Simulation for MBR Optimization
Designing a membrane bioreactor (MBR) facility for optimum performance is challenging. Diurnal flow profiles, basin volumes and equipment capacities are unique to each project. Programmed delay times and process lag times make it impossible to analyze the following critical process interactions with a simple spreadsheet analysis—controlling basin instantaneous volume; the facility’s diurnal influent flow profile; and MBR capacity staging as a function of controlling basin level.
Accurately evaluating those relationships at the design stage is essential. Enviroquip’s EQLogix simulator is a software tool that dynamically simulates facility processes and provides data to support utilization and energy estimates at the design stage. The simulator also provides control settings that may be used at the startup of a new MBR facility or to optimize performance of an existing facility.
A typical MBR facility is shown in Figure 1. Screened influent is fed into the equalization basin. From there, it is pumped to the anoxic basin, where it is mixed with recycled activated sludge. The slightly oxic sludge is quickly depleted of oxygen, allowing denitrification to occur. The denitrified blended sludge is pumped forward into the pre-aeration basin where additional biological treatment occurs.
The MBR system is configured such that membrane capacity is incrementally taken in and out of service based on actual demand. Capacity staging enables the plant to run more energy efficiently by operating fewer membrane basins at higher flux rates. In simple terms, the programmable logic controller running the plant evaluates the controlling basin level and automatically selects the flux and number of membrane basins necessary to process influent flow efficiently. This approach is also referred to as energy matching.
Even though some membrane basins are idle at periods of low flow, mixed liquor is still recycled through them and air is periodically pulsed into the basins in order to keep the biology aerobic and the membrane units ready for service. This is referred to as the intermittent mode.
Maintaining the MBR basins in a combination of intermittent mode and filter mode reduces energy consumption by avoiding inefficient operation. Inefficient operation is characterized by low flux operation (higher kW per gallon processed) or excessive on-off cycling (equipment wear and tear). Return activated sludge stays on at all times to promote consistent mixed liquor suspended solids concentrations between membrane basins in different modes of operation (intermittent or filter mode).
Because a typical facility must accommodate a wide range of waste and hydraulic loads, automatic controls are provided to maintain idle membrane basins in a condition that will allow them to be staged on and off as required by influent flow. Lead and lag membrane basins are automatically rotated to equalize utilization.
EQLogix is a modular, standards-based program that ensures predictable operation of every Enviroquip MBR facility. EQLogix incrementally stages membrane basins in response to diurnal flow variations, thereby allowing the MBR system to efficiently and appropriately process incoming flows.
Appropriate capacity staging matches the amount of energy put into the system to the amount of wastewater being treated, minimizing energy costs and maximizing the intervals between membrane maintenance cleanings. The capacity-staging algorithm monitors level in the controlling basin, and as level increases or decreases in response to influent flow, MBR basins are automatically placed into and out of service.
The simulator is a modified version of the EQLogix control system programming. It dynamically simulates the facility’s diurnal flow and calculates mass balance around the controlling (varying level) basin. The mass balance calculation drives a simulated level transmitter that engages the control logic to place the required number of MBR basins into filter mode at the required fluxes. The user enters setup information on the simulator’s graphical user interface. (Figure 2)
In addition to hydraulic simulation, the simulator exactly reproduces the equipment sequencing of an operating MBR system. Simultaneous hydraulic, mechanical and process simulation performed using the simulator is accurate, repeatable and self-validated, leading to complete confidence in the resulting simulation data.
The diurnal flow profile, either empirical or hypothetical, is the primary variable that determines instantaneous membrane basin and permeate flux requirements. The user loads a diurnal flow CSV file resolved to one-minute intervals.
A mass-balance calculation accounting for programmed delay timers and process lags dynamically determines the instantaneous value of the controlling basin level, which is typically an inline partial equalization basin or an anoxic basin with surge capacity. The user enters equalization or anoxic basin dimensions.
MBR capacity is variable in two ways: First, the number of parallel MBR basins in filter mode, and second, the instantaneous flux state (gal per day per sq ft) of each permeate header in filter mode. A typical permeate header has three flux states: nominally low, medium and high. The number of permeate-capacity increments in an MBR system is equal to the number of flux states times the number of permeate headers. The user enters flux set points corresponding to low, medium and high flux states.
The simulator evaluates level in the controlling basin against user-entered intermittent mode and flux-state transition levels.
The MBR aeration requirement varies in proportion to the flux state. The user enters the aeration set points for low, medium and high flux states (expressed as sq cu ft per minute per membrane cartridge).
A mode-change time delay determines how long an MBR waits before making a transition from intermittent mode to filter mode. The timer also delays changing an MBR’s flux state when a mode or state transition point is crossed by the controlling basin level. This delay reduces the likelihood of a false transition arising from a transient change in the level signal. Additionally, it reduces equipment wear and tear by minimizing unnecessary transitions.
Trends & Data
The simulator produces multivariable trend graphs that allow the user to observe the relationships among process variables. Additionally, by using pump and blower work equations (adjusted to account for vendor efficiency specifications), the simulator calculates energy consumption for the aeration and pumping subsystems.
Membrane operating fluxes are specified at the design stage to warrant sustainable operation under a specified set of design conditions. In general, operation at higher fluxes is more energy efficient than operation at lower fluxes. Other things being equal, there is a quantifiable correlation between operational flux rates and maintenance cleaning frequency.
For every facility, there is an optimum point of operation that can be described by a set of time-weighted flux rules. Operation close to the rule limits will yield MBR performance that minimizes energy consumption while ensuring sustainable flux between prescribed maintenance cleaning intervals.
One simulator output is time-weighted flux data for each MBR permeate header. If the data are not optimal, adjustments can be made to the controlling basin volume, the mode-change time delay settings or the varying basin-level flux-transition set points. After adjustments are made, the simulation may be iterated as necessary until time-weighted flux optimization is achieved.
Integrated Design is Essential
The efficiency of an MBR system is a function of component selection and plant configuration. An integrated design approach considering biology, hydraulics (referred to as biohydraulics) and controls is essential to efficient operation of an MBR system across a range of conditions. Dynamic process simulation allows a plant designer to evaluate a proposed design over a range of conditions and to verify that sufficient flexibility is built into the process prior to construction, saving time and money.
By providing sufficient aeration and process flexibility—or turndown—operators can better match the amount of energy put into the system to the actual pollutant load using features built into controls. A simulator based on EQLogix can be used to accurately estimate energy at numerous duty points using adjustable diurnal curves, evaluate equipment operation and optimize other operating costs including membrane cleaning.