When Jack traded the family cow for magic beans, he didn't get what he thought he was buying. Sometimes, livestock producers suffer the same fate when they buy solid separators to sort their waste flow. Expectations and reality don't match.

The reality about solid separators is they only separate a portion of the solids, says Doug Hamilton, Oklahoma State University waste management expert. Hamilton recently organized a guide to solid separators at the request of the National Pork Board.

“What they should be using solid separators for is making their manure handling easier so the manure will flow down a pipe, or removing the floaters from dairy lagoons or scum from hog lagoons,” Hamilton says.

He adds that standard solid separation, from a purely mechanical standpoint, will not affect nutrient content of the effluent or solids very much. Nor will it make a large difference in sludge buildup in lagoons.

The two basic designs for mechanical solid separators are:

  • screens and filters, and
  • centrifuges and spinners.

Settling basins are a third type, but they won't be covered here other than to note they typically remove about 60% of the solids if properly designed and operated.

Screen separators

The efficiency of screen separator systems is purely logical. How much is removed depends on the fineness of the screen and the type of manure being filtered. “Hog farmers are usually very disappointed when they buy a solid separator,” Hamilton says.

Particle shape and size determines if they will fit through the holes in a screen. Simple gravity is the primary power involved.

Hog feed is so finely ground these days that most hog manure exits the hog as slurry with very fine particles. So, it takes a very fine screen to capture much of it.

For example, a pork producer might catch 50% of the solids with a very fine screen, while a dairy farmer might catch 10% of the solids with a coarse screen. “But the dairy farmer may see a greater effect because it's those stringy 10% of the solids that create most of his headaches,” he says.

This illustrates a very important point about screen-type separators: The finer the screen, the slower they operate.

Most manufacturers have washing or vibrating equipment to deal with this issue, Hamilton says. The simplest screening devices remove an average of 10-20% of the solids and leave them in a rather wet form.

A subcategory of screen separators is the pressure screen. One type forces the manure against a screen, usually with an auger that essentially squeezes out the liquid. Others use various combinations of permeable belts and rollers to squeeze out liquids.

One benefit of pressure screens is they usually present the solids in a rather dry form that is stackable and movable with equipment such as a front-end loader.

The most common pressure-screening device in agriculture is the screw press. On average, screw presses catch 20-30% of solids that pass through them. Results with hog manure might be in the lower end, while dairy manure might be in the higher end.

To get the most from a screw press, the in-flow material must be fairly thick, and the rate of flow may be significantly lower than the machine's maximum rating since it was most likely established using thinner slurry.

Other filtering options

Fabric filters function in the same way as screens, although the openings are much smaller so separation efficiency is higher.

A belt press filter uses a continuous belt of filter fabric to move material through the system. Press rollers squeeze moisture from the solids, and a rotary brush removes solids that stick to the belt.

Vacuum drum filters drop the in-flow mixture across a rotating drum made of filter fabric. A vacuum inside the drum draws moisture into it by negative pressure. Solids stick to the filter fabric and are removed from the fabric with a metal edge as the drum rotates.

Media filters use sand or synthetic material to trap solids. They can achieve a high rate of separation efficiency and remove smaller particles than other filters. Thin media filters, such as sand beds, dry as well as separate. Deeper media filters usually perform biological treatment, in addition to separation.

Centrifugal separators

Because centrifuges can effectively increase the gravitational force on particles, and because centrifugal forces can be greater than the earth's gravitation field, Hamilton says centrifuge solid separators can achieve efficiencies approaching that of settling. They also leave the solids, sometimes called cake, dry and manageable.

Two types of separators use centrifugal force: centrifuges and hydrocyclones. Both rotate the solid/liquid mixture and force particles to move to the outside of the rotating motion. In effect, an artificial gravity is created, and particles move by the force of gravity as they do during settling.

Decanting centrifuges are horizontal or vertical cylinders continuously turned at high velocities. Centrifugal force presses solids onto the inside wall of the cylinder. An auger, which turns slightly faster than the cylinder, removes the cake.

Decanting centrifuges can attain high separation efficiency producing semi-solid cake. They require influent total solids concentrations in the 5-8% range, and are considerably less efficient when operated with more dilute influent.

Hydrocyclones are cone-shaped separators with no moving parts. Slurry is pumped into them at an angle near the top of the cone, creating a vortex motion. The swirling motion increases the settling of solids to the bottom of the cone. Liquid leaves the cone from the top.

Separation efficiency of hydrocyclones is not as great as that of decanting centrifuges, but they're fairly good at separating dense particles.

The efficiency of nearly all these separators can be improved by adding one or two chemical agents.

Coagulants are used to cause the particles to coagulate or join together. When manure particles are dispersed in water, they carry a small negative electrical charge that keeps small particles separated. Adding positively charged particles or chemicals to a mixture collapses the negative charges and allows the particles to move closer together.

The most common class of coagulants is salts of positively charged metals such as aluminum sulfate, ferric chloride and lime. Although these substances make the particles stick together, they create a gooey substance that can clog equipment.

Metallic salts are very effective coagulants and, at the right rates, will virtually remove phosphorus from manure slurries through sedimentation. Lime and ferric chloride are caustic and must be handled with care to protect people and equipment.

Some metallic salts are toxic to plants and animals, so they must be used judiciously to avoid problems during land application.

Flocculants are chemicals that bind particles together. Most flocculants used by farmers are organic polymers that carry positive charges.

Flocculants are weak coagulants and are relatively ineffective at removing phosphorus from manure. But, by binding particles together, flocculants create larger particles that are more easily screened. As particle size becomes heavier and stronger, they're more easily removed by centrifugal force.

Polyacrylamide (PAM) is the only commercially available flocculant useable for this purpose. It can increase efficiency of screen filtration dramatically — some research shows up to 95% removal. PAM also improves centrifugal separation.

PAM is non-toxic to plants and animals, and creates strong, easily filtered solids. Cationic polymers are most effective at pH higher than 7.

Although coagulants and flocculants increase the amount of solids settled, they also increase the time needed for a cloud of solids to settle. Chemical coagulants and flocculants can greatly improve the performance of solid separators, but they do it at a higher cost.

Buy big enough

Buy a separator with enough capacity, Hamilton warns. It's critical to calculate flow rates of material passing through the separator accurately, thereby preventing it from becoming a bottleneck in your manure handling flow.

“Outflow must equal inflow. Remember, a solid separator takes a single waste stream and creates two waste streams,” he says.

Forethought and wisdom may be the most important part of the analysis. Solid separation should make handling easier, but it must fit into the manure handling system and timing requirements of the operation, he cautions.

Animal feeders must not choose solid separators based on an arbitrary standard such as percent efficiency, Hamilton says. The most effective system is likely one that combines benefits of multiple subsystems.

Above all, don't forget the cost-benefit analysis.

Table 1. Characteristics of common solid separators
Device Separation Efficiency (%) Mass Removal Efficiency (%) Cake Total Solids (%) Factors Affecting Performance
Settling Naturally: Up to 65
Chemically: Up to 95
Naturally: Up to 75
Chemically: Up to 95
1 to 3 Influent total solids concentration, Settling time and/or overflow rate
Gravity Screens Naturally: Up to 40
Chemically: Up to 95
Typically: 10 to 25
Naturally: Up to 45
Chemically: Up to 95
Incline: 8 to 22
Vibrating: 5 to 22
Rotating: 5 to 16
Screen opening size, Influent total solids concentration influent flow rate
Pressure Screens Naturally: Up to 40
Chemically: Up to 95
Naturally: Up to 55
Chemically: Up to 98
Screw Press: 20 to 30 Screen opening size, Influent total solids concentration, influent flow rate
Fabric Filters Up to 60 Up to 65 15 to 20 Fabric opening size, Influent total solids concentration, influent flow rate
Decanting Centrifuge Up to 65 Up to 70 25 to 40 Influent solids content, drum speed, auger speed, influent flow rate
Liquid Cyclone Up to 30 Up to 40 Less than 10 Influent solids content, influent flow rate
Source: Doug Hamilton, Oklahoma State University