Rock Dryers pollution event
Hyder Consulting
Rock Dryers is an Imerys kaolin (china clay) dewatering and pressing plant east of the village of Bugle in Cornwall. Kaolin is introduced into the plant as slurry which is then progressively heated, dewatered and pressed in tube presses into the final product. Slurry water, containing a high proportion of suspended kaolin is recycled back into the plant or is collected in a filtrate sump from whence it enters an effluent treatment plant, where it is treated with caustic soda and a polymer to raise the pH and remove iron respectively. This is carried out in a series of settlement lagoons where a high proportion of the suspended kaolin settles out before the remaining water is treated and passed through oil traps before being discharged to the Rock Stream.
During 2013 a redundant tank was cleaned out and the remnants in the bottom of the tank were washed out into the sites’ effluent system and the settlement lagoons. This contained approximately 660 litres of the polymer Jayfloc 85, which it is believed would have bonded with the kaolin in the lagoons and been considerably diluted by the high flows during a period of prolonged wet weather at the time. However the Environment Agency subsequently reported a substantial fish kill along a reach of the Par River Par into which the Rock Stream eventually flows via the Rosevean Stream. At the same time South West Water’s Luxulyan Waste Water Treatment Works (WWTW) experienced problems due to the high flows, with the works discharging poorly treated effluent to the Par River. Further investigation found that one of five surface aerators at the works had been out of service for a considerable time.
Subsequently Imerys requested surveys of aquatic invertebrates in the Par River, Rock Stream and the Rosevean Stream, in order to ascertain their status and to identify any indications of impact on the fauna of the three watercourses which could be attributed to either the Rock Dryers discharges, the incident at Luxulyan WWTW, or a combination of the two. RICT analysis of the data enabled WFD ecological status categories to be assigned to each site and identified bad ecological quality within the Rock Stream and on the lower Rosevean Stream, downstream of the confluence of the Rock Stream. Relatively diverse communities were recorded along the Par, with little, difference in the status upstream and downstream of Luxulyan WWTW.