Trinity River Authority of Texas Starts Move from Lime Treatment to Thermal Hydrolysis and Anaerobic Digestion

The Trinity River Authority (TRA) Central Regional Wastewater System (CRWS) provides wastewater collection and treatment services to all or portions of 21 jurisdictions in the Dallas-Fort Worth (DFW) metroplex, as well as the DFW International Airport. The solids treatment facilities at the CRWS Wastewater Treatment Plant (WWTP) include thickening, dewatering and lime stabilization. The WWTP has a permitted capacity of 7.1 m3/sec (162 mgd) and a biosolids production of 150 tons/day. The lime stabilized solids are currently sent to a landfill.The CRWS WWTP biosolids program has drawn increased scrutiny from regulators and citizens in recent years. To be proactive, TRA embarked on a Solids Management Improvements (SMI) program in the late 2000s. The key driver for the development of the plan was concerns regarding the long-term sustainability of the WWTP’s solids management program. Another important driver was the need to ensure reliable, low cost solids handling within the plant. After evaluating several alternatives, the master plan recommended changing the stabilization process at the WWTP from lime stabilization to mesophilic digestion, preceded by thermal hydrolysis as a pretreatment step.Thermal hydrolysis is a process that heats and pressurizes sludge in order to break down sludge cell walls. This enhances digestion and solids destruction; and consequently, reduces the amount of solids to be moved offsite, thus reducing transportation costs. Biogas production is also enhanced. The rupturing, or “lysis” of cell walls makes the material in the cells more available— and hence, biodegradable. When used in combination with downstream mesophilic digestion, the process generates Class A biosolids.Implementation of the SMI was broken down into four phases. Phases I and II focuses on modification and expansion of existing facilities to meet critical operational needs at the plant. Phase III of the SMI program includes a comprehensive process change away from lime stabilization to a more reliable process of advanced anaerobic digestion. The selected process includes thermal hydrolysis, anaerobic digestion and dewatering to produce stable, Class A biosolids that are reliably low in odor and suitable for multiple outlets. In addition, further consideration is being given to combined heat and power generation, nutrient harvesting and a number of other options for further reducing the biosolids volume—with the overall aim of using the biosolids as a valuable resource. Phase IV will be implemented in several years to meet future growth needs. The design phase of the Phase III SMI project is complete, and construction on the thermal hydrolysis system and digesters is currently underway.This paper will discuss the challenges encountered and lessons learned during the design phase of this landmark project that could be beneficial for other utilities and designers who are considering thermal hydrolysis and digestion in the future. Additionally, while thermal hydrolysis has been implemented at more than 30 facilities worldwide, there is only one operating facility in North America located at the Blue Plains Advanced Wastewater Treatment Facility in Washington, DC. The CRWS design is based upon the general process used at DC Water, with many lessons learned garnered through the design, construction and operation of that facility. This paper will also present the recent advances in the thermal hydrolysis process along with the improvements and differences in the process between the TRA and DC Water facilities. A summary of the current status of the project and the procurement challenges faced with changing general contractors after initiating construction will also be described.