Let Gravity Do The Work: Use of Siphon and Partial-Siphon Permeate Design for MBR Systems

Introduction
Most membrane bioreactors (MBRs) use pumps to create a vacuum within the lumen or on the back side of the membranes to draw permeate through them. However, if the downstream head is sufficient, a siphon or partial siphon configuration can be used to draw permeate through the membranes by gravity. The use of a siphon can provide numerous benefits including lower capital and operating costs, as well as reduced equipment complexity and maintenance. Despite these benefits, the use of siphon and partial-siphon for permeation remains rare in MBR designs. This paper will explore siphon design and operation and draw on case studies to highlight potential benefits of siphon design in MBR systems.
Background
In an MBR, membranes are used to separate purified effluent (i.e. permeate) from mixed liquor following an activated sludge biological wastewater treatment process. Trans-membrane pressure (TMP) is the difference in pressure between two sides of a membrane. The TMP is influenced by many factors including the flux (flow per surface area) across the membrane, water temperature, mixed liquor concentration and characteristics, and biological and chemical fouling of the membrane surface. MBRs are typically designed to operate over a TMP range from as little as 1 to 2 psi (0.07 to 0.14 bars) with clean membranes at lower flux rates to higher TMP operation as flux rates increase and membranes become fouled. The vast majority of MBR installations rely on permeate pumps to overcome the TMP and hydraulic losses through the MBR system. Pump operation is usually controlled based on a combination of influent flow and the water level in the tank where the immersed membranes are located. As water level increases, permeate pump speed increases to maintain the water level in the tank within the desired operating band. However, when hydraulic conditions allow, permeate can be pulled through the membranes without the use of permeate pumps. Siphon Design Options In a full-siphon design, sufficient downstream head is available under all operating conditions for permeating through the membranes. Flow is typically regulated through the membranes by control valves on the permeate piping from each set of membrane cassettes. The control valves are throttled to maintain water level in the membrane tanks within the design operating band and control flow through the membrane system. In a partial siphon design, flow passes through the membranes by gravity under lower TMP operating conditions. If required, pumping is used under higher TMP operating conditions to generate additional negative pressure within the lumen or on the back side of the membranes. Two types of pumping arrangements are possible. In a closed-system partial-siphon design, conventional permeate pumps are used on the permeate piping to draw vacuum in the permeate piping when required. In an open-system partial siphon design, the permeate flow is conveyed to a lower tank open to atmospheric pressure where effluent pumps are used to draw down the water level in the lower tank when required. In the closed-system arrangement, bypass piping and valves are required around the permeate pumps to utilize the available head without pumping, while no bypass is required for the open-system arrangement. The three types of siphon designs are illustrated schematically in Figure 1. Note that all three types may require a backpulse tank to provide sufficient permeate storage to supply clean water flow back through the membranes periodically during maintenance or recovery cleans if reverse filtration is used for backpulsing or membrane cleaning by the membrane manufacturer.
Case Study 1: Brush Creek Water Pollution Control Facility (WPCF) The Brush Creek WPCF is a 6.0 mgd (22.7 MLD) treatment facility owned and operated by the Cranberry Township, PA. During upgrade of the original conventional activated sludge process to MBR in 2019, abandonment of the existing secondary clarifiers and effluent filters provided approximately 19 ft (5.8 m) of available driving head between the new membrane tanks and the existing chlorine contact tanks. After accounting for piping losses, this is sufficient to allow gravity permeation up to 4.8 psi (0.33 bars) membrane TMP. The required TMP across the membrane is a function of design operating conditions. For the Brush Creek WPCF, the optimum design was found to require a maximum TMP of 8 psi (0.55 bars). To accommodate the required maximum TMP at PHF through a full-siphon with maximum hydraulic losses would have required 29 ft (8.8 m) of driving head. The maximum required TMP could have been reduced through increasing the membrane surface area or lowering the maximum design mixed liquor suspended solids (MLSS), but this would have resulted in increased capital cost to purchase and house additional membranes or for larger BNR reactor volume. An open-system partial siphon arrangement was selected. Operational results for June 2019 through December 2021 are shown in Figures 2 and 3. TMP averaged about 0.5 psi (0.03 bar) during this period. The effluent pumps were required to operate only 115 hours during this 30 month period (mostly during high flow events), during other times all permeate flow was by gravity. Estimated construction and operating cost savings at design flow are shown in Tables 1 and 2.
Case Study 2: Linwood Water Reclamation Facility (WRF) The Linwood WRF is a 4.6 mgd (17.4 MLD) MBR facility owned and operated by the City of Gainesville, GA. Permeate flow from the immersed hollow fiber membranes is via full-siphon design since facility startup in 2008. The original membranes were replaced in 2017 through 2019. Operational results for 2017 through 2021 are shown in Figures 4 and 5. TMP averaged less than 1.0 psi (0.07 bar) more than 70% of the time during this period and never exceeded 3.6 psi (0.25 bar).
Conclusions
As shown in data reviewed from the Brush Creek WPCP and the Linwood WRF, the membranes operated for the majority of the time with less than 1.0 psi (0.07 bar) TMP. Therefore, if wastewater treatment facilities have as little as 3.3 ft (1.0 m) of driving head available downstream of the membrane tanks, they may be able to operate in siphon mode the majority of the time. This means that many facilities which may not have sufficient head to support full siphon operation may be still able to achieve substantial energy cost savings through partial siphon operation. In addition, a siphon system is more reliable and will continue to permeate even during a power outage. A siphon system also may reduce the amount of equipment and/or the rate at which pump and motor wear occurs. These benefits should be balanced against the added operational complexity of a partial siphon control system.