This study is mainly focused on the phytotoxicity improvement within five to six weeks of thermophilic composting of biowastes. Two sets of experiments were conducted involving both sawdust and rice husk as bulking agents, which were composted in self-heating reactors with potato-peel industrial waste and grass clippings as organic materials. The main variables observed over time were temperature, oxygen uptake rate (OUR), biodegradability, and germination index (GI). The effects of compost water extracts on seed germination and primary root growth of garden cress (
Nowadays, the natural resources are consumed at unsustainable rates, and consequently a huge amount of waste is produced, which can cause environmental damage if it is not well managed. In particular, the European Union policy states that the diversion of biowastes from landfill to anaerobic digestion or to composting should be improved for reducing pollution and recovering energy or soil conditioner materials.
Composting can be defined as the aerobic microbial decomposition of organic matter, under controlled conditions that allow the development of thermophilic temperatures as a result of the heat released in the biochemical reactions, leading to a stabilized and sanitized final product (free of pathogens and seeds), commonly known as compost [
In this context, it is important to ensure that the starting composting mixture is sufficiently biodegradable to promote aerobic breakdown of organic matter with consequent development of thermophilic temperatures [
The finished compost can be applied as soil improver, mulch, or as a medium to grow plants. However, it is very important to reinforce end users’ confidence in order to create a marketable product. For that, the stability and the quality of the finished compost should be well established. Indeed, it is necessary to understand the characteristics of the end product in order to avoid any undesirable effect [
Some studies have reported the linear dependence between germination tests (maturity parameter) and aerobic biological activity of the compost measured by static respirometric methods, expressed in terms of oxygen consumption (stability indicator) [
In fact, in theory one may expect these two parameters to be negatively correlated, which means that as the biological activity increases, the germination indexes decreases indicating a higher phytotoxicity potential [
Therefore, the main novelty of the present work is to evaluate the phytotoxicity improvement during the composting of biowastes and to appraise the existence of linear dependence between the germination index and on-line measured biological activity during process evolution, expressed as cumulative OUR.
In addition, the biodegradability levels of the starting mixtures and along the process were investigated as well. The mixtures tested were obtained by combining potato peel industrial waste (PP), grass clippings (GCs), sawdust (SD), and rice husk (RH) in different proportions.
Experiments were performed in a system of self-heating reactors arranged in parallel, each with 120 L of internal volume (0.7 m high and 0.44 m in diameter) and isolated with a rubber-based elastomeric material (Aeroflex MSR). The compressed air flow enters at the bottom of the reactors, and is uniformly distributed across a perforated Perspex plate. On the top of the reactors there are two openings for the introduction of the temperature sensor and for releasing the gases generated in biological reactions as well as the feed air not consumed. The reactors were equipped with a data acquisition system for on-line monitoring of temperature and oxygen concentration, and the feed air flow rate was measured with rotameters. In general, the air flow rate was controlled in order to maintain the oxygen concentration of the outlet gas in the range of 5% to 15% (v/v) and to control the temperature of the composting material in the reactor as well.
Potato peel (PP) was gathered at a Portuguese industry of potato chips, sawdust (SD) provided by a local pine sawmill, rice husk (RH) collected in a rice husking factory, and grass clippings (GCs) obtained from a garden maintenance service of a national football stadium. These materials were used in composting experiments immediately after their collection. Grass clippings were sieved through a 5 cm mesh, in order to obtain a homogenous material in size and shape, and the other materials did not require any specific treatment. The initial composition of each mixture tested is shown in Table
Initial composition and properties of starting mixtures.
Mixtures | Composition (%w/w) | Mixture properties | ||||
---|---|---|---|---|---|---|
PP | GC | SD | RH | VS (%w/w) | Lignin content (% VS) | |
SD1 | 53.0 | 39.8 | 7.2 | 0 | 90.8 | 23.9 |
SD2 | 66.0 | 28.0 | 6.0 | 0 | 92.3 | 21.3 |
SD3 | 38.7 | 53.0 | 8.3 | 0 | 91.0 | 25.5 |
RH1 | 58.7 | 19.4 | 0 | 21.9 | 86.7 | nd |
RH2 | 50.6 | 32.4 | 0 | 17.0 | 85.9 | nd |
RH3 | 66.0 | 8.1 | 0 | 25.9 | 86.2 | nd |
nd: not determined.
The composting experiments inside the self-heating reactors lasted about 20 days for SD and 25 days for experiments involving RH. The biowastes were periodically mixed on a daily basis to achieve homogeneous conditions. Once a week, samples were taken for further analysis.
After composting in self-heating reactors, mixtures were allowed to conclude maturation phase in small piles that were homogenized and moisture corrected every week, until 39 days of composting.
The moisture content was determined as the loss in weight in an oven at 105°C to constant weight. The organic matter content was considered equal to the weight loss on dry incineration (VS) of 2.5 g (dry weight) of sample. Incineration was conducted in a muffle furnace at 550°C for at least 4 h, until differences between two successive weighing were less than 0.01 g.
Lignin content was determined by the Klason method, which involves gravimetric measurements using solid samples, air-dried and ground to
Phytotoxicity was assessed by evaluating the effects on seed germination and root growth of aqueous extracts prepared from 10 g of solid (dry basis) with 100 mL of distilled water (
The conversion of organic matter obtained experimentally,
The maximum conversion of organic matter,
In this study, other empirical correlations to predict mixture biodegradability from lignin content were also evaluated. Haug [
However, in line with results of Komilis and Ham [
One-way analysis of variance (ANOVA) was used to test the equality of three or more means at one time based on variances. As null hypotheses, it was assumed that the mean values of all populations are equal, and the alternative hypothesis is that at least one mean is different. The level of significance considered was
The phytotoxicity test allows us to determine whether a material contains any detrimental substances to seed germination or to plant growth. Table
Physicochemical properties and phytotoxicity of materials potentially usable in composting.
Materials | Physicochemical properties | Phytotoxicity parameters | Classification | |||
---|---|---|---|---|---|---|
H (%) | pH | EC (mS/cm) | C/N |
|
||
Organic wastes | ||||||
PP | 78.68 | 5.39 | 0.84 | 25.5 |
|
Phytotoxic |
GC | 70.09 | 8.19 | 0.46 | 11.3 |
|
Nonphytotoxic |
Bulking agents | ||||||
SD | 12.25 | 5.50 | 0.23 | 68.0 |
|
Very phytotoxic |
RH | 10.61 | 6.75 | 0.87 | 57.5 |
|
Moderately phytotoxic |
In fact, this property is also reflected on seed growth inhibition of the composting mixtures (SD1 to SD3) used in this study, once all of them are classified as very phytotoxic (results shown in the next section). Rice husk was also used as bulking agent, and when tested by itself, it led to the classification of moderately phytotoxic.
As biological degradation proceeds, several changes take place at physical, chemical, and biological levels in the mixture, and thus the monitoring of certain variables is required to assess the evolution of the composting process. The temperature is usually regarded as a key parameter of composting, once it reveals whether the mixture is heating up due to exothermic oxidation reactions promoted by microorganisms. The temperature profiles observed during the composting processes in the exact centre of mixtures are illustrated in Figure
Temperature profiles in reactors (a) SD1 to SD3 and (b) RH1 to RH3.
In both sets of experiments, these results presented the expected behaviour over time for thermophilic composting, where the three classic phases can be well observed [
For oxidizing organic materials during composting, it is required to maintain a certain oxygen concentration (about 12%–20%) in the interparticle spaces. As biological reactions progress, the oxygen concentration for respiration often decreases, and thus this variable should also be monitored. In the present study, the oxygen uptake rate (OUR) profiles were monitored and presented in Figure
Temporal profile of oxygen uptake rate for reactors (a) SD1 to SD3 and (b) RH1 to RH3.
Organic wastes have been classified by Barrena and coworkers [
It is also well established that composting mixtures with an OUR lower than 500 mg O2 kg OM-1 h−1 [
Besides the temperature and OUR variables, also the biodegradability was assessed over time for reactors SD1 to SD3. This is an important property in composting and can be defined as the extent to which organic matter is degraded, (
Figures
Conversion of organic matter obtained experimentally,
The correlation often referred in the literature to appraisal the potential of biodegradation,
During the composting processes, complex biochemical reactions take place, leading to not only the breakdown of certain organic compounds and the formation of simpler molecules and ions (e.g.,
GI profiles during composting processes (a) SD1 to SD3 and (b) RH1 to RH3.
The evolution of GI for mixtures with sawdust (SD), Figure
When rice husk was used as bulking agent, Figure
In Figure
Relationship of germination index at day 18 of composting and cumulative OUR after thermophilic phase.
From these results it was possible to find that for each bulking agent used, there is a proportional relationship between the cumulative OUR attained after the thermophilic phase (
In summary, as the biological activity progresses during composting, the phytotoxicity of the ingredients tested was more successful for mixtures where rice husk was used as bulking agent, although the composts obtained after 39 days were still immature. This means that more composting time is necessary to obtain final products with low levels of phytotoxicity.
This study mainly analysed changes in phytotoxicity during composting of two sets of experiments involving sawdust and rice husk as bulking agents. Different biowastes were combined in order to obtain balanced mixtures. The analysis of the phytotoxicity of individual ingredients through the germination index revealed that all of them impair germination of
All mixtures revealed regular progress in terms of temperature profiles over time inside the reactors, where the three characteristics phases were well observed. The oxygen uptake rate (OUR) profiles were monitored as well, and the mixtures tested were categorized as moderate to high biodegradable. The biodegradability was followed over time for the set of experiments involving sawdust. The results obtained from the quantification of lignin were in good agreement with the observed conversion of organic matter. Some empirical correlations proposed in the literature adequately predict the maximum biodegradability for specific mixtures.
The germination index profiles along time may be significantly different between experiments with sawdust or with rice husk as bulking agents. Sawdust seems to impair seed germination to a greater extent, and after 39 days of composting, GI remained below 30%. In the case of rice husk, it was possible to reach 70% of GI at the end of the same period. Our results revealed that, at a certain point, there is a proportional relationship between the cumulative oxygen uptake rate and the germination index. Indeed, higher cumulative OUR provoked lower germination index.
The phytotoxicity test using seeds of
CERNAS is supported by National Funds through Foundation for Science and Technology (FCT ) under the project PEst-OE/AGR/UI0681/2011.