As a stand-alone system, the Air Grid^® air distributor provides reliable, economical air-water backwashing. Each Air Grid is analyzed and custom designed for the filter basin in which it is installed. In new construction, the Air Grid is used in conjunction with any of the Roberts underdrain systems, providing the advantages of air-water washing, without the shortcomings commonly found in "combination" air-water underdrain designs. In retrofit applications, the Air Grid makes it possible to convert to an air-water backwash cycle, without removing and replacing the existing underdrain system.