Concentración de metales en hueso de lobo marino

digestate, compost and vermicompost, which were the focus of this research. The production mechanisms and the properties of each of these soil amendments are detailed in the following sections.

Digestate

The anaerobic digestion process

Anaerobic digestion is a microbial degradation process made up of a succession of stages involving different bacteria in an environment free of oxygen. Some of the bacteria involved in the process are methane-producing bacteria (Pfeffer, 1980). The resulting product is biogas, a methane-rich gas which can be used in similar ways to natural gas. Digestate is the other product of the process, which contains all the nutrients present in the raw material before digestion and

therefore constitutes a valuable fertiliser. Nutrients are broken down from the original organic matter through the chain of reactions that occur during anaerobic digestion, the components of the initial organic material become degraded into their building block basic components (Pfeffer, 1980). Nutrients end up in their most soluble form at the end of the digestion process, readily available for plants.

Nutrient content of digestate

During the digestion process no nutrients are lost, only the chemical form they are present in changes (Möller and Müller, 2012; Loria and Sawyer, 2005) compared the nutrient content of raw and digested swine manure these were found to be statistically identical and their long-term effect on soil was found to also be identical (Loria and Sawyer, 2005).

The process of anaerobic digestion reduces the concentration of organic N because it becomes mineralised in the digestion process. Kirchmann and Witter (1992) compared the ammonium content of fresh, aerobically and anaerobically decomposed animal manure and found that only anaerobically digested waste contained more than half of N in the form of ammonia, the most readily available form for plants. Digestate can therefore behave in similar ways to chemical fertilisers because the nutrients are in their most soluble and chemically available form. Furthermore Morris and Lathwell (2004) reported that the application of digestate yielded better growth of maize than chemical fertiliser in the early stage of crop growth.

Nutrients are present in mineral form after anaerobic digestion so it is hypothesised that this type of amendment is more susceptible to nutrient leaching than other organic amendments (Nkoa 2014). Chantigny et al. (2008) concurred with this hypothesis after growing maize on plots treated with swine manure digestate and plots treated chemical fertilisers and obtaining similar corn grain yields and grains with similar N and P content on all plots. Ammonia can easily volatilise if not stored in a closed environment, storage conditions and application rates for digestate therefore need to be carefully controlled to avoid N losses; it is even recommended to apply digestate in several doses to avoid nutrient leaching in soil (Smith et al., 2014). The origin of anaerobic digestate substrates

also directly affects the composition of the digestate obtained and so there is a higher risk of heavy metal contamination from digestate use than chemical fertilisers (Nkoa, 2014). There are few studies that have been carried out on the fertiliser value of digestates and as Möller and Müller (2012) point out, more detailed investigations are required to characterise the nutrient content of digestates with different feedstocks and their effect on soil.

Compost

• Overview of the composting process

Composting is an aerobic degradation process; it is a spontaneous microbial exothermic process involving a wide range of microorganisms and small invertebrates. The process consists of 4 successive stages that yield carbon dioxide, water and a humus-like material rich in organic matter which constitutes a valuable soil amendment. The temperature during the composting process initially rises rapidly and spontaneously through the microbial activity of the bacteria consuming the readily degradable material such as sugars and protein.

Thermophilic microorganisms then become predominant once the temperature in the pile has reached between 50-70^oC degrading more complex materials such as fats, lignin and cellulosic materials. A reduction in temperature then takes place giving rise to a recolonization of the medium by mesophilic organisms and further degradation and stabilisation of the organic matter (Diaz et al., 2011).

During these successive stages organic compounds are oxidised, nutrients are released and immobilised and new compounds are synthesized through microbial action (Insam and de Bertoldi, 2007).

• Compost operating parameters

Several operating parameters need to be controlled for an optimal composting process, namely pH, Carbon to Nitrogen ratio (C/N), moisture and porosity. One of the main factors contributing to the efficiency of composting is the C/N of the initial organic material; the initial balance of C and N concentration in material prior composting should be between 20 and 30 in order to provide the appropriate

ratio of nutrients to the composting microorganisms during their growth. pH should be maintained between 6 and 7.5 to maintain a good operating environment for microbes and prevent N volatilisation which occurs if the pH is above 7.5 (Diaz et al., 2011). Moisture is needed for bacterial growth and an optimal range is between 40 and 60% by weight; excess moisture inhibits oxygen transfer and anaerobic conditions can develop as a result inhibiting the composting process. Porosity in the composting pile is important for ensuring good aeration and oxygen transfer and the free pore space should ideally be between 35 and 50% (Bernal et al., 2009).

• Compost nutrient content

The qualities of compost as a soil conditioner are known, its application leads to an increase in the organic matter and nutrients are provided through slow and gradual release (Insam and de Bertoldi, 2007). The latent nutrient benefits of compost are sometimes seen as a drawback by farmers, especially in poorer countries where farmers expect to see fast results from their investment. Several researchers therefore recommend enriching compost with chemical fertilisers providing additional nutrients in order to optimise crop yields after compost application (Adamtey et al., 2009; Useni et al., 2013).

One of the advantages of compost application is that nutrients are present in organic forms and therefore less prone to leaching. Maynard (1993) found that nitrate leaching to ground water occurred with compost applications (average 3.4 mg.L^-1nitrate concentration in water) but nitrate concentrations in ground water were higher beneath control plots fertilised with chemical fertilisers (4.2 mg.L^-1), concentrations of groundwater nitrate remained under the regulatory limits with both treatments. Basso and Ritchie (2005) however reported higher mean annual nitrate leaching from plots treated with compost (35 kg NO3–N ha^-1in alfalfa–

maize and 30 kg NO3–N ha^-1 in maize–alfalfa) than those treated with chemical fertilisers (33 kg NO3–N ha^-1 in alfalfa–maize and 25 kg NO3–N ha^-1in maize–

alfalfa) in a 6 year experiment growing maize and alfalfa in rotation.

Compost production and application to soil has also been used as a measure for soil carbon sequestration in the context of climate change mitigation. Increases

in soil carbon concentration have been measured after repeated applications of compost over several years (Eghball, 2002; Monaco et al., 2008; Kukal, Rehana-Rasool and Benbi, 2009; Sodhi et al., 2009). There is however evidence of greenhouse gas emissions (carbon dioxide and nitrous oxide) during composting, which could counteract the climate change mitigation benefits of compost application (Hao et al., 2004).

Vermicompost

• Vermicompost process overview

The first commercial application of vermicomposting was developed in the UK in 1982 and has now spread worldwide for transformation of a wide range of organic materials using several species of earthworms (Edwards et al., 2011).

Vermicomposting is a degradation process with similarities to composting, which utilises earthworms, usually Eisenia Fetida, to degrade organic matter. Organic matter is digested jointly by worms and microorganisms and produce vermicompost, a humic-like substance with finer structure than compost. The microorganisms biochemically degrade the organic matter but the worms are instrumental in aerating and fragmenting the substrate and therefore increase microbial activity (Edwards et al., 2011). Unlike compost, vermicompost occurs at ambient temperature, although these need to be close to mesophilic (10- 32^oC) for worm optimal worm activity and survival (Edwards et al., 2011). Stabilised vermicompost should have a pH ranging between 5.5 and 8, moisture between 30 to 50%, organic matter content be greater than 20-25%, and C/N ratio below 20. Nutrient contents of vermicompost depend on their parent material but as an indication, Total N can range from 0.1 to 4%, NH4-N should not exceed 10% of Total N, and P concentrations higher than 0.5% are desirable (Edwards et al., 2011). Vermicompost is said to have advantages over compost, such as heavy metal reduction through assimilation by the worms (Atiyeh et al., 2000; Pereira et al., 2014). Nutrients are more readily available for plants in vermicompost because the process achieves a higher degree of mineralisation; it has been

shown for instance to have a higher concentration of nitrates than compost (Atiyeh et al., 2000; Sinha et al., 2010).

• Effect of vermicomposting on soil and crops

Several studies have shown the beneficial effects of vermicompost on the productivity of several crops such as tomato (Atiyeh et al., 2001; Gutiérrez-Miceli et al., 2007), aubergine (Gandhi and Sundari, 2012), rice (Kale et al., 1992) and pepper (Rodríguez-Canché et al., 2010). Atiyeh et al. (2000) however found that excess application of vermicompost lead to reduced yields and growth of tomato plants and Roberts et al. (2007) reported that the addition of vermicompost to soil did not significantly increase fruit number, weight or yield of tomatoes. Arancon and Edwards (2011) produced vermicomposts from a range of organic materials (paper waste, cow manure, food waste) and tested their effect on a range of crops (tomatoes, strawberries, grapes and peppers) and showed yields from plots treated with vermicomposts were higher than those treated with chemical fertilisers but they also showed that the properties of vermicompost depend on the parent material and their effect varies depending on the crop type. It has also been shown that vermicompost can have disease preventing properties for crops through the action of bacteria and funghi present in vermicompost as well as promoting the microbial activity in the soil (Szczech, 1999; Masciandaro et al., 2000; Gutiérrez-Miceli et al., 2007). Sinha et al (2010) also claim that the lower operating temperatures of vermicomposting prevent ammonia volatilisation and hence allow for lower N losses and lower emissions of Nitrous oxide (a much more powerful greenhouse gas than carbon dioxide) than during the composting process. Frederickson and Howell (2003) however found that vermicomposting systems had the capacity to emit high levels of nitrous oxides, which were comparable to those from other waste processing methods. Nigussie et al. (2016) combined composting and vermicomposting and showed that the addition of a vermicomposting stage significantly reduced N losses and greenhouse gases emissions from composting. Worm feeding ratio during vermicomposting was also shown to have an impact the NO2emissions during treatment, higher feeding ratios lead to higher nitrous oxide emissions (Nigussie et al., 2017)

The potential of fertiliser production from human excreta

A wide range of end products can be obtained from human excreta depending on the treatment applied on FS. These include direct products such as fertilisers, biofuel and building materials as well as products indirectly derived from excreta such as fish and plants, protein material (Kengne, et al., 2014). The processes to produce organic fertilisers include anaerobic digestion, composting and vermicomposting.

2.3.1 Risks associated with human excreta reuse