The Perfect Fit

(May 2006, Alan E. Rimer for WE&T)

This practical decision matrix can help any utility create a viable water reuse program

As concerns about water shortages grow, many municipalities have begun exploring water reuse programs. Some states now require municipalities to consider water reuse before building a new water or wastewater treatment plant or upgrading an existing one. Reusing water can keep water tables from dropping, lakes from shrinking, and wetlands from disappearing. In effect, the reused water becomes a new resource.

Urban areas have many potential uses for reclaimed water, including manufacturing and cooling processes, toilet flushing, dual-system utility supplies, recreational lake supplementation, and irrigation of golf courses, parks, cemeteries, and large landscaped areas. The challenge is determining whether potential industrial, commercial, and residential customers would be best served by a centralized or a decentralized water reuse system.

Early water reuse programs were centralized. They typically involved upgrades to an existing wastewater treatment plant and elaborate storage and distribution systems, which often made them difficult to justify economically. So, design engineers began considering satellite water reclamation facilities - compact systems built near potential customers and major sewer trunk lines. This decentralized option can be more cost-effective if the customer base isn't too diffuse.

To determine whether a centralized or a decentralized water reuse system would best suit their needs, municipalities will need to answer the questions outlined below.

What are our long-term needs and goals?
When deciding whether to base a reuse program on an existing treatment plant or on satellite water reclamation facilities, a municipality should begin with its wastewater master plan. This comprehensive plan should include:

• existing and projected wastewater flows,
• a map of the existing collection and treatment system,
• a description of the system's current condition and the specific improvements needed, and
• a capital improvement program for meeting future wastewater collection and treatment needs.

Where are our reuse customers?
The municipality then should conduct a market study to identify potential uses for reclaimed water, potential customers, and the volume of water these customers would need. The potential customers should be pinpointed on a collection system map to determine whether they are clustered in specific parts of the service area.

Table 1. Alternative Screening Matrix

Rating criteria (1=unfavorable, 5=favorable)	Weight factor	SWRF alternative/score		Regional plant alternative/score
Ease of operation	15	3		4
Operational complexity		45		60
Control requirements
System Reliablity	25	4.5		4
Equipment		112.5		100
Title 22
Ease of implementation	15	3.5		4.5
Design		52.5		67.5
Construction
Permits
Aesthetics	5	4.5	22.5	5	25
Capital Costs	20	5	100	4	80
O&M costs	20	5	100	3	60
Total	100	432.5		392.5
Rank		1		2

Source: Rimer et al., 2003

Should we decentralize?
Then, the municipality should decide whether to produce reuse-quality water at the existing wastewater treatment plant (upgrading it, if necessary) or build satellite facilities at strategic points throughout the service area. Before comparing alternatives, however, the municipality should develop appropriate evaluation criteria and assign each a "weight" based on the community's priorities (see Table 1, above). The total sum of the weights should equal 100.

When evaluating an alternative, the municipality should rate each criterion from 1 (unfavorable) to 5 (favorable), multiply each score by the criterion's weight, and add all the weighted scores together to get a total score. Then, a simple comparison of totals will reveal the best option.

Cost obviously matters, and municipalities should not forget distribution piping and pumping costs when evaluating alternatives. Annual pumping energy costs, for example, may make the centralized option surprisingly more expensive than building, operating, and maintaining three new satellite facilities (see Table 2, below).

Table 2. Summary of Total Present Worth

Capital cost	SWRF alternative	Regional plant expansion alternative
SWRF-1	$6,300,000
SWRF-2	$1,100,00
SWRF-3	$2,600,000
Regional expansion alternative		$3,035,000
Pipelines	$2,203,000	$8,696,000
Resevoir and pump station		$1,310,000
Total capital cost	$12,203,000	$13,041,000
O&M cost
Annual O&M cost	$432,000	$572,000*
Present worth O&M cost	$4,955,000	$6,561,000
Total present worth cost	$17,158,000	$19,602,000

* *About $100,000 of the total annual O&M cost is the additional cost for boosting flow back into the reclaimed water distribution system.
Source: Rimer et al., 2003

Which treatment technologies should we consider?
Generally, advanced treatment technologies with small footprints, such as biological aerated filters, integrated fixed-film activated sludge systems, membrane bioreactors (MBRs), and moving-bed biofilm reactors, are appropriate for an existing treatment plant or satellite facility intended for a water reuse program. All require fine screening of the influent, but the degree of screening varies with the technology. MBRs require the most stringent screening (a 1- to 3-mm mesh screen, depending on the membrane manufacturer).

Alternatively, a conventional activated sludge facility could be retrofitted with shallow- or deep-bed tertiary filters to produce reuse-quality water.

Afterward, the treated water can be disinfected via ultraviolet irradiation or hypochlorite, depending on whether it is more important to have a compact footprint or a chlorine residual in the distribution system.

Which technology is best for our project?
The choice of treatment technology should be based on such issues as aesthetics, effluent quality, footprint, life-cycle costs, odors, and operations and maintenance (O&M) requirements - as well as the life-cycle costs of the related reclaimed water distribution system.

Aesthetics. An enclosable system that can be designed to "blend in" with the surrounding neighborhood is important if public exposure is high.

Effluent quality. The treatment system must meet local water quality requirements. Currently, most compact advanced processes meet California Title 22 water reuse standards.

Footprint. Satellite facilities generally have limited space, so the smaller the system, the better.

Life-cycle costs. When comparing life-cycle costs of the centralized and decentralized water reclamation options, municipalities shouldn't forget the related distribution system costs.

Odors. Many compact advanced technologies do not require primary or secondary clarifiers, so odor emissions are minimal compared to a conventional wastewater treatment plant with large, open basins.

O&M requirements. Ideally, a satellite facility should be automated and require minimal staff attention.

Is this a good satellite facility design?
Most water management professionals know that a wastewater treatment facility should be built in the lower reaches of a watershed, but where should satellite water reclamation facilities be built? The optimum location is where demand is greatest, sewer trunk lines are close, and sufficient wastewater flow is available (see Table 3, below). One important construction goal is to minimize both the suction line from the sewer and the distribution lines to customers.

When analyzing plans for satellite facilities, municipalities should evaluate them based on several economic and noneconomic factors, including aesthetics, reliability, capital costs, O&M costs, ease of implementation (how easily it can be designed, constructed, and permitted), and ease of O&M.

Oak lsland's Experience
Oak Island, N.C., is a rapidly growing beach resort. The town currently has about 7200 full-time and 32,000 summer residents. Within 20 years, it expects to have nearly 18,700 full-time and 42,000 summer residents.

The Oak Island wastewater treatment plant, which is on the mainland, currently serves about 10% of the area's existing homes. The collection system includes about 24 km (15 mi) of sewers, 13 lift stations, and about 900 connections.

Table 3. Satellite Water Reclamation Facility Design Flows

Satellite water reclamation facility	Tributary trunkline sewage flow (gal/d)	Recycled water demand (gal/d)	SWRF capacity (mgd)	Identified user
SWRF-1	650,000	625,000	0.63	Community college
SWRF-2	450,000	115,000	0.11	Industry - process
SWRF-n
SWRF-n+1	500,000	260,000	0.26	Industry - cooling
Total	1.6 million	1 million	1.0

SWRF = satellite water reclamation facility
Source: Rimer et al., 2003
While this table only identifies point-of-sale customers, it also could apply to dispersed irrigation or other customers near the satellite facility

The rest of the homes on the island are served by septic tanks, which have begun discharging untreated wastewater to the surrounding Intracoastal Waterway. Given expected growth, increasingly stressed groundwater supplies, and persistent septic tank failures, the town needed to expand its wastewater utility to reduce fecal coliform contamination and restore dissolved-oxygen levels in the waterway. Also, Oak Island already produces reclaimed water in its mainland portion and considers development of more reclaimed water capacity to be a prudent use of available resources.

After answering the questions listed above, the project team determined that the most cost-effective option would be an island-wide vacuum collection system and a new 1500-m3/d (400,000-gal/d) satellite facility that could produce reusable water for irrigation and cooling towers. The existing treatment plant already produces reclaimed water, but building a distribution system back across the lntracoastal Waterway would have cost nearly $4 million. And according to the market study, there are enough customers on the mainland to use all of the plant's reclaimed water.

The satellite facility, which will be in a park next to Town Hall, will use an MBR to treat wastewater so it can be reused by island residents. The project team also intends to build about 3000 m (10,000 ft) of distribution mains to various reuse customers. At press time, construction was expected to begin in June or July.

This facility - the first in North Carolina to use an engineered MBR - and a small portion of the reuse distribution system are estimated to cost about $2.6 million. The project was not eligible for funding through the North Carolina Clean Water Revolving Loan Fund, so the team applied for and received a grant from the North Carolina Clean Water Management Trust Fund to design and construct the project. The project met a number of the trust fund's goals, including reducing pollution of state waters by eliminating failing septic tanks.

Bangkok's Experience
Samut Prakarn province south of Bangkok, Thailand, has a 526-000-m3/d (139-mgd) wastewater treatment plant and hundreds of kilometers of collection lines, but none of this system has been placed into service.

The extended aeration plant has a large pretreatment system that includes four 12.5-mm bar screens, four vortex grit collectors with agitators, and three 336-m x 157-m x 3.84-m pretreatment basins. This system is expected to achieve 60% solids removal and 50% biochemical oxygen demand (BOD) removal. The plant also includes eight 55-m-diameter secondary clarifiers that function at overflow rates up to 2.2 m/h and solids loadings up to 10 kg/hom3.

Sludge handling is limited to temporary storage while disposal options are developed. Theoretically, the primary storage ponds can hold about 2 years' worth of sludge, while the secondary lagoons can hold the equivalent of 1 year's worth of sludge. The related odor issues have not been addressed.

Overall, the treatment plant is designed to produce effluent with 20 mg/L of BOD, 50 mg/L of total suspended solids, and less than 5 mg/L of ammonia-nitrogen. It was planned to discharge to the Gulf of Thailand. However, concerns about the ecologically sensitive gulf, which is home to green mussels and other commercially valuable aquatic life, prompted the government to "mothball" the wastewater utility while it evaluated other uses for the reclaimed water.

Initial market studies suggested that a water reuse program might be effective. A detailed market evaluation revealed four possible options: industry, agriculture, aquaculture, and groundwater recharge.

Samut Prakarn has nearly 6000 factories, which collectively use about 165,000 m3/d of water. At the moment, however, many are reluctant to use reclaimed water for product manufacturing because of concerns about variability in water quality, related regulatory and export limitations, and customer acceptance. Nevertheless, the project team believes that industrial reuse will become a viable option during the next 30 years, because groundwater use will be tightly controlled or eliminated in the future, and water will be scarce in the dry season.

The province also has large agricultural and aquacultural areas in the east and near the existing treatment plant. The project team estimated that local farms will need 265,500 m3/d and local fish farms will need 149,900 m3/d during the 6-month dry season.

Meanwhile, overpumping has been depleting local aquifers, and the government wants groundwater withdrawals to cease within the next 5 years. To supplement and rebuild these aquifers, the project team recommended starting by building 10 recharge wells, which would add 30,000 m3/d of reclaimed water to groundwater supplies.

To ensure that the reclaimed water would be suitable for agriculture, aquaculture, or groundwater recharge, the project team considered several treatment and distribution options and determined that adding disk filters to the existing treatment plant would be the most cost-effective alternative.

This option isn't the best choice for industrial reuse, however, because too much pumping would be required. Instead, the team recommended that several satellite water reclamation facilities be constructed to serve these customers, beginning with a 10,000-m3/d demonstration facility. This facility, which would use an MBR, would promote public acceptance and build industry confidence by showing that reclaimed water can be a high-quality, reliable source of water.

These case studies illustrate the many decisions that any community faces as it begins to evaluate using reclaimed water on a communitywide basis. A structured approach to decision-making that involves all aspects (from aesthetics to financing) provides the best opportunity to make the right decisions.

Alan E. Rimer is practice leader for water reuse in the Cary, N.C., office of Black & Veatch (Kansas City, Mo.).

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