Complex sentence: The first three in the list above require the use of flocculant chemicals to work effectively whilst slow sand filters can produce very high quality water free from pathogens, taste and odour without the need for chemical aids.
Split into simpler sentences: The first three in the list above require the use of flocculant chemicals to work effectively. Slow sand filters can produce very high quality water free from pathogens, taste and odour without the need for chemical aids.