(a) In addition to income withheld for the current support of a child, income shall be withheld from the disposable earnings of the obligor to be applied toward the liquidation of any child support arrearages, including accrued interest as provided in Chapter 157.
(b) The additional amount to be withheld for arrearages shall be an amount sufficient to discharge those arrearages in not more than two years or an additional 20 percent added to the amount of the current monthly support order, whichever amount will result in the arrearages being discharged in the least amount of time.