Water, water everywhere, and not a drop to waste...
Michael J. Graves
Project Manager and Wastewater Operations Specialist
The availability of water is vastly becoming a top priority of municipal planners and utility administrators. With an increasing population and the subsequent demand for water resources, a new focus on wastewater reuse is emerging—and not just in the arid climates of the deep Southwest. A shrinking freshwater supply and increasingly stringent discharge criteria have many utility providers seeing the millions of gallons discharged by their wastewater treatment facilities as a supplemental supply and potential revenue.
In June of 2006, the National Drought Mitigation Center at the University of Nebraska in Lincoln reported that more than 60 percent of the United States is experiencing abnormally dry or drought conditions. More recently, the southeastern part of the United States has reported extreme drought conditions, specifically in north and western Georgia. State climatologists and University of Georgia officials predict little hope of major recovery and say that there is a good probability that by the spring of 2008 southeastern Georgia may also experience severe drought conditions.
Water scarcity and expanding metropolitan populations are encouraging potable water providers to identify and develop additional surface and groundwater supply sources. Officials are planning billion-dollar projects to move unallocated raw water hundreds and thousands of miles for treatment and distribution to major population centers. In some instances water utilities are competing for water rights in neighboring states in an effort to quench their thirst for water supply reserves.
This quest for more water, and increasing regulatory standards on sewage plant effluents, has driven municipalities to consider wastewater reuse as a viable long-term solution to reduce potable water demand. Reclaimed wastewater has long been an acceptable product for agriculture and golf course irrigation, industrial processes and aquifer recharge. Polished wastewater effluents are also being utilized for indirect reuse or discharge to a surface water supply. In order to realize sustainability in these and other wastewater reuse alternatives a utility must have confidence in its ability to reliably produce high-quality wastewater effluent.
Advancing Water Reuse Technology
Over the past 10 years treatment of wastewater with membrane filters has grown. Membrane filtration delivers superior water quality, and with a growing number of installations, capital and operational costs are more closely resembling those associated with traditional wastewater treatment processes. In addition to the production of reuse quality wastewater, membranes offer unique advantages during wastewater facility expansions as a result of their reduced treatment process unit footprint and operational simplicity.
Membranes are a man-made filter material that provide a physical barrier. The membrane's pore size is designed to capture target pollutants. Table 1 indicates the range of pollutant capture as compared to membrane pore size. Membrane manufacturers offer a number of physical configurations ranging from a flat sheet of membrane material to a spiral-wound tube or even a hollow core fiber.
Most of today's wastewater treatment facilities employ a biological process in which the facility itself is an engineered environment. They are designed to optimize the biodegradation of organic waste and the separation of suspended solids from the incoming wastewater. If a utility desires to produce a reuse quality product, or if a discharge water quality target cannot be met with conventional treatment, membrane treatment of wastewater effluents is a practical application to consider. Membrane treatment can also occur in a combined process with the conventional biological treatment plant.
This is the concept behind the Membrane Biological Reactor. MBR treatment plants are unique in that the membranes are integral to the biological process, and in some cases, submerged within the existing biological process tanks. The MBR scenario offers the previously mentioned capital cost benefits by eliminating the solids settling (clarifiers) component of a traditional wastewater treatment plant. However, there are also many added operational considerations of the MBR. Because the membranes take the place of the secondary clarification step, operators are no longer burdened with sludge bulking and the negative impacts of filamentous or Nocardia bacteria. Operational issues are further enhanced by the MBR's ability to operate at higher suspended solids concentrations, thus reducing sludge yields and the establishment of a more stable biomass that is potentially resistant to variable organic and toxic loads.
Table 2 (above) represents a comparison of three different wastewater treatment trains consisting of the following:
Water Reuse Case Study
MBR applications have dramatically increased and, with more installations, more confidence has been established in the MBR's ability to produce reliable high-quality effluent. A recent Midwest case study evaluated two pilot-scale MBR plants that were targeting extremely low-level effluent phosphorus concentrations. The pilot project verified that the target effluent concentrations can be met and effluent water quality is sufficient for reuse applications.
|Figure 1: Pilot Plant Process Schematic|
The proposed treatment process to meet the effluent phosphorus goal of 0.037-mg/L was an MBR that integrated biological, chemical and membrane processes for nutrient removal into an activated sludge treatment process. Further, the proposed treatment process targeted an effluent nitrate level of less than 10 mg/L. There are inherent limitations to minimum effluent phosphorus concentrations that can be achieved through biological means. As such, the biological process was enhanced with a chemical nutrient removal process prior to membrane filtration. Tertiary phosphorus removal was achieved by chemical precipitation with a metal coagulant (alum). The membrane system could then provide superior solids and precipitate removal and replace conventional secondary clarifiers and granular filters. The general schematic of the proposed process treatment train is provided in Figure 1.
The pilot study was conducted in four phases over a six-month period. The four phases included:
Hollow Fiber Membrane Module
A detailed monitoring, sampling, and analysis plan was developed for each phase of the pilot study to ensure proper data collection and similarity of results. In general, analytical data was collected from 24-hour composite samplers, and an independent certified testing laboratory performed necessary testing. Operational data was collected via online electronic instrumentation, and access to the data was provided to project team members through a secure Internet connection. The data was evaluated for the ability to reliably maintain target effluent phosphorus and nitrate concentrations. Also, the data was analyzed for ammonia, COD, TSS, turbidity and total coliform.
Both MBR pilot plants proved to be an effective barrier to particulate matter, including unhydrolyzed solids, suspended solids and chemical precipitants. Given the effectiveness of the membranes, the pilot units operated at an elevated suspended solids concentration (9,000 to 11,000 mg/L), thus providing the operational benefits anticipated from a higher solids concentration.
During the demonstration phase, both pilot units achieved significant phosphorus reduction. Overall, the pilot units achieved 99.49% phosphorus removal when comparing pilot unit influent to the pilot unit's effluent (membrane permeate). As shown in Table 3, the pilot units comfortably provided effluent below the extremely stringent phosphorus limit of 0.037 mg/L. Similar to the total phosphorus, the piloted treatment processes achieved nitrate levels well below the target set for the pilot study. As demonstrated, the pilot units produced an effluent with less than 7 mg/L nitrate. Table 3 summarizes the effluent water quality from the pilot units during the demonstration phase as a 30-day running average.
Given the water quality achieved by the pilot units, MBR technology, combined with biological and chemical nutrient removal, should be considered where nutrient requirements of NPDES permits are very stringent and/or for reuse programs. For instance, the Safe Drinking Water Act targets a drinking water finished turbidity value of <0.1 ntu. From Table 3, the piloted MBR processes produced an effluent turbidity value an order of magnitude less than the drinking water standard.
Water Reuse Scenario
With discharge restrictions driving municipalities to produce near drinking water quality from their wastewater effluents, a new paradigm may be in order. For most cities, it seems a shame to expend the effort required to produce a high-quality effluent only to throw it away. The reality is that the effluent produced by an MBR plant will have significant value. Many municipalities in the arid Southwest and along both coasts already consider treated wastewater as a valuable resource and practice reuse in one or more of the following ways:
When assigning a value to this new resource, it is important to consider at least two key points:
For illustration purposes, consider the MBR equipment payback period if a mid-sized municipality were to practice non-potable reuse of 1.0 mgd of WWTP effluent:
Under this scenario it is evident that the capital costs associated with advanced reuse technology can offer a rapid return on the city's investment.
Water Reuse Implementation
Before implementation of wastewater reuse, municipalities should consider preparation of a Water Reuse Plan and the associated economic analyses. Such a plan can help educate the public, develop target users, establish a rate structure, determine investment return and identify the necessary distribution infrastructure.
Although typically a tougher sale to the public, the greatest economic potential would be development of the plant effluent as a source of raw water supply. As most utility professionals have experienced, when it comes to indirect potable reuse of wastewater the general public is not convinced that today's water supply shortage warrants promotion of wastewater discharges into water supply reservoirs.
As source water supplies continue to diminish and wastewater effluent requirements become more stringent, wastewater reuse will become more paramount. MBR technology has proven a viable option for production of reliable reuse quality water. As utilities consider ways to offset the stresses on water supplies, it appears a priority shift to reuse of one of their most valuable commodities—wastewater effluents—is on the horizon.
The author thanks the Water Business Line staff at Garver Engineers for their input to this article.
Michael J. Graves can be reached at (405) 329-2555 or MJGraves@garverengineers.com.