Hi,
We used Abanaki belt skimmers with the separator system. We did try poly belts and steel belts. You may find that the effectiveness depends on the coolant chemistry and the type of Tramp oil. It isn't an exact science, and we made a few mistakes.
How deep do you have the lower portion of the belt? Too shallow and you don't pick up enough oil and too deep you will wash off some of what you pick up. The oil wiper should be close enough to wipe oil from the belt but not to cause drag on the belt. We did have steel belts and had more than a few break.

Are you allowing the tank to settle - so there is zero agitation at the point of pickup? I realise this will need the machine to stop between shifts. We had non-stop operation and found a lot of oil that was picked up was partially emulsified and needed to settle before separation. It meant we lost a lot of coolant this way.

I am surprised your supplier has advised against adding a biocide. I wonder what they are advising you to do in this situation. Your problem is quite common. Don't forget bacteria can be aerobic and anaerobic so Oxygenation may not help. I didn't try UV light but I think it would work if you can light to travel through the fluid, not just the surface.

Is it possible to stop the machine long enough to take the sump from the machine and remove all traces of sludge? If you replace the coolant without cleaning the machine you will find the bacteria problems return. It was possible to lose a 1,000 litre sump in 1-2 days without cleaning. We found dead spots in the conveyor which needed scraping and pressure washing to get rid of the sludge. That sludge is soap, emulsified with oil and coolant emulsion - bacterial heaven. Also we found dead spots of swarf and coolant, like nests, under some of the machine covers. These also act as hot spots for bacteria. Our dip slide tests for bacteria were always taken in subsurface coolant - it depends on the depth of your tank/sump but you must be below the surface.
Our best results were from a 'mechanical' clean, meaning the scraping of sludge from surfaces, the brushing of all sheet metalwork, and wiping with a suitable oil and disinfectant ( a dedicated machine cleaner). Prior to a mechanical clean we always used a cleaning additive to flush the machine while in use. Machining performance was not affected but was in the machine for a day prior to removal and coolant disposal. We always lost a complete shift for this process.

Biocides and other additives (like Abanaki Coolant Mints) are a temporary solution.

If you need some information about what can go wrong with coolant and the consequences to people have a look on the HSE (Health and Safety Executive in the UK) website for a copy of the Longbridge Report. It was nearly 20 years ago that over 100 people were made ill by the poor maintenance of coolant in just one factory.

Also we are at an advantage in the UK with regard to water quality. All UK mains water is drinkable, therefore bacteria free. If your water isn't of that quality you have dimension to your problem that we don't have. You may have to start solving your problem in a different way.

The UK coolant market has been altered of late, mostly by the consequences of Longbridge. Larger coolant supply companies now offer integrated coolant maintenance contracts as part of the deal. In a small company you have to learn how to deal with your situation as you find it - it is difficult to be accurate with help as your situation is unique. In my view your supplier isn't doing enough to help you - you might be able to find one that does know how to help. In my old company we changed suppliers twice before we found someone who offered real help on an on-going basis. It was better commercially in the longer term as well.

Good Luck