With serious drought and water scarcity issues plaguing numerous regions of the US, the implementation of wastewater reuse is changing how the public, industry, and governments manage water supplies. The prospect of running out of fresh water has been a sobering reality for far too many communities, placing a greater emphasis on conservation and reuse. Once underappreciated and undervalued, water reuse is now at the forefront of many water-related conversations. Whether it is for irrigation, groundwater recharge, or potable reuse, the practice of recycling wastewater for beneficial uses is growing at a rapid pace. Mother Nature has been recycling wastewater for an awfully long time. Advances in membrane bioreactor (MBR) technology have allowed us to do the same thing, but at a much more rapid and efficient rate.
MBR technology is well positioned to play a critical role in solving some of today’s most critical water issues. Technological advances over the past 10–15 years have lowered MBR costs dramatically, making it the most efficient and effective wastewater treatment technology. From retrofits to decentralized plants, MBRs are a versatile treatment platform that can help change the perception of water reuse.
DECENTRALIZED TREATMENT
Solving some of today’s more pressing water challenges will require innovative thinking, advances in treatment technology, and most importantly, fresh perspectives. The historical approach to wastewater treatment has been to construct large, centralized wastewater treatment plants (WWTPs) that serve a single or multiple communities. Since wastewater has always been viewed as a nuisance rather than a resource, much effort has been placed on minimizing costs. With this sole purpose, large, centralized plants have worked well as treatment and operating efficiencies are improved with large-scale plants. However, with today’s pressing water needs and challenges, communities are rethinking how best to handle wastewater and wastewater treatment. More importantly, wastewater is now often viewed as a resource driving innovation in both technology and treatment approaches. Communities and agencies are now focused on how to best utilize such a valuable asset and how to utilize its benefits.
Solving today’s water challenges will require innovative thinking, new technology, and fresh perspective. The conventional approach to urban wastewater treatment, which has worked well for more than a century, has been to build a large, centralized WWTP that serves a community, while solving a wastewater problem. In times of plenty, both of these solutions worked, but in the current climate, communities and homeowners are realizing that wastewater treatment (WWT) is not just about solving a problem, it’s about reclaiming a valuable resource. Turning a nuisance into revenue stream, at the point of use, is now often the most cost-effective way of managing our most important resource.
One type of decentralized WWT, often referred to as “scalping,” is gaining in popularity because it creates reuse opportunities near or at the point of use. As water reuse practices continue to grow, industry experts and practitioners are also beginning to see that centralized treatment isn’t always practical or cost effective. The cost to install pipelines that carry recycled water from a centralized treatment plant to the point of use can far exceed the cost of a satellite treatment plant. MBR technology is particularly well-suited for scalping plants due to its compact footprint and high effluent water quality. Advances in MBR technology have positively impacted capital and operating economics. In today’s water industry, MBR is cost competitive, if not cheaper, than conventional treatment approaches such as sequencing batch reactors (SBR) or conventional activated sludge (CAS).
For these reasons, and many others, the City of Midland, TX, installed an MBR Water Reclamation Plant to augment its water supply and reduce the total cost of providing water to the Midland College campus.
MIDLAND COLLEGE (MIDLAND, TX)
College campuses are meant to be vibrant environments filled with students and faculty eager to discover new ideas. The look and feel of a campus can play an important role in the discovery process and, in some cases, be fertile ground for bringing new ideas to life.
Many universities around the country are well-kept and beautifully landscaped. Unfortunately, maintaining a healthy and vibrant landscape at a university can consume large quantities of valuable potable water, a luxury the drought-stricken Southwest does not have. The City of Midland saw an opportunity through water reuse to not only maintain the look and feel of a college campus, but more importantly, alleviate stress on its severely depleted drinking water supplies.
The city of Midland relies on a combination of surface water and groundwater for its potable water supplies. With surface storage levels dipping below 15% and groundwater supplies high in arsenic and fluoride, the city started looking for ways to cut demand for fresh water.
Midland College was an obvious candidate for reuse with usage exceeding 1 million gallons per week during the summer. Adding to concerns about current demand, projections showed future water needs increasing by as much as 44% (Table 1).
After a joint 2004 study between Midland College and the City of Midland, a decision was made to enter into a partnership to construct a water reclamation plant next to the campus. A satellite treatment plant was the most effective solution, both in terms of cost and time, as the prospect of constructing pipelines from the existing WWTP through populated areas simply was not feasible. The campus’ close proximity to a main sewer line made the decision for decentralized treatment even more attractive.
Aerial view of Windlands Park
REGULATORY APPROVAL
In 2007, the city of Midland petitioned the Texas Commission of Environmental Quality (TCEQ) to create regulations that would simplify and shorten the permitting process for satellite treatment plants. In late 2008, Chapter 321 P was implemented in response to Midland’s petition, allowing the city to move forward with its water reclamation plans. TCEQ Chapter 321 requires that any reclaimed water production facility (RWPF) upstream in the collection does not add capacity to the city’s discharge permit. In addition, any and all sludge generated by a satellite RWPF must be treated offsite.
SELECTION OF MBR
Compared to conventional wastewater treatment technologies, MBRs provide significant technical and economic advantages, making it an obvious choice for the Midland RWPF. Advancements and efficiency improvements in MBR technology over the last decade have resulted in lower life cycle costs compared to conventional activated sludge (CAS) plants for reuse applications.
Unlike a CAS treatment process that relies on sedimentation processes for solids separation, MBRs provide a definitive boundary layer that provides complete solids retention. Both solids and pathogens are retained by the membrane, providing a highly disinfected, low-turbidity effluent.
Another key difference between CAS processes and MBR is footprint. Since CAS systems rely on clarifiers to remove solids, they are limited in the biomass concentrations in the bioreactors. The use of a membrane barrier layer to remove solids allows decoupling of a plant’s hydraulic and solids retention time (HRT and SRT). Therefore, MBRs can be operated at much higher mixed liquor suspended solids (MLSS) concentrations, shrinking plant footprint dramatically.
The combination of highest effluent quality, smallest footprint, and lowest life cycle costs made MBR the best available technology for the Midland RWPF.
FACILITY LOCATION AND DESIGN
The city had originally identified Midland Air Park, which is located directly next to campus, as the location for the RWPF. However, this location had its disadvantages, as the lift station and return flow sections would have to be located at Windlands Park, just south of the Air Park. It was decided that the best location for the RWPF was within the 55-acre Windlands Park, where piping to and from the lift station would be minimized. Windlands Park is a multi-use area containing 4 baseball diamonds, 13 soccer fields, trails, playgrounds, and picnic tables. Locating a WWTP within a park created several design challenges, from an aesthetics, noise, and odor-control perspective.
Even though TCEQ Chapter 321 P allows open treatment facilities without odor control, as long as there is a 300-foot buffer surrounding the plant, it was vitally important to the city that the public experience within the park not be negatively impacted by the presence of a wastewater treatment plant. Therefore, the city chose to construct an enclosed facility with odor and noise control while maintaining a 450-foot buffer. A masonry building containing the treatment plant, offices, control room, and chemical and blower rooms was designed to blend in with the surrounding park.
MBR DESIGN AND SELECTION PROCESS
The RWPF design at full capacity is 200,000 gpd, which represents only 5% of the total flow in the main sewer line at that location. This ensures that there is adequate flow to return all generated sludge from the MBR plant back to city’s wastewater collection system, even during low diurnal flows at night. This feature allows the MBR to operate at full design capacity at all times to meet weekly water demand, resulting in a smaller and more efficient plant design with lower capital and operating costs.
Based on the necessary flow demand, influent loading characteristics (Table 2), and effluent quality requirements (Table 3), a set of design specifications was developed and request for proposals delivered. It was decided to break the project down into two equal phases, with each phase delivering 100,000 gpd (0.1 mgd) in treatment capacity.
Since nutrient removal was not required to meet Texas reuse requirements, the MBR design, as shown in Figure 1, proved to be straightforward as no nitrification/denitrification basins would be needed.
The MBR configuration consisted of aeration basins followed by the membrane basins. A breakdown of the number of basins by phase is shown in Table 4, while details of the MBR design specification can be seen in Table 5. Each basin was sized so that an additional 20% in treatment capacity could be easily added to the RWPF in the future.
Installation of submerged membrane units (SMUs)
Proposals were evaluated using two net present worth analysis (NPW). The first analysis looked at capital, operating, and maintenance costs, but did not include membrane replacement costs. The second NPW looked at all four cost considerations. The plant owner chose to go with an Ovivo MBR system using a pre-selection process as it helped ensure that the offering did in fact provide lowest net present worth. Construction of the RWPF began in 2011 followed by commissioning in 2012.
FUTURE RECLAIMED WATER DEMANDS
The implementation of the MBR RWPF creates an opportunity to expand the city’s reuse efforts in the northern areas of Midland. Numerous schools and city parks near the RWPF consume nearly three times the water that Midland College does, totaling nearly 1 million gallons per day. If additional reclaimed water can be delivered to those entities, then additional fresh water resources can be preserved.
MBR treatment plant
