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This chapter is an account of previous studies and data on humic acids, heavy metals, their origins in compost, basic conditions for good composting, characteristics of compost materials and method of composting applied.
2.0 Composting and its benefits
Composting is the controlled process of decomposition of organic matter. Usually, degradation of organic matter occurs naturally by the action of micro-organisms, but during composting, the optimum conditions necessary to accelerate this decomposition is made available.
Since there is more than 50 % of the MSW in Mauritius which is organic in nature, composting is the most common recycling method apply so as to decrease the load in the landfill and also to discourage the use of fertilizers in crops which may be detrimental to health as mentioned by Dr. Zoubeir Joomye in the Week-End newspaper (pp. 12, 2010). Compost is also cheaper as compared to fertilizers which are another factor in favour of use of compost as a soil enrichment to promote plant growth.
Compost has the ability to regenerate soil by promoting the production of micro-organisms which further degrade organic matter into humus like substances, helps the soil to retain moisture. It also provides sufficient nutrients to the plant slowly such that the plant is not exposed to relatively large amount of nutrients at one go.
Composts also increase biodiversity in soil by bringing bacteria, fungi, insects, worms and more support for healthy plant growth. Compost changes the structure of the soil such that there is less erosion and soil splattering on plants. It also encourages roots health so that there is less run off occurring.
2.1 Composting methods
There are many composting techniques which are static piles, turned windrows, forced-aeration static piles, in-vessels and close-housing systems (Anon, 2007). Usually, the windrow type and static piles are favoured due to their low cost. In the US, windrow, aerated static pile and in-vessel composting techniques are used according to Agardy, Nemerow and Salvato (pp. 232, 2009).
Windrow composting is the process of placing mixed material in long narrow piles with regular agitation to avoid anaerobic condition building up inside the windrow and also to prevent combustion to due overheating. (Rhonda, pp.5, 2010). Usually there is an optimum size that a windrow can be made such that there is adequate flow of oxygen and also to maintain the proper temperature (Richards, 2000). The windrow is usually 1-2 m high and 2-5 m at the base. This allows for aeration to take place but need some mechanical mixing from time to time for proper aeration to Agardy, Nemerow and Salvato (pp. 233, 2009). Agardy, Nemerow and Salvato suggested that the windrow be turned mechanically every four to five days so that the temperature of the windrow dropped to about 38 OC from about 60 OC.
Yuh Ming Huang also suggested that the height of the windrow be 1.5-1.8 high with a width of 2-3 m. She also suggested that the ceiling temperature of the windrow be kept at about 45-55 OC (YUH-MING HUANG, 2005) for maximum biodegradation to occur.
Static piles are similar to windrows but they are piled as a round construction (YUH-MING HUANG, 2005) and in-vessels composting is achieved in a reactor with control measures such as aeration and moisture content (Agardy, Nemerow and Salvato, pp. 235, 2009). Windrow height may vary between 90 cm for dense material to 360 cm for light materials with width of 300-600 cm. (Anon, 2010).
2.2 Conditions for composting
Composting is the degradation of organic matter by the action of microorganisms, bacteria and fungi which work best under certain conditions. They need moisture to be able to decompose the organic matter and also good aeration as they need oxygen, nutrient balance, pH and temperature. These are the key parameters that need to be monitored for composting to occur.
2.2.1 C: N ratio
The C stands for carbon while the N stands for nitrogen. Carbon is essential for building blocks of life for microorganisms and also a source of energy while nitrogen is necessary for protein, cell structure and genetic materials (Cortesia, 2008). Decomposition of organic matter is increased when the proper C: N ratio is made available in the compost. This can be made by adding up different amount of carbonaceous materials, called brown material due to their dryness and nitrogen rich material called green material since they are fresher and has relatively higher moisture content.
According to Jenkins, J. (1999), a good C: N ratio for compost pile is between 20/1 to 35/1 which means that 20 parts of carbon to 35 parts of carbon to one part of nitrogen in compost is one of the optimum condition for the degradation of organic matter by the action of microorganisms. If there is excess nitrogen, the microorganisms will not consume all nitrogen but the excess will be converted to ammonia gas and will be loss to the atmosphere and thus there will be loss of valuable nutrients for plants.
In MSW, there is variation of the C: N ratio according to the constituents of the waste, this is even the case for Mauritius whereby under different studies different composition of MSW which did not differ much since almost 50 % of the wastes was organic in nature but the fraction of kitchen waste and yard waste were different such as according to Mohee (2002) the amount of garden waste was 43% and kitchen waste was 23 % while the ministry of environment found that garden waste was 25% and kitchen waste 45 % in 2004. This variation in organic composition of the MSW does not guarantee a fixed C: N ratio for all MSW in Mauritius.
2.2.2 Moisture content
Like C: N ratio, moisture is another key parameter that needs to be controlled for compost. The level of moisture in the compost regulates the activity of the microorganisms and also the condition of degradation of organic matter. If too little water is present, the microbial activity will be limited as it is necessary for the microorganisms. At higher levels, the degradation of organic matter by microorganisms may occur under anaerobic conditions. This situation arises due to the fact that spaces inside the compost will be occupied by water mostly giving less or no spaces for air to be in pores inside the compost thus preventing air to penetrate giving rise to absence of oxygen causing anaerobic condition (Anon, 1996).
The ideal moisture content of compost is between 40 - 60 % (Anon, 2009). At this moisture level, the compost is like a sponge in which the water has been squeezed out and only a film of water is covering the compost at this point. At this level of moisture, the microorganisms work best and also there is prevailing aerobic conditions due to present of air in pores.
Temperature is another important factor in aerobic degradation of organic matter that is in windrow composting. The temperature is an indication of microbial activity taking place inside the composting pile. The temperature rises because of degradation of organic matter by microorganisms which produce some energy. In case the temperature is too high or too low, or there are no nutrients or enough water, microbial activities will stop resulting in absence of degradation of organic matter. Usually, the optimum temperature is between 43-66 OC according to Burton S., Harwood M., Moser V., and Thompson C. (2009). Below is a table showing the effect of temperature in compost and its effect (Burton S., Harwood M., Moser V., and Thompson C. 2009).
Table 1: Temperature and its effects on composting
55 - 70Â°C
Pathogens are inactivated
50 - 60Â°C
Weed seeds are inactivated
Slow composting rate
As can be seen above, the different conditions prevailing at different temperatures are given showing that for temperatures between 40 to 70 OC, the composting rate is relatively higher but it should also be taken in consideration that microorganisms are destroyed beyond 70 OC.
Since the decomposition of organic matter is done in presence of oxygen by aerobic microorganisms, it is necessary to provide good aeration for the compost windrow. The aeration not only provides oxygen to the microorganisms but also prevent odours by preventing formation of anaerobic digestion at high temperatures. There are several techniques of aeration but the most common one is turning (Anon, 2010). Turning the pile also ensure that outside materials are put in the centre while turning s as to be subject to the high temperature.
The frequency of turning the windrow is dependent upon many factors such as moisture content and type of material. Moisture is considered the most important since high moisture content reduces the air space inside the pile ad reduces the strength of the structure of the material. The figure below illustrates the turning frequency effects on composting.
Figure 1: Turning frequency effect on composting
Source: Washington State University, Compost fundamentals, 2010
As can be seen from the above figure, the higher the turning frequency the higher is the temperature of the pile and also the faster is the rate of decomposition. This figure illustrates that the turning frequency should be high enough to accelerate the rate of decomposition but it should be noted that if the C: N ratio is optimum; the pile need not be aerated so often.
The composting process is relatively insensitive to pH, within the range found in mixtures of organic materials, largely because of the broad spectrum of microorganisms involved (Rynk et al., 1992) cited in "Role of pH in composting" (Campbell, A., 2002). The optimum range of pH in composting as stated by Campbell A. (2002) is in the range of 5.5-9.0. The composting takes place best at a pH of neutral, which is at pH 7.0. pH is more important when the composting material has a high nitrogen content, this accelerates the loss of nitrogen in the form of ammonia gas. To lower the pH so that there is minimum loss of nitrogen right balances of material should be achieved. Campbell cited in role of pH in composting, the moisture of the outer layer of a windrow should be around 60 % so that the ammonia escaping from the centre of the pile is cooled and transformed into less mobile nitrogen compounds (Rynk et al, 1992).
During the initial stages, the ph of the compost tends to fall due to release of organic acids but as the composting process continues, the pH rises due to the beak down of organic acids by microorganisms until the pH approaches neutral pH. The neutral pH is achieved when maturation of the compost has arrived. (Campbell A., 2002)
2.2.6 Electrical conductivity
Electrical conductivity is a measure of soluble salt content from compost. It measures the amount of salt which are soluble in water and can leached out from the compost on addition with water.
2.3 Humic substances
Humic substances are the product of decayed organic matter. Humic substances as defined by Zadow Ryan (2009) is the organic matter that is very stable which has undergone the humification process and is more resistant to microbial degradation.
Humic substances as stated by Davies, G. and Ghabbour, E. A (pp. 19, 2001)
"Humic substances comprise an extraordinary complex, amorphous mixture of highly heterogeneous, chemically reactive yet refractory molecules, produced during early diagenesis in the decay of bio-matter, and formed ubiquitously in the environment via processes involving chemical reaction of species randomly chosen from a pool of diverse molecules and through random chemical alteration of precursor molecules"
The above is stated to be the first principle of humic substances from which according to Davies, G. and Ghabbour, A. E. (pp. 19-20, 2001) five corollaries can be obtained, which are;
Humic substances are devoid of a regularly recurring, extended skeletal entity.
Humic substances cannot be purified in the conventional meaning of purity.
The essence of humic substances resides in the combinations of their molecular heterogeneity and pronounced chemical reactivity.
Humic substances from different sources display a remarkable uniformity in their gross properties.
It is not possible to write a molecular structure, or set of structures, that fully describes the connectivity within the molecules of a humic substance.
The first corollary signifies that there is no skeletal structure that could be assigned particularly to humic substances and the fourth corollary denotes that humic substances are a category of organic material in itself.
There are three types of humic substances that exist which differ slightly in acidity and chemical composition. They are: (Anon, 2008)
Fulvic acid, and
Humic acids are that fraction of the HS which are only soluble in high alkalinity condition; high pH and insoluble in acidic condition. They have high molecular weight and are brown to dark in colour. (Zadow, R, 2009).
Fulvic acids are substances which are soluble in water under any given pH; they remain in solution after removal of humic acid under acidification and are orange to yellow-orange in colour.
Humin are that fraction of humic substances which are insoluble in any conditions and are black in colour.
The figure 2 shows the chemical properties of the three different components of humic substances. As can be seen from the figure, the colour intensity increases from fulvic acid to humic acids to humin and also, there is an increase in degree of polymerisation, molecular weight and carbon content. It suggests that the structure of molecules become bigger from fulvic acid to humic acid and to humin. It also shows that the solubility of molecules, oxygen content and acidity exchange decreases from fulvic aicd to humic acid to humin.
Figure 2: chemical properties of humic substances
Source: http://karnet.up.wroc.pl/~weber/kwasy2.htm (Weber Jerzy, 2010)
Figure 3: Model structure of humic acid
Source: http://karnet.up.wroc.pl/~weber/kwasy2.htm (Weber Jerzy, 2010)
The above structure is a hypothetical model which has free and bound phenolic -OH groups, quinone structures, nitrogen and oxygen as bridge units and carboxylic groups - COOH placed at various locations on the aromatic structure.
Humic and fulvic acids are very reactive. They have an abundance of carboxylic groups and also possess weakly attached acidic phenolic groups which contribute to the complexation and ion-exchange properties of humic substances. Since they possess free radicals, they are able to bind small molecules through hydrogen bonding and non-polar interactions. They also exhibit both hydrophobic and hydrophilic properties enabling them to attach to mineral surfaces.
According to Inbar et al. (1990) in Characterisation of NaOH-extracted humic acids during composting of a biowaste (Veeken et al., 2000), humic acids are mainly produced in the last stage of composting process which is also known as the "maturing stage " which requires from several weeks to a few months.
Humic acids, according to Trubetskoj, Oleg et al. (2007) have some interesting photochemical properties and absorb light which upon irradiation undergo numerous reactions during which radicals or excited states capable of degrading pollutants are produced. This is supported by Zepp et al. (1985) and Kim et al. (1998), Hustert et al. (1999) and Richard et al. (2004) who stated that these naturally occurring absorbing macromolecules can mediate the photochemical transformation and degradation of organic chemicals such as pesticides, herbicides in soils and aquatic environments, cited by Trubetskoj et al. in evaluation of photochemical properties of compost humic-like materials (pp. 5090, 2007).
2.3.1 Past studies related to humic acids in compost
In a study to determine the chemical and physicochemical characterisation of humic acid- like materials from compost, Ouatmane et al, (2002) HA- like materials are produced through humification processes which are initiated during proper composting to partially stabilise raw organic matter. They also found that the amount of HS and HA-like fraction increased confirming a high rate of humification occurring during composting.
In another study, study of the organic matter evolution during municipal solid waste composting aimed at identifying suitable parameters for the evaluation of compost maturity (Alberti et al, 2005), there was an increase in HA concentration specially during the curing phase which was very small as compared to the pre-existing humified fraction and they concluded that the humic acid concentration could not be used alone as an index to estimate the degree of organic matter evolution during composting.
According to Lhadi, E. K. et al., in organic matter evolution during co-composting of the organic fraction of municipal waste and poultry manure, found that the humic acid content increases during composting. Different mixtures of organic fraction of municipal solid waste and poultry manure were used; two different ratio of different mixed of poultry manure and organic fraction of municipal solid waste was prepared for composting.
In another study, Tomati, U., Madejon, E. and Galli, E. (2000) found that humic acid formation was detectable only after 20-30 of composting. They had composted olive-mill wastewater mixed with chopped wheat straw and olive-mill pomace mixed with chopped wheat straw. This study relates to the fact that humic acid formation does not take place the instance composting start but start to form after certain days of composting.
2.4 Heavy metals in compost
Compost is used as an amendment in soil for plant growth, having qualities such as moisture holding capacity among others. However, if the compost is contaminated by any source such as plastics, glasses, metal cans, textile wastes and others, the compost may become unusable for agricultural purposes depending on criteria of pollutants that should be present in the compost. These criteria are specific to countries or states.
Some standards for compost are given in the table below for European countries and the United States.
Table 2: Heavy metals standards in the European region and US
EU- Range (mg/kg)
USA biosolids (mg/kg)
0.7 - 10
70 - 200
70 - 600
0.7 - 10
20 - 200
70 - 1,000
210 - 4,000
Source: Compost quality and standards available from: http://compost.css.cornell.edu/Brinton.pdf (pp.15, 2000)
As can be observe from the different standards regarding heavy metals in compost in different countries, it is important to notice the difference between the standards applied around Europe and the United States. While some standards of the US are in the range of that of the European countries, some are not in the range.
The amount of heavy metals in compost may have detrimental effects on plant growth and development according to Dimambro, Lillywhite and Rahn (2007). They cited from various authors the effect of heavy metals on plants such as copper and zinc content in corn above ground tissues (Paino et al., 1996), cadmium, chromium, lead and nickel accumulation in MSW compost treated vine (Pinamonti et al., 1999) among others.
2.4.1 Studies related to heavy metals evolution in compost
In a study to find the heavy metal distribution in soil and plant in municipal solid waste compost amended plots, Ayari F. et al., (2010) found that there was heavy metals present in the compost that was used. The compost was made from MSW and the amount of heavy metals was as follows; cadmium 5.17 mg/kg, lead 411.5 mg/kg, chromium 78.87 mg/kg, nickel 90.80 mg/kg copper 337 mg/kg and zinc 1174.5 mg/kg. It was also found that there was significant leaching of heavy metals from compost into the soil however, the uptake of these heavy metals tended to be stagnant from the second year.
According to Logan, T.J., Henry, C.L., Schnoor, J.L., Overcash, M., and McAvoy, D.C., (1999) found that the concentration of lead exceeded, in some cases, the standards for heavy metals in compost based on EQ biosolids lead concentration for unrestricted use and US EPA 1990. The concentration of other heavy metals was within the norms for MSW composting.
Ayari, F., Chairi, R. and Kossai, R. (2008) found that during the composting of urban waste there was a reduction of exchangeable metal elements. The total content of heavy metals extracted, mainly for zinc, lead and copper, decreased after 2 months. These elements which are in ions forms at the initial stage of composting undergo complexation with organic matter during composting. Thus, zinc, lead and copper are present mostly in organic fraction during composting than in ion form. Cadmium and chromium also have low exchangeable fraction content since they form complexes in inert matrices of urban wastes.
Castaldi, Paola, Santona, Laura and Meli, Pietro,(2005) found that the concentration of some heavy metals decreased significantly which was in disagreement with results obtained from other authors. They noted that cadmium, copper and zinc concentrations decreased significantly during the first 21 days of the composting process. These decreases were deduced to be from leaching and process of composting used. However, lead concentration did not change much during the composting process and was assumed to be due to bonding of lead ions with organic components which reduced leaching.