On-line Monitoring Helps Optimize Colorado BNR Plant
Biological nutrient removal processes can be both effective and economical, but they can also be a challenge to operate.
Biological nutrient removal processes can be both effective and economical, but they can also be a challenge to operate. In its BNR system, the Pinery Wastewater Treatment Plant is using advanced instrumentation and control to achieve lower effluent nutrients and improved treatment efficiency.
More advanced instrumentation and control systems are today allowing the monitoring and tuning of wastewater treatment processes to an extent that would never have been considered manually. This is allowing BNR treatment plants such as Pinery to consistently meet stringent phosphorous limits while improving plant efficiencies.
The Pinery Subdivision is located along Cherry Creek, approximately three miles south of Parker, a suburb of Denver. The 2 mgd wastewater treatment plant, built in 1990, includes a 5-Stage Bardenpho advanced biological nutrient removal system. Processes include fermentation, first anoxic, aeration, second anoxic, re-aeration, final clarification, filtration and disinfection. The BNR facility is used to remove both phosphorous and nitrogen as its primary objective. Biological Oxygen Demand (BOD) and total Suspended Solids (TSS) are removed by the longer retention times in the facility.
The 2 mgd Pinery Wastewater Treatment Plant includes a 5-Stage Bardenpho advanced biological nutrient removal system. Photo by Kevin Clark.
Plant effluent is either discharged into Cherry Creek or used to recharge local groundwater. The creek eventually flows into Cherry Creek Reservoir and the plant is under the oversight of the Cherry Creek Basin Water Quality Authority.
“The Authority has a goal for the reservoir of 40 μg/L of total phosphorus, and in August 2004 it reduced wastewater treatment plant (WWTP) direct discharges to 0.05 mg/L phosphorus,” said Kevin Clark, wastewater superintendent of The Pinery Wastewater Treatment Plant. “Phosphorous removal was the major driver in the design of our plant, and currently there’s a lot of anticipation that ammonia and nitrate removal will become even more predominant.”
The plant’s current nitrate discharge limit is 11 mg/L and ammonia is 35 mg/L.
From the pretreatment processes, wastewater enters a fermentation zone where it is combined with return activated sludge from the plant’s secondary clarifier to produce the appropriate organism stress condition (occurring in the absence of dissolved oxygen and nitrates) that allows large quantities of phosphorous to be removed in subsequent aerobic stages.
Following the fermentation stage, flows enter the first anoxic zone, which is primarily designed for denitrification, and then proceed into an oxidation ditch that serves as the plant’s aerobic zone. From there, flows enter a second anoxic zone, which is a polishing step to remove any residual nitrate. Prior to final clarification, flows enter a re-aeration zone to ensure that phosphorous is retained in the sludge.
Automating Monitoring/Control Functions
Advanced control and instrumentation offers tremendous potential for achieving lower effluent nutrients and improved treatment efficiency in BNR facilities. Key areas for optimizing control include aeration, nitrate recycle stream, and SRT control. A recent expansion project at the plant included the addition of a number of sensor and control technologies to enhance monitoring and process control of this BNR facility, including the implementation of SCADA and automation support.
Integral to the plant’s new data communication and process control infrastructure are six Hach sc100 Universal Controllers that continuously read the on-line LDO® (Hach Luminescent Dissolved Oxygen Measurement Technology), ORP and pH probes and communicates this data via 4-20 mA signals to the plant’s SCADA system. The controllers also have built-in dataloggers that collect measurements at user selectable intervals (1 to 15 minutes), along with calibration and verification points, alarm history, and instrument setup changes for up to six months. The controllers are designed to receive data from up to two sensors simultaneously, and plug-and-play capabilities and multiple-parameter functionality allow operators to easily switch probes between different processes.
“Prior to this upgrade, we worked with lots of grab sample data, but nothing that really allowed us to look at the complete cycle of the facility like we do now with on-line monitoring,” Clark said.
Integral to the plant’s new data communication and process control infrastructure are six Hach sc100 Universal Controllers (right) that continuously read the on-line LDO®, ORP and pH probes. Photo by Kevin Clark.
Two ORP sensors, two LDO sensors, five pH sensors and two phosphorous analyzers have been added at the facility, each reporting continuously to the plant’s SCADA system.
An example of improved BNR monitoring and control through on-line measurement technologies can be found in the plant’s first anoxic stage. An on-line ORP sensor continuously monitors the process. In this stage, mixed liquor containing nitrates from the plant’s third stage (the nitrification stage) is recycled back and mixed with conditioned sludge from the fermentation stage. In the absence of oxygen, bacteria uses BOD in the influent to reduce nitrates to gaseous nitrogen.
“Here, our nitrate mixed liquor recycle rate is now controlled to real-time ORP values,” Clark said. “When the redox value becomes greater or less than operator-definable set-point values, the nitrate recycle pump rate is increased or reduced proportionally.”
With the new on-line meters and plant control systems providing a real-time and historical picture of DO and ORP values at different organic/hydraulic loading rates, recycle rates, solids retention times and seasonal conditions, operators are better equipped to establish trends to further optimize individual processes as well as determine the cause of any transient conditions.
“In a nitrification/denitrification facility like ours, we’re primarily looking at DO and ORP to determine when nitrification occurs and at what point denitrification takes place,” Clark said. “We’re now using continuous measurements to compile an extensive operating history to help us optimize our system at these critical process points.”
In the plant’s oxidation ditch, real-time ORP or DO control the speed of the mechanical aerator VFD drives serving the process, and a LDO probe located in the plant’s re-aeration zone, installed just upstream from the final clarifiers, continuously monitors the process to ensure there’s sufficient DO in flows prior to final clarification and discharge to the filters. The SCADA system alarms operators if DO levels here drop below 1 mg/L.
New DO Probe
As part of the instrument installation upgrade last year, the plant chose an on-line DO metering probe that uses a completely new technology over conventional membrane-type metering probes. Hach LDO luminescent technology DO probes, unlike galvanic and polarographic DO sensors, do not consume oxygen as part of the measurement process. Membrane-type DO probes rely on the consumption of oxygen at one electrode and the resulting current flowing through electrolyte to the second electrode. This oxygen consumption often creates a fouling buildup in membrane sensors and an oxygen gradient that slows down response.
“We like the fact that the LDO probe can read lower DOs,” Clark said. “In our treatment process, we definitely need to read lower DOs when we start to denitrify. Measurement-wise, we’re going to places that most DO probes don’t like to go. But with our LDO probes, we’re often reading down to the 0.2 ppm range, and it’s very responsive at this level. The membrane-type DO probes we had previously used didn’t perform well at these lower levels. Plus, there are no membranes or reagents that need replacing with the LDO probes, and we haven’t had any maintenance issues with them, nor with our Hach ph or ORP probes.”
Four on-line pH probes have been added to the influent side of the plant’s AWT filters, which now allow operators to better monitor chemical addition there. A fifth pH meter is located in the final effluent channel and, through an alarm established with the SCADA, provides an early warning system in the event of a pH excursion. Two Hach Series 5000 phosphorous analyzers also communicate to the plant’s SCADA via 4-20 mA signal, with one analyzer reading the effluent channel and the other reading secondary effluent into the filters.
The new SCADA system provides an integral picture of the treatment processes at the Pinery Wastewater Treatment Plant. Photo by Kevin Clark.
“Our system alarms us if our secondary phosphorous levels start to get into the 0.5 mg/l range,” Clark said. “That way we can make adjustments to the AWT filters, whose primary purpose is to provide phosphorous removal.”
Information = Optimization
Due to increasingly stringent regulations and the need for greater efficiencies, a wastewater treatment plant must be continually modified in order to maintain a high level of optimization. For an existing BNR facility, it is often in the area of advanced control and instrumentation that the greatest improvements can be achieved.
“We’ve seen definite improvements since the addition of our new instrumentation and process control equipment,” Clark said. “With continuous, real-time data we can now look at it as often as we need to, plus our processes are more easily adjusted because we can now see what our diurnal flow patterns are doing to our actual DO and ORP. We have four full cycles of water temperatures in Colorado, and each of these cycles present us with different DO requirements. We now have the necessary information and tools to help us year-around.”
About the author:
Phil Kiser is Technical Applications Manager for Hach Inc., Loveland, Colo.