Benjamin Franklin famously said there are only two things in this world to be certain of: death and taxes. Wastewater operators can add a third: sludge.
Sludge can be just a normal, innocuous byproduct of wastewater treatment, or it can be a real nuisance, disrupting treatment efficiency by reducing retention time, causing odor complaints, and requiring expensive remediation. In this article, we’ll discuss what sludge is, the problems it can cause, and how robust lagoon aeration and mixing can keep sludge accumulation in check.
What is sludge?
Sludge is defined as the biosolids that accumulate at the bottom of the lagoon, composed mostly of dead bacteria, algae, plants, sand, silt, gravel, and insoluble metals. (According to a recent study from Arizona State University, valuable metals like gold and silver can be found in sewage sludge!) The biodegradable components will digest over time; the rest will have to be removed eventually.
One of the benefits of a lagoon-based wastewater system is the absence of daily sludge handling. The vast majority of wastewater treatment lagoons are partial-mix systems, designed to accommodate sludge buildup. Typically, the bottom 1–2 feet of the lagoon is designated as a sludge storage area, allowing for the slow anaerobic digestion of solid influent material.
Because sludge storage is built into the cell’s capacity, most lagoons can go several years—even decades—without requiring dredging. Excessive accumulation of sludge, however, can cause all sorts of problems:
Higher BOD, TSS, and Ammonia Effluents—If wastewater lagoon sludge is allowed to accumulate in excess, it can short-circuit the treatment process by reducing the volume of water a lagoon can hold, thereby lowering the retention time. The lower the retention time, the less time there is for proper treatment to occur. This can lead to an increase in the effluent concentrations for BOD and TSS. In some circumstances, anaerobically digesting sludge can actually release ammonia into the water column of the lagoon, a process known as benthal feedback. This can lead to a situation where the effluent ammonia concentration is higher than the influent.
Eventually, excess lagoon sludge could lead to a violation of permit effluent levels or even a complete system failure.
Lagoon Odors—Large accumulations of sludge create anaerobic conditions in the lagoon. Biological decomposition will continue, but much more slowly, and with a byproduct of odorous gases. Once these gases are released, you can count on complaints from your neighbors.
Lagoon Turnover—Most common in spring and fall, lagoon turnover occurs when the temperatures at the surface and the bottom of the lagoon equalize, freeing the sludge and foul gases trapped at the bottom. Our article How to Prevent Spring Lagoon Turnover and Odors covers this subject in more detail.
There are a couple of less extreme methods of dealing with excessive sludge:
Reposition aerators: If there are surface aerators, they can be repositioned to disrupt the drifts of sludge that settle around them due to insufficient mixing. This will resuspend some of the settled solids so they can come into contact with oxygen and bacteria and decompose. Late spring and summer is the time to reposition aerators, because the natural increase in DO allows the lagoon to handle the added load.
Bioaugmentation: Adding special blends of bacteria, especially in conjunction with aeration, can jump-start the digestion process and help break down the sludge over time.
Excess sludge accumulation must be dealt with. At some point, mechanical removal, which is costly, may be the only option. Read our article, Wastewater Lagoon Sludge: Treatment or Removal? for more information.
MARS Lagoon Aeration prevents excess sludge accumulation
Ben Franklin also famously said, “An ounce of prevention is worth a pound of cure.” The best way to prevent excess sludge accumulation is through aeration and mixing. A well aerated lagoon keeps biodegradable organic solids in suspension, where they are in constant contact with dissolved oxygen and bacteria, instead of settling to the bottom of the lagoon as sludge.
Triplepoint’s MARS Aerator provides lagoon aeration and mixing in a single unit. Because it sits at the bottom of the lagoon, it provides robust mixing from the floor to the surface to prevent sludge drifts from forming and causing short-circuiting. Efficient fine bubble aeration maximizes dissolved oxygen levels and biological treatment.
The combined aeration and mixing capability of the MARS aerator accelerates the natural removal of sludge—preventing sludge buildup, optimizing the capacity and service life of a lagoon, and reducing the need to dredge.