City of San Marcos Wastewater Treatment Plant

On December 17th, 2019, I visited the City of San Marcos Wastewater Treatment Plant (WWTP) in San Marcos, TX. This facility was first built in January 1969 and is currently permitted for a daily average flow of 9 million gallons per day (MGD). Since September 2005, Jacobs Engineering (formerly CH2MHill) has operated and maintained the WWTP.

First, raw wastewater is sent to the preliminary treatment processes, including bar screens and grit chambers, to remove larger materials, then to primary clarifiers to remove settleable suspended solids. The primary effluent is then sent a secondary process (biological treatment) consists of the anaerobic/oxic (A/O) process, which is a variation of activated sludge process to remove nutrients, and secondary clarifiers. A small flow of activated sludge is returned to the anaerobic cells to facilitate denitrification. If needed, ferric chloride is added to supplement phosphorus removal. The secondary effluent is treated by the tertiary treatment consists of sand filters and ultraviolet (UV) disinfection. The final effluent is re-aerated and discharged to San Marcos River. A portion of tertiary effluent is chlorinated and reused as reclaimed water for irrigation in the city. Primary and secondary sludge is held in two sludge holding tanks and then processed by the sludge dewatering facility including three centrifuges. The dewatered sludge is sent to a landfill for disposal. The WWTP is also equipped with extensive odor control facilities as well.

In 2014 the WWTP operation staff have conducted an extensive study to fine-tune the enhanced biological phosphorous removal within the A/O process to reduce the chemical cost associated with phosphorus removal by ferric chloride addition. The final effluent meets and exceeds a 30-day average total phosphorus concentration of 1.0 mg/L.