Many old vans are fitted with SOG cassette toilet systems by previous owners, but unlike the standard cassettes they are not maintenance free, so if you have one it is important to know how they work.
How it works is this. On a non sog system - ie the original Thetford unmodified system, when you slide the cassette into the toilet, an air vent on the cassette is depressed which allows air to flow in and out. This is to allow equalisation of pressure as you go up and down mountains - without this, there could be unwanted venting of air when the flap is opened after a change in altitude. The blue chemical is used to control odour.
On a sog system, the cassette is modified to disable this vent to make it permanently closed. Then a hole is drilled in the outlet pipe, and a tube is attached, which runs to an extractor fan, and then to the filter panel mounted in the door. On some vans there is a conical filter rather than a door panel filter. This filter contains a charcoal filter, which is supposed to clean the air so that it doesn't smell when expelled to the outside. This filter is a consumable and should be changed regularly - how often depends on how much it is used, but for an average holiday van - once a year is generally about right.
When you open the flap on a sog system, the fan is activated, by the same switch that activates the pump, and draws air down through the main flap opening - ie where you poo, and out through the pipe and fan and filter to the outside. This prevents foul air from rising up from the contents into the bathroom. When the flap is closed, the fan cuts out, but the cassette contents are properly vented because of the pipe, and therefore safe during altitude changes.
A properly installed and maintained sog system does not need the blue chemical, but with the caveat that when you empty the cassette, it does tend to pong more than if there was blue chemical added. Basically it is more eco friendly - so the theory goes.
When emptying the cassette, the procedure is to pull out the air tube from the cassette, and plug the hole with a plug which should be stored in the toilet compartment. It is most important that you don't mislay this plug, in fact it is best to keep a spare, they have an annoying habit of coming out and disappearing down the disposal hole.
The potential problems with sog systems - especially those fitted in older vans are as follows. 1. A previous owner has replaced the cassette with a new one, not knowing that the cassette has to be modified. 2. Over the years the wiring fails and the fan stops operating. 3. The charcoal filter never gets replaced, resulting in foul smelling air outside. 4. The procedure for removing and emptying the cassette is slightly more complex because you have to unplug the air pipe, plug the hole, empty, then the same procedure in reverse. You generally get a bit of a dribble which you have to be willing to clean. A pair of gloves and a packet of baby wipes stored in the toilet compartment makes the whole procedure as pleasant as possible.
In my experience of 20 years, and I am now a full timer, the sog system works well, and for full timing can save you around £20 a month in blue chemical. But even with all this technology installed it can still tend to pong a bit in hot weather, and I have found myself still using a bit of chemical now and then - I also use the cheap washing machine capsules from Lidl. However we tend to do a lot of wildcamping and use the cassette to the max quite often. If you use sites nearly all the time, where you can empty the cassette regularly before it gets full, then this problem is minimised. I also think that for heavy users, a new cassette every year is a good investment - a well used cassette, several years old, will eventually pong regardless of how much you rinse it out. My advice to buyers of old vans is always to buy a new cassette!
However, because there are a lot more components to the sog system, it does tend to need to be kept in working order, and to do this you need to know how it works. So for this reason I believe that for normal holiday use in the northern european climate, the case for a sog system is 50/50. But if you are an eco / anti chemical warrior, then sog is the only way to go.
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