Section 42C. If a rate or charge for which a lien is in effect under section forty-two B has not been added to or committed as a tax and remains unpaid when the assessors are preparing a real estate tax list and warrant to be committed by them under section fifty-three of chapter fifty-nine, the board or officer in charge of the water department, or the town collector of taxes, if applicable under section thirty-eight A of chapter forty-one, shall certify such rate or charge to the assessors, who shall forthwith add such rate or charge to the tax on the property to which it relates and commit it with their warrant to the collector of taxes as a part of such tax. If the property to which such rate or charge relates is tax exempt, such rate or charge shall be committed as the tax.