BENEFICIAL REUSE OF TREATED WASTEWATER: A COOPERATIVE PUBLIC-PRIVATE EFFORT

Transcription

1 BENEFICIAL REUSE OF TREATED WASTEWATER: A COOPERATIVE PUBLIC-PRIVATE EFFORT Stephen L. Simpson, P.E. Black & Veatch Robert K. Willet, P.E. Black & Veatch INTRODUCTION Hanover County s Wastewater Treatment Plant (WWTP) in Doswell, Virginia was recently modified to supply treated wastewater to a new independent power production facility. The Doswell WWTP treats municipal wastewater to NPDES secondary standards and is permitted to discharge treated effluent to the North Anna River in the York River Basin. The modifications include facilities for interception, storage, and pumping the treated wastewater to the power plant for use as process water. The power production facility was constructed by Doswell Limited Partnership (DLP), a private enterprise, to generate and sell electricity to Virginia Power Company. This combined cycle facility, which has more than 600 megawatts of power generation capability, requires up to 1.2 mgd of process water for operation. Utilizing the new facilities for wastewater reuse will provide several benefits. First, the amount of plant effluent discharged to the river will decrease, thus reducing the waste loading on the river. In addition, a large industrial customer will have an economical source of process water, and the limited potable water supply will be conserved for other consumers. This project illustrates a successful balancing of environmental awareness with resource allocation through cooperation between a public utility and a private company. BACKGROUND The Doswell area of Hanover County is located in the north-central part of the county, approximately 20 miles north of Richmond. The area includes a mix of agricultural, commercial, and industrial land uses. The Hanover County government includes a conventional public water and sewer utility for the Doswell area. The Department of Public Utilities operates and maintains 435

2 a potable water treatment plant and distribution system, and a municipal wastewater collection system and wastewater treatment plant for this service area. The utility is operated as an enterprise fund, with revenues dedicated to pay for all expenses.. Before the power production facility was constructed, the primary customers in the County s Doswell service area were King s Dominion (KD), an amusement/theme park, and Bear Island Paper Company (BIPCO), a paper mill. Although the service area also includes a commercial center adjacent to King s Dominion and several widely scattered residential customers, approximately 90 percent of the County s water production and wastewater treatment capacity was allocated to the two primary users. BIPCO utilizes a fairly consistent amount of water from Hanover County, and augments that supply through its private wells. BIPCO operates its own wastewater treatment plant, whose effluent is discharged in an outfall combined with the effluent from the County s Doswell WWTP. In general, BIPCO s water use and wastewater flows are somewhat higher during the winter months of October through March. $ In contrast, the water use and wastewater discharges of King s Dominion are extremely variable. This is attributable to the seasonal nature of the theme park. Flows are highest from May through September when the park is open daily, particularly on weekends. Moderately high flows occur in April and October when the park is open only on weekends, and in the months of February and March when the facility is being prepared for the coming season. In November through January, KD water use and wastewater discharges drop to approximately 10 percent of the peak levels. The water use and wastewater discharges from the commercial center adjacent to KD, as would be expected, retlects the KD pattern. The Doswell water supply is limited by the allowable withdrawal from the North Anna River, the source of the supply. The withdrawal limits are set by state permits to ensure that adequate flows remain in the river for assimilation of treated wastewater discharges. The NPDES permit for the combined Hanover County/BIPCO discharge limits the strength of the treated wastewater entering the river to correspond with the permitted withdrawal. 436

3 * The allowable water supply withdrawal and treated wastewater discharge limits are shared between the County and BIPCO. Doswell Limited Partnership was formed by Diamond Energy, Inc., under Virginia law to design, construct, and operate a power production facility for supplying electricity to Virginia Power Company during peak demand periods. The concept of nonregulated independent power producers dates back to the Public Utility Regulatory Policies Act (PURPA) of Privately owned energy companies can now sell electricity to investor-owned utilities. The decision to locate the DLP generating facility in Hanover County was based on a number of economic and technical factors. These included suitability of the site for large-scale industrial development, availability of large quantities of natural gas and oil for fuel, proximity to the Virginia Power transmission system, and an economical supply of adequate process water. During planning for the new power production facility, use of potable water from the County s Doswell WTP to satisfy the process water needs of the power plant was given first consideration. However, in the summer months, the 2.0 mgd capacity of the WTP was fully allocated to existing customers. Therefore, expansion of the WTP to the County s 3.5 mgd permit limit was necessary to accommodate this increased demand. However, Hanover County did not want to permanently reserve this extra potable water supply for DLP, since the County is actively recruiting other firms to complete the planned development of the Doswell area. The projected water demand for this area, when fully developed, will exceed the available supply, considering the limited withdrawal from the river and the demands of existing customers. Consequently, the idea of using treated effluent from the Doswell WWTP was advanced. Since the power production facility requires demineralized water for steam generation, DLP had planned to construct an onsite water treatment plant for potable water. With limited additional treatment, wastewater plant effluent could be used to supplement, or even replace, the potable water supply for steam generation. To verify the feasibility of this approach from both technical and economic standpoints, the availability of wastewater, the capabilities of the existing wastewater treatment plant, the power production facility water needs, and the criteria for additional treatment were evaluated. In 1990, Hanover County and DLP entered into an agreement for water service. The County agreed to supply potable water for operation of the 437

4 power plant, and DLP agreed to accept treated wastewater from the County s WWTP to meet its needs during peak demand periods and to reduce its potable water demand when additional customers connect to the County s system. TREATMENT PROCESSES Use of treated wastewater posed a treatment challenge for both the power plant as the customer and the County as the supplier. Process water in the power plant is used primarily to generate steam for the steam turbine cycle. High quality demineralized water is required for this application to prevent scaling and mineral deposits in the equipment. Therefore, even potable water must be treated at the power plant to remove dissolved solids. Since the treatment facilities were needed at the power plant in any case, DLP felt that it would be in its best interests to provide the additional treatment necessary to utilize wastewater. Treatment responsibility is divided as follows: Hanover County provides storage, aeration, and mixing, and pumps the effluent (treated to secondary standards) to the DLP site. Facilities for chlorination of the water beyond normal effluent disinfection are also available for control of algae and regrowth of microorganisms. DLP provides advanced wastewater treatment and demineralization to suit its process needs. This division of responsibility is advantageous to both parties. DLP can control the level of treatment that the wastewater receives to serve its purposes and economic interests, while Hanover County maintains the option of supplying treated wastewater to other customers in the future. EXISTING WASTEWATER TREATMENT FACILITIES The Doswell WWTP is a secondary treatment plant with a design capacity of 1.0 mgd. The plant currently treats an average annual tlow of approximately 300,000 gpd. However, during the summer, daily flows of 600,000 to 700,000 gallons per day are common due to the additional flows generated at KD, and hourly flows of 1.1 mgd have been recorded. 438

5 - The treatment processes at the Doswell WWTP consist of screening, grit removal, primary clarification, activated sludge, secondary clarification, disinfection, and post treatment aeration. The activated sludge facilities are typically operated in an extended aeration mode. The treatment plant process schematic is shown in Figure 1. The effluent flows from the County s Doswell WWTP and BIPCO s treatment works are combined for aeration and discharge into the North Anna River. The National Pollutant Discharge Elimination System (NPDES) permit for the combined discharge is issued by the Virginia Department of Environmental Quality. The combined effluent is limited to a biochemical oxygen demand (BOD,) and total suspended solids (TSS) concentrations of 30 mg/l each on a monthly basis, with any weekly average not to exceed 45 m@. The plant performance consistently meets these permit limits, which are typical for secondary treatment plants. Table 1 lists the annual average flows and effluent characteristics for 1990 through 1993, the four most recent years of record. Parameter Doswell WWTP Annual Average Flow and Effluent Characteristics Flow, kgd BOD,, m a TSS, m a NH,, m&l TKN, m a Phosphorous, mg/l _ In addition to the regularly monitored quality parameters for the wastewater treatment plant effluent, special monitoring was performed to 439

6 identify the levels of other parameters of interest. these values. Table 2 summarizes Table 2 Doswell WWTP Effluent Chemical Analysis Range* Parameter Minimum, ~ a x j ~ Design ~ ~ Value,, mgl* * mg/l m a Na I I I K Ca Mg co I c1 I I so NO PO Fe Mn SiO, TDS TOC

7 * The range for minimum and maximum values was based on twelve samples collected over a period of nine months. ** The values indicated were used as the basis for designing the DLP treatment plant. The sulfate content controlled the sizing of the RO unit process. TREATED WASTEWATER SUPPLY FACILITIES Facilities for wastewater reuse handling involve diversion of the plant effluent from the existing outfall sewer plus storage and pumping. The reuse handling facilities were sized to accommodate the maximum water demand of the power plant at full capacity, which was estimated to be 1.2 mgd. Storage of the treated effluent is required to accommodate the wide variability of plant flows so that a relatively constant supply of water is available on demand. A storage volume approximately equal to the estimated daily requirement was provided. The treated wastewater must be pumped to the DLP site since it is at a higher elevation than the WWTP site, the distance between the DLP site and the WWTP is approximately one mile, and DLP desires a residual pressure at its site. Based on the elevation of DLP s onsite storage tanks and the service water piping connection, a delivery pressure of 50 psi above ground elevation was specified. Since the treatment facility at the DLP site was being designed for a specific capacity and level of treatment, a two-phased approach was used to ensure that WWTP effluent quality consistently met DLP s expectations. First, the design effluent quality was specified in the supply contract between the County and DLP. Since the existing treatment plant has consistently been in compliance with its permit, the quality requirement was based directly on the plant s NPDES permit limits. Second, design of the treated wastewater supply facilities incorporated means to minimize water quality deterioration during storage, as well as provisions for flexibility in handling the treated wastewater. 441

8 Minimizing water quality deterioration was addressed in several ways. First, mixing and aeration equipment was installed in the treated wastewater storage facilities to maintain a well mixed, aerobic supply, preventing the development of septic conditions. Second, the storage facilities were configured to be operated in a flow-through mode to minimize the average detention time. Third, sampling facilities were provided to enable monitoring the water quality in the storage tanks. Provisions for flexibility in handling treated wastewater were important from both quality and operational standpoints. To maintain compliance with permit discharge limits, the capability for recycling stored wastewater to the head of the plant or to the plant discharge was provided. The piping configuration was also arranged to permit backflushing the treated wastewater supply pipeline so that the potential for septic conditions in the supply pipeline is minimized when the power plant is not in operation for extended periods. Even with the provisions for mixing, aeration, and treated wastewater handling, the potential for deterioration of water quality exists. To address this problem from an operational standpoint, additional disinfection facilities were provided for breakpoint chlorination of the treated effluent. However, additional chlorination could have detrimental effects on the activated sludge process if the treated wastewater was recycled through the plant, or could cause noncompliance with the discharge permit when treated effluent was discharged. Therefore, provisions were made for chlorinating either the stored water or the treated wastewater in the supply pipeline. Figure 2 shows the treated wastewater supply facilities schematic. DLP ONSITE TREATMENT The water quality mandated by this use required onsite treatment facilities; Figure 3 shows a schematic of the DLP process water treatment facilities. As can be seen in this schematic, the DLP water treatment facilities are configured to accept either potable water or treated wastewater. Clarification and filtration are the only treatment processes for treated wastewater prior to those required for potable water. 442

9 * Because of the concerns over water supply and wastewater discharges, the DLP plant was constructed as a zero discharge facility to minimize the quantity of process water needed and eliminate discharges. Therefore, a second treatment train was provided to recycle water which would normally be discarded; Figure 4 illustrates the recycle sidestream treatment processes. IMPLEMENTATION HURDLES Several obstacles had to be overcome to successfully complete this project, including an accelerated construction schedule, coordination among all of the participating entities, budget constraints, and resolution of contract and legal issues. The DLP power plant was designed and built on a fast-track schedule, in less than 18 months. This is not unusual for private sector projects, but it represents a very tight time frame for public works facilities. Speed is hampered in the public sector by the requirement for competitive bidding, as well as the financial and legal reviews required by the public agencies contracting the work. These procedures are mandated to protect taxpayers interests by preventing misuse of public funds and ensuring costeffective government services. They are time-consuming, however, and time equals money to private investors. Since construction of the treated wastewater facilities was dnven by DLP s need for process water at startup of the power plant, the County s schedule was dependent on DLP s established project schedule. To accommodate this schedule, the County s project was divided into multiple construction and procurement contracts. Work with short design time requirements was scheduled for bidding sooner than work requiring extensive design time and a lengthy approval process. In addition, equipment requiring a long lead time for fabrication was purchased independently from the construction contracts under which it was to be installed. During critical stages of the project, contractors progress meetings were held as often as weekly to ensure that all field issues were resolved promptly and construction tasks were completed on schedule. As a result of the close cooperation among all parties involved, the project was completed on schedule. 443

10 As with most projects, budget constraints were a significant issue. Following the preliminary project definition, a budget cost was developed for each of the new facilities, and DLP arranged financing through its lenders. The project had to be constructed within the established budget, since no additional funding, public or private, was readily available. The final overall project cost totalled four percent below the budget amount. Effective cost control was a result of Black & Veatch s detailed project definition during the preliminary engineering phase, production of welldetailed and comprehensive drawings and specifications, thorough review of contractors shop drawing submittals, and continuous onsite observation of all construction activities. This project involved parties who normally do not participate in public works projects. Close coordination of the activities of all participants was paramount to successful completion. The owner, the engineer, construction contractors, and governmental permitting agencies performed their usual functions, but each was impacted by the presence of DLP as a private developer along with its engineering and construction firms, program manager, and financial backers. DLP wanted to finance only those facilities needed for its water supply. Negotiations were held to identify the facilities that were essential to meet DLP s needs and those that the County sought to enhance its systems. The contract conditions for engineering services and construction of the project had to be carefully defined to include certain rights for DLP, without making it a direct party to the contract. DLP conducted additional reviews of design work and construction changes, and its review comments had to be considered and responded to. Also, progress payments for completed work had to be approved by DLP, which extended the time of payment to each company. These examples represent only a few aspects of the coordination required throughout this project. The project also involved significant contractual issues. A long-term water service agreement was required to protect the interests of both the County and DLP. The agreement covered reservation of capacity and a use rate structure for allocation of facility costs. The use rates for treated wastewater were designed to recompense the County for the administrative, operation, and maintenance costs associated with the 444

11 t treated wastewater supply facilities, but do not include the cost of treating the wastewater for compliance with the NPDES permit, since these treatment costs are required outside the scope of supplying treated wastewater to DLP. The use rates also do not include reimbursement for debt service attributable to the County s existing facilities, since the capital costs of the new facilities were funded by DLP in exchange for reservation of system capacity. CONCLUSIONS Direct reuse of treated wastewater as a water supply has not yet found widespread acceptance in the US. However, a variety of beneficial reuses for treated wastewater have been successfully implemented. Treated wastewater has been used for agricultural irrigation, municipal irrigation applications such as parks and golf courses, and development of wetlands and other fish and wildlife habitat areas. Treated wastewater has also been used for groundwater recharge, recreational applications such as pond establishment, and industrial process water. Although treated wastewater reuse is much more prevalent in the arid western and southwestern areas of the U.S., industrial reuse was a technically achievable and economical option for DLP and Hanover County. One of the determining factors for siting the DLP power production facility was the ability to supply large quantities of process water. The proximity of the selected plant site to an existing WWTP and the availability of treated wastewater for process water presented an excellent opportunity for mutually beneficial reuse. By taking advantage of this opportunity, Hanover County was able to economically satisfy a new demand on its system while preserving the limited supply of potable water for future customers. In addition, Hanover County benefits from the reduction in waste load discharged to the North Anna River and reservation of effluent assimilative capacity in the river to meet future wastewater service needs. The benefits of reusing treated wastewater apply not only to the waterstarved areas in the western and southwestern U.S., but also to the relatively water-rich east coast. Although the current economic realities of water supply in Virginja may not justify wastewater reuse in all 445

12 situations, great benefits may be realized for both the user and the supplier, as demonstrated by this project. Implementation of beneficial reuse of treated wastewater requires a proactive approach by public utilities to identify potential customers and to actively market this resource. In addition, a coordinated effort by all parties involved is critical to the success of the project. 446

13 HANOVER COUNTY WASTEWATER TREATMENT FACILITIES INFLUENT FLOW SCREENING ACTIVATED SLUDGE b POST TREATMENT AERATION EFFLUENT PROGRESS BY DESIGN Figure t

14 ~ HANOVER COUNTY TREATED WASTEWATER SUPPLY FACILITIES m DISINFECTION I - L POST TREATMENT AERATION I PUMPING TREATED WASTEWATER SUPPLY TO DLP b PROORESS BY DESlWl Figure 2 t

15 WASTEWATER DOSWELL COMBINED CYCLE FACILITY TREATMENT PROCESSES PUMPING FILTRATION POTABLE r -- WATER SUPPLY k REVERSE OSMOSIS \ \ STORAGE \ --b \ \.) \ DEGASSIFIER t - STORAGE DEMIN WATER TO PROCESS Figure 3

16 / (~ \ 4 \ RECYCLE \ \ \ R 0 \ + REVERSE OSMOSIS h -----y THICKENER SOLtDS TO LANDFILL BLACK & VEATCH PROGRESS BY DESIGN EVAPO RAT0 R/ CRYSTALLIZER Figure 4 1 i