(Benefits of Aeration & Anoxic Control Strategies)
With the need for managing our resources ever so important today, mitigating the impact of discharging processed wastewater into “receiving waters” and utilizing the benefits of recycling wastewater is much more than just a good idea. In California, where droughts come and go on a regular basis – California has been in one of the worst recorded droughts for over three (3) years as of the time of this writing – reusing once considered “wastewater” from sanitation facilities has become a necessity. In order for water users such as farmers, ranchers, golf courses and parks to be able to rely on recycled water, it’s important for that water to be usable and practical to store in reservoirs. But wastewater is not all waste. Even though it will be “sanitized”, it’s typically loaded with nutrients from raw influent and nitrites and nitrates from processing. And therein lies a new problem to be solved created by this opportunity to reclaim water from water pollution. For example, California state regulations set discharge eutrophication limits for a number of biological parameters including nutrients into “receiving waters”, which can lead to the depletion of oxygen and acceleration of algae growth and even put humans at risk when discharged water can enter drinking water systems as via rivers. And, storing water in open body reservoirs exposed to sunlight, creates an environment in which algae can bloom and thrive well beyond an acceptable level making stored water quality management essential.
Past Solutions - Limited Technology
To date, one method used for stored water protection has been adding chemicals to create an artificial built-in shade cover to water beneath the surface thereby substantially slowing algae growth. And while this is a good solution, it leads to more costs and the use of more chemicals than what’s actually necessary for proper management. Clearly the amount of exposure to the sun cannot be reduced for large bodies of water without adding chemicals, so that leads us to determine a practical and cost-effective way to process and ideally remove the nutrients before storing it. But these chemicals do not remove the nutrients, so the use of inorganic chemicals alone is not a viable solution to complying with state mandated discharge requirements.
Throughout the years of operation at a wastewater treatment facility located in the beautiful wine country of Sonoma Valley, every now and then the aeration process had been disrupted putting the wastewater into an anoxic state due to various reasons such as a power failure or perhaps a system shutdown to install new equipment or to solve some technical problem. During these times operations and lab personnel have observed and documented what was historically considered an adverse effect on the plant and in particular the aeration basins where the essential biological process occurs breaking down the raw sewage entering through the facility’s influent. This organic autotrophic bacteria (aka “bugs”) living in the aeration basins typically need about 2.0 mg/L oxygen to survive and thrive, so the longer the oxygen supply was disrupted the more adverse the impact on this necessary bacteria. Deprived of oxygen long enough, a wastewater treatment plant just might need a new colony of healthy autotrophic bacteria to get started again – a situation considered totally unacceptable. Consequently, most wastewater treatment plants historically have over-aerated, and many still are over-aerating to this day. As a side effect to aeration, the autotrophic bacteria in the aeration process easily converts gaseous ammonia or ammonium (the presence of which is pH dependent) to nitrites which are then converted to nitrates. So with inspiration and insight from extended education in the advances of wastewater technology, and with astute observations and analytical review, Sonoma County Water Agency’s then Regional Coordinator, Brian Anderson, Grade V Operator, who was responsible for supervising the facilities daily operations and maintenance, began the pursuit of nutrient removal using the biological means and equipment already available to him at his facility in Sonoma, California.
What is Our Plant Capable Of? (Research, Trials and Testing)
Aware of the potential opportunity to take advantage of the process, Brian and his staff began running tests using several method’s to monitor the process and their progress in trying to maximize the effectiveness of the oxygen deprived aeration basins in removing nutrients. Monitoring the processes would consist of observing visual changes, odor differences, and the use of instrumentation readings such as oxygen reduction potential (ORP) and other lab tests. The ORP probe readings are a helpful tool for monitoring and making decisions regarding adjustments. While an ORP reading of about 200 is typical for normal conditions, readings in the range of -50 to +50 are generally accepted to be indicative of an anoxic state.
Carried out over eight (8) months, various trials were conducted such as adjusting DO set points and adjusting MLSS concentrations through wasting activated sludge. ORP meter readings were taken multiple times daily noting in particular the conditions in which readings fell into a specific desired range - the point at which it is believed the process had reached a denitrified state (when the heterotrophic bacteria begins consuming the oxygen molecule found in ammonia and ammonium). For Sonoma Valley, a reading of -150 was considered the sweet spot for its denitrification cycle; however, it is expected that the desired ORP reading will vary for each facility, which is why conducting various trials and testing are critical to the success in determining the required control strategy changes. In short, Brian and staff found that being able to target times of day with high Biochemical Oxygen Demand ( BOD ) loading periods and controlling the duration of low dissolved oxygen periods and increased MLSS concentrations the system produced more consistent nitrate values below 10 PPM approximately about one-third below the norm for their plant.
Some challenges were encountered of course such as from storm events that increased flow due to I&I (inflow and infiltration). Storm water flows are typically much colder and highly oxygenated which require an even higher MLSS concentration to get dissolved oxygen levels to drop into the desired target range. Typically, these challenges were resolved by increasing the mixed liquor concentration during winter conditions for this treatment facility. For a more in-depth analysis and explanation of denitrification, Steven Myers has a very informative video titled Nitrogen Removal Basics.
Collaboration & Implementation
Assisted by ZSI, Inc, ( zsii.com ), a process control and automation consulting firm, Sonoma County Water Agency ( SCWA ) began collaborating with ZSI on how the existing aeration system control strategies could be modified to enable Operations personnel to operate the system differently using the knowledge gained through their tests in order to implement a biological nutrient removal (BNR) cycle into the existing infrastructure and equipment without having to have a dedicated denitrification zone. After detailed discussions and research, the facility was eventually able to obtain funding to automate DO control to target peak BOD loading times and set points for necessary DO concentrations.
The control strategy design for Sonoma Valley Wastewater Treatment Plant used two (2) time periods to obtain the maximum benefits available from the heterotrophic bacteria feeding on the organic carbon source or BOD found naturally in the decaying matter throughout the raw sewage coming into the plant.
Having ensured the autotrophic bacteria accomplished its task, the control strategy then severely restricts the oxygen injected into the raw sewage at the aeration basins inducing an anoxic state, whereby the process can then take advantage of the available oxygen found in the raw sewage and from aeration. This control strategy effectively denitrifies the water by diminishing the normal 2.0 PPM dissolved oxygen state allowing the heterotrophic bacteria, which thrives in oxygen deprived environments, to consume the nitrate’s oxygen molecule turning it to nitrogen where it can vent to atmosphere. Inducing this anoxic state also leads to further untended benefits.
Unintended Benefits Harnessed through Automation
While the original objective was primarily the reduction of nitrates, once the control strategy changes were established several other benefits became apparent such as a more stable biological process which isn’t so affected by seasonal offsets like water temperature and oxygenation, alkalinity restoration, pH stability and lower utility cost due to an approximate 50 percent reduction in air supply during the anoxic cycle. Also, secondary turbidity values well below 2.0 were experienced. And then in turn the lower turbidity requires less tertiary filters to be in service decreasing maintenance costs and further reducing energy costs. Because this cycle also has the benefit of getting alkalinity back naturally, the process is able to achieve an enhanced buffering effect, which was found to lead to less dramatic pH variation and easier management of pH, thus reducing the application or inorganic chemicals. The end result was an achievement of residual nitrates in the 3-4 PPM (new regulations in California require less than 10), leaving the ammonia / ammonium concentration always less than 5 PPM, a low enough value for the chlorine injected to burn it off. Similarly, an additional benefit realized was found in the diminished need for inorganic chemicals to control algae growth in reclaimed water reservoirs.
Because the chlorine dosage would typically pick up when leaving the anoxic state, the process typically experienced a 3 – 4 hour sine wave cycle in affect for the DO content while catching up after an anoxic cycle creating an additional demand for chlorine.
Potential Additional Benefits
With the experience and knowledge gained, Brian has continued researching various potential additional benefits likely to be achievable by implementing a more complex cyclical control during the typical diurnal period. So instead of a longer cycle once or twice a day as in Sonoma, at another facility located in the Sonoma County wine country he’s enhanced the strategy by adjusting the anoxic state intervals and mixed liquor concentrations in which the plant is taken in and out of its anoxic state thus achieving nitrates of 5 PPM or less and approximately 0.1 mg/L of ammonia, thereby also reducing the amount of chlorine required from otherwise having been in a longer anoxic state.
Lastly, further laboratory test results suggest that a well thought out anoxic control strategy may also have the benefit of stripping out certain metals such as mercury and nickel, but this is for research to be addressed in a separate publication.
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