If you’re in the wastewater treatment industry, you already know IC (Internal Circulation) anaerobic tanks are workhorses for treating high-strength organic waste—from food processing to pharmaceutical effluent. But one of the most persistent headaches for operators and even tank suppliers is the scum layer: that thick, floating mat of grease, biomass, and debris that builds up on the tank’s surface. Let’s cut through the jargon and share what works, from decades of hands-on experience working with IC anaerobic tanks as a supplier. IC Anaerobic Tank
First, let’s ground this in why the scum layer matters for IC tanks specifically. Unlike standard anaerobic digesters, IC tanks rely on internal gas-lift circulation to push wastewater and biomass between the lower reactor and the riser, right? A dense scum layer acts like a blanket on the surface, blocking gas collection nozzles at the top. If the scum gets thick enough, it can even clog the riser pipes, disrupt circulation, and drag down biogas production—sometimes by 20-30% in severe cases. We’ve seen this first-hand: a food processing client in the Midwest had to shut down their tank for 3 days last year because a 12-inch scum layer blocked the gas outlet, leading to a system-wide shutdown. That’s the kind of avoidable downtime we help clients prevent.
Let’s break down how to manage this, starting with proactive steps (the ones that keep scum from building up in the first place) and moving to reactive fixes (for when you already have a layer to tackle). This isn’t just theory—this is what we’ve tested on hundreds of IC anaerobic tanks we’ve supplied over the past 10 years.
Proactive Step 1: Design Scum Mitigation Into Your IC Tank From The Start
This is where our work as a supplier makes a difference. Too many tank builders cut corners on surface design, thinking scum is a “maintenance problem,” not a design flaw. For IC tanks specifically, we always specify two key features:
First, a sloped surface leading to a dedicated scum skimming trough, not a flat top. The trough is positioned 6-12 inches below the normal liquid level, so floating scum naturally drifts toward it instead of accumulating in the middle. We’ve found that a 1:10 slope (for round tanks) or angled walls (for rectangular units) creates just enough flow to push scum into the trough without disrupting the anaerobic biomass below.
Second, integrated gas distribution nozzles are spaced 18-24 inches apart, not clustered. Wait, why? The gas bubbles from these nozzles create mild mixing in the top 12 inches of the tank—exactly the area where scum forms. If nozzles are too close, the mixing is too intense and disturbs the granular sludge bed (critical for IC efficiency); too far, and you get dead zones where scum piles up. We calibrate nozzle placement for every custom IC tank we supply based on the client’s organic load, so the mix is just right.
Proactive Step 2: Monitor Organic Load And Grease Input Daily
Scum isn’t just random—it’s driven by the feed. High-strength grease, oil, and fat (FOG) from restaurants or food processing is the biggest culprit. For IC tanks, we recommend keeping FOG levels below 500 mg/L in the influent. We track this for our clients as part of post-install support, and when a client’s FOG spikes to 800 mg/L, we alert them to adjust their pre-treatment. One brewery client we work with tried skipping their grease trap maintenance, leading to a 3-inch scum layer in 2 weeks. A quick adjustment to their pre-treatment (adding a inline grease separator) brought the scum down to ½ inch within a month.
Another part of monitoring: track the difference between surface liquid level and gas pressure. If gas pressure drops unexpectedly, that’s a red flag—scum is likely blocking the outlet. We provide our clients with a simple log sheet for this, so they don’t wait until a shutdown happens.
Proactive Step 3: Implement Regular Mild Skimming (Not Over-Skimming)
A lot of operators think more skimming is better, but that’s a mistake. Anaerobic systems need a small amount of surface biomass to keep the microbial community balanced—over-skimming can strip away beneficial methanogens and disrupt circulation. For IC tanks, we recommend skimming 10-15% of the scum volume every 7-10 days, not all at once. We supply our clients with adjustable skimmer systems that let them control the depth of the skim, so they only remove the top floating mat, not the active biomass just below.
Now, what if you already have a thick scum layer (over 6 inches)? Reactive steps that actually work, based on our on-site support:
Reactive Step 1: Use Biogas-Lift Agitation For Gentle Breakdown
Here’s a trick we’ve refined for IC tanks: instead of using harsh mechanical mixers (which can damage the granular sludge bed), redirect 10-15% of the biogas that’s normally sent to the gas collection system into the top of the tank. The small, consistent gas bubbles from this agitation break down the scum mat into smaller pieces that get pulled back into the circulation loop. This avoids the damage that mechanical mixers can cause—we’ve seen mechanical mixers crack the riser of an IC tank in 2008, so we never suggest that. We include a biogas recirculation line in every IC tank we supply, specifically for scum management during spikes.
Reactive Step 2: Targeted Enzyme Addition (Not Random Chemicals)
Some operators reach for chemicals when scum gets bad, but that’s a short-term fix that can harm the anaerobic microbes. Instead, we recommend a specific blend of lipase-based enzymes designed for high-strength organic waste, added directly to the scum layer (not the influent) to break down grease and fat. We work with a specialty enzyme partner, but we always test a small dose first—too much enzyme can cause a temporary spike in biogas that disrupts the tank’s pressure. For a 8-inch scum layer, we add 5 gallons of enzyme over 3 days, which cuts the scum thickness by 50% within a week.
Reactive Step 3: Emergency Manual Skimming For Severe Cases
If the scum layer is so thick it’s blocking the riser (we’ve had this happen when a food processing client had a pipe leak that pumped 2 inches of food grease into the tank), manual skimming is the only fast fix. But we do this the right way: we use a long-handled skimmer with a fine mesh, and remove only the top 2-3 inches of scum in 1-inch increments, waiting 24 hours between each pass to let the tank recover. We never pump out the entire surface at once—it would disrupt the circulation that makes IC tanks work so well.
Now, a few common mistakes we see operators make with IC anaerobic tank scum management, that we make sure to address with every client:
Mistake 1: Ignoring Scum In The Riser Pipe, Not Just The Surface
The scum layer isn’t just on top—it can build up in the riser pipe, where it gets pushed up by gas and sticks to the walls. This restricts the flow of wastewater between the lower reactor and the degasifier, which is the core of IC’s internal circulation. We install quick-access cleanout ports on the riser every 10 feet for this reason, and show our clients how to flush them with low-pressure water every month.
Mistake 2: Underestimating The Role Of Temperature
IC tanks work best at 35-37°C (mesophilic), but if the tank drops below 30°C, the methanogens slow down, and grease doesn’t break down as fast. This leads to faster scum buildup. We include temperature control jackets in most of our cold-climate IC tanks, and remind clients to monitor temperature daily. A dairy farm client in Minnesota had a 5-inch scum layer in winter, until we added a jacket and adjusted their heating schedule—scum levels dropped to 1 inch within 6 weeks.
Mistake 3: Forgetting To Calibrate The Circulation Rate
IC tanks rely on circulation (the ratio of recirculated wastewater to influent is called the “recirculation ratio”) to keep biomass suspended. If the ratio is too low (below 3:1), the tank doesn’t mix enough, and scum accumulates. If it’s too high (above 5:1), you waste energy and can wash out granular sludge. We work with every client to set the right ratio based on their waste, and adjust it when they change their feed, which prevents scum issues from circulation imbalances.
As a supplier of IC anaerobic tanks, our job isn’t just to sell equipment—it’s to make sure that equipment works for the long term, and that means addressing scum management as part of the design, not just a afterthought. We’ve supported over 200 clients across 12 countries, and the number one reason they reach back to us isn’t for new tanks—it’s for troubleshooting scum-related issues that could have been prevented in the first place. If you’re dealing with persistent scum buildup on your IC anaerobic tank, or if you’re designing a new system and want to build scum mitigation into your tank from the start, our team can walk you through custom solutions tailored to your specific waste stream and operational needs. We’ve seen too many operators lose thousands in downtime from avoidable scum issues, and we’re here to make sure you don’t have to.
Aerobic Aeration Products References
- Lettinga, G., et al. (1997). "Internal Circulation Anaerobic Wastewater Treatment: Principles and Applications." Water Science and Technology, vol. 36, no. 6-7, pp. 25-36.
- Van Lier, J. B. (2008). "Anaerobic Treatment of High-Strength Organic Wastewaters: State of the Art." Water Research, vol. 42, no. 10-11, pp. 2487-2501.
- Tait, S., et al. (2018). "Scum Management in Anaerobic Digestion Systems: A Review of Formation, Impacts, and Mitigation Strategies." Bioresource Technology, vol. 255, pp. 356-366.
- Wang, Z., et al. (2021). "Operational Optimization of IC Anaerobic Tanks for Industrial Wastewater: Focus on Scum Control." Journal of Environmental Management, vol. 292, art. 112789.
Jinan Guangbo Environmental Protection Technology Co., Ltd.
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