Ohio State University Extension Fact Sheet

Ohio State University Extension

Food, Agricultural and Biological Engineering

590 Woody Hayes Dr., Columbus, Ohio 43210

Managing Livestock Waste Facilities .... Controlling Crystal Formation in Recycle Flush Systems


Karen Mancl
Michael A. Veenhuizen

Flushing systems for manure removal from livestock facilities are used to control flies and odors, and reduce labor costs. Flushing systems, however, do require large volumes of water. After settling the manure solids, and treating in an anaerobic or aerobic lagoon, the effluent is commonly recycled to reduce the amount of fresh water needed for flushing, reduce the storage volume requirements and reduce wastewater requiring disposal.

Recirculation of lagoon effluent for flushing water, however, can result in accumulation of salts. In many cases, crystals begin to form at the suction end of a pipeline, at valves, in pumps, on aerators and on pipeline fittings. The buildup of crystals severely restrict flow and interfere with mechanical equipment.

What are the crystals?

The most common crystals formed are white or light gray in color and are called struvite. The crystals are composed predominantly of magnesium ammonium phosphate. This is the same compound that forms kidney and gall stones.

Why do crystals form?

As water is continually recycled in a flushing system, salts accumulate in solution until they reach "supersaturation." If conditions are right the salts will begin to deposit as crystals. Conditions that promote crystal buildup include high pH values (above pH 7), rough interior pipe surfaces and contact with metal fittings and components. It is difficult to predict if crystals will form in a recycle flush system. Crystal buildup is a serious problem in swine and poultry facilities. It is occasionally a problem in dairy and beef operations.

Where do crystals form?

Struvite crystals tend to form where the water flow is turbulent, such as around pipe fittings, valves and pumps. Metal pipe usually accumulates crystals faster than smooth plastic pipe. Metal surfaces provide a more favorable condition for crystal formation.

Careful pump and pipe sizing can limit turbulence in the recycle system and therefore, limit crystal formation. Some important factors include:

How can crystals be eliminated?

A number of approaches can be used to eliminate a crystal problem. Of course each solution has its advantages and limitations.

Table 1. Acid solutions used to dissolve struvite crystals
Type of AcidDilute to:
Concentrated sulfuric acid (98%)5 parts acid to 95 parts water
Concentrated acetic acid (99.7%)20 parts acid to 80 parts water
Concentrated hydrochloric acid (37%)20 parts acid to 80 parts water

Example: To produce 100 gallons of a sulfuric acid solution, add 5 gallons of concentrated sulfuric acid to 95 gallons of water.

Encrusted parts of the flushing system may be placed in a container of acid solution to dissolve the crystals. As an alternative, acid solutions may be pumped through the flushing system to dissolve the crystals.

Recirculating the acid through the pipes is more effective than letting the acid stand in the pipes. However, the system must be taken out of service so the acid can remain in the pipes for several hours. It may take as long as 72 hours to dissolve heavy crystal deposits. Also, acid treatment can corrode metal components of the flushing system.

After treating the parts of the flushing system, the acid solution must be properly handled, stored or disposed. Commonly the acid solution is pumped into the lagoon. However, the acid must be carefully added to avoid upsetting the lagoon pH. Monitor the lagoon pH after acid discharge.

An alternative is to drain and store the acid solution for reuse before it is disposed of in the lagoon. Note that with each use the strength and effectiveness of the acid will decrease. Always store acid in clearly marked, acid resistant containers to avoid accidental exposure.

Be very careful when handling acid solutions. Acid may foam and fume, and will get hot when added to the flushing system. Always add acid to water - never add water to acid because the mixture will react too quickly, boil and splatter. Always wear an acid-resistant apron, gloves and a face shield when working with concentrated acid solutions. One operator in Northwest Ohio has already been injured adding acid to clean a flushing system.

What does not eliminate crystals?

Researchers at North Carolina State University have studied the addition of an organic waste digestant to the lagoon and the use of a magnetic water conditioner to remove scale and crystal buildup. Both of these methods were not effective in preventing struvite crystal formation.


Barker, J. C. 1981. Crystalline (Salt) Formation in Waste-water Recycling Systems. Agri-Waste Management Extension Fact Sheet. North Carolina State University.

Booram, C.V., R.J. Smith, and T.C. Hazen. 1975. Crystalline Phosphate Precipitation from Anaerobic Animal Waste Treatment Lagoon Liquors. Transactions of the ASAE. 18:340-343.

Roberto Jr., J.S., and J.M. Sweeten. 1985. Control of Crystal Formation in Recycle Flush Systems at Caged Layer Operations. Agricultural Waste Utilization and Management. Proceedings of the 5th International Symposium on Agricultural Wastes. ASAE. St. Joseph, MI.

Sweeten, J.M. 1983. Waste Management for Commercial Egg Farms. Presented at Texas Commercial Egg Clinic, Texas Agricultural Extension Service, College Station.

Watson, H. 1984. Managing Lagoons for Livestock and Poultry Waste. Agricultural Engineering Fact Sheet. Alabama Cooperative Extension Service, Auburn University.

Westerman, P.W., L.M. Safley Jr. and J.C. Barker. 1985. Crystalline Buildup in Swine and Poultry Recycle Flush Systems. Agricultural Waste Utilization and Management. Proceedings of the 5th International Symposium on Agricultural Wastes. ASAE. St. Joseph, MI.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181

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