SMC packaging as an obstacle - The Irish mushroom industry mainly utilises the 'bag and tunnel' production system developed by Teagasc. Fresh compost is packaged in 20 kg polyethelene bags. The plastic bags stay with the compost during the production cycle and contain the SMC when it is discarded from the tunnels. Approximately 15 million bags are discarded annually. Discussions with farm plastics recyclers revealed that these could not be economically recycled due to the amount of debris that adheres to the bag surfaces. The bags impede the ultimate management of SMC as they must be separated from the SMC.

Laboratory investigations of a biodegradable alternative to plastic bags demonstrated the potential of hemp for this purpose. Eighty bags (40 plastic, 40 hemp) were placed in a typical mushroom tunnel and evaluated for a full growing cycle of 12 weeks in a statistically designed (Latin Square) trial (Fig. 1). Twenty bags of each material were placed on the concrete floor as per commercial production practices; twenty bags were placed on pallets to allow water drainage and air circulation. Samples of all bags were evaluated for tensile strength weekly. Hemp bags consistently demonstrated a greater tensile strength (indicating better structural integrity) than plastic bags for the duration of the test (Fig. 2).

Fig 1. Experimental design for comparing plastic (light colour) and hemp (dark colour) compost bags.

Fig 2. Comparison of tensile strength for hemp and plastic compost bags under typical mushroom production conditions over a period of 10 weeks.

SMC moisture content as an obstacle: The high moisture content of SMC (ca. 70% w/w) is a disincentive to long distance SMC transport as well as some potential end uses, notably waste-to-energy conversion. Laboratory studies were conducted to examine low cost SMC drying strategies. Samples of SMC were spread in uniformly deep piles of 4, 6 and 8 cm in a randomised trial that examined drying under ambient climatic conditions with and without exclusion of natural precipitation. While it is technically feasible to dry SMC under ambient conditions if rainfall is excluded (Fig. 3), the operational problems and related costs of this approach are unacceptable. Without drying, SMC has an unacceptably low net energy value and would be unacceptable as a fuel.

Fig. 3. Typical drying curve of SMC under ambient climatic conditions (Dublin in October/November), rainfall excluded.

Strategy for overcoming logistical constraints to SMC management: The satellite nature of the Irish mushroom industry dictates that a centralised approach be taken. Strategically located depots for SMC collection, storage and possible processing would facilitate a number of alternative end uses for SMC. Such facilities would need to be managed to a high standard and would require prior planning permissions (and possibly waste or IPC licences).

Landspreading and landscaping are the most suitable SMC end use options for immediate implementation. The quantities utilizable by each stream would have to be ascertained through a market survey of the landscapers and horticulturists and with an awareness campaign amongst farmers with suitable spreadlands. SMC is, as yet, an unknown product to some farmers (particularly those far removed from mushroom producing areas) but the results of agronomic trials being conducted by UCD and Teagasc are identifying its agronomic benefits.

In order to avoid dependence on a single option and to maximise the amount of SMC to be re-used a combination of end uses should be considered. Costs involved in an operation such as this are dependent on the scale and type of operation. 'Gate fees' could be used to help finance operation of the scheme. In the future, once a centralised SMC handling scheme is established, it may be feasible to divert SMC into other smaller-scale end uses (e.g. vermiculture). The more end uses and number of users that are available the less dependent the scheme becomes on any single outlet.