The Influence of Gypsum in Animal Slurry Systems on the ... · PDF fileslurry systems on the generation of ... The influence of gypsum in animal slurry systems on the generation of

Prepared by the Health and Safety Laboratoryfor the Health and Safety Executive 2015

Health and Safety Executive

The influence of gypsum in animal slurry systems on the generation of hydrogen sulphide

RR1041Research Report

Ian Smith, Gillian Frost, Alan BeswickHealth and Safety Laboratory Harpur HillBuxtonDerbyshireSK17 9JN

The aim of this study was to determine the influence of gypsum (calcium sulphate) on bacterial populations in slurry systems and, as a consequence, its potential to enhance hydrogen sulphide (H2S) generation. The study focused on three slurry / bedding types: slurry from dairy farms that use either a non-slatted collection system or a slatted system, and dry, soiled bedding recovered from sheds used to house beef cattle over winter.

There is clear evidence that the presence of gypsum in slurry will enhance the potential for generation of toxic H2S gas. The levels of the gas produced, even from the small, contained systems, would be toxic to anyone exposed to equivalent concentrations on a larger scale. Therefore, if gypsum residues enter slurry this could increase the risk of H2S gas accumulation in confined spaces in the close vicinity of slurry systems. It is important therefore that this is taken into account in managing risk. Importantly, the levels of H2S gas produced from unamended slurry and bedding (no gypsum added) would still be sufficient to constitute a hazard to anyone exposed to it, though the addition of gypsum further increased the level of H2S gas production.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.


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© Crown copyright 2015

First published 2015

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ACKNOWLEDGEMENTS

We are grateful for the advice provided by NFU to help source farms with slatted and nonslatted slurry systems that were happy to support HSL with this work.

Thanks also to farmers in Derbyshire, Shropshire and Leicestershire for supplying HSL with the required test material (slurry and bedding) in order to facilitate this work.

HSL would also like to thank members of the Environment Agency Gypsum Quality Protocol Technical Advisory Group for providing the commercial and reprocessed gypsum test material used in this project.

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EXECUTIVE SUMMARY

Objectives

The aim of this study was to determine the influence of gypsum (calcium sulphate) on bacterial populations in slurry systems and, as a consequence, its potential to enhance hydrogen sulphide (H2S) generation. H2S is produced by sulphate reducing bacteria within the slurry material, is highly toxic and can cause asphyxiation, especially in confined spaces that are poorly ventilated. Gypsum has the potential to enter cattle slurry systems if it is used as a bedding material, and with it there is a potential for enhanced H2S generation, putting farmers at increased risk of exposure if this is not recognised and controlled.

For the current study, the following approach was taken to assess the potential for cattle slurry and bedding to generate H2S:

• Initial development of a contained storage system using a volume of slurry that could be easily handled but was also safe for staff to monitor and of sufficient scale to allow measurable levels of gas to be generated and recorded;

• To then extend the scale of work in order to look at additional material from animal sheds used for over-wintering and from slatted/unslatted slurry produced at different dairy herd facilities;

• Use accurate gas measurement devices to monitor H2S and methane (CH4) production over an appropriate timescale, in order to assess maximum levels of gas production and likely duration of production, and

• Determine whether the measured levels were of potential risk to human health and whether the addition of gypsum to the slurry had any measurable influence on the levels of H2S gas produced.

Main Findings

This project focused on three slurry / bedding types: slurry from dairy farms that use either a non-slatted collection system or a slatted system, and dry, soiled bedding recovered from sheds used to house beef cattle over winter. These materials were successfully sourced from independent farms with help from NFU. Experiments comprised quantities of slurry in sealed (20 litre) vessels with mechanical stirrers, and H2S was monitored in the head space above the slurry before and after stirring. Experiments were subdivided to examine the effect of H2S generation with the addition of commercial and reprocessed gypsum and monitored daily for 2 – 3 weeks. The ‘slatted’ slurry experiment was further subdivided into ‘fresh slatted slurry’ (collected directly from the parlour) and ‘farm-added-gypsum slatted slurry’, which was obtained from further ‘downstream’ at the same farm premises, where gypsum had been added by the farm.

The main findings from this study are outlined below:

• Unamended cattle slurry (no gypsum added) is capable of emitting high levels of H2S gas above both the short-term and long-term workplace exposure limits (WEL; 10ppm and 5ppm respectively). Before mixing, H2S levels were below both WELs for slatted slurry, non-slatted slurry and bedding, however following mixing / agitation all measurements for non-slatted slurry and the majority of observations for slatted slurry were above both WELs. The much drier bedding material showed no H2S generation above the WELs following

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mixing / agitation. However with the addition of water, and its subsequent availability to any resident anaerobic microorganisms, 92% of H2S observations were above both WELs.

• There is a statistically significant difference in the amount of H2S emitted by slurry (non-slatted) & bedding spiked with gypsum, compared with slurry (non-slatted) / bedding without gypsum. The observations show strong evidence that the amount of H2S gas emitted after stirring depended on whether or not gypsum was present for both the non-slatted slurry and bedding experiment (excluding dry bedding control). ‘Fresh slatted slurry’ did not demonstrate this pattern, with no evidence that H2S generation depended on the addition of gypsum. However ‘farm-added-gypsum slatted slurry’ did show evidence that H2S levels were greater than those for ‘fresh slatted slurry’. Furthermore, addition of gypsum to ‘farm-added-gypsum slatted slurry’ in our experiments produced even greater levels of H2S than those not spiked.

• There is no statistically significant difference between the amount of H2S produced by slurry or bedding spiked with commercial gypsum, in comparison with slurry or bedding spiked with reprocessed gypsum.

• Trends in H2S generation over time varied between the non-slatted slurry, slatted slurry and bedding, and with the presence / absence of gypsum. Non-slatted slurry with gypsum added showed emission levels increase to a peak at around day 15 before falling. Bedding time trends were not influenced by presence / absence of gypsum and H2S levels peaked also at around day 15, before falling.

• There is a statistically significant difference in the levels of H2S gas emitted from non-slatted slurry in comparison with slatted slurry. Emissions of H2S from the non-slatted slurry were greater than those from slatted slurry, irrespective of whether gypsum was present / absent. However these differences may be attributed to differences in the age of the slurry at point of collection.

In summary, there is clear evidence that the presence of gypsum in slurry will enhance the potential for generation of toxic H2S gas. The levels of the gas produced, even from the small, contained systems, would be toxic to anyone exposed to equivalent concentrations on a larger scale. Therefore, if gypsum residues enter slurry this could increase the risk of H2S gas accumulation in confined spaces in the close vicinity of slurry systems. It is important therefore that this is taken into account in managing risk.

Importantly, the levels of H2S gas produced from unamended slurry and bedding (no gypsum added) would still be sufficient to constitute a hazard to anyone exposed to it, though the addition of gypsum further increased the level of H2S gas production.

CONTENTS PAGE

1. INTRODUCTION ................................................................................................ 1

2. METHODOLOGY ............................................................................................... 4 2.1 Experimental set-up 4 2.2 Location and risk mitigation 5 2.3 Slurry and bedding preparation 5 2.4 Sample mixing 6 2.5 Gas monitors used 7 2.6 Sequence of measurement 7 2.7 Statistical analysis methods 7

3. RESULTS ........................................................................................................... 9 3.1 Context of results 9 3.2 Results summary 9 3.3 Bedding and slurry data (non-slatted & slatted) 11

4. DISCUSSION AND CONCLUSIONS ............................................................... 21

5. REFERENCES ................................................................................................. 23

6. APPENDIX ........................................................................................................ 24 6.1 Bedding and slurry data (non-slatted & slatted) continued 24 6.2 Results for interim method optimisation report; The influence of gypsum on hydrogen sulphide production from cattle slurry 28

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1. INTRODUCTION

Tanks or lagoons on dairy farms are a necessary facility for storage and subsequent disposal of slurry associated with the housing of cattle (Figure 1). Similar systems are also used for handling pig slurry. The facilities provide a controlled means by which the micro-organisms in animal waste degrade and stabilise the waste prior to subsequent use as a soil fertiliser. However, bacteria within the matrix generate gaseous by-products that can include methane, carbon dioxide and hydrogen sulphide (H2S). The latter is known to be produced by naturally occurring microorganisms known collectively as ‘sulphate reducing bacteria’ (SRB). In confined spaces these gases can lead to oxygen depletion and present an asphyxiation hazard for workers, but even more importantly, H2S is highly toxic, acting as an inhibitor to normal respiratory function.

Figure 1. Slurry lagoon

The consequence of H2S inhalation is likely to be an initial local irritant effect followed by arrest of cellular respiration. However, exposure to high levels may lead to immediate unconsciousness. H2S forms a complex bond to iron-containing proteins in the body and thus can arrest aerobic metabolism in an effect similar to cyanide toxicity. Coupled to this is very high lipid solubility which allows it to penetrate easily through biological membranes. H2S has a poisonous effect on all organs, but particularly the central nervous and pulmonary systems. Health effects depend on the concentration and duration of exposure, summarised as follows:

• At a concentration as low as 0.0047 ppm 50% of humans can detect the characteristic ‘rotten egg’ odour of H2S;

• The Workplace Exposure Limit (WEL) for H2S is 5 ppm for long term exposure (8-hour Time Weighted Average reference period; TWA), or 10 ppm for short term exposure (15 minute reference period);

• At 150-250 ppm the olfactory nerve is paralysed after a few inhalations so that the sense of smell disappears, often together with awareness of danger;

• Exposure to 530-1000 ppm can cause strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing;

• 800ppm is the generally accepted lethal concentration for 50% of an exposed human population after 5 minutes exposure;

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• Concentrations over 1000 ppm can cause immediate collapse with loss of breathing, even after inhalation of a single breath.

Exposure to high enough concentrations to cause H2S -induced acute central toxicity can lead to reversible unconsciousness, also referred to as a “knockdown”. While it is likely that repeated or prolonged knockdowns are associated with chronic neurologic health effects, of more immediate concern is that knockdowns can be fatal as a consequence of respiratory paralysis and inability to escape from an asphyxiant atmosphere. As a result of the above, there have been a number of fatalities associated with slurry tanks and lagoons, most recently in Northern Ireland in September 2012. In some instances there have been multiple fatalities, where one person has been overcome and their colleague has attempted a rescue, only for them in turn to be overcome.

It is therefore important to understand the circumstances in which H2S may be generated from slurry, and factors influencing it. Gypsum (calcium sulphate) has the potential to enter cattle slurry systems if it is used as a bedding material. In a recent case, the presence of gypsum in slurry was thought to contribute to the deaths of 4 cows from H2S suffocation (Farmers Guardian news item; 16th February 2012; http://www.farmersguardian.com/home/livestock/cattle-slurry-deaths-prompt-warning-to-farmers/44924.article. In this case waste plaster board had been used to dry up straw bedding and heavy rain had washed gypsum into the slurry store. The news item stated that "gypsum...contains high levels of sulphur which encourages the bugs to produce even greater amounts of H2S from the slurry".

Three possible sources of gypsum exist. First is virgin gypsum, i.e. natural gypsum as mined. Second is synthetic gypsum, e.g., that which is derived from flue gas desulphurisation (FGD) systems at electric power plants. Sulphur dioxide emission control systems used by coal-fired power plants remove sulphur from combustion gases using "scrubbers." One particular type of scrubber that uses lime or limestone reagent and a forced oxidation system produces "FGD gypsum," which is chemically almost identical to mined natural gypsum. The third source is reprocessed gypsum recovered from waste plasterboard. The landfilling of gypsum and other high sulphate-bearing wastes with biodegradable waste has been prohibited in England and Wales since July 2005 to reduce H2S generation as well as encouraging recycling (http://www.environment-agency.gov.uk/business/topics/waste/32148.aspx). This means there is a material of which there is an increasing supply, from FGD and reprocessed material, and a desire to develop markets for its use.

In agriculture, gypsum has traditionally been used as a soil conditioner, and it has been considered for use as a cattle bedding material. Powdered gypsum could potentially be used as bedding material for dairy cows as an alternative to straw or sawdust, and is capable of keeping the cows dry, sticks to their hooves less than sawdust, is non-caustic, non-abrasive, non-dusty, inert and antibacterial. Consequently, its use for this purpose could mean it being introduced in significant quantities into slurry. There is the potential for enhanced H2S generation in slurry systems on farms using powdered gypsum as bedding, potentially placing farmers at increased risk if this is not recognised and if exposure is not managed.

The Environment Agency is responsible for overseeing the disposal and/or re-use of waste gypsum. Its Position Statement on the use of waste gypsum in animal bedding states the following:

“There are no exemptions which allow the use of waste gypsum and waste plasterboard as animal bedding. Therefore the use of waste gypsum or plasterboard as animal bedding without an environmental permit is an offence. We will not accept the use of plasterboard and waste

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gypsum as animal bedding until there is clear scientific evidence to demonstrate that it does not pose a risk to animals, humans and the environment from hydrogen sulphide generation.”

A further concern is the corrosive nature of hydrogen sulphide in contact with ferrous metal. On-farm anaerobic digester systems utilising slurry could be at greater risk of corrosion and structural failure brought on by enhanced hydrogen sulphide generation from SRB. This corrosive mechanism is one already recognised in the oil industry, where SRB may also grow under anaerobic conditions (Muyzer & Stams, 2008).

The current position in industry is that there is a surplus of gypsum from waste plasterboard with only limited outlets available. This has created interest in its suitability for use as an animal bedding material. Further information on the risks associated with the use of gypsum in animal bedding and slurry systems is required to inform future regulatory approaches.

HSL staff were commissioned by HSE, with additional funding from Environment Agency and HSE Northern Ireland, to research the potential for enhanced H2S generation from cattle slurry and bedding to which gypsum was added. This report describes the findings of this study.

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2. METHODOLOGY

2.1 EXPERIMENTAL SET-UP

In order to simulate the environment of a slurry tank or lagoon, HSL used a closed-system tub approach based on a principle used for other experimental systems. Work completed by Masse et al. (2003) investigated methane emissions from slurries using 232L closed plastic barrels; barrels were filled with manure or 50:50 manure and tap water, and held in test chambers at controlled temperatures. Gaseous emissions were then measured via a tube in the head space. Rodhe et al. (2009) investigated conditions in slurry systems on farms using a similar approach; pilot scale experiments used 1 metre high by 1.92m diameter containers with insulation for outdoor storage and fixed lids for head space gaseous emission capture. An alternative approach adopted by Marie Cornell (PhD thesis; University of Southampton) used stirred semi-continuous 5L feed digesters, batch fed with 4L of cattle slurry. Karim et al. (2005), also used digesters (3.73L working volume), both stirred and unstirred, with both 5% and 10% dilutions of slurry.

Based on the above, HSL used a similar approach with slurry samples stored in a sealed system (Figure 2), each comprising a 20 litre plastic tub, with rubber sealed screw-top lid. Maximum slurry volumes used were 15 litres - to minimise handling of heavy slurry volumes, reduce exposure risk to staff and to allow for head space measurement of any emitted gases. Stirring apparatus utilised a plasterer’s stirring paddle and drill, with the paddle shaft accommodated via a small aperture in the lid of each slurry tub and sealed with malleable sealant. An additional access hole was drilled in the lid for gas monitoring and this remained plugged between measurements. All experiments were run at ambient temperature with temperature measurements taken during periods of gas monitoring. Preliminary work on method development and optimisation, together with the results from this work, is presented in the Appendix (6.2).

Figure 2. 20L plastic tubs with rubber sealed screw-top lids and stirring paddles.

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2.2 LOCATION AND RISK MITIGATION

Because of the potential gas and odour associated with this work, all experimental testing was performed in an outbuilding at HSL, i.e. some 800m away from the main laboratory building. All monitoring work was undertaken using a buddy system of two staff, with mobile phone contact with the main HSL building. Low and high level H2S monitors were used during all room entry to ensure no accumulation of toxic gas within the air of the building. Personal protective equipment, including disposable over suits, disposable gloves and overshoes were worn during monitoring visits and appropriate respirators were available during gas monitoring work. The building’s windows were kept permanently open at all times to promote room aeration, and this was enhanced by opening two end doors to the building during all gas monitoring activity.

2.3 SLURRY AND BEDDING PREPARATION

Three separate slurry / bedding material were sourced to form the basis of the three experimental runs. Through the Safe Farm Partnership, the NFU participant was asked to help identify farms willing to participate by supplying slurry and bedding. Dry bedding was sourced from over-wintering sheds at a beef and sheep farm in Ludlow, Shropshire and consisted largely of a layered mix of soiled hay. ‘Non-slatted’ slurry was sourced from a local farm near Buxton and was of a largely watery consistency. ‘Slatted’ slurry was sourced from a farm in Leicester. Although the original intention was to use slurry that did not have gypsum already added, this last farm actually used FGD gypsum as bedding in some cattle sheds. This provided additional experimental data and slurry was collected in two forms. One, termed ‘fresh slatted slurry’, was collected directly from the slats, and therefore in effect comprised mainly fresh manure. The second, termed ‘farm-added-gypsum slatted slurry’, was collected from further down the slatted system and therefore comprised cattle slurry which included bedding and therefore any gypsum that had been incorporated into the bedding.

Each slurry / bedding material was initially investigated for baseline H2S levels in ‘neat’ form (termed ‘unamended’). In all instances, no further dilutions of the slurry were required and experiments were performed using ‘neat’ slurry / bedding. These were then compared with the same material with gypsum added (termed ‘amended’).

Gypsum was supplied through contact with the Environment Agency Gypsum Technical Advisory Group. A commercial supplier of gypsum participating in the Advisory Group provided samples of the three different types of gypsum powder as described in the Introduction.

At the method development stage (see data in Appendix; 6.2) gypsum was added to slurry at concentrations of 5% and 1% weight per volume. These concentrations were chosen fairly arbitrarily as amounts of gypsum significant enough to make a difference to the slurry. Although there is no direct relationship, this is similar to on-farm gypsum usage as a soil conditioner (data from Gypsum Technical Advisory Group). In subsequent discussion with a farmer who had used gypsum in bedding, his estimate of gypsum tonnage used and volume of slurry produced in a year equated to around 1% weight per volume. Supported by preliminary data from early testing, the 1% concentration therefore was used in all further tests.

All slurry / bedding materials were subdivided into test combinations run in triplicate (Table 1). ‘Non-slatted’ slurry was subdivided into 3 test combinations; (1) slurry only (unamended), (2) slurry + 1% commercial (FGD) gypsum, and (3) slurry + 1% reprocessed gypsum. Due to the

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expected lack of H2S produced from dry bedding, the bedding was subdivided into a total of 4 test combinations, to include the addition of water for this much drier material; (1) dry bedding, (2) bedding + 5L water, (3) bedding + 5L water + 1% commercial gypsum, and (4) bedding + 5L water + 1% reprocessed gypsum. ‘Slatted’ slurry was subdivided into 5 test combinations to incorporate both the ‘fresh slatted slurry’ and ‘farm-added-gypsum slatted slurry’; (1) ‘fresh slatted slurry’ only (unamended), (2) ‘fresh slatted slurry’ + 1% commercial (FGD) gypsum, (3) ‘fresh slatted slurry’ + 1% reprocessed gypsum, (4) ‘farm-added-gypsum slatted slurry’ (unamended), and (5) ‘farm-added-gypsum slatted slurry’ + 1% commercial gypsum.

Table 1. Analysis matrix

TEST MATRIX UNAMENDED

AMENDED (GYPSUM ADDED)

Commercial (1%)

Reprocessed (1%)

Slurry (non‐slatted)

Slurry (slatted)

fresh

farm‐added‐gypsum X

Bedding

dry X X

wet

2.4 SAMPLE MIXING

Hydrogen sulphide measurements were taken pre and post mixing at intervals outlined below (sequence of measurement). Mixing was performed using plaster mixing paddles set up in each tub, and manipulated either manually (dry bedding and ‘non-slatted’) or automatically using the electric drill (‘slatted’), each for a period of ten seconds at a set speed (Figure 3).

Figure 3. Sample mixing using low-speed electric drill.

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2.5 GAS MONITORS USED

A calibrated GeoTech GA5000 was supplied by Analytical Science colleagues, and is capable of detecting up to several thousand ppm of H2S, but also % CH4 levels. A Portasens II monitor was used during all pilot tests for background (safety related) levels of H2S and the GeoTech monitor for test sampling of head space gases. Both monitors are capable of measuring gases in room air, or via the aperture in the slurry tubs (Figure 4).

Figure 4. Portasens II monitor connected to head space for gas emission measurement.

2.6 SEQUENCE OF MEASUREMENT

Regular measurements were taken from the slurry / bedding tub headspaces to allow an accurate assessment of gas emission over time. Measurements were completed daily for up to three consecutive weeks, excluding weekends (due to the requirement for 2 staff to be present). These have allowed an informative assessment of H2S emission for the various slurry/gypsum combinations.

2.7 STATISTICAL ANALYSIS METHODS The slurry data and the bedding data were analysed separately, but both followed the same process.

Observed hydrogen sulphide levels were summarised using the median plus minimum/maximum, frequency distributions, and box and whisker plots. The overwhelming majority of measurements taken before stirring observed zero hydrogen sulphide emissions, thus statistical analysis focussed on the levels observed after stirring. Dry bedding after stirring did not have any non-zero measurements, and so this was also excluded from statistical analysis.

A number of samples were measured over time, and so mixed effects models with the sample as the random effect were used to analyse the data. This took into account any variability in the results due to different samples being used. The dependent variable was the logarithm (base 10) (log10) of the observed hydrogen sulphide level. The logarithm was used to normalise the data and reduce heteroscedasticity in the model residuals, that is, to reduce effects that may result from sub-populations that have different variabilities from others. The logarithm of zero is not defined and so, in the rare cases where a zero was recorded for the hydrogen sulphide level after stirring, the logarithm of half of the smallest observed non-zero level was used.

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To investigate any differences in hydrogen sulphide levels with the type of gypsum introduced, a categorical variable (a variable that can take on one of a limited, and usually fixed, number of possible values) was included as an independent variable. For slurry data from a non-slatted system, this had the categories of ‘none’, ‘1% commercial’, and ‘1% reprocessed’; for the bedding the data, the categories were ‘wet’, ‘wet + 1% commercial’, and ‘wet + 1% reprocessed’; and for slurry data from a slatted system, the categories were the same as those for slurry from an non-slatted system, but with the addition of ‘farm-added-gypsum slatted slurry’ and ‘farm-added-gypsum slatted slurry + 1% commercial’. The overall statistical significance of the type of gypsum was tested using the likelihood ratio test (LR-test). Individual comparisons between groups were tested using the Wald test.

The change in hydrogen sulphide levels over time was investigated by including the day of the measurement using a restricted cubic spline (with five knots). This statistical treatment estimated the data curve fit for a sample point by taking into consideration the previous and next point, thus allowing for irregular data collection, i.e., samples were not taken daily, and improved curve fitting to the data. The model also included the type of gypsum incorporated and the interaction between the type of gypsum and day of measurement, which would allow a different curve to be fitted for each type of gypsum. The statistical significance of the interaction, and hence whether or not the time trend was different depending on the type of gypsum, was tested using the likelihood ratio test.

Finally, the cow slurry data from the slatted and non-slatted systems were combined to investigate if there was a difference in the hydrogen sulphide levels emitted by the two systems. Measurements from the slatted system were collected over 25 days, whereas data from the non-slatted system was collected over 21 days. Additionally, there were more types of gypsum added to cow slurry from the slatted system than for slurry from the non-slatted system. Therefore, to ensure comparability of data from the two systems, only the first 21 days observations were included and only for those samples with no added gypsum, with 1% commercial gypsum, or with 1% reprocessed gypsum. The mixed effects model included a categorical variable for the type of gypsum, and a categorical variable for the type of system (slatted or non-slatted) as independent variables. The model also included the interaction between the type of gypsum and the type of system, which would allow the difference between the two systems to vary from one type of gypsum to another. The statistical significance of the interaction was tested using the likelihood ratio test, and comparisons within gypsum types were undertaken using the Wald test.

Results from the statistical models are presented in charts that show the predicted or expected mean hydrogen sulphide level on the log10 scale.

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3. RESULTS

3.1 CONTEXT OF RESULTS

In interpreting the results, reference is made to Workplace Exposure Limits (WELs). It is recognised that a small scale experimental system will not necessarily scale up to a full scale slurry storage system, therefore an assumption is being made about the potential for H2S gas to collect in a head space above a full scale slurry storage system. The use of WELs provides a point of reference to occupational exposure and health risk.

3.2 RESULTS SUMMARY

3.2.1 Is cattle slurry capable of emitting dangerously high levels of hydrogen sulphide gas? [NB. Short term workplace exposure limit (WEL) = 10ppm; long term WEL = 5ppm]

Slurry (non-slatted system). Before stirring and without gypsum, there were no observations over the 21 day period that were above the long term WEL or the short term WEL. Adding commercial or reprocessed gypsum resulted in similar numbers of observations over the WELs before stirring: 10% (95%CI 3.5-22.7%) and 13% (95%CI 4.7-25.2%) of observations being above the long term WEL of 5 ppm respectively, and 10% (95%CI 3.5-22.7%) and 6% (95%CI 1.3-17.2%) above the short term WEL of 10 ppm respectively. The addition of commercial and reprocessed gypsum resulted in maximum ppm values of 205 and 312 respectively. The measurements were always over both the short and long term WELs after stirring, whether or not gypsum had been added, with maximum measured ppm values of 330, 1618 and 1580 for unamended, commercial and reprocessed gypsum respectively. Median values ranged from 126 – 690 ppm (Appendix, table 1).

Bedding. Before stirring, there were no observations over the 18 day period that were above the long term WEL or the short term WEL. Even after stirring, dry bedding did not have any emissions above the WELs. However, once wet and after stirring, 92% (95%CI 77.5-98.2%) of observations for pure wet bedding, wet bedding with added commercial gypsum, and wet bedding with added reprocessed gypsum were above both the long term and short term WELs, with median values of 108, 971 and 573 ppm, and maximum values of 1190, 3899 and 3940 respectively (Appendix, Table 3).

Slurry (slatted system). Before stirring, all observations over the 25 day period showed zero hydrogen sulphide emissions, whether or not gypsum had been added. After stirring, the majority of observations were over both the long and short term WELs (median values ranged from 24 – 346 ppm, and maximum H2S levels ranged from 358 – 1772 ppm; Appendix, table 5). The percentage of observations over the long term WEL and the short term WEL depended on the type of gypsum added. The percentage of observations over the long term WEL ranged from 57% (95%CI 43.2-70.8%) when commercial or reprocessed gypsum was added, to 96% (95%CI 87.3-99.5%) when commercial gypsum was added to ‘farm-added-gypsum slatted slurry’. The percentage of observations over the short term WEL ranged from 52% (95%CI 37.8-65.7%) when commercial or reprocessed gypsum was added, to 94% (95%CI 84.6-98.8%) when commercial gypsum was added to ‘farm-added-gypsum slatted slurry’.

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3.2.2 Is there any statistically significant difference in the amount of H2S gas emitted by cattle slurry/bedding amended with gypsum, compared to unamended bedding/slurry? Is there any significant difference in the levels of H2S gas emitted by bedding/slurry amended with reprocessed gypsum, as opposed to commercial gypsum?

Slurry (non-slatted system). There was strong evidence that the amount of hydrogen sulphide gas emitted after stirring depended on whether or not gypsum was added. The amount of hydrogen sulphide emitted when the slurry contained 1% commercial gypsum was on average 5 times (0.70 log10 ppm; 95%CI 0.59-0.81 log10 ppm) greater than that emitted from pure slurry, and was on average 4.5 times (0.65 log10 ppm; 95%CI 0.54-0.76 log10 ppm) greater when the slurry contained 1% reprocessed gypsum. There was no evidence that the type of gypsum (commercial or reprocessed) affected the amount of hydrogen sulphide emitted.

Bedding. The results were similar to those for slurry (non-slatted system). Excluding dry bedding (which always observed zero emissions), there was strong evidence that the amount of hydrogen sulphide gas emitted after stirring depended on whether or not gypsum was added. The amount of hydrogen sulphide emitted when the wet bedding contained 1% commercial gypsum on average 4.6 times (0.66 log10 ppm; 95%CI 0.20-1.11 log10 ppm) greater than that emitted from pure wet bedding, and was on average 5.5 times (0.74 log10 ppm; 95%CI 0.28-1.19 log10 ppm) greater when the wet bedding contained 1% reprocessed gypsum. There was no evidence that the type of gypsum (commercial or reprocessed) affected the amount of hydrogen sulphide emitted.

Slurry (slatted system). There was no clear evidence that the amount of hydrogen sulphide gas emitted after stirring was increased by adding gypsum to ‘fresh slatted slurry’. However, there was evidence that ‘farm-added-gypsum slatted slurry’ without gypsum (unamended) had levels that were greater than for ‘fresh slatted slurry’ without or with gypsum. ‘Farm-added-gypsum slatted slurry’ amended by the addition of 1% commercial gypsum had levels that were much greater than those for the same slurry without added gypsum and those for ‘fresh slatted slurry’ without or with gypsum. The amount of hydrogen sulphide emitted from ‘farm-added-gypsum slatted slurry’ amended to contain commercial gypsum was on average 15.8 times (1.20 log10 ppm; 95%CI 0.81-1.59 log10 ppm) greater than that emitted from the same slurry without gypsum, and was on average 4.8 times (0.68 log10 ppm; 95%CI 0.29-1.07 log10 ppm) greater than ‘fresh slatted slurry’ amended by addition of 1% gypsum. ‘Farm-added-gypsum slatted slurry’ without gypsum (unamended) showed hydrogen sulphide levels that were on average 3.3 times (0.52 log10 ppm; 95%CI 0.13-0.91 log10 ppm) greater than ‘fresh slatted slurry’ without gypsum.

3.2.3 What are the trends in hydrogen sulphide emissions over time?

Slurry (non-slatted system). The levels observed after stirring stayed relatively constant over the 21 day period when no gypsum was added. However, for slurry when either commercial or reprocessed gypsum was added, the emissions increased up to a peak at around day 15 before starting to reduce. This reduction could be due to a drop in ambient temperature, but the effect of decreasing emissions with decreasing temperature was not consistently observed across all slurry types as would be expected, and so this was not adjusted for in the analysis.

Bedding. In contrast to the slurry results, there was no evidence that the time trend in emissions was different depending on whether or not gypsum was added. The hydrogen sulphide levels

11

increased over time, peaking at around day 15 before reducing slightly. There were sparse data after day 15, and so this drop may not be reliable.

Slurry (slatted system). The time trend over the 25 day period depended on the type of gypsum added. With unamended ‘fresh slatted slurry’, the hydrogen sulphide levels initially decreased before increasing from around day 8 onwards. When commercial or reprocessed gypsum was added to ‘fresh slatted slurry’, the levels again initially decreased until around day 8, but then increased to around day 20 before there was again a slight decrease in levels. With amended ‘farm-added-gypsum slatted slurry’, the levels increased sharply to around day 5, and then stayed relatively stable. There was a slight dip in emissions at around day 10 and at the end of the 25 day period, but this could be due to sparse data at these time points.

3.2.4 Is there any difference in the levels of hydrogen sulphide gas emitted from slurry collected via slatted systems compared to non-slatted?

There was strong evidence that emissions from slurry from the non-slatted system were greater than those from slurry from a slatted system, whether or not gypsum was added. However, there was also evidence that the differences in emissions from slurry from the two systems depended on whether or not gypsum was added. When no gypsum was added, hydrogen sulphide levels from the non-slatted system were on average 8.3 times (0.92 log10 ppm; 95%CI 0.64-1.20 log10 ppm) greater than those from the slatted system. The differences between the two systems were greater when gypsum was added, but they were similar for both gypsum types: levels were on average 97.7 times (1.99 log10 ppm; 95%CI 1.71-2.27 log10 ppm) greater for the non-slatted system compared to the slatted system when commercial gypsum was added, and 87.1 times (1.94 log10 ppm; 95%CI 1.66-2.22 log10 ppm) greater when reprocessed gypsum was added.

3.3 BEDDING AND SLURRY DATA (NON-SLATTED & SLATTED)

Refer to Appendix (6.1) for tabulated results data and statistical explanations.

3.3.1 Slurry data (non-slatted system)

The levels of hydrogen sulphide observed in cow slurry from a non-slatted system are summarised in Figure 5, as box and whisker plots for levels after stirring.

12

Figure 5 Box and whisker plots showing the levels of hydrogen sulphide observed in cow slurry over a 21

day period after stirring, by type of gypsum added

The results from the mixed effects analysis investigating the differences between gypsum types after stirring are shown in Figure 6. Overall, there was a statistically significant association between the observed hydrogen sulphide levels and whether or not gypsum was added (LR-test p<0.001). There was a statistically significant increase in hydrogen sulphide levels when gypsum was added compared to without gypsum: the level was on average 5 times (0.70 log10 ppm; 95%CI 0.59-0.81 log10 ppm, p<0.001), greater when commercial gypsum was added and 4.5 times (0.65 log10 ppm; 95%CI 0.54-0.76 log10 ppm, p<0.001) greater when reprocessed gypsum was added. The difference in the hydrogen sulphide levels between the two gypsum types was not statistically significant (p=0.342).

Figure 6 Predicted mean hydrogen sulphide levels for cow slurry over a 21 period after stirring, estimated

using a mixed effects model

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Error bars represent 95% confidence intervalsNumbers on horizontal axis represent statistically significant pairwisecomparisons (p<0.05)

13

Figure 7 shows how the levels of hydrogen sulphide changed over time. The interaction between type of gypsum added and time was statistically significant (LR-test p<0.001), providing evidence that the time trend differed between the groups. The interaction between type of gypsum and time was not statistically significant when the samples without added gypsum were excluded (LR-test p=0.236), and so there was no evidence that the time trend for samples with commercial gypsum or reprocessed gypsum differed. For samples without added gypsum, the hydrogen sulphide levels remained relatively constant over the 21 day period. However, for samples with added gypsum, the hydrogen sulphide levels increased to a peak at around day 15 before starting to reduce. Note that this reduction ties in with a reduction in the air temperature at the same time. But the effect of temperature was not found to be consistent across all datasets – i.e. there was an increase in hydrogen sulphide level with decreasing temperature for the bedding data – and so it was decided not to adjust for temperature when investigating the time trend.

Figure 7 Time trends for the amount of hydrogen sulphide emitted by cow slurry over a 21 day period

after stirring. Fitted using a mixed effects model with a restricted cubic spline for day plus interaction with type of gypsum added.

3.3.2 Bedding data

The levels of hydrogen sulphide observed in cow bedding are summarised in Figure 8, as box and whisker plots for levels after stirring.

050

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14

Figure 8 Box and whisker plots showing the levels of hydrogen sulphide observed in cow bedding over an

18 day period after stirring, by type of gypsum added

The results from the mixed effects analysis investigating the differences between gypsum types after stirring are shown in Figure 9. Overall, there was a statistically significant association between the observed hydrogen sulphide levels and whether or not gypsum was added (LR-test p=0.001). There was a statistically significant increase in hydrogen sulphide levels when gypsum was added to wet bedding compared to without gypsum: the mean level was 4.6 times (0.66 log10 ppm; 95%CI 0.20-1.11 log10 ppm, p<0.001) greater when commercial gypsum was added, and 5.5 times (0.74 log10 ppm; 95%CI 0.28-1.19 log10 ppm, p<0.001) greater when reprocessed gypsum was added. The difference in the hydrogen sulphide levels between the two gypsum types was not statistically significant (p=0.720).

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15

Figure 9 Predicted mean hydrogen sulphide levels for bedding over an 18 day period after stirring,

estimated using a mixed effects model

Figure 10 shows how the levels of hydrogen sulphide changed over time. The interaction between type of gypsum added and time was not statistically significant (LR-test p=0.179), providing no evidence that the time trend differed between the groups. It is easier to see that the time trends are similar when plotted on the log10 scale (Figure 10). For all gypsum types, there was an initial steep increase in hydrogen sulphide levels, which was probably due to the zero observations on the first day, followed by a more steady increase up to day 15. There was a slight decrease in hydrogen sulphide levels after day 15, but this was based on sparse data after this day.

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Error bars represent 95% confidence intervalsNumbers on horizontal axis represent statistically significant pairwisecomparisons (p<0.05)

16

Figure 10 Time trends for the amount of hydrogen sulphide emitted by bedding over an 18 day period

after stirring. Fitted using a mixed effects model with a restricted cubic spline for day plus interaction with type of gypsum added.

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17

3.3.3 Slurry data (slatted system)

The levels of hydrogen sulphide observed in cow slurry from a slatted system are summarised in Figure 11, as box and whisker plots for levels after stirring.

Figure 11 Box and whisker plots showing the levels of hydrogen sulphide observed in cow slurry from a

slatted system over a 25 day period after stirring, by type of gypsum added

The results from the mixed effects analysis investigating the differences between gypsum types after stirring are shown in Figure 12. Overall, there was a statistically significant association between the observed hydrogen sulphide levels and whether or not gypsum was added (LR-test p<0.001). There was no statistically significant difference in hydrogen sulphide levels between samples with no gypsum, samples with commercial gypsum, and samples with reprocessed gypsum (all p>0.05, Figure 12). The greatest hydrogen sulphide levels were observed for ‘farm-added-gypsum slatted slurry’ with added commercial gypsum, and this was statistically significantly greater than all other gypsum types (all p<0.05) and on average 15.8 times (1.20 log10 ppm; 95%CI 0.81-1.59 log10 ppm, p<0.001) greater than when no gypsum was added. Unamended ‘farm-added-gypsum slatted slurry observed levels that were statistically significantly lower than ‘farm-added-gypsum slatted slurry’ + 1% commercial gypsum (difference [diff] 0.68 log10 ppm, 95%CI 0.29-1.07 log10 ppm, p=0.001), but statistically significantly higher than the other gypsum types (all p<0.05) and on average 3.3 times (0.52 log10 ppm; 95%CI 0.13-0.91 log10 ppm, p<0.009) greater than when no gypsum was added (Figure 12).

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18

Figure 12 Predicted mean hydrogen sulphide levels for slurry from a slatted system over a 25 day period

after stirring, estimated using a mixed effects model

Figure 13 shows how the levels of hydrogen sulphide changed over time. The interaction between type of gypsum added and time was statistically significant (LR-test p<0.001), providing evidence that the time trend differed between the groups. The time trends were not statistically significantly different for commercial gypsum and reprocessed gypsum, and also for unamended ‘farm-added-gypsum slurry’ and ‘farm-added-gypsum slurry’ plus commercial gypsum. The similarities/differences between time trends are easier to see when plotted on the log10 scale (Figure 13). When no gypsum was added, or when commercial or reprocessed gypsum was added, there was an initial decrease in hydrogen sulphide levels, which bottomed-out at around day 8. Increasing levels were observed after this, up until around day 20 when gypsum was added, when a slight decrease in levels was observed. For ‘farm-added-gypsum slatted slurry’, both with and without commercial gypsum, there was an initial steep increase in hydrogen sulphide levels up until day 5, when the emissions started to level out. There was a slight dip in emissions at around day 11, and a slight decrease at around day 20.

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Type of gypsum

FSS: Fresh slatted slurryFAGSS: Farm-added-gypsum slatted slurry Error bars represent 95% confidence intervalsNumbers on horizontal axis represent statistically significant pairwisecomparisons (p<0.05)

19

Figure 13 Time trends for the amount of hydrogen sulphide emitted by cow slurry from a slatted system over a 25 day period after stirring. Fitted using a mixed effects model with a restricted cubic spline for day plus interaction with type of gypsum added

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20

3.3.4 Cow slurry – non-slatted versus slatted system

The results from the mixed effects analysis investigating the differences between cow slurry from slatted and non-slatted systems are shown in Figure 14. The interaction between the system and gypsum type was statistically significant (LR-test p<0.001), providing strong evidence that the differences between the two systems depended on the type of gypsum that was added. For all experimental treatments, i.e., with or without added gypsum, cow slurry from the non-slatted system produced levels of hydrogen sulphide that were statistically significantly above those produced by cow slurry from the slatted system (Figure 14). The difference between the systems was smallest when no gypsum was added; the hydrogen sulphide levels for the non-slatted system were on average 8.3 times (0.92 log10 ppm; 95%CI 0.64-1.20 log10 ppm) greater than the slatted system. The difference between the two systems was statistically significantly greater when gypsum was added (p<0.001 for both), and there was no evidence that the difference between the two systems was related to the type of gypsum (commercial versus reprocessed, p=0.804). The estimated difference in hydrogen sulphide levels between the non-slatted and slatted systems was 97.7 times (1.99 log10 ppm; 95%CI 1.71-2.27 log10 ppm) greater for commercial gypsum and 87.1 times (1.94 log10 ppm; 95%CI 1.66-2.22 log10 ppm) greater for reprocessed gypsum.

Figure 14 Predicted mean hydrogen sulphide levels for slurry from both slatted and non-slatted systems

over a 21 day period after stirring, estimated using a mixed effects model

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* indicates difference between slatted and unslatted system wasstatistically significant (p<0.001)

21

4. DISCUSSION AND CONCLUSIONS

The presence of gypsum in slurry tanks or lagoons has the potential to enhance H2S generation and therefore could put farmers at increased risk if this is not recognised and the exposure not managed.

The observations recorded during this project confirm cattle slurry is capable of emitting high levels of H2S that – particularly in a confined space – has the potential to expose workers to H2S levels above both short-term and long-term WELs (10ppm and 5ppm respectively). Following agitation, levels of H2S were measured in excess of 250 ppm (tables 2,4,6), a level which can cause the olfactory nerve in an exposed individual to become paralysed after a few inhalations, leading to a loss of sense of smell; also leading to a lack of awareness of danger. However, these values must be kept within context; measurements were taken within a closed system with a modest headspace versus slurry volume and are not therefore directly representational of a large slurry tank or well-ventilated lagoon. Nonetheless, the data suggests cattle slurry has the ability to generate levels of H2S above WEL limits to dangerously high levels – particularly when in a confined or poorly ventilated space – and with it a risk to the health of those who might be exposed.

The data provides strong evidence that the inclusion of gypsum has the ability to greatly enhance H2S generation, further increasing exposure risks. Following agitation, levels of H2S were measured in excess of 1000 ppm (table 2, 4 & 6), which can cause immediate collapse with loss of breathing, even after inhalation of a single breath. Again, these values must be kept within context as mentioned above, but these observations do confirm that the inclusion of gypsum at concentrations of just 1% has the ability to greatly enhance the H2S generation beyond WELs and to increase them to dangerously high levels.

There was no statistically significant differences in H2S generation between the types of gypsum added (commercial or reprocessed gypsum), suggesting the overall effects were similar, since all of the tested material is essentially calcium sulphate. Therefore, any use of reprocessed gypsum is likely to be as potent, but no more so, than using commercial gypsum in terms of increased H2S generation, i.e. within the context of gypsum as bedding or within a slurry system (e.g. use on the parlour floor).

Observations from this project also suggested that emissions from the non-slatted slurry were greater than those measured for the slurry collected from a slatted system. However, any differences between these two slurry sources can most likely be attributed to the differences in the slurry itself. For this project, the slurry collected from the non-slatted system was sourced directly from the slurry tank, where it had been stored for a period of time, and mixed with slurry from previous days / weeks. The material was also heavily diluted with water and therefore likely to be rich in active sulphate-reducing bacteria (SRB). In comparison, the ‘fresh slatted slurry’ was collected straight from the parlour only a short time after animal defecation, therefore unlikely to contain the same levels of SRB as the slurry from the non-slatted system.

Trends in H2S generation over time vary between the three different slurry and bedding types and do not all follow a uniform pattern. Slurry sourced from the non-slatted system and dry bedding with water added both showed signs of peak H2S levels after 15 days, followed by a fall in gas levels thereafter. In the case of non-slatted slurry, these time trends corresponded with the period when gypsum was added. In the case of dry bedding (plus water), these time trends applied for all H2S levels, whether gypsum was added or not. The variation in bedding could be related to the addition of water, enhancing the anaerobic conditions required of the SRB but also elevating H2S generation in all test tubs through the process of mixing, irrespective of presence /

22

absence of gypsum. Trends seen with the non-slatted slurry indicate a 15 day period during which the SRB have an excess of available calcium sulphate (gypsum) leading to increasing levels of H2S, before peaking and presumably reducing as the calcium sulphate levels are exhausted. This association between gypsum and H2S production is further supported by much lower observed H2S levels in the gypsum free control. Increased H2S generation as a result of gypsum addition is therefore likely to occur in the days following that addition; in the context of a slurry tank or lagoon that is constantly fed with slurry from a slatted or non-slatted system, it is most likely that H2S levels will generally spike following any large addition of gypsum, or generally have higher levels as they are ‘drip-fed’ calcium sulphate through the slurry systems.

The observation that release of H2S from bedding was greatly enhanced by addition of water irrespective of the presence of gypsum also has practical on-farm implications. Bedding could become wet in localised areas of cattle sheds, such as near a water feeder / trough, through animal urination or rainwater / water leakage through roofs leading to increased bacterial metabolism and the potential to generate H2S. Disturbance of the bedding then lead to localised increased H2S levels.

In summary, there is clear evidence that the presence of gypsum in slurry will enhance the potential for generation of toxic H2S gas. The levels of the gas produced, even from the small, contained systems, would be toxic to anyone exposed to equivalent concentrations on a larger scale. Therefore, if gypsum residues enter slurry this could increase the risk of hydrogen sulphide accumulation in confined spaces in the close vicinity of slurry systems. It is important therefore that this is taken into account in managing risk.

Importantly, the levels of H2S gas produced from unamended slurry and bedding (no gypsum added) would still be sufficient to constitute a hazard to anyone exposed to it, though the addition of gypsum further increased the level of H2S gas production.

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5. REFERENCES

Muyzer, G. and Stams, AJM. (2008). The ecology and biotechnology of sulphate-reducing bacteria. Nature Reviews Microbiology. 6; 441-454. Abstract at: http://www.nature.com/nrmicro/journal/v6/n6/abs/nrmicro1892.html

Massé D.I, et al (2003). Methane emissions from dairy cow and swine manure slurries stored at 10°C and 15°C. Canadian Biosystems Engineering, volume 45, 6.1-6.6. Rodhe, L. et al. (2009). Emissions of greenhouse gases (methane and nitrous oxide) from cattle slurry storage in Northern Europe. IOP Conf. Series: Earth and Environmental Science 8 (2009) 012019. P.1-18. Cornwell, M. Improvement of the digestion of cattle slurry via the process of codigestion. University of Southampton, The School of Civil Engineering and the Environment, PhD Thesis, P.1-178. Karim, K. et al. (2005). Anaerobic digestion of animal waste: Waste strength versus impact of mixing. Bioresource Technology. Volume 96; issue 16, P.1771-1781.

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6. APPENDIX

6.1 BEDDING AND SLURRY DATA (NON-SLATTED & SLATTED) CONTINUED

6.1.1 Slurry data (non-slatted system)

The levels of hydrogen sulphide observed in cow slurry from a non-slatted system are summarised in Table 2.

The majority of observations before stirring showed no hydrogen sulphide, whether or not gypsum was added: just one (2%) observation was non-zero when no gypsum was added, 6 (13%) were non-zero after adding commercial gypsum, and 9 (19%) were non-zero after adding reprocessed gypsum (Table 2). Non-zero levels of hydrogen sulphide were measured in all samples after stirring (Table 2). The minimum observed hydrogen sulphide level after stirring was 49 ppm without added gypsum, 170 ppm with commercial gypsum, and 133 ppm with reprocessed gypsum (Table 2), which were all higher than both the long and short term WEL. Table 2 Levels of hydrogen sulphide observed in cow slurry over a 21 day period before and after stirring,

by type of gypsum added

Added gypsum Count Hydrogen sulphide levels (PPM) Non‐zero levels

Median Min Max Count % Before stirring None 48 0 0 4 1 2% 1% commercial 48 0 0 205 6 13% 1% reprocessed 48 0 0 312 9 19% After stirring None 48 126 49 330 48 100% 1% commercial 48 690 170 1,618 48 100% 1% reprocessed 48 560 133 1,580 48 100%

Table 3 shows the number and percentage of observations that were over the long and short term WELs before stirring. No observations were over either WEL when there was no added gypsum. When commercial gypsum was added, 10% (95%CI 3.5-22.7%) of observations over the 21 day period were above both the long term and short term WEL (Table 3). When reprocessed gypsum was added, 13% (95%CI 4.7-25.2%) of observations were above the long term WEL, and 6% (95%CI 1.3-17.2%) above the long term WEL (Table 3). There was no statistically significant difference in the number of observations over the WELs between the two gypsum types (Fisher’s exact test: long term WEL p=1.000, short term WEL p=0.714).

25

Table 3 Percentage of observations where the hydrogen sulphide levels observed in cow slurry before

stirring were over the long term and short term Workplace Exposure Limits (WELs)

Added gypsum Over the long term WEL of

5 ppm Over the short term WEL of 10 ppm

Count % 95% CI a Count % 95% CI a None 0 0.0 0.0‐9.7 0 0.0 0.0‐9.7 1% commercial 5 10.4 3.5‐22.7 5 10.4 3.5‐22.7 1% reprocessed 6 12.5 4.7‐25.2 3 6.3 1.3‐17.2 a, 95% exact binomial confidence intervals

6.1.2 Bedding data The levels of hydrogen sulphide observed in bedding are summarised in Table 4. The majority of observations before stirring showed no hydrogen sulphide, whether or not gypsum was added: just one (3%) observation was non-zero when no gypsum was added, 2 (6%) were non-zero when wet bedding was used, 4 (11%) were non-zero after adding commercial gypsum, and 6 (17%) were non-zero after adding reprocessed gypsum (Table 4). Non-zero levels of hydrogen sulphide were measured in the majority of samples after stirring (92%), excluding dry bedding which never observed any hydrogen sulphide emissions after stirring (Table 4). It was only the measurements taken on day zero that showed no hydrogen sulphide emissions after stirring. The maximum observed hydrogen sulphide level before stirring was 2 ppm for dry and wet bedding, and 3 ppm when gypsum was added (Table 4), which were all lower than both the long and short term WEL.

Table 4 Levels of hydrogen sulphide observed in cow bedding over an 18 day period before and after stirring, by type of gypsum added


Median Min Max Count % Before stirring Dry 36 0 0 2 1 (3%) Wet 36 0 0 2 2 (6%) Wet + 1% commercial 36 0 0 3 4 (11%) Wet + 1% reprocessed 36 0 0 3 6 (17%) After stirring Dry 36 0 0 0 0 (0%) Wet 36 108 0 1,190 33 (92%) Wet + 1% commercial 36 971 0 3,899 33 (92%) Wet + 1% reprocessed 36 573 0 3,940 33 (92%)

Table 5 shows the number and percentage of observations that were over the long and short term WELs after stirring. No observations were over either WEL when there was no added gypsum and the bedding was dry. When the bedding was wet and/or any type of gypsum added,

26

then 92% (95%CI 77.5-98.2) of observations were above both the long and short term WELs (Table 5). Table 5 Percentage of observations where the hydrogen sulphide levels observed in bedding after stirring that were over the long term and short term Workplace Exposure Limits (WELs)

Added gypsum Over the long term WEL of 5 ppm

Over the short term WEL of 10 ppm

Count % 95% CI a Count % 95% CI a Dry 0 0.0 0.0‐9.7 0 0.0 0.0‐9.7 Wet 33 91.7 77.5‐98.2 33 91.7 77.5‐98.2 Wet + 1% commercial 33 91.7 77.5‐98.2 33 91.7 77.5‐98.2 Wet + 1% reprocessed 33 91.7 77.5‐98.2 33 91.7 77.5‐98.2

a, 95% exact binomial confidence intervals

6.1.3 Slurry data (slatted system) The levels of hydrogen sulphide observed in cow slurry from a slatted system are summarised in Table 6, with box and whisker plots for levels after stirring shown in Figure 11. There were no observations before stirring that showed any hydrogen sulphide emissions. The majority of observations after stirring showed hydrogen sulphide, whether or not gypsum was added; this ranged from 67% of observations when commercial gypsum was added, to 100% of observations when the slurry contained farm-added-gypsum and commercial gypsum (Table 6). Table 6 Levels of hydrogen sulphide observed in cow slurry from a slatted system over a 25 day period,

before and after stirring and by type of gypsum added


Median Min Max Count % Before stirring FSS 51 0 0 0 0 0% 1% commercial 51 0 0 0 0 0% 1% reprocessed 51 0 0 0 0 0% FAGSS 51 0 0 0 0 0% FAGSS + 1% commercial

51 0 0 0 0 0%

After stirring FSS 54 24 0 740 46 85% 1% commercial 54 11 0 395 36 67% 1% reprocessed 54 12 0 358 37 69% FAGSS 54 89 0 449 52 96% FAGSS + 1% commercial

54 346 1 1,772 54 100%

FSS = Unamended fresh slatted slurry FAGSS = farm-added-gypsum slatted slurry

Table 7 shows the number and percentage of observations that were over the long and short term WELs after stirring. The proportion of observations over the long term WEL differed depending on what type of gypsum was added (Chi-squared test with 4 df, p<0.001), and similarly for the short term WEL (Chi-squared test with 4 df, p<0.001). The proportion over the

27

long term WEL ranged from 57% (95%CI 43.2-70.8%) when commercial or reprocessed gypsum was added, to 96% (95%CI 87.3-99.5%) when the slurry contained farm-added-gypsum and commercial gypsum was added (Table 7). The proportion over the short term WEL ranged from 52% (95%CI 37.8-65.7%) when commercial or reprocessed gypsum was added, to 94% (95%CI 84.6-98.8%) when the slurry contained farm-added-gypsum and commercial gypsum was added (Table 7).

Table 7 Percentage of observations where the hydrogen sulphide levels observed in cow slurry from a slatted system after stirring that were over the long term and short term Workplace Exposure Limits

(WELs)

Added gypsum Over the long term WEL of 5 ppm

Over the short term WEL of 10 ppm

Count % 95% CI a Count % 95% CI a FSS 41 75.9 62.4‐86.5 39 72.2 58.4‐93.5 1% commercial 31 57.4 43.2‐70.8 28 51.9 37.8‐65.7 1% reprocessed 31 57.4 43.2‐70.8 28 51.9 37.8‐65.7 FAGSS 47 87.0 75.1‐94.6 47 87.0 75.1‐94.6 FAGSS + 1% commercial

52 96.3 87.3‐99.5 51 94.4 84.6‐98.8

a, 95% exact binomial confidence intervals FSS = Unamended fresh slatted slurry FAGSS = farm-added-gypsum slatted slurry

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6.2 RESULTS FOR INTERIM METHOD OPTIMISATION REPORT; THE INFLUENCE OF GYPSUM ON HYDROGEN SULPHIDE PRODUCTION FROM CATTLE SLURRY

6.2.1 Method Methodology used for the optimisation pilot study mirrored those described in section 2. METHODOLOGY of the main report, with the exception of a requirement for slurry dilution as outlined below: SLURRY PREPARATION: Initially, ‘neat’ slurry was prepared for baseline H2S measurement. The use of neat slurry was initially chosen because the use of undiluted slurry was seen as the preferred option, to minimise any interference with the consistency of the test matrix and of the SRB activity within. Slurry was decanted in to 12 litre volumes in several tubs and allowed to settle for 24 hrs. Baseline levels of gas were then measured – both prior to and after stirring for 10 seconds with an electric drill and mixing paddle - to establish the residual H2S head space levels prior to any addition of gypsum. Initial levels of H2S gas were found to be far higher after stirring than anticipated (details below). These levels were at the limit of accurate detection using available monitoring equipment, and it was suspected that any further increase in H2S emission due to gypsum addition might be difficult to quantify. In view of this 1 in 4 dilutions and 50:50 dilutions of slurry were prepared and monitored over several days, initially without gypsum and later with addition of Newark gypsum at either 1% or 5%. In each case, duplicate tubs of slurry were prepared, to minimise measurement error and to allow mean values to be used for subsequent data analysis. The levels of methane (CH4) were also monitored along with H2S, mainly for ignition risk purposes.

6.2.2 Results Initial levels of H2S gas within the headspace of undisturbed, neat slurry tubs were found to be low or absent (0 ppm to 4 ppm). However, the action of stirring the slurry with a plasterer’s paddle and mixing drill for 10 seconds generated immediate H2S at levels in excess of 1000 ppm, and in some cases levels of between 3,000 ppm to 4,000 ppm were noted. Although 10 x 12 litre tubs of neat slurry were initially prepared for testing - in as near identical fashion as possible - some variation in H2S levels was evident. Methane levels (measured as % CH4) also varied between 0 and 0.5% for this undiluted slurry material; well below the ignition level (~3%). High H2S levels were therefore evident even prior to the addition of any gypsum. In view of the potential challenge of measuring these high levels accurately, a decision was made to dilute the slurry 1 in 4 with tap water (1 part slurry to 3 parts tap water; total volume 12 litres). Although this dilution factor inevitably altered the constituency of the slurry, and might potentially influence SRB behaviour, it was seen as the only logical way to reduce H2S emission levels to within a more measurable range. After preparation of control tubs of this type (1 in 4, no gypsum), test samples containing 1% and 5 % gypsum were also prepared and H2S gas measurements have been successfully made over the course of several days. (Figure 15). Data readings were not taken every day, but for Figure 15 it was possible to provide a most likely fit for a data line passing through points for each day. This was achieved by taking the data from the three days before and the three days after each actual data reading, along with the day itself, and then calculating the mean of the 7 days - this smooths out short-term fluctuations and

29

highlights longer-term trends. A 7 day resolution was chosen because this was the smallest time period possible to ensure that the 7-day window always had at least two data points within it (Figure 15). An additional plot is shown at Figure 16. This illustrates that adding gypsum almost always resulted in greater levels. For each day, the measured H2S levels for the slurry with gypsum were plotted against that of the control. The blue line in Figure 16 represents the reference line; if the gypsum slurry measured the same as the control slurry then the point lies on the line. If the point is above the blue line then this means that the sample with added gypsum is greater than the control on that particular day. If the point is below the blue line then the sample with gypsum was lower than the control. It can be seen from Figure 16 that the samples with gypsum were almost always greater for measured H2S than the control samples. In parallel to the 1 in 4 slurry dilutions, an additional assessment has involved the use of slurry diluted 50:50 with tap water (total volume 12 litres). This less dilute slurry was prepared because, initially, the 1 in 4 dilutions appeared to emit very little H2S, even in the presence of gypsum, and there was a concern that the 1 in 4 slurries might not provide the required SRB activity. The 50:50 slurry dilutions were similarly amended with either 1% or 5% Newark gypsum and gas emissions noted over several days (Figure 17). As with the 1 in 4 slurry dilution data trend indication (Figure 16), a similar graphical representation was also prepared for the 50:50 dilution data. These are shown in Figure 18 and the data were prepared in a similar way. Spot checks for sulphate reducing bacteria (SigSulphide agar culture tests) confirmed their presence.

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Figure 15. H2S generation (ppm) vs. number of days monitored for 1:4 diluted slurry without

gypsum, and with gypsum added at 1% and 5% concentration.

Figure 16. Observable trend for H2S generation at 1 in 4 dilution vs. control (blue reference line)

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Figure 17. H2S generation (ppm) vs. number of days monitored for 50:50 diluted slurry without

gypsum, and with gypsum added at 1% and 5% concentration.

Figure 18. Observable trend for H2S generation at 50:50 dilution vs. control (blue reference line)

Published by the Health and Safety Executive 01/15



RR1041

www.hse.gov.uk

The aim of this study was to determine the influence of gypsum (calcium sulphate) on bacterial populations in slurry systems and, as a consequence, its potential to enhance hydrogen sulphide (H2S) generation. The study focused on three slurry / bedding types: slurry from dairy farms that use either a non-slatted collection system or a slatted system, and dry, soiled bedding recovered from sheds used to house beef cattle over winter.

There is clear evidence that the presence of gypsum in slurry will enhance the potential for generation of toxic H2S gas. The levels of the gas produced, even from the small, contained systems, would be toxic to anyone exposed to equivalent concentrations on a larger scale. Therefore, if gypsum residues enter slurry this could increase the risk of H2S gas accumulation in confined spaces in the close vicinity of slurry systems. It is important therefore that this is taken into account in managing risk. Importantly, the levels of H2S gas produced from unamended slurry and bedding (no gypsum added) would still be sufficient to constitute a hazard to anyone exposed to it, though the addition of gypsum further increased the level of H2S gas production.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

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