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Possibilities of aerobic and anaerobic biological treatment of organic waste (1998)

Antonín SLEJŠKA

The decision on what type of treatment will be used depends on:

1. The type of waste,
2. The kind of assortment,
3. The amount of waste,
4. Economy of the whole process.

Ad 1: The most important properties of waste for biological treatment are:

• C/N, content of N, P, K and other nutrients and content of toxic substances (mainly of heavy metals),
• Presence of microbes and hygienic danger,
• Porosity, water content, volumetric weight and other physical properties.

According to properties of biowaste can be decided which technology is the best. Then can be optimised parameters of the process. Unfit properties of the material can be balanced by addition of material which has opposite properties. In this way can be optimised C/N, content of nutrients, presence of aerobic / anaerobic microbes and physical properties. If You want compost some material with low porosity, You can add e.g. straw, but not more than 5% because it significantly increase volume of the mixture.

When are some hygienic problems presupposed, then should be used technologies with higher temperatures; that is bioreactors for composting (usually ~70°C, 1 week) and thermophilic (55°C) process for AD. If there is no hygienic risk, as during AD of plants, we can use mesophilic (35°C) process, which is more stable, produce approximately the same amount of methane and doesn't consume so much heat.

In some European countries are these requirements for sanitation of material during composting (ORCA 1992¹,ČSN 46 5735²):

Generally is the process of composting influenced by temperature in this way:

Ad 2: There are these kinds of assortment of municipal waste:

1. No assortment - results in necessity of waste-dumping or incineration. These methods of waste treatment are not usable for longer periods of time.
2. Mechanical assortment - is operated at the area of waste treatment plant. This method enable some recycling, but the organic product from AD and/or composting should not be used as a fertiliser, because it will probably posses to high contents of toxic substances mainly heavy metals.
3. Assortment by dwellers - is the only way how to obtain good organic fertiliser after AD and/or composting. Organic waste can be assorted by several ways:
4. 1. Classical bins for biowaste, which are periodically emtied. The biowaste is collected on a place where it is treated by AD or composting.
   2. Biowaste can be collected by a sewage system and then anaerobically digested.
   3. Composting of biowaste can be operated for several blocks of flats in a composting bin which is served by a person.
   4. Treatment in family sized reactors.
   5. 1. Dwellers can operate their own vermicomposting reactors on their balconies,
      2. composting reactors on their gardens,
      3. anaerobic digesters for treatment of excreta from their animals, toilet and of organic residues. These are popular mainly in developing countries.

Ad. 3: The amount of waste is important in particular for AD. The minimum amount of substrate needed for AD depends on its type. Generally we can say that in developed countries with high hygienic and safety requirements is the minimum amount of biowaste 5000 t/year (it is biowaste from a city with 50 000 - 70 000 dwellers). Composting can be operated in all scales.

Ad. 4: Investment and operation is cheaper for larger plants, but the transport is cheaper for smaller plants (on a unit of material - ton or m³). In the cost of plant should be calculated possible environmental impacts (cost of remediation). In countries with low wages is better to use more manual force than mechanical. Sometimes is possible to use prisoners who can be motivated also by reduction of their prison term.

Our current research and projects

In our laboratory we now try to find the best way of AD of grass. Our current conviction is that grass should be digested by a two stage mesophilic process with liquid recirculation. Recirculation of liquid phase serves as inoculation and saves heat. Process can be intensified by addition of some elements, e.g. Co (Jarvis et al. 1997).

We also design composters for specific purposes and test composting technologies.

Literature

ČSN 46 5735: (1991): Průmyslové komposty. Vydavatelství norem, Praha.

GORODNIJ, N.M.; MELNIK, I.A.; POUCHAN, M.F.: (1990): Biokonversija organičeskych otchodov v biodinamičeskom chozjajstve. Urožaj, Kijev, 254 p.

HARTENSTEIN, R.; BISESI, M. S.: (1989): Use of earthworm biotechnology for the management of effluents from intensively housed livestock. Outlook on agriculture, 18 (2), pp. 72-76.

JARVIS, A.; NORDBERG, A.; JARLSVIK T.; MATHISEN, B. and SVENSSON, B.H. : (1997): Improvement of a grass-clover silage-fed biogas process by the addition of cobalt. Biomass & Bioenergy 12, p. 6.

KARKI, A.B. and DIXIT,K. (1984): Biogas Fieldbook. Sahayogi Press, Tripureshwar, Kathmandu, Nepal, 87 p.

MEYNELL, P.J. (cit. Stafford et al. 1981, p. 162): (1976): Methane: Planning a Digester, Prism, Detroit, 1976.

ORCA: (1992): A review of compost standards in Europe. ORCA technical publication No 2, published by ORCA, Brussels, December 1992.

STAFFORD, D.A.; HAWKES, D.L. and HORTON, R.: (1981): Methane production from waste organic matter. CRC Press, Florida, 285 p. - (p.113-114).

VÁŇA,J.: (1994): Výroba a využití kompostů v zemědělství. Institut výchovy a vzdělávání MZe ČR v Praze, Agrodat, 40 p.

ZAJONC, I.: (1992): Chov žížal a výroba vermikompostu. Animapress, Povoda, okres Dunajská Streda, 59 p.