The secrets of sludge It takes up space in manure lagoons, but sludge is actually a valuable nutrient-dense fertilizer source. by Mahmoud Sharara and Stephanie Kulesza
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ludge is a nutrient-rich by-product of manure storage and treatment. Sludge accumulates at the bottom of the lagoon as a result of two processes: the biological treatment by microbial communities and the solids settling from gravity. When compared to manure, sludge contains more solids and less organic matter. Different species collect sludge at different rates, with typical values between 0.22 and 0.54 gallon per pound of total solids (TS) entering the lagoon (Gal. /lb. TS). With continued use for manure storage and treatment, the volume of sludge in the lagoon grows until clean out is required. Lagoon design standards specify volumes required for manure treatment and storage, as well as rainfall, occasional storms, and sludge storage (Figure 1). To determine the sludge design volume, the operator specifies a period of time, from five to 15 years, after which the sludge will be removed.
impacts, it is recommended to regularly track the volume of sludge in the lagoon by conducting a sludge survey. Tracking sludge volume helps producers anticipate and plan for sludge removal when it becomes necessary. Surveying lagoon sludge can be accomplished manually or using remote tools. Surveys require measuring the distance from the liquid surface to the sludge at multiple points (eight to 12) distributed on a grid in the lagoon. A disc tied to a rope or connected to a pole can be used to measure depth to sludge. Remote methods such as a
Figure 1. A single-stage anaerobic manure treatment lagoon (adapted from USDA-NRCS).
(to divert surface water)
Freeboard (1 foot minimum) 25-year, 24-hour storm storage
Start pump Irrigate
Current lagoon level
Heavy rainfall factor Temporary liquid storage zone
Stop pump Lagoon liner
Sludge accumulation zone
Permanent liquid treatment zone
Accumulated sludge
The sludge that settles at the bottom of a manure pit is extremely high in several nutrients, including phosphorus, calcium, magnesium, zinc, and copper.
8 | Journal of Nutrient Management | August 2020
F2 8-9 Aug 2020 Sludge.indd 1
Lagoon level marker
Lagoon berm
Take a measurement As volume of accumulated sludge proliferates, treatment and storage volumes are reduced. This can be a significant constraint to producers, particularly in wet regions with heavy rainfall and rainstorms where overtopping or inundation can occur. Sludge removal is also necessary to maintain lagoon treatment performance and minimize the potential of objectionable odors. To avoid these sludge-related
remote-controlled boat equipped with sonars or a fishing rod with a fish finder can also be used. Acoustic methods, like sonar, are sensitive to the amount of suspended solids and can report an error if the distance to the sludge is too shallow (less than 2 feet). Therefore, it is always helpful to have a backup tool for surveying sludge. Prior to removing sludge, it is necessary to collect sludge samples to determine nutrient concentration and which fields and crops would be best suited to receive the removed sludge. In North Carolina, annual sludge surveys are a
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7/30/20 12:32 PM
