Considerations for manure separation

University of Wisconsin Biological Systems Engineer Dr. Joseph Sanford recently discussed the types and benefits of manure separation during the university’s Separator Week.

Manure separation efficiency can be improved with storage, added treatments such as sand and/or solid removal, digestion and other more advanced treatment processes.

“Depending on the animal it comes from and methods for collecting manure, manure is expensive to transport,” said Sanford. “It contains a lot of water and water is not cheap to move. That’s a challenge with manure and one of the reasons we separate solids. Separation can also help with odor issues, nutrient distribution and improving storage efficiency.”

The primary reason producers separate manure is for nutrient management.

Separated solids can be moved farther from the main farm area; the liquid portion can be used closer to the farm.

Separation also adds storage and handling options. Solid separation can potentially allow the operation to add animals without having to expand manure storage capacity.

Manure is also separated for handling purposes. “We can pump manure to improve efficiency in moving manure to where it needs to go,” Sanford said.

“With more advanced processing such as ultrafiltration, reverse osmosis and clean water systems, we need separation at the front end of those technologies.”

One separation method is the fairly simple process of separating solids from liquids. Manure goes into the system and fiber or sand is separated from the liquid. Passive separation relies on gravity: sedimentation from solid materials drops out and liquid flows over the top. Active systems rely on mechanical separation using force or pressure to remove the solids.

Many dairy producers who use sand bedding have passive systems that take advantage of gravity and sedimentation.

Manure flows downward in long, shallow lanes that are designed for a certain depth and flow velocity.

The sedimentation process separates either sand or solid particles (fiber) in manure.

Passive systems are free draining and have a specific retention time during which manure flows, allowing the sedimentation process to occur. The process usually takes 20 to 30 minutes depending on how the system is designed as well as the manure characteristics and goals.

An advantage of passive systems is low construction cost. Although passive systems are effective for sand separation, they require a large area. There’s less control and lower efficiency with a passive system. The resulting fiber for bedding is often high in moisture and requires time, and sometimes turning, for sufficient drying.

Active systems use force or pressure to separate solids from liquids. Most active systems operate continuously or semi-continuously. Producers provide power for active systems but have more control over the system. Active systems require a higher upfront investment and more maintenance.

One type of active separation is a screen separator. A screen is placed at an incline, and can be stationary, vibrating or rotating. As manure flows over the screen, liquid passes through and solids are retained on the surface. Some solid particles may drop through, but screen size can increase efficiency. Screen separation is most efficient with liquid manure at 2% to 4% dry matter content.

Screen separators have a simple design and fairly low maintenance that includes screen cleaning. Because there aren’t a lot of moving parts, maintenance is comparably lower than other technologies. The initial cost of screen separator systems is usually higher than passive systems, and energy consumption is higher.

The primary limitation of screen separators is that the system is only as efficient as its design. Screen size, slope of the screen and manure flow rate influence efficiency. Nutrient separation can be variable and depends on screen size.

Screw press separators are becoming more popular for separating solids for bedding. Manure flows into the system and is pressed slowly with rotating screws. The liquid portion is pushed through a screen as solids move through the screw. Liquids drain at the bottom and solids are pushed out through the end of the separator.

The main advantage of a screw press separator is drier solids. At the end, total solids will typically range between 20% to 30% moisture. This system is relatively compact and allows for continuous operation. Screw presses generally cost more than slope screen systems but are highly reliable.

Screw presses have higher maintenance costs due to numerous moving parts and wear on screens and screws. Sanford suggested having spare parts on hand for quick replacement and minimal down time.

The centrifuge separator uses rotational force to separate solids from liquids. Centrifuge separators typically move at high speeds, from 3,000 to 4,000 rpm, to settle out heavier solids.

The major limitation of centrifuge separators is high energy consumption and high upfront cost. There’s also significant wear on parts and potentially more down time due to wear and tear resulting from force.

Hybrid separator systems combine physical, chemical and hydraulic forces to boost system performance. They’re designed to enhance nutrient capture and separation efficiency. Pretreatment chemicals such as flocculants create larger, heavier particles to hasten sedimentation and improve filtration. Coagulants help bind smaller particles and boost efficiency.

All separators have a specific daily manure handling capacity based on screen size and manure characteristics. Large farms may require multiple systems to separate the entire manure stream.

Sanford outlined options to consider when choosing separation technology, starting with determining the desired outcome. The farm may want to separate solids for bedding, improve nutrient capture or add a clean water system. Important considerations include how much manure the farm generates and the solid content of manure.

Prior to visiting separator dealers, Sanford suggested producers have a list of questions to ask dealers. “What type of system is the dealer offering? Is it a hybrid system? What is the solids capture, and what is the nutrient separation efficiency? What is the dry matter content of the final product for bedding?” It’s also important to ask about space required and other farm-specific constraints.

Because dealer data on separators are from their own trials, it’s a good idea to ask whether other farms with similar solids have implemented the technology. Be sure to ask about routine maintenance costs.

“There are a lot of separation technologies out there,” said Sanford. ““Technologies all have advantages, so it’s important to look at the end goal when deciding on a system. This isn’t a one-size-fits-all – it depends on the farm’s goals, manure characteristics and farm resources.”

by Sally Colby