A Look at Protozoa in Wastewater Treatment Systems

Wastewater treatment is fundamentally a biological process. When influent enters the microbial ecosystem of a treatment plant, nutrient removal is accomplished through the consumption of organic matter by microorganisms. The bulk of all nutrient removal is performed by bacteria, however protozoa and metazoa balance these bacterial populations and offer insight into wastewater conditions. Operators who understand the varying roles of wastewater microbes and the conditions that favor their growth can foster an ecosystem that promotes optimal treatment. In this week’s blog post we will review the niche protozoa fill in wastewater systems to enhance monitoring efforts and inform process control.

Roughly four percent of a wastewater system’s microbial ecosystem is made up of protozoa. Protozoa are single celled microbes both larger in size than bacteria and more complex. The most common types of wastewater protozoa include amoeba, flagellates, and ciliates. By consuming free bacteria and small, unsettled floc, protozoa enhance the clarity of the final effluent. Observing protozoa populations under a microscope can also alert operators of treatment conditions and sludge age.

Amoeba are predominant under a young sludge age because they require high nutrient levels or low competition to grow. Under shock loads of biochemical oxygen demand (BOD), high concentrations of particulate matter, toxic conditions, or low dissolved oxygen (DO), amoeba can also dominate. The latter two conditions generally trigger the amoeba to develop a protective gelatinous shell that gives them an advantage over other microbes. Furthermore, their slow movement reduces oxygen demand required for growth and reproduction.

Flagellates are typically present under a young sludge age as well. Since flagellates compete poorly with bacteria for the same soluble nutrients, their growth is favored at the younger sludge age before bacteria have had a chance to populate. As such, a wastewater sample relatively high in flagellates can indicate high soluble nutrient levels also known as a high food to mass (F:M) ratio.

Ciliates are favored under a healthy sludge age. While they do not consume organic matter, they do feed on bacteria making them excellent indicators of healthy floc formation and useful clarifying agents. Without ciliates, bacteria and algae populations can grow out of control in the wastewater microbial ecosystem. Among the three types of ciliates common to wastewater, each group has different conditions under which their populations are favored.

Swimming ciliates start to form as flagellates disappear. They may experience a spike in population when levels of free bacteria are abundant for predation. If too many free bacteria are present, the ciliate population surge can ultimately result in a cloudy effluent. Crawling ciliates dominate when those free bacterial populations begin to stick together forming floc through a secreted slime layer. This slime layer is produced when dissolved nutrients become limited. Since swimming ciliates cannot readily pick off bacteria within the floc, crawling ciliates begin to out-compete them. As they feed on bacteria, crawling ciliates can improve flock structure. A more mature sludge age with reduced BOD allows stalked ciliates to compete with crawling ciliates. Stalked ciliates anchor themselves to floc using the cilia surrounding their mouth structure to create currents that draw in bacteria. Once their food levels have diminished significantly more, stalked ciliates begin to branch into colonial units to acquire food more efficiently. If sludge continues to age, stentors and vaginocola protozoa grow in abundance.

For more information on wastewater protozoa and how to monitor them, we’d like to recommend the following documents. These resources and others like them can be found using our online, resource library.

Bacteria Protozoa – Toni Glymph
The guide overviews basic wastewater microscopy, slide preparation, sample collection, and the microbiology of activated sludge plants.

Wastewater Microbiology & Process Control - Wisconsin Wastewater Operator’s Association
Learn the about microscopes, slide preparation, and the microorganisms found during wastewater treatment.

Protozoan Count – Toni Glymph
This guide describes how to sample protozoa for observation under the microscope.

Featured Video: Wastewater Treatment Process Control Testing

This week’s featured video was produced by the Athens Wastewater Treatment Plant. The plant serves a small town of approximately 1,050 people in West Virginia. In an effort to educate their small town and others across the country, Athens WWTP has developed a series of videos. In this particular recording, the plant will demonstrate several process control tests they use to evaluate their wastewater conditions. You’ll learn how Athens performs a settleometer test and monitors pH, temperature, dissolved oxygen, oxygen reduction potential, mixed liquor suspended solids, and volatile suspended solids.

Tests likes these are valuable for troubleshooting the dynamic environment of wastewater treatment processes and meeting regulatory compliance. As such, it’s important for sampling to be performed accurately, consistently, and in a location that is representative of the wastewater quality as a whole. The types of tests you perform, the number of samples taken, and the laboratory methods used to analyze these samples will depend on your system’s treatment type, chemical usage, equipment, and raw water quality. Results from the analysis will promote process optimization. A detailed copy of your facility’s sampling and testing procedures should be accessible in the utility Operations and Maintenance Manual for reference.

To provide more information on process monitoring, we’d also like to recommend:

• How to Utilize Your Lab Data to Optimize Process Control – Kim Riddell
• Wastewater Laboratory Basics – Illinois Water Environment Association
• Activated Sludge Process Control and Troubleshooting Chart Methodology – Ohio EPA
• Activated Sludge Process Control – Michigan Department of Environmental Quality
• Wastewater Sampling – U.S. Environmental Protection Agency

Focus on Chemical Feed Control

Chemical dosing at the water treatment plant is a critical, but often underrated step in producing safe drinking water. Historically, process control points have focused on the hazards present in incoming source water - with emphasis on the filtration and disinfection steps to minimize microbial risks. But while many hazards do indeed enter the plant with the raw water, it is just as important to identify the multiple risks associated with treating this raw water.   

Featured Video: Wastewater Microbiology