Inveresk's Somerset mill is the UK's sole manufacturer of resin-impregnated paper which is printed and used as the decorative surface for chipboard. The resins give rise to traces of ammonia which are discharged in the mill's effluent to the river Axe.
The river is designated as a salmonid fishery under EC legislation, and Inveresk has struggled for some years to ensure that its discharges do not give rise to breaches of the ammonia concentration limit laid down by these rules. According to the Environment Agency, the limit was exceeded downstream of the firm's discharge in three of the past four years.
In 1994, under pressure from the Agency's predecessor, the National Rivers Authority, Inveresk invested £0.75 million in a biological aerated filter to remove ammonia from the effluent. However, the plant failed to meet the guarantees promised by its supplier, Microbac, despite 12 months' effort to improve its performance by Microbac itself and a further six months' work by a consultancy. Inveresk has since won a civil claim against Microbac but subsequently found that the firm lacked the resources to pay it compensation.
Meanwhile, the Agency lost patience over the discharge after detecting ammonia at concentrations of 21.8, 15.1 and 8.4mg/l in three effluent samples taken late last year. The ammonia limit in the mill's consent is 5mg/l.
On 16 May, Inveresk pleaded guilty before Mendip magistrates to three charges of breaching its discharge consent, contrary to section 85(6) of the Water Resources Act 1991. It was fined £3,000 on each charge, and ordered to pay costs of £250.
Since the prosecution was initiated, Inveresk has completed improvements to the treatment plant which have enabled it to operate within its consent. Air compressors have been installed to improve the supply of oxygen to the process, and bacteria have been identified which degrade ammonia effectively even during periods of low temperature. The company has also addressed the problem at source - for example, by switching away from a wet strength agent which contributed part of the effluent's ammonia load.
Another more radical change has brought the mill close to the paper industry's Holy Grail of closing up its water and effluent system.
Inveresk's consent allows it to discharge almost 3,500m3 of effluent per day - close to half the flow of the Axe in dry periods. Until recently its discharge was around half this figure - but with a recent investment of £90,000 in a recirculation system it is now reusing most of its clarified effluent so that its discharges are no higher than about 600m3 per day, and zero for lengthy periods. According to a spokesman, the mill is aiming to achieve total effluent recirculation consistently, but because the plant operates on a batch basis this would require a large buffer lagoon to store effluent to match demand.