Hello Sarah and welcome, you found the right place! Congratulations with your new lottie, you now officially joined the ranks of allotmenteers.
My guess about the mysterious substances in the bins is that it's probably good stuff. Could be comfrey in water to make liquid fertilizer, or manure for the same purpose. It seems a bit too wet for making leaf mould but who knows?
If I were in your place, I would tip it all onto my lottie but....... I would not do it in an area where you next grow veggies that are eaten raw. If you have an area with flowers, like a border or something, that would be my first choice.
I wish you all the luck with your new lottie. Do you know what all you are going to grow yet? All the best, -Ina