FLEXIBILITY Houston's Approach to Regional Biosolids Handling

The City of Houston has been beneficially reusing biosolids since 1921 and currently reuses over 90% of the biosolids that are produced citywide. The first two activated sludge treatment plants, known as the North and South Side Plants, were constructed in 1916, and the first heat dryer was installed at the City's North Side Plant in 1921. The end product was distributed to agricultural end-users, including local rice farmers, in the Houston area. Heat-dried material from the City is marketed as “Hou-Actinite,” a registered trademark, since the 1930s.The City has experienced significant growth over the last two decades and this growth generally occurred through annexation of developed areas adjacent to the City limits. Private developers to provide water and sanitary services to areas outside the City limits created numerous municipal utility districts. These utility districts constructed wastewater treatment plants of various sizes ranging from small package plants to larger permanent facilities. As the City annexes these areas, it also acquires the wastewater treatment facilities. Some of the wastewater treatment plants are eliminated from the system and the sewage is pumped to larger existing regional facilities, while other facilities are incorporated into the system.As Houston continues to grow through annexation, so grows the wastewater treatment system. As a result, the transfer, stabilization, and beneficial use or disposal of the biosolids has become a significant challenge. Because of this continuously changing facility inventory, flexibility is the key to Houston's successful regional biosolids management handling program.The City of Houston operates a uniquely challenging regional biosolids management program due to the size and magnitude of the utility system. The City owns and operates 39-wastewater treatment facilities located throughout the City in a large geographical area. The service area contains over 105,221 hectares (260,000 acres) that includes a customer base of over 1.7 million citizens. The City's treatment plants together treat an average of 908,000 m3/d (240 MGD) of wastewater each day. From this flow an average of 147 dry metric tons per day (162 dry tons per day) of biosolids are produced. One-third (16) of the City's facilities prepare biosolids for either land application or disposal. Sludge from some of the smaller wastewater treatment plants are pumped or hauled to six larger regional preparer facilities for stabilization. In 2000, output for the three largest facilities was: 69th Street WWTP – 77 dry metric tons per day (86 dtpd), heat dried; Sims Bayou North WWTP – 23 dmtpd (25 dtpd), lime stabilized; and Almeda Sims WWTP – 23 dmtpd (25 dtpd), heat dried. In addition, thirteen (13) other facilities prepare biosolids on-site. In total, the City prepares approximately 53,600 dry metric tons (59,100 dry tons) of biosolids annually.Based on this complex and dispersed network of treatment plants, the City of Houston has developed an effective regional approach to processing and beneficial reuse of biosolids. Building on this regional framework, the City has an ongoing program of facility consolidation and upgrade and has developed a beneficial use program that features two alternate disposal pathways – distribution and marketing of heat dried material and land application of lime stabilized or aerobically digested material.This flexibility by the City of Houston has contributed to the success of their biosolids reuse program.