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Newsletter and Technical Publications
<Municipal Solid Waste Management>

Sound Practices
Composting

1.4.1 Introduction

Composting solid waste for use as a soil amendment, fertilizer, or growth medium is important in many countries. Asian countries in particular have a long tradition of making and using compost. In Western Europe, a range of modern technologies is used to produce compost.

At the same time, composting has the distinction of being the waste management system with the largest number of failed facilities worldwide. In cities of developing countries, most large mixed-waste compost plants, often designed by foreign consultants and paid for by aid from their home countries, have failed or operate at less than 30% of capacity.

The problems most often cited for the failures of composting include: high operation and management costs, high transportation costs, poor quality product as a result of poor pre-sorting (especially of plastic and glass fragments), poor understanding of the composting process, and competition from chemical fertilizers (which are often subsidized). In many urban places, collection systems are too unreliable for urban authorities to consider running composting facilities efficiently.

But these differing and sometimes conflicting explanations miss the central issues in compost failures and successes, and leave some questions unanswered:

  • When there is so much small- and medium-scale composting operating successfully in India and China, why are there still so many failures in other places?
  • Why is centralized composting a successful, cost-effective, environmentally sound waste management approach in Europe (and increasingly in North America), when it has not fared as well elsewhere?

The answers to these questions are key to understanding what constitutes sound practice in the use of composting as a waste management approach.

What causes compost systems to fail

Compost systems have failed for economic and technical reasons. What these failures have in common is a failure to understand the role of composting as part of an overall waste management system.

Economic failure
Many compost plants have failed for economic reasons, related either to the ability to secure waste or to the need to market the compost that is produced.

Failure to secure waste. In many parts of Asia, where there is a long tradition of successful composting, the availability of inexpensive MSW disposal in dumps or landfills does not seem to impede composting. Composting takes place both informally and in an organized fashion.

In much of Latin America and Africa, however, efforts to organize composting have failed to secure enough waste. When dumping or landfilling is inexpensive and not subject to effective environmental controls, composting is relatively expensive. In Europe, where landfilling is subject to controls and fees and land is very limited, composting is much more attractive. Furthermore, European political culture gives government a monopoly over the waste stream, so a policy decision to give composting a priority over landfilling can force waste to a composting facility even when it is not cost-competitive. (With increasing privatization of waste management services in Europe, this may change.)

Marketing failure. The second economic failure is on the marketing end. Finished compost can become, but is not automatically, a valuable commodity: its value depends on external demand for soil enhancers, on perceptions of its value, on its quality, and on its accessibility to potential users in the immediate vicinity. It also depends on what alternatives to compost are available to farmers and cultivators in the region, and on the cost of those alternatives from chemical fertilizer to wastewater sludge relative to the cost of the compost.

  • Compost marketing works when:
  • the farmers or gardeners are located close to the source of the compost;
  • the entity producing the compost is willing to transport it to the users; and
  • the compost is priced below the price of commercial fertilizers, or is given away.

Technical failure
Composting has experienced two kinds of technical failures: first, a failure of the mechanical systems that manipulate waste streams before composting itself begins, and secondly, a failure of the decomposition process itself, largely attributable to failure to create the environment for the biological process to thrive.

On another level, the failure of composting technology is a failure of the waste management sector to understand the nature of the waste stream or the biological composting processes, and to attempt to solve problems with over-designed machines.

Failure of mechanical pre-processing. The technological failure of composting is primarily a failure of the mechanical pre-processing systems, and not of the biological composting process itself. Biowaste composting facilities have generally relied on complex mechanical pre-processing to remove non-compostables. These systems have by and large failed at their tasks. It is an open question as to whether there is any mechanical system which could ever adequately identify and separate all of the materials that occur in mixed waste, but no existing systems do this sufficiently to ensure good compost quality.

Pre-processing techniques based on manual separation aided by human eyes and hands have consistently produced the best compost in developing countries, and often in industrialized ones as well. There are small-scale biowaste composting facilities in both industrialized and developing countries that are successful because of the high degree of manual pre-processing. The larger facilities dependent on mechanical separation cannot accommodate the diversity of the waste stream.

High organic content is essential. While many biowaste composting facilities are failures, the great preponderance of source-separated composting systems are successes. Yard, garden, restaurant, and market waste composting projects quietly thrive in every corner of the globe. The biological composting process is so basic that it is very likely to succeed if there is an appropriate input stream and proper handling.

In developing countries, the high animal and vegetable waste content of the waste stream, combined with existing materials recovery systems, means that the mixed waste stream is sufficiently compostable to produce good compost at a small or medium scale. Support and enhancement of existing materials recovery activities and (where otherwise reasonable) limitation of new types of packaging can maintain the compostability of the waste stream and result in the production of good quality compost.

Failure of biological processes. Where there is a failure in the composting process itself, this relates to the failure to understand the nature of biological processes. Compost bacteria, insects, and microorganisms require certain environmental conditions to thrive. If these are absent or interrupted, they must be corrected.

Critical lessons in sound composting practice

This analysis of compost system failure yields the guidelines for sound composting practice that are listed in the accompanying box. Each of these points is discussed below.

a. The material to be composted must be compostable in order to produce a marketable product:

  • Both large- and small-scale systems can work well with highly compostable waste streams. In industrialized and transition countries, this will usually require source separation. In developing countries, the same degree of compostability can be accomplished by facilitating the recovery of non-compostables and reducing the introduction of new packaging into the waste stream (bearing in mind that the public health benefits of good packaging are significant).
  • In most cases (a) analyzing the waste stream and (b) if necessary, designing the separation protocol and separate collection system are as important to the success of composting as the selection of the technique itself.

b. Mechanical pre-processing of mixed solid waste does not work well enough in most cases; therefore:

  • large-scale systems should be limited to source-separated streams unless the waste stream is already highly compostable and contaminating materials can be manually removed.

c. Manual pre-processing of mixed waste does work on a small to medium scale for the highly compostable waste streams in developing countries, but also in very small projects in industrialized countries; therefore:

  • manual or manual-assisted processing is the soundest approach to biowaste composting that can be sustainable over the long term in a technical sense;
    However,
  • manual processing may not be either pleasant or safe for workers.

d. The economic viability of composting depends on three factors; failure of any of the three can cause the system to fail:

  • In the absence of a tradition of composting, landfilling or dumping must be controlled and sufficiently expensive to make the moderate cost of composting (US$20 - 40 per ton) competitive with the cost of dumping.
  • There must be a market or use for the compost at the quality it is produced. This market does not have to produce net income, but it has to be factored into the cost of composting as a positive or negative. The closer the market is, the more likely composting is to be sustainable.
  • The waste streams composted have a large effect on the quality and marketability of the compost. Support and enhancement of materials recovery of non-compostables is a necessary step in many developing countries, and manual picking and final screening can help. In industrialized and transition countries, the emphasis in sound practice is on source separation.

e. Technical viability depends on three factors:

  • There must not be excessive dependence on mechanical pre-processing.
  • The scale of composting must not be too large. In general, the more complex the input stream, the smaller must be the scale to ensure proper composting process and a good product.
  • The entire system from separation and collection to final screening must be designed together to deliver the appropriate input streams and to support the biological processes of the bacteria and other organisms.

     

    Guidelines for sound composting practice
    • The waste stream must be compostable.
    • Mechanical pre-processing of mixed solid waste does not work well enough in most cases.
    • Manual pre-processing of mixed waste does work on a small to medium scale for the highly compostable waste streams in developing countries.
    • Economic factors related to landfills, markets, and materials recovery (see text) must support composting.
    • Separation and composting techniques and scale must be appropriate for the input stream.

 

BacteriaÕs central role in composting

Composting is the biological decomposition of complex animal and vegetable materials into their constituent components. Composting is a natural process of bacteria and other organisms eating what they like in a favorable environment. The most common form of composting, aerobic composting, takes place in the presence of oxygen. Aerobic bacteria require a mix of approximately one part nitrogen to at least 30 and no more than 70 parts carbon in their food supply. Aerobic bacteria also require at least 40% but not more than 60% water in their environment, and a plentiful supply of oxygen. In the absence of any one of these four factors, the composting process will fail. The products of aerobic composting are steam, carbon dioxide, and decomposed organic material, called humus.

Anaerobic composting, also called anaerobic digestion, takes place in the absence of oxygen. Anaerobic bacteria live in the absence of oxygen and can consume mixtures with a higher proportion of nitrogen and lower proportion of carbon. Anaerobic digestion can also occur at higher levels of moisture. The products of anaerobic digestion are methane gas and decomposed organic material. To recover the gas, anaerobic systems are enclosed in a pressurized environment.

Composting bacteria operate on the surfaces of compostable materials. That means that composting works well with small particles of waste and poorly with large pieces of organic material. For this reason, size reduction or shredding is frequently required prior to composting to allow for adequate bacterial decomposition.

All solid waste composting is based on one or both of these biological processes. Differences in technology relate to input materials, pre-processing techniques, and the way in which the environment for bacterial action is created and maintained, but not to the composting process itself.

Composting is the only solid waste management technology that depends on bacteria for its correct functioning (wastewater treatment also depends on bacterial action, but it is not a solid waste system). The task of maintaining the correct environment for bacteria requires significantly different areas of knowledge and different management strategies than operating a collection system or incinerator or running a landfill. This is important to consider in any analysis of the success or failure of composting systems.


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