Gas-liquid distribution technique for water or wastewater filters using underdrains

A method of distributing gas and liquid through an underdrain system, the method comprising:

• a) positioning the underdrain system on a filter floor, the underdrain system comprising at least one underdrain block and filter media positioned on top of the at least one underdrain block;
• b) distributing gas sufficiently evenly through the underdrain system during the backwashing process at a rate of less than 2 scfm/ft2 by distributing a liquid at a rate of less than 2 gpm/ft2 concurrently with the gas through an interior of the at least one underdrain block and into the filter media of the underdrain filter system;
• c) increasing a distribution rate of the liquid after step b), or increasing the distribution rate of the liquid and a distribution rate of the gas after step b);
• d) stopping a distribution of gas;
• e) further increasing the distribution rate of the liquid after step d);
• and
• f) decreasing the distribution rate of the liquid after step e).