The McFarland Creek Wastewater Treatment Plant (WWTP), located in Bainbridge Township, Ohio, recently underwent an expansion of overall plant treatment capacity from 1.2 million gal per day (mgd) to 1.8 mgd over a 20-year design plan.
The plant was provided with an MBR conversion rather than an expansion because most of the existing tanks at the plant were reused for the new facility, significantly reducing construction time and cost.
Solids Handling Process
The McFarland Creek WWTP uses an aerobic digestion process known as PAD-K  (pre-thickened aerobic digestion with Kubota membranes) for solids stabilization. The process is a sludge handling system that reliably and effectively produces a stabilized and homogeneous Class B sludge with minimal operator involvement.
The PAD-K  process consists of two aerobic digesters operating in conjunction with two membrane thickener (MBT) units. They are used in a two-step process that brings the sludge concentration in the first step digesters to 1.5 to 2% and 3 to 4%, respectively. Because the aerobic digesters and the MBT work in sync, the digested sludge is continuously thickened while undergoing both nitrification and denitrification through aeration cycling in the digester basin.
Rather than an expansion, solids handling at the facility was again converted to use the PAD-K process. The same existing tankage was used for the larger flows but it was optimized by the use of the MBT/PAD-K for a much higher solids concentration than previously done in the conventional aerobic digestion system.
The system was designed to operate at temperatures ranging from 10 to 12°C, meeting the objective of Class B and reducing the sludge disposal cost by more than 20%. Digesters in this facility are able to operate both in series and in parallel based on the loading rates to the plant and dewatering cycle. The plant gained exceptional solids processing savings of 41% reduction in cubic yards produced, 41% cost savings in polymer by using the MBT system and 41% savings in sludge disposal by using the system (Figure 1).
The American Council of Engineering Companies of Ohio has awarded CT Consultants an Outstanding Achievement Award for their lead role in the design of the McFarland Creek WWTP for 2007.