Both trickling filter and activated sludge processes are common methods of secondary wastewater treatment. While commonly perceived as competing technologies, did you know that these two processes can actually work together to improve performance, reduce operational cost, and simplify operation? When we think of trickling filter and activated sludge processes as partners instead of competitors, we end up with a best–of–both–worlds approach to produce high-quality final effluent at the lowest possible cost. Let’s take a closer look:
What is a Trickling Filter?
Trickling filters are non–submerged fixed-bed biological reactors. Trickling filters utilize fixed media surfaces that serve as a host for microorganisms to attach to and form biofilm. A rotary arm distributes wastewater over the media where pollutants are effectively removed. Most trickling filters rely solely on natural draft ventilation to supply aeration. This means the oxygen required for the process requires no large blowers or aeration system. Filters with aeration systems use low-power fans that are usually run intermittently or when needed.
What is Activated Sludge?
Activated sludge processes eliminate organic pollutants from wastewater through microorganisms that are suspended in an aeration basin. In activated sludge processes, oxygen is supplied via mechanical or diffused aeration. Organic matter and ammonium nitrogen are removed via the suspended microorganisms in the wastewater.
Benefits of Combining Processes
Both processes are aerobic in nature with a shared goal of removing organic compounds from wastewater. The level of treatment that can be achieved with either independent process is comparable. By combining processes, wastewater treatment plants can capitalize on the benefits of both technologies while minimizing their respective shortfalls.
Often times, trickling filters precede activated sludge to provide a stabler, more robust system. A trickling filter can increase activated sludge capacity and help absorb shock loads. As trickling filters typically only require power for pumping, incorporating a trickling filter into an activated sludge system can reduce the amount of energy required by 25%-50%. Activated sludge technology can also help to improve effluent quality when paired with a trickling filter system.
Combining processes can also reduce overall system footprint. As illustrated below, a trickling filter system already yields an approximate 20% reduction over a traditional activated sludge system. But when combining processes, even greater footprint savings can be achieved – from 30% with a trickling filter/solids contact process up to 49% if employing a roughing filter/activated sludge process.
When it comes to combining two technologies, either process can be used as the first stage in a two-part series, or the two systems can operate in parallel. Combined processes are not limited to trickling filter and activated sludge technologies. Trickling filters can also complement rotating biological contactors (RBC), lagoons, or wetlands. The effectiveness of a combined approach ultimately depends on wastewater characteristics, a plant’s design and operation, and local environmental regulations. It is important to take a situational approach when designing a combined system layout.
Combining systems can yield more comprehensive and effective treatment. While combining processes can minimize the shortcomings commonly associated with two independent technologies, it will not eliminate all issues. Like any marriage, there will still be issues that exist. However, most issues can be easily remedied with proper design and operation. Keep in mind that there are many possible combinations when it comes to complementary processes and to evaluate your site-specific needs when determining an appropriate pairing.
In November 2022, we launched a monthly, ten-session webinar series titled Trickling Filters for the 21st Century. This series was created for wastewater professionals to learn more about trickling filter system design, advantages, and integrations with complementary technologies. We devoted an entire session to discussing the benefits and limitations of combined processes and explored data from various plants in the United States and Canada that have successfully combined processes. Hear an in-depth review of this topic from John Harrison, P.E., Consulting Engineer at John R Harrison Consulting, by visiting our webinar series webpage and clicking on the session titled Better Together: Combined Trickling Filter and Activated Sludge Processes.