Hydraulic collection and transport is a preferred method of handling livestock manure for many producers. Hydraulic methods tend to reduce labor and equipment requirements and take advantage of gravity in the movement of manure. However, most manure, as excreted by the animal, is too thick (solids content is too high) to move hydraulically. Therefore, a typical practice is to use recycled effluent from the manure lagoon to dilute manure so that it can be collected and transported hydraulically to the lagoon.New 04/03/3.5

Fulhage, Charles

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$.50 EQ 353 Printed on recycled paperMU GuideENVIRONMENTAL QUALITYPUBLISHED BY MU EXTENSION, UNIVERSITY OF MISSOURI-COLUMBIA muextension.missouri.edu/xplor/Nutrients and Bacterial WasteCrystallization in Lagoon Effluent Recycle LinesCharles FulhageAgricultural Engineering ExtensionHydraulic collection and transport is a preferredmethod of handling livestock manure for many produc-ers. Hydraulic methods tend to reduce labor and equip-ment requirements and take advantage of gravity in themovement of manure. However, most manure, asexcreted by the animal, is too thick (solids content is toohigh) to move hydraulically. Therefore, a typical practiceis to use recycled effluent from the manure lagoon todilute manure so that it can be collected and transportedhydraulically to the lagoon. Figure 1 is a schematicdiagram of a typical recycle system. Pit recharge systemsfor swine and flush systems for dairies are typical exam-ples of the use of recycled lagoon water for dilution andhydraulic transport of manure (Figure 2).A common problem in these manure-handlingsystems using recycled lagoon water is the buildup ofcrystals in various components of the system. Pumpimpellers, piping, flush tanks, flush gutters and drainlines are all susceptible to crystal buildup. Crystalformations can cause leaking seals and degrade perfor-mance of pumps. Accumulations in a pipe reduce flowarea and can eventually stop flow in the pipe (Figure 3).The severity of the crystal problem depends on manyfactors, including chemical characteristics of the lagooneffluent, selection and design of system components,and effluent flow rates in the recycle system.Crystal accumulation can be a major hindrance inthe use of recycled lagoon effluent to collect and trans-port manure hydraulically. To date no feasible means ofpreventing crystal buildup has been developed.However, by using certain design and maintenanceprocedures a producer can usually use recycled lagooneffluent in an acceptable manner.Crystal formationThe troublesome crystals that form in pipes andother components of systems using recycled anaerobiclagoon effluent have been identified as a compoundcalled magnesium ammonium phosphate (commonlyreferred to as “struvite”). This compound has beenknown and experienced for many years in the municipalwastewater systems. Crystallization has also becomeprevalent in the agricultural sector with the develop-ment of manure recycle flush and handling systems.Many factors contribute to the formation of themagnesium ammonium phosphate crystal “struvite”(MgNH4PO4 ˙ 6H2O), including the following:• The concentration of magnesium, ammonium andphosphate ions in the recycled lagoon effluent.Nearly all anaerobic manure lagoons contain theseions in sufficient concentration to support crystalformation to some degree. Generally the higher theconcentration of these ions, the more likely it is thatcrystal formation will occur.Figure 2. This dairy flush system uses effluent recycled from ananaerobic lagoon.Recycle pump Recycle lineConfinementbuildingDrain lineLagoonFigure 1. A typical recycle system for manure collection andremoval.Archive vrsion -- See extension.missouri.edu• The pH of the lagoon effluent. Most anaerobicmanure lagoons exhibit pH in the range of 6.5 to 8.0.Crystal formation is common in this pH range.• Materials used in the recycle system. Experiencehas shown that crystals are much more likely tooccur when metallic structural or piping compo-nents (black iron, galvanized iron, steel, cast iron,brass, bronze, copper) are used in the system.Plastic, rubber, or other synthetic materials do notbecome encrusted as readily.• Contact time and volume of effluent handled inthe recycle system. A recycle system designed forcontinuous pumping at the rate of 50 gallons perminute will circulate more than 26 million gallonsin the course of one year. Such intense use affordsample opportunity for crystal formation.• Physical configuration of the recycle system. Pipefitting components such as elbows, tees, valves andreducers, whether metallic or plastic, tend to haveedges or protrusions, which can trap grit or otherparticulate matter. Such trapped material providesa base for crystal formation and growth. Crystalgrowth is commonly expected to be more prevalentat these points in the system. Encrustation is lesssevere but also occurs in smooth portions of plasticpipe. Typically, crystals begin to form in the bottomof the pipe and then proceed around the circumfer-ence and longitudinally along the length of the pipe.In any given case, there may be other factorscontributing to crystal formation. The complex interac-tion of the many possible factors makes the severity ofcrystal buildup difficult to predict. When anaerobiclagoon effluent is recycled in a manure managementsystem, the possibility of crystal problems should beexpected, and measures should be taken to minimizepotential problems.SolutionsAlthough no feasible means of preventing crystalbuildup has been identified or developed, experiencehas shown that certain considerations can minimize theproblem to an acceptable level. These considerations canbe categorized as (1) design considerations and (2) oper-ation and maintenance considerations.Design considerationsA number of design considerations have beenshown to reduce problems of crystal buildup in lagoonrecycle systems.• Proper lagoon design. The lagoon should be prop-erly designed and sized for the number of animalscontributing manure to the system. Undersizedlagoons develop higher concentrations of magne-sium, ammonium and phosphate ions, and thus areassociated with more severe crystal problems.• Avoid the use of metallic components in thesystem wherever possible. Use pumps with plasticvolutes and impellers, plastic pipe, and plasticvalves and fittings when designing the pumpingsystem.• Minimize the number of fittings in the system tothe extent possible. Elbows, valves, tees, short 90-degree bends and other fittings tend to accumulatecrystals more quickly than smooth pipe. Use flexibleplastic pipe to the extent possible to eliminateelbows. Long-radius 90-degree elbows are prefer-able to short bends.• Use larger pipe sizes than might be indicated bytraditional hydraulic design. A larger pipe sizeprovides greater flow area or cross section per unitof wetted perimeter or circumference where crystalscan occur. For example, a 4-inch pipe has twice theflow area per inch of circumference as does a 2-inchpipe. Further, larger pipe sizes can accommodatesome crystal growth without degrading flow char-acteristics significantly. Consider that one-quarterinch of crystal growth around the periphery of a 4-inch pipe reduces the cross sectional flow area byabout 23 percent, while the same one-quarter inchbuildup around the periphery of a 2-inch pipereduces the flow area by about 45 percent. Currentrecommendations suggest sizing the recycle pipe sothat flow velocities are 2.0 to 2.5 feet per second. Donot use pipe smaller than 2 inches in diameter.Operation and maintenance considerationsOperation and maintenance considerations includeprocedures to remove crystals on a regular, periodicbasis after they have formed in the system. These proce-dures should be implemented on a schedule such thatcrystals are removed before growth becomes so exten-sive that pump performance or flow rate is impaired.The most widely used procedure in this category is acidwashing, or the use of a commercial de-scaling productto dissolve crystals from the system on a periodicschedule. Magnesium ammonium phosphate crystalsreadily dissolve in a low-pH environment, and this char-acteristic can be used to remove crystals in a recyclesystem.Page 2 EQ 353Figure 3. Crystals have reduced the flow area in this 2-inch PVCpipe by about 75 percent.Acid washing systems. Acid washing involves theintroduction of a diluted acid into the entire recyclesystem to dissolve crystals. Simply filling the pipes andother components of the recycle system with an acidsolution and letting it “sit” for a period of time todissolve the crystals is not as effective as circulating anacid solution through the system for a period of time.Hence, an acid washing system should include thenecessary piping, valves, pumps and other componentsso that the acid solution can be circulated in the recyclesystem as needed to dissolve the crystals. Figure 4shows how an acid circulation system might beimplemented.With reference to Figure 4, the system is operated asfollows:• Under normal recycling operations, valves 1, 2, 3and 4 are closed, and valves 5, 6 and 7 are open. Thisconfiguration allows lagoon effluent to be pumpedto the flush tanks or other holding areas.• When preparing to circulate acid, first allow asmuch lagoon effluent as possible to drain back to thelagoon from the pipes. This will minimizeunwanted dilution of the acid solution.• Close valves 5, 6 and 7 to isolate the lagoon andflush tanks or holding areas from the system.• Open valves 1 and 2 to form the closed-loop circu-lation system. Use the acid circulation pump ormain recycle pump to circulate the acid solutionthrough the system.Valve 4 should be located at a “high” point in thesystem and can be used to release or purge air from thesystem when preparing to circulate the acid solution.Excess lagoon effluent can be purged from the systemby diverting the acid return line to the lagoon until acidstarts arriving back to the acid storage reservoir (plasticcontainer in Figure 4).The acid return line should be at least half the diam-eter of the main recycle line, and the acid storage reser-voir should be, at minimum, large enough to hold thevolume of acid solution required to fill the piping in thesystem. Table 1 shows the volume per foot of lengthcontained in pipes of various sizes.Table 1. Volume of a pipe per foot of length.Pipe size Volume containedinches gallons per foot length2.0 0.163.0 0.174.0 0.656.0 1.47Example. A system with 400 feet of 4-inch mainrecycle line, and 400 feet of 2-inch acid return line wouldrequire an acid reservoir size of at least 400  0.65  400 0.16  324 gallons. Hence, an acid reservoir with avolume of 350 to 400 gallons should be selected.A separate acid circulation pump may not be neces-sary if the main recycle pump can be used to circulatethe acid solution. However, if the main recycle pump isa submersible pump in the lagoon, or in a wet well, itmay be more convenient to provide a separate acidcirculation pump, which stays plumbed into the system.The submersible pump is then acid washed separately,usually by placing the pump in a container of acidsolution.Almost any acid of sufficient strength will dissolvethe magnesium ammonium phosphate crystals. Glacialacetic acid mixed 1:50 with water has been used success-fully. Muriatic acid (generally available at concreteplants) diluted at the rate of 400 gallons of water per 55-gallon drum of acid has also been used effectively.Observe appropriate safety precautions when working withacid. It can cause injury! The acid reservoir should beplaced within a secondary containment facility tocontain any spills or leaks. All components of the systemcontaining acid should be labeled as a potential hazard.Acid-wash the system on a regular schedule asneeded before excessive crystal buildup occurs. Crystalsare much easier to dissolve and remove before flow inthe system is reduced significantly. If crystals areallowed to completely block flow, acid washing will notwork, and the system will have to be replaced. Acidused to dissolve and remove crystals will eventuallybecome neutralized (pH elevated) until it is no longereffective in dissolving crystals. When this conditionoccurs, the acid may be drained to the lagoon, and a“fresh” volume of acid is used for the next washing.Acid washing systems can be implemented in mostrecycle systems and are particularly applicable tocomplex systems involving multiple flow circuits orbranches or long flow paths that cannot be easilycleaned by other means.Hydraulic crystal removal. Some success has beenexperienced using water under high pressure to breakup or erode crystal formations from the components ofrecycle systems. With this approach, a high-pressurewasher with pressure and flow characteristics in theEQ 353 Page 3AcidcirculationpumpFlushtanksLagoonAir bleedvalvePlasticcontainerDrain valveMain recycle return linePlastic lineLagoon7235146Figure 4. Schematic of an acid recirculation system.range of 2,000 to 2,500 psi and 4 to 5 gallons per minuteis used with a special “sewer nozzle” to clean crystalsfrom pipes (Figure 5).The sewer nozzle has a forward-facing port thatdischarges water at a high velocity to erode and breakup crystal formations. The nozzle also has rear-facingports that drive the nozzle forward along the length ofpipe as it is cleaned. Figure 6 is a conceptual sketch ofthis type of nozzle.Since the sewer nozzle is fed by a high-pressurehose (typically 1⁄4 inch in diameter), this approach islimited to systems, which will allow movement of thenozzle and hose within the pipe. Smooth, continuouspipe and long-radius elbows must be used in the pipingsystem so the sewer nozzle and hose can be inserted andtravel along the length of the pipe. With this approach,cleaning should begin at the discharge end of the recyclesystem, and the recycle pump should be operatedduring cleaning to flush the eroded crystals from thepipe.SummaryThe formation of magnesium ammonium phos-phate struvite crystals in recycle systems using anaero-bic lagoon effluent is a common and problematic occur-rence. To date, no feasible means of preventing thedevelopment of these crystals has been identified.However, with proper design and management of thesystem, including the lagoon, the crystal problem can becontrolled in an acceptable manner. Acid washing atregular intervals is the most commonly used method ofcontrolling crystals in recycle systems.Page 4 EQ 353 New 04/03/3.5M■ Issued in furtherance of Cooperative Extension Work Acts of May 8 and June 30, 1914, in cooperation with the United States Department ofAgriculture. Ronald J. Turner, Director, Cooperative Extension, University of Missouri and Lincoln University, Columbia, MO 65211. ■ UniversityOutreach and Extension does not discriminate on the basis of race, color, national origin, sex, religion, age, disability or status as a Vietnamera veteran in employment or programs. ■ If you have special needs as addressed by the Americans with Disabilities Act and need this publicationin an alternative format, write ADA Officer, Extension and Agricultural Information, 1-98 Agriculture Building, Columbia, MO 65211, or call(573) 882-7216. Reasonable efforts will be made to accommodate your special needs.OUTREACH & EXTENSIONUNIVERSITY OF MISSOURICOLUMBIAFor further informationBooram, C. V., R. Smith, and T. Hazen. 1975. Crystalline Phosphate Precipitationfrom Anaerobic Animal Waste Treatment Lagoon Liquors. Transactions of theAmerican Society of Agricultural Engineers. American Society of AgriculturalEngineers, St. Joseph, Mich.Published with funds provided to the Missouri Department of NaturalResources and the Environmental Protection Agency, Region VII. To learnmore about water quality and other natural resource issues, contact theMissouri Department of Natural Resources, P.O. Box 176, Jefferson City, MO65102. Toll free 1-800-361-4827.Figure 5. This sewer nozzle is made of stainless steel for long-term durability.FrompressurewasherDirection of nozzle travelRear-facing ports drive nozzle forwardFront-facing ports erode crystalsFigure 6. Schematic drawing of a sewer nozzle used to cleancrystals from a recycle pipe.

Crystallization in lagoon effluent recycle lines (2003)

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