University of Missouri Extension

G1158, Reviewed October 1993

Editor's note
The printed version of this publication includes illustrations.

Recycling Lagoon Water for Manure Flushing Systems

Charles D. Fulhage
Department of Agricultural Engineering

Flushing systems for manure collection and transport from animal confinement buildings have the advantages of improved building environment and reduced labor. Flush water comes from either the domestic water supply or as recycled water from the lagoon receiving the wastes. The advantages of using recycled water are a decreased demand on the domestic water supply and less hydraulic or "fresh water" loading on the lagoon.

Recycle pumps

A typical recycling system consists of a pump, recycle line or pipe to carry water from the lagoon to the confinement building, and the appropriate controls. These components are connected in a closed-loop configuration with the flushing gutter and the drain line from the confinement building (Figure 1).

Figure 1
Basic components of a recycle system

Pump selection and location
These factors are important in developing a reliable recycling system. In most cases, locate the pump at or near the lagoon to minimize elevation and pipe friction losses on the suction side of the pump. Although self-priming pumps located in or near the confinement building are accessible and inexpensive to install, they may become difficult to maintain because of the high suction head, the resulting loss of prime, and dry running. For pumps located at the lagoon, you may supply the power by laying underground feeder electric cable in the same trench with the building drain or recycle line. A high voltage (220V) operation is preferred to minimize wire size.

A pump selected for recycling lagoon water should have an open or semi-open impeller to decrease the risk of plugging due to solids ingestion (Figure 2). Most sewage or trash pumps have semi-open impellers, capable of handling the solids normally encountered in recycling systems.

Figure 2
Three types of impellers commonly used in centrifugal pumps

However, in applications where fibrous, stringy material (such as hay or silage in a dairy lagoon) is present, use a cutter or chopper pump. These pumps have a cutting or chopping device (located just outside the pump inlet) that rotates with the impeller and shreds fibrous material as it enters the pump.

Pumps and installation
Pumps most often used for recycling lagoon water are either submersible centrifugal (commonly called sewage or sewage ejector pumps), or self-priming centrifugal pumps.

Submersible pumps may be more difficult to install, but have the advantages of continuous positive priming and no freezing because all components are located below the freezing level.

Self-priming pumps may be located on the lagoon berm, or in a wet well adjacent to the lagoon. With either location, you must provide freeze protection. You also would need to provide a means of automatically shutting off the pump if lost prime causes overheating. Figure 3 shows two methods of installing self-priming recycle pumps.

Figure 3
Two methods of installing self-priming recycle pumps

Install submersible sewage pumps by suspending them about 2 feet below the water surface, so they remain free of surface debris and bottom sludge. Figure 4 shows the preferred method of installing a submersible pump.

Figure 4
Submersible recycle pump installation using a catwalk and hand winch for ease of pump retrieval

The catwalk should extend out far enough into the lagoon so that the pump will stay in 3 to 4 feet of water when the lagoon is pumped down. This installation has the advantages of freeze protection, positive prime, and the pump is easy to retrieve for service and repair. The greater time and expense required for this type of installation will be returned many times over through its convenience in maintenance and through no freezing or loss of prime.

Figure 5 shows an alternative for installation when the lagoon is filled before the catwalk arrangement of Figure 4 can be built. A float suspends the pump 1 to 2 feet below the water surface. To prevent movement and stress on the recycle line, moor the floats to the lagoon berm with two or three stainless steel cables. This installation is easier than the one shown in Figure 4, but it is more difficult to remove this pump from the lagoon for service or repair. Often, you will need a boat or chest waders to retrieve a pump installed in this manner.

Figure 5
Submersible recycle pump installation using a float for pump support

Whichever method is used, always locate the pump on the side of the lagoon opposite the discharge pipe from the building. This location will ensure maximum retention and treatment of the water before it is recycled back to the confinement unit.

Pump controls

Recycle pump controls may be manual or automatic. In most installations, the pump runs continuously and the flow rate is adjusted by valves or appropriate restrictions. These controls ensure that the flush tank or tanks fill within the flush frequency time period.

Operators are sometimes reluctant to restrict or throttle the discharge from a pump for fear they will make the pump "work harder." However, most centrifugal pumps consume less power at higher pressures and lower flow rates. As long as enough water moves through the system to prevent heat build-up in the impeller housing, the pump may be operated at any flow rate consistent with the flush frequency requirements of the confinement buildings.

Installation of pump controls
In systems requiring more or less continuous pump operation, a manual on-off switch is usually satisfactory. If the pump must be turned on and off each day, an automatic device is usually used. This device may be a float switch or timer.

A float switch located in the flush tank automatically shuts off the pump when the tank is full. If more than one flush tank is in the system, float valves can stop the flow of water to all tanks except the one with the float switch. If the flow into each tank is properly adjusted, the tank containing the float switch fills at a slower rate than the other tanks and all tanks will be full when the pump shuts off.

An alternative is to install a float switch in one of the tanks in a multiple tank installation. No float valves are installed in the remaining tanks. The fill rate of each tank is then carefully adjusted through the use of valves or orifices, so each tank fills at a slightly faster rate than the tank containing the float switch. These tanks overflow for a short period of time until the float switch shuts the pump off.

Another alternative is to use a timer to control the pump. The timer is set to run the pump until all the flush tanks are filled. Although there may be some overflow, it is usually not a critical problem if recycled lagoon water is being used.

Continuous pump operation usually is used when flush tanks are designed to discharge automatically. Automatic pump controls are usually used when flush tanks are dumped manually at a specific time during the day (as in a dairy operation).

Recycle pipe

PVC or polyethylene pipe is most suitable for recycling lagoon water. If possible, all joints, couplings and fittings also should be plastic. Flexible plastic pipe for turning corners is preferable to sharp 90-degree elbows.

Valves or orifices can accomplish flow adjustment at the point of discharge into flush tanks. Orifices are more difficult to size, but they are not as likely to plug with crystal build-up or solids. To make orifices, glue a cap on the end of the recycle pipe and drill a hole in the cap until it is large enough to achieve the desired flow rate.

With partially open gate valves, the flow of water passes through a relatively long, narrow slot or crack that may be more susceptible to plugging than the orifice. However, once installed, the flow is easier to change with a gate valve than with an orifice.

System design

Recycle system design and equipment selection require a knowledge of hydraulic head, pipe friction losses and other factors. The following steps are only a guide for estimating component sizes. Consult your local MU Extension center or other qualified individuals for tailed design assistance.

Table 1
Selecting pipe size for recycle systems

Flow rate Pipe size
0 to 15 gallons per minute 1-1/2 inches
15 to 25 gallons per minute 2 inches
25 to 50 gallons per minute 2-1/2 inches
50 to 80 gallons per minute 3 inches
80 to 150 gallons per minute 4 inches

For example, a 30-gallon-per-minute system 350 feet long with 2-inch pipe would have a friction loss of 2.36 x 350/100 = 8.26 feet of head.

Table 2
Pipe friction loss (in feet) of water per 100 feet of plastic pipe

Flow rate Pipe size
1-1/2 inches 2 inches 2-1/2inches 3 inches 4 inches
5 gallons per minute 0.35        
10 gallons per minute 1.26        
15 gallons per minute 2.65 0.65      
25 gallons per minute 6.83 1.68 0.57    
30 gallons per minute   2.36 0.80    
40 gallons per minute   4.02 1.36    
50 gallons per minute   6.07 2.05 0.84  
60 gallons per minute     2.87 1.18  
80 gallons per minute     4.89 2.01 0.50
100 gallons per minute     7.40 3.04 0.75
125 gallons per minute       4.60 1.13
150 gallons per minute       6.45 1.59

Table 3 shows typical power requirements of submersible sewage pumps for various pressures (total head) and flow rates. Most sewage pumps in the 1/2 to 1 horsepower range have head limitations of 25 to 40 feet. When pumping heads are greater than 25 to 40 feet, you may need a special high head sewage pump.

In some cases, the head-flow rate requirements of the system may indicate that pumps smaller than 1/2 horsepower would be adequate. However, pumps smaller than 1/2 horsepower have limited solids handling capacity and do not have the "heavy duty" characteristics necessary for operating in the lagoon environment.

Table 3
Typical power requirements for submersible sewage pumps at various pumping heads and flow rates

Pumping head feet of water Gallons per minute flow rate
10 20 30 40 50 60 80 100 120
Horsepower
5 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2
10 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 3/4
15 1/2 1/2 1/2 1/2 1/2 1/2 1/2 3/4 1
20 1/2 1/2 1/2 1/2 1/2 3/4 3/4 1  
30 3/4 3/4 3/4 1 1 1      

Crystallization

Crystalline deposits in pumps, pipes and plumbing may develop in some recycle systems. Although crystallization may not occur, such deposits can stall the pump, ruin pump seals, or close off recycle pipes, severely restricting the flow. It is difficult to predict the formation of these crystals. However, crystals appear to form most often in lagoons that are highly concentrated or overloaded with manure.

The causes of crystallization
The crystals result from concentrations of magnesium, ammonium and phosphate ions in anaerobic lagoon water. These ions form the magnesium ammonium phosphate crystalline complex.

Crystals form most quickly on metal components. Steel, cast iron, bronze and brass appear to be equally susceptible. Although deposition in plastic pipes has been observed in several cases, plastic components are not as susceptible.

The crystals usually begin to form on some grit or other solid material lodged in the pipe. After the initial formation, the crystals build upon themselves quite readily. The deposit accumulates both along the length of the pipe and across the diameter of the pipe.

Precautions
Crystal build-up seems to start most readily at joints, elbows and fittings. Grit and solid material tend to lodge at these locations and provide a base from which the deposit can grow.

For this reason, recycle pipes should be a minimum of 1-1/2 inches in diameter. If possible, flexible plastic pipe should be used for direction changes rather than short- or even long-sweep plastic elbows.

Systems in which the only metal parts are the pump housing and the impeller are the least susceptible to crystalline deposits.

Dissolving crystals
Since crystals may form in any system despite these precautions, give consideration to methods of maintaining the system if build-ups do occur.

Crystals can be dissolved with an acid if the acid solution is kept in contact with the crystals. A 1:50 dilution of concentrated acetic acid with water has proven effective in dissolving crystals. This solution will not harm the lagoon.

A recirculation pipe installed along with the recycle pipe provides the capability for circulating an acid solution through the system (Figure 6).

Figure 6
Recirculation system for dissolving crystals in recycle line

An acid mixing tank, sized to hold enough acid solution to fill the system, is located at the discharge end of the recycle pipe. The recirculation pipe carries the acid solution from the mixing tank to the inlet of the pump. On submersible pumps, weld a collar or coupling to the pump inlet for connecting the recirculation pipe. The size of the recirculation pipe is not critical because high recirculation flow rates are not required, but the pipe should not be less than 1-1/2 inches in diameter.

Examine the recycle system regularly to determine the pattern and rate of crystal build-up. Then use the recirculation system as needed to prevent excessive build-up and subsequent failure of the system.

Installation of recirculation capability will increase the cost of the recycle system. However, the probability that total failure of the system could occur if crystal build-up becomes excessive appears to justify the additional cost.

Lagoon management
Careful lagoon management can minimize the potential for crystal formation. Overloading or periods of high evaporation with little rainfall can result in concentrations of salts that may cause crystal problems. Pump the lagoon regularly and add fresh water as needed, especially during dry periods, to keep salt concentrations below critical levels.

G1158, reviewed October 1993

G1158 Recycling Lagoon Water for Manure Flushing Systems | University of Missouri Extension

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