Crystalline (Salt) Formation in Wastewater Recycling Systems

Prepared by:
James C. Barker, Professor and Extension Specialist
Biological and Agricultural Engineering
North Carolina State University, Raleigh, NC

Published by: North Carolina Cooperative Extension Service

Publication Number: EBAE 082-81

Last Electronic Revision: March 1996 (JWM)

Widespread implementation of flush system technology has improved waste handling from swine production facilities by reducing pit gases and odors and improving animal performance, herd health, and in-house working environments. Implementation of this technology has also revealed the buildup of a grayish-white crystalline salt on the internal pump and piping surfaces 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 is first detected in the internal pump components gradually moving outward into discharge pipes. Deposition also occurs at pipe joints, elbows, valves or imperfections because grit and manure solids tend to lodge at these points providing a base for the salt to grow. Predicting the occurrence of salt crystallization and deposition is difficult since exact causes are not well defined.

This problem has been extensively studied. Research and field trials have identified design, maintenance and management techniques which, in many cases, reduces the buildup to more acceptable levels.

Pump and Piping System

High-quality low-pressure, self-priming centrifugal or submersible pumps give the best service. Avoid significant oversizing of pumps in relation to the piping network to minimize high pipe flow velocities and excessive pump cavity turbulence caused by restricting the discharge. Consider placing pump controls on a timer. Ensure that the suction line is large enough to prevent pump cavitation. (Rule of Thumb: The suction pipe diameter should be one standard size larger than the discharge pipe.) Locate the pump as close to the high water level of the lagoon as possible to minimize suction lift. Replace fine mesh suction intake strainers with wire screens or baskets 1-inch mesh or larger 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 3-5 feet per second. The minimum pipe diameter at any point throughout the system except at the immediate vicinity of the discharge point should be 1.5 inches. Sharp pipe bends such as elbows and tees should be minimized using instead flexible plastic pipe and long sweep elbows for direction changes.

The pump and piping system should have enough capacity to allow it to operate only one-half to two-thirds of the time. Piping systems not in continuous use should be planned for draining between pumping events.

Electrostatic Charges

Direct grounding of the pump housing discharges any static charges or stray voltage believed to contribute to salt deposition. A metal rod should be driven 10-12 feet into moist soil near the lagoon edge. Cable connections at the ground rod and pump should be checked periodically for corrosion.

Lagoon Management

Primary lagoons should be properly sized with adequate treatment capacities. A new lagoon should be charged at least half full of water prior to startup and the liquid level brought up to design levels as soon as possible. Rainfall during normal years dilutes lagoon liquid concentrations while extended periods of hot, dry weather increase nutrient and salt levels and the rate of salt buildup in recycling systems. During these periods, flushing with fresh water or irrigating a portion of the lagoon contents replaced by fresh water may help.

Acid Cleaning

Salts can be dissolved from internal pump and pipe surfaces with dilute acid treatments. Several dosings followed by flushing of the spent acid solutions will be needed for removal of heavy buildups. A more thorough, but costly, treatment can be obtained by installing an acid recirculation loop.


                        Inside Pipe       Gallons per
                      Diameter, inches   Foot of Length
                      _________________________________
                            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

A 150-gallon acid-resistant tank could serve as the acid recirculation reservoir. The tank should supply enough total solution to fill the pipe length to be cleaned as calculated from the table plus some reserve to keep the recirculation pump primed. To reduce tank size, line sections can be isolated with valves so that acid is circulated through only one section. The flush pump suction is switched from the lagoon and connected to the bottom of the acid tank with a quick-connect coupling. A 1-inch line returns acid from the end of each treated pipe section to the tank.

Muriatic (hydrochloric) acid (30% (20o) technical grade), purchased at most chemical supply houses or paint stores, is diluted (one gallon acid added to 9 gallons water). Extreme caution must be exercised since mixing of acids with water can be very hazardous. 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. Heat will be generated. Eye protection is advisable. This solution should be recirculated overnight for a heavy salt buildup. This dilution should not hurt metal although prolonged contact should be avoided. A heavy buildup may render the acid usable only one time; although it should be retained after the first use and reused to see how much strength remains. Spent acid may be dumped to the lagoon.

Example acid costs (Worth Chemical Company, Durham, NC) are $13.80 for a 15- gallon drum plus a $25 deposit on the drum, or $32.50 for a 50-gallon drum plus a $50 drum deposit.

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

EBAE 082-81

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