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

Sludge 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.

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 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 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 regulatory requirement for permitted operations.

Operators use various techniques to collect sludge samples. The choice of sampler is important to ensure the collected sample captures the composition of the sludge that will be removed for land application or utilized later. Producers typically use a sampling bottle connected to a long pole to grab multiple sludge samples (10 to 20 samples) then combine them before sending a composite sample for analysis.

Analyzing duplicate or triplicate samples help reduce any sampling bias. It also allows producers to accurately determine nutrient amounts that will be managed.

Nutrients are concentrated

Sludge contains more solids and minerals (phosphorus, zinc, copper, and magnesium) than fresh manure or lagoon liquid. Typically, lagoon liquid contains less than 1% total solids, while sludge contains from 10% to 20% total solids.

Another difference in sludge from fresh manure is the nitrogen, phosphorus, and potassium (N-P-K) ratio. While fresh swine manure has an N-P-K ratio of around 5-1-2, the swine lagoon sludge N-P-K ratio is 5-5-1. This means that sludge application on an N-basis risks introducing significant amounts of phosphorus that can raise soil P levels and elevate P runoff.

A study tracking distribution of nutrients within a lagoon found that more than 90% of lagoon P, calcium, and magnesium was in the sludge, with average concentrations of 32, 38, and 10 pounds per 1,000 gallons, respectively. When we compared multi-year analysis results of lagoon liquid and sludge samples, we found zinc (Zn) and copper (Cu) concentrations in the sludge were 30 and 35 times greater than in the liquid.

These findings reveal both a challenge and an opportunity for animal producers using lagoons for manure management. The first challenge is the high cost of sludge removal, whether agitating and pumping or dredging, when compared to lagoon liquid irrigation. In cases where sludge removal was delayed, the cost of sludge removal and utilization can become prohibitive. Another constraint in managing sludge is the high concentration of Zn, Cu, and P, which can require very low application rates and large acreage to utilize these nutrients agronomically. These fields are typically found farther from the lagoon, which also raises the cost of handling.

Opportunities for use

Despite these challenging attributes, sludge offers producers an opportunity to improve their nutrient use efficiency retroactively. This is mainly because the sludge in lagoons represents the accumulation of nutrients from many years of past production. Implementing a dewatering step using solid-liquid separation can greatly improve nutrient use efficiency. Although sludge dewatering raises the nutrient concentration in the solids, it reduces the cost of transportation.

Sludge solids can also be composted, or co-composted, which improves their agronomic properties and opens other utilization avenues. The use of separated solids as an energy production feedstock is another option to concentrate sludge minerals while recovering energy to offset drying requirements.

Several producers, integrators, and technology developers are exploring new approaches beyond direct land application to remove, process, and utilize sludge. Coordinating such efforts in areas with a high density of lagoons can greatly reduce the cost of adoption by using centralized or collaborative processing schemes. This could generate value-added by-products, which would further offset the cost of adoption. ■

Additional resources

Sludge sampling in anaerobic treatment swine lagoons

Available online at bit.ly/JNM-sludge-sampling

Sludge management in anaerobic swine lagoons: A review

By Owusu-Twum, M. Y., and Sharara, M. A.

Published in the Journal of Environmental Management , 271, 110949

Sharara is an assistant professor and extension specialist in the department of biological and agricultural engineering, and Kulesza is an assistant professor and extension specialist in the department of crop and soil sciences at North Carolina State University, Raleigh, N.C.