The improvements at Cumbernauld's Dunnswood Waste Water Treatment Works have been completed by Scottish Water.
It involved a whole new waste water treatment process being introduced alongside the existing system, to comply with new standards, and to provide for future population growth in the area as a result of development in and around Cumbernauld.
Bill Elliot, Scottish Water's Community Team manager for Lanarkshire, said: "Dunnswood Waste Water Treatment Works has served the community well for many years.
"This significant investment has brought it up to 21st century standards.
"We have effectively constructed a whole new waste water treatment works alongside the existing one.
"This involves pumping stations, a sampling chamber, giant underground tanks, a sludge treatment and storage facility, electrical controls and maintenance of existing kit on site as well."
A number of drainage scheme systems have also been introduced, with the creation of a new pond near Wardpark Industrial Estate and several more ponds on the other side of the road.
Mr Elliot added: "Since forming in 2002, Scottish Water has invested heavily in water and waste water infrastructure for North Lanarkshire and has established itself as a valued and trusted business.
"We are continuing to deliver value for money to communities across Scotland. Our £2.5 billion investment in 2010-2015 will continue to deliver clearer, fresher drinking water and reduce interruptions to supply by improving our mains network."
Meanwhile, more than 9000 people can expect clearer, fresher drinking water thanks to a £10m upgrade to the Corsehouse Water Treatment Works, near Stewarton, in East Ayrshire.
Scottish Water customers in Stewarton, Dunlop, Lugton and Fenwick will benefit from the investment.