Reuse, reduce, recycle
Water-reuse seems a logical solution to increasing water shortages. But would industrial, commercial and residential customers best be served by a entralised or decentralised water-reuse system? Alan Rimer, Cindy Wallis Lage and Frank Rogalla of Black & Veatch report
In the US, water consumption per person is about twice as high as in Western Europe. Yet by far the largest percentage of indoor water use occurs in the bathroom, with up to 40% of the water used for toilet flushing, and an additional 50% of the domestic water use dedicated to flushing, washing and cleaning.
It would, therefore, seem logical to connect the showers and sinks to the toilet - a concept adopted more indirectly in large Japanese cities. In Tokyo and Fukuoka, a dual-distribution and water-reuse system is mandated for buildings with a floor space of more than 3,000m2 - grey water is collected and treated for toilet flushing, reducing by 30% the fresh water needs.
Where other non-potable supplies are available, lesser-quality water can be used for lower quality needs, as it was practised for the Millenium Dome, saving up to 50% of water use. A similar concept will be integrated in Terminal 5 at Heathrow Airport. In Hong Kong for instance, seven million people, about three quarters of the population, use seawater for toilet flushing, reducing the fresh water for this purpose about 80%.
Urban areas, therefore, 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 centralised or decentralised water-reuse system.
Early water-reuse programs were centralised, involving upgrades to an existing WwTW and elaborate storage and distribution systems, which often made them difficult to justify economically. To reduce the cost of additional distribution infrastructure, satellite water-reclamation facilities are considered -- compact systems built near potential customers and major sewer trunk lines.
This decentralised option can be more cost-effective if the customer base is reasonably concentrated. Several key questions determine whether a centralised or decentralised water-reuse system would best suit a municipality's needs:
Long-term needs and goals?
When evaluating a reuse program and deciding on the existing treatment plant or on satellite water-reclamation facilities, a comprehensive wastewater master plan will help to determine future wastewater collection and treatment needs and identify:
- Existing and projected wastewater flows
- A map of the existing collection and treatment system
- A description of the system's current condition
- The specific improvements needed
- A corresponding capital improvement programme
A market study to identify potential uses for reclaimed water, potential customers, and the volume of water these customers as well as their quality needs is essential. The potential customers should be pinpointed on a collection system map to determine if there are clusters in specific parts of the service area.
Decentralisation or one plant?
Once the reuse potential and the location is identified, it needs to be decided whether to produce reuse-quality water at the existing WwTW (upgrading it, if necessary) or build satellite facilities at strategic points throughout the service area. Before comparing alternatives, however, appropriate evaluation criteria need to be established, assigning each a weight based on the community's priorities.
When evaluating an alternative, each criterion from 1 (unfavorable) to 5 (favorable) can be multiplied 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 distribution piping and pumping costs need to be incorporated when evaluating alternatives. Annual pumping energy costs, for example, may make the centralised option surprisingly more expensive than building, operating, and maintaining three new satellite facilities.
Compact treatment technologies?
Generally, advanced treatment technologies with small footprints are particularly appropriate for a water-reuse programme, both for an existing treatment plant or for satellite facilities. The processes can be based on biofilms, such as biological aerated filters (BAFs), moving bed biofilm reactors (MBBRs), integrated fixed-film activated sludge (IFAS) systems, or include membranes, such as membrane bioreactors (MBR). All require fine screening of the influent; however, and the degree of screening varies with the technology. The MBR requires the most stringent screening using a 1-3mm mesh screen depending on the membrane manufacturer.
Alternatively, a conventional activated sludge facility could be retrofitted with tertiary filters or membrane microfiltration or even reverse osmosis to produce reuse-quality water. The treated water can be disinfected via ultraviolet (UV) irradiation or hypochlorite, depending on which is more important: a compact footprint or a chlorine residual in the distribution system.
The choice of treatment technology should be based on such issues as aesthetics, effluent quality, footprint, life-cycle costs, odours, and 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 neighbourhood is important if public exposure is high
- Odours: Many compact advanced technologies do not require primary or secondary clarifiers, so odour emissions are minimal compared with conventional large, open basins
- Footprint: Satellite facilities generally have limited space, so the smaller the system, the better, as it also facilitates the insertion of the system into the neighbourhood and reduction of odours and noise
- Effluent quality: The treatment system must meet local water-quality requirements, but can be tailored to the customer's needs and even include several streams of different quality. Currently, most compact advanced processes meet California Title 22 Water Reuse Standards
- Life-cycle costs: When comparing life-cycle costs of the centralised and decentralised water reclamation options, the related distribution system costs can be a major part
- O&M requirements: Ideally, a satellite facility should be automated and require minimal staff attention
Typically, wastewater treatment facilities are built in the lower reaches of a watershed to allow use of gravity flow, 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. One important construction goal is to minimise both the suction line from the sewer and the distribution lines to customers.
When analysing plans for satellite facilities, several economic and non-economic factors need to be evaluated, including aesthetics, reliability, capital costs, ease and cost of operations and maintenance, but also the ease of implementation concerning design, construction, and planning permission.
Oak Island's experience
Oak Island, North Carolina, is a rapidly growing beach resort in the south-eastern US. Right now, the town has about 7,200 full-time and 32,000 summer residents. Within 20 years, it is expected to add about 10,000 both full-time and summer residents. The Oak Island WwTW, which is on the mainland, currently serves about 10% of the area's existing homes, and already produces reclaimed water for the mainland portion of the town. The collection system includes about 24.1km of sewers, 13 pumping stations, and about 900 connections. The rest of the homes on the island are served by septic tanks, some of which are leaking untreated wastewater to the surrounding Intracoastal Waterway. Given expected growth and persistent septic tank failures, the wastewater utility needed to be expanded to reduce fecal coliform contamination and restore dissolved oxygen levels in the waterway.
In addition, coastal groundwater supplies are increasingly stressed, and because Oak Island already uses reclaimed water, the development of additional reclaimed water capacity was viewed as a prudent use of available resources. After answering the questions listed above, a project team determined that the most economical option would be an island-wide vacuum collection system and a new 1,500m3/d satellite facility that could produce reusable water for irrigation and cooling towers. 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 3,000m of distribution mains to various reuse customers. Construction is imminent and expected to begin the autumn of 2006.
This facility -- the first MBR application in North Carolina -- and a small portion of the reuse distribution system are estimated to cost about £1.4M. This per capita investment of £189/PE is within normal cost expectations of advanced treatment facilities.
Peoria Southern Water Reclamation Facility
The City of Peoria, Arizona, performed a feasibility study, site selection and design for a water reclamation facility (WRF). The alternatives included expanding the city's capacity in a regional WwTW versus options for a satellite water-reclamation plant and effluent reuse. The study considered collection system and reclaimed water distribution system improvements along with treatment costs for several potential treatment processes. The economic comparisons also considered the value of retaining reclaimed water as a resource.
As a result, a new WRF with a capacity of 38,000m3/d is under construction to serve the south-eastern area of this growing city, with additional infrastructure to treat a future flow of 50,000m3/d. A membrane-bioreactor treatment process was chosen to minimise the required site footprint in a rapidly developing residential area and provide superior effluent quality for aquifer recharge and reuse. Effluent quality design requirements are summarised in Table 1.
The City of Peoria bought 16ha for its WRF, whereas an adjacent surface twice as large was also purchased by the city for a regional park. The small footprint of an MBR facility provided an attractive benefit and made it compatible with a regional park. As shown in Figure 2, the architecture and landscaping was designed to be a good neighbour and the WRF occupies less than 3ha. A bulk of the site purchased for the facility can be used as park space.
The waste solids generated by the MBR will be pumped directly from the aeration basins to dewatering centrifuges. Dewatered solids will be hauled to an area landfill for ultimate disposal.
The elevated solids retention time and mixed liquor concentration (+/-10,000mg/l) allow solids to accumulate in the secondary process over several days.
This provides the benefit of operating without an intermediate solids holding basin, reducing cost and operating complexity. In the initial operation period, the plant will treat additional solids contained in the influent from another upstream water reclamation facility in Peoria, which allows for flexibility of future expansion.
Both these case studies illustrate the many decisions that a community (large or small) faces as it begins to evaluate the use of reclaimed water on a community-wide basis. A structured approach to decision making that involves all facets of decision making form (aesthetics to financing) provides the best opportunity to make the right decisions.
As concerns about water shortages grow, many more water-reuse programmes will take shape, as in some countries water reuse is considered before building a new water or WwTW 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.