Rasing the Bar
The city of Delphos, Ohio evalu-ated several treatment options, including an oxidation ditch, when considering how to upgrade its aging trickling filter facility. Some of the factors considered in the decision-making process were current and future permit compliance issues, fluctuating loading conditions and treatment capacity.
The city selected an Enviroquip membrane bioreactor (MBR) system using Kubota submerged membrane equipment. At times treating more flow than any other MBR system in the world, the Delphos plant is the second-largest plant in the U.S. in terms of rated capacity. Since startup in September 2006, the plant has already been put through its paces and demonstrated its capability to reliably meet permit limits. When asked about some of the drivers that lead to the selection of an Enviroquip MBR system, City Council President Robert Ulm stated, “Our decision to use this technology for our wastewater treatment was based on the need to have a system that could easily adapt to considerable fluctuations in our plant intake depending on industrial activity and Midwestern thunderstorms that can often deposit several inches of rain in a short period of time on our relatively flat terrain.” Ulm said the council also needed technology that could more than adequately deal with the demands of its partially combined storm and sanitary sewer system and the plethora of environmental waterway protection regulations.
“Our community was facing a reality of reoccurring EPA violation fines and faced the prospect of continued regulation violations without a major plant upgrade,” Ulm said.
Delphos has a sewer system that is 70% combined and 30% separate. This means that 70% of the sewers collect storm water as well as domestic and industrial waste. As a result, the new MBR system had to be designed to handle a wide range of waste and hydraulic loading.
Using the concept of biohydraulics, the MBR system was designed to exceed biological treatment objectives over the range of expected operating conditions. Designed using the Storm Master configuration, the plant is also equipped with SymBio technology to promote simultaneous nitrification and denitrification (SNdN) in the supplemental aeration zone. Operating at low DO in SNdN mode can reduce operating costs and ensure optimum biological process performance. The Delphos WWTP comprises five independent process trains that include an anoxic zone, SNdN zone and MBR (see Figure 2). In each MBR there are 26 double-deck Kubota submerged membrane units (SMU), as shown in Figure 1.
The Storm Master design is an important feature of the Delphos WWTP because it further reduces overall plant operating costs by putting offline membrane capacity to beneficial use. For example, during extremely low flows, one of the five process trains is used to treat incoming wastewater while another is used to digest and thicken solids to 3% before further treatment. As flows increase, the plant computer automatically brings all trains on line to treat peak flows.
Flexible & reliable
Using Kubota SMU, Enviroquip designed the MBR system to handle average daily flows of 3.83 mgd and peak daily flows up to 12 mgd. However, by employing the Storm Master design, Enviroquip actually automated the plant to handle flows ranging from 300 gpm (0.4 mgd) to a maximum net capacity of 8,328 gpm (12 mgd). As a result, the effective turndown ratio of the plant is 28:1, allowing for minimum recorded flows of ~400 gpm and the peak flows of ~8,000 gpm already experienced during periods of high rainfall.
Unlike many MBR plants that are significantly underloaded in anticipation of future flows, the Delphos WWTP has already treated sustained flows in excess of 10 mgd. In addition, these flows were handled at mixed liquor suspended solids (MLSS) concentrations averaging 20,000 mg/L, with a maximum concentration of greater than 25,000 mg/L in one reactor during startup (see Figure 3). Most MBR plants are designed to handle average flow conditions and occasional well-defined peak events. However, MBR systems must be capable of handling fluxes far above average capacity for long periods of time during any given day and operating over a broad range of MLSS concentrations.
The Delphos MBR System was designed to operate at MLSS concentrations ranging between 8,000 mg/L and 18,000 mg/L, and to automatically handle diurnal peaks and storm events. The capacity of the system to handle diurnal flow patterns during periods of high flow was illustrated over a 10-day period in January.
The maximum monthly capacity of the plant is 6 mgd. In membrane terms, this equates to a flux (filtration rate) of 13.4 gfd. In Figure 4 the colored bars show how the plant treated flows in excess of 10 mgd (23.4 gfd) for ~2 to 16 hours per day during this time even as MLSS concentrations hovered around 20,000 mg/L (see Figure 2).
The ability to handle such a wide range of operating conditions gives operators time and confidence to handle system upsets. However, the ability to operate the plant manually in the event of an emergency provides another level of reliability that can be invaluable.
The Delphos plant is fully automated using a customized SCADA system that allows for remote monitoring and control. However, unlike many membrane systems that require complex automation and constant adjustments, the plant can be run in a hand mode to protect against power surges, brownouts or a loss of network communications that may temporarily interrupt automation (even with necessary built-in redundancy).
Effluent data collected since startup has already demonstrated the ability of the plant to easily meet permit limits over a range of influent loading conditions. In addition, measured mercury levels have dropped to non-detectable amounts (<0.5 mg/L), most likely through adsorption onto wasted solids (see Table 1). With a total installed cost of approximately $31 million (including digestion, solids handling and UV) and a construction time of less than two years, the Delphos WWTP has set the bar for new large MBR facilities in the U.S. and around the world. More importantly, the competitive cost and proven capabilities of the system to produce reuse quality effluent over a broad range of loading conditions demonstrates the ability of MBR systems to be a reliable alternative to conventional technologies.
“This new facility puts the city ahead of most other communities in terms of new regulations that we know are coming down from the EPA,” Safety Service Director Greg Berquist said. “Luckily we were on the front end of the wave with the EPA.”
Delphos truly is on the front end of the wave—not just with the EPA, but with the wastewater treatment industry as well.