The Duckbill Valves

John Carollo joined Sam Headman and Ben Ferguson in setting up the engineering firm of Headman, Ferguson, and Carollo in downtown Phoenix in 1933.[3] In 1957, after Sam Headman died and Ben Ferguson retired and sold his interest to John Carollo, the firm became John A. Carollo, Consulting Engineers. The corporation's name was later changed to Carollo Engineers.

During the course of Carollo's history, the firm has completed more than 15,000 engineering projects for the public sector and now employs over 1,000 employees. Carollo is currently ranked within Engineering News Record's (ENR) top 100 design firms, and is ranked 13 overall for design firms in sewage and solid waste.[4] Almost half of the employees are Registered Professional Engineers and Carollo Engineers is a Registered Professional Corporation. In 2016, Carollo was named California's Design Firm of the Year by ENR California.[5]