With sponges, like other cleans, you really need to give it a good go with a vac cleaner ie Sebo BS36, to get rid of all the loose bits.
The sponges are easy to apply, scatter by hand, or there is a machine to do it (bit like a garden grass seed distributor). You agitate these in (hence the machine), wait 30 minutes, then when dry you vac out - or as much as you can.
Very dirty carpets may need a 2nd application, and you can apply a prespray (lightly) to help break up the sticky soiling (microsplitters ok as is MPower/Nemesis).
I have done the odd job with sponges but prefer to how water extract - but if your back gonna play up it may be good for you.
Sponges are woolsafe.
If you target the high end market it may prove lucrative, but you do need a lot of sponges and the price per sq yd will be high - hence you charge a premium.
A new Envirodri costs about £1000, Host have brought a model onto the market at about the same price.
You can get different brushes (2 in a set) for different type of pile. Envirodri costs about £90, Host about £180 a pair.
Higher end models of Host have a vac pod sat on top of the brush machine, doing away with a seperate vac, and Restormate does a Flexi 5 also with a vac pod considerably cheaper then a Host (or used to be cheaper - bit out of touch with prices.)