Raw and digested municipal waste compost leachate as potential fertilizer: comparison with a commercial fertilizer
Introduction
More than 260 million metric tons of municipal solid waste (MSW) are generated in Europe every year with more than 25% of organic matter (EIONET, 2011). In order to avoid the deposition of the organic fraction of municipal solid waste (OFMSW) on landfill, compost production in tunnels is a current method used in the mechanical and biological treatment plants (MBT plants) (Hyde et al., 2001, Henningsson et al., 2004, Aranda et al., 2012). The main pollution issue associated with this compost treatment is the production of a liquid leachate that is characterized by high levels of salts and NH4–N as well as high organic load (Moreno et al., 2010).
Although municipal waste compost leachate is one of the problems of compost production, it may be considered as a source of nutrients and water. Compost leachate from municipal wastes contains carbon, nitrogen, phosphorus, potassium and trace elements that can be used as nutrients by plants.
The advantages of the use of organic wastes such as compost leachate as fertilizers are evident. Their use would reduce the consumption of commercial fertilizers which need in their production high cost and energy (Hall et al., 1992). Leachate is a waste product, which means that no direct production costs are associated. The use of leachate in agriculture also means that the costs at waste treatment plants can be reduced, since processes for nitrogen and phosphorus removal would not be necessary.
However, due to the nature of the leachates, they may contain, in addition to elements of interest, heavy metals, phytotoxic substances such as ammonia, organic compounds of low molecular weight and/or high salt content (Zhang et al., 2009). From the agricultural point of view, the presence of these substances can mask by negative effects the benefits of leachate (content of organic matter and nutrients).
For an exhaustive analysis about the effects of the incorporation of amendments or products such as leachate to the ground, particularly when products applied are based on the organic matter, content of heavy metals, macro and micronutrients present are needed to be studied, but also their phytotoxicity to predict the behaviour of the organic waste after the application of the amendment.
Precisely this has been the aim of this preliminary study. Compost leachate has been characterized for use as potential liquid fertilizer checking its composition and phytotoxicity. An anaerobic process has been used for leachate treatment in order to reduce the organic load and to improve the fertilizing properties. Results have been compared with results for a commercial liquid fertilizer.
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Sample collection and analysis
For this investigation raw leachate was collected from the compost leachate pool at the MBT plant of Zamora (Spain) (Fig. 1). The samples were stored in air-tight 25 L polyethylene bottles. They were transported to the laboratory and kept at 4 °C prior to analysis.
Samples of raw and digested leachates and commercial fertilizer were analysed for pH (standard method 4500-H+, APHA, 1992) and electrical conductivity (standard method 2510 B, APHA, 1992). Chemical oxygen demand (COD) was determined
Raw leachate
Chemical properties of compost leachate, summarized in Table 1, show a high organic load in COD of the leachate used. This is a result of the large organic matter content of this liquid which may increase the COD value up to 110,000 mg L−1 (Mokhtarani et al., 2012). High content in solids observed is another typical characteristic of compost leachate which presents amounts of solids ranging from 3000 to 50,000 mg L−1 (Pitarch et al., 2007).
The major fraction of the TKN was in form of ammonia (
Conclusions
Experimental comparison of raw and digested leachates with a commercial fertilizer shows that the formers have better fertilizing qualities than the latter despite this is a marketed product. In addition, raw and digested leachates can be considered viable as fertilizers from an economic point of view, according to the obtained results.
This preliminary study proves that raw leachate could be used as potential fertilizer because it meets the requirements for commercial fertilizers: low
Acknowledgements
The authors gratefully acknowledge the financial support of the Junta de Castilla y León (Spain) and Aprovechamiento Integral de Residuos S.L. (Spain). The authors would also like to thank Dr. Carlos García Izquierdo and his team from CBAS-CSIC Murcia (Spain) for technical support on germination assays and Dr. Juan I. Reguera Useros and his team from Burgos University (Spain) for technical support on microbiological assays.
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