Pre-project Conditions

The wastewater treatment plant (WWTP) constructed in 1978 had an average daily flow capacity of 80,000 gallons. The effluent was discharged into Sonoita Creek, a tributary of the Santa Cruz River. In November 1996, EPA issued a notice of violation because the plant's effluent exceeded permitted parameters for several contaminants, and its discharge monitoring reports indicated that the facility was not consistently removing total nitrogen as required by its discharge permits. The sewer system was installed in 1965 and served more than 400 residential connections. Deteriorated sections of the system were allowing infiltration of excess water volumes, especially during heavy rainfall events. The resulting wastewater flow rates exceeded plant capacity, allowing solids and pollutants to mix with the effluent. In addition, excessive volumes of untreated wastewater were diverted into the existing lagoons, which were not lined and thus posed the risk of polluting the aquifer.

Project Scope

The project was divided into two phases. Phase I involved the construction of a 110,000 gallon-per-day, extended aeration WWTP. Phase II involved replacing approximately 612 linear feet (lf) of 6- and 8-inch asbestos cement sewer lines with new PVC sewer pipes, as well as the construction of approximately 6,545 lf of sewer pipes ranging from 6 to 10 inches in diameter.

Benefits

The improved treatment facilities are producing a better quality effluent that complies with U.S. norms for ambient water quality. Rehabilitation of the sewer system reduced the amount of untreated wastewater leaking from deteriorated lines and infiltrating into the groundwater or pooling at the surface, as well as reduced peak flows to the treatment plant.