Introduction

The loss of soil fertility, the high mineral fertilizer prices, the decrease of soil organic matter, and the insufficient nutrient supply in plant production in the developing countries declare the need for alternative fertilizers. The high amount of household waste both in developed and developing countries is an actual problem. Composting allows organic waste to be recycled and returned to the soil as fertilizer and provides a solution for managing much of waste. Besides, compost can be used in landscaping, horticulture, and agriculture as a soil conditioner. It is also useful for erosion control, land and stream reclamation, wetland construction, and as landfill cover. Compost application is a very actual task considering the EU soil protection priorities, where one of the most important priorities is the improvement of organic matter content of soil. Compost serves as a growing medium: a porous, absorbent material that holds moisture and soluble minerals, providing support and nutrients in which most plants will flourish. To maximize the fertilizer effect of compost it can be useful to dilute compost with soil or peat to reduce salinity or to add neutralizers to bring the pH closer to 7, or additional nutrients like fertilizers or manure, wetting agents, and materials to improve drainage and aeration, such as sand, grit, bark chips, vermiculite, perlite, or clay granulates. Composting of municipal solid waste (MSW) and its subsequent application to agricultural land is gaining popularity because of the environmental concerns associated with the disposal of these materials in landfills. Several studies have shown that the use of MSW compost in agriculture has many benefits to soil, crops, and environment (Hortenstine and Rothwell 1973; Maynard 1995; Hicklenton et al. 2001; Rodd et al. 2002). Compost influences plant growth and health indirectly via the grooving conditions, by providing nutrients, especially micronutrients and by improving soil conditions and water retention capacity. Composts are not inert materials; they are carriers of living organisms. If the fermentation is correctly managed, pathogens are killed during the heat period (Bollen 1993; Engeli et al. 1993). At the same time, antagonists develop during maturation of the compost. Therefore, composts can reduce the incidence of variant plant diseases (Fuchs 1995, 2002; Hoitink et al. 1997).

Composting has a long history in connection with the animal manure management, but the modern composting originates in the organic farming in the early twentieth century. In organic farming, the mineral fertilizers are not allowed to use, but compost as an organic fertilizer has high importance. However, the more recent application of composting is the large-scale waste reduction. Because of the European Landfill Directive from 1999 the countries of the European Union have to meet specified targets for landfill reduction, principally by establishing alternate disposal and treatment of organic materials. Today, composting has three important aspects: (i) mechanical-biological treatment of biowaste, (ii) management of animal manure and plant residues, and (iii) home composting.

Recycling is the reprocessing of materials into new products. It prevents generally the waste of potentially useful materials, reduces the consumption of raw materials and reduces energy usage, and hence greenhouse gas emissions, compared to virgin production. Recycling is a key concept of modern waste management and is the third component of waste hierarchy. Recyclable materials may originate from a wide range of sources including the home and industry. They include glass, paper, aluminum, asphalt, iron, textiles, and plastics. Biodegradable waste, such as food waste or garden waste, is also recyclable with the assistance of microorganisms through composting or anaerobic digestion. Recyclates are sorted and separated into material types. Contamination of the recyclates with other materials must be prevented to increase the recyclates' value and facility. This sorting can be performed either by the producer of the waste or within semi-or-fully-automated materials recovery facilities. Recycling is beneficial in two ways: it reduces the inputs (energy and raw materials) to a production system and reduces the amount of waste produced for disposal. For economic, agricultural, and environmental reasons, composting is frequently used for organic waste recycling (Lemunier et al. 2005).

Biodegradable waste is a type of waste, typically originating from plant or animal sources, which may be broken down by other living organisms. Waste that cannot be broken down by other living organisms may be called nonbiodegradable. Biodegradable waste can be commonly found in municipal solid waste as green waste, food waste, and biodegradable plastics. Through proper waste management, it can be converted into valuable products by composting or energy by waste-to-energy processes, such as anaerobic digestion and incineration. Composting converts biodegradable waste into compost. Anaerobic digestion converts it into several products, including biogas and soil amendment (digestate). Incineration as well as biogas can be used to generate electricity and/or heat. Biodegradable waste can be recycled into useful materials by biological decomposition. There are two mechanisms by which this can occur: The most common mechanism of recycling of household organic waste is household composting or municipal curbside collection of green wastes sent to large-scale composting plants. Alternatively organic waste can be converted into biogas and soil improver using anaerobic digestion. Here organic wastes are broken down by anaerobic microorganisms in biogas plants. Biogas can be converted into renewable electricity or burnt for environment-friendly heating. Advanced technologies, such as mechanical biological treatment, are able to sort the not-recyclable elements of the waste out before biological treatment by composting, anaerobic digestion, or biodrying.

The collected and treated amounts of organic material differ much in the European Union (EU) countries. Around 35% of the estimated 49 million tons biowaste is presently collected separately (ECN 2008). In Germany, 30% of the household waste insists of biowaste and between 1990 and 2004 the highest increase is also documented for biowaste: from two million tons in 1990 to around eight million tons in 2004. The German Federal Compost Quality Assurance Organisation (BGK) assumes the amount in 2007 to 9-10 million tons, but in this year alone six million tons biowastes were delivered to composting plans connected to the BGK.

There are three disposal ways of the biowastes:

• Composting and digestion

• Incineration or mechanical-biological treatment with the aim to landfilling

• Landfilling of untreated biowastes

In Germany, 60-70% of the biowastes are collected separately and recycled by composting or digestion, but in the EU-15 countries only 6% and 22% are incinerated, and 72% is landfilled (Kehres 2008). For composting the selective collection of waste is important because of the separation of biowaste.

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