ALABAMA COOPERATIVE EXTENSION SERVICE, AUBURN UNIVERSITY, ALABAMA 36849-5612

Controlling Salt Buildup In Wastewater Recycling Systems

Widespread use of wastewater recycling systems has im­

proved 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 outwrurd 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 nor well defined.

Salt buildup has been exten­sively studied in other livestock­producing 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 low­pressure, 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.

• Locat~: 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 l-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

CIRCULAR ANR-860

the immediate vicinity of the dis­charge point, should be 11/z inch­es.

• Minimize shrurp 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 la­

goons to provide adequate treat­ment capacities.

• Charge a new lagoon at least half full of water prior to start­up. 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 fi.ll 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 l-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 somP 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

1.0 1.5 2.0 2.5 3.0 4.0 6.0

Gallons Per Foot Of Length

0.06 0.13 0.20 0.29 0.43 0.70 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.

The

E:.."1Aiabama lil7Cooperative

Extension Service AUBURN UNIVERSITY

Circular ANR-860 This publication was prepared by Ted W. Tyson and James 0. Donald, Extension Agricultural Engineers. It was adapted from "Crystalline (Salt) Formation In Wastewater Recycling Systems," EBAE 082-81. James C. Barker. Biological, and Agricultural Engineering, North Carolina State University, Raleigh, NC.

Use chemicals only according to the directions on the label. Follow all directions, precautions, and restrictions that are listed.

For more information, call your county Extension office. Look in your telephone directory under your county's name to find the number.

Issued In furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension Service, Auburn University, Ann E. Thompson, Director, offers educational programs and materials to all people without regard to race, color, national origin, sex, age, or handicap and is an equal opportunity employer. ACES, 3M, New 5:94, ANR-860