Many wastewater facilities are either planning or are currently implementing biological nutrient removal (BNR) processes as part of the treatment schemes in response to the Colorado Water Quality Control Commission’s regulations 85 and 31. The Voluntary Incentive Program (Policy 17-1) is pushing facilities who have already implemented BNR processes to improve performance. Under proper conditions, conventional BNR is capable of producing effluent with concentrations of 0.7 mg/L total phosphorus (TP) and 7 mg/L total inorganic nitrogen (TIN), which are the required annual median effluent concentrations to fulfill Policy 17-1 regulations. Producing regulation-approved effluent requires plant operations staff to balance carbon and nutrient needs, leading to carbon that is utilized efficiently in order to biologically remove phosphorus and nitrogen. Too often, it’s assumed that municipal wastewater influent is properly suited to drive BNR, and consideration is not given to influent conditions during design.

Last year, I presented at the Rocky Mountain Water Conference on this topic and discussed processes at four different wastewater treatment facilities (WWTF) to compare and contrast their influent conditions, treatment process, solids digestion, recycle streams, and effluent quality. All four facilities are less than five million gallons per day (mgd) in size, and all are currently producing effluent that meets the full requirements of the Voluntary Incentive Program (Policy 17-1). During my presentation, I focused on the impacts of the different influent conditions and how they impact process design and operations.

Clifton Regional Wastewater Treatment Facility

The Clifton Regional Wastewater Treatment Facility is an oxidation ditch with a rated capacity of 2.5 mgd and 7,287 pounds per day (ppd) five-day biochemical oxygen demand (BOD₅). Influent concentrations of BOD₅, ammonia, and phosphorus average 313, 35, and 7.4 mg/L, respectively. The existing treatment process consists of headworks with mechanical screens and vortex grit separator, influent screw lift pump station, two parallel oxidation ditches, two secondary clarifiers, and ultra-violet (UV) disinfection. Clifton also has aerobic digesters and centrifuge dewatering. In response to regulatory drivers, the facility was upgraded to include enhanced biological phosphorus removal (EBPR) in 2015. EBPR improvements included the addition of a three-cell anaerobic reactor upstream of the oxidation ditches. The anaerobic reactor consists of a three-cell reactor (one for each oxidation ditch). The anaerobic reactor is valved such that it can operate as a Johannesburg or conventional process anaerobic zone.  Due the influent screw pump station, influent to the anaerobic zones has a dissolved oxygen concentration of 2.5 mg/L; consequently, additional carbon is required to drive biological phosphorus removal. The oxidation ditch performs nitrogen removal (nitrification and denitrification). The plant dewaters biosolids every other day and utilizes ferric chloride to reduce impacts of phosphorus recycling.

City of Evans Consolidated Wastewater Treatment Plant

The City of Evans Consolidated Wastewater Treatment Plant consists of a headworks, influent submersible pump lift station, secondary biological process, UV disinfection, and an anaerobic sludge lagoon for biosolids digestion. The plant is rated for 2.86 mgd and 6,024 ppd BOD₅; influent concentrations of BOD₅, ammonia, and phosphorus average 370, 43, and 7.0 mg/L, respectively. The Johannesburg secondary process consists of a recycle activated sludge (RAS) denitrification zone, anaerobic zone, anoxic zone, swing zone (anoxic or aerobic), and aerobic zone. The secondary biological process is configured as a Johannesburg nutrient removal process with a RAS denitrification zone and an anaerobic zone (where the influent mixes with the denitrified RAS). The RAS denitrification zone is 34,000 gallons and the anaerobic zone is 50,000 gallons. Approximately one third of the influent to the wastewater treatment plant enters via gravity, while two thirds of the flow is pumped through a five-mile-long force main. Supernatant from the anaerobic sludge lagoon is pumped back to the secondary process for treatment. Due to the anaerobic conditions of the sludge lagoon and the significant phosphorus release, alum is added to the supernatant prior the secondary process to remove the phosphorus recycle load to the biological process.

City of Evans Consolidated Wastewater Treatment Plant

Santa Rita Water Reclamation Facility

The Santa Rita Water Reclamation Facility (SRWRF), located in Durango, Colorado, is rated for 3.26 mgd and 7,825 ppd BOD₅. The SRWRF has the lowest BOD₅ concentration in the secondary process of these four facilities. Influent concentrations of BOD₅, ammonia, and phosphorus average 327 (270 to the secondary process due to 25 percent removal in primary clarifiers), 36, and 7.4 mg/L, respectively. The facility includes a headworks, primary clarifiers, secondary biological process, UV disinfection, RAS thickening, anaerobic digestion, dewatering, and sidestream treatment. While the collection system has 16 lift stations, a majority of the lift stations have short forcemains. The lift station with the longest forcemain in the distribution system feeds directly into the headworks and provides an estimated 20% of the influent flow. The secondary biological process is a Johannesburg process where RAS endogenously denitrifies prior to mixing with the influent. Each process train has two anaerobic zones, an anoxic zone, swing zone, and an aerobic zone. Alum is added to the filtrate produced in the dewatering process to remove phosphorus prior to combining with RAS in a small, 200,000-gallon filtrate and RAS reactor tank prior to the RAS denitrification zone.

City of Louisville Wastewater Treatment Facility

The City of Louisville Wastewater Treatment Facility is rated for 2.54 mgd and 5,515 ppd BOD₅. The facility consists of a headworks, influent lift station with submersible pumps, secondary biological process, UV disinfection, RAS thickening, aerobic digestion, and dewatering. All flow to the treatment facility is by gravity, although there is one small lift station in the city, but it has a short force main and is at the far end of the collection system. Influent concentrations of BOD₅, ammonia, and phosphorus average 300, 25, and 7.8 mg/L, respectively. The system has a Johannesburg process; each train has a RAS denitrification zone, two anaerobic zones, anoxic zone, swing zone, and aerobic zone. Biosolids are dewatered two to three times per week and centrate is recycled back to the secondary process.

 City of Louisville Wastewater Treatment Facility

No Two Facilities Are the Same

No two wastewater treatment facilities are the same; they have different influent conditions, operations ability, energy costs, and infrastructure. Each of the four case study facilities mentioned above have different influent conditions, each has its own unique biological process, and each accomplishes biological nutrient removal. Two items that need special attention: (1) anaerobic zone sizing and (2) understanding the impacts of nutrient recycles due to intermittent biosolids dewatering. The anaerobic zone size is different at each facility to account for the site-specific influent conditions, i.e. larger anerobic zones are needed for fresh influent and influent with high dissolved oxygen. Nutrient recycling, even with aerobic digestion, can have a large impact on effluent due to the infrequency of dewatering activities.  

Wastewater treatment facilities with effluent nutrient limits must balance their influent carbon in order to biologically remove nutrients. The staff at smaller treatment facilities often wear many different hats and don’t have the time—and the resources—to dedicate themselves full time to optimizing their nutrient removal. Designing smaller facilities for site-specific conditions, while providing operational flexibility, is the key to assisting smaller facilities with their carbon balancing and helping resilient and sustainable biological nutrient removal at smaller WWTFs to be achieved.