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Why sludge matters and what to do about it

Kelli Boylen for Progressive Dairy Published on 28 February 2022

There is liquid manure, solid manure and then there is sludge. How much sludge is in your lagoon and why does it matter?

Dr. Mahmoud Sharara has focused his research on sustainable management of agricultural waste, as well as reducing ag-related impacts on ecosystems and generating value-added byproducts. He is an assistant professor and extension specialist in the North Carolina State University Biological and Agricultural Engineering Department. He recently presented “Cleanout for Lagoons and Anaerobic Digesters” through the Livestock and Poultry Environmental Learning Community (LPELC). 

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Sharara says sludge is comprised of more solids and less organic matter compared to regular manure solids. It also contains more minerals and metals, such as magnesium, calcium, phosphorus, zinc and copper. Sludge ends up at the bottom of the lagoon due to biological treatment by microbial communities and gravity.

Manure ponds that are emptied on an annual basis do not treat the manure or store the solids long enough to form sludge, he explains. Lagoons, on the other hand, are designed to treat manure and to continuously have a treatment volume occupied with manure. According to Natural Resources Conservation Service (NRCS) design standards, the sludge accumulation ratio (SAR) for dairy cattle is 545 gallons of sludge for every 1,000 pounds of total solids (TS), or 0.545 gallons per pound TS.

Most lagoons and digesters are typically designed to accommodate a certain amount of sludge buildup between cleanings, which typically occur every five to 15 years. But how much sludge is too much in a storage structure? There are a few ways to answer this question. Sharara explains, “First, the design standard for that structure has specified volume to accommodate generated sludge. With regular surveys, they can detect when that volume is exhausted, and the new sludge is occupying treatment volume.

“Another checkpoint is the regulation relevant to the state, which can specify a percentage of treatment volume that must be sludge-free. For example, in North Carolina that is 50 percent of the treatment volume. And finally, from operational perspectives, the producers can often tell by changes in the lagoon color and smell, or in the recycling pump performance, that a higher-solids liquid is being circulated. In the case of digesters, the decline in biogas production is a tell-tale sign that the treatment volume is occupied with sludge.”

A buildup of sludge in lagoons and digesters interferes with the function of the structure. “The sludge takes over space in the structure that should be occupied by microbial communities and could result in incomplete treatment or digestion of manure,” Sharara says. In addition to an increase in odor emission from the lagoon, a buildup of sludge can also result in increased odors and nutrients in recycled liquid, decreased barn air quality and pathogen issues when effluents are used to flush barns, which can affect cow health. When excess sludge builds up, the cost of sludge removal and utilization can become prohibitive.

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There are two ways to determine the amount of sludge in a manure structure, Sharara says. One is to survey the amount by measuring depth and determining the volume. In an open lagoon, a weighted plate or disc can be used. First, measure the depth to the bottom of the lagoon, and then use the weighted plate to see where the top of the sludge layer is located.

Or you can use a commercial product such as Sludge Judge, which is a large clear tube used to get a column sample of the lagoon and visually measure the layers. Sonar/acoustic tools can also be used and may be a good method if you have a covered lagoon. It is common to have different levels of sludge in different areas of a lagoon, so it is important to check in different areas of the structure. “We recommend a minimum of six points per acre. Using sonars or fish finders can allow the operator to gather tens or hundreds of readings during the survey,” Sharara says. “This helps provide a more accurate picture of the sludge volume in the lagoon.”

The next step is to determine the amount of nutrients in the sludge. There are several different ways to obtain samples including a tube method or a clamshell sampler. Multiple samples are required to accurately determine nutrient levels. Composite samples are sent to a lab for analysis. Some states require annual sludge surveys as part of the regulatory requirements for permitted operations.

Although nitrogen and phosphorus are important in waste-management plans, planning for the higher levels of zinc and copper in sludge is critical to avoid nutrient losses and crop stress/toxicity, Sharara says. The high amounts of minerals and metals in sludge generally require low application rates and large acreage to utilize these nutrients agronomically.

“Once the amount of sludge is determined and the nutrient levels are known, a plan can be created, keeping in mind the end-use plan and the distance the sludge needs to travel,” he says. Equipment availability, cost of operation and the end goal for removal and removed product are all considerations for determining the best method for cleaning sludge out from a manure structure. “The travel distance of the waste has a big impact on what methods are best for a particular operation,'' Sharara says.

In most cases, a PTO pump is used as it is widely available, easy to operate and the goal is often to remove solids from an area of accumulation near the lagoon bank to apply on nearby fields. “This is a critical process that requires a trained operator to protect the integrity of the lagoon berm and the equipment,” he says. A floating pump can also be used in situations where PTO pumps may not be able to reach.

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If the sludge needs to be dewatered and transported longer distances, sludge removal will likely be carried out using a dredge followed by a dewatering step, or by an excavator. Excavators are typically suited for smaller lagoons (less than 1 acre). Larger lagoons will have more areas inaccessible to on-bank excavators. Depending on the region, one of these apparatuses (dredge and dewater, or excavator) might be more available to contract, and that would determine the choice. “This is a more-involved, and expensive process, but [it] works well in areas with limited access to land to spread solids,” Sharara says.

Some producers using a dredge apply polymers to the lagoons. “The choice to use a polymer depends on many factors, including what is the end use for the separated liquid and solids and if the separation could be successful without that polymer,” Sharara says. “In general, polymer is helpful when the goal is to separate fine solids – like sludge solids – which cannot be separated effectively using screens or centrifuges alone. Adding the polymer, however, adds cost to the process, which can be a disincentive, especially if there is no clear end use for the separated solids. Also, as many dairy and swine growers are interested in targeting organic fertilizer markets with manure and manure byproducts, adding traditional polymers disqualifies the solid fraction from organic certification. This can be overcome by pursuing organic polymers, of course, but they are relatively less accessible at this point.”

Regardless of how you choose to manage and monitor sludge, consider your safety first and plan accordingly. Sharara says, “Agitating lagoons involves many mechanical, chemical and biological hazards. Gas emissions from manure storage structures have been the cause of many deaths on animal farms. Be sure to have wearable gas detectors for hydrogen sulfide, consider wind direction and avoid weather inversion conditions that trap emissions near the ground.”

Click here for more information, or go to the LPELC website.  end mark

PHOTO: High-solid sludge can settle at the bottom of manure lagoons. Containing higher amounts of minerals and metals, it should be monitored and removed when quantities take over too much space in the storage vessel. Photo by Karen Lee.

Kelli Boylen is a freelancer based in northeast Iowa.

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