A state Superior Court judge has granted class certification for a lawsuit on behalf of more than 9,000 Metropolitan District Commission customers who claimed the nonprofit municipal corporation charged them an unlawful annual water surcharge.

The ruling allows the litigation to proceed on behalf of a class of consumers from East Granby, Farmington, Glastonbury and South Windsor, who were charged a $158 annual surcharge in 2012, then $423 the following year and $199 in 2014.