Constructed treatment wetlands have become an established technology for treatment of municipal and industrial wastewaters around the world. Wetland systems can provide treatment similar to conventional tertiary wastewater facilities if sufficient land is available but with lower energy and maintenance requirements. Ancillary benefits of treatment wetlands include wildlife habitat, passive recreation, and public education. A 165-acre treatment wetland facility was created by draining, grading, and planting an existing facultative oxidation pond at the City of Stockton Regional Wastewater Control Facility (RWCF) in California during a recent treatment plant upgrade project. The treatment wetland facility is designed primarily to reduce total suspended solids concentrations from upstream ponds and provide additional treatment of biochemical oxygen demand and ammonia-nitrogen upstream of new nitrifying biotowers and tertiary treatment facilities. The mid-process placement of the wetland is designed to manage solids loading to the biotowers and eliminate the need for power-intensive dissolved air flotation for algal solid separation.
The design and sizing of this system was based upon treatment wetland performance modeling calibrated to geographically and climactically relevant data sets from a similarly-loaded treatment wetland system. Initially challenging site conditions resulted in difficulty with wetland construction and startup, but solutions are presented that provide an instructional case study in treatment wetland design and establishment.
The City of Stockton (the City) RWCF, constructed in 1922, is the primary treatment facility serving the City. Since construction, the RWCF has been incrementally expanded to accommodate City growth. Since 1989, wastewater strength has increased due to water conservation practices. This increase in strength has reduced the effective capacity of the RWCF from 48 million gallons per day (mgd) to 38 mgd.
In 2001, a new National Pollutant Discharge Elimination System (NPDES) Wastewater Permit was issued to the RWCF with more restrictive requirements than past permits. The new requirements imposed more strict ammonia (NH3) and nitrogen limitations on the RWCF’s effluent discharged to the San Joaquin River. The City implemented the RWCF Upgrade Project to construct the necessary facilities to meet NPDES ammonia and nitrogen limitations. The project provides the additional benefit of restoring the RWCF’s treatment capacity to 48 mgd.
The project has upgraded treatment facilities and processes at the RWCF to bring the discharged effluent into compliance with permitting requirements. Upgrades include enhanced primary treatment, refurbished primary scum thickener, nitrifying biotowers, tertiary filters, modifications to the chlorine contact channel and injection system, new and upgraded support facilities, and conversion of a large wastewater pond (Pond 4) into a treatment wetland.
This paper addresses key design, construction, and startup aspects of the wetlands and also describes solutions developed to solve important issues during the first year of operation.