Estel·la Pagans^{1, **}, Rita Domíngues¹, Anton Philip van Harreveld ¹

¹ Odournet SL. Crta Esglesia 60B. 08017 Barcelona.

^* Contact author: epagans@odournet.com

    Summary

   Odour emissions from intensive livestock activities are currently one of the biggest problems in areas with a high density of livestock compared with the human population density. At the same time, odour emissions make expanding these units of production difficult, and if they are to be maintained or even increased, it will be essential to reduce these emissions. In this context, it is essential to use the best available techniques (BAT), primarily to prevent and reduce the generation of odours through strategies integrated into the process, such as best practices and the drafting and implementation of an Odour Management Plan (OMP). In case of continued discomfort caused by odours, the application of end-of-pipe technology treatment will be necessary. This article presents the most common methodology to assess the impacts of odours from a livestock operation as well as the main strategies and treatment systems aimed at reducing odour emissions and their impact.

    Key words

   Best Practices, Management, Impact, Odours, End-of-pipe technologies treatment

    IMPACT ASSESSMENT METHODOLOGY FOR ODOURS IN THE LIVESTOCK SECTOR

   The procedure for approaching an odour impact study includes identifying the sources of odour emissions, quantifying the rate of odour emissions and assessing the risk of generating an impact in the vicinity.

    Inventory of sources: review of the production system

    The majority of emissions generated by intensive livestock operations are produced as a result of natural processes, such as the animal's metabolism and the degradation of slurry or manure. In general terms, the main stages that act as odour generators on a livestock operation are activities in the housing, storage and management of livestock waste.

    Emissions are highly variable and depend on many factors, especially those associated with the design and maintenance of the facilities, as well as the type of management performed during the storage, treatment and agricultural application of the waste. Table 1 shows a summary of the sources and the factors that influence the generation and emission of odours on a farm.

    Usually, the quantity, quality, and the composition of the livestock waste and the way it is handled and stored are the main factors that determine the emissions levels of polluting and odorous substances. It is therefore of vital importance to consider all factors that influence the characteristics and composition of the manure and slurry produced on a farm.

    These factors are affected primarily by the type of feed supplied, defined by the concentration of nutrients and the efficiency with which the animal can transform it into product. Due to the high variability in feed characteristics, the concentration of nutrients in livestock waste is also variable.

    The measures later applied and associated with the housing and the waste collection systems, the storage and treatment applied, will equally affect the composition and final characteristics of the waste as well as emissions derived from its agricultural application.

   Table 1. Summary of sources and potential odour generating factors and with a high risk of impact

   As a result of this high degree of variability the need becomes apparent for each facility to identify their own odour generating sources and include them in an Odour Management Plan (OMP), to be able to influence them when reduction or impact minimisation strategies are proposed.

    Estimation of odour emissions: emissions factors

    The most advisable methodology to calculate the rate of odour emissions derived from livestock operations is based on making use of emissions factors, mainly because the majority of facilities are not enclosed and possess numerous fugitive odour sources that are difficult to quantify.

    Emissions factors are numerical values based on multitude of samples from different livestock facilities analysed by dynamic olfactometry, which have provided an emissions rate per animal unit. In this way, quantification is performed by multiplying the odour emissions rates established for each type of animal by the total number of animals of the same type at the facility.

    There are a number of publications that provide odour emissions factors for this sector (Clarkson and Misselbrook, 1991; EA, 2003; EPA, 2001; Melse and Ogink, 2005; Peirson and Nicholson, 1995). Table 2 shows the applicable emissions factors in livestock operations according to the Annoyance from Odours and Livestock Regulation in the Netherlands (VROM, 2006).

Table 2. Emissions factors for a livestock operation

    Impact assessment: modelling of atmospheric odour dispersion

     The impact of odour emissions in the vicinity of an operation will essentially depend on the magnitude of the emissions rate at the facility, the proximity of sensitive receptors and the local topography and prevailing weather conditions. In order to take all of these factors into consideration and determine the odour immission values generated by the activity, the application of mathematical models to simulate odour dispersion must be used.

    Most of the atmospheric dispersion models used to characterize impacts in small-scale situations (> 10 km) are Gaussian models, for example: ISCST (Industrial Source Complex Short Term), Aermod, ADMS or Pluimplus (regulatory model in the Netherlands). Gaussian models are steady state models that calculate the distribution of ground level concentrations downwind of the emission source. This approach has several limitations that usually result in an overestimation of the impact, such as the limited capacity to properly simulate low wind speed conditions.

    There is a new generation of models that more closely simulate real emissions, improving on many of the limitations of the Gaussian models. This applies to the Calpuff model, which renders the emissions as a series of discharge points ("puff") that are collected by the wind flow and disperse as they move along the surface layer of the atmosphere. In this sense, the Calpuff model allows for modelling situations that are very common in Mediterranean climates (low wind speeds and high percentages of calm) without overestimating the odour impact area (Van Harreveld et al., 2009).

    The results of the dispersion models are presented in maps that show the odour concentration in the vicinity of the emitting source, identifying those areas where the concentration exceeds the impact criteria deemed to be acceptable. These maps are made by isopleths of odour concentration, lines connecting points at a frequency that matches the odour occurrence. These contours indicate, for example, that the area in which 98% of the hours of the year the maximum ground level concentration (in immission) averaged in 1 hour is x ou_E·m^-3. In abbreviated notation:C _{98, 1-hour} = x ou_E·m^-3.

    Immission standards and current legislation

    Currently there is no specific European Union legislation that establishes air quality standards for odours. However, in some member states there are guides and regulations which limit the odorous emissions for each activity and establish standards for acceptable air quality (EA, 2002; EA, 2005; EPA, 2001; MWLA, 2005).

    The only reference in Spain was issued by the Regional Ministry of Environment of Catalonia, Spain in the Initial Version of the Draft Law against Odour Pollution (DMAH, 2005). The nature of the aforementioned document is preventative and refers to all activities that potentially produce odour, including livestock activities. It also sets the maximum odour immission values in affected residential areas by using the best available techniques (BAT), and the application of best management practices or with the implementation of corrective measures. According to this regulation, the indicative impact standard for odours from livestock activities is set at C_{98, 1hour} = 5 uo_E·m^-3. This standard can be translated to say that for 98% of the hours in a year, the maximum ground level odour concentration as an hourly average cannot exceed 5 uo_E·m^-3, as odours below this level are unlikely to cause discomfort in the exposed population.

    An association with discomfort from odours in the surroundings of a specific activity has been determined at the 98^th percentile by dose-effect epidemiological studies, carried out especially in the Netherlands. Thus, the effect of the livestock odours on the population has been established through questionnaires and the collection of complaints over time, differentiating between the upper and lower levels of tolerance to the odour among subsamples of the population.

    In the case of the Netherlands, for odours from livestock activities the indicative impact standard according to the Regulation of Annoyance from Odours and Livestock is set at C_{98, 1 hour} = 8 ou_E·m^-3(VROM, 2006).

    There are other studies, carried out in the Netherlands and in England, where the impact that can be caused by livestock activities has been assessed in relation to the distance between these activities and potential recipients (EPA, 2001). This is also the case in Germany, where these separations are regulated depending on the type of animals, stabling, food provided, etc., and the productive capacity of the operation (VDI3473p1, 1994). For example, according to the cited regulation, for an operation of 1,200 large calves, the minimum legal distances between an operation and a residential area vary between 540 and 740 m depending on the type of stabling, feeding, etc.

    Another interesting standard worth mentioning is the one that sets the environmental evaluation guide for livestock operations by the EPA in Australia (EPA, 2008). In this case, a livestock operation with more than 200 heads of calves for fattening must be located more than 500 m from a residence and more than 1,500 m from an urban centre.

    ODOUR EMISSIONS CONTROL OPTIONS IN THE LIVESTOCK SECTOR

    In any sector, and according to the IPPC Directive, the use of BAT for combating pollution is fundamental. The Current European Reference Document of BATs for the Intensive Breeding of Poultry and Pigs (BREF, 2003) is being reviewed by the technical working group and is expected to submit a first draft of the version reviewed in July of 2010 (CD, 2009).

    The BATs to control odour emissions in the livestock sector focus on prevention through the application of best practices that allow the reduction in odour generation, its transfer into the atmosphere and onto the exposed surface of the waste. If after having applied all reasonable measures of prevention there is still discomfort caused by odours, it will be necessary to consider end-of-pipe technologies for control that would be adequate based on the nature and type of emission source. This fact requires confining odour emissions at the point of generation and extracting them to a treatment system with a minimum of fugitive emissions.

    Some preventative practices and end-of-pipe techniques for odour control applicable in the livestock sector are mentioned below.

    Implementation of best practices

    General aspects

    One of the first stages where impact from odours is prevented is during planning and territorial zoning, placing livestock activities at a sufficient distance from isolated houses or urban centres.

    Likewise, several decisions made by the farmer, such as the location of the slurry and manure storage areas, the times in which certain operations are performed to avoid adverse weather conditions (prevailing winds in direction of potential recipients, atmospheric stability, days of the year in which recipients perform outdoor leisure activities), etc., can play a very important role in reducing the odour impact.

    Other essential considerations are the implementation of a PGO (detailed later) and the establishment of maintenance and cleaning programmes that ensure that equipment remains in good condition and that the facilities and animals are clean.

    Application of nutritional techniques

    Nutrition techniques seek to avoid an excess of ingested nutrients and improve the efficiency of their use by the animal. Reducing the excretion of nutrients (nitrogen and phosphorus) and therefore, their concentration in the waste, can reduce emissions that would be generated throughout the process (housing, storage, management and agricultural application). The adoption of these techniques has been shown to reduce ammonia emissions up to 50% in pigs, birds and cattle (Melse et al., 2009). However, they either have a null effect on odour (Lyngbye et al., 2006) or a limited one, between 25-30% (EPA, 2001). In the same way, the efficiency of odour removal through the use of additives has not been shown (EA, 2005).

    The application of improvements in the design and management of housing

    The design of the housing, i.e., the combination of the type of soil and the system of collection and disposal of waste, largely determines the ammonia emission level. In this sense, the improvements applied are aimed preventing and reducing this compound, reaching reduction efficiencies of between 30% and 80% (BREF, 2003; Melse et al., 2009). At the same time, they also reduce odour levels (EPA, 2001). The most prominent techniques are the reduction of the surface of waste emissions (odour emissions increase along with the grid floor surface area), the quick withdrawal of the waste outside the housing, the cooling of the slurry surface, the increase of straw content and its restocking to better retain the N in the case of manure, or the modification of the physico-chemical properties of the waste (pH reduction).

    Optimisation of the ventilation of livestock sheds

    The ventilation and air conditioning systems must conform to the needs and comfort of the animals. The ventilation rate varies depending on the season. When external temperatures are high, the indoor air renewal rate tends to increase, along with odour emissions. Some studies indicate that rate of odour emissions is between 3 and 5 times higher during the summer (Lyngbye et al., 2006), when there is a greater risk to cause discomfort from bad odours in neighbouring communities, compared to the winter. Some experiences have demonstrated reductions in the odour emissions rate through the cooling of entry air (Lyngbye et al., 2006).

    On the other hand, poor ventilation rates can cause excess humidity and an excessive accumulation of odour. In the case of pig operations, an insufficient renewal of air with an excessively high temperature inside the housing can modify the behaviour of animals causing an increase in ammonia emissions.

    Optimisation of the atmospheric dispersion of odours

    In sheds with forced ventilation, fans that direct emissions up above the roof are better than those located in the side walls, as the former achieve greater dispersion of the residual air in the wind. In the mechanically ventilated housing, if the point of emission is raised 4 -5m above the roof the turbulence effects are limited and there is an increase in the dilution of odours downwind. This fact is especially noticeable at relatively short distances, up to 300 m.

    Directing the ventilation through a single chimney emitting at a greater height will further improve the dispersion of odours, although this can cause the emissions to be detected at greater distances. In general, increasing the height of the chimney between 10 and 25 m, delivers very significant benefits in terms of downwind dilution. However, the calculation of the minimum required height of the chimney requires the use of odour dispersion models.

    In short, residual air must be evacuated through a chimney with sufficient height, ascending vertically through the roof and without covers that would impede its circulation. At the same time, by providing an additional push to the exhaust air, in other words, by optimising the vertical speed of the emission flow, there is an increase in the height of effective odour emissions and thereby benefits downwind dilution. Optimum exit speeds are between 10 and 20 m/s^{- 1}.

    Application of odour modifying agents

    When modifying agents are applied, certain volatile substances are discharged into the environment that mingle directly with the odorous air in order to mask, counter or neutralize it. However, there are few data that quantitatively demonstrate its effectiveness in terms of odour reduction. These systems may be beneficial during short periods of time in specific problem situations. In the long term they may be counterproductive, as the masked odour can become a nuisance, especially if the intensity is greater than the original odour.

    Application of BATs during the storage and handling of manure and slurry

    During storage and under adverse conditions, small volumes of highly concentrated air can travel very far and be detected downwind of the source causing a significant impact. Just above the surface where the slurry is stored, for example, odour concentrations that exceed the tens and even hundreds of thousands ou_E·m^-3, can be reached while emissions from housing ventilation rarely reach values of 5,000 ou_E·m ^{-3. In this context, there are opportunities to minimise odour impacts in both the storage and handling of waste.}

    The storage of slurry in farms can occur in tanks or ponds. To reduce odour emissions it is important to reduce the evaporation of gases from the surface and locate the ponds based on the prevailing winds and far from residential areas. The level of evaporation can be maintained at a low level if the disturbance and turbulence of the waste water is minimal, favouring the emergence of a crust on the surface. Emissions from a surface with turbulence, compared to an undisturbed surface, increases in order of magnitude by 10. So, in open systems the filling of the slurry should be made from the base and provide disturbance only before its application to minimise its heterogeneity. Another alternative is to reduce the surface of the storage area through deeper ponds of smaller diameter.

    The most effective technique to minimise the rate of emission is achieved by covering the storage systems, although sometimes this involves management problems and costs. The covers can be fixed, rigid, flexible, or floating. Neck covers must be airtight, except when associated with the production of biogas, in order to avoid the accumulation of toxic gases and the risk of explosion.

    In the case of the fixed covers, the odour concentration in the headspace can be extremely high, and when the cover is withdrawn if the odour does not dilute properly, significant discomfort can be caused to nearby recipients. Floating covers have the advantage of not having headspace, however, they do not work properly if turbulence on the surface is not minimised. There are some new configurations of permanent floating covers, for example a 1 mm thick reinforced plastic (PVC) canvas, which includes an emissions extraction system.

    To reduce odour generation during the storage of solid waste, such as manure, it must remain dry and in a barn or covered area, with impermeable soil and sufficient ventilation. Walls (wood, cement or brick) can be constructed that act as screens against the wind, placing the opening of the warehouse downwind of the predominant wind direction.

    Application of BATs during the application of manure and slurry to fields

    Odorous emissions from the agricultural application of manure are one of the main reasons for discomfort and complaints from nearby communities. During application odours can be detected at distances between 1,000 and 3,000 meters from the field. However, there are several factors that affect this impact during and after application, such as the odour potential of the waste (especially if they contain some other type of residue), duration and method of storage and pre-treatment, equipment and methods of application, the applied dose and weather conditions at the time of the application.

    The application with a spreader on a conventional surface, with a diffuser plate system, produces small drops that maximise the release of odorous compounds. The larger the drops, the lower their trajectories and lower the release of odours at the time of the application, although problems may remain afterwards. In this sense, when potential recipients exist nearby, this type of application technique is not recommended.

    Some application techniques that can minimise odour emissions with their corresponding reduction level in comparison with the use of a spreader on a conventional surface, are: hose systems that directly apply the slurry on the surface of the ground with a 55% - 60% reduction in odour, a disk system that applies the slurry through a shallow cleft in the ground with a 55% - 60% odour reduction, and the injection of the slurry into the field, with an 85% odour reduction in surface injections with closed groove (50 - 80 mm) and deep (120 - 130 mm).

   Odour Management Plan (OMP)

The OMP sets the guidelines to follow to achieve the proper management of odours. At minimum the following aspects should be included:

• The identification of staff with roles and responsibilities to ensure that the odour management system is established, implemented, maintained and improved.
• Identification of odour sources and the quantification of their emissions rate.
• Enumeration of preventive and/or corrective measures as well as best practices applied to minimise odour emissions in each previously identified source.
• Description of the monitoring mechanism that allows the supervision of the effectiveness and the critical parameters of each odour control element.
• Emergency protocol that define all potential situations that could occur due to odours and the appropriate response if they were to occur.
• Definition of all of the processes for the registration and management of incidents and complaints about odours. Especially define how complaints are received, documented and responded to.
• In this context, to be able to demonstrate the application of the BATs, all practices and odour reduction strategies should be reflected in the OMP.

    End-of-pipe technologies for odour control

    The implementation of end-of-pipe technologies for odour control in the sector is generally difficult. This is because they can only be applied in housing with forced ventilation systems and that the treatments, though they are technically feasible, are not economically profitable. The additional costs involved in this application should be financed, for example, through a higher level of productivity and improvement in the quality of the meat (Lyngbye et al., 2006). It is also necessary to reduce the costs in both the investment and maintenance of treatment equipment and improve the control and monitoring process to ensure proper efficiency of odour removal in the long term (Melse et al., 2009).

    Currently, the most widely used technologies are chemical, such as chemical washing, or biological, such as biotrickling filters or biofiltration. In the Netherlands, more than 900 operations apply this type of treatment, purifying a total of 79 million m³·h^{- 1}. Currently there is a growing trend in the use of these technologies in Northern Europe, which coincides with the increase in the scale of operations to reduce these treatment costs, and the need to comply with European air quality environmental regulations.

    In chemical wash systems the air to be purified from the housing comes into contact with an acidic solution that generally flows upstream, so that the ammonia is transferred to the aqueous phase. In this system sulphuric is acid the washing liquid that is mainly used, though hydrochloric acid can also be used. Part of the washing solution can be recirculated while the rest will require later treatment. In the case of the biofiltration the air to be treated is uniformly distributed over the surface of a bed of porous organic or inorganic filler, where a microbial biomass is developed that is able to degrade pollutants, which have been previously adsorbed and absorbed into the filter material. The biotrickling filters have the same operating principle as the biofilters, with the exception that an aqueous phase is continuously recirculated on the filling, which tends to be inert material. In this way, a microbial biomass is developed on the filling, which has adapted to metabolise the soluble contaminants that have been transferred from a gas to a liquid state.

    Numerous studies show ammonia removal effectiveness above 90% and between 50% - 90% when using acidic cleaners and the biotrickling filters respectively to deal with emissions of from the housing of livestock operations (BREF, 2003; HAHNE et al ., 2000; Melse et al., 2005). However, they are more limited with respect to odour reductions.

    The efficiency of odour removal from air treatment equipment in a livestock operation designed to remove ammonia exclusively tend to be of the order of 30% for the chemical cleaners and 45% for the biotrickling filters (Melse et al., 2005). This is because the odour coming from farms is a complex mixture of many compounds (Aarnink et al., 2005) which includes not only those odours which are easily removed, such as soluble and easily biodegradable compounds in the case of biotrickling filters, and alkaline compounds in the case of acid scrubbing.

    The efficiency of odour removal by this equipment can be improved by adjusting the design and operating strategy of control units, thus occasionally reaching efficiency rates of up to 90-95%. However, these improvements imply higher investment and maintenance costs per volume unit of treated air.

    An improvement would be to add more than one phase of treatment, so that each one seeks to remove a few target compounds and at the same time to increase the retention time of the gases inside the purification equipment. This is the case of employing a biofilter after having previously treated emissions through chemical washing. Although biofilters have been widely used for livestock operations (especially in the Netherlands), high concentrations of ammonia and dust particles may limit its long-term effectiveness. This pre-treatment would avoid the problems of acidification and plugging of the filler medium. Following the same approach, it is possible to add a second phase of oxidative treatment instead of biofiltration, through for example hydrogen peroxide, ozone or UV radiation. Finally, another alternative that is still in the research phase and operates only on some farms, is a prototype that combines the concepts of acid scrubbing, biotrickling filters, water curtains and biofiltration (Ogink et al., 2008).

   The performance of odour elimination in the case of the biotrickling filters can be improved with adequate control and monitoring of the process, for example avoiding the accumulation of NH₃ and NO₂^- in the system, compounds that can be toxic and inhibit microbial activity.

   Finally, further development is needed in this field to study the main causes of the low efficiency of odour removal with treatment equipment and achieve an efficient odour treatment technology that is also economically profitable for the sector. A useful tool to do this can be found in studies that combine olfactometry analysis with the identification and quantification of the chemical compounds responsible for odours by gas chromatography and mass spectrometry (GC-MS).

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