The acquisition of an oil skimmer materialized following lengthy periods of under performance from a primary Krofta flotation clarifier earlier in the year. Average daily oil losses of 200 liters per machine, the majority of which was making 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 resulted in a higher loading and increased strain on a secondary treatment process. On top of its' negative effect on the Krofta's operation, some oil was actually getting through to the secondary system, where it was an ideal food source for several varieties of filamentous bacteria. With such a plentiful food source, the bacteria can spread quickly and wreak havoc with sludge settlement rates within the secondary clarifier.
Diagnosing what was affecting the Krofta efficiency wasn't that straightforward, mainly because there are a number of chemicals on site that have the ability to affect operation. After numerous checks of all the chemical systems on site, and many hours spent ensuring that the Krofta was mechanically sound, the only option was to conclude that the problem was caused by excessive oil drainage.
Oil to drain is nothing new but the primary plant's main drain pit (which feeds the Krofta) has always been a collection point for coal dust, grit, oil, and other debris. Although the volume of oil to drain was not overly higher than normal, the pits capacity had been reduced somewhat with the gradual build-up of rubbish. The oil was therefore not being allowed to float in the pit but was instead being pulled into the pumps and fed direct to the Krofta. Oil being a natural defoamer and its presence in the Krofta's feed water, was counter productive towards achieving the desired solids floatation, when the air was added later in the process.
Several attempts were made to remedy this problem including eliminating or reducing oil leakage, and capturing as much oil as possible at the source, which meant installing containers and collection drums in the key areas of oil loss under all machines. Another solution was to clean the main drains pit to increase its capacity, thus allowing the oil to enter the pit, float, and remain on the surface, rather than being dragged into the pumps. Lastly, was to hold back or 'trap' the oil before it reached the Krofta, which was done quite successfully using a piece of old felt wrapped around a wooden pallet.
When 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, one of which was an oil skimmer.
The trial unit was a belt skimmer design, consisting of a 4' 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 oils disliking for water means it will quite happily bond 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 form the pulley during high flows. On ordering a similar model to the trial unit, a cage was also purchased to prevent this from happening.
The skimmer is currently working well. At the rate it's going, the skimmer should pay for itself in just over a year.