Composting, AD and MBT

Composting

We can design and manage the construction of composting facilities, see www.compost.me.uk.

AD

Anaerobic digestion is set to increase over the next few years as food wastes start to be collected. For a beginner's guide see www.anaerobic-digestion.com.

MBT

Useful information is available from http://www.compostinfo.info/

How to interpret compost analysis

In the UK, peat-based compost has traditionally been extracted with water on a 1:6 v/v basis. New EU methods are to be followed which are based on a 1:5 ratio. Interpretation needs to be modified accordingly, but some research work needs to be done to compare the results. The PAS100 standards are based on the new EU methods. Other methods of measuring the availablity of plant nutrients may be better than water extraction, such as CAT, which is recommended for compost to be used in growing media, but research needs to be carried out to compare extractants.

Bulk density

Green waste compost has a relatively high bulk density, between 250 (low) and 750 g/l (high) This affects transport costs if it is incorporated into a growing medium, as it may limit the number of pallets loaded onto a lorry. For inclusion in a growing medium, bulk density should be less than 500 g/l, at a moisture content of 40%. Post composting processing affects this.

Moisture content

Green waste compost should be screened when it has a moisture content of about 40 to 45%. This avoids dust problems when it is drier. Dry compost may also not be stable as microorganisms are generally inactive when the moisture content falls to 35% and hence some materials may not have been fully decomposed. Re-wetting can re-activate microbial action.

pH (pH 1 = very acid, pH 7= neutral and pH 14 = very alkaline)

Green waste compost typically has an alkaline pH between 7.5 and 8.5. This can fall with maturity as the process of ammonium nitrogen conversion to nitrate nitrogen is acidifying. The compost has a small neutralising value for soils (around 10% that of chalk). The pH in a growing medium should, for most plants, be less than 6.5. Green waste compost therefore needs to be mixed with materials to create a growing medium with a lower pH. Peat often has a pH of around 4. Wood and bark based materials are also slightly acidic.

Organic matter content

Organic matter content is usually measured by loss on ignition. For green waste compost screened to <10mm,>Carbon:nitrogen ratio (C:N)

Green waste feedstock has a C:N ratio of between 25:1 and 35:1. As composting proceeds, carbon dioxide is evolved and the ratio slowly falls to a final value of around 12:1. If the feedstocks were high in nitrogen, e.g grass clippings or wet food wastes, ammonia may be evolved and cause an odour problem. The final C:N ratio may also be below 12:1 if the feedstocks are high in nitrogen. With very woody feedstocks, the final C:N ratio may be over 20:1 indicating that the compost may absorb nitrogen from the soil or growing medium system when mixed in.

Conductivity

This is a refelction of the salt content in the compost. Plants growing in soil or a growing medium can tolerate varying levels of salts depending on a number of factors such plant species and size, watering regime, environment, etc.

Green waste compost may have conductivity (1:5 v/v water extract) of as high as 1500 microS/cm and therefore need diluting. Seeds can germinate when the growing medium has a low conductivity e.g. less than 400 microS/cm whereas larger plants potted on may grow well when the conductivity is over 800 microS/cm.

Conductivity is particularly affected, and raised, by ammonium-nitrogen levels if the compost is immature.

Conductivity is reported as microSiemens/cm or milliSiemens/m - so divide the former to get the latter.

Water soluble nutrients

Water solubility of nutrients may not reflect the full availability of those nutrients to plants.
Ammonium and nitrate nitrogen are usually low (less than 50 mg/l in the fresh compost) with green waste compost. The ratio of these two forms of nitrogen may help to indicate maturity as immature composts have relatively higher levels of ammonium nitrogen.

Phosphorus is nearly always low due to the high pH levels.

Potassium and chloride levels are usually high as they are not bound up with organic matter and are very soluble, although potassium has been found to be only 50 to 80% extractable even by CAT extraction. These ions contribute to the high conductivities normally seen in green waste compost.

Total nutrients

The total nitogen content of a green waste compost is usually 1% plus or minus 0.5% in the dry matter This is only slowly released at a rate of about 10% a year.

The phosphorus content is also of low availability but the potassium is fairly soluble but only 50-80% may be available according to CAT analysis.

31 tonnes per hectare of green waste compost may apply 250 kg/ha of total nitrogen (25 kg/ha available in year 1), 100 kg/ha of phosphate (60 kg/ha available in year 1) and 200 kg/ha potash (half available).

Particle size

Screening mature compost gives materials such as a mulch grade (10-40 mm) or a soil improver (0-10 mm). Any plastic contaminants tend to be held in the over 10 mm fraction unless extarcted by fans.

For use in a growing medium too much fine material below 1 mm may result in poor aeration.

Air Filled Porosity (AFP)

AFP is a measure of the air space in a compost after free drainage. For use in a growing medium, AFP should be between 10 and 20% although for nursery stock outdoors in wet weather higher AFPs may be suitable.

Potentially Toxic Elements (PTEs)

as ppm in dry matter:

PAS 100 limits

Copper (Cu) 200
Zinc (Zn) 400
Lead (Pb) 200
Nickel (Ni) 50
Chromium (Cr) 100
Cadmium (Cd) 1.5
Mercury (Hg) 1.0

Physical Contaminants

Total glass, metal and plastic > 2mm should not exceed 0.5% m/m of the total air-dried sample (of which plastic should not be more than 0.25%).

Stones > 4mm should not exceed 8% of the total air-dried sample apart from 'mulch' grades in which stones > 4mm should not exceed 16%.

These levels may be unacceptable in many applications, such as growing media or for potato production, and are further specified in industry specific guidelines.

Microbiological analysis

Salmonella should be absent per 25 g sample
E. coli to be <>Stability and maturity

Compost stability and maturity are important issues for the use of compost in growing media.

The BSI PAS 100 2005 suggests that compost with less that 16 mg CO2/g VS day is stable.

Options for compost measurement include Solvita, respiration techniques (CO2 evolution or O2 uptake), C:N ratio, ammonium:nitrate-N ratio, etc.