Influence of wheat straw addition on composting of poultry manure

Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 206–212

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Influence of wheat straw addition on composting of poultry manure b c ˇ ˇ Ivan Petric a,∗ , Almir Sestan , Indira Sestan a b c

Department of Process Engineering, Faculty of Technology, University of Tuzla, Univerzitetska 8, 75000 Tuzla, Bosnia and Herzegovina Department of Chemistry, Faculty of Science, University of Tuzla, Univerzitetska 4, 75000 Tuzla, Bosnia and Herzegovina Department of Physical Chemistry, Faculty of Technology, University of Tuzla, Univerzitetska 8, 75000 Tuzla, Bosnia and Herzegovina

a b s t r a c t Poultry manure is a significant source of nitrogen, but small amount of carbon needs to be added for faster degradation of organic matter in composting process. Composting of poultry manure mixed with wheat straw was carried out in specially designed reactors (32 L) under controlled laboratory conditions over 13 days. The aim of the study was to determine the influence of wheat straw addition to poultry manure on performance of composting process in terms of the following: the substrate temperature, carbon dioxide, ammonia, pH, electrical conductivity and organic matter. According to the results, the mixture of 83% poultry manure and 17% wheat straw (dw) among three different mixtures used in this research provided the best conditions for composting process. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Composting; Poultry manure; Wheat straw; Reactor

1.

Introduction

Nowadays, there are many poultry farms in many countries throughout the world which daily produce huge amounts of manure. According to Agricultural Waste Characteristics (United States Department of Agriculture, Natural Resources Conservation Service, 2008), poultry waste characterization is described by the following data (all in units per day and per animal): weight 0.19 lb/d-a, volume 0.0031 ft3 /d-a, total solids 0.049 lb/d-a, volatile solids 0.036 lb/d-a, nitrogen 0.0035 lb/d-a, phosphorus 0.0011 lb/d-a, potassium 0.0013 lb/da. For instance, in 2007 the total live poultry stock in Bosnia and Herzegovina was 14,989,229 birds (Agency for Statistics of Bosnia and Herzegovina, 2008a). Therefore, waste characteristics are the following: weight 2,847,953.5 lb/d (1294.65 tons/d), volume 46,466.6 ft3 /d (1315.75 m3 /d), total solids 734,472.2 lb/d (333.88 tons/d), volatile solids 539,612.2 lb/d (245.30 tons/d), nitrogen 52,462.3 lb/d (23.85 tons/d), phosphorus 16,488.2 lb/d (7.50 tons/d), potassium 19,486 lb/d (8.86 tons/d). Very often, farmers use this kind of manure as organic fertilizer in agriculture without any preliminary treatment. Consequently, a considerable emission of harmful gases is released into atmosphere (greenhouse gases), nitrogen losses from manure are large, and contamination of soil with pathogens is pos-

∗

sible. Manure handling, storage, and disposal continue to present major problems for poultry producers throughout the world. Problems related to fly control, odour, urban encroachment, limited proximate land base for manure disposal, and increased regulatory pressures necessitate the development of alternatives to traditional scrape, haul, and spread manure management system. Raw or liquid stored manures, on the other hand, have limited uses, can be applied to land just a few times during the year and are expensive to transport (Jongbloed and Lenis, 1998). Manures contain high levels of ammonia and nitrogen loss is usually attributed to ammonia volatilization and leaching (Dewes, 1999; Barrington et al., 2002) and to nitrous oxide and nitrogen volatilization (He et al., 2000; Veeken et al., 2002). Nitrogen losses occur during many phases of manure handling including during accumulation and storage in the barn, during removal, mixing, processing and finally during and after land application. It is difficult to compare overall nitrogen losses from different manure handling systems to minimize ammonia losses (Gibbs et al., 2002) since in some systems the majority of the losses occur during land application (liquid manure and anaerobic digests) while for others it occurs during processing and storage (solid storage and composting).

Corresponding author. Tel.: +387 35 320 766; fax: +387 35 320 741. E-mail addresses: [email protected], [email protected] (I. Petric). Received 28 March 2008; Received in revised form 19 February 2009; Accepted 25 February 2009 0957-5820/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2009.02.002

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Composting is an effective and safe way for reduction of the manure’s mass and volume, for destruction of pathogens and stabilization of nutrients and organic matter in it (Tiquia et al., 2000). Composting of manures has provided options for other livestock producers, including reduced odour and fly problems, reduced manure volume and moisture content, and production of a more uniform, saleable and easily transportable fertilizer product. The costs of the process can be offset by the value added nature of composts. For example, composts enhance soil fertility, increase crop yields (Dick and McCoy, 1993) and reduce diseases caused by soil borne plant pathogens (Hoitink and Boehm, 1999). Furthermore, as compared to raw manure and synthetic fertilizers, composted animal manures can reduce nutrient leaching when applied to agricultural fields. Composting animal manure can reduce greenhouse gas (GHG) emissions in two ways; by reducing nitrous oxide and methane emissions during manure storage, processing and application to the field, and by reducing the amount of manufactured fertilizers and the GHG associated with their production and use (Paul et al., 2001). It should be noted that methane is 20 times and nitrous oxide is 310 times more polluting than carbon dioxide regarding greenhouse effect. The method of composting does impact GHG emissions, where GHG emissions are higher from outdoor windrow composting systems than from controlled aerated systems. Greenhouse gas emissions from the anaerobically stored farmyard manure were about 4.5 times higher than from the composted farmyard manure (Amon et al., 1998). Ammonia emissions during storage from the anaerobically stored farmyard manure were about four times lower than from the composted farmyard manure, but after spreading of the compost no ammonia emissions were detectable. After spreading of the anaerobically stored farmyard manure there was significant emission of ammonia. Certain physical and chemical characteristics of the poultry manure are not adequate for composting and could limit the efficiency of the process: excess of moisture, low porosity, high N concentration for the organic-C, which gives a low C/N ratio, and in some cases high pH values. The addition of a bulking agent for manure composting will optimise substrate properties (moisture, porosity, C/N ratio, pH). Barrington et al. (2002) showed that for six bulking agents (shavings, shavings and soybean, hay and urea, hay, wheat straw, oat straw), the moisture and the aeration regime had no consistent significant effect on nitrogen and carbon losses by volatilization. Only the type of bulking agent had a significant effect (99.5% confidence level). Amongst the controllable factors which influence manure composting, the selection of appropriate bulking agents plays an essential role in controlling the decomposition rate and favouring nitrogen retention within the compost (Bernal et al., in press). There are a lot of bulking agents that could be used as a carbon source (barley straw, oats straw, olive waste, cotton straw, wood chips, wood shavings, corn leaves, sawdust, newsprint, rice straw, etc.). Farmers should make their choice on the basis of the availability and quantity of specific bulking agent. However, some of the bulking agents have very high content of lignin which resists biological degradation. These bulking agents have less readily degradable C source. For instance, lignin contents for some bulking agents are the following (all data on a dry matter basis): barley straw 11%, corn leaves 3.8%, cotton straw 15%, oats straw 14%, olive waste 28%, newsprint 20.9%, rice straw 12.5%, wheat straw 8.9%, pine 27.8%, red maple 23%,

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spruce 28.6 (Richard, 1996). The composting system and conditions, characteristics of both the bedding material and the bulking agent added for composting have a great influence on the mineralisation of the organic matter during composting. For instance, the use of woodchips instead of cereal straw as bedding material in beef manure reduced the organic-C loss during composting (Hao et al., 2004) due to the combination of larger particle size, higher C/N ratio and the recalcitrant nature of the woodchips. Similar results were shown by Hansen et al. (1989) and Michel et al. (2004) in composting of poultry manure (corncobs vs. sawdust) and cow manure (wheat straw vs. sawdust), respectively; they obtained a lower decomposition of the composting substrate when employing amendment materials (sawdust in both studies) with recalcitrant organic matter such as lignin. The main reason for choosing the wheat straw as a bulking agent in this study is its availability for poultry farms since huge amounts of wheat straw exist in Bosnia and Herzegovina. For instance, in 2007 the total wheat production in Bosnia and Herzegovina was 257,441 tons from harvested area 73.968 ha (Agency for Statistics of Bosnia and Herzegovina, 2008b). Straw makes up about half of the yield of cereal crops such as barley, oats, rice, rye and wheat. The main applications of straw in Bosnia and Herzegovina are as bedding livestock and as animal feed. Since wheat straw is available in poultry farms, there are no transportation costs. Composting poultry manure with wheat straw could offer many environmental and economic benefits for the regions where both poultry manure and wheat straw are available in great amounts. Some of these benefits are the following: elimination of pathogens and weeds, microbial stabilization, reduction of volume and moisture, removal and control of odours, production of good quality fertiliser or substrate, ease of storage, transport and use, etc. The aim of the study was to determine the influence of wheat straw addition to poultry manure on performance of composting process.

2.

Materials and methods

2.1.

Composting materials

Fresh poultry manure and wheat straw were used as experimental materials and were collected in polyethylene bags ˇ from poultry farms near Gracanica in the Tuzla Canton, Bosnia and Herzegovina. Day-old chicks were purchased from hatcheries that specialize in hatching egg-production pullets (young hens). Pullets were reared to 19 weeks of age by egg producers or pullet growers until they are ready to begin laying eggs. The egg-production cycle lasted about 1 year. Both pullets and hens were primarily raised in cage systems in environmentally controlled barns. Feeding, watering and egg collection were automated on almost all production sites. Used grower (KN-16.5%, 16.5% proteins) was a mixture consisted of the following materials: cereals, by-products of milling industry, by-products of oil industry, by-products of starch, by-products of alcohol and fermentation, dried plant products, minerals, etc. Poultry manure was collected directly from transportation band, while wheat straw was purchased by bale. Visual evaluation deemed the straw to be of good quality with no apparent signs of mould or decay. Immediately after transport to the laboratory, the materials were analyzed (Table 1) and prepared for filling the reactors. The straw was cut on pieces 2.5 cm long. The purpose of size reduc-

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Table 1 – Characteristics of raw composting materials (values represent a mean followed by the standard error of three replicates). Parameters Moisture content, %ww Organic matter, %dw pH Electrical conductivity, dS m−1 Total organic carbon, %dw Total nitrogen, %dw C/N

Poultry manure

Wheat straw

72.59 ± 0.97 78.07 ± 1.83 8.37 ± 0.06 3.94 ± 0.10 43.37 4.55 9.53

10.87 ± 0.95 87.91 ± 1.11 7.18 ± 0.05 1.91 ± 0.03 48.84 0.55 88.80

ww: wet weight; dw: dry weight.

tion was to allow uniform mixing of the manure and straw, and to facilitate sampling and analysis of the composting solids. The manure and straw were manually mixed in plastic boxes to achieve better homogenization of material. Three different mixtures (for composting reactors 1, 2 and 3) were prepared with following percentages of manure and straw (by dry weight): (1) 73.5% manure and 26.5% straw (reactor 1), (2) 83% manure and 17% straw (reactor 2), and (3) 88% manure and 12% straw (reactor 3).

2.2.

Experimental set-up and procedure

A laboratory-scale composting system as shown in Fig. 1 was used for this study. The investigation was conducted using three identical 32 L reactors (0.48 m high and 0.30 m in diameter) made of high-density polyethylene. The reactors were insulated with a layer of polyurethane foam (1 cm of thickness). A vertical rotating axis with blades mixing on intermittent schedule, fixed at perforated plate made of chrome, ensures the complete mixing of the composting mass. The reactors were equipped with a valve for dropping the leachate and condensate. A perforated steel plate was installed in the reactor 0.08 m above the bottom. Each reactor was connected with an air compressor EURO 8/24 (Einhell, Germany), which provided air into the reactor at a controlled rate (0.9 L min−1 kg−1 OM) at the bottom. Measurement of airflow was carried out using airflow meter (Valved Acrylic Flowmeter, Cole-Parmer, USA). Each reactor was equipped with thermocouple type T (DigiSense, Cole-Parmer, USA), which was mounted 0.2 m above the perforated steel plate to monitor inside temperature of

the composting materials at 15-min intervals. The acquisition module (Temperature Data Acquisition Card Thermocouple CardAcq, Nomadics, USA) was used to record the temperature data from reactors and ambient. Before inlet to reactors, the air had been introduced into solution of sodium hydroxide in order to remove traces of carbon dioxide. The incoming air was moisturized by blowing through a humidifier i.e. 1-L water bottle prior to entering the reactor to minimize drying of composting materials. At outlet, the gas mixture passed through a condenser, a gas washing bottle with 1 M sodium hydroxide and a gas washing bottle with 0.65 M boric acid, in order to remove the condensate, carbon dioxide and ammonia, respectively. The gas washing bottles were changed daily because for determination of carbon dioxide and ammonia. The additional water was not added to composting material during the process.

2.3.

Sampling and analysis

The composting material was mixed several times per day (for 15 min each time). After mixing, samples (about 50 g) were taken every day at the same time, from different places in the substrate (top, middle, and bottom). The analysis of the fresh samples was performed immediately after taking them out of the reactor. For determination of carbon dioxide content, an aliquot volume of sodium hydroxide solution (used as a “trap”), with the indicator of phenolphthalein, was titrated by standard solution of 1 M hydrochloric acid. The difference in titration between blank and sampled probes was used for calculation of the mass of the “trapped” carbon dioxide. For determination of ammonia content, an aliquot volume of boric acid solution (used as a “trap”), with the indicator of bromcresol green-methyl, was titrated by standard solution of 1 M hydrochloric acid. The difference in titration between sampled and blank probes was used for calculation of mass of the “trapped” ammonia. The moisture content of the experimental material was analyzed by drying oven method at 105 ◦ C for 24 h (APHA, 1995). The organic matter content (volatile solids) of the material was measured by burning oven at 550 ◦ C for 6 h (APHA, 1995). Kjeldahl nitrogen determination was performed according to APHA (1995). The carbon content (%C) was calculated from the ash fraction (%ash), according to the Eq. (1) (Haug,

Fig. 1 – Schematic diagram of reactor system for composting process: (1) aquarium pump, (2) airflow meter, (3) gas washing bottle with solution of sodium hydroxide, (4) gas washing bottle with distilled water, (5) reactor, (6) thermocouple, (7) condenser, (8) graduated cylinder, (9) gas washing bottle with solution of sodium hydroxide, (10) gas washing bottle with solution of boric acid and (11) laptop.

Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 206–212

1993): %C =

(100 − %ash) 1.8

(1)

Electrical conductivity and pH were measured in aqueous extract. This aqueous extract was obtained by mechanically shaking the samples with distilled water at a solid to water ratio of 1:10 (w/v) for 1 h. The suspension was centrifuged and filtered through a Whatman no. 42 filter paper. The pH and electrical conductivity measurements were carried out using a PC 510 Bench pH/Conductivity meter (Oakton, Singapore) with two separate electrodes. The loss of organic matter (k) was calculated from the initial and final organic matter contents, according to the Eq. (2) (Diaz et al., 2002; Külcu and Yaldiz, 2004) k=

[OMm (%) − OMp (%)] × 100 OMm (%) · [100 − OMp (%)]

(2)

where OMm is the organic matter content at the beginning of the process; and OMp is the organic matter content at the end of the process. Each analysis was done in triplicate with calculation of the mean value.

2.4.

Statistical analysis

Statistical analysis (ANOVA analysis, and the least significant difference for mean at 95%) was performed with Statgraphics statistical package (STATGRAPHICS, 1996) on data obtained in the composting mixture at the different composting times.

3.

Results and discussion

3.1.

Temperature profiles

At the outset, after filling the reactors, a fast increase in temperature was produced in all reactors, indicating a marked microbial activity. The temperature profiles of the reactors are illustrated in Fig. 2. The lag period on temperature curve was not recorded because the original substrate was rich in microorganisms. Thus, after several hours the temperature in the composting mass started to rise due to intense biodegradation. The composting process reached the maximum temperature of 63.85 ◦ C after 2 days in reactor 2, whereas reactors 1 and 3 reached lower maximum temperatures (56.12 ◦ C after 0.5 days and 62.36 ◦ C after 1.25 days). These maximum temperatures have been maintained very shortly (only few hours) so they are below the temperature

Fig. 2 – Changes of temperature in the reactors during composting process.

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range where microbial inhibition can start (>65 ◦ C). It can be noted that the aeration system controls the temperature of composting material. This substantially reduces the risk of overheating and microbial inhibition. However, if temperature becomes greater than 65 ◦ C, the fungi, actinomycetes and most bacteria are inactive and only spore-forming bacteria can developed (Gray et al., 1971). At 75 ◦ C, spore-forming bacteria are the predominant, or perhaps the only organisms (Epstein, 1997). Although there were no differences between temperature for reactors 1 and 3, there were statistically significant differences in between reactors 2 and others according to the results (P < 0.05). During the most of the composting period, temperatures were the highest in reactor 2. The temperature in this reactor was maintained above 55 ◦ C about 2 days, which should be sufficient to maximize sanitation (Stentiford, 1996) and to destroy pathogens (Strauch and Ballarini, 1994). The temperatures in other two reactors decreased immediately after reaching their maximum values. The secondary temperature peaks that occurred in the reactors were possible a result of delayed microbial growth in the lower portions of composting mixtures, either due to water leached from the top, or lower heat removal rate due to the predominance of smaller, constant aeration rate, or both. Shin and Jeong (1996) explained the secondary temperature peaks as indication of degradation of cellulose after readily degradable matter is consumed. At the end of the process, when there is a smaller amount of easily degradable organic matter (reactor 3), the composting mixture cools down and the temperature of the decomposing material becomes comparable to the ambient temperature (Solano et al., 2001). This is not the case with bulk temperatures in the reactors 1 and 2. It was probably the result of the secondary fermentation in the prolonged duration induced by more available organic components and active microbes. Secondary temperature peaks are indicators of degradation of cellulose (after readily degradable matter is consumed) or recovered thermophilic microbial population. Also, this could be the result of improved aerobic conditions. Turning of a compost material may cause a secondary temperature rise brought about by the replenishment of the exhausted oxygen supply. Cellulose assays were not done in this study but literature data for cellulose content in wheat straw ranges from 30.5 to 42% (on a dry matter basis). Also, considering wheat straw literature, data ranges from 7 to 18% for lignin, and from 27.6 to 31% for hemicellulose. There are three different groups of microorganisms which are active during secondary fermentation. Predominant microorganisms are mesophilic bacteria, while actinomycetes and fungi decrease. Lignocellulose is mainly composed of a mixture of cellulose (ca. 40%), hemicellulose (ca. 20–30%), and lignin (ca. 20–30%) (Tuomela et al., 2000). Lignocellulose is gradually degraded during composting. The decomposing trend of hemicellulose is similar with that of cellulose. They are partially degraded during the initial stage of composting. Then the degrading ratio is almost unaltered due to the high temperature followed by the large decomposition during the temperature falling phase and initial stage of the secondary fermentation. Lignin is a highly branched, irregular three-dimensional organic polymer, which provides plant strength and resistance to microbial degradation. Its degradation is quite different from that of cellulose or hemicellulose. Lignin is slightly decomposed during the initial stage of composting. When the temperature is lower than the maximum value during thermophilic phase, lignin is greatly degraded until the tem-

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perature begins to fall. According to Tuomela et al. (2000), the most effective lignin degraders in composting conditions may be thermophilic fungi, which have an optimum growth at temperatures between 40 and 50 ◦ C. Tomati et al. (1995) found that 70% of lignin was degraded when the temperature of the compost was kept at 50 ◦ C. Windrow composting is the most commonly used of farm scale composting methods. The experiments in this study were undertaken in laboratory reactors in order to simulate conditions in windrow composting. According to Petiot and de Guardia (2004), this volume of laboratory reactor (with provided external insulation) should allow the self-heating of the substrate and therefore even to simulate full-scale composting. Although it seems that 13 days is too short period for composting (windrow composting usually takes 30 days or more), temperature profiles of the reactors (Fig. 1) has demonstrated very accelerated process especially in the first few hours of composting, indicating a marked microbial activity. Therefore, Fig. 1 shows three of four phases in aerobic composting process: mesophilic (initial) phase, thermophilic phase and cooling phase. Maturation phase has not been shown because it was not the aim of this study (only active, high-rate composting in reactor).

3.2.

Evolution of carbon dioxide and ammonia

Fig. 3 shows the results of the carbon dioxide changes inside each of the reactors. The mass of the produced carbon dioxide increased in all reactors proportionally to microorganisms’ activity during the process and it was higher in reactor 2 than in the other reactors. The greatest mass of the carbon dioxide was generated after the first 3 days. After the third day, easily degradable organic compounds were degraded and the microbial activity decreased. Therefore, the amount of produced carbon dioxide was reduced. It was observed that there were no statistically significant differences in carbon dioxide evolution between reactors (P < 0.05). The increased emission of ammonia was observed in all reactors (Fig. 4). Ammonia represents even 98% of emission of nitrogen from composting material (Beck-Friis et al., 2001). The emission was the greatest in the reactor 3, which could be explained by the greatest content of manure in the initial mixture than in other reactors. It was observed that there were significant differences between reactor 3 and other reactors (P < 0.05). The high straw additions in the reactors 1 and 2 decreased C/N ratio in the mixtures and losses of ammonia. On the other hand, high straw additions reduced moisture content of the mixtures. Therefore, pores filled with air became greater, which made better conditions for degrada-

Fig. 3 – Changes of carbon dioxide mass in the reactors during composting process.

Fig. 4 – Changes of ammonia mass in the reactors during composting process. tion. As a consequence, modification of NH4 + /NH3 equilibrium could have been expected. Temperature rose as the amount of straw in mixture increased, which helped production of ammonia because of increasing the ammonia vapour pressure. Ammonia volatilization was related to interstitial carbon dioxide concentration, which itself was influenced by both microbial activity and air exchange rate based on multisolute CO2 –NH3 –H2 O aqueous system (Liang et al., 2004). Therefore, it is obviously there are very complex interrelationships between pH, emission of carbon dioxide and ammonia, moisture content, temperature and aeration rate. These interrelations are the reason for very strange changes of ammonia in reactors.

3.3.

Patterns of pH and electrical conductivity

The evolution of pH is presented in Fig. 5. The pH of the material in reactors increased significantly in the decomposition stage (in reactors 1 and 3 for the first 2 days, in reactor for the first 4 days). This increase was related to the increase in ammonia in this period. Sundberg and Jönsson (2008) showed that increased aeration rates at the start of the process resulted in large heat losses, higher microbial activity, faster decomposition and a faster rise in pH values. The pH of the material in reactor 2 increased slightly in the initial stage. The pH of mixtures in all reactors rose to levels remaining alkaline during composting. These alkaline pH values could contribute to nitrogen losses and ammonia odours during the process because the high temperature and pH seemed to favour nitrogen volatilization (Michel et al., 1993). It was observed that there were significant differences between reactor 3 and other reactors (P < 0.05). The final pH values were higher than those demanded for optimum plant growth and, according to Campbell et al. (1997), could cause nutrient defi-

Fig. 5 – Changes of pH in the reactors during composting process.

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Fig. 6 – Changes of electrical conductivity in the reactors during composting process. ciencies in the plant. In order to avoid these negative effects it would be advisable to restrict the utilization of this compost on acidic soils. The evolution of electrical conductivity during composting of poultry manure with straw in the different reactors is shown in Fig. 6. In all cases, electrical conductivity values increased during composting. This increase may be due to the loss of weight and release of other mineral salts such as phosphate and ammonium ions through the decomposition of organic substances (Abid and Sayadi, 2006). Increased concentration of soluble salts reflected the progressive mineralisation of the organic matter (Cáceres et al., 2006). The electrical conductivity value reflected the degree of salinity in the co-compost, indicating its possible phytotoxic/phyto-inhibitory effects on the growth of plant if applied to soil (Huang et al., 2004). The greater amount of poultry manure would contribute to higher electrical conductivity. At the end of the process, the composting material in reactor 3 showed the highest electrical conductivity during composting (5.65 dS m−1 ), whereas the material in reactor 1 had the lowest electrical conductivity (4.66 dS m−1 ). These values are in agreement with results of Broddie et al. (2000). There were significant differences between reactor 3 and other reactors (P < 0.05).

3.4.

Degradation of organic matter

During composting process, organic matter is oxidized and converted to carbon dioxide, water, ammonia and new microbial biomass. The rate of organic matter loss is an indicator of the overall composting rate. During the study, both volume and mass of composting material within the reactors were decreasing significantly. This decrease was mainly the result of degradation of organic matter during composting process. The reactors were filled with approximately 13 kg of compost material. Amount of each sample, daily taken from the reactors, can be neglected if compared with the whole composting mass within the reactors. Therefore, volume and mass of composting material have changed significantly due to decrease of moisture and organic matter contents in composting mixture (not due to removed samples). After 13 days of composting organic matter losses were 34.3, 38.8 and 25.1% for reactors 1, 2 and 3, respectively. Initially, the organic matter content decreased somewhat faster in reactor 2 than in the others (Fig. 7), since a substantial loss of total organic matter occurred during composting. The smallest loss of organic matter was achieved in reactor 3. The mixture in this reactor had the smallest addition of straw, so the carbon content was very low which did not provide a favourable condition for the growth and biological activity of microorganisms. It was observed that

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Fig. 7 – Changes of loss of organic matter in the reactors during composting process.

the organic matter degradation was more intense in reactor 3 than reactor 1 during the first 9 days. This can be explained by higher porosity of the material in reactor 1 as a consequence of the higher proportion of wheat straw. Thus the rate of degradation was higher both because of the higher carbon content and because of the action of a bulking agent. The role of wheat straw as a bulking agent was to increase C/N ratio and to provide degradable organic carbon. Adding more wheat straw to poultry manure gave higher carbon content and therefore C/N ratio was higher. Wheat straw exerted a great influence on composting performance since appropriate conditions of the physical environment for air distribution must be maintained during the process. The addition of wheat straw for poultry manure optimised substrate properties such as air space, moisture content, C/N ratio, particle density, pH and mechanical structure, affecting positively the decomposition rate. From the tenth to thirteen day, the organic matter content decreased faster in reactor 1 than reactor 3. There were statistically differences between k values for reactor 2 and other reactors (P < 0.05).

4.

Conclusions

Composting of poultry manure and wheat straw was characterized by determinations of such parameters as temperature, pH, electrical conductivity, organic matter conversion, changes in carbon dioxide and ammonia mass. Wheat straw exerted a great influence on composting performance since appropriate conditions of the physical environment for air distribution must be maintained during the process. According to the experimental results, the highest values of organic matter degradation, temperature and carbon dioxide emission, were obtained in reactor 2, so the mixture ratio in reactor 2 was more suitable for composting than other mixture ratios. Moreover, only the reactor 2 provided conditions for sanitation and reducing pathogens. Therefore, a mixture of 83% poultry manure and 17% wheat straw (dw) among three different mixtures used in this research provided the best conditions for composting process. Future experiments should give the answer how to control pH and ammonia emission.

Acknowledgements This work has been funded by the Federal Ministry of Education and Science of Bosnia and Herzegovina, and by the Ministry of Education, Science, Culture and Sport of the Tuzla Canton.

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