Composting versus digestion of biowaste

by W van Zanten, the CADDET Dutch National Team

Two technologies can be used to turn the separated organic fraction of household waste into useful soil improver: composting and anaerobic digestion. An extensive monitoring programme in the Netherlands is comparing these two technologies in detail by studying a sample plant of each type.

Background

Here in the Netherlands our environmental policy aims to minimise the amount of waste we produce. To accomplish this, an order of priority has been set for solid waste management. Highest priority is given to waste prevention followed by (in descending order of preference) re-use, incineration with energy recovery and sanitary landfilling. The implementation of this policy also takes into account the reduction of CO2 emissions. Waste treatment contributes to CO2 emission reduction with 3.5 to 4.5 million tonnes in the year 2000.

In the Netherlands, municipalities have been committed to collecting the organic fraction of household waste separately from other waste fractions since the beginning of 1994. We call this biowaste fraction GFV-waste (Garden, Fruit and Vegetable waste). To comply with the waste prevention policy this waste is processed into GFV-compost. Using the resulting compost as a soil improver achieves re-use of the stable organic material and makes a positive contribution to CO2 emission reduction.

There are two technologies to turn GFV-waste into a soil improver: composting and anaerobic digestion. Both processes meet the waste treatment policy of the Netherlands. As anaerobic digestion also produces biogas, it could contribute further to reducing CO 2 emissions. Several authorities, together with Novem, commissioned an extensive monitoring programme to compare an anaerobic digestion with a composting plant.

The Valorga digestion plant

The Tilburg anaerobic digestion plant uses the Valorga technology and has been in operation since 1994. It is a one-phase digestion system operating under mesophilic (anaerobic) conditions. The design capacity is 50,000 tonnes/year.

Fresh GFV-waste is pre-treated by screening, removal of iron, grinding and manual separation of non-organic components. Then the waste is mixed with already digested material and fed into one of two cylindrical tanks - the digestion reactors. Steam heats the mixture to the process temperature of 37°C and the material stays in the reactors for about 18 days. A combination of a screw-press, a hydro cyclone and a belt-filter-press de-waters the digested mixture. The hydro cyclone also removes sand from the mixture. After maturing and screening, the digestate is ready for use as a soil improver.

Mass balance of the Valorga and the Bühler process.

The Bühler composting plant

The Medemblik composting plant uses Bühler technology. This is an aerated pile composting system, which takes place in closed buildings. Operational since the end of 1990, the total plant capacity of 60,000 tonnes/year is divided over two units.

Pre-treatment of the fresh organic waste consists of crushing , screening and iron separation. The resulting material is mixed with large fragments from the end of the process which are not fully composted. These fragments 'open up' the pre-treated material, allowing air to penetrate. Conveyor belts carry the mixture to the composting building where the conveyor automatically heaps the material. The heap is turned once a week. Composting takes 6&shy;11 weeks during which the heap reaches temperatures up to 70°C. The material is then transported to the post-treatment section at the end of the process. Screens and a hard plastic separator divide the composted material into three organic fractions with various particle sizes (<10 mm, 10 - 40 mm and > 40 mm).

Monitoring

An extensive monitoring programme compared the anaerobic digestion plant with the composting units. A first evaluation took place in August and September 1997; the final evaluation will occur before the end of 1998. The following conclusions are drawn from the preliminary results.

Both plants produce high-quality compost (see table). The compost meets the general requirements for maturity and is free of weed seeds. The anaerobic digestion process produces 305 kg of waste water per tonne of treated GFV-waste, much more than the amount produced by the composting process (120 kg/tonne of GFV-waste). The anaerobic digestion process also produces much more inert material (95 kg/tonne GFV-waste), mainly sand, compared to only 5 kg of inert material per tonne from the composting plant, in the shape of glass, stones, sand, etc.

Despite consuming more than four times more primary energy, the anaerobic digestion plant is a net energy producer. The process produces about 90 kg of biogas, with a content of 55% methane, for each tonne of GFV-waste treated. During the twelve months' evaluations the anaerobic digestion plant produced 366 MJ of net energy per tonne of GFV-waste, whereas the composting plant consumed 261 MJ per tonne of GFV-waste.

Conclusion

One of the Netherlands' environmental goals is to reduce CO2 emissions. When peat is used as a soil improver it produces about 2 kg of CO2 as it degrades. The organic component of compost can substitute for dry peat as a soil improver. In this way, both composting and anaerobic digestion can contribute to reducing CO2 emissions. Taking into account the organic part in the compost production and the net energy consumption, the total CO2 reduction can be calculated as 173 kg CO2/tonne GFV-waste treated for the anaerobic digestion plant in Tilburg and 158 kg/tonne for the composting plant in Medemblik. This gives a favourable outcome for the Valorga anaerobic digestion over the Bühler composting technology for mitigation of CO2 emissions.

For more information contact the CADDET Dutch National Team in Sittard.

 

Anaerobic digestion Tilburg1

Composting Medemblik2

Dutch Regulations3

Community Eco-label

Heavy metals (mg/kg dry mass)

       

Chrome

40

32

50

140

Nickel

15

14

20

50

Copper

29

37

60

75

Zinc

169

164

200

300

Cadmium

<0.5

<0.5

1

1.5

Mercury

0.16

0.10

0.3

1

Lead

61

87

100

140

Arsenic

2.8

4.0

15

7

Macro contaminants (% d.m.)

       

Plastic, metal, rubber >2 mm

0.62

0.09

<0.2

-

Glass >2 mm

0.07

0.02

<0.02

-

Stones > 5 mm

1.18

0.12

<2

-

Glass > 16 mm (detected yes/no)

3 yes/5 no 0

1 yes/11 no

0

-

1 Results are the average of eight values
2 Results are the average of eleven values
3 Regulations set in the KIWA guideline BRL K256/02 and the "decree other organic fertilisers (BOOM)"

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.