Stored Spent Mushroom Compost and Hydrogen Sulphide Gas
Helen Grogan, Gerry Walsh and Tom Kellegher, Teagasc, Horticultural Development Unit, Kinsealy, Malahide Road, Dublin 17.
Hydrogen sulphide gas (H2S) is released from stored organic material that becomes anaerobic such as animal slurry, organic waste and also spent mushroom compost.
On livestock farms, the dangers of hydrogen sulphide gas release from agitated slurry tanks, prior to removal and spreading on land, is well known. Fifteen deaths were associated with slurry tanks in one 10 year period, with many of the fatalities being children.
Hydrogen sulphide gas is a toxic gas (Table 1) and those involved in storing and handling organic wastes, including spent mushroom compost (SMC), must take the appropriate precautions to protect the health and safety of operators.
The Health and Safety Authority issue guidance on safe levels of exposure to harmful compounds.
For hydrogen sulphide gas the Short Term Exposure Limit or STEL is 15 ppm over a 15 minute period; i.e. an operator should not spend more than 15 minutes in an area where the concentration of H2S is 15 ppm. This has been reduced to 10 ppm in the UK and will be reduced to this level in Ireland in due course.
Teagasc have conducted some preliminary work monitoring H2S gas emissions during the handling of spent mushroom compost when it is removed for land spreading.
Two heaps of SMC were studied, one stored undercover in a barn (Figure 1) and the other stored outdoors. The barn-stored heap was smaller, approx 200 tonnes, while the outdoor-stored heap was larger at approx 1000 tonnes.
The SMC was delivered weekly to both sites from the end of October to the end of March, a period of approx. 20 weeks. Each week the fresh SMC was tipped from the trailer and pushed up with a front-end loader to approx. 3m high. After 4-5 weeks, the height settled to 2 to 2.5m. After 20 weeks of deliveries and storage, the smaller heap was removed and spread on land in one day while the larger heap took over 3 days to move and spread.
From our preliminary work the following points emerged:
(1)Hydrogen sulphide gas (H2S) was consistently released from the excavated face of the SMC as the front-end loader was taking loads of SMC (Figure 2). It then decreased once the front-end loader stopped and the trailer load of compost was driven away for spreading. This complete activity took 15 to 20 minutes. The maximum average concentration of H2S above the SMC face of the smaller heap (stored under cover) rose to between 10 and 80 ppm when the front-end loader was removing the SMC and then decreased to near zero when the trailer-load of SMC was driven away to spread on land.
(2)Maximum H2S concentrations above the SMC face of the larger, outdoor, heap were much higher and were consistently > 250 ppm when the front-end loader was removing SMC. It took longer for the concentration to drop when the loaded SMC was being spread on land and it seldom went below 10-50 ppm.
(3)Lower concentrations of H2S were detected around the perimeter of both heaps with average concentrations registering generally between 5 and 60 ppm.
(4)Hydrogen sulphide gas (H2S) was detected at very low average concentrations of 2 ppm or less in the cab of the front-end loader working on the smaller heap. The Short Term Exposure Limit (STEL) never went above 1 ppm indicating that the working conditions at this location on the day in question were safe for the operator. The operator had kept the cab door and windows shut throughout the day.
(5)Hydrogen sulphide gas (H2S) was detected at higher concentrations in the cab of the front end loader working on the larger outdoor heap with average concentrations being between 3 and 28 ppm when the front-end loader was active at the SMC face. The STEL value in the cab was consistently at 3 ppm for most of the day but it rose to 8-10 ppm on three occasions when the H2S monitor in the cab had registered short duration H2S peaks of between 10 and 28 ppm. The operator had left the cab door and window open on several occasions. These results suggest that the working conditions for the front-end loader operator at this location on the day in question were close to being unsafe, particularly given the fact that the STEL value has already been reduced to 10 ppm in the UK.
In conclusion, there is a real risk of hydrogen sulphide gas poisoning when working in close proximity to large heaps of stored SMC. The gas will naturally and quickly dissipate into the atmosphere however if weather conditions are such that there is little air movement then the gas, which is heavier than air, may linger and give rise to localised high concentrations. It is imperative that the driver of the front end loader wears a personal H2S gas monitor to alert him if the concentration in the cab reaches an unsafe level. The cab driver must also ensure that the door and windows are always kept closed when he is operating the loader at the face of the SMC heap.
It is not known how large heaps of SMC will behave in terms of H2S emissions if stored under cover. Covered and uncovered storage on concrete is planned in Ireland to comply with the Nitrates Directive. SMC that is stored outdoors absorbs large quantities of rainfall, making it wetter and more likely to be anaerobic. Further work is needed to identify what factors influence the level of H2S emissions from stored SMC during handling.
Teagasc are continuing to research the dynamics of H2S gas emission from stored SMC in order to identify the key risks involved and to develop health and safety guidelines for those working with this material.
Table 1. Concentration of H2S gas associated with various situations H2S concentration Parts per million (ppm)
General atmosphere 0.00011 - 0.00033 Detectable by human 0.02 - 0.13 Offensive odour, rotten egg smell 3 - 5 Olfactory fatigue 100 Short and long term effects on humans (headache, eye irritation) 150 Damage 500 Lethal (few minutes exposure) 700
Occupational exposure standards: 8 hour time weighted average - TWA 10 ppm for 8hrs*
15 minute short term exposure limit - STEL 15 ppm for 15 mins*
* The TWA and STEL have been reduced to 5 and 10 ppm, respectively, in the UK.
Figure1. Heap of Spent Mushroom Compost stored under cover in a barn
Figure2. Concentration of H2S in the air above a heap of SMC throughout the day as it was being removed by a front end loader, filled into a trailer and spread on land. Peaks reflect front-end loader activity. The SMC was stored under cover in a barn.
6/16/2013 9:19:23 AM can we keep the spent mushroom storage on the mushroom farm or it may harm the growing mushroom? Do we have to take it away from the farm immediately? Johnny Sciberras, Malta
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