Queen Mary of sludge

A handy, remote-controlled boat removes the risks of measuring sludge depth in lagoons.

The electronic fish-finder nearly fills the inside of Mark Rice's boat — not because the electronics are so large, but because the boat is so small.

The vessel is a remote-controlled model of a tugboat, just a bit longer than a yardstick. The fish-finder mounted inside the boat allows Rice to measure the sludge depth of lagoons without the danger of launching a regular-sized boat on them.

Measuring sludge is a legally mandated imperative in North Carolina where Rice works as a North Carolina State University (NCSU) agricultural engineer. In some states, the tracking of sludge depth is mandated only at problem sites.

“The remote-controlled boat is faster and safer than getting out on the lagoon in a boat,” Rice says.

In about 10 minutes of navigating the boat across a lagoon, Rice takes some 30,000 measurements, backed by global positioning system (GPS) locations. The data is stored on a memory card similar to those used in digital cameras, then plugged into a computer and uploaded to calculate an average depth.

When done by hand, Rice says NCSU recommends a minimum of eight measurements to determine average depth, which certainly would take longer than 10 minutes. Per measurement, those eight hand readings technically would be more accurate, but the fish-finder allows many more points to be measured.

“So you have a much more accurate sludge survey with the remote-controlled boat and the fish-finder,” he says.

Safer Sampling

Rice first raised the question of using a fish-finder for sludge-depth measurements at a meeting where swine company representatives were present. After he began his work documenting the concept's accuracy potential, one swine company began surveying lagoons with a fish-finder mounted on a johnboat.

But Rice felt a remote-controlled boat would better address safety concerns.

“My original concept was to put together common, relatively cheap components that producers could duplicate,” he adds.

A fish-finder with GPS capability costs about $900. The boat can range from as little as $300 to the $2,000 Rice spent to have the model boat completely prepared and the fish-finder mounted.

The university is in the process of protecting Rice's “sludge boat” design from patents or trademarks so it can remain in the public domain. To contact Rice, call 919-515-6794 or email him at jmrice@unity.ncsu.edu.

Sludge measurements

A guide to proper lagoon sludge measurements is available from North Carolina State University at: http://www.bae.ncsu.edu/programs/extension/manure/sludge-survey/sludge_survey.pdf

Separating fact from fiction

Realistic equipment expectations will make your solid separator purchase more successful.

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.

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


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.

Irrigation network provides flexibility

An extensive network of irrigation piping lets this Oklahoma hog operation pump effluent and fresh water to 20 center pivots, up to three miles away.

When the Hitch family got into hog production 10 years ago, they wanted more options for applying lagoon slurry to cropland. Their goal was to spread slurry over more acreage, at the times and rates they needed.

Curtis Raines, Hitch Farms manager, and employee Cecil Goetz designed and built a system that ties most of the pivots and wells together and takes effluent from the hog lagoons to the pivots.

Hitch Farms' swine effluent system uses 27 hog lagoons, five portable pumping units, 15 booster pumps, 20 center pivots, 30 wells and 50 miles of underground pipe. It allows more cropping flexibility and makes the timing of effluent pumping less critical.

Raines and his crew manage a variable rotation of corn, wheat, sunflowers and silage corn under the pivots, plus dryland production of wheat in the corners outside the pivots' influence.

“We try to put it on the circles closest to the lagoon,” Raines says, “but if something comes up, we can put it wherever we want it.”

The slurry is 40% effluent, 60% fresh water. An environmental specialist conducts effluent and soil analyses on a carefully regulated schedule.

Hog effluent is the only animal waste flowing through this part of the system. Effluent from Hitch's cattle feedlot lagoons flows into another, smaller system.

A starting place

Five, trailer-mounted, portable pumping units move the effluent. Propane powers the engines. A trailer is parked on the bank of a lagoon and the inlet hose is put into the effluent. Eventually, Raines plans to have a permanent inlet hose at every lagoon to eliminate that step.

A hand-priming device draws fluid into the pump, then the engine is started and the pump activated. Some units have a chopper ahead of the pump. A solid separator puts larger material back into the lagoon through a 2-in. hose.

An in-line meter records gallons per minute (gpm) and acre-feet pumped so operators can record the information for the environmental department's work. A check valve prevents back-flow of freshwater into the lagoon in the unlikely event the engine should shut off.

The pumps have a capacity of 500 gpm but Raines runs them at 200-250 gpm to provide the right amount of flow for the standard mixture of fresh water and effluent to his pivots.

As it leaves the portable pumping unit, the effluent enters the underground system through a portal at each lagoon and is directed to the desired pivot or pivots through the system of pipes, valves and booster pumps.

The 1,000-head finishing houses are mostly grouped in threes, so the effluent from 3,000 hogs goes into each lagoon. Raines says the portable pumps usually stay at each location four to five days.

A web of underground pipes

The underground piping — mostly 8-in. PVC pipe, with some 10-in., and most rated for 50 lbs. of pressure — is the heart of the system. But, three, old, quarter-mile sections with 30-lb. lines are still in the system, so booster pumps are doubly important when water must be moved long distances.

With the area's slope mostly northwest to southeast, that's the general direction of flow for the system. In principle, the system begins with two central lines running from northwest to southeast, with one cross-over point near the high point and many branch lines to pivots and lagoon pumping points along the way (see diagram at right).

Pivots and pumps

For years, Raines has run his pivots at 20-lb. pressures and 500 gpm, but he ischanging nozzles to 700 gpm.

“There were just too many times we couldn't keep up at 500 gpm,” he says. “If we got a little bit behind, or if the transpiration rate was extremely high, especially with corn, we didn't have enough capacity.”

The higher pumping rate will allow application of enough water when irrigation is needed and shut down when it's not. Gypsum blocks are used to monitor soil moisture.

“When the soil profile is full, we shut off the irrigation,” Raines explains.

The 15 booster pumps are indispensable. Some sections of the underground system require low pressure, and friction loss from sending water long distances also can be high.

“My underground water system is really just a reservoir, and I use booster pumps to pick up the water and put it through the pivots,” Raines says. Booster pumps can serve up to eight pivots.

A series of valves at each location allows the water and effluent to be directed where it's needed.

Connections are important

The wells on this portion of Hitch Farms vary from 250-500 gpm. The better-producing wells are on the north and northwest ends, the poorer ones at the southern end, Raines says. All pivots were in place before the swine-finishing units were built. Before hogs, there was some sharing of well water between pivots, but water stayed on the section where it was pumped.

Some states don't allow groundwater to be moved about in this manner. Raines says he's lucky Oklahoma regulations allow it.

The combined piping available for each pivot and the combined use of water for hog and crop production give Hitch Farms options other operations may not have. Raines maximizes water usage through strip-tillage and by limiting irrigation to crop needs. The gypsum blocks used to monitor soil moisture confirm his water conservation efforts.

“My goal is to conserve every drop of water Mother Nature gives me and grow something with it,” Raines says.

Arrested undulations

This Oklahoma pork producer found a simple solution to wave problems.

One very windy day three years ago, Vic Little drove to the lagoon near one of his 3,400-head hog nurseries and was horrified to see huge waves rolling across the surface and crashing against the northeast shoreline.

“It almost looked like you could surf them,” says Little, of Rosston, OK. “I thought about how much those waves were increasing the surface area of the lagoon and how much more odor was coming off it. And also, how much the wave action was stirring it and probably messing up the biological activity.”

Little says his first concern when he got into contract hog production with Murphy Farms in 1998 was odor and its effect on neighbor relations. In fact, Little's emphasis on getting along with neighbors and taking care of the environment earned him an environmental stewardship award in 2003 from the National Pork Board and National Hog Farmer.

“I had to find some way to slow down the wave action,” Little explains.

He first considered large, floating squares of PVC pipe tethered in the lagoons to interrupt wave action, but realized they would be too expensive and cumbersome.

Then he struck upon a brilliantly simple solution. He bought a roll of 1½-in., black polyethylene pipe and cut it to fit the nearly full width of his lagoons. He heated, mashed down and sealed each end of the pipe. Next he drilled a hole through each end of the pipe to which he tied a piece of nylon rope.

Finally, he drove steel T-posts on either side of the lagoon and draped the piece of pipe across and tied it to the T-post on the opposite side. He allowed just enough play for the pipe to float on the surface of the effluent.

It worked perfectly, even on the windiest days.

Little uses just two of these wave levelers on each of his 250 × 325-ft. lagoons. The levelers are spaced so they nearly divide the lagoons into thirds.

“That first one really slows the wave action. Before it can get going good again, it hits the next one. After that, it's not much farther to the edge and it can't get much of a wave going in that amount of space,” Little says. “It's really just a buffer on the surface of the lagoon.”

The banks of Little's lagoons are lined with riprap so bank erosion isn't much of a danger. Many lagoons aren't, so wave action can damage bank slopes and impair the lagoon's function, say lagoon-engineering experts.

Let sleeping sludge lie

Research says old lagoons might best be filled in and monitored.

The old saying that “only the rocks live forever” reminds us that all of our man-made inventions have an end-point. That includes animal waste lagoons.

Most of today's large-scale, animal-feeding facilities are too young to have reached the end of their useful life, yet at some point they will. And, their expiration may pose quite an expense for animal feeders who use them.

An estimate from North Carolina says it costs an average of $43,000/acre of lagoon surface to haul the wet sludge and apply it to land elsewhere. Other states show similar figures.

North Carolina State University (NCSU) is studying an alternative closure method that shows promise. Researchers say unneeded lagoons might be covered with soil and planted to trees to help draw out the moisture, as well as the nutrients. The process, known as phytoremediation, is essentially the same process being used on many municipal landfills across the U.S.

The two lagoon sites in the research project are planted to hybrid poplar trees provided by Ecolotree, an Iowa company specializing in reclaiming landfills and contaminated sites.

Native species of trees may also be considered, but have not been yet planted, says NCSU agricultural engineer Frank Humenik.

Monitoring wells at the sites check for seepage of the nutrients away from the lagoons.

Only one year of data has been drawn from the site, Humenik says, but the second-year data was being gathered at press time. If the process is successful, it could be vitally important as North Carolina is considering the elimination of all lagoons, pending the outcome of a large research project on alternative waste handling technologies.