Biological Treatment 101: Suspended Growth Vs. Attached Growth

Optimal biological behavior is the linchpin of successful secondary wastewater treatment. Here’s an overview of two popular methods for gaining “bug control.”

No one knows it all. Not in life, and not in the complex world of wastewater treatment. Nearly every day I talk to industry practitioners — from front-line operators and engineers to PhD’s and CEOs — and I’m reminded that we’re all at different points on the learning curve. For those who aren’t well-versed in secondary (biological) wastewater treatment and the differences between traditional suspended-growth processes and attached-growth processes, this article is for you.

An assist comes from Orenco Systems’ Grant Denn, Senior Manager – Engineering Projects, who provided a “layman’s description” of how the technologies differ (from his perspective as a supplier of attached-growth solutions). For balance, as well as a bit more technicality, I also consulted research papers evaluating the two processes. It remains, however, a “101” — plainspoken and hopefully helpful.

A Short Primer

During secondary treatment, biodegradable soluble organics are degraded through aerobic biological processes. Beneficial microorganisms (bacteria and protozoa) feed on the contaminants, increasing their population (biomass) as food and oxygen are supplied. Biochemical oxygen demand (BOD) indicates a high level of microbial activity and is used as a measure of wastewater strength; as organic contaminants are removed, BOD decreases.

Fundamental Differences

In a suspended-growth system, such as activated sludge processes (also aerated lagoons and aerobic digestion), the waste flows around and through the free-floating microorganisms, gathering into biological flocs that settle out of the wastewater. The settled flocs retain the microorganisms, meaning they can be recycled for further treatment.

By contrast, attached-growth systems use a medium to retain and grow microorganisms. Trickling filters and rotating biological contactors (RBCs) are two common types.  The trickling filter (AKA trickling biofilter, biofilter, biological filter, etc.) consists of a fixed bed of gravel, peat moss, ceramic, plastic, or textile media — to name just a few — over which sewage passes and creates a biofilm that becomes thick and falls off (called “sloughing”). RBCs consist of a series of circular disks rotating through the wastewater flow, partially submerged. These disks, usually plastic, are the media on which the biofilm develops and eventually sloughs off.

Vertically aligned textile sheets

Bug Behavior

Successful treatment is tied directly to proper bug management, but the type of organic load coming into the system is an important factor.

According to Denn, suspended-growth technologies often struggle with highly variable waste streams. “The bugs get used to eating a certain amount of food, and when that food is taken away, they look at each other as food,” he explains. “They're all in contact with each other, so they start cannibalizing and reducing your total number of organisms.”

“With attached-growth,” he continues, “they don't go hunting down other bugs. When the food reduces, they basically learn to eat less, just like you or I would. If we're used to eating a lot, but for some reason we couldn't afford to eat as much as we did, we would learn to survive on less food. It's the same thing.”

Pros And Cons

Denn further states (and multiple scholarly sources confirm) that attached-growth systems generally require less energy, simpler operation, and less equipment maintenance than suspended-growth systems because there is less technology involved.

However, disadvantages attributed to attached-growth processes (on the whole) include a larger land requirement, odor issues associated with clogging of certain media, and the inability to handle high volumes of wastewater. Consequently, urban facilities often opt for suspended-growth processes, while attached-growth processes thrive in small- to medium-sized operations.

Orenco therefore focuses on small communities and decentralized opportunities, where efficacy and cost-effectiveness are at their peak. The sweet spot for the company’s AdvanTex system is 50,000 to 100,000 gallons per day (GPD).  “Once you get over 100,000 GPD, our footprint becomes a disadvantage and other technologies start to close the gap,” admitted Denn.

Of course, variations and innovations will continue to alter the landscape of biological wastewater treatment. For example, Orenco utilizes a patented textile fiber media that shrinks footprint by maximizing surface area, air space, and water-holding capacity. Other technology providers are similarly looking for their edge, but there will never be a one-size-fits-all solution.

As stated at the outset, wastewater treatment is complex, and biological wastewater treatment is particularly detail-oriented. The intent here is to acknowledge that everything (and everyone) has a starting point.