Sludge Advice: Basics of Lagoons and Anaerobic Digester Clean Out

Sampling and cleaning lagoons and anaerobic digesters is a dirty, but important job. Here are a few safety considerations and tips to remember when it comes to dealing with sludge.

What is sludge?
Sludge is the residue of biological manure treatment that accumulates in lagoons and digesters. It is lower in organic matter and higher in solids and minerals because of the longer treatment time in the manure storage structure. This also makes sludge denser than manure. Sludge has a lower odor intensity than manure.

Why clean out the sludge?
Sludge storage in the facility reduces the treatment volume of the structure. When mixed with the liquid portion of manure, sludge can increase the nutrient content of recycled liquid pumped out of the facility. Issues with barn air quality and pathogen issues have also been noted in manure storage facilities. Finally, the high levels of zinc and copper in digester sludge can affect the microbe populations and the treatment performance.

How can you measure sludge volume and nutrients?
Test the depth in multiple locations across the facility as some areas will be deeper than others. To measure sludge depth, use a weighted plate or disc with a rope attached such as a Sludge Judge type of product or an acoustic tool like a fish finder. Sludge sampling can be done with a clamshell sampler, a Sludge Judge, or other custom solutions. Send these samples off to get tested by a lab. Plan to apply the sludge to fields that show a need for the nutrients in the sludge, particularly copper, zinc and phosphorus. Expect the cost of collecting information on sludge depth and nutrients to cost around $325 per million gallon according to a study by the University of Missouri.

How should you clean out the sludge?
In lagoons it is common to use PTO powered pumps and floating pumps or boats. Dredging and dewatering is done less often but can be used if sludge needs to travel a significant distance to be spread.

Covered lagoons and digesters can be more difficult to pump out. Submerged pump-out lines with ports as well as sludge suspension and pumping are usually the best method in this situation. In some cases, partial or complete removal of the cover may be necessary. It is estimated that the cost to agitate and land apply is $151,000 or approximately $0.0188/gallon at a million gallons a day.

Lagoon solids removal can be tricky especially if there is a high level of solids in the sludge. This may result in the need to dilute, agitate and dilute again before pumping and land application.

Finally, train employees to be safe around manure gases that can be released when agitated. Other potential hazards include PTO shafts, confined spaces and pressurized hoses.