Presented by: Moore and Associates, Inc.
The town of Payson, Arizona is a mountain community that required hours of travel from Phoenix until the completion of the Beeline Highway in 1958. The completion of State Highway 87 opened opportunities for city dwellers in the Salt River Valley to escape the hot summers and enjoy winter snows in the Mogollon Rim area. With the greater accessibility came an increase in development in the area.
The Northern Gila County Sanitary District was established in 1965 in response to the need to protect the environment by planning and constructing a central collection, treatment and discharge of the community wastewater. The first wastewater treatment facility was constructed in 1972 to treat and discharge 350,000 gallons per day. The rapid growth of the community soon caused problems in meeting the effluent discharge standards set forth by the Arizona Department of Environmental Quality (ADEQ), especially with regard to nutrients such as nitrogen and phosphorous. The facility discharges to the American Gulch which is a tributary to the East Verde River. The East Verde River is a tributary to the Verde River which provides potable water to the Salt River Valley; namely, the City of Phoenix.
In 1983, with the support of the community, the District began the design and construction of a 1.7 million gallon per day (MGD) facility that removes nitrogen and phosphorous from wastewater as well as normal organic substances found in domestic wastewater. This facility was designed to meet the wastewater treatment requirements for the next 20 years. The 1.7 million gallons per day facility was completed and placed in operation in 1984. Soon after operation began it was determined that nitrogen and organic compounds could be biologically removed with relative ease. However, phosphorous removal was inconsistent and needed an additional unit process to achieve consistent phosphorous removal. The developer of the biological process that was employed had completed further research to address the process problem. In early 1989, a volatile fatty acid basin was constructed that resolved the issues of consistent phosphorous removal.
In 1989 the District partnered with the Town of Payson to facilitate the construction of Green Valley Park. The construction of the Park allowed the District to use effluent to recharge the groundwater in that area. It also allowed the District to facilitate the delivery of effluent to Payson Meadows, the Pioneer Cemetery,Payson High School, Chaparral Pines Golf Course, Payson Golf Course and the Rim Club Golf Course for reuse.
In 1997 additional automatic backwash filters and ultra-violet disinfection units were added to assure adequate treatment during wet weather flows (periods of high storm water infiltration). In 2010 a secondary sedimentation basin was added and modifications to the two existing secondary sedimentation basins were made to adequately receive and treat high flows that occurred during times of infiltration during the wet weather.
In 2009, based upon actual plant operating data, the process was mathematically resized and proven that it can now handle an average dry weather flow of 2.2 MGD
In 2010, biosolids treatment was addressed by adding a biosolids dewatering building with two 1.0 meter sludge filter presses. The original headworks with screening and grit removal could not handle the high wet weather flows at times without adding sand bags to restrain influent flows. There were times that the capacity of the influent flow meter was exceeded. To resolve that issue a new headworks was constructed and made operational in 2012. The new headworks consisted of a new fine bar screen, flow meter and grit removal system that was designed to handle the future capacity of the facility with allowance for wet weather flows.
There are three regulatory permits that have been promulgated by the USEPA and the Arizona Department of Environmental Quality which outlines the limits of treatment at the American Gulch Water Reclamation and Reuse Facility. The policy of the Arizona Department of Environmental Quality is that a facility must be in planning and design of the expansion of such when average daily flows (or committed flows) to that facility reach 80% of it’s design capacity. The regulatory permits that apply at the Northern Gila County Sanitary District are: 1) the Aquifer Protection Permit for protection of Arizona groundwater aquifers; 2) the AZPDES Permit for discharge to surface waters of the United States; and 3) the EPA 503 Regulatory Permit for biosolids treatment and disposal.
Aquifer Protection Permit
The requirements for obtaining an ADEQ Aquifer Protection Permit (APP) are spelled out in Arizona Administrative Code (A.A.C.) R18-9. The permit contains requirements that allow the permittee to comply with two key requirements of the Aquifer Protection Program: 1) meet Aquifer Water Quality Standards at a designated Point of Compliance; and 2) demonstrate Best Available Demonstrated Control Technology (BADCT). BADCT’s purpose is to employ engineering controls, processes, operating methods or other alternatives to reduce discharge of pollutants to the greatest degree achievable before they reach the aquifer or to prevent pollutants from reaching the aquifer.
Since the American Gulch Water Reclamation and Reuse Facility pumps their effluent to Green Valley Park for groundwater recharge and also conveys it’s effluent to various irrigation sites throughout the District’s service area the APP sets effluent limits for recharge and reuse.
The routine discharge monitoring constituents relate to fecal coliform standards and nitrogen standards. E. coli (fecal coliform0 limits are non-detectable (zero) in four out of the last seven samples and a single sample maximum of 15 E. coli units. These samples are required to be taken daily and reported monthly. Total nitrogen is measured by adding nitrate as N, nitrite as N, and total Kjeldahl nitrogen as N as a 5-sample rolling geometric mean. The discharge limit is set at 10 mg/l Total Nitrogen and is sampled monthly and reported quarterly. An “alert limit” has been established at 8.0 mg/l. This limit acts as the alarm that triggers a process investigation to determine why the nitrogen is about to exceed 10 mg/l and warrants action or process change to avert the discharge limit. Other monitoring requirements include quarterly sampling of Antimony, Arsenic, Barium, Beryllium, Cadmium, Chromium, Cyanide, Fluoride, Lead, Mercury, Nickel, Selenium, and Thallium and have alert limits and discharge limits. In addition, volatile organic compounds (VOC’s), mostly carcinogenic compounds, are measured semi-annually and have alert and discharge limits.
The APP contains the reclaimed water quality monitoring requirements for Total Nitrogen, E. coli and turbidity. The discharge limit for Total Nitrogen is 10 mg/l (five sample rolling geometric mean), sampled monthly and reported quarterly. E. coli limits are non-detectable (zero) in four out of the last seven samples and a single sample maximum of 23 E. coli units. These samples are required to be taken daily .and reported quarterly. Turbidity is required to be taken every day and reported quarterly. The turbidity limits are 5.0 NTU single read and 2.0 NTU on a 24-hour average.
The American Gulch Facility has consistently met the discharge limits in every category.
Arizona Pollutant Discharge Elimination System
The State of Arizona has established stringent water quality standards for the Verde River and its tributaries. The AZPDES program regulates discharges to surface waters of the United States(specifically within Arizona) and establishes effluent standards of discharge to protect these waters. The effluent quality standards for the American Gulch Facility include Nitrogen limitations of less than 1.0 mg/l Total Nitrogen and a total Phosphorous limitations of less than 0.1 mg/l.
In order to meet the stream standards with the discharge from the American Gulch Facility, American Gulch was declared “an effluent dominated stream” and the District was given a “variance” for their discharge. The District was allowed to employ a 500 meter mixing zone in the East Verde River at the confluence of the East Verde and American Gulch where the effluent is mixed into the flows of the East Verde.
With the implementation of the effluent reuse and groundwater recharge program the impact of discharging to the East Verde River is minimized. The District discharges to American Gulch only during periods of peak wet weather flow.
The District is required to monitor the quality of the effluent discharge both at the effluent discharge and at the end of the mixing zone in the East Verde River. Total Nitrogen and Total Phosphorous are sampled monthly and reported to ADEQ monthly.
Biosolids Disposal Permit
The regulation of biosolids treatment and disposal comes under the AZPDES Permit. Arizona is required to follow the USEPA 503 regulation promulgated by Congress. These regulations are applicable to the generator and disposer and/or end user of biosolids. The District is permitted to dewater biosolids in a 1.0 meter sludge filter press and currently contracts with Waste Management for it’s ultimate disposal. Both the District and Waste Management are required to test the biosolids and submit results to ADEQ on an annual basis. The biosolids from the District must pass a “paint filter test” to assure that excessive liquids are not present prior to being transported by Waste Management. In addition, the District must perform annual testing for heavy metals.
PROJECTED FACILITY CAPACITY
The District employs TetraTech to maintain their collection system Master Plan. The Plan divides the service area into drainage areas and projects wastewater flows in each area based upon projected future population, the Towns’ Land Use Plan, and existing water use data. Storm water infiltration/inflow projections are based upon gallons per day per acre. The total dry weather wastewater flows for the entire service area at build out is projected to be 3.15 MGD. Peak wet weather flow for the service area is projected to be 9.19 MGD. Since zoning in Payson is subject to change based upon the needs of the community, the Districts’ engineer is recommending that future capacity be established at 3.5 MGD. Flow data shows that the plant peaking factor for dry weather conditions is between 1.9 and 2.0 peak to average daily flow. Therefore the peak dry weather capacity of the plant will be 7.0 MGD. An examination of infiltration/inflow has not been possible since the prior influent flow meter was not large enough to measure actual flows at peak wet weather conditions. It is estimated that storm water flows are in the region of 3,000 to 3,400 gpm or 5.0 MGD. A review of the interceptor entering the plant establishes a line capacity of 12.0 MGD at minimum slope before surcharging of the line. Therefore, the future additions to the treatment facility should be such that the facility can adequately treat a peak dry weather flow of 7.0 MGD and be adequate to handle peak wet weather flows of 12.0 MGD.
According to District records, approximately 1.9 MGD of its present 2.2 MGD capacity is committed to wastewater generators in its’ service area. Since 86% of the facilities capacity is committed the District has already begun implementing the studies, designs and construction to increase the facility capacity to 3.5 MGD.
RECOMMENDED FACILITY ADDITIONS
In 2012, the District completed the first phase of its’ 3.5 MGD expansion. The original headworks with automatic screening and grit removal could not handle the high wet weather flows at times without adding sand bags to restrain influent flows. There were times that the capacity of the influent flow meter was also exceeded. To resolve these issues a new headworks was constructed and placed in operation. The new headworks consisted of a new fine bar screen, flow meter and grit removal system that was designed to handle the future capacity of the facility with allowance for wet weather flows (namely 12.0 MGD).
The remaining expansion of the facility to 3.5 MGD should take place in phases as funds are available. The following is a description of the needed additions to achieve a design capacity of 3.5 MGD.
1. Construction of an additional 1.3 MGD modified Bardenpho train. The District has executed a design contract with Moore and Associates, Inc. for this phase which will include:
- Construction of the third process train with a new flow splitter box upstream of the secondary sedimentation basins.
- Replacement of fine bubble diffusers in the existing nitrification and reaeration basins. The existing PVC piping attached to the diffusers, after 27 years of operation, has become brittle. Fine bubble diffusers will be installed in the new nitrification and reaeration basins.
- Slope floor of all nitrification basins (existing and new) to assist in easier cleaning of these basins.
- Addition of 24” diameter sluice gates at critical locations that allow easier cleaning of all basins.
- Installation of three submersible pump sumps with a single portable pump and associated piping to transfer liquid between nitrification basins and transfer solids to a manhole outside of the structure that will allow a more complete cleaning of process basins.
- Replace existing centrifugal blower with high speed turbine blowers that are variable speed controlled to reduce power costs. These blowers will vary the delivery of air based upon the dissolved oxygen in the aeration basins and the need for oxygen.
- Remove the existing flow splitter box upstream of the three secondary sedimentation basins and replace with the new flow splitter box in the third process train structure.
- Abandon existing underground piping between the new headworks and the process trains. Install a new mixing basin that blends the flow of the influent, return sludge recycle, effluent filter backwash water, sludge press underflow, and volatile fatty acid effluent. The basin will be followed by three flow meters and sluice gates that will allow the influent flow to be proportionately divided between the three main process basins.
- Upgrade of existing process control instrumentation and SCADA hardware and software.
2. Modifications to and /or construction of new VFA basin. A portion of the existing engineering contract is to evaluate the existing VFA basin to determine whether or not the existing VFA basin is adequate to handle all future flows or will a new VFA basin need to be added between the new headworks and the process trains.
- If the existing basin is adequate, covers and odor control will need to be added. In addition changes in sludge pumping and VFA effluent pumping will be needed.
- If a new VFA basin is required, the design will include gravity flow from the headworks to the process trains, covers and odor control and sludge pumping for elutriation and transfer of solids to the process trains.
- Final analysis may lead us to conclude that both VFA basins are necessary.
3. Addition of a new 65’ diameter secondary sedimentation basin. The next phase should include the construction of a new 65 ft. diameter secondary sedimentation basin. The basin will include Stamford baffles for hydraulic control and ducking skimmer for scum removal. In addition, the existing return sludge pump station cannot be modified to handle the return sludge from the basin. Therefore, a new return sludge sump with dual submersible pumps will be installed in a concrete structure adjoining the basin and a sludge pipe will be installed to deliver the sludge to the mixing basin ahead of the process trains.
4. Replace existing effluent sand filters with cloth disk filters. The existing effluent sand filters are not hydraulically adequate to treat the newly projected plant flow. New cloth disk filters are available to fit in existing structures at minimal cost. However, the existing underground piping between the secondary sedimentation basins are not able to handle the higher flows. Therefore, the pipes will be abandoned and an open flow channel will be installed between these structures that will eliminate the hydraulic problem.
5. Construct a new ultraviolet disinfection station. The existing closed UV chambers in the UV building are not adequate to handle the anticipated flows at build-out. Therefore, a new concrete structure with vertical UV units will be installed between the filters and the effluent pump station. The station will include an effluent flow meter.
Anticipated project costs are as follows:
Projects 1 & 2. – $9,000,000
Project 3 – $3,000,000
Projects 4 & 5 – $4,000,000
Figure 1 show the location of the new improvements that will expand this facility to it’s build-out capacity of 3.5 MGD.