Biological Wastes 30 (1989) 133-147

Struvite Deposits in Pipes and Aerators

Mohaj i t ,* K. K. Bhat ta ra i , E. Paul Ta igan ides & B. C. Y a p

Pig and Poultry Research and Training Institute, Sembawang Road, 2776 Singapore

(Received 22 July 1987; revised version accepted 7 May 1989)

ABSTRACT

Struvite created maintenance and operational problems in the Ponggol Pigwaste Plant ( PPP) in Singapore a few months after the commissioning of the plant. Struvite growth was studied in the laboratory and in pipes and aerators at the PPP. Struvite solubility and removal with addition of chemicals and physical cleaning have been studied. Sulfuric acid removes struvite cheaper and faster than acetic acid. The struvite crystals are MgNH4PO 4 . 6H20. A general formula for the prediction of the growth rate and control of deposition of struvite crystals in the shaft section of floating aerators in the aerated anaerobic lagoon (AAL) in the PPP was developed .from experimental data.

I N T R O D U C T I O N

The Ponggol Pigwaste Plant (PPP) is used for the treatment of wastes from the Industrial Farm (Pte) Ltd pig farm in Singapore which has a standing pig population (SPP) of 35 000 pigs. The average capacity of the plant is 500 m 3 day - t. The major unit operations of the plant are: a raw waste sump, primary and secondary sedimentation tanks, extended aeration oxidation ditches, 1500 m 3 anaerobic sludge digester, an aerated anaerobic lagoon (AAL), sand filter beds, treated water recycle pump, chemical dosing equipment, decanting centrifuge, and pump stations at several points (Taiganides, 1986). The sludge produced can be dried in the sand filter beds or dewatered using the

* Present address: Mombertstr. 10, 7500 Karlsruhe, FRG.

133 Biological Wastes 0269-7483/89/$03"50 ,f) 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

134 Mohajit, K. K. Bhattarai, E. Paul Taiganides, B. C. Yap

centrifuge decanter. The recycled water is stored in a pond and is used to flush the waste, using automatic flushing-siphons. The two areas of severe struvite deposition are in the digester overflow pipe and in the floating aerators in the aerated anaerobic lagoon.

The problem of struvite (magnesium ammonium phosphate crystals) in the PPP was first detected when the digester overflow pipe choked. PPP had been commissioned for full operation three months earlier. Besides the struvite in the digester overflow pipe, struvite crystals were also found in the floating aerators, raw waste-pump pipelines and in the outfall pipe. It was noticed that the struvite deposits occurred in places subject to a higher level of turbulent flow, e.g. on elbows, valves, flange connections, shafts and propellers.

According to Borgerding (1972) struvite crystal is composed (on a weight basis) of 9.8% magnesium, 7.3% ammonium, 38.8% phosphate, 44-1% water and organic compounds. Furthermore, concentrations of magnesium, ammonium and phosphate required for struvite to precipitate at Ksp = 3-9 x 10- 1o are 17-5, 13.1 and 69.5 mg/liter, respectively. He hypothesized that the factors which may be responsible for struvite formations are, first, relative surface to volume ratio. Crystal growth may also be enhanced when the sludge is not being withdrawn and the fluid in the pipeline is in a quiescent state. Secondly, interior surface roughness aids crystalline growth, and the increase in energy caused by vibration or turbulent flow and the raising of pH when CO2 is released as a result of pressure decrease also contribute. Struvite has a low solubility in water, is highly soluble in dilute acidic solutions and highly insoluble in alkaline solutions, so the most favorable environment for struvite precipitation occurs at pH 10.7 (Caldwell-Connell Engineers, 1985). According to Westerman et al. (1985), PVC and PE plastic pipes and PVC fittings reduce the struvite buildup compared to metal pipes, but struvite can occur in all systems.

This paper reports on studies carried out to measure and predict struvite growth and to test different methods for its removal from pipes and aerators.

METHODS

Field studies

Samples of raw waste, primary sludge, digester effluent supernatant, aerated anaerobic lagoon (AAL) liquor and crystal deposits in pipes and aerators, were analysed for pH, Mg 2+, NH£ and PO 3- and other parameters while the PPP was in full operation. Laboratory analyses were carried out at the Waste Analysis Laboratory of the Pig and Poultry Research and Training Institute (PPRTI).

Struvite deposits in pipes and aerators 135

To measure the growth of struvite in aerators, the number of running hours and days were recorded. The aerators were manually cleaned by scraping out the deposits. Times needed for the removal of the aerators from their mooring in the AAL, cleaning and replacement were recorded. Struvite crystals from different parts of the aerator were collected and washed, dried in the sun, and the thickness and density determined. Their composition was tested in the PPRTI laboratory.

Two experiments were run to measure the growth of struvite in pipes. In the first experiment a new pipe and bend were installed at the digester overflow. One meter of vertical PVC pipe was connected to a 90 ° PVC elbow which was connected to a meter-length horizontal acrylic pipe; the PVC bend and pipes were of 150 mm diameter while the acrylic pipe was of 142 mm diameter. The primary-sludge pumps were run to feed the digester and then stopped after several days. The flow rate of the digester effluent was measured. The pipes and bend were then removed, the crystals were manually removed, and the thickness and density measured.

In the second experiment, new PVC pipes (25.4mm internal diameter) were connected to the digester overflow and flow rates in the pipes were measured. The pipes were removed after 24days and crystal thickness measured. After the pipes were put back in their original positions they were washed with acid solutions and the time required for completely removing the struvite was recorded.

Laboratory experiments

Laboratory experiments to determine struvite solubility in sulphuric and acetic acids were run. A large piece of struvite crystal (about 3-5 g) was weighed and placed in the acid solution, which was occasionally stirred. After some time the solution was filtered through a filter paper and the struvite was removed, dried at 100°C for 20-30 min and the crystal weighed to determine its solubility. The crystals were dried for only 20-30 min as it was found that after 30 min of drying the crystals decomposed.

Laboratory studies on struvite formation in wastewater at different pH levels were also carried out. The waste sample from the digester outlet was collected and taken to the laboratory where three samples in replicate were prepared in beakers. The first sample consisted of 600 ml of wastewater. To 300 ml of wastewater 300 ml of distilled water was added (sample no. 2) and 30 ml of 1N sulfuric acid was added to 600 ml of wastewater sample (sample no. 3). The samples were first stirred by a magnetic stirrer at maximum speed for 25 min. The stirring was stopped and 20-30 g of struvite crystals were added to all the samples to act as catalysts. The wastewater samples were then analyzed for magnesium after 24 h.

2 4

~_

5 i 8 ~6

Immm2m4

6 B

lO

12

14

16

18,

20

22

24

26 ~

~K)kmm3"21

. m

mm

mm

/m/m

mm

mm

m/m

mm

mm

mm

mm

T

s=32

m ~

2 3

4.

5 6

'7

B 9

I0

II

12

13

14

15

16

17

m |

mm

m

m m

m m

m m

m |

m mlm|~,:3~.

1 2

3 ~,

5

6 7

8,

9 10

11

12

13

lk

m

m

m

mm

m

m

m

m

m

m

,~

m

m

m

1 2

3 t,

5 6

7 7.

75

I I

I I

I ITs

=31

I 2

3 4

4.4

1 2

3 4

I I

T,:Z~

ml

Ts=4

2

Ts :

Tot

al g

row

th ti

me

Run

ning

T

in

r~

[

Uni

t S

topp

ed T

ime

,T

I D

ECEM

BER

198

,6

JAN

UA

RY

198

,?

I F

EB

RU

AR

Y 1

98,?

Fig.

1.

Exp

erim

enta

l de

sign

for

six

flo

atin

g ae

rato

rs o

pera

ting

on a

sto

p-an

d-ru

n sc

hem

e. A

erat

or 1

was

sto

pped

1 w

hole

day

bef

ore

bein

g re

star

ted

(T=

l).

It w

as s

topp

ed f

or 3

2 di

ffer

ent d

ays

du

rin

g th

e ex

peri

men

tal p

erio

d. A

erat

or 6

was

sto

pped

for

6 w

hole

day

s be

fore

it w

as r

esta

rted

(T

= 6

). It

was

sto

pped

on

four

occ

asio

ns f

or 6

day

s pe

r st

op g

ivin

g a

'sto

pp

ed' t

ime

of

24 d

ays

whi

ch is

T s.

The

sam

e ap

plie

s to

the

oth

er a

erat

ors,

e.

g. A

erat

or 3

was

sto

pped

for

3 d

ays

on

eac

h oc

casi

on i

t w

as n

ot

run

nin

g. I

t w

as s

topp

ed 1

4 ti

mes

for

a T

s o

f 42

day

s =

(14

) (3)

. The

du

rati

on

of

run

nin

g is

the

sam

e as

tha

t o

f th

e 's

top

ped

' ti

me

for

each

of

the

six

aera

tors

.

Struvite deposits in pipes and aerators 137

Laboratory experiments to remove magnesium as Mg(OH) 2 from the wastewater were carried out to determine the feasibility of using lime to inhibit struvite formation. The tests were run as follows. Raw waste samples were collected and four replicate samples were prepared in beakers. To the four samples of 200 ml of wastewater, 1, 1.5, 2 and 2-5 g of lime were added. The samples were stirred by a magnetic stirrer at medium speed for 30 s. The stirring was stopped and the flocs formed were then settled for 30min. Suspended-solids removal was then determined.

Struvite formation in aerators

To measure the growth of crystals of struvite in aerators, six aerators were taken out, cleaned and then replaced in the AAL. The six aerators were numbered 1-6 with Aerator Number 1 being stopped for one day (full 24-h period) before running it again. Similarly, Number 2 aerator was stopped for 48h before starting. The Number 6 aerator remained at a standstill for 144 h before being started again. At the end of the experiment, each aerator was taken out of the AAL. A series of experiments was set up for a period of three months, as explained in Fig. 1. The thickness of struvite crystals at the aerator shaft was measured and samples were taken for laboratory analyses. The shaft is the most critical part because buildup of struvite in this section means more energy is required to force through the same amount of flow in a smaller space. In other words, there is higher load on the motor. Burned electrical motors are a common problem with floating aerators which are subject to severe struvite formations.

RESULTS AND DISCUSSION

Struvite composition

Analyses of wastewater samples showed that the average concentrations of magnesium, ammonium and phosphate were above the concentration product reported by Borgerding (1972) to be sufficient for struvite formation. So it is not surprising that struvite crystals grew in the raw waste pipe and digester overflow pipe, as well as in the floating aerator. Results of the sample analyses are presented in Table 1.

Quantitative analyses of the crystals showed that the crystal was not pure struvite. Although all elements were not present in stoichiometric proportions, it was assumed that the crystalline material was struvite (magnesium ammonium phosphate). The results of the analyses are shown in Table 2.

oo

TA

BL

E 1

A

ver

age

Co

nce

ntr

atio

n a

of

Mg

2 +

, N

H~

an

d P

O 3

in

Po

ng

go

l P

igw

aste

Pla

nt

(PP

P)

for

the

1986

an

d

1987

E

xp

erim

ents

Sam

ple

pH

T

empe

ra-

Mg

2 +

tu

re (

°C)

Aug

. 86

F

eb.

87

Aug

. 86

A

ug.

86

Feb

. 87

A

ug.

86

Feb

. 87

Aug

. 86

F

eb.

87

Raw

was

te

6-75

6"

74

28-8

17

4 17

9 95

6 91

9 1

532

1 57

6 P

rim

ary

slu

dg

e 7.

06

--

28.0

90

--

1

697

--

674

--

Dig

este

r ef

flue

nt

dig

este

r o

utl

et

7.12

--

32

.0

164

--

1 05

4 --

l

103

--

dig

este

r o

utf

all

to l

ago

on

7.

22

7-16

32

.0

122

122

987

1 08

0 91

9 1

040

Aer

ated

an

aero

bic

lag

oo

n

7.51

7.

36

29-5

37

51

1

028

611

306

195

Co

nce

ntr

atio

n u

nit

s ar

e in

mg

lit

er

~.

--,

no

t d

eter

min

ed.

NH

; PO

?,-

Struvite deposits in pipes and aerators

TABLE 2 Composition of Crystalline Deposits at the PPP

139

Constituent Actual composition Theoretical composition of (wt %) struvite (wt %)

Mg 2÷ 10"25 9"8 NH~ 5'01 7'3 PO 3- 38"99 38"8 H20 44"66 44"1 (H20 and organics)

Growth and removal of struvite in aerators

All the necessary activities, tools and times required for the cleaning of struvite from the aerators are given in Table 3. It was determined that the time and manpower required to 'tow an aerator to the jetty, lift it out by crane, dismantle, scrape off the crystal deposits, clean, put back together and tow it back to its place in the lagoon (AAL) was 8.25-10.15 man-hours. It takes three people to coordinate the whole operation. These three co- ordinators are included in the man-hours calculation. After removing the struvite from the various parts of the aerator (Fig. 2), the aerators were placed back in the AAL. Table 4 shows the amount of struvite accumulated in different aerator parts.

Growth and removal of struvite in pipes and bends

Struvite deposits in pipes and bends for experiments run in 1986 are shown in Table 5. The data indicate that struvite deposits can occur even within 3 weeks as compared to 6 weeks that was reported by Westerman et al. (1985). It is to be noted that during the 24 days of the experiment there were no struvite deposits in the acrylic pipe, whilst the PVC pipe accumulated 10-6 mm of struvite during the same period. Struvite density is around 1"5 kg liter- 1. Struvite deposits in the pipe bends were greater than in other pipe portions.

For the experimental studies ofstruvite growth in pipes and bends in 1987, three pipes were installed in parallel to receive flow from the digested- supernatant overflow. The same flow rate was maintained in each of the three pipes. The flow rate varied from 0.15~)'501iters s-1. When the flow rate was less than 0-15 liters s-1, the pipes choked frequently due to the sludge. For flows above 0.50liters s -1, it was assumed that the struvite formed would be removed due to the high fluid velocity. The average crystal thickness was 2.58 mm and 2"80 mm for 24 days operation in two pipes (1, 3)

140 Mohajit, K. K. Bhattarai, E. Paul Taiganides, B. C. Yap

TABLE 3 Procedures and Time Required to Remove Struvite Deposits from Floating Aerators

]tem Aerator

Number 8 Number 6

Date of cleaning 7 Aug. Manpower needed (persons) 3 Time taken (min): - - t o prepare equipment for the whole process 5 - - t o release wire and cable of the aerator from the boat 11 - - to row and bring aerator to the bank 5 - - to fasten aerator on to the crane 7 - - t o bring aerator to the cleaning platform 8 - - t o remove motor and place on the platform 12 - - to clean aerator (one person) 120 - - to fasten motor back in place 23 - - to fasten aerator to the crane and tow to the lagoon 17 - - to put aerator back in its place in thellagoon 15 - - t o connect wire and cable back to the aerator 55 - - to row back to the bank 5

Total time (h): --before and after cleaning 8.15 - - for cleaning 2.00 Total time: man-hours aerator i 10.15

Equipment needed for cleaning only: ---chisel - -hammer --scraper --brush

1986 12 Aug. 1986 3

5 9 5 4 7

15 180

10

27 5

13 5

5.25 3.00 8.25

The same equipment was used for cleaning both aerators

and 3.48 mm for 20 days operation in the other pipe (2). Pipe Number 2 choked after 20 days of operation but was removed for weighing, measuring and cleaning on the same day as the other pipes. The struvite formed in the pipes was flushed by sulphuric and acetic acid solutions in pipes Numbers 2 and 3, respectively. Flushing pipe Number 2 twice with normal sulfuric acid removed 3.48 mm in 4 h, while flushing pipe Number 3 three times with acetic acid removed 2.8 mm in 6 h. Sulfuric acid was more effective than acetic acid in completely removing struvite. This verified the results of laboratory experiments.

Struvite solubility in acids

The results of the struvite solubility laboratory tests at 29°C with respect to detention time in acids are shown in Fig. 3. In 120min detention time,

Struvite deposits in pipes and aerators 141

TABLE 4 Struvite Format ion in Aerators in the PPP

Date Aerator of number

cleaning

Position Weight Thickness Struvite Number Number of (ram) density of hours t~f days

struvite kg liter-1 aerator aerator (g) max. mean was run was in

lagoon

8 Aug. 86 8

19 Aug. 86 1

inlet 255.0 9.10 7.39 1.71 outside 1 149.07 10.95 8-74 1.65 propeller a 12.92 6"35 5'88 1.76 shaft s 38.34 1-85 1.05 1.55 outlet None

inlet 3096.87 13"10 10.00 1.59 outside 1 753.35 11'10 8-38 1.70 propeller 890-42 14"05 12.00 1-60 shaft 1 617.42 17'60 14.44 1"58 outlet 3434.84 12.15 11'25 1.49

95.30 220

1 793.65 367

a Fibreglass-coated.

4

5 ~

Fig. 2. High-speed floating aerator used in the aerated anaerobic lagoon (AAL) showing sections which were studied. 1, Motor: 2, outlet: 3, shaft: 4, propeller: 5 and 6, outside: 7, inlet.

l,J

,--4

o-

D

~q

0

i00 90

80

70.

60.

50.

40.

30-

20"

10"

1 N sulfuric acid, pH = 0.91

//

0.

75

N sulfuric acid, pH = 1.93

/

/

....

0.50 N sulfuric acid, pH = 1.16

~-~-~

1.0

N acetic acid,

pH = 5.84

~i

ii

i

N sulfuric acid' pH = 1"29

N sulfuric acid, pH = 1.47

1"5

3"o

Fig.

3. 4"

5 6o

7"5

9'o

1o5

i~o

TIME (minutes)

Solu

bili

ty o

f st

ruvi

te i

n su

lfur

ic a

nd a

ceti

c ac

ids.

Struvite deposits in pipes and aerators

TABLE 5 Struvite Formation in Pipes for Digester Effluent in the PPP (1986)

143

Date Position Weight Thickness Struvite of of (ram) density

cleaning struvite (kg l - 1) (g) max. mean

Running time

(days)

18 Feb. 87 90 ° Elbow 1 633.54 69.43 28.38 1-43 11 Aug. 86 90 ° Elbow 1 820.27 19.85 12-52 1.61 2 Sep. 86 90 ° Elbow 999"71 13-40 8"81 1-56

11 Aug. 86 Horizontal 2066"19 2 0 - 0 6 12"55 1.40 2 Sep. 86 VerticaP 1402"28 13"25 10'65 1"51 2 Sep. 86 Horizontal b 2.52 Negligible - -

169 174 24

174 24 24

a Height of digester effluent vertical pipe, 7-6 m. b Pipe material transparent acrylic; the material for the other pipes was PVC.

Date Average flow rate of digester effluent (liter s-1)

12 Aug. 86 2.294 13-23 Aug. 86 5.754 24 Aug. 86 2.294

solubility ofstruvite in acetic and sulfuric acids reached respective saturated values. Struvite solubility in sulfuric acid was greater than in acetic acid for the same normality (IN). Based on market prices for acids as of January 1987, it was eight times cheaper to use sulfuric acid (US$0.17 kg- 1 struvite removed) than acetic acid (US$1.41 kg -~ struvite removed) for struvite flushing.

Struvite crystallization

Data on the formation of struvite crystals in the diluted wastewater and in the presence of acids obtained from laboratory experiments are given in Table 6. In Sample 1, magnesium concentration in the solution decreased from 158 ppm to 68 ppm, which shows that struvite formation occurred. There was no evidence of struvite formation in Samples 2 and 3.

Removal of magnesium as Mg(OH)2

As the pH is increased above 10 to 11, Mg 2+ will start precipitating out as Mg(OH)2, thus lowering the available Mg 2+ concentration (Caldwell- Connell Engineers, 1985). If Mg 2+ could be removed using lime it could

144 Mohafit, K. K, Bhattarai, E. Paul Taiganides, B. C. Yap

TABLE 6 Laboratory Test on Struvite Formation at Room Temperature

Sample Total Liquid added number* volume to wastewater b

(mr)

pH Time of Weight of Magnesium concentration stirring struvite (mg liter- l) (min) added (g)

(initial) (final) c

1 600 None 8.02 25 22.998 158 68 2 600 300ml distilled water 7.88 25 30-260 79 78 3 630 30ml 1N sulfuric acid 7.03 25 22-208 150 182

Experiment was done using a magnetic stirrer at maximum speed, and small pieces of struvite were added as an auto-catalyst. ° See text. b Wastewater sample was collected at the digester overflow outlet. See Methods. c After 24h.

inhibit struvite crystallization. Thus, laboratory experiments were per- formed to precipitate Mg 2÷ from raw waste using different lime dosages.

The results of the experiments are presented in Table 7. Data show that a lime dosage of 5 g liter- 1 removes 98"5% of the suspended solids. If used daily, it would be uneconomical to use lime to precipitate magnesium so as to inhibit struvite growth (see Table 7).

Formula for Struvite growth rate

The average struvite thickness and growth rates in the shaft section of the aerator are tabulated in Table 8. The struvite thickness for each aerator was measured at the end of each experiment with a vernier calliper at 18-22

TABLE 7 Removal of Magnesium Using Lime in Laboratory Tests

Item Sample number

1 2 3 4 5

Volume (ml) 200 200 200 200 200 Lime added (g) 0 1 1-5 2 2-5 Dose (g/liter) 0 5 7.5 10 12.5 pH of solution 7.83 8'8 10'30 11'71 11'76 Suspended solid removal (%) 0 98.5 98.5 96.3 90-4 Cost to precipitate Mg per m 3 of

waste (US$ per m 3) - - 0.33 0.50 0.66 0"83

Struvite deposits in pipes and aerators 145

TABLE $ Struvite Growth Rate and Thickness in Aerator Shaft

Unit Growth time, Total thickness Growth rate, Thickness stopped-time, T s ± SD G T per unit T

T (days) L s (ram d- ~) x 10- 3 L (days) (2) (ram) (4) = (3)/(2) (ram) x 10- 3

(1) (3) (5) =(4) x (1)

1 32 3'02 ___ 0"40 94.4 94.4 (19)

2 34 4-14 + 0'52 121"8 243-6 (22)

3 42 11"52 + 1"07 274"3 822"9 (21)

4 31 4-24 + 0'99 136-8 547"2 (20)

5 22 3"08 + 0"79 140"0 700"0 (18)

6 24 3-40 + 0-6 1417 8502 (20)

Figures in brackets represent the number of measurements taken in order to calculate the mean shown. The + numbers are the standard deviation (SD).

different points on the shaft in order to arrive at the average thickness indicated in column (3) of Table 8.

The general formula for struvite growth rate can be expressed as:

/ Gr = G s - - (1)

T

where

G r = growth rate at unit stopped-time T (mm day-1)

Gs = maximum growth rate (mm d a y - 1)

l = constant (mm)

T = unit stopped-time (days)

From Fig. 4 the values of Gs and l are 0-151 mm day -1 and 0.058 mm, respectively (the line of best fit has been drawn using linear regression; the coefficient of correlation r = 0"999...). Substituting the values of Gs and 1 into eqn (1) then,

0.058 Gr = 0.151 - ~ - -

1 4 6 Mohajit, K. K. Bhattarai, E. Paul Taiganides, B. C. Yap

1000

800

T o x 600

E

z. oo

t~

200

A T

~L

r = 0 . 9 9 9 .. . .

Tz

I 2 3 ~ 5 6 7 8 9 10

UNIT STOPPED TIME, d (T)

Fig. 4. Struvite thickness in the aerator shaft at different unit stopped-times showing the two constants; the minimum stopped-time duration (Tz) before significant deposition of struvite occurs and the maximum thickness (l) which is expected to be scraped offdue to the

start of liquid movement when the propeller is put in motion.

ACKNOWLEDGMENTS

The work reported here was carried out as part of the two-month Research and Training Programme in Waste Resource Recovery Biotechnologies and Pollution Control, sponsored by the International Development Research Centre (IDRC) of Canada in collaboration with the Primary Production Department (PPD) at the Pig and Poultry Research and Training Institute (PPRTI), Singapore. The facilities used were those developed under projects partially funded by the United Nations Development Programme (UNDP/FAO-SIN/74/006) and the Australian Development Assistance Bureau (ADAB) (GCP/SIN-001/AUL), IDRC, and Industrial Farm (Pte) Ltd.

REFERENCES

Borgerding, J. (1972). Phosphate deposits in digestion system. J. Water Poll. Cont. Fed., 44, 813--19.

Struvite deposits in pipes and aerators 147

Caldwell-Connell Engineers (1985). Disposal of Digested Sludge from Glenelg STW. Engineering and Water Supply Department South Australia Report 4-1-4-5.

Taiganides, E. P. (1986). Animal farming effluent problems--an integrated approach: Resource recovery in large scale pig farming. Water Sci. Tech., 18, 47-55.

Westerman, P. W., Safley, L. M. Jr & Barker, J. C. (1985). Crystalline buildup in swine and poultry recycle flush systems. In Proceedings of the 5th International Symposium of Agricultural Wastes. American Society of Agricultural Engineers, St Joseph, MI, 16-17 December 1985, pp. 613-23.