Introduction
Malodor and Odorants: Source, Generation and Emission
Source
Generation and emission
Malodor and malodorants
Technologies and Techniques for Controlling Malodor and Malodorants in Livestock Facilities
Animal feeding: diet modification (reducing crude protein (cp))
Air treatment: washing walls and wet scrubbers
Air treatment: dust reduction
Air treatment: Ozonation
Air treatment: bio-filtration
Manure handling
Manure handling: Storage covers
Manure handling: Bedding
Manure handling: Additives
Manure handling: Solid/ liquid separation of manure
Manure handling: Aerobic treatment
Manure handling: Anaerobic digestion
Conclusions
Introduction
Livestock facilities are the most important sources and cause of malodor nuisances that have human health impacts in rural areas (Cole et al., 2000; Wing and Wolf, 2000). Expansion in livestock production, besides, animal feeding and manure handling operation results are responsible for odor problems. Consequently, malodor causes social issues, negative impacts on the local economy, human health, and living standards in rural areas (Thu, 2002). The enhancement of the pig production industry has caused the malodor problem to become an environmental constraint (Feddes et al., 2001). According to Hardwick (Hardwick, 1985), there are three basic areas of malodor emission: livestock buildings (30%), manure storage facilities (20%), and land application (50%) (Mielcarek-Bocheńska and Rzeźnik, 2019). However, reduction of malodor or malodorant production in livestock facilities, referred to as FIDO (Frequency, Intensity, Duration, and the Offensiveness) of the malodors, was suggested by Watts and Sweeten (Brancher et al., 2017; Watts and Sweeten, 1995). Malodor management and mitigation from livestock facilities depend on few vital factors: the malodor, malodorants (the chemical compounds responsible for creating unpleasant odors), their sources and emission factors, the sense of malodor, and malodor management techniques and technologies. In this study, an attempt has taken to summarize these factors and to focus on malodor mitigation techniques and technologies with respect to the problem of malodorants in livestock (swine).
Malodor and Odorants: Source, Generation and Emission
Source
Swine odor sources could classify into the following three categories: buildings and facilities, manure storages, and land application sites. Malodorant composition and emission strength from pig facilities are due to the synchronization of many variables, including animal species, type of production, housing system, and feed and feeding system, as well as the method of manure storage and application and weather conditions (Song et al., 2013; Ubeda et al., 2013).
Generation and emission
Slow, incomplete degradation of the organic matter contained in manure, such as proteins, fermentable carbohydrates, and fats (Varel, 2002), are the primary basis of malodor emission. Livestock malodors originate from the microbial decomposition of the organic matter that remains in the animals' digestive tracts of and their manure under anaerobic conditions (Guffanti et al., 2018; Mackie et al., 1998). Anaerobic microorganisms use organic compounds as their electron donors and as sources for cell synthesis and metabolism. And that results in the production of various malodorous gasses and volatile compounds (Hartung and Phillips, 1994). It has been shown that starch fermentation dominates in cattle manure whereas both protein and starch fermentation take place in swine manure (Miller and Varel, 2003). In animal Manure proteins are the precursors of sulfurous, indolic, and phenolic compounds, while volatile fatty acids (VFAs), ammonia (NH3), and volatile amines in manure are some of the major malodorous compounds (Jang and Jung 2018; Aarnink et al., 2007; Mackie et al., 1998).
Malodor and malodorants
Mixtures of more than 411 identified compounds (Janni, 2020; Schiffman et al., 2001) are extracts into the air from livestock facilities. Among them, 168 were identified as volatile compounds (Chen et al., 2009). However, the interactions between them are not yet well understood. Table 1 reports malodorant heterogeneity modified by Schiffman et al. (2001) and Liu et al., (2014). The principal components responsible for malodor production are NH3, amines, sulfur-containing compounds, VFAs (Page et al., 2014; Zhang and Zhu 2003) indoles, skatoles, phenols, alcohols, and carbonyls, as reported by (Cho et al., 2013; Parker et al., 2013; Trabue et al., 2011) (Table 1). Among the constituents, sulfur-containing compounds (i.e. hydrogen sulfide (H2S), dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide) have been found to have the highest correlation (Jo et al., 2015).
Table 1.
Odorous compounds: group, example compound, chemical formula, and characteristics (Liu et al. (2014); Schiffman et al. (2001))
Technologies and Techniques for Controlling Malodor and Malodorants in Livestock Facilities
Muehling (1970) mentioned livestock production facilities malodor control should be assigned to 1) counter the cause, 2) treat the emissions, and 3) possibly deal with both. However, NH3 and H2S are the main gasses emitted from manure in swine houses (Blanes-Vidal et al., 2009). In this paper, some very important but commonly used techniques and technologies for malodor control have been discussed. Table 2 summarizes the technologies for controlling malodor by diet modification and air treatment and their application and air pollution from livestock (swine) facilities.
Table 2.
Odor control techniques of diet modification and air treatment and their application in livestock (swine) facilities
| Practices | Methods | Efficiency | Odor reduction | Comments | Benefits | Difficulties | References |
|
Animal feeding (diet modification) |
Reduction of CP (crude protein) |
Moderate ~ Low |
9 out of 10 odorants were significantly reduced including NH3 and H2S |
Low CP content diets and/or feed additives |
Use of synthetic amino acids to reduce diet CP; cost is well established; should be considered as a BMP |
Reduced excretion of N and thus can reduce manure NH3 releases; no effects on animal performance | Hobbs et al., 1996; Kay and Lee, 1997; Ha and Kim, 2015 |
| Air treatment |
Washing Walls/ Wet scrubbers | Moderate |
Dust: －50% and NH3: 33% |
Effectiveness depends on solubility of odorants |
50% reduction of dust and 33% of NH3 when the ventilation rate is low |
Wastewater treatment needed | Moore Jr et al., 2018; Estelles et al., 2011; Keener et al., 1999 |
|
Oil Sprinkling |
Low ~ moderate |
Dust levels: 37－89% |
Create slick flooring for pigs and people; health concern on oil misting |
Effectively reduces dust and odor levels |
Residue on the floor and pen partitions, which increases labor required for cleaning | Jacobson et al., 1998; Zhang et al., 1996 | |
|
Windbreak Walls |
Moderate ~ high | 20% to 67% |
Many odorous compounds are absorbed on dust particles |
May reduce odor and dust emissions |
Periodic cleaning of dust on walls is necessary for sustained odor control | Lemay et al., 2000 | |
| Ozonation | Moderate |
Swine barn: 50%, Tunnel vent: 58%, Exit fan: 63% |
Reduced disease and mortality, improved growth are experimentally confirmed |
May have positive health effective |
Effectiveness for odor remediation currently unproven | Watkins et al., 1997 ; Bottcher et al., 1998 ; Keener et al., 1999 | |
| Bio-filtration |
Moderate ~ high |
H2S: 88%, NH4: 50%, odor threshold: 81% |
Operating conditions; moisture: 40%－65%, temperature: 25°C－50°C, media porosity: 40%－60% |
Effectively reduces odors and H2S emissions |
Requires replacing existing ventilation fans | Bottcher et al., 2000 ; Sun et al., 2000 ; Rahman et al., 2012 |
Animal feeding: diet modification (reducing crude protein (cp))
Manure produced by growing and finishing pigs contains protein and carbohydrates that undergo incomplete microbial degradation and thereby produce malodorous compounds (Recharla et al., 2017; Sutton et al., 1999). Several authors mentioned that reducing CP from 21% to 41% can reduce around 19%- 47% of N and 13% to 9% of NH3 (Cho et al., 2015; Hobbs et al., 1996; Kay and Lee, 1997).
Air treatment: washing walls and wet scrubbers
This technique involves the use of water sprays to remove dust particles from the air. This system reduced the total dust and NH3 levels by as much as 65% and 67% at low airflow rates and 20% and 15% at high airflow rates, respectively. Acid scrubbers can reduce NH3 by 70% to over 90% (Melse and Ogink, 2005; Estelles et al., 2011), but they are much less effective in reducing typical malodors (the overall average was 27%) (Moore Jr et al., 2018; Melse and Ogink, 2005).
Air treatment: dust reduction
Toxic and malodorous gasses could get absorbed by dust particles. This is why reducing the dust concentration inside buildings can also lower malodor and gas emissions. Sprinkling with oil, air filtration, and washing walls and other wet scrubbers are a few of the techniques that have been suggested to reduce dust reduction. Oil sprinkling reduces malodor and H2S levels both inside a building and in ventilated air (MPCA, 2003; Jacobson et al., 1998), with an observed reduction in dust levels from 37 to 89% compared to an unsprayed control.
Air treatment: Ozonation
Keener et al. (1999) reported that the NH3 level was reduced to 58% and the total dust mass at the exit fan by 63% in a building with an ozonation system and maximum tunnel ventilation compared to a non-ozonated building (Schiffman et al., 2000). Priem (1977) found that O3 reduced the NH3 level in a swine barn by 50% during cold weather and by 15% under hot ventilation conditions due to high ventilation rate reduced the retention and reaction time of ozone (Banhazi et al., 2002). Remondino and Valdenassi (2018) reported that ozonation creates anti-oxidant and immune-stimulating action among 700 pigs and 0.1–0.2 ppm ozone in the air prevents the transmission of genes via aerogenic (Prrss, Mycoplasma, Influenza, Actinobacillus, and Streptococci).
Air treatment: bio-filtration
Exhaust air from swine housing and sub-surface pits for manure storage possibly be treated with air-cleaning technology composed of organic materials. Biofilters can reduce malodorous emissions of NH3 and H2S from ventilation fan exhausts by up to 90% (Chen and Hoff, 2012; Sun et al., 2000). The general, the recommended operating conditions for biofilters are a moisture level of 40% to 65%, a temperature of 25°C to 50°C, and media porosity of 40% to 60% (Nicolai et al., 2005; Rahman and Borhan, 2012). Well-designed and managed biofilters can reduce odors and H2S by as much as 95% and NH3 by 80% (Chen and Ho, 2012; Lim et al., 2012).
Manure handling
Technologies available to reduce the malodor from waste storage and treatment facilities can classify into two groups: 1) those intended to add to existing systems and 2) those designed to replace the existing systems completely. These systems are divided into seven categories: bedding, manure additives, solid/liquid separation, composting, aerobic treatment, anaerobic treatment lagoons, and biogas production. Table 3 summarize most common malodor removal techniques.
Table 3.
Odor control techniques and their application in livestock (swine) manure management
| Practices | Methods | Efficiency | Odor reduction | Comments | Benefits | Difficulties | References |
|
Manure handling | Straw |
Moderate ~ high |
Thickness- 25.4cm: －90% |
Inexpensive, adaptable, and immediately useable |
Odor reduction |
Temporary solution; straw sinks after a certain period | Vander-Zaag, 2008 ; Schmidt et al., 2004 |
|
Plastic Cover |
Thickness- 0.4cm: －76% |
Slow release of gases from storage |
Reduce odor and H2S emissions. |
Significant capital cost | Williams, 2003 ; Jacobson et al., 1999 ; Stenglein et al., 2011 | ||
|
Clay balls/ Leka rock |
Thickness -3.8－4.0cm: 60%－80% | - |
Helps reduce odor and hydrogen sulfide emissions |
Care must be taken during agitation and pumping; capital cost | Vander-Zaag 2008 ; Williams, 2003 ; Bottcher et al., 2000 | ||
| Bedding |
Low ~ moderate | 16%－63% |
Carbon-based systems, physical structure by absorption, evaporation, and composting |
Significant odor reduction; partial composting of bedding in place. |
Must change after certain time; increased volume of manure to haul | Chastain, 1999 ; Ritter, 1989 | |
| Additives |
Low ~ moderate | - |
Categories for malodor control agents: masking, biological additives, absorbents, digestive and chemical deodorants |
Acidification can effectively reduce NH3 emission; Oxidizing agents are effective for the short-term |
Digestive additives are effective for only one or two malodorants | McCrory and Hobbs, 2001 ; Kai et al., 2008 | |
|
Solid/ liquid separation | Moderate |
Total odorants: 26%, Combine with aeration: 55% |
Reduce odor by mechanical or gravitational separation; efficiency is highly variable |
Can reduce odor in liquid manure storage pits; better works with aeration |
Significant operational and capital costs. Requires management of the solid waste fraction | Szögi and Vanotti, 2007 | |
|
Aerobic treatment |
Moderate ~ high | 50－80% |
Solids decomposition and odor control by inhibiting VFA accretion and other odor generating compounds |
Effectively reduces odor, nutrients and organic matter |
Significant capital and operating costs | Zhu et al. 2008 ; Ndegwa et al., 2002 ; Williams et al., 1989 ; Kroodsma, 1986 | |
|
Thermophilic aerobic oxidation (TAO) |
Moderate ~ high |
VFA: 95%, NH3:77%, H2S: 99% |
Thermophilic aerobic oxidation 3-5 days operation, maintain 50－60°C |
Effectively reduces odor; nutrient recovery; volume reduction | - | Lee et al., 2000 ; Lee and Cha, 2003 | |
|
Anaerobic treatment lagoon | Moderate | VFA:79－97%, |
Digestion of only swine manure was not very promising due to high content of NH3 |
Reduces nutrients and odor |
Odor releases when weather changes; higher energy consumption; cost nominal | Westerman and Zhang, 1997 ; Powers et al., 1999 ; Zhang et al., 2000 |
Manure handling: Storage covers
Li et al.(1997), Clanton, and Schmidt (2001) found that covering a manure surface with low-cost materials (6 or 10 inches of chopped straw, 0.4 inches of vegetable oil, or 8 inches of floating clay balls) is helpful to reduce malodor emissions like H2S and VOC and sulfur compounds. Organic bedding, straws clay balls, and geotextile membranes are classified as permeable covers and impermeable covers, include plastic, rubberized, or concrete covers (Stenglein and Clanton, 2011a, 2011b). Most researchers have agreed that a straw cover thickness of > 200 mm is needed to reduce malodor by more than 60% (Vander-Zaag et al., 2008; Williams, 2003; Meyer and Converse, 1982).
Manure handling: Bedding
Bedding systems create solid manure by placing the animals’ bedding and dung on a deep pack of cornstalks, straw, or other material. The system has shown advantages for malodor control, and pig health and production. Bedding systems have two categories: 1) carbon-based and 2) sand-based (Richard and Smits, 1998; Brumm et al., 1997).
Manure handling: Additives
There are five categories for malodor control agents: masking agents, biological additives, digestive deodorants, absorbents (e.g. zeolite and bentonite) (Ritter, 1989), and chemical deodorants (acids, disinfectants, or oxidizing agents) (Chastain, 1999; McCrory and Hobbs, 2001). Digestive additives are effective for only one or two target malodorants (McCrory and Hobbs, 2001). Oxidizing agents (potassium permanganate (KMnO4), hydrogen peroxide (H2O2), and ozone (O3) are efficient for the short-term reduction of malodor (McCrory and Hobbs, 2001). Slurry acidification can effectively reduce NH3 emission and improve sulphur and nitrogen fertilizer amounts in the treated slurry (Kai et al., 2008).
Manure handling: Solid/ liquid separation of manure
The idea behind solid/liquid separation is separated solid matter has a much smaller volume than the liquid portion so the liquid portion has lower biodegradable organic matter for anaerobic degradation and thus less malodor generation. Solid/liquid separation alone can reduce malodor by 26% but can reduce it by 55% when combined with aeration (Pain et al., 1990). However, frequent manure scraping, which can reduce NH3 emissions by approximately 50% (Swierstra et al., 2001) has proved to be the most promising among the other techniques. However, some studies show that only Solid/liquid separation is not enough to reduce the malodor from swine manure (Zhu et al., 2001; Zhang and Lei, 1998).
Manure handling: Aerobic treatment
Aerobic bacteria are capable of decomposing much more of the organic compounds in manure than anaerobic bacteria. Aerobic treatment prevents VFA and other malodorous compound accumulation by solid decomposition of the treated manure (Zhang et al., 2004; Williams et al., 1989). Typically, the classification of an aeration system is either aerobic or facultative. By aeration, 5 to 35% of the organic N in the slurry can convert to NH3. After solid-liquid separation with five days of aeration, VFA reaches the acceptable level of 230 mg/l (Ndegwa et al., 2002). Zhu et al. (2008) studied a low-cost surface aeration system that showed the VFA concentration of 230 mg/l and biochemical oxygen demand (BOD) of 171 mg/l, after 83 and 74 days of operation respectively. Lee and Lee (1996) showed that phototropic bacteria in aerated liquid swine manure for 10 days reduced the VFA by 90.2% and the BOD by 42.6%. The thermophilic aerobic oxidation (TAO) system, which is an aerobic thermal system maintained at 50－60°C for liquid manure treatment, can remove VFA components by up to 95% (1,538 to 72.9 mg/L) (Lee and Lee, 2000). Meanwhile, another study on the TAO system showed that it dose remove 77% of ammonia ions (NH3+) and 99% of the VFAs (Lee and Cha, 2003). The Liquid Manure Circulation System (LMCS) which is a combination of aeration and liquid manure circulation system became a common practice to reduce manure malodor problem in Korea pig farms. Ha (Ha and Kim, 2019) mentioned that LMCS can reduce odor strength by 2.4 to 2.3 in inside of barn to the border of the firm. He also shows the CO2 can reduce from 584 to 718ppm, ammonia 6 to 10 ppm and H2S 0.0 to 0.1 in inside of the barn to the border of the farm respectively In another study Botermans et al., (2010) showed that without LMCS NH3 and H2S concentration was in barn outside of pig house was 18.8ppm and 3.97ppm whereas, with LMCS NH3 reduced to 6.3ppm and 0.4 in barn and outside respectively and H2S reduced to 1.06 in barn and 0.14 outside.
Manure handling: Anaerobic digestion
Anaerobic digestion is a widely applied technology for the stabilization of organic waste and the production of biogas and is one of the most effective end-of-pipe methods of reducing malodor and air pollutants from swine manure (Chantigny et al., 2009). Moreover, there is a 22% reduction of NH3 anaerobically treated manure after surface application compared to untreated manure (Hansen et al., 2006). Hjorth et al. (2008) mentioned that anaerobic digestion reduced the VFA content which caused an increase in pH and that escalates the potential of NH3 volatilization. The VFA contents could be reduced by 79 to 97% when anaerobic digestion has been used (Hwang et al., 2018; Hansen et al., 2006). Pain et al. (1990) found that odor intensity from anaerobic digestate reduced by 70% and 80% compared to untreated (anaerobic) manure during land application.
Conclusions
A good number of researches related to livestock malodor nature, measurement techniques, dispersion modeling, mitigation techniques, and removal technologies have been conducted, albeit with some limitations. These limitations are mainly the effectiveness of controlling malodor, the complexity of the operation, high capital and operating costs, and the expertise required for some mechanized systems effectively. Diet manipulation of CP has shown some promising results in reducing nitrogen emission and has proved to be a low-cost approach that reduces NH3 emissions effectively. Biofilters have great potential as the most promising and cost-effective technic specifically for swine houses and manure storage comparing to ozonation or wet scrubbers. Malodor emission from outdoor storage can be reduced by using a lagoon cover. Although, covering materials regulate their effectiveness. Both of these techniques need careful maintenance for effective performance. Solid-liquid separation is a very effective process to mitigate malodor by separating manure particles but that is difficult to separate the easily degradable finer particles that generate malodor during anaerobic digestion processes. Anaerobic digestion is a good option for controlling malodor from swine manure, but NH3 inhibition is of great concern. Moreover, anaerobic digestion is not cost-effective for small and medium scale swine operations. The aerobic treatment uses aerobic bacteria to decompose the organic compounds and prevent the accumulation of VFA; conversion of organic N. The aerobic treatment reportedly reduces the BOD from liquid manure. The TAO treated manure is more malodor free than the other aeration treatment. Only a single method may not be enacted to control the malodor problem for in-house swine production, manure storage, and/or manure treatment. The LMCS could be a potential technique for long term liquid manure treatment and malodor reduction method in pig farms. However, for future suggestions- the odor generation sources should reduce in the first place, such as create and follow a manual for quick manure collection. Secondly, the fusion between existing technologies. Combining various methods is highly recommended for a sufficient and significant reduction in malodor in pigs and other livestock facilities. In Particular, for manure handling, adding additives under plastic and/or straw cover during storage could be promising to the reduction of malodor emission. And for manure treatment, solid/ liquid separation, liquid manure circulation system (LMCS), and the thermophilic aerobic oxidation (TAO) system could be a potential fusion technique, which will reduce the malodor emission besides transforming manure from waste to fertilizer.
