Coping with inconsistencies
Nutrient dosing helps waste treatment plant tackle variable flowsReducing pollution and improving the environment following decades of industrialisation has been a priority for the Scottish Environment Protection Agency (SEPA). The fact that many rivers once again support salmon and trout is testimony to its efforts in recent years.
But eroding profits and the rising cost of production in industry sectors throughout Scotland, could see a reversal of SEPA's achievements without the cooperation of businesses that to some degree have been responsible for polluting Scotland's waterways.
Westfield, a paper manufacturer based in West Lothian, is seeking to minimise the environmental impact of its operations by trying to improve the river on which it depends to run the mill. With a current annual output of more than 37,000t of one-sided, wood-free paper products Westfield Mill returns more than 1,200m3 of treated water into the River Logie every day.
Before discharge by-products of the paper manufacturing process: clay, fibre, chemical additives and paper coating have to be removed from the waste stream. Most of the solid materials are taken to landfill after primary settlement and pressing on a belt press.
According to Westfield's technical manager, Mark Smith, a problem synonymous with paper mills is the variable flow rate, which can affect the performance of the effluent treatment plant. At Westfield treatment is based on primary clarification followed by an activated sludge system through which all the raw effluent from the paper mill is processed.
"The nature of the paper manufacturing industry means that coping with high and low flow rates is more the norm than rarity," said Smith. "It makes management of the effluent treatment plant very difficult. The performance of the activated sludge plant is significantly impaired when flow rates are varied. Too little flow of water and sludge restricts the biological activity of the bugs, while too much increases the hydraulic load on the final clarifier and the overall load to the plant. In an ideal world, with plenty of capital and space on site, a large buffering system would be installed to help maintain a continuous and steady flow."
Omex Environmental, a King's Lynn-based company specialising in microbiological industrial wastewater treatment devised a solution to the problem. According to the company's managing director, Ian Farr, a biological treatment plant cannot achieve its designed performance if the wastewater does not contain the correct proportions of nutrients and micronutrients for the growth of biomass.
"It was clear to us that in Westfield paper mill's case variable flow rates were causing havoc with the bugs' natural activity which was not being optimised. We advised adding Ferromex to the sludge for two reasons," said Farr. "Firstly, it contains a cocktail of essential nutrients for the bugs which ensures they always work at peak performance. This is particularly important during low flow rates when there is not enough food for the bugs to function effectively, as well as during high flow rates when the sludge settling properties of the Ferromex ensure the material does not get washed out through the end of the plant back into the river."
"At Westfield the waste comes from the paper mill and enters the primary clarifier where solids are removed and pressed before being taken to the landfill site. Overflow from the clarifier goes into the activated sludge plant, where Ferromex and Nutromex - a liquid nutrient providing N and P to optimise micronutrient supply - are added. A carousel design ensures a thorough mixing before entry into the final clarifier.
According to Farr: "The bugs need oxygen and nutrients such as phosphorus, carbon, and nitrogen which is supplied by the waste in the effluent as Nutromex, while the Ferromex provides other valuable food nutrients, aids flocculation, and improves the bugs activity and vigour."
The introduction of Ferromex was not straight-forward. Pressure to reduce chemical costs and problems with dosing pumps forced a reduction in the recommended rates of Ferromex from 40ppm to 10ppm. The result was no significant improvement in plant performance. Increasing the rates made a significant contribution towards a 40% reduction in chemical costs throughout the plant.
"Water quality prior to the Ferromex addition was still within our consent limit. However, since dosing to the recommended Ferromex levels the plant's performance has continued to improve and stabilise with the effect that we are very happy with the discharge water quality," said Mark Smith. "It is this stability that is of paramount importance to the running of the plant."
According to Farr, correct dosing in the long run is more cost-effective because altering the rates means that to correct the imbalance overcompensating is necessary. Associated problems do not always become apparent for sometime, by which time there is a tendency to blame something else for poor performance.