Publications

On-farm Composting of Municipal and Commercial Organics as an Environmentally and Socially SustainableResource Recovery Scheme for Rural Communities

June 2003,
Environment Australia

7 Compost Quality

• 7.1 Australian Standard for Composts and Soil Conditioners
• 7.2 Organic Production Standards
• 7.3 Quality of Generated Compost

Generally speaking, compost quality and the use of compost are not regulated, unless the input material contains biosolids. As Queensland does not have any state regulations concerning the use of biosolids, the NSW EPA Environmental Guidelines for the Use and Disposal of Biosolids Products (NSW EPA, 1997) are used as default regulatory framework in Queensland. However, if the composted product (or any other waste product) does not contain biosolids, there are neither regulations that stipulate minimum product qualities nor guidelines that set maximum nutrient and contaminant loads applied to land. It seems that at this point in time only those potential environmental problems are addressed that are associated with the processing of organic waste materials but not those that can arise from use of inferior quality products or from improper use. However, following last year's scandal with the land application of highly contaminated industrial waste (Sydney Morning Herald, 6-10.5.2002), it seems that regulatory bodies are addressing this issue (e.g. DPI, 2002).

There is the Australian Standard for Composts and Soil Conditioners but compliance and accreditation, i.e. regular audits and compulsory product analysis, are voluntary.

7.1 Australian Standard for Composts and Soil Conditioners

Standards Australia has published the Australian Standard for Composts, Soil Conditioners and Mulches (AS 4454-1999). The standard is currently revised and extended to include also vermicompost. It is anticipated that the revised Australian Standard for Composts, Soil Conditioners and Mulches will be published later this year.

AS 4454 specifications regarding contaminant limits and pathogen reduction requirements refer to National or State guidelines for use and disposal of biosolids products that are for unrestricted use. Due to the absence of Queensland guidelines or regulations governing the use and disposal of biosolids, the NSW EPA Environmental Guidelines for the Use and Disposal of Biosolids Products (NSW EPA, 1997) are applicable by default. Relevant requirements contained in the NSW Biosolids Guidelines regarding heavy metal limits are included in Table 4, which specifies AS 4454 compost quality criteria.

The Australian Standard for Compost furthermore stipulates that composted products should be free of plant propagules (weeds) and comply with physical and chemical quality criteria outlined in Table 4. The new draft standard (Standards Australia, 2001) differentiates between 'composted' and 'pasteurised' products. Compost is defined as 'an organic product that has undergone controlled aerobic and thermophilic biological decomposition to achieve pasteurisation and a specified level of maturity (< 40 °C in self heating test)'. Pasteurised products are defined as having also undergone controlled aerobic and thermophilic biological decomposition to achieve pasteurisation but without reaching a high level of stability. The minimum requirement for achieving pasteurisation is the appropriate turning of the outer material to the inside of the windrow so that the whole mass is subjected to a minimum of three turns with the internal temperature reaching a minimum of 55 °C for three consecutive days before each turn.

An overview of physical and chemical quality criteria for composted and pasteurised products according to the draft Australian Standard AS4454 is provided in Table 4.

7.2 Organic Production Standards

Since the farmer operated a fully certified organic farm (certified by Biological Farmers of Australia [BFA]), the utilised feedstock, the composting process and the quality of the finished product had to comply with requirements and specifications outlined in the Australian Organic Standard (Australian Certified Organic, 2002).

Aspects of the standards that are relevant for the undertaken project specify the following:

• Biosolids arising from multi-source urban areas are prohibited from use on organically certified lands (with some exceptions possible in non-food production).
• Feedstock for composting shall be selected so as not to pose contamination risk on-farm. This includes prevention of GMO feedstock into compost heaps on-farm.
• Where feasible, heaps shall be turned to enable heat transmission to all parts of the heap.
• As a guideline, heaps should reach 65°C for a minimum of three or more continuous days to enable destruction of harmful microbes, disease pathogens, weed seeds and to assist in the breakdown of potential residual chemicals. The process should be maintained for a sufficient interval to ensure effective and complete composting has occurred throughout the heap.
• As a guideline, the temperature should remain between 50-65°C for as long as the process requires.
• At all times food safety of end food products shall be a core consideration for all compost use.

Furthermore, the organic standard provides guidance on the maximum level of heavy metals allowed in manure and fertiliser as well as in the soil (Table 5).

It can be noted that, except for arsenic, heavy metal levels tolerated in farm inputs through the Australian Organic Standard are considerably higher than limits stipulated for Grade A product in the NSW Biosolids Guidelines (compare Table 4).

7.3 Quality of Generated Compost

Compost that is utilised in agriculture does not necessarily have to be fully stabilised and matured, while it should be pasteurised. There are various examples where immature but pasteurised compost is produced specifically for the agricultural market (Ebertseder, 1997, Halm, 1994, Petersen and Stoeppler-Zimmer, 1996). The reduced processing time allows commercial composters to produce a low cost product that is suitable for agricultural use.

Regardless of this option, the composting process employed during the trial was designed to produce fully matured compost.

Apart from the intensity and duration of the composting process, compost quality is largely determined by the quality of processed input materials. As far as impurities and contaminants such as heavy metals are concerned, quality of the input material is the sole determining factor. As the kerbside collected kitchen and garden organics showed very little contamination and as the incoming material was not shredded (abrasion from shredding machinery can also result in increased heavy metal levels), the resulting compost was not expected to have elevated contaminant concentrations. This was confirmed by laboratory analysis of three compost samples (Table 6). Compost I showed relatively high arsenic levels (compare Wilkinson et al., 2000) but it seems that this was due to an analytical error, as no arsenic was found in any of the other samples. Pond sludge contained 200 mg zinc per kg dry matter, which is equal to the AS4454 limit. However, none of the compost samples exceeded this limit.

At levels between 6.15 and 7.65 dS/m all compost samples showed elevated electrical conductivity, which is a measure for salinity. Wilkinson et al. (2000) reported a range between 0.8 and 5.1 dS/m for composted garden organics. It is well known that compost that is derived from garden and kitchen organics has higher salt levels than compost derived solely from garden organics (as long as grass clippings are not too dominant). Stoeppler-Zimmer et al. (1993) reported salinity levels between 1.0 and 3.5 g KCl per L) green organics compost and levels between 3.0 and 8.0 g KCl for compost derived from garden and kitchen organics.

Since the collected organic materials contained relatively low levels of kitchen organics, it was not expected that the finished compost would show elevated conductivity levels. However, it seems that the pond sludge had relatively high salt levels (10.3 dS/m) and this fact may have contributed to elevated conductivity levels of the compost.

It was noted that pH values for all compost samples were unusually low at levels between 4.8 and 5.5. Mature compost usually has a pH value of about 7.0 or slightly above. Even composting of citrus processing residues (pulp and peel) with a pH of 3.2 resulted in a composted product that had a pH above 8 (Tittarelli et al., 2003). No explanation can be given as to why the compost generated had such low pH values.

What was said for conductivity also applies for nutrient levels: as the compost was derived mainly from garden organics and contained little kitchen organics, nutrient levels were not expected to be high. Total nutrient levels (on a dry matter basis) of 0.83-0.95% nitrogen, 0.32-0.39% phosphorus, 0.55-0.64% potassium, 0.37-0.5% magnesium and 510-550 mg/kg manganese closely resemble average values reported for compost derived from green organics (Wilkinson et al., 2000).