Diffusion model simulation of odor pollutants in livestock and poultry farms based on environmental monitoring of the Internet of Things

: Livestock pollution is one of the main sources of agricultural pollution, which has a negative impact on the global environment. Monitoring, simulation and early warning of major pollutants emitted from livestock production is of great significance for reducing agricultural pollution. Especially, real-time comprehensive monitoring and early warning of the concentration and distribution of harmful gases could improve the harm of livestock production to people, livestock itself and the environment, and increase the safety level of livestock production. This study focused on the perception and monitoring of the discharge status of livestock farming simulation technics, mainly to carry out Internet of Things-based monitoring of the main components of livestock culture pollutants, and to use odor gas air dispersion software “ModOdor” to simulate the spread of pollutants. This study was aimed to determine the characteristics of pollutant diffusion in typical farms, which could provide decision reference to odor hygienic buffer zone and minimum shelter distance to achieve the ecological and safety objectives of livestock farming.


The main components of odor pollution and the release source intensity
The main odor pollutants produced by livestock and poultry farms include ammonia (NH3) and hydrogen sulfide (H 2 S).Nitrogen-containing organic matter in animal and poultry manure breaks down to produce ammonia under the effect of urea enzymes.The evaporation of ammonia can occur at all stages of livestock and poultry production, but mainly in the process of fecal urination and fecal storage, as well as after usage into soil.Microorganisms in livestock farms break down sulfur-containing organic matter from sulfates and feces in water into H2S gases inanaerobic environments.The sources of H 2 S mainly include feces that are not cleaned in the animal house in time, as well as manure stirring, biogas pumping out fertilization, marsh slag cleaning and so on.
The odorous composition of livestock and poultry is complex, which mainly include volatile fatty acids (VFAs), phenols, alcohols, aldehydes, ketones, esters, ethers, amines, hydrocarbons, halogenated hydrocarbons, sulphides, nitrogen heterocyclics, and aromatic [1][2][3][4][5] .Various types of VOCs contain different substances that can be further subclassed into subclasses.In addition to organic components, odorous substances include inorganic components such as ammonia and hydrogen sulfide [6] .Due to the complexity and diversity of animal and poultry odorous substances formed under different conditions, and the easy migration and transformation in the atmosphere, it is not entirely clear how to pay tribute to the stench of livestock and poultry under all kinds of odorous substances.From the existing studies, it is generally believed that the substances that play a key role in the odor of livestock and poultry are mainly volatile fatty acids, sulfur-containing compounds, aromatic compounds (mainly phenols and benzpyrole), as well as ammonia and volatile amine [7][8][9] .
VFAs are considered to be the most important component of animal and poultry odors.About 60% of VFAs are acetic acid, followed by acetic acid, orthobutyric acid, isobutyric acid, isoprene acid, orthoic acid, and oxalic acid.Sulfur-containing compounds generally have a strong irritating odor and are an important component of the odorous substances of livestock and poultry.Of the substances with the lowest odor thresholds identified, 6 are sulphide-containing compounds.The main sulfur-containing compounds are hydrogen sulfide, methanol, propylene thiol, dimethyl ether, dimethyl dithion ether.Most of the sulfur in livestock droppings is distributed into the air in the form of hydrogen sulfide and methanol, and 2 substances account for 70% to 97% of volatile sulfur-containing odors in livestock and poultry.Pig droppings Volatile hydrogen sulfide mass concentration reached 90 μgꞏm -3 [10] , while pig house air methanol mass concentration reached 3.6×10 4 μgꞏm -3 , which is 947 to 120 ×10 6 times the self-smell threshold [11] , because methanol is considered to contribute the most to the odor of livestock and poultry sulfur compounds .Under normal circumstances, the concentration of sulfur-containing compounds is significantly higher than VFAs, and the odor threshold of sulfur-containing compounds is lower than VFAs, so sulfur-containing compounds are generally considered to play a stronger role in odor than VFAs.
Ammonia has a strong irritating odor, and compared with other odorous components, ammonia has a relatively high odor threshold (0.3×10 -6 -53×10 -6 ), so in early studies ammonia was once considered as the only indicator of the odor of livestock and poultry [12][13] , so far many countries have also used ammonia as the main indicator of odor evaluation [14] .However, it has also been found that there is no significant positive correlation between ammonia and odor [15] .Volatile amines emitted during livestock and poultry breeding are mainly methamidine, acetamine, triamcinolone, carcassamine and rotting amine.Volatile amine accounts for a small proportion of volatile nitrogen compounds, and there are very few studies on the determination of volatile amine emission concentration.
From an environmental toxicity perspective, the 50 VOCs emitted from livestock and poultry are classified into 9 categories, of which 5 are dangerous atmospheric pollutants.In addition, different types of livestock and poultry produce different odor components, pig manure discharge odor substances to volatile low-grade fatty acids mainly [16] , chicken manure odor components to NH 3 , dimethyl sulfur and hydrogen sulfide [17][18] , cow dung odor to low-grade fatty acids [19] .

Release source intensity of odor pollution from dairy farms
Studies had shown that the release of NH 3 varies greatly in the same species of animals during different growth periods and under different breeding conditions.Under the condition that the farmers raise dairy farms in bulk, under the condition that the "solid floor type" (the floor of the cowhouse is made of stone or cement) is raised, the NH 3 release of the cowhouse is greater than that of the storage process, while under the feeding conditions of the "straw mat type" (the floor of the cowhouse is paved with straw and other materials), the release amount of NH 3 during the storage process is greater than that of the cowhouse.Under intensive breeding conditions, the release of NH 3 during cattle sheds and storage is equivalent.The specific release source is strong in Table 1.At present, domestic and foreign research on the release of livestock and poultry H 2 S, especially long-term and high-frequency monitoring is less, the existing research mainly focused on the concentration of H 2 S in pigpens.

Release source intensity of odor pollution from laying hens farms
In 2002, Lim et al. measured H 2 S concentrations of 4.71μg/m 3 and 29.9μg/m 3 [20] in vents and exhaust vents of a nearly 250,000-scale laying hens farm.Studies shown that the volatile amount of NH 3 in livestock and poultry feces is generally 0.007%-0.354% of the total amount of feces, while the volatile amount of NH 3 in cow dung is 18.6% of pig manure, while the NH 3 volatility of chicken manure is 1.8 times that of pig manure.3 ModOdor-Odor Gas Air Dispersion Software Based on convection diffusion equation, ModOdor can be applied to the numerical solution model software for the atmospheric diffusion of local-scale odorous gases and can be used for atmospheric diffusion simulation & concentration prediction of odorous gases.ModOdor realized the numerical simulation method of the finite difference between the three-dimensional atmospheric diffusion of pollutants, focusing on the atmospheric diffusion of pollutants in medium and small-scale research areas.

Internet of Things-based monitoring of livestock and poultry farming environments
ModOdor enables multiple pollutant composition simulations that can simultaneously simulate atmospheric diffusion of up to 60 compositions.At the same time, ModOdor is able to simulate up to 3000 wind speed field scenarios simultaneously when the wind speed field is selected for the same wind speed across the region.
To make the software easier for users to use, ModOdor uses a consistent window form to build the solution, as shown in Fig. 2. The user interface of the solution consists of 2 sub-windows: the upper sub-window is a conditional window, mainly used for parameter entry, conditional selection, running operations, etc., and the sub-window is composed of a simple text editor in RTF format, which is mainly used to display the results of the calculation.When ModOdor completes the calculation, the new calculation is placed in this window.ModOdor provides default data support system, full data entry interface system, entry error automatic identification system, entry parameter legitimacy check system, function and formula entry system, calculation results illustration system, help system, users can easily, easily and quickly enter parameters, implement calculations and draw calculation results of the drawing (Fig. 3.).

Governing Equation
ModOdor uses convection-diffusion spreads as a calculated governing equation, under the above assumptions, the coordinate origin is located in the lower left rear corner of the study domain, the x-axis points to the right, the y-axis points forward, the z-axis points up (Figure 4), and can establish a mathematical model of the non-steady state flat current diffusion of the foul-smelling gas, the micro equation is as follows: w ( ) ( , , ) 0 The first type of boundary condition ( , , ) 0 ( , , ) 0 (5) C(x, y, z, t): concentration of odor gases, gm 3 ; K x , K y , K z : the principal value of the turbulent diffusion coefficient tensor, m 2 ꞏs -1 ; W: gas sink intensity, volume of gas taken away per unit of time sink volume (input positive value), m 3 ꞏ(m 3 ꞏs) -1 , whose concentration is equal to C(x, y, z ×,t); S：source sink term, gm 3 s 1 ，the mass of odor gases emitted per unit of time unit of source volume; k: first-level chemical reaction constant, s -1 ; k w : removal factor in wet deposition, s -1 ; u x , u y , u z : coordinates of wind speed, ms -1 ; C 0 (x, y, z): The given concentration on the first type of boundary condition  1, gm 3 ; f(x, y, z, t): A given turbulence diffusion pass on the second type of boundary  2 , gm 2 s 1 , for the odor gas mass that enters the study domain vertically through the unit boundary area per unit of time due to turbulence diffusion; g(x, y, z, t) :The amount of flat and turbulent diffusion diffusion given on the third type of boundary  3, gm 2 s 1 , the odor gas mass perpendicular to the study domain perpendicularly through the unit boundary area under the joint action of wind speed and turbulence diffusion; cos(n, x), cos(n, y), cos(n, z): direction cosine; G: Spatial research domain; : Boundaries of the study domain,  1 + 2 + 3 =; x, y, z: the position of the calculated point, m; t: calculated time, s.

The division of the time domain and the spatial domain
ModOdor allows you to set "in-domain mesh" and "out-of-domain mesh" in differential meshes, a feature that allows ModOdor to finely simulate complex changes in terrain and ground buildings, making our models widely applicable.The term " out-of-domain mesh " refers to which grids in the differential grid are outside the boundaries of the study domain.Out-of-domain mesh do not participate in simulation calculations because they are outside the study domain.All grids except the out-of-domain grid are in-domain grids.An in-domain grid is a grid that implements simulated calculations.Fig. 4. shows ModOdor's situation in which complex terrain conditions are depicted by setting an out-of-domain mesh.If the simulation area has terrain fluctuations, the height of the lowest point in the simulation area is calculated to be z 0, and the study domain issectioned to form a differential mesh.Based on the intersection of differential mesh and terrain, the mesh is determined to be in-domain or out-of-domain mesh: the mesh below the surface of the earth is an out-of-domain mesh, and the reverse is an in-domain mesh.In this way, the ups and downs of the surface (including buildings) are approximated by the changes in the folds of the differential mesh.The finer the differential mesh, the higher the accuracy of this approximation.(Fig. 5.)

Basic information on livestock and poultry farms
In this study, three different types of intensive farms (RM Ranch, GDK Pig Farm, LDK Chicken Farm) of medium size in Z County, Province B, were selected as the research objects.The scale and basic conditions of farming are in table 3.

Weather conditions
The farm selected in the simulation case is located in B City Z County, S Province, is a warm temperate humid continental monsoon climate zone, the annual average temperature of 13.1 ℃, relative humidity of 64%, the dominant wind direction is west wind (wind frequency 9.76 percent), static wind frequency is 9.69%, the average annual wind speed is 2.6 m/s.

The ModOdor model simulates input conditions
The concentration distribution of the main odor pollutants (NH 3 ) in the downwind of three farms was simulated using the ModOdor model, and the input conditions of the model were as follows: -

Results
The results of software simulation show that the odorous pollutants of livestock and poultry farms are mainly produced in the "house-fecal storage" link.In the farm, when the feces moisture is too much or no fresh air, it will form a local oxygen-free environment in the feces.
Thereby producing and releasing odor gas.Similarly, sewage discharged from farms produces odorous gases when oxygen is scarce.
Compared with the three main breeding categories, dairy farms are in a semi-open environment, the resolution storage time is relatively short, the peak of odorous gas is steep, which means that ammonia diffusion is faster, that is, the impact on the surrounding environment is eliminated faster.While pig breeding and egg and chicken breeding for feeding, feces storage time is relatively long, the peak obvious width is large, the impact on the surrounding environment is larger, diffusion intensity convergence is slower.

Classification management by breeding scale
From the simulation results and foreign experience, developed countries and regions are mostly concerned about the scale of animal husbandry pollution control.The United States would take farms of a certain scale as point source management, to achieve continuous discharge under emission standards.Non-point source pollution management is according to the national breeding industry non-point source pollution prevention and control plan to establish all levels of government's non-point source pollution management plan, and to improve the non-point source pollution monitoring, census & evaluation system and implementation of the integrated watershed management plan.Combined with the characteristics of small-scale farming in China, the idea of classification management in developed countries, as well as the policy and practical experience in pollution control of non-point sources (e.g.BMPs and TMDL programs in the United States) have some reference significance.

Reasonable regional breeding planning
Developed countries generally attach importance to the way of combination of farming and animal husbandry.Considering the development trend of animal husbandry and the characteristics of China's meat industry, we can learn from the "balanced regional integration of breeding" livestock and poultry farming methods which first implemented by EU countries, which is to combine with regional vegetation resources, farmland area, soil fertility, human resources and other conditions of comprehensive planning of the area's livestock capacity.In large and medium-sized intensive livestock and poultry farms, we could promote the establishment of the application of organic fertilizer green crop planting base, and the implementation of relevant preferential policies, so that the application of organic fertilizer green products to obtain rich market returns.

Encourage the resource utilization of livestock and poultry faeces
From the simulation results, the storage and disposal facilities of farmed environmental feces are the main sources of odor emissions.How to properly treat with the farm manure is an important aspect of controlling the environmental pollution of animal husbandry.Providing high-quality and efficient organic fertilizer sources to agricultural production is a common approach in developed countries.The animal and poultry manure in intensive breeding industry is treated harmlessly and made into multi-potable organic bio-fertilizer for agricultural production.Minnesota farms, for example, use livestock and poultry waste and garbage to generate electricity, not only to deal with garbage, but also to provide residents with new energy.The pollution load of wastewater in animal husbandry is very high, and the cost of direct biological treatment is high.It is encouraged that biogasification, acidification and precipitation and at the end using biological ponds and land treatment systems.

Strengthen environmental monitoring of animal and poultry pollution
The current development of livestock and poultry breeding industry in China has caused serious pollution to surface water and groundwater in some areas, and environmental monitoring of pollution in livestock and poultry farming industry should be strengthened.Investigation of the current situation of animal and poultry pollution with the stakeholder feedback should be carried out.Ecological protection information should also be provided for the prevention and control of livestock and poultry pollution as the basis for the establishment of scientific management procedures.

Conclusion
Software "ModOdor" could simulate the spread of odor pollutants well under different breeding conditions based on environmental monitoring data of the Internet of Things system.According to the simulation result, the characteristics of pollutant diffusion in typical farms could provide decision reference to odor hygienic buffer zone and minimum shelter distance.
This study provides an effective exploration for achieving the ecological and safety objectives of livestock farming.

3. 1
Introduction to the ModOdor model Odor gas air dispersion software (Modeling of Odor gas air dispersion software, ModOdor v1.0, 2014) is from a special key project of Environmental Protection Public Welfare Industry Research of the Ministry of Environmental Protection "Solid waste disposal Facilities Environmental Safety Evaluation Technology Research" (2012.1-2014.12)supported by Tsinghua University, which is developed by Tsinghua University for solid waste disposal facilities and other pollution sources generated by the atmospheric diffusion simulation and concentration forecast.ModOdor, although developed for atmospheric diffusion of foul-smelling gases, is also suitable for the simulation of atmospheric diffusion of other gaseous pollutants on a small and medium scale.

Fig. 4 .
Fig. 4. Three-dimensional spatially limited differential section mesh (a)Out-of-domain mesh simulates terrain changes (b)In-domain compute grid

Fig. 5 .
Fig. 5. ModOdor sets up an Out-of-domain mesh to simulate a complex topographic interface map

Fig. 6 .Fig. 7 .Fig. 8 .
Fig. 6.Distribution of the concentration of ammonia in the wind under the Inma Ranch

Table 2 .
NH3 release source intensity (kgNH3/ head × years) in different breeding conditions in China

Table 3 .
Basic information table for farms