The IRIS scheme is one of the biggest and most expensive projects undertaken by the Manx government in years, costing around ?85 million so far.
Original plans for centralised sewage treatment works have been abandoned in favour of regional plants.
Meary Veg is already serving the east of the Island and two more plants are to be built in the north and west.
Today, the Transport Minister David Anderson will ask Tynwald for almost ?2 million towards the scheme, with more than half the sum for consultants' fees.
Mr Anderson says it's a vital expense:
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