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In conventional activated sludge (CAS) plants that rely on sedimentation, turndown (the effective operating range) is often very limited. When properly designed, operated and controlled, submerged membrane bioreactor (MBR) systems that use filtration for separation of solids can have higher turndown capabilities and ultimately be more energy-efficient than competing technologies.
What is turndown? In simple terms, turndown can be described as the range over which a system can operate, or the ratio between the maximum amount of wastewater that can be treated and the minimum amount.
There are many factors to be weighed when considering how much turndown a plant will need and how to make sure provisions are in place to achieve that turndown. Six keys to ensuring an MBR system will have the flexibility needed to operate efficiently over a wide range of operating conditions can be summarized as follows:
1) Install multiple smaller blowers connected to a common header or dedicated to reactors, as opposed to one or two large-duty machines.
2) Equalize or partially equalize influent to keep process equipment operating at or near their best efficiency point (BEP).
3) Incorporate multiple process trains so that unnecessary equipment can be put into a temporary idle/ intermittent mode.
4) Design and operate membrane filtration capacity in small groups (banks or trains).
5) Utilize the oxygen in air scouring to meet process demand.
6) Optimize and integrate controls for the entire plant to bring on capacity incrementally, dynamically match aeration to loading and avoid spikes in demand that lead to energy cost premiums.
On paper, the turndown of the Delphos, Ohio, Wastewater Treatment Plant (WWTP) is about 3:1, but in reality, this Energy Pro system has an effective turndown exceeding 30:1. A variation of the Storm Master concept that utilizes offline membrane capacity for treatment of biosolids, an Energy Pro system is designed to automatically take process trains (including filtration capacity) in and out of service to match demand and adjust membrane air scouring intensity as a function of throughput.
By utilizing a pump-forward recycle strategy and five parallel trains, the operator has more control of the process than in conventional designs that use a combined-pumped internal recycle.
Even with enough turndown built into the plant, the flexibility of the MBR system was not fully realized until the EQLogix SCADA system was modified and operating parameters field-optimized to account for low dry-weather diurnal flows. The current flow-based control strategy brings on a single process train—one of five—only when flows exceed an operator-adjustable low-flow setpoint (300 to 400 gal per minute [gpm]) and allows lower flows to accumulate or partially equalize in the anoxic zone until a low-level setpoint is reached and filtration begins.
In early 2007, flow control was purely based on level, and all trains functioned as a single unit transitioning between discrete low, medium and high setpoints. The SCADA system is now equipped with both control algorithms to protect against instrument failures (e.g., loss of flow transmitter signals) and provide another level of flexibility to handle the highly variable input coming from the combined sewer overflow system.
Following the SCADA system upgrade, the total plant’s energy costs were reduced by approximately 50% and the overall plant efficiency increased at low flows by several fold. Energy bills show the cost of treatment decreased from an average of $24.68 per 1,000 gal in 2007 (April through June) to less than $12.87 per 1,000 gal over the same period in 2008.
Even with the optimization of the system that has occurred since startup, energy usage trends still show how the plant gains in efficiency as flows approach or surpass the plant’s average daily flow, 3.83 million gal per day (mgd). At 2.60 mgd the plant usage was 1.95 kWh/m3, and at 3.46 mgd the average demand for the month dropped to 1.59 kWh/m3.
What happens if a plant is not equipped with sufficient turndown to handle lower than expected flows?
The Dundee, Mich., WWTP was upgraded from sequencing batch reactor (SBR) technology to MBR technology in order to increase hydraulics capacity and meet increasingly stringent permit limits. Without increasing the footprint of the plant, the average day flow capacity was doubled from 0.75 to 1.5 mgd (3.5 mgd peak day flow) and pollutant loading to the receiving stream was decreased during summer months.
The retrofit to an MBR system was accomplished by converting two SBR tanks into new process tankage and building four new common wall MBR tanks. New and existing process equipment was integrated into the MBR system to manage upgrade costs. The total constructed cost of the retrofit was $6.55 million. Construction began on the MBR system in June 2004 and was completed within 12 months.
At the time of the retrofit, the design goals of the plant were increased capacity and improved water quality on a budget. Both of these goals were met and exceeded. Unfortunately, only one of the keys to turning down energy usage was implemented—using air scouring to offset process oxygen needs.
The system is not equipped with offline equalization and has virtually no surge capacity. In fact, under typical operating conditions, the average side water depth in the AX/PA zones is less than 12 in. from the top of the tank wall and fluctuates over a 3-in. band. Even this small amount of equalization is used to treat incoming flow in a semibatch-like mode to avoid frequent step changes in flow/membrane flux.
The current programming tries to match demand by taking one of two filtration trains in and out of service based on the level in the AX/PA zone. Each train is made up of two reactors. The strategy works well given the small operating band but can often bring on a lot more filtration capacity and attendant air scouring than necessary to treat the actual flow coming into the plant. For example, during dry-weather months, average daily flows can be on the order of 0.5 mgd, while the low permeate setpoint capacity of one train has historically been 700 gpm, or roughly 1 mgd. While only 0.5 mgd is coming into the plant, two to four times the necessary filtration capacity can be online depending on the level in the PA/AX zone.
Based on a survey of electrical loads, it was estimated that the combined system energy usage should be on the order of 0.84 kWh/m3 of wastewater treated. In January 2006, at an average flow of 1.07 mgd, the actual total energy requirement was 1.23 kWh/m3; in January 2007, at a flowrate of 1.5 mgd, usage was cut in half to 0.66 kWh/m3. The reason for the discrepancy between predicted consumption and actual energy usage is most likely the result of several factors, including unaccounted-for loads, varying operating conditions and turndown. But a seven-year trend indicated strongly that turndown was the most critical factor in determining overall process efficiency.
In Figure 1, plant energy usage (based on utility bills) is plotted against average daily flows. Normalized plant energy demand increases as flows decrease, and the achievable energy efficiency is better than for the old SBR system (average 0.80 kWh/m3). The problem is that plant production will not be pegged at 1.5 mgd or higher year-round, so what can be done to improve energy efficiency?
To keep retrofit costs down, the old equipment—including blowers—was repurposed and the existing large SBR tanks made into process tankage. Compartmentalizing the large converted SBR tanks into smaller process trains, upgrading the old equipment and adding some amount of equalization would all go a long way toward increasing plant turndown and energy efficiency.
Short of these capital improvements, more advanced programming is being implemented that will automatically adjust aeration to match demand. This type of membrane aeration strategy is an integral part of an Energy Pro MBR system and modulates air scour intensity as a function of flux. Referred to as proportional aeration, the plant computer will automatically adjust air scour intensity using a mathematical correlation based on three operator inputs defining low, medium and high setpoints. In Figure 1, air scour intensity is varied according to influent flow rate given in terms of Q (equivalent to maximum monthly flow, which for the Dundee plant is 2.0 mgd). Assuming this type of diurnal profile, energy requirements can be reduced on the order of 20% or more using proportional aeration.