The corporation shall be a nonprofit corporation, and no part of its net earnings remaining after payment of its expenses shall inure to the benefit of any individual, firm, or corporation; except, that in the event the board of directors of the corporation shall determine that sufficient provision has been made for the full payment of expenses, bonds, and other obligations of the corporation, then any net earnings of the corporation thereafter accruing shall, at the option of the board of directors, be used to pay the cost of extensions and improvements to any of its projects or paid to the municipality with respect to which the corporation was organized.