Biological Wastes 26 (1988) 247-259

Compost ing of Agricultural Wastes for their Use as Container Media: Simulation of the Compost ing Process

Y. Inbar, a Y. Chen, a Y. Hadar b

aThe Seagram Center for Soil and Water Sciences, bDepartment of Plant Pathology and Microbiology, Faculty of Agriculture,

The Hebrew University of Jerusalem, PO Box 12, Rehovot 76100, Israel

&

O. Verdonck

Laboratory of Soil Physics, Faculty of Agricultural Sciences, State University of Gent, Belgium

A B S T R A C T

Separated cattle manure ( S M ) and grape marc ( G M ), and mature composts o f these raw materials, were incubated in a composting simulator for 10 days. The effects o f moisture content and addition o f nitrogen on the rate o f composting were measured in terms q f oxygen consumption. Moisture content had a major effect on 02 consumption. The higher the water content, the higher was Oz consumption, which reached a maximum at 60 70% water content. Below 50% moisture, the microbial activity seemed to decline sharply. The 0 z consumption was extremely low for the mature composts and was unaffected by the moisture content. No effect was recorded when nitrogen

B

was added to SM. However, the addition o f 0"25% (w/w) nitrogen to G M at all moisture levels increased significantly the 0 2 consumption, except for the mature compost. The most prominent effect o f nitrogen addition was exhibited by G M that was composted at 60% moisture.

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

The production of healthy, uniform plants is a basic requirement of modern greenhouse agriculture. Container media must be homogeneous, aerated, reproducible and pathogen-free. In order to meet the need for a substrate

247 Biological Wastes 0269-7483/88/$03.50 © 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain

248 Y. lnbar, Y. Chen, Y. Hadar, O. Verdonck

with proper air and water capacity growers use many types of organic and inorganic materials.

Increasing demand and rising costs for peat used as a substrate in horticulture have led to a search for high quality, low cost substitutes, i.e. composts derived from organic wastes such as bark, leaf mould, town refuse, sewage sludge, sawdust, spent mushroom compost and treated animal excreta. These organic materials have been introduced as peat substitutes in container media after proper composting (Cull, 1981; Bik, 1983; Lohr et al., 1984; Verdonck, 1984; Inbar et al., 1985; Raviv et al., 1986).

Both cattle manure and grape marc are readily available agricultural wastes in many countries worldwide. Therefore, these two materials were selected and studied for their chemical and physical properties and their possible use as peat substitutes in horticulture (Inbar et al., 1986; Chen & Hadar, 1987).

The major parameters determining the rate of decomposition of organic matter during the composting process are: aeration; water content; temperature; and C/N ratio. The effects of these parameters can be studied on pilot-plant, full-scale composting systems or in simulation systems. The latter has advantages of flexibility, short time requirements and low operational costs.

A variety of bench-scale composters for the control and evaluation of composting have been described. Two basic designs were employed: rotating drums generally of sufficient volume to allow the development of thermophilic conditions (Jeris & Regan, 1968; Galler & Davey, 1971; Bagstam et al., 1974); and smaller-scale static systems where temperature levels are imposed from an external source (Cappaert et al., 1976a, b, c; Clark et aL, 1977; Deschamps et al., 1979; Ashbolt & Line, 1982; Hong et al., 1983). Both designs incorporate forced aeration and water replacement or retrieval (using cooled condensers).

Two aspects related to the composting and use of grape marc (GM) and separated manure (SM) were investigated in this study. These were the optimization of the composting process in a simulation system with respect to temperature, water content and nitrogen amendment, described here, and suppression of soil-borne plant pathogens, described in the following paper.

METHODS

The following materials were employed in the study on the composting process (t = composting time, days):

--Separated cattle manure--freshly separated (SM, t = 0 days).

Composting of manure and grape marc 249

--Composted separated cattle manure (CSM, t = 150days; aerobically composted in windrows).

- -Grape marc--fresh (GM, t = 0 days). - -Composted grape marc (CGM, t = 360 days; aerobically composted in

windrows).

These materials were analyzed for their organic matter, total nitrogen and solution pH, electrical conductivity (EC) and NO3 concentration following methods described by Chen et al. (1988).

Composting simulator

A composting simulator similar to the one described by Cappaert et al.

(1976a) was used. The simulator was operated in the laboratories of Soil Physics, State University of Ghent, Belgium. The effect of moisture content and addition of nitrogen on the rate ofcomposting was measured in terms of oxygen consumption, which was calculated from concentrations of oxygen in the gas released from the reactor.

The respiratory apparatus consisted of an air-tight reactor of 4 liters capacity. After filling with the organic material, air was blown through the reactor. Oxygen and CO2 concentrations were measured in the air leaving the reactor. The simulator consisted of twelve reactors and the measure- ments were performed with a fully automated gas chromatograph connected to the reactors.

Each reactor consisted of two spherical, concentric flasks of 4 (internal) and 6 (external) liters, respectively. The bottom part of the internal flask consisted of a porous glass plate through which the air supply to the reactor was provided. The cover of the reactor was fitted with an opening for a thermocouple and a condenser. The space between the internal and external flasks served as a passage way for temperature-controlled water during the composting experiment. The system was designed to simulate a temperature increase in a compost pile during the initial stages of heating. Therefore, the temperature was increased linearly at a rate of 5°C per day from 20°C in the first day until a temperature of 65°C was achieved (10days).

The gas released from the composting organic matter contains water vapors that interfere with CO z and O2 determination. Therefore, pre- liminary drying was performed by passing the released gas through a cold (2°C) condenser. The resulting gas was water-vapor free, while the condensed water was directed back into the composting material.

Air was forced through the twelve reactors at a constant flow rate. The gases leaving the condenser were directed to an electromagnetic three-way valve. At constant intervals controlled by an electronic control unit the valve

250 Y. lnbar, Y. Chen, Y. Hadar, O. Verdonck

was opened and the gas was then led to a sampling loop. Three minutes later (the time required to remove all traces from previous sample analysis) the sample was pneumatically introduced into the gas chromatograph (GC). Every 15 min gas from one reactor was sampled and analyzed by the GC. Following a consecutive sampling of all reactors, a reference gas, of a known composition, was analyzed to eliminate variability of sample analysis. Each reactor was sampled every 3-5 h. During the composting period of 10 days each reactor was sampled 68 times.

The analysis of 02 and CO2 in the released gas was conducted with a Packard model 4X7 Becker GC. The separation of the gas was performed by means of two parallel columns, one filled with Porapack Q to separate 0 2 from N 2 and CO 2, and the other filled with Molecular Sieve 5A to separate CO2 from N 2 and 0 2. The results of 02 and CO 2 concentrations were automatically recorded on an integrator and further processed with a computer.

Experimental conditions

Three hundred and seventy-five grams of organic matter (on a dry matter basis) were weighed into each of the reactors. Either air drying or water addition was employed to bring the material to the desired moisture level. Fresh grape marc (GM) was studied at moisture contents of 30, 40, 50 and 60% (on wet-weight basis) with (+N) or without ( - N ) the addition of 0-25% (w/w) nitrogen as urea. Composted grape marc (CGM) was tested at moisture contents of 40 and 50% with or without added nitrogen. Fresh separated manure (SM) was studied at moisture contents of 40, 50, 60 and 70% with (+N) or without ( - N ) the addition of 0"25% nitrogen as urea. Composted separated manure (CSM) was tested at moisture levels of 40 and 50% with or without the addition of nitrogen. The maximum moisture content was adjusted to levels close to the water holding capacity at a suction of 10 mbars. The air supply rate was 8 liters h-1 kg-1 dry material.

RESULTS

Some characteristics of the organic materials are shown in Table 1. The organic matter content of GM is significantly higher than that of SM. Composts of grape marc and separated manure contain 89-1 and 48.4% organic matter, respectively. From these values and from the changes in the C/N ratios when fresh and composted materials are compared, it is obvious that the stability of GM towards decomposition is higher than that of SM. In addition, high levels of NO3 are formed in CSM, whereas NO3 levels in

Composting 0[" manure and grape marc 251

T A B L E 1 Some Characteristics of the Used Composting Materials

Property Units Separated Composted Grape Composted manure separated marc grape

manure marc

Organic matter % 78.6 48.4 96-2 89.1 Carbon % 45.6 28.1 55.8 51-7 Total nitrogen % 1.7 3-5 2.1 2.6 C/N 27"2 8"0 26"6 19-9 pH a 8"0 6.7 3.5 7.7 ECzs a dSim m - 1 2.3 5.6 3.3 1-7 NO3 a meq/100 g 0'3 40.0 0-2 1.5 Water content % of total

at sampling wet weight 86.0 56-0 64-0 38-6 Bulk density g/cm 3 0-12 0.25 0.23 0.36

a Measured in a 1:10 (solid:water) extract.

CGM are extremely low. The pH values of the fresh materials differ significantly: that of SM is basic (8.0) whereas that of GM is acidic (3"5), probably due to free organic acids formed from alcohols and sugars in GM. After composting, both materials exhibit slightly basic pH levels. The electrical conductivity (ECz5) of CSM extract is significantly higher than that of CGM indicating a stronger need for salt leaching in CSM prior to its use as a substrate.

Figure 1 shows the cumulative oxygen consumptions during the course of 10 days of composting simulation. Moisture content had a major effect on the O2 consumption. The higher the water content the higher was 0 2 consumption, which reached a maximum at the highest water content that was tested (60-70%). The 02 consumption was extremely low for the composted materials and was unaffected by the moisture content. No effect was recorded when nitrogen was added to SM at low water content levels. At 60% moisture the addition of nitrogen slightly enhanced O 2 consumption, whereas at 70% moisture a slight reduction in 02 consumption as a result of nitrogen addition was recorded. However, the addition of nitrogen to GM increased significantly the total O 2 consumption at all moisture levels, except for the mature composted material. The most prominent effect of nitrogen addition was exhibited by GM that was composted at 60% moisture, which seemed to be the maximal attainable water level for this product.

Oxygen consumption rate is a basic parameter in the determination of microbial activity and organic matter decomposition rate. Figures 2 and 3

252 Y. Inbar, Y. Chen, Y. Hadar, O. l/erdonck

S E PA RATED MOISTURE ._140 MANURE ~ - 7 0 % -N]

- - - - / ,,.60% -N/ ~; . / / ~ ' Q - 6 0 % + N |

(~100 / . / e l . - - - 50% "," N ~. X. .C 7 , . " / " - 50 % - N r ~

f '" " / 80 ,,-.,, ,

4 0 -- / ~ / / ~"~ / / / /

0') 2 0 s/ ~ ~ ~-~-" 4 0 - 6 0 % + N CSM

! Z MOISTURE

ELI t 6 0 F (.9 ) - X 440 L.

~ - 6 0 % ÷ N ] _G R~ p~ M__~A RC .-" . -" /

f / R / `/ ~ 6 0 % - N |

I - ,," _., .~--- 50% *N/ o ,2ot- . . " . - . . ~ . 3 - ~ ,o , -N I

" ' " "

| /.,/.s / / . . . . . . 4 0 % ÷ N / 80 r ~" _ _ ~ - - - = ~ S~ 40 Y° -N

,o i _..--";//- - " " ~ . . . . . . . . . . . . 3 0 % . , / I z ¢ ' . / ' / . / " / . - " - - 3OO/o _N |

(--) 40 F . f ~ ' ( ~ . J

0 ~ . . . . . . . . . . 0 1 2 3 4 5 6 7 8 9 I0

C O M P O S T I N G T I M E ( D A Y S ) Fig. 1. Cumulative oxygen consumption vs composting time of fresh and composted (CSM, CGM) separated manure (A) and grape marc (B) at various moisture contents with ( + N) and

without ( - N ) the addition of nitrogen as urea.

represent the influence of moisture content o n 0 2 consumption rate as a function of temperature in GM and SM, with and without the addition of nitrogen. In both materials, three maxima were observed at the following temperatures: 22°C; 35-40°C (mesophilic range); and 55°C (thermophilic range). Between these peaks two zones with low O2 consumption rate were observed. This low activity results probably from changes in the microbial population during transition from mesophilic to thermophilic conditions.

Cornposting o f rnanure and grape marc 253

,-° I

o

e,I o

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o

0.8

0.6 ̧

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SEPARATED MANURE / ~ Moisture

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/ / .-,,.,/~ i ,~. ~OO, o

/ / / X ''- 70 %

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B /

5O~/o 70 Vo

~ ~'~ ~ 40%

0 I I I I

20 30 40 50 60 70

T E M P E R A T U R E (°C) (RAISED AT A RATE OF 5°C /DAY)

Fig. 2. Oxygen consumption rate vs temperature of composting of fresh and composted separated manure at various moisture contents with and without the addition of nitrogen

254 Y. lnbar, Y. Chen, Y. Hadar, O. Verdonck

"C t-

O 13n

N o

E hl I- n~

Z 0 I

I- O_

D Z 0 c.) I N o

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0.6

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0.8

0.6

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0

GRAPE MARC A NO NITROGEN

ADDED ~ h

I I I I

GRAPE MARC 0.25% NITROGEN IP ' = ' ~ a ADDED . . . . .~, ~ ~ Fresh

~ e , ~ ~~, M°ist u r--'~e

Fresh ' ~ ~ 60%

, " ~ 50 °/*

4 0 % " ~ 3 0 * / .

Fig. 3.

" ~ I I I /

20 50 40 50 60 70 T E M P E R A T U R E (°C)

(RAISED AT A RATE OF 5*C/DAY)

Oxygen consumption rate vs temperature of composting of grape marc at various moisture contents with and without the addition of nitrogen.

Composting o f manure and grape marc' 255

Above 60°C all the materials exhibited a marked reduction in respiration rate.

The optimal microbial activity in SM to which nitrogen was not added was observed at the thermophilic range. The addition of nitrogen to the SM resulted in two intensified peaks at both mesophilic and thermophilic regions with lower 02 consumption rates in between (Fig. 2). The maximal decomposition rate in the GM to which nitrogen was not added was observed mainly in the mesophilic region. Higher water contents resulted in a maximum in the thermophilic range. The addition of nitrogen to GM increased 0 2 consumption rate at all moisture levels.

Both CO2 and O2 concentrations in the gas reaching the exhaust were measured. The lowest O2 concentration for GM was 12.3% (at 40°C, 60% moisture, + N), and for SM was 14.3% (at 40 and 60°C, 70% moisture, - N). The corresponding CO 2 concentrations were 9"3 and 7.6%, respectively. In mature composts, measured O2 concentration was close to ambient (20-5%). These results indicate that oxygen was not a limiting factor throughout the experiment.

DISCUSSION

The maximal biodegradation rate was obtained in this study at moisture levels of 60-70% for SM and 60% for GM. Below 50% moisture, the microbial activity seemed to decline sharply. These results are in close agreement with those reported for the composting of solid municipal wastes (Schulze, 1962; Suler & Finstein, 1977), and for the composting of dairy manure (Senn, 1971; Hong et al., 1983).

Haug (1980) stated that the optimal air content for a composting process as measured by the free air space should be at least 35% by volume. The volumetric water content (calculated from the bulk density) at the highest moisture content was 28% for SM and 35% for GM, leaving free air space levels of 65 and 50%, respectively. It may be concluded that both materials exhibited high enough air space and can therefore be maintained at opt imum moisture levels by the addition of water.

Oxygen concentration in decomposed organic matter depends on the biological activity and the rate of air supply, and can therefore indicate the aeration conditions. In order to achieve aerobic conditions the minimum 02 concentration reported in the literature is 10%, while the opt imum 02 levels are 14-17%. Carbon dioxide concentration should not exceed 4-7% (Spohn, 1970; Jeris & Regan, 1973; Suler & Finstein, 1977; Hoitink, 1980; Diaz et al., 1982). The results of this experiment indicated that opt imum aeration conditions were maintained in all the reactors.

256 Y. lnbar, Y. Chen, Y. Hadar, O. Verdonck

Contradicting data about the optimum temperature for composting have been published. Schulze (1962) reported that the 02 consumption rate during composting of city refuse and sewage sludge mixes at temperatures ranging from 20-70°C increased logarithmically. Hong et al. (1983) reported that the maximum decomposition of dairy manure in a batch-type closed composting unit was obtained at 65-70°C. Jeris and Regan (1973) reported optimum CO 2 production and 0 2 consumption rates at temperatures that did not exceed 60°C, for composting of mixed city refuse and sewage sludge in bench-scale composters and in shake-flask incubators held at a constant temperature. They also reported that the microbial activity dropped sharply when the temperature exceeded 60°C. Subsequently, Suler and Finstein (1977) found maximum CO 2 evolution at 56-60°C in city refuse composted in one liter containers at constant temperatures. In this case also the microbial activity was found to fall sharply at temperatures higher than 60°C. McKinley et al. (1985a, b) reported that the microbial activity was significantly higher when sewage sludge was composted at temperatures that did not exceed 58°C. Kuter et al. (1985) measured the rates of CO2 evolution during the composting of municipal sludge in a full-scale vessel system. They found that the highest composting rates were achieved when the average compost temperature was maintained below 60°C.

In the experiment described here two peaks of O 2 consumption were obtained for both raw grape marc and separated manure at temperature ranges of 35~2°C and 55-60°C. The decomposition rate of GM was higher over the lower temperature range, whereas similar rates were recorded in the mesophilic and thermophilic ranges for SM. Above 55-60°C a sharp decline in O 2 consumption rate was observed. These results are in close agreement with literature reports for other organic materials as described in the previous paragraphs. Grape marc showed an initial peak at the beginning of the composting process. This may be attributed to the initial population found in GM which is mostly comprised of yeast strains. Streichsbier et al.

(1982) found that the high sugar, acid and alcohol concentrations found in fresh GM favour the development of yeasts that are naturally present in the material and the suppression of accompanying microbial flora. In the course of yeast metabolism residual sugars are used and the high metabolic activity results in temperature increases beyond the optimum range for yeast activity. The inhibiting alcohol content simultaneously decreases sharply, because the alcohol is partly vaporized by the relatively high temperatures. Autolysis and subsequent binding of the residual alcohol by esterification reactions enable rapid appearance of a mixed population of bacteria. The temperature increase continued by this bacterial flora favors growth of a thermophilic fungal flora, which is mainly responsible for the microbial decomposition process (Streichsbier et al., 1982).

Composting of manure and grape marc 257

The optimum initial C/N ratio recommended for composting of organic materials is 25-35:1. Higher values may slow down the rate of decomposition and lower ones result in nitrogen loss (Gray et al., 1971; Alexander, 1977; Golueke, 1977). In composting of carbon-rich materials such as bark, up to 1.5% nitrogen is frequently added (Cappaert et al., 1976b). The results of the present work show that the addition of 0"25% nitrogen to GM increased the respiration rate and the total O2 consumption at all moisture levels. Separated manure was less affected by the addition of nitrogen. The initial C/N ratio of both materials was similar (ca 27), while the final ratio was stabilized at a low level for CSM (ca 8), and at a higher level for CGM (ca 20). The differences in behavior between the two materials may derive from differences in their structure and chemical composition. The SM is a uniform fibrous material, while the GM contains two different components, seeds and skin of grapes, each of which initially comprise about 50% of the total weight.

This research provides data for the optimization of composting of grape marc and separated manure which are readily available agricultural wastes. Results that were published earlier (Inbar et al., 1986; Chen et al., 1988) show that these products when composted in windrows can serve as high-quality partial substitutes for peat in container media.

ACKNOWLEDGEMENT

This study was partially supported by a grant from the Nieders~ichsischen Ministry of Science and Technology, FRG.

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