The Better Biology Behind Wastewater Phosphorous Removal
Ed Kobylinski, Wastewater
August 18, 2022
You visit a lake in the heat of the summer and wonder if you should have brought a golf cart instead of a ski boat. Or maybe you notice dead fish bobbing in the slack, rank waters near the shore.
You’re likely seeing the effects of a toxic algae bloom.
Algae can be beneficial to water ecosystems, but a scummy, thick film on the water (often with a blue-green tint) indicates the presence of cyanobacteria, which are single-celled organisms that use chlorophyll to draw energy from sunlight. As blooms of cyanobacteria die off, they can deplete oxygen from the water, emit a stench, and produce toxins that cause serious illness or death in fish, wildlife, and humans. Think the dead zone in the Gulf of Mexico.
Clearly, we don’t want this muck our waterways.
The way to combat bad blooms is to reduce the primary nutrients that fuel their growth – nitrogen and phosphorus. Environmental regulators are increasingly ratcheting down the level of these nutrients that can be released by wastewater treatment plants. Fortunately, there is a proven strategy for reducing phosphorus called biological phosphorus removal (BPR).
Unfortunately, not all wastewater plants have the right conditions to support BPR. Many plants, however, can be enhanced by adding a sidestream fermenter so that BPR works more reliably and consistently.
As a process engineer, I helped develop and implement sidestream fermentation systems alongside Dr. James Barnard, who pioneered the science behind removing phosphorus and nitrogen from wastewater.
To better understand what sidestream fermenters can do, let’s quickly review phosphorus and BPR.
Phosphorus Feeds the World, So Why the Fuss?
A highly reactive nonmetal element, phosphorus is extracted from minerals to make fertilizer for large-scale crop production. Excess phosphorus washes into streams and lakes, throwing aquatic systems out of balance. Phosphorus is also a nutrient that passes through humans, which is how it enters a wastewater treatment plant.
Wastewater plant operators can reduce phosphorus by adding aluminum salts, iron, and/or calcium, but the cost and storage demands of these additives can be substantial. What’s more, the chemicals produce large amounts of sludge, the removal of which consumes even more of a plant’s budget.
BPR relies on producing a robust population of microscopic bacterial organisms with the ability to absorb and retain phosphorus. They’re called phosphorus accumulating organisms or PAOs. The process is pretty simple – create the right conditions for the microbes to flourish and the effluent will meet the requirements of a plant’s environmental permit. As a bonus, BPR is more affordable than chemical treatment and it produces significantly less sludge.
The right conditions are achieved by passing influent wastewater through the proper sequence of holding tanks that alternate between anaerobic (oxygen absent) and aerobic (oxygen present) conditions.
Another key factor is having enough food (in the form of soluble carbon like organic acids) in the influent water for the good microbes to populate and take up excess phosphorus. If your community has a food producer, soda factory, or brewery that releases sugars into the influent wastewater, you could be set up perfectly for BPR.
Some wastewater plants have it all and work great with a standard BPR system. Some function well during periods of normal precipitation and warm temperatures, but they struggle during periods of heavy rain and winter temperatures. Other plants lack one key element or another and just aren’t right for BPR. The way to find out where your plant falls is through influent wastewater sampling.
Many of the shortcomings mentioned above can be addressed by designing a sidestream fermenter enhanced biological phosphorus removal (S2EBPR) system.
Fermenting Cleaner Wastewater
Similar to the way a wine fermenter produces alcohol, a S2EBPR system creates volatile fatty acids, essential for the good microbes (PAOs) to grow. The fermenter is essentially a food-management system, but it also performs other critical roles.
Designing a sidestream fermenter into a standard biological phosphorus removal system provides a way to consistently cut phosphorus levels in a plant’s effluent, regardless of seasonal temperature changes or stormwater runoff.
For example, the fermenter acts as an anerobic incubator where the good microbes can rapidly replicate. The operator can control the retention time in the fermenter; a flow-through anaerobic zone has a much shorter retention time and performs poorly during high runoff events. Gaining better control of the environment and fermentation zone results in better process stability and reliability. In other words, the system does its job rain or shine, hot or cold.
Wastewater plants with flow-through anaerobic systems have achieved phosphorous levels consistently below 1 milligram per liter (mg/L), but during flooding rains, phosphorus removal falters. At these times, it is common to add iron or aluminum to polish phosphorous removal or rely on chemical precipitation to meet permit limits. Adding chemicals also increases sludge production and operating costs.
The results from plants designed with fermenters have been excellent. S2EBPR plants can produce effluent total phosphorus concentration well below 0.5 mg/L without effluent filtration. In addition, S2EBPR systems can achieve the desired results on their own, without chemical supplementation. One S2EBPR plant in Kansas has produced an effluent averaging between 0.1 and 0.2 mg/L total phosphorus (TP) without adding iron to polish phosphorus removal or supplemental carbon to enhance production of volatile fatty acids. This plant has been in operation for four years.
Enhanced BPR systems allow good microbes to chug along and digest phosphorus around the clock, reducing your wastewater plant’s budget and meeting the limits of your environmental permit. And ultimately, the plant does its part to combat aquatic dead zones.
Ed Kobylinski, PE, is subject matter expert in wastewater with more than three decades of experience in evaluation and design of municipal and industrial treatment facilities. He specializes in biological treatments, batch/continuous pH control systems, wastewater reuse and segregation projects. He is currently serving as an adjunct professor at the University of Kansas where he teaches a course on wastewater plant design. Reach Ed at 913.381.1170