Adapting Our Thinking to Fit a New Facility and a New Process
Infrastructure Alternatives operates a wide variety of wastewater treatment plants, both large and small. Many of these wastewater treatment plants are “typical” activated sludge plants, oxidation ditches and sequencing batch reactors (SBRs). Of course, after you have been in the wastewater business for a while, you realize each wastewater treatment plant, even the “typical” ones, are unique in some way. It is an operator’s responsibility to embrace the uniqueness of their plant and to find ways to enhance operational performance through a wastewater treatment plant’s distinctiveness.
IAI does have state-of-the-art, cutting edge wastewater treatment plants among its Contract Operations facilities. One of these plants is in Leoni Township just east of Jackson, Michigan. The David J. Phelps Clean Water Plant is a 3.0 MGD Membrane Reactor (MBR) wastewater treatment plant, and I have been privileged to serve as the Manager of this facility since its start up in June 2010. The effluent from the Leoni Township MBR plant typically has TSS of less than 2.0 mg/L, BOD of less than 2.0 mg/L, ammonia of less than 0.1 mg/L and total phosphorus of less than 0.33 mg/L. Consistently low effluent values are the norm for a MBR plant.
In its most basic form, a MBR wastewater treatment plant is a Biological Nutrient Removal (BNR) activated sludge system. The twist in this system is in the solids removal process following the BNR process. Every operator of an activated sludge system has dealt with the problem of solids escaping from the secondary clarifiers. So, instead of secondary settling, the MBR process filters the mixed liquor through a membrane. This is a fail-safe way to keep the mixed liquor solids from exiting the wastewater treatment plant in the effluent. The MBR process uses computer control to manage and synchronize all of the actions required to keep the process operating.
Capital cost is high, which is the primary disadvantage of the MBR process. But that must be weighed against the exceptionally high quality effluent, the consistency of the effluent quality and the small footprint of an MBR plant. Leoni Township considered these advantages and disadvantages and decided that the MBR process was the best option for their community.
Leoni Township built their new wastewater treatment plant from the ground up, to replace an old, outdated aerated lagoon facility. The entire wastewater upgrade for Leoni Township cost over $32 million, which includes approximately $6.0 million in capital costs for the MBR equipment. This upgrade not only included the MBR process, but also centrifuges for sludge dewatering, effluent disinfection, pumping and reaeration and a six mile effluent discharge line. Even considering the cost, Leoni Township was convinced that the MBR process best fit their need for a system that would provide reliable, high quality effluent. The Township was expecting MDEQ to impose a phosphorus limitation of 0.1 to 0.33 mg/L and the MBR process was one of the few options to consistently produce an effluent with that degree of phosphorus removal.
The start-up of the MBR process went smoothly. Over 1 million gallons of return sludge was hauled from the City of Lansing WWTP to seed the Leoni Township process. We were not able to start processing wastewater at Leoni Township until the mixed liquor concentration in the MBR process was at least 8,000 mg/L. The return sludge from Lansing was combined with raw wastewater from Leoni Township and this mixture was constantly recycled through the return sludge system until the MLSS concentration reached 8,000 mg/L in the MBR basin.
In mid-June 2010, we began operating the MBR system 6 to 8 hours per day. The remaining influent flow was processed through the existing aerated lagoon system. We slowly ramped the process up over a period of 3 ½ months. During this period, all mechanical equipment and the PLC control system was tested and debugged. By mid-October 2010, we were processing all of Leoni Township’s wastewater flow through the new MBR system.
The most serious start-up issue that we faced involved the six process blowers. These blowers began to exhibit significant issues including failure of pressure relief valves, belt failures and metal cracking in filter housings. These failures left us without backup equipment from several days to several weeks on a number of occasions. Finally, all six blowers were replaced in late 2011 and early 2012. The replacement blowers have been reliable and relatively maintenance free.
The MBR process challenges the training and experience of a wastewater operator. Many of those long-standing “rules” of operation of an activated sludge process are essentially thrown out the window with a MBR process. For example, wastewater operators are taught that the mixed liquor concentration in the activated sludge process should be 1,500 to 3,000 mg/L. The mixed liquor concentration in the Leoni Township MBR plant averages about 12,000 to 14,000 mg/L, similar to the mixed liquor concentration of an aerobic sludge digester.
The Mean Cell Residence Time (MCRT) of a typical activated sludge plant will be from 5 days to 15 days. As a result of the high mixed liquor concentration in the MBR process, MCRT is significantly higher. The MCRT of the Leoni Township WWTP is greater than 100 days.
Process control in an MBR plant is also challenging. In a typical activated sludge system, the operator bases many operational decisions on the settling characteristics of the mixed liquor. Since settleability is irrelevant to an MBR plant, process control decisions are made based on two factors: a determination of wasting rates to maintain the target mixed liquor concentration; and filterability test results, to assure that the separation of solids and effluent proceed efficiently.
I have relished the unique challenges of starting up and operating this new MBR plant in Leoni Township, along with the rest of the WWTP staff (pictured, from left: Operations Specialist Jason Freeman, Plant Manager and blog post author Tom High, and Operations Specialist Chris Crenshaw). We feel fortunate to be involved with such a unique, leading edge facility.Share this: