the on-line data is visible for process personnel who can now see the effect that their activities are having on the effluent loads being discharged to the site treatment plant. Greencore has also linked the on-line results provided by the BioTector to an alarm system so that production personnel know when the TOC levels have increased, enabling them to react more quickly and thereby identify and address any issues as they arise.

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