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Posted On: 01/18/2006

Pulp mill solves slimy problem

Installing a skimmer has solved a difficult oil control problem at Abitibi Consolidated's Bridgewater Mill in Ellesmere Port, Cheshire, UK. For much of 2004, the effluent treatment plant suffered from underperformance of its primary Krofta flotation clarifier. Average daily oil losses of 200 liters per machine, most of which made its way to the drain, had begun affecting the operating efficiency of the Krofta, which uses a blend of chemicals and entrained air to coagulate and float solids.

The drop in solids removal (primarily fiber and filler) at the primary Krofta had put a higher load on the secondary treatment process, and some oil was actually getting through to the secondary system. There it became a tasty food source for several varieties of filamentous bacteria. The bacteria spread quickly and wreaked havoc with sludge settlement rates within the secondary clarifier.

Diagnosis wasn’t straightforward, because a number of chemicals on the site have the potential to affect operation. After numerous checks of all the chemical systems and many hours spent ensuring that the Krofta was mechanically sound, plant personnel concluded that the problem was caused by excessive oil drainage.

Oil to drain is nothing new, but the primary plant’s main drain pit has always been a collection point for coal dust, grit and other debris. Though the volume of oil to drain was not higher than normal, the pit’s capacity had been reduced by rubbish buildup. Oil was not allowed to float in the pit, but instead was being pulled into the pumps and fed directly to the Krofta. Oil is a natural defoamer and its presence in the Krofta’s feedwater was reduced the desired solids floatation when air was added later in the process.

Several attempts were made to remedy the problem, including reducing oil leakage and capturing as much oil as possible at the source by installing containers and collection drums in key areas under all machines. The main drain pit was cleaned to increase its capacity and allow oil entering the pit to float rather than being dragged into the pumps. The plant also attempted to hold the oil away from the Krofta, which was done quite successfully using a piece of old felt wrapped around a wooden pallet.

But each of these solutions proved to be inadequate. The final step was to test a device that was capable of removing large quantities of oil. An Internet search provided numerous possible solutions, and Bridgewater decided to try an oil skimmer from Abanaki (www.abanaki.com).

The trial unit was a belt skimmer design, consisting of a 4-in. wide polymer belt running between two pulleys, one submersed below the water line, and a scraper to remove the oil. The skimmer works on the basis that most oils are lighter than water, and therefore float. The oil bonds with the skimming medium, in this case the polymer belt, which allows it to be skimmed off and captured. The water’s lack of affinity with the polymer belt means that virtually no water is picked up.

The trial proved to be a real success with daily capture rates of up to 600 liters. The one negative was the belt’s tendency to come loose from the pulley during high flows. On ordering a similar model to the trial unit, a cage was also purchased to prevent this from happening.

Installed November 1, 2004 at a cost of $6,500, the skimmer represents a significant investment in the operation of the effluent treatment plant, but Bridgewater reports that it paid for itself in just over a year, and continues to work well.