Monitoring mill effluent

One of the largest paper mills of its type, Aylesford Newsprint's plant on the stretches of the River Medway broke a world record for newsprint production in December 2000.

Sold under the Renaissance brand name, its products are 100 per cent recycled and are used by Europe's leading newspaper publishers. The mill recycles about half a million tonnes of recovered newspapers and magazines a year to produce around 400 000 tonnes of premium grade newsprint.

With that level of production comes an equally high level of environmental responsibility. Paper production has been estimated as accounting for around 20 per cent of UK waste and, not surprisingly, mills such as Aylesford are regulated by the Environment Agency under the IPPC (Integrated Pollution Prevention and Control) regulations.

The importance of this aspect of paper production is emphasised by Aylesford's ceo Alan McKendrick, who lists the plant's environmental performance as a key factor in making the business a success. As part of the company's ethos of achieving the highest possible operational and environmental standards, Aylesford is accredited to ISO 14001 for its environmental management.

The water treatment plant at the mill deals with around 16 500m3/d of process water, mainly from the fibre preparation plant. Effluent from the mill is first filtered and then cooled before entering an activated sludge plant, from where it goes for clarification and discharge to the River Medway.

In the primary filtration treatment stage an aqueous solution of cationic polyacrylamide is injected prior to three parallel drum filters to agglomerate suspended solids and aid drainage. Separated solids are discharged via a chute into the mixed sludge sump, with filtered water being deposited in the primary treated water (PTE) sump.

The temperature of the PTE can be as high as 45 degrees C, which could inhibit the secondary treatment activity and cause the consent limit on final water temperature to be threatened. The PTE is therefore cooled to about 27 degrees C in a single-pass open evaporative cooling tower.

Secondary treatment

The secondary treatment plant consists of up to seven aeration tanks, fed with oxygen from either a LOX storage vessel or an on-site oxygen generator. Two clarifiers are used for separation of the activated sludge from the cleaned effluent, which is then discharged to the Medway. The retention time in the aeration section is about 10 hours and the selectors can be run anoxic or aerobic.

Under normal operating conditions, the aerobic plant removes approximately 85 per cent of the incoming COD and 98 per cent of the BOD, giving BOD figures of less than 10mg/l and CODs of about 250mg/l - levels that are considered normal for a de-inking mill producing newsprint. Suspended solids level is generally about 15mg/l.

There are no water recycling tertiary treatment stages at Aylesford but there is a drum filter available to treat solids carryover from the clarifiers if needed, although this is normally by-passed. Any solids separated at this stage are transferred into the mixed sludge sump.

The hard COD remaining in the final effluent is typically 90 per cent lignin with the remainder being extractives, consisting of chemicals such as waxes, resins and oils that normally come from adhesives attached to the recovered paper.

Typical discharge performances monitored at Aylesford are less than 20mg/l suspended solids and less than 10mg/l BOD, with daily volumes around 14 500m3 - all well within consent limits.

When it comes to choosing instrumentation to monitor these discharges there are a number of factors that come into play. For example, monitoring may be necessary for operational reasons, or for compliance purposes. Operational monitoring needs to demonstrate that genuinely valid measurements of water quality are being recorded in a manner that is consistent with accepted procedures. However, measurements to demonstrate compliance with a specific discharge consent should take place in the units and format of the consent.

For example, BOD is a common measurement of pollution load in discharge to controlled waters, and COD is a common measurement for discharges to sewers. This situation has not changed since the introduction of the IPPC Directive.

'Monitoring under IPPC follows similar principles to IPC [the earlier Integrated Pollution Control regulations],' says Tim James of the Environment Agency's PPC Policy Unit, 'except that the Agency has now defined standards for instrumentation in the new MCERTS scheme.'

MCERTS has already been established for air quality and ensures that any techniques used for monitoring are tested and approved by the relevant authorities before being accepted for compliance monitoring. The MCERTS scheme for water quality is still in its infancy, but will become the source of reference for all those involved with compliance monitoring.

The Environment Agency's technical manager for monitoring and assessment process, John Tipping, further says that the emphasis in the future will move toward self-monitoring [further information at www.mcerts.net].

For the past few years, Aylesford has used UV probes from Lange to measure organics in primary and final effluent streams. Prior to this, says the company's utilities process engineer Frank Holton, they had used traditional TOC monitors but these required a high level of maintenance and often failed. Lange's UVAS monitors, however, have proved themselves to be robust, self-cleaning and need no calibration.

The UVAS (available from the recently merged Hach-Lange company) measures UV light extinction (the proportion of light at 254nm absorbed by dissolved organics in the effluent) in situ by flashing two beams of light across an aperture in the probe housing. The effect of turbidity is compensated for by a reference measurement. The resultant reading is known as the spectral absorption coefficient, SAC, which has been shown to correlate well with COD or TOC measurements.

The UVAS probes can be connected to a multifunctional display unit, the Multi Unit Plus, from which the continuously measured and logged value is immediately available. Users can request hourly, daily, weekly or monthly progress reports, shown graphically or downloadable via an RS232 connection. Other Lange monitors, such as pH, turbidity, dissolved oxygen and sludge can be included to form a complete monitoring system.

Aylesford believes that 'from an operator's point of view, the UVAS system is simple to operate, reliable, robust and requires very little maintenance. It fulfils a vital role in the protection of the Medway.'