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The filter beds might have been made up with gravel sized slag pieces - lots of surface area for bugs to live on whilst they eat the gloop. Some sites I've visited had piles of old slag lying around. Treated sludge, fine black soil - has anyone ever found pathogens in soil from sewage works? I never have, its just one of those things that is in the industry profile so you feel obliged to test for it. Tomato plants, in abundance.