Nitrogen transfer from grey municipal solid waste to high quality compost
Introduction
In tropical areas, clearing of wood and plantations induces ruptures of nutritional and biological equilibrium through the destruction of soil organic matter and soil protection. Fertiliser shortage is principally a restitution problem, mostly for N, P, K, Ca and Mg (De Leenheer and Waegemans, 1970). Inputs of nitrogen into the agricultural system may be in the form of N-fertiliser, or be derived from atmospheric N2 via biological N2 fixation Atlas and Bartha, 1992, Dick and McCoy, 1993, Peoples et al., 1995. As nitrate is subject to leaching in soils, it should be dosed to cover the immediate needs of crops. Organic residues can contribute to slowly available N throughout the year. They can furthermore improve the level of other essential nutrients, the soil water retention capacity, the Cation Exchange Capacity (CEC), the pH and the structural stability. Mature compost directly affects the soil pH and the supply of mineral nutrient elements, and is thus expected to increase the fertility of a soil to which the compost is applied (Kostov et al., 1996). Moreover, it does not have some of the disadvantages of chemical fertilisers such as potential burning effects and high salt content (Jarvis, 1996). Due to high costs of industrial fertiliser in certain tropical areas alternative solutions to increase the N availability for the crop and soil fertility have to be explored.
The dry anaerobic treatment of municipal solid waste with the Dry Anaerobic Conversion (DRANCO) process produces a N-rich residue (Gellens et al., 1995). The process is an anaerobic solid-state fermentation of concentrated substrates. It produces both energy in the form of biogas and a humus-like end-product called humotex.
A new concept is presented in Fig. 1. The concept is based on four process steps: DRANCO digestion, NH3 exchange coupled to nitrification in the recipient compost, air drying and finally incineration. Moreover, the concept is based on the principle that one collects biowastes at source and produces from the latter high quality compost. In addition to that, one collects grey wastes, digests these to produce biogas and recovers the NH3 they contain. Hence, one has two separate lines of solid waste treatment which complement each other.
In the case where heterogeneous waste (defined here as grey municipal solid waste) is used for biogas production, the remaining paste after anaerobic digestion exhibits an inherent risk of containing a high concentration of heavy metals, and is thus of low quality for agricultural soil conditioning. However, the contained ammonia can be recovered from the residue. For example, after mixing the material with CaO or ash, the NH3-N is set free and can be transferred to a mature nitrifying compost. The ammonia-free cake can be air-dried and used as a type of Refuse Derived Fuel (RDF) for heat production. The N-enriched compost can be used as soil fertiliser. The ash obtained after burning the dried cake serves again to start a new cycle of the treatment. The whole recycling process is referred to as the `Multiple Integrated Reuse Of Refuse (MIROR)' process, as shown in Fig. 1. It especially fits the socio-economic conditions prevalent in non-industrialised regions, where the availability of industrial fertilisers is limiting and separation at source of garbage is not possible.
The objectives of the study of the proposed MIROR process were :
- 1.
To evaluate the possibility of obtaining a NO3− enriched compost by treating a conventional vegetable–fruit–garden (VFG) compost with ammonia released from DRANCO wet product;
- 2.
To study the nitrogen transformations in the VFG compost;
- 3.
To investigate the possibility of obtaining NH3 release when using the ash after incineration of the dried anaerobic compost. The latter could be an alternative way to reduce the costs related to CaO addition.
Section snippets
DRANCO and VFG compost
The wet anaerobic material (ca. 2% DM) was obtained from a full-scale anaerobic digester treating waste at thermophilic conditions (55°C) (Six and De Baere, 1992).
The high quality aerobic compost was obtained from an aerobic composting plant (VLAR, Grimbergen, Belgium) treating VFG waste (Maricou et al., 1998). The VFG compost originated from source separated domestic waste. The compost contained 42–60% DM on wet weight basis at the beginning of the experiment.
NH3-N transfer from DRANCO paste to a mature VFG aerobic compost
Three kg of fresh mature VFG
Ammonia transfer from anaerobic paste to aerobic compost
The aerobic compost lying on the anaerobic compost showed a factor 4 greater mineral N content than the control. The evolution of the NH4+–N and the NO3−–N during 120 days is plotted in Fig. 3 for the treated compost and for the control. The treated aerobic compost had an increase of NH4+–N from 2.75 to 288 mg/kg DW during the five days of contact. NO3−–N in the treated compost increased to a value of 967 mg/kg DW after 120 days. The NH4+–N concentration decreased below detectable limits. In
Conclusion
N-enrichment of compost can be incorporated in a recycling scheme labelled as `Multiple Integrated Reuse of Refuse (MIROR)'. The new approach is proposed to produce actively nitrifying compost. The treatment presented in this work has two advantages:
- 1.
Recovery of energy (biogas, RDF) and NH3 from MSW.
- 2.
Making high quality N-enriched compost from source selected bio-wastes. The latter is a convenient source of N for crop growth. It provides opportunities for administration of plant nutrients,
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