nitrogen losses from manure storages

Agricultural Wastes 4 (I 982) 41-54

NITROGEN LOSSES FROM MANURE STORAGES

R. E. MUCK & T. S. STEENHUtS

USDA, Agricultural Engineerh*g Department, Cornell University, Ithaca, NY 14853, USA

ABSTRACT

A simple computer model to predict nitrogen (N) losses from anaerobic manure storages was developed and validated with both laboratory and literature data. Using the model, we investigated the effects of temperature, pH, and loading rate on N losses from bottom-loaded and top-loaded manure storages. Bottom-loaded storages provided good N conservation (less than 15 % loss) under all conditions studied, whereas the top-loaded storages had a wide variation in N losses (3-60 % ammonia loss). At low temperature, low pH, and/or high loading rate, the top-loaded storages conserved N almost as well as the bottom-loaded storages. In anaerobic storages, diffusion limits ammonia movement in the storage and possibly explains why bottom-loaded storages provide excellent N conservation.

INTRODUCTION

Commercial nitrogen (N) fertiliser is a major energy input to the production of crops in the US (Pimentel, 1975). However, as the supplies of natural gas and oil decrease, pressure to efficiently use manures instead of commercial fertiliser will increase. Efficient manure usage involves applying manure to the crops at times of opt imum crop use. Thus, manure storage is required.

A principal method of storing manure is to push or pump relatively undiluted fresh manure into a storage facility. The storage facility can take several forms such as an above-ground metal tank, a concrete tank, or an earthen storage which is loaded from the bot tom by a ram pump. A cheaper manure storage is possible by loading an earthen storage from the top from a pushof f ramp or by a gutter cleaner. In all of these storages the manure is anaerobic and is either a slurry or a semi-solid.

41 Agricultural Wastes 0141-4607/82/0004-0041/$02.75 © Applied Science Publishers Ltd, England, 1982 Printed in Great Britain

42 R. E. MUCK, T. S. STEENHUIS

While the manure is in storage, significant N losses may occur, reducing the value of the manure to the crops. Nitrogen losses frona anaerobic lagoons and storages have been studied by several investigators (Willrich, 1966; Koelliker & Miner, 1973; Smith et al., 1971 ; Jones et al., 1973; Booram et al., 1975; Safley, 1980); however, the wide range of reported results do not provide an accurate basis for comparing one storage design with another. A bottom-loaded manure storage, because of its crust, is generally believed to conserve N better than a top-loaded storage, but how much better is a bottom-loaded metal tank over a bottom-loaded earthen storage? Answers to such questions are needed to provide the farmer with economical and efficient designs.

Thus, our objectives were (1) to develop a model for simulating N losses from anaerobic manure storage and to validate this model with laboratory and literature data, and (2) to identify the design parameters and management strategies that will minimise N losses from manure storage.

MODEL

The model assumed that the predominant mechanism for N loss from an anaerobic manure storage was NH 3 volatilisation from the exposed surface. This assumption is valid since little N loss occurs through seepage from manure storages (Davis et al., 1973) and nitrification-denitrification will not occur under anaerobic conditions.

In this study, N losses from manure storage were predicted through simulation of N H 3 - N movement in a storage facility by a finite difference program with a time step of 1 h. A storage was divided into 1 cm depth elements, and the N H 3 -N concentration in each element was assumed to be uniform throughout the element. This model also had provisions for loading the manure storage at various times, locations, and rates. Additions of manure to the storage resulted in the creation of one or more new depth elements at the desired loading location. For example, the bottom loading of 2 cm of manure would create 2 new elements at the bottom of the storage while pushing up the previously added elements.

Since manure storages are not physically mixed until unloading and since thermal mixing with a solids concentration of 6 to 12 ~o is unlikely, the transport of N H3-N between elements within the storage facility was by diffusion. Diffusion of NH3-N between elements in the model was determined using Fick's Law:

dC J = - D d ~ (1)

where: J = the flux of NH3-N, mg cm-2 h-1; D = the diffusion coefficient, cm 2 h - l ; C = the concentration of NHa-N, mg cm-3; z = the depth, cm.

N H a - N loss from the top element into the atmosphere is dependent on NH 3 concentration, temperature, pH, and wind velocity. The theoretical volatilisation

NITROGEN LOSSES FROM MANURE STORAGES 43

loss of NH 3 from a stirred liquid (i.e., no concentration gradient within the liquid) was described by Hashimoto & Ludington (1971).

C = C O exp ( - Kgaft/H) (2)

where: C = t h e concentration of N H 3 - N at time t, mg era-3; C o = t h e concentration of NH3-N at time 0, mg c m - 3; Ks = the mass transfer coetficient, h - 1; a = the surface to volume ratio, cm 2 cm-3 ; f= the fraction of unionised NH3-N; H = the Henry's Law constant, c m - t ; t = time, h.

Since the NH a concentration in the top element in our model was uniform throughout the element, eqn. (2) was appropriate to describe the loss of NH 3 from that element into the atmosphere. Thus, for our model, the new concentration in the top element after At hours of volatilisation was:

C, + A, = Ct exp ( - KsafAt/H ) (3)

where: C t = concentration of N H 3-N in the top element at time t, mg cm - 3; Ct + at = concentration of NHa-N in the top element at time t + At, mg cm- 3; At = time step, h.

The values of K v f and H can be estimated by either theoretical or empirical relationships; however, the following empirical relationships were used for simplicity. Haslam et al. (1924) found Kg was a function of wind velocity and temperature:

Ks = 4930V0.ST - 1.4 (4)

where: K S = the mass transfer coefficient, h - l ; V = wind velocity, km h - l ; T = temperature, K.

The fraction of unionised NH3-N in poultry manure slurries was reported by Hashimoto (1972) to be a function of pH and temperature.

f = {1 + {[H+]/[0-81 x 10-1°(1-07)(T-293)]}} -1 (5)

where [H + ] = molar hydrogen ion concentration. Finally, the Henry's Law constant is a function of temperature (Hashimoto, 1972).

H = 1.013(1.053) ~29a-T) (6)

where: H = Henry's Law constant, cm- 1. Another mechanism involved in making ammonia available for volatilisation is

biological mineralisation of organic N to NHa-N. The rate of mineralisation after 1 or 2 days of storage, however, may not be very significant. In dairy manure, for example, 40-50% of the N is in the form of urea (Loehr, 1974), but the urea is converted to NH3-N in less than 1 day, except at temperatures at or below freezing (Muck & Steenhuis, 1980). Other forms of organic N are mineralised at much slower rates, as has been indicated by anaerobic digestion studies with dairy manure (Jeweil et al., 1978). Thirty days of mesophilic digestion produced only small changes in the NHa - N and organic N levels. Thus, in a manure storage where biological

44 R . E . MUCK, T. S. STEENHUIS

decomposition of the manure is not optimised, the mineralisation of non-urea organic N should be small. Consequently, in our models, we assumed the ammonia concentration was the sum of ammonia plus urea and ignored biological activity.

In summary, a conceptualisation of our model is shown in Fig. i. The manure storage was divided into 1.0 cm depth elements. Ammonia volatilisation occurred from the top element and was predicted using eqns. (3) to (6). Using a literature value (American Institute of Physics, 1957) for the diffusion of ammonia in water of 0"065 c m 2 h-1 at 25°C, diffusion between elements was predicted using eqn. (1). Ammonia volatilisation and diffusion were predicted for all elements simul- taneously at 1 h increments. A copy of the computer program may be obtained from the authors.

/ / / \ \ \ SIDE OF / NH 3 VOLATILISATION \

STORAGE" / I 4 4 , \

Fig. 1.

x / / / / / / / / / / "~- I -cm DEPTH ELEMENTS

Conccptualisation of the manure storage for the model.

Analytical check Since the selection of 1.0 cm depth elements and 1 h time steps for the model is

arbitrary and based on convenience, their effect on the model's results had to be determined. This was done by running the model for a case that could be solved analytically. Such a case is diagrammatically shown in Fig. 2; it is a semi-infinite column with vertical sides, and the initial NH a concentration is uniform throughout the column. The concentration with depth for any time, as adapted from Crank (1975), is:

Coo - 1 - 2(D~),/~- +e×PLD H + ~ j

x erfc 1/2 + Dt) 1/2 (7)

where: x = the distance from the surface, cm; D = the diffusion coefficient, c m 2 h - ~ ; erfc ( ) = the complementary error function.

The model was run for a time period of 13 days using typical values for all parameters. Figure 3 indicates that the model results closely matched the values for the analytical solution.

NH 3 VOLATILISATION

f //=/[, (// WALL /

/ / /

/

t ,

F . WALL T X

MANURE \ \ \ \ \

Fig. 2. Analytical case: semi-infinite column with an initial concentration of C O and no loading.

IOO

90

z _0 6 0 i,.-

i- 7 0 z w z 6 0 0 u

J 50

I,-

_~ 40

Q

z w ¢lg W

I0

I I I I I I I | I i O IO 20 30 40 50 60 70 80 90 I00

DEPTH FROM S U R F A C E , cm

Fig. 3. Compar ison o f concentration w~lth depth as determined by 2 the model and by the ana ly ica l result when t =13 days, K s = 2 . 5 h - , f = 0 . 0 1 , D=0 .065cm sec- , a n d H = l . 0 c m - .

46 R.E. MUCK, T. S. STEENHUIS

Validat ion Data from two sources were used to validate the model: N losses from manure

storages reported by other investigators and the results we obtained from several laboratory experiments. A schematic diagram of the laboratory set-up is shown in Fig. 4. Air from a blower was scrubbed of NH 3 and humidified by spraying with a 0.2 ~o boric acid solution. The air, flowing at about 1.6 km h-1, passed over a container with manure 12 cm deep, 40 cm long, and 6 cm wide. Ammonia volatilising from the manure was caught by two boric acid scrubbers. Measurements made with NH4C1 solutions instead of manure indicated that the two scrubbers together captured 89 of the HN 3 volatilised.

~ PUMP

FAN /

BORIC ACID

Fig. 4. Laboratory set-up for measuring ammonia loss from dairy manure.

The experiments were conducted over a 4 day period in a controlled temperature room at 20 °C using dairy manure from different treatment systems (raw manure, oxidation-ditch mixed liquor, anaerobic digestor effluent). These systems provided manure with different pH values, NH3-N concentrations, and solids contents. Twice daily during the run, the boric acid in the scrubbers was adjusted for volume and sampled to determine NH3-N content and to provide an NH 3 loss curve with time. The pH of the manure at the surface was measured daily. The initial and final NH3-N, organic N, Total Solids, and Volatile Solids concentrations in the manure with depth in the container were determined using standard procedures (American Public Health Association, 1975).

Using the temperature, air velocity, and pH data from the experimental runs, NH 3 losses were simulated using the computer model. In a similar manner, the N losses from three different manure storages and lagoons reported by others (Willrich, 1966; Converse et al., 1975; Safley, 1980), were predicted using average monthly weather data from weather stations closest to the storage sites. Table 1 describes these storages and the manner in which the nitrogen losses from each were determined.


0

~o ~ Z

~z ~ z ~ ~ . . ~ . z .~ =

. ~ 0 0 ~ " " - ~

~ o o

0

r~

e~ 0

M

~ ~ X 0 o~Q_

0 ~

Z

"~°=

L~

o o ~ . ~

r~

c~

o

~D G~

0

0 " -

~ o


Figure 5 presents the predicted versus observed losses for both the laboratory and literature data. Only one literature value (Willrich, 1966) was predicted poorly. In our modelling of this swine lagoon, we assumed that effluent from the lagoon consisted solely of the whole top layer of liquid in the lagoon. In fact, the outlet was an elbow so that the overflow from the lagoon was removed approximately 15 cm below the lagoon's surface. Thus, the effluent consisted of some liquid which was considerably below the surface. Because of this, our model overpredicted the loss.

I/) 0 J

- r Z

a IllJ t--

a I~J n- O.

2 0

15

I 0

0 0

Y~X

x / E] r~

9: / - I I I I

5 I0 15 2 0

ACTUAL NH 3 LOSS,%

Fig. 5. Predictions of ammonia loss from actual manure storages and from laboratory experiments as compared with actual losses. (17, (,~), (A). Laboratory Results; (Q) Safley, 1980; ( x ) Converse et al.,

1975; (Q) Willrich, 1966.

Since the fraction of effluent which is taken from a particular depth was not known, we could not make a more accurate prediction. Nevertheless, the model predicted the other points well despite the variety of manures used and their solids contents (2.5-13 ~o)- Thus, it appears that the model reasonably predicts nitrogen losses from manure storages of any solids concentration as long as there is no thermal or physical mixing of the storage; however, it must be noted that the model is not well validated for storages which are loaded from the top.


RESULTS

We used the model to s imula te a wide var ie ty o f condi t ions . Unless specified, the values o f the pa rame te r s for each run were: t empera tu re , 10°C; air velocity, 6.2 km h - 1 ; ra te o f manure add i t i on to s torage, 2 cm d a y - x (volumetr ic load ing rate per uni t surface area) ; and p H 7. These values were chosen as typical average values for each pa ramete r . F o r each set o f cond i t ions , the model was run assuming two different load ing locat ions , top and bo t tom. In top- load ing , the fresh manure was a d d e d da i ly in a un i fo rm layer at the t op o f the s torage, s imula t ing a s torage loaded on the surface off a push r a m p or by a gut ter cleaner. Bo t tom- load ing s imula ted a s to rage loaded f rom the b o t t o m by a ram p u m p or s imilar device and assumed a un i fo rm layer was added to the b o t t o m of the s torage, pushing up the previously added manure . F o r each run, N loss at a pa r t i cu la r t ime was ca lcu la ted as a percentage o f the cumula t ive N H 3 - N a d d e d to the s torage f rom the s tar t o f filling to that pa r t i cu la r time.

Temperature effects Figure 6 shows the effects o f t e m p e r a t u r e on N losses f rom top- and b o t t o m -

loaded manure s torages. As expected, the N losses f rom the two types o f s torage

70

60 30 ° C TL

40 Z 20*C TL

z 3o ,/_..-..

" ' i " GC 201 "-.~

~ .~ ~ ~ - - I0 ° C TL

t3_ I0 / ~ - - ~ ~ - ~ - - - . . . . __~ ~ ~__~ ~ " - - ~ ' - ~ ' ~ ' - ~ - - - ~ - ~ . . ~ . _ ~ _ _ ~ : 30 °20°cc BLBL

- - - - - - IO = C BL 0 ° C TL o e c BL

0 i I I I I i | I

0 5 I0 15 20 25 :30 35 40

TIME FROM START OF FILLING, cloys Fig. 6. Effects of temperature on nitrogen losses from top-(TL) and bottom-loaded (BL) manure storages. Air velocity = 16.2 km h - t, fall 7, manure loading rate = 2 cm day- 1. Nitrogen losses expressed

as a percentage of the cumulative NH3-N added to the storage up to a particular time.

50 R. E. MUCK, T. S. STEENHUIS

were quite different. With the top-loaded storage the percentage N volatilised levelled off after 20 days of filling, so that a constant fraction was lost each day. This, however, was not true for the bottom-loaded storage. Although the initial losses from a bottom-loaded storage resembled those from a top-loaded storage, after several days, the amount of N lost each succeeding day decreased. Thus, as manure was added daily to the storage, the percentage of the cumulative N added that was lost decreased with time.

Because of the difference in the placement of flesh manure in the storage, the two storages varied considerably in their response to temperature. With the fresh manure placed on top, high temperatures rapidly caused large N losses from the fresh manure. When the manure was added at the bottom, N from the fresh manure had to move to the surface by diffusion, a process which is much less affected by temperature than volatilisation and which is apparently slow. Thus, the effects of temperature on N losses from the bottom-loaded storage were more muted than those on N losses from the top-loaded storage.

Despite the wide variation in N losses for the top-loaded storage, Fig. 6 does show that a top-loaded storage may be useful under certain circumstances. When temperatures were 10 °C or lower, N losses from the top-loaded storage were about the same as those from the bottom-loaded storage. Thus, the top-loaded storage might be the best choice for a farmer who wants to store manure only during the cold season of the year. Such a storage would provide good N conservation cheaply.

pH effects The effects o f p H on N losses are shown in Fig. 7. These results closely resembled

those for the varying temperatures. The pH markedly affected N losses from a top- loaded storage but had little effect on the N losses from a bottom-loaded one. Like temperature, pH has little effect on diffusion, but a major effect on the NH3-N volatilisation rate. Thus, fresh manure having a high pH and loaded on the surface of a storage can lose much N by volatilisation before it is covered by succeeding days' additions. With bottom-loading, however, since the diffusion rate for NH3-N is slow, little NH3-N from freshly added manure moves to the surface to be lost. Therefore, pH has almost no effect on N losses from a bottom-loaded storage.

Although the effects of pH on N losses resembled those for temperature, their practical implications are different. At pH values below 7, both types of storages performed well. When the pH value was high, there was a potential for high N loss with a top-loaded storage. A bottom-loaded storage is then preferred unless an inexpensive form of pH control is available. Thus, pH becomes a design consideration only when the manure is of a high pH.

Loading rate effects Loading rate is the depth of manure added daily to the storage or, in other words,

the volumetric loading rate per unit surface area. Figure 8 shows the effects of


(n (n 0 ..J

70

60

50

40

/ - - pH 8 TL

z

IwO -r N z 3C \ \

z ~ b.I .,. U 2 0 " ~ LU ~", , _ pH 7 TL

. . . . . . . . . . . . pH 8 BL

pH 7 BL

- - - pH 6 BL,TL

1'5 2"0 ' ' ' ' 0 5 I0 25 30 35 40

T I M E F R O M S T A R T OF F I L L I N G , doys

Fig. 7. Effects of pH on nitrogen losses from top4TL) and bottom-loaded (BL) manure storages. Temperature = l0 °C, air velocity = 16.2 km h - l, manure loading rate = 2 cm d a y - i. Nitrogen losses

expressed as a percentage of the cumulative N H a - N added to the storage up to a particular time.

loading rate on N losses from top- and bottom-loaded storages. Again, top-loaded storages have greater N losses, but apparently for either top- or bottom-loading the loss of N was approximately inversely proportional to loading rate. This is not surprising because when the loading rate is doubled, the average distance that the NH3-N in the manure has to travel to reach the surface of the storage is doubled. In the design of a manure storage for a particular farm, loading rate is determined by the surface area of the proposed storage since the daily volume of manure is considered constant. As the proposed surface area is reduced, the loading rate increases. Thus, there is an advantage in designing a storage with a small surface area (therefore, making it deeper) to reduce N losses.

With bottom-loaded storages, the advantage of smaller surface areas in conserving N, however, is not great. For the environmental conditions used in these simulations, there was only a difference of 4 ~ between the 2- and 4-cm day- l loading rates. Since neither temperature nor pH has a large effect on N losses from a bottom-loaded storage, it is unlikely that normal environmental conditions will produce a large difference in N losses between loading rates. Thus, an above-ground metal storage, which usually has one half the exposed surface area (or i.e., twice the


18

2 cm/d0y TL

15

3 TL cm/doy

0 i - - i i I "~__.~__- ' . --..__~__-- 3 C m/d oy B C I

4 cm/doy BL

0 5 IO 15 2=0 25 30 15 40

TIME FROM START OF FILLING, days

Fig. 8. Effects of loading rate on nitrogen losses from top- (T_L 1 and bottom-loaded (BL) manure storages. Temperature= 10°C, pH 7, air velocity= 16.2kmh- . Nitrogen losses expressed as a

percentage of the cumulative NH3-N added to the storage up to a particular time.

20

16

Z

,9, ,2

o

u- 8 0 h- Z bJ ¢J (x 4 bd n

TOP LOADING

5 cm DEPTH

~ T H 20 cm DEPTH

0 5 tO t5 20 25 30 35 40

TIME FROM INITIAL LOADING, days

Fig. 9. Effects of loading the storage a constant depth beneath the surface on nitrogen losses. Temperature = 10 °C, pH 7, air velocity = 16-2 km h-t, manure loading rate = 2 cm day-t. Nitrogen losses expressed as a percentage of the cumulative NH3-N added to the storage up to a particular time.


depth) of an earthen storage of the same volume, must cost only slightly more than a bottom-loaded earthen storage to be justified solely on the basis of N conservation.

Floa t ing inlet

Finally we tested the effects of loading a manure storage at a constant distance below the surface crust of the stored manure. Figure 9 shows the results of these simulations. When the manure is injected only a small distance (10 cm depth) below the surface, N losses are reduced significantly. Clearly, the diffusion of NH 3 within the manure is slow, limiting NH 3 loss from the surface. This implies that a crust is not necessary on a manure storage for good N conservation. A crust may, indeed, help N conservation, but the crust is not needed to explain the differences in losses between top- and bottom-loading. Without mixing in the storage, NH 3 in the manure does not move quickly to the surface of the storage.

CONCLUSIONS

We developed a simple computer model to predict N losses from anaerobic manure storages that considered only two processes occurring in the storage: NH 3 volatilisation at the surface and diffusion o f N H 3 within the storage. The model used physically based parameters and thus required no calibration. The model was validated using laboratory and literature data and indicated that it reasonably predicted both data sets.

Using the model, we investigated the effects of loading rate, pH, and temperature on the N losses from bottom-loaded and top-loaded manure storages. Bottom- loaded storages provided good N conservation under all conditions studied, suggesting that design, management or ambient conditions have little effect on N losses. Top-loaded storages had a wide variation in N losses; however, under low temperatures, low pH values, and/or high manure loading rates, such storages will provide good N conservation. Finally, the model indicated that diffusion limited the movement of NH 3 in the storage and is perhaps more important than the surface crust in explaining why bottom-loading provided excellent N conservation.

ACKNOWLEDGEMENTS

We wish to express our appreciation for the technical assistance of P. G. Kellner and D. R. Wood.

REFERENCES

AMERICAN INSTITU'rE Or PHYSICS. (1957). American Institute of Physics Handbook (I st ed.). McGraw-Hill Book Company, NY.

AMERICAN PUBLIC HEALTH ASSOCIATION. (1975). Standard methods for the examination of water and wastewater. American Public Health Association, New York.


BOORAM, C. V., HAZEN, T. E. & SMITH, R. J. (1975). Trends and variations in an anaerobic lagoon with recycling. In: Managing livestock wastes. Am. Soc. Ag. Eng., St. Joseph, Michigan, 53%40.

CONVERSE, J. C., CRAMER, C. O., LARSEN, H. J. ~ JOHANNES, R. F. (1975). Storage lagoon versus underfloor tank for dairy cattle manure. Trans. Am. Soc. Ag. Engr., 18, 558-63.

CRANK, J. (1975). The mathematics of diffusion (2rid ed.). Clarendon Press, Oxford, England. DAVIS, S., FAIRBANK, W. ~ WEISHEIT, H. (1973). Dairy waste ponds effectively self-sealing. Trans. Am.

Soc. Ag. Eng., 16, 69-71. HASLAM, R. J., HERSHEY, R. L. & KEEN, R. H. (1924). Effect of gas velocity and temperature on rate of

absorption. Ind. and Eng. Chem., 16, 1224-30. HASHIMOTO, A. G. (1972). Ammonia desorption from concentrated chicken manure slurries. Ph.D.

Thesis, Cornell University, Ithaca, NY. HASHIMOTO,A. G. & LUDINGTON, D. C. (1971). Ammonia desorption from concentrated chicken manure

slurries. In: Livestock waste management and pollution abatement. Am. Soc. Ag. Eng., St. Joseph, Michigan, 117-21.

JEWELL, W. J., CAPENER, H. R., DELL'ORTO, S., FANFONI, K. J., HAYES, T. D., LEUSCHNER, A. P., MILLER, T. L., SHERMAN, D. F., VAN SOEST, P. J., WOLIN, M. J. & WUJC1K, W. J. (1978). Anaerobic fermentation of agricultural residue: potential for improvement and implementation Report EY-76- S-02-2981-7, US Dept. of Energy, Washington, D.C.

JONES, R. E., NYE, J. C. & DALE, A. C. (1973). Forms of nitrogen in animal waste. Paper No. 73-439 presented at Annual Meeting of Am. Soc. Ag. Eng., June 17-20, Lexington, KY.

KOELLIKER, J. K. & MINER, J. R. (1973). Desorption of ammonia from anaerobic lagoons. Trans. Am. Soc. Ag. Eng., 16, 148-51.

LOEHR, R. C. (1974). Agricultural waste management: problems, processes, and approaches. Academic Press, NY.

MUCK, R. E. & STEENHUIS, T. S. (1980). Nitrogen losses in free-stall dairy barns. Presented at Fourth Int. Symposium on Livestock Wastes, April 15-17, Amarillo, Texas.

OVERCASH, M. R., HUMENIK, F. J. Hr. HOWELL, E. S. (1977). Lagoon pretreatment: swine waste loading rate and response to loading rate change--laboratory scale. Trans. Am. Soc. Ag. Eng., 20, 348-52.

P1MENTEL, D. (1975). World food, energy, man and environment. In: Energy, agriculture and waste management. (Jewell, W. J. (Ed.)), Ann Arbor Science, Ann Arbor, Michigan.

SAFLEY, JR., L. M. (1980). Analysis of an above-ground storage tank for handling as-produced dairy manure. Presented at 4th Int. Sym. on Livestock Wastes, April 15-17, Amarillo, Texas.

SMITH, R. J., HAZEN, T. E. & MINER, J. R. (1971). Manure management on a 700-head swine finishing building; two approaches using renovated waste water. In: Livestock waste management and pollution abatement. Am. Soc. Ag. Eng., St. Joseph, Michigan, 149-53.

VANDERHOLM, D. H. (1975). Nutrient losses from livestock waste during storage, treatment and handling. In: Managing livestock wastes. Am. Soc. Ag. Eng., St. Joseph, Michigan, 282-5.

WILLRICH, T. L. (1966). Primary treatment of swine wastes by lagooning. In: Farm animal wastes. Am. Soc. Ag. Eng., St. Joseph, Michigan, 70-4.

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