ANR-860 Controlling Salt Buildup In Wastewater Recycling Systems
ANR-860, New May 1994. By
Ted W. Tyson, Extension Agricultural Engineer, Associate Professor,
and James O. Donald, Extension Agricultural Engineer, Professor;
both in Agricultural Engineering at Auburn University.
Controlling Salt Buildup In Wastewater Recycling Systems
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Widespread use of wastewater recycling systems has improved
waste handling from poultry layer and swine production facilities. Wastewater
recycling reduces pit gases and odors and improves poultry and swine performance,
flock and herd health, and in-house working environments.
However, wastewater recycling also causes buildup of a grayish-white
crystalline salt on the internal surfaces of pump and piping systems used
for lagoon liquid recycling. This compound is predominantly magnesium ammonium
phosphate, sometimes referred to as struvite. Deposits occur most frequently
on metallic surfaces but also occur in varying degrees on plastics. Among
metal components, steel, cast iron, bronze, and brass appear to be about
equally susceptible.
Salt buildup first occurs in the internal pump components and gradually
moves outward into discharge pipes. Buildup also occurs at pipe joints,
elbows, valves, or imperfections because grit and manure solids tend to
lodge at these points, providing a base on which salt can grow. However,
predicting the occurrence of salt crystallization and deposition is difficult
since exact causes are not well defined.
Salt buildup has been extensively studied in other livestock-producing
states. Based on the research and the field trials from other states, this
publication identifies design, maintenance, and management techniques to
reduce salt buildup.
Pump And Piping System Design
Choose a high-quality low-pressure, self-priming centrifugal or submersible
pump for the best service.
Avoid using a significantly oversized pump. Using a pump that is too
large for the piping system will restrict the discharge and increase pipe
flow velocities and pump cavity turbulence.
Locate the pump as close to the high water level of the lagoon as possible
to minimize suction lift.
Consider placing pump controls on a timer.
Ensure that the suction line is large enough to prevent pump cavitation.
(A good rule of thumb is the suction pipe diameter should be one standard
size larger than the discharge pipe.)
Replace fine mesh suction intake strainers with wire screens or baskets
with 1-inch or larger mesh and a diameter at least 5 times the suction
pipe diameter.
Use nonmetallic pipe and fittings.
Consult pump experts to size pipe diameters large enough to maintain
flow velocities between 3 to 5 feet per second. The minimum pipe diameter
at any point throughout the system, except at the immediate vicinity of
the discharge point, should be 1 1/2 inches.
Minimize sharp pipe bends such as elbows and tees. Instead use flexible
plastic pipe and long sweep elbows for direction changes.
Choose a pump and piping system with enough capacity to allow it to
operate only one-half to two-thirds of the time.
If a piping system will not be in continuous use, design it so that
it can be drained between pumping events.
Electrostatic Charge Elimination
Eliminate static charges or stray voltage that may contribute to salt
buildup by directly grounding the pump housing. Drive a metal rod 10 to
12 feet into moist soil near the lagoon edge.
Periodically check cable connections at the ground rod and pump for
corrosion.
Lagoon Management
Properly size primary lagoons to provide adequate treatment capacities.
Charge a new lagoon at least half full of water prior to start-up.
Bring up the liquid level to design levels as soon as possible;
Maintain lagoon levels during weather extremes. Rainfall during normal
years dilutes lagoon liquid concentrations. Extended periods of hot, dry
weather increase nutrient and salt levels and the rate of salt buildup.
During these dry periods, flush with fresh water or irrigate a portion
of the lagoon contents and replace with fresh water as needed to dilute
lagoon salt concentrations.
Acid Cleaning
- Dissolve salts from internal pump and pipe surfaces with dilute acid
treatments. Dilute 1 gallon muriatic (hydrochloric) acid in 9 gallons of
water. Muriatic acid (30 percent [20°] technical grade) can be purchased
at most chemical supply houses or paint stores. (Muriatic acid costs approximately
$27 for a 15 gallon drum plus a $25 drum deposit and $1 drum cleaning fee.)
NOTE: Exercise extreme caution
when mixing acids with water. Never try to add water to the concentrated
acid. Always partially fill the tank with water; then add the acid to the
water very slowly. Mixing acid and water generates heat. Wear eye protection
against possible spattering. Although this dilution should not hurt metal,
avoid prolonged contact of the acid solution and metal.
- To remove heavy salt buildup, install an acid recirculation loop for
thorough treatment. Install a 1-inch line to return acid from the end of
each treated pipe section to the tank. Consider installation of a permanent
return line for convenience. Use a 150-gallon acid-resistant tank as the
acid recirculation reservoir. Make sure the tank is large enough to supply
enough total solution to fill the pipe length to be cleaned plus some reserve
to keep the recirculation pump primed (See table to calculate gallons needed
to fill pipe).
Calculating Gallons Per Foot Of Length Based On Inside Pipe Diameter.
Inside Pipe Diameter, Inches
|
Gallons Per Foot Of Length
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| 1.0 |
0.06 |
| 1.5 |
0.13 |
| 2.0 |
0.20 |
| 2.5 |
0.29 |
| 3.0 |
0.43 |
| 4.0 |
0.70 |
| 6.0 |
1.53 |
Isolate pipe line sections with valves so that acid can be circulated
through only one section. Isolating pipe sections will reduce the amount
of acid solution needed and, therefore, reduce tank size. Install a quick-connect
coupling to switch the flush pump suction from the lagoon to the bottom
of the acid tank.
- If the salt buildup is heavy, recirculate the solution overnight. After
it is used to dissolve a heavy salt buildup, the solution may have to be
discarded. An inexpensive pH test kit can show how much strength remains.
If the solution is weak, discard it. Spent acid may be dumped in the lagoon.
For more information, call your county Extension
office. Look in your telephone directory under your county's name to find
the number.
Information was adapted from "Crystalline (Salt) Formation In Wastewater
Recycling Systems," EBAE 082-21. James C. Barker, Biological, and Agricultural
Engineering, North Carolina State University, Raleigh. NC.
Use chemicals only according to the directions of the label.
Follow all directions, precautions, and restrictions according to the directions
on the label.
For more information, contact your county Extension office. Visit http://www.aces.edu/counties or look in your telephone directory under your county's name to find contact information.
Issued in furtherance of Cooperative Extension work in agriculture and
home economics, Acts of May 8 and June 30, 1914, and other related
acts, in cooperation with the U.S. Department of Agriculture. The Alabama
Cooperative Extension System (Alabama A&M University and Auburn
University) offers educational programs, materials, and equal
opportunity employment to all people without regard to race, color,
national origin, religion, sex, age, veteran status, or disability.
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