Stringent phosphorous standards lead to new MBR plant in Hyrum City, Utah
In 2004, Hyrum City, Utah, population 7,800, upgraded its wastewater treatment plant to a 2-million-gal-per-day (mgd) membrane bioreactor (MBR) system. The old plant used oxidation ditch technology and had served the northern Utah community since 1978. Hyrum’s treated effluent discharges to Spring Creek, a tributary of the Bear River that flows to the Great Salt Lake. In 2002, a total maximum daily load (TMDL) study was completed on Spring Creek. The findings required more stringent phosphorus standards at the water reclamation facility. The application of microfiltration was selected as the best phosphorus removal technology to meet the more stringent standard.
The TMDL study completed by Utah’s Department of Environmental Quality (DEQ) resulted in phosphorus limitations for Hyrum of 0.1 mg/L on a 30-day average with a total maximum discharge of 563 kg per year. To meet these requirements, the city elected to construct a new plant using MBR technology with the potential to deliver effluent complying with Type I standards. Type I effluent is required for municipal irrigation purposes and other uses where human contact is likely. Type I effluent requirements specify additional filtration and disinfection measures to produce an effluent that is higher in quality than Type II effluent. Type II effluent is suitable for irrigation purposes where direct contact with humans is unlikely.
The MBR process utilizes aluminum sulfate (alum) as the primary phosphorus removal coagulant. Between 2005 and 2011, the cost of this chemical rose 60%, prompting the city to look for ways to trim part of its $90,000 annual alum expenditure from the wastewater budget without violating permit limits.
Chemical Cost Savings
Under Mayor Dean Howard’s direction, the city looked for ways to cut chemical costs while recovering between 600,000 and 900,000 gal of water per day for use in the city’s piped irrigation system. Utah law stipulates that effluent applied to lawns, yards and pastures inside city limits and not discharged into the waters of the state is exempt from treatment for phosphorus. The city determined that reuse of its treated effluent would yield a chemical cost savings of approximately $45,000 for the six-month period effluent is diverted to the city’s irrigation system. This savings covers approximately 60% of the annual debt service on the bonds issued for the $995,000 project. Additional savings are realized through reduced biological oxygen demand and total suspended solids testing. Whole effluent toxicity (WET) testing is eliminated altogether during quarters that effluent is reclaimed, helping the city reach its goal of paying for the reclamation system without increasing user fees.
Revenue from the sale of reuse water for irrigation more than offsets the cost of electrical power to pump the treated effluent through the irrigation distribution system to the city’s 35-million-gal reservoir. Reuse water is delivered to the reservoir well before surface water is available. This allows the city to begin the irrigation season with a full reservoir and a charged distribution system, which eliminates the several days of delay in irrigation service that existed before reuse water was available.
Marisa Egbert with the Utah Div. of Water Resources steered the city through the funding application process, and the board approved a loan for $684,000 at 3.4% interest. The city provided the balance of the capital from water reserves. Plans were produced by Aqua Eng. Inc. of Bountiful, Utah. Counterpoint Construction of Layton, Utah, submitted the low bid for project construction, which began in the spring of 2011 and ran through early winter of the same year.
After treated effluent is diverted to the irrigation system, the phosphorus treatment component is deactivated and drained. This makes it impossible for the plant operators to quickly revert back to a discharging system in an emergency without violating the city’s discharge permit. The design team was tasked with creating a storage buffer to accommodate a minimum of 24 hours of storage for treated effluent to give operators time to reactivate the phosphorus removal system and divert the effluent flow to discharge. In addition, the design team wanted to simplify operation and control by creating enough suction storage to buffer the inflow and provide a level-based pumping system with a narrow operating band to approximate a flow rate-paced system.
To accomplish this, Aqua’s engineers made use of two abandoned 150,000-gal clarifiers to provide suction storage and converted the unused 1-million-gal oxidation ditch to store emergency overflow. After coring inlet and outlet penetrations in the wall of each clarifier, a new diversion structure was completed to allow one clarifier to be taken out of service during operation. Stainless steel screens were installed on the pump intake lines to protect the pumps and system from any debris that accumulate in the uncovered tanks. In the event of an abnormal flow or pressure condition, an alarm is triggered and effluent is automatically diverted to the oxidation ditch through a motorized/slave valve assembly. This gives the operators time to assess the nature of the alarm and determine the proper course of action. Emergency overflow then is gradually returned to the front of the treatment process.
The pumping station is equipped with three close-coupled vertical turbine pumps capable of conveying a total of 2,100 gal per minute (3 mgd) to the irrigation reservoir. Each pump is controlled with an Allen Bradley variable-frequency drive controller. This allows the pumping rate to approximate the effluent rate discharging from the treatment plant, which reduces system pressure fluctuations and stop/start events that would occur in a constant-speed pumping configuration with limited suction storage.
Because the pumping station is located at the lowest point in the irrigation distribution system, Aqua’s engineers were faced with the problem of providing sufficient pumping capacity without creating excessive system pressures. In order to achieve a maximum pressure of 155 psi at the pumping station, which results in a pressure of 125 psi at the lowest service lateral, Counterpoint Construction laid about 1/2 mile of 10- and 14-in. pipe in order to keep pressures below the design threshold at the maximum pumping rate.
The city’s piped irrigation system is supplied by a canal conveying water from mountain streams. Normal irrigation water consumption during July and August is approximately 7 mgd. Reuse water provides only about 10% of the total during this peak demand period. In its first year of operation, however, more than 154 million gal of treated effluent were delivered to the city’s lawns and gardens. This represents 18% of the total demand over the irrigation season. It is estimated that the project will conserve approximately 75 acre-ft of groundwater that would have been diverted to supplement surface water resources in a typical irrigation year.
With the city’s growing need for water, effluent reuse makes sense, especially after considering chemical and related cost savings and the conservation of drinking-water-quality groundwater.