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ARTICLE
REVIEW AND ANALYSES

Additives to Reduce Ammonia and Odor Emissions from Livestock Wastes

A Review

Soils and Agroecology, Institute of Grassland & Environmental Research, North Wyke, Okehampton, Devon, UK

Corresponding author (daniel.mccrory{at}bbsrc.ac.uk)

Received for publication February 15, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION AMMONIA EMISSIONS OFFENSIVE ODORS FURTHER WORK CONCLUSIONS REFERENCES

 
This paper reviews the use of additives to reduce odor and ammonia (NH₃) emissions from livestock wastes. Reduction of NH₃ volatilization has been shown to be possible, particularly with acidifying and adsorbent additives, and potential exists to develop further practical and cost-effective additives in this area. Masking, disinfecting, and oxidizing agents can provide short-term control of malodor, but as the capacity of these additives is finite, they require frequent reapplication. Microbial-based digestive additives may offer a solution to this problem as they are regenerative, but they appear to have been developed without a thorough understanding of microbiological processes occurring in livestock wastes. Currently, their use to reduce odor or NH₃ emissions cannot be recommend. If the potential of these types of additives is to be realized, research needs to shift from simply evaluating these unknown products to investigating known strains of bacteria or enzymes with known modes of action. To protect the farmers' interest, standard independent test procedures are required to evaluate efficacy. Such tests should be simple and quantify the capacity of the additive to perform as claimed. The principle use of additives needs to be identified and addressed during their development. Producers may not use effective additives in one area if they further compound other problems that they perceived to be more important. There is the potential to use additives to treat other problems associated with livestock wastes, particularly to improve handling properties, reduce pollution potential to watercourses, and reduce pathogenic bacteria. Further work is required in these areas.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION AMMONIA EMISSIONS OFFENSIVE ODORS FURTHER WORK CONCLUSIONS REFERENCES

 
IN the last few decades livestock practices have evolved considerably. Highly integrated farms, notably in cattle (Bos taurus), pig (Sus scrofa), and poultry production, have largely disappeared, replaced by intensive systems using confined rearing methods. Management of the large volumes of excreta produced from these systems has meant bedding is minimized and slatted floors are employed, allowing feces and urine to collect as slurry containing approximately 3 to 12% solids. As intensive farming methods have proven economically effective, many adverse effects of handling livestock wastes, particularly as slurry, have become evident. The main problems were summarized by Pain et al. (1987):

(i) Ammonia volatilization.

(ii) Offensive odor release.

(iii) Handling problems due to the formation of crusts and sediments during storage.

In addition, other issues, such as the pollution of watercourses via surface runoff and the spread of pathogens, are becoming ever-increasing concerns. The importance of all these problems varies according to the nature of the waste, concerns of the farmer, distance of neighbors, vulnerability of the surrounding environment, and current legislation.

Present technology provides a wide array of innovative treatments for managing livestock wastes, a full description of which is beyond the scope of this review. Among these, the majority of research has concentrated on biogas (methane) production, anaerobic and/or aerobic purification, and solids separation. While these methods have proven effective (Woestyne and Verstraete, 1995), their use is limited, primarily due to the high cost and expertise required to operate these mechanized systems effectively. In recent years, attention has focused on treatment methods that alleviate associated problems but are also both practical and economically viable to the farmer. One treatment approach that appears to fit these criteria is livestock waste additives.

An additive can be defined as a substance that is applied to a livestock waste with the intention of alleviating one or more of the problems previously outlined. Numerous types of additives have been investigated over the last three decades including bacterial–enzymatic preparations, plant extracts, oxidizing agents, disinfectants, urease inhibitors, masking agents, and adsorbents. However, the effectiveness of additives, particularly commercially available products, has been the subject of debate (Pain et al., 1987; Ritter, 1989; Zhu et al., 1997a).

This review will concentrate on the use of additives in alleviating NH₃ and odor emissions from livestock wastes. A further section will briefly discuss the potential of additives to treat other associated problems. In each section, attention will focus on the processes by which these problems arise, the proposed mechanisms by which additives can ameliorate these problems, and a discussion of their effectiveness.

	AMMONIA EMISSIONS

TOP ABSTRACT INTRODUCTION AMMONIA EMISSIONS OFFENSIVE ODORS FURTHER WORK CONCLUSIONS REFERENCES

 
Livestock slurry is a valuable fertilizer source for crop production but its value is reduced over time by significant losses of nitrogen (N), attributed mainly to the volatilization of NH₃ (Lauer et al., 1976; Pain et al., 1987; Hartung and Phillips, 1994). In addition to the economic loss, NH₃ emission and subsequent deposition can be a major source of pollution, causing N enrichment, acidification of soils and surface waters, and the pollution of ground and surface waters with nitrates (Hartung, 1992; Sutton et al., 1995; Pain et al., 1998). In the housed environment, NH₃ emissions can also adversely affect the health, performance, and welfare of both animals (Donham, 1990) and human attendants (Donham et al., 1977; Donham and Gustafason, 1982).

During the last 30 years NH₃ emissions in Europe have increased by more than 50% (ApSimon et al., 1987; Sutton et al., 1995). Intensification in livestock production has been identified as the primary contributor to this increase and is estimated to account for 80% of yearly emissions (Buijsman et al., 1987; Pain et al., 1998). Consequently, many European countries have implemented legal constraints on the spreading of livestock slurry (Burton, 1996), necessitating an increase in storage capacity. Storage of livestock slurry has been recognized as a major source of NH₃ emissions (Hartung and Phillips, 1994), with reported N losses ranging from 3 to 60% of initial total N (Muck and Steenhuis, 1982; Dewes et al., 1990).

Factors Influencing Volatilization
The concentration and type of N in livestock slurry varies according to animal species, diet, and age. Typically, livestock use less than 30% of N contained in their feed, with 50 to 80% of the remainder excreted in the urine and 20 to 50% excreted in the feces. Urea is the major nitrogenous component in urine, accounting for up to 97% of urinary N. The exception is poultry manure, where uric acid is excreted instead of urea. Urea is hydrolyzed by the enzyme urease, found in the feces, to ammonium (NH⁺₄) and bicarbonate ions. Hydrolysis occurs rapidly, with complete conversion of urea N to NH⁺₄ possible within a matter of hours, depending on environmental conditions (Muck and Richards, 1980; Beline et al., 1998). Fecal N typically consists of 50% protein N and 50% NH⁺₄. Mineralization of fecal protein N mainly occurs through the activity of proteolytic and deaminative bacteria, initially hydrolyzing proteins to peptides and amino acids and finally by deamination to NH⁺₄. This process occurs at a far slower rate than the hydrolysis of urea and is thought to be a relatively unimportant source of NH⁺₄ where livestock slurry is stored for a short period of time (Muck and Steenhuis, 1982). However, where livestock slurry is stored for long periods, especially at higher temperatures, it becomes the dominant pathway for NH⁺₄ production (Patni and Jui, 1991).

Reactions that govern NH₃ volatilization may be represented by the following summarized equation (Freney et al., 1981):

[1]

The driving force for NH₃ volatilization is considered to be the difference in NH₃ partial pressure between that in equilibrium with the liquid phase and that in the ambient atmosphere. In the absence of other ionic species, this is predominately influenced by the NH⁺₄ concentration, pH, and temperature, although any displacement of the equilibrium will affect NH₃ emission.

Numerous additives have been investigated to reduce NH₃ volatilization. The most common can be categorized, according to their modes of action, into the following five groups: digestive additives, acidifying additives, adsorbents, urease inhibitors, and saponins from Mohave yucca (Yucca schidigera Roezl ex Ortgies). These are discussed below.

Digestive Additives
Digestive additives are an example of a waste treatment approach known as bioaugmentation. They consist of selected microbial strains and/or enzymes that enhance the biodegradation of livestock wastes. Strains are generally isolated from environmental samples and selected by conventional enrichment techniques. They are grown in a nutrient media that contain the specific targeted organic chemical as the sole source of carbon and energy or N. Selected strains are grown by fermentation and concentrated by centrifugation or filtration, and then preserved by lyophilization, drying, or freezing.

Unfortunately, information on the composition of commercial products, or their mode of action, is not available because of confidentiality and is limited to the marketing literature supplied. Marketing statements generally claim that volatilization of NH₃ is reduced by stimulating immobilization of NH⁺₄ by microorganisms, thus reducing its concentration in livestock slurry. To immobilize NH⁺₄, it can be postulated that products must stimulate the degradation of decomposable organic compounds with low N content, such as lipids, fatty acids, and simple carbohydrates, or recalcitrant organic compounds with high C to N ratios, such as lignin or cellulolytic complexes.

In addition to reducing NH₃ emissions, other claimed benefits of using digestive additives include reduced malodor emissions and improved handling properties of livestock slurry, both of which are discussed later. A summary of the effectiveness of digestive additives in controlling NH₃ emissions from livestock wastes is presented in Table 1. Generally, the performance of this type of additive appears to be poor, though field trials indicate more success than lab trials. Ritter (1981) postulated that the reason for this was because a greater proportion of additive to livestock waste is required as the volume of the livestock waste is reduced. Conceivably, with a lower sample volume, a greater proportion of the livestock slurry is exposed to the atmosphere and thus through oxygen diffusion a greater proportion is in an aerobic state. Should the added microorganisms be obligate anaerobes, their survival will decrease with decreasing livestock slurry volume.

Although evaluation of commercial digestive additives is clearly beneficial to the farmer, it offers little information to the researcher. Greater insight into the benefits of bioaugmentation would be gleaned if specific microorganisms and enzymes with known modes of action were investigated and the viability of the added or stimulated microbial community monitored.

Acidifying Additives
The pH of livestock slurry controls the equilibrium between NH₃ and NH⁺₄ in solution. The percentage of NH₃ in solution at pH 6, 7, 8, and 9 is approximately 0.1, 1, 10, and 50, respectively (Court et al., 1964). Thus, there is more potential for NH₃ volatilization at higher pH. Molloy and Tunney (1983) found that NH₃ volatilization effectively stopped at pH 5.0 for pig slurry and at pH 4.0 for cattle slurry. The effectiveness of an acidifying additive is determined by its capacity to neutralize the alkaline nature of the livestock slurry (Husted et al., 1991). Several types of acidifying additives have been investigated and these can be divided into three groups: acids, base precipitating salts, and substrates that induce acid production.

Acids
The use of acids to reduce NH₃ volatilization and the associated literature is voluminous. Various acids have been proven to be consistently effective, such as sulfuric (Molloy and Tunney, 1983; Stevens et al., 1989; Pain et al., 1990), hydrochloric (Husted et al., 1991; Hoeskma et al., 1993; Martinez et al., 1997), nitric (Kroodsma and Ogink, 1997), phosphoric (Safley et al., 1983), and lactic acid (Berg and Hornig, 1997). The practicalities of some of these additives are questionable; for instance, phosphoric acid has proven to be effective but not economic (Safley et al., 1983). Sulfuric, hydrochloric, and nitric acids are cheaper than phosphoric acid but are hazardous to use and corrosive. Consequently, they would require complex application systems that would add additional costs.

Base Precipitating Salts
Base precipitating salts can be used to control the pH of livestock slurry. Chloride and nitrate salts of magnesium and calcium are used most often, although other soluble magnesium or calcium salts are also suitable. Sulfate salts are not sufficiently soluble and considered less effective (Witter, 1991). Several researchers have observed reductions in pH and NH₃ volatilization from livestock slurry or manure using base precipitating salts (Heck, 1931; MacKenzie and Tomar, 1987; Witter, 1991; Husted et al., 1991; Al-Kanani et al., 1992; O'Halloran and Sigrest, 1993; Vandre and Clemens, 1996). However, they are less effective in reducing and sustaining low pH values than acids (Husted et al., 1991; Vandre and Clemens, 1996), as Eq. [2] to [4] below demonstrate with calcium chloride (CaCl₂). The addition of acid converts all the bicarbonate into CO_2(gas) (Eq. [2]), whereas only half is converted into CO_2(gas) by the addition of a CaCl₂ (Eq. [3] and [4]) (Husted et al., 1991):

[2]

versus:

[3]

[4]

Base precipitating salts cannot completely stabilize the pH of livestock slurry and will only reduce pH to the point where equilibrium is reached between precipitation and dissolution of CaCO₃ (Husted et al., 1991). In order to sustain a reduced pH, frequent reapplication of these additives is required. However, where short-term reductions in NH₃ volatilization are sought, base precipitating salts offer the advantages of being relatively cheap and nonhazardous.

Labile Carbon
Labile carbon sources also offer a less-hazardous alternative to direct acid addition, inducing a reduction in livestock slurry pH by stimulating the indigenous anaerobic microorganisms to produce organic acids. Subair (1995) used several different sucrose concentrations to indirectly reduce the pH of pig slurry. The highest addition rate of 11% reduced slurry pH to 3.5 and NH₃ volatilization by 98%. A less-expensive carbon source was investigated by Hendriks and Vrielink (1997), who used milled wheat (Triticum aestivum L.) and potato (Solanum tuberosum L.) starch and achieved reductions in NH₃ volatilization from stored pig slurry by up to 42%. In an attempt to use the carbon source more efficiently, Eekert and Wijbenga (1992) used a lactic acid producing bacteria (Lactobacillus plantarum) in addition to glucose to reduce the pH of pig slurry by 1 pH unit. Similarly, Hendriks and Vrielink (1996) used Lactobacillus plantarum with glucose to reduce the pH of pig slurry from 8 to 6 and subsequently reduce NH₃ emissions by 50%. Unfortunately, these studies did not indicate if the added lactic acid bacteria assisted or modified the pH decline induced by the glucose. Currently, the quantity of substrate required to induce a significant pH decline makes this type of additive uneconomic. However, if the production of acid can be optimized, possibly by using suitable lactic acid bacteria, it would offer an effective and safe means to prevent NH₃ volatilization.

Adsorbents
A variety of additives adsorb NH₃, NH⁺₄, or both. The most commonly employed are clinoptilolite and peat.

Clinoptilolite
Zeolites are naturally occurring alumino silicate minerals with high cation exchange capacities. There are more than 50 different types of natural zeolites, each with a unique crystalline structure. Species differ in their selectivity towards different cations, with one species, clinoptilolite, having a specific affinity for NH⁺₄ ions (Beck, 1974; Barbarick and Pirela, 1984; Bernal and Lopez-Real, 1993; Komarowski and Yu, 1997).

Clinoptilolite has been investigated as both a livestock feed and waste additive. The early research in this area originates from Japan. In a review of the possible applications for natural zeolites, Mumpton and Fishman (1977) describe much of this work, including examples where conservation of manurial N was achieved through the addition of clinoptilolite to both feed and excreta of poultry and pigs. More recently, Miner et al. (1997) found that the application of 1 to 4% (w/v) finely ground clinoptilolite to dairy slurry, immediately before spreading through a sprinkler system, reduced NH₃ emission rates by up to 60%. Similarly, in two separate experiments, Nakaue et al. (1981) evaluated clinoptilolite first as a broiler-litter additive and then as a broiler-feed additive. Results showed that an application rate of 5 kg m^-2 to broiler litter could reduce aerial NH₃ concentrations by up to 35%, while an incorporation of 10% clinoptilolite to the feed of the birds throughout their lifetime reduced aerial NH₃ concentrations by up to 8%. These findings are in agreement with several other researchers, who have reported reductions in NH₃ emissions from livestock wastes using clinoptilolite directly (Miner, 1984; Witter and Kirchmann, 1989) or through the feed (Koelikker et al., 1978; Airoldi et al., 1993; Krieger et al., 1993). Generally, application directly to the waste appears to more effective in reducing NH₃ emissions. Addition through the feed, however, is a more practical method of application.

Peat
Peat, in particular Sphagnum fuscum peat, appears to show a high N adsorptive capacity. In contrast to clinoptilolite, sphagnum peat is more effective in adsorbing NH₃ rather than NH⁺₄ and can adsorb 2.5% of its dry weight in NH₃–N (Peltola, 1986). This ability is derived from the physical and chemical properties of the dead leaves and stems. The leaves offer a very large surface area and are no thicker than a single layer of cells and are highly porous. Thus, sphagnum peat can adsorb from 15 to 20 times its own weight in water (Barrington and Moreno, 1995).

Field trials conducted by Barrington et al. (1990) demonstrated that a cover of floating sphagnum peat moss over pig slurry could reduce N losses by up to 80%. Similarly, in a lab-based study, Patni (1992) found a 20-cm floating cover of sphagnum peat significantly reduced NH₃ emissions from pig slurry. However, sphagnum peat can sink. Barrington and Moreno (1995) solved this problem by drying the sphagnum peat at 105°C for 5 h to render it hydrophobic to pig slurry, achieving 76% N conservation when a 20-cm depth was used. These findings are in agreement with several other researchers who have reported reductions in NH₃ emissions from livestock slurry and manure using sphagnum peat (Peltola, 1986; Daigle et al., 1987; Witter and Kirchmann, 1989; Al-Kanani et al., 1992).

The advantages of using either clinoptilolite or peat for conservation of livestock slurry N are that they are nonhazardous and act as good soil conditioners when spread with slurry. In addition, there is some evidence that NH⁺₄ retained in the structural channels of clinoptilolite is physically protected from oxidation to nitrate by nitrifying bacteria (Barbarick and Pirela, 1984; Huang and Petrovic, 1994), which may prevent leaching losses. However, both additives increase the dry matter content of the livestock waste, making handling more difficult where waste is managed as slurry. Maintaining a floating sphagnum peat cover on a slurry store may also be hard to achieve in practice.

Urease Inhibitors
Additives that inhibit the urease enzyme have been developed in order to reduce NH₃ emissions from soil after the application of urea fertilizers (Pain et al., 1987). Since the majority of NH⁺₄ in livestock slurry originates from urea hydrolysis (Muck and Richards, 1980; Muck and Steenhuis, 1982; Patni and Jui, 1991), they have been investigated as potential additives. Ogink and Kroodsma (1996) observed reductions of NH₃ emissions from a cow cubical house by flushing with dilute formalin instead of water. Varel et al. (1997) investigated the urease inhibitors phenyl phosphorodiamidate (PPDA), cyclohexylphosphoric triamide (CHPT), and N-(n-butyl) thiophosphoric triamide (NBPT) on cattle feedlot slurry. All were very effective but required frequent applications. Urease inhibitors are currently too expensive and too easily broken down or inactivated to bring any economic or practical benefit to the farmer.

Saponins from Yucca
Several commercial additives are based on saponins that are extracted from the sap of the yucca plant. Saponins are high-molecular-weight glycosides, consisting of a sugar part linked to a triterpene or steroid aglycone. Saponins arouse interest as they are reputed to be responsible for the yucca's novel capability of conserving NH⁺₄ (Headon and Walsh, 1993). The mechanism is unclear, but it was suggested that they act as a urease inhibitor (Mader and Brumm, 1987). However, this theory is no longer widely supported and it is now believed that they act by binding or converting NH⁺₄ (Headon et al., 1991; Kemme et al., 1993).

Saponins are added either to the livestock feed or slurry. Headon and Walsh (1993) reported two trials in piggeries where a significant reduction of NH₃ concentration was observed. Amon et al. (1995) also reported a significant reduction in NH₃ emissions (26%) in a piggery, which was similar to the 23% reduction reported by Kemme et al. (1993). However, the latter was obtained using six times the manufacturer's recommended dose. No effects where observed with the recommended dose. Amon et al. (1997) achieved a reduction of 50% in NH₃ emitted when a saponin-based additive was fed to broiler hens. However, Johnston et al. (1981) found no reduction of NH₃ emissions when using saponins added to the feed of broiler hens. Similar negative results were reported when using saponins as livestock slurry additives (Andersson, 1994; Martinez et al., 1997). It is difficult to ascertain why in some cases this type additive has not worked. Further clarification of the way the saponins bind or transform NH₃ should be identified.

	OFFENSIVE ODORS

TOP ABSTRACT INTRODUCTION AMMONIA EMISSIONS OFFENSIVE ODORS FURTHER WORK CONCLUSIONS REFERENCES

 
Offensive odor emanating from livestock production is of concern for intensive systems and confined operations as the number of complaints continue to rise (Jongebreur, 1977; O'Neill and Phillips, 1991; Misselbrook et al., 1993). Odors from livestock slurry are due to a complex mixture of volatile compounds arising from anaerobic degradation of plant fiber and protein (Spoelstra, 1980; Hammond, 1989). Chemical analysis has identified approximately 170 volatile compounds (Spoelstra, 1980; Yasuhura et al., 1984; O'Neill and Phillips, 1992). According to O'Neill and Phillips (1992), the most important odorous components emitted from livestock slurry appear to be the volatile fatty acids (VFAs: p-cresol, indole, skatole, hydrogen sulfide, and NH₃), by virtue of either their high concentrations or their low odor thresholds.

In order to develop additives for odor control it is necessary to assess the effectiveness of a treatment by quantifying human olfactory response. Odor can be assessed by two criteria: strength, which is measured as concentration or intensity, and offensiveness (i.e., the perceived quality). Relationships between the known volatile compounds and perceived olfactory responses have also been sought by many researchers (e.g., Schaefer, 1977; Williams, 1984; Pain et al., 1990; Mackie, 1994; Zhu et al., 1997b). At present, though, no compound has been found suitable as a marker to predict olfactory response. Based on olfactory measurements, the problem of odor nuisance can be tackled by reducing either the perceived strength or offensiveness (O'Neill and Phillips, 1991). Reducing odor strength implies destroying or diluting odorants, whereas reducing odor offensiveness implies modifying odorants emitted from livestock slurry.

Additives that reduce odor nuisance originating from livestock slurry have been investigated for many years. The most common can be categorized according to their modes of action into the following five groups: digestive additives, disinfecting additives, oxidizing agents, adsorbents, and masking agents. These are discussed below.

Digestive Additives
In addition to reducing NH₃ volatilization, manufacturers of digestive additives also claim that their products reduce the release of odorous compounds. As the composition or mode of action of these products is unknown, it can be assumed that they alter the microbial community in such a way as to enhance the degradation of odorous volatile compounds or reduce their production.

A summary of the effectiveness of digestive additives in controlling odors emissions from livestock wastes is presented in Table 1. As with NH₃ emmisions, results indicate that these additives are generally ineffective for odor control. Ritter (1989) postulated that the general ineffectiveness of digestive additives was that some may only eliminate one or two types of odorant; if these are not the major contributing odorants, the product will not work. Clearly, targeting specific odorants cannot guarantee a reduction in the perceived malodor from livestock slurry, as no consistent correlation between these parameters has been identified. To effectively manipulate the microbial community from one that emits malodors to one that does not requires a fundamental understanding of the processes that produce odors. Several recent reviews in this area have attempted to identify many of the bacteria and the pathways that produce odors (Mackie et al., 1998; Zhu and Jacobson, 1999; Zhu et al., 1999), with Zhu and Jacobson identifying Eubacterium and Clostridium as the most important genera for odor production. However, all reviewers conclude that a great deal more research is required before these pathways they can be fully understood. As with the use of these products to control of NH₃ emissions, overall failure in the viability of digestive additives could also be a valid explanation for their inconsistency. Until specific microorganisms and enzymes with known modes of action are investigated and the viability of the added or stimulated microbial community monitored, no conclusive reason for the performance of this type of additive can be identified.

Disinfectants
In contrast with digestive additives, disinfecting additives are indiscriminate in their mode of action, reducing the formation of odorants by attempting to inhibit all microbial mediated processes occurring in livestock slurry. Ulrich and Ford (1975) found the commercial additive Ozene (active ingredient: orthodichlorobenzene) to be effective in controlling odors from a cattle feedlot. Similarly, Warburton et al. (1980) reported that additive Tec II, containing orthodichlorobenzene, was successful in reducing the odor offensiveness of pig slurry but not odor strength. However, Cole et al. (1976) found that orthodichlorobenzene was not effective in reducing sulfides or odor in long-term tests on both cattle and pig slurry. Chlorine was effective in reducing odors in liquid pig manure but can be expensive (Hammond et al., 1968; Warburton et al., 1980). Hydrogen cyanamide (H₂NCN) has been investigated as disinfectant. Pain et al. (1987) reported that Alzogur, which contains 50% H₂NCN in solution, reduces odor emissions. Patni (1992) also found that a 50% solution of H₂NCN applied at 1900 mL m^-3 of pig slurry completely eliminated H₂S emissions.

Although there is some scope for disinfectants that can produce short-term reduction in emissions, such chemicals are often toxic and therefore impractical as well as uneconomic.

Oxidizing Agents
Oxidizing agents decrease odorant concentration in livestock slurry and also disinfect to inhibit the formation of odorants by microorganisms. The most widely investigated oxidizing agents are potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂), and ozone (O₃).

Potassium Permanganate
Faith (1964) was the first to report the effectiveness of oxidizing agents when he found a 1% solution of KMnO₄ to be highly effective in controlling odor from a cattle feedlot. Emanuel (1965) reported similar results when KMnO₄ was used to control the odor from stockyard manure. Ritter et al. (1975) reported that KMnO₄ was effective in controlling odors for 72 h in dairy slurry at concentrations of 480 and 240 mg L^-1. Cole et al. (1976) found that KMnO₄ concentrations of 500 mg L^-1 were effective in controlling odors from pig slurry during short-term tests. Ulrich and Ford (1975) reported KMnO₄ to be the most economical out of six additives in completely suppressing odor emissions from a cattle feedlot.

Hydrogen Peroxide
Hollenback (1971) reported that H₂O₂ was effective at concentrations of 50 and 100 mg L^-1 for reducing both H₂S and odor emissions from cattle slurry. Cole et al. (1976) found that H₂O₂ was effective in reducing odor offensiveness and H₂S emissions in liquid pig slurry when applied at 500 mg L^-1, and these results agree with other researchers (Kibble et al., 1972; Ritter et al., 1975).

Ozone
Ozone is generally produced from dry air or oxygen using an electrical discharge generator or ultraviolet radiation. As ozone is a gas, effective application is only achieved if it is bubbled through the livestock waste. Watkins et al. (1997) found that dosage levels of 1 g L^-1 pig slurry significantly reduced odor intensity. Similarly, Wu et al. (1998) found that the same dosage level significantly reduced odor intensity from stored pig slurry. These results are consistent with other researchers who have found a reduction in malodor from livestock wastes using O₃ (Jutras et al., 1980; Wu et al., 1999). Although ozone is highly effective, it requires a complex application system that increases the cost and calls into question the practicality of this additive to the farmer.

In general, oxidizing agents are effective in reducing malodors, but only for a short period of time (Hollenback, 1971; Ritter et al., 1975; Cole et al., 1976). This is probably because large volumes of organic matter in livestock wastes require large quantities of reagents for complete oxidation. As with disinfecting additives, their use appears to be limited to short-term reduction of odor emissions.

Adsorbents
Reduction of malodor emanating from livestock slurry and manure has been claimed for products containing the alumino silicate mineral group, zeolites, and the clay mineral bentonite. Their mechanism of odor control has been attributed to their high adsorptive capacities (Pain et al., 1987), but limited success has been reported in the literature. In a review of possible applications for natural zeolites, Mumpton and Fishman (1977) describe several instances where zeolites have been used to control odors from both poultry and pig manure. Dewes (1987) reported that Buchgraber (1983) found that the bentonite containing additive, Agriben, reduced odor offensiveness. In contrast, Elsasser and Kunz (1988) found Agriben and bentonite ineffective in controlling the odor from cattle slurry. Miner and Stroh (1976) found zeolites ineffective in reducing odor intensity from a cattle feedlot.

It is hard to give conclusive reasons for the general ineffectiveness of these additives. It can be postulated that a particular adsorbent is likely to exhibit a preference over the odorants it adsorbs. If these do not contribute significantly to the overall malodor of the livestock slurry, only limited effects will be observed through their use.

Masking Agents
Masking agents are mixtures of aromatic oils with particularly strong odors that overcome malodor with a more desirable one and can be applied to the manure or sprayed directly into the surrounding area. There has been some success. Burnett and Dondero (1970) concluded that masking agents were most effective for controlling odors from poultry slurry. Ritter et al. (1975) found that Alamask 518B and 151A partially masked odors from dairy slurry. Smith et al. (1980) found Alamask M-X effective in improving odor quality from stored pig slurry but noted that this was reduced over time. Masking agents are liable to degradation by microbes in livestock slurry, which may explain why their use is only appropriate for short-term odor reduction (Smith et al., 1980; Warburton et al., 1980).

	FURTHER WORK

TOP ABSTRACT INTRODUCTION AMMONIA EMISSIONS OFFENSIVE ODORS FURTHER WORK CONCLUSIONS REFERENCES

 
Despite the general inconsistent performance of commercially available digestive additives in controlling NH₃ and odor emissions from livestock wastes, they continue to be the most widely available and popular type of additive on the market. One possible explanation for this may be the extra benefits they offer the farmer in addition to emission control. These generally include improvement in the handling properties, a reduction in surface water pollution, and in some cases a reduction in the levels of pathogenic bacteria. Although only limited independent research has been conducted in theses areas, there appears to be some scope for additives and possible approaches are discussed below.

Handling Properties
Where livestock waste is handled as a slurry, handling problems are often encountered due to the formation of crusts and sediments during storage that make removal for timely and accurate applications to land difficult (Pain et al., 1987). The rheological properties of a livestock slurry are dependant on its total solids content (Chen, 1986). Reducing total solids reduces viscosity and so reduces power and cost when pumping. The composition of solids varies considerably among animal species, age, physiological state, and diet, but generally consist of undigested plant fiber and protein. Commercial digestives claim to reduce total solids by stimulating their degradation. Only limited investigations have been reported in this area and the key findings are presented in Table 1. Again, the performances for this type of additive are poor. However, stimulating the microbial degradation of total solids would appear to be a more feasible application of digestive additives than either control of NH₃ or odor emissions, as the targeted organic compounds are readily identified. Work is needed to discover the microbial decay patterns of theses organic compounds in livestock slurries and identify the responsible enzymes and bacterial genera.

Researchers have evaluated various other additives (for example, saponin extract, finely divided iron fillings, H₂NCN, peat, masking agents, and oxidizing agents) for their effectiveness to reduce total solids in livestock wastes, but no beneficial effects have been reported. Indeed, several of these additives cause an increase in total solids relative to controls over an incubation period. These are generally additives that directly add additional solids, such as clinoptilolite or peat, or additives that inhibit the natural degradation of solids by the indigenous microbial population, such as disinfecting or acidifying additives (Panti, 1992). Developers of new additives need to recognize that where the primary concern of the producer is the performance of the waste handling system, it is unlikely that such products would be used, even if they were particularly effective in tackling another problem.

Pollution to Surface Watercourses
Today there is considerable pressure on farmers to avoid water pollution. On entry to a watercourse, livestock wastes exert a high biochemical oxygen demand (BOD) and cause eutrophication due to high levels of nutrients, particularly N and phosphorous (P). Although it is unlikely that a producer would purchase an additive based on the possibility of such an event, there appears to be scope for the development of additives that reduce its effect and also control NH₃ or odor emissions. For instance, Williams (1983) found that the volatile fatty acid (VFA) fraction of livestock slurry accounted for up to 70% of its BOD. The VFA fraction of livestock wastes has also been identified as a primary contributor to odor (Zhu et al., 1997c; Mackie et al., 1998; Zhu and Jacobson, 1999; Zhu et al., 1999). Enhancing the degradation of this fraction reduction may well also lower the BOD. However, further understanding of the microbiology pathways in livestock wastes is required before this can be achieved. Phosphorus runoff from land receiving slurry is another major environmental problem, particularly from sites receiving poultry manure. The majority of P runoff is from the dissolved reactive P fraction. Moore et al. (1995) reported that various additives, such as alum, calcium carbonate, and ferrous sulfate, reduced both water-soluble P concentrations from poultry slurry and loss of NH₃ during storage.

Reduction of Pathogens
Many of the bacteria in livestock slurry are pathogenic and pose a heath risk. In a review on the inactivation of viruses in pig slurry, Turner and Burton (1997) described instances where additives such formalin, chlorine, and potassium permanganate have been used successfully to inactivate pathogens and viruses in stored slurry. Such additives may also offer the benefits of short-term odor control. However, like disinfecting additives, they are hazardous to use and high concentrations are often required. Any additive, though, that alters environmental parameters within the slurry, such as pH regulators, are likely to alter the bacteria present and may cause an indirect reduction in pathogenic bacteria. The possibility of a dual effect to the producer may cause such an additive to be more readily employed, so further work is area is warranted.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION AMMONIA EMISSIONS OFFENSIVE ODORS FURTHER WORK CONCLUSIONS REFERENCES

 
Current literature suggests that the development of effective livestock waste additives requires an understanding of the fundamental mechanisms involved in NH₃ and odor emissions. Reduction of NH₃ volatilization has been shown to be possible, particularly with acidifying and adsorbent additives, and there appears to be considerable potential to develop further practical and cost-effective additives for this purpose. However, only short-term control of malodor has been achieved using masking, disinfecting, and oxidizing agents, as the capacity of the additive is finite and requires frequent reapplication. Microbial additives may offer a solution to this problem as they are regenerative, but they appear to have been developed without a thorough understanding of livestock waste microbiology. At present, their use to reduce odor or NH₃ emissions cannot be recommend. Development of digestive additives remains the domain of the commercial sector and is subject to confidentiality regarding product information. If the potential of these types of additives is to be realized, research needs to shift from simply evaluating these unknown products, which often yields little valuable information, to investigating known strains of bacteria or enzymes with known modes of action.

Although additives can only be considered to be at an early stage of development, the fact that there is a commercial market suggests that they are a popular treatment approach for farmers. The principle use of additives needs to be identified and addressed during their development. Producers may not use effective additives in one area if they further compound other problems they perceive to be more important. Standard independent test procedures need to be adopted for the evaluation of additives that relate to a given set of environmental circumstances, livestock, and diet. Such procedures have been proposed (Zuh et al., 1996) but none have been widely adopted. Once established and approved, a voluntary product registration scheme leading to a recognized "award" based on performance could be introduced. Such a scheme has recently been successfully introduced for silage additives in the UK.

	ACKNOWLEDGMENTS

 
The authors would like to thank The Ministry of Agriculture, Food and Fisheries (UK) and the Biotechnology and Biological Sciences Research Council (UK) for supporting this review.

	REFERENCES

TOP ABSTRACT INTRODUCTION AMMONIA EMISSIONS OFFENSIVE ODORS FURTHER WORK CONCLUSIONS REFERENCES

 

• Airoldi, G., P. Balsari, and R. Chiabrando. 1993. Odour control in swine houses by the use of natural zeolites: First approach to the problem. p. 701–708. In E. Collins and C. Boon (ed.) Livestock Environment IV, 4th Int. Symp., Univ. of Warwick, Coventry, UK. 6–9 July 1993. Am. Soc. Agric. Eng., St. Joseph, MI.
• Al-Kanani, T., E. Akochi, A.F. MacKenzie, I. Alli, and S. Barrington. 1992. Organic and inorganic amendments to reduce ammonia losses from liquid hog manure. J. Environ. Qual. 21:709–715.[Abstract/Free Full Text]
• Amon, M., M. Dobeic, T.M. Misselbrook, B.F. Pain, V.R. Phillips, and R.W. Sneath. 1995. A farm scale study on the use of De-Odorase for reducing odour and ammonia emissions from intensive fattening piggeries. Bioresour. Technol. 51:163–169.
• Amon, M., M. Dobeic, V.R. Phillips, R.W. Sneath, T.M. Misselbrook, and B.F. Pain. 1997. A farm scale study on the use of clinoptilolite zeolite and De-Odorase for reducing odour and ammonia emissions from broiler houses. Bioresour. Technol. 61:229–237.
• Andersson, M. 1994. Performance of additives in reducing ammonia emissions from cow slurry. Swedish Univ. of Agric. Sci. Dep. of Agric. Biosyst. and Technol. Rep. no. 93. JBL Publ., Sweden.
• ApSimon, H., M.M. Kruse, and J.N.B. Bell. 1987. Ammonia emissions and their role in acid deposition. Atmos. Environ. 21:1939–1946.
• Barbarick, K.A., and H.J. Pirela. 1984. Agronomic and horticultural use of zeolites: A review. p. 93–103, 257–262. In W.G. Pond and F.A. Mumpton (ed.) Zeo-agriculture: Use of natural zeolites in agriculture and aquaculture. Westview Press, Boulder, CO.
• Barrington, S.F., and G.R. Moreno. 1995. Swine manure nitrogen conservation in storage using sphagnum moss. J. Environ. Qual. 24:603–607.[Abstract/Free Full Text]
• Barrington, S.F., P. Schuepp, R. Capp, and J. Blanchette. 1990. Peat moss to conserve swine manure nitrogen. p. 434–441. In Agriculture and food processing waste. Proc. 6th Int. Symp. on Agric. and Food Processing Wastes, Chicago, IL. December 1990. Am. Soc. Agric. Eng., St. Joseph, MI.
• Beck, D.W. 1974. Molecular sieves structure. Chemistry and use. John Wiley & Sons, London.
• Beline, F., J. Martinez, C. Marol, and G. Guiraud. 1998. Nitrogen transformations during anaerobically stored ¹⁵N-labelled pig slurry. Bioresour. Technol. 64:83–88.
• Berg, W., and G. Hornig. 1997. Emission reduction by acidification of slurry—Investigations and assessment. p. 459–466. In Proc. of the Int. Symp. on Ammonia and Odour Emissions from Animal Production, Vinkeloord, the Netherlands. 6–10 Oct. 1997. NVTL, Rosmalen, the Netherlands.
• Bernal, M.P., and J.H. Lopez-Real. 1993. Natural zeolites and sepiolite as ammonium and ammonia adsorbent materials. Bioresour. Technol. 43:27–33.
• Bonazzi, G., C. Fabbri, and L. Valli. 1996. Options for controlling ammonia emissions from pig housing. In Proc. of Workshop on European Co-Operative Research Network on Animal Waste, Godollo, Hungary. 9–11 Oct. 1996. WRC Ref. CP 783. Hungarian Inst. of Agric. Eng., Godollo, Hungary.
• Buchgraber, K. 1983. Vergleich der Wirksamkeit konventioneller und alternativer Dugungssyssteme auf dem. Gruland hinsichtlich Etrag, Futterqualitat und Gute des Pflanzenbestandes. Diss., Vienna.
• Buijsman, E., H.F.M. Mass, and W.A.H. Asman. 1987. Anthropogenic NH₃ emissions in Europe. Atmos. Environ. 21:1009–1022.
• Burnett, W.E., and N.C. Dondero. 1970. Control of odours from animal wastes. Trans. ASAE 13:221–231.
• Burton, B.H. 1996. Processing strategies for farm livestock slurries to minimise pollution and to maximise nutrient utilisation—An EU collaboration. p. 5–10. In G. Parafait et al. (ed.) Ingenieries EAT—Animal Manures and Environment in Europe. Dec. 1996. Antony, Cemagfef, France.
• Chen, Y.R. 1986. Rheological properties of sieved beef-cattle manure slurry: Rheological model and effects of temperature and solids concentration. Agric. Manure 15:17–33.
• Cole, C.A., H.D. Bartlett, D.H. Buckner, and D.E. Younkin. 1976. Efficacy of certain chemical and biological compounds for control of odor from anaerobic liquid swine manure. J. Anim. Sci. 42:1–7.[Abstract/Free Full Text]
• Court, M.N., R.C. Stepphen, and J.S. Waid. 1964. Toxicity as a cause of the inefficiency of urea as a fertilizer. J. Soil Sci. 15:42–48.
• Daigle, J.Y., A. Arseneau, and A. Robichaud. 1987. Sphagnum peat: A tool for liquid hog manure management. p. 173–176. In Wetlands/Peatlands Symp. 1987, Edmonton, AB, Canada. Wetlands/Peatlands `87 Publ., Ottawa, ON, Canada.
• Dewes, T. 1987. Chemical and microbial changes during the fermentation of liquid cattle manure treated with Agriben and its ingredients. p. 323–329. In Proc. from Agric. Waste Manage. and Environ. Protection. 4th Int. Symp. of CIEC, Braunschweig Federal Republic of Germany. 11–14 May 1987. Vol. 2. Goltze-Druck, Gottingen, Germany.
• Dewes, T., L. Schmitt, E. Valentin, and E. Ahrens. 1990. Nitrogen losses during the storage of liquid livestock manures. Biol. Wastes. 31:241–250.
• Donham, K.J. 1990. Relationships of air quality and productivity in intensive swine housing. J. Agric. Practice 10:15–18.
• Donham, K.J., and K.E. Gustafason. 1982. Human occupational hazards from swine confinement. Ann. Am. Conf. Gov. Hyg. 2:137–142.
• Donham, K.J., M.J. Rubino, T.D. Thedell, and J. Kammermeyer. 1977. Potential health hazards to agricultural workers in swine confinement buildings. J. Occup. Med. 19:383–387.[ISI][Medline]
• Eekert, M.H., and A.D.J. en Wijbenga. 1992. Microbiele verzuring van varkensdrijfmest. Nederlands Instituut voor Koolhydraat Onderzoek. NIKO-TNO. September 1992. Univ. of Groningen Press, the Netherlands.
• Elsasser, M., and H. Kunz. 1988. Effects of slurry with and without additives on forage yields and quality of a meadow in the alpine foothills. Wirtschaftseigene Futter 34:48–65.
• Emanuel, A.G. 1965. Potassium permanganate offers new solutions to air pollution control. Air Engineering. September 1965.
• Faith, W.L. 1964. Odour control in cattle feed yards. J. Air Pollut. Control Assoc. 1411:459–460.
• Freney, J.R., O.T. Denmead, I. Watanbe, and E.T. Craswell. 1981. Ammonia and nitrous oxide losses following applications of ammonia sulfate to flooded rice. Aust. J. Agric. Res. 32:37–45.
• Grubbs, R.B. 1979. Bacteria supplimentation what it can and can't do. Paper presented at the 9th Eng. Foundation Conf. in Environ. Eng. in the Food Processing Ind., Pacific Grove, CA. 27 Feb. 1979. ASCE, Reston, VA.
• Hammond, E.G., C. Heppner, and R. Smith. 1989. Odour of swine waste lagoons. Agric. Ecosyst. Environ. 25:103–110.
• Hammond, W.C., D.L. Day, and E.L. Hansen. 1968. Can lime and chlorine suppress odours in liquid hog manure? Agric. Eng. 49:340–343.
• Hartung, J. 1992. Emissions and control of gases and odorous substances from animal housing and manure stores. Zentralbl. Hyg. Umweltmed. 192:389–418.[Medline]
• Hartung, J., and V.R. Phillips. 1994. Control of gaseous emissions from livestock buildings and manure stores. J. Agric. Eng. Res. 57:173–189.
• Headon, D.R., K. Buggle, A. Nelson, and G. Killeen. 1991. Glycofractions of the yucca plant and their role in ammonia control. p. 95–108. In T.P. Lyons (ed.) Biotechnology in the feed industry. Allttech, Nichoasville, KY.
• Headon, D.R., and G. Walsh. 1993. Yucca schidigera extracts and ammonia control. p. 686–693. In E. Collins and C. Boon (ed.) Livestock Environment IV, 4th Int. Symp., Univ. of Warwick, Coventry, UK. 6–9 July 1993. Am. Soc. Agric. Eng., St. Joseph, MI.
• Heck, A.F. 1931. Conservation and availability of nitrogen in farm manures. Soil Sci. 31:335–363.
• Hendriks, J.G.L., D. Berckmansand, and C. Vincker. 1997. Field tests of bio-additives to reduce ammonia emission from pig houses. p. 707–714. In Proc. of the Int. Symp. on Ammonia and Odour Emissions from Animal Production, Vinkeloord, the Netherlands. 6–10 Oct. 1997. NVTL, Rosmalen, the Netherlands.
• Hendriks, J.G.L, and M.G.M. Vrielink. 1996. Anzuren van varkensmest via het voer. Praktijk-oderzoek varkenshoulderij. June 1996. Rosmalen, the Netherlands.
• Hendriks, J.G.L., and M.G.M. Vrielink. 1997. Reducing the emission from pig houses by adding or producing organic acids in pig slurry. p. 493–501. In Proc. of the Int. Symp. on Ammonia and Odour Emissions from Animal Production, Vinkeloord, the Netherlands. 6–10 Oct. 1997. NVTL, Rosmalen, the Netherlands.
• Hoeskma, P., N. Verdoes, and G.J. Monteny. 1993. Two options for manure treatment to reduce ammonia volatilisation from pig housing. p. 301–306. In European Assoc. for Animal Prod. Publ. 69. EAAP, Wageningen, the Netherlands.
• Hollenback, R.C. 1971. Manure odour abatement using hydrogen peroxide. Rep. no. 5638-R. Food Machinery Corp., Princeton, NJ.
• Huang, Z.T., and A.M. Petrovic. 1994. Clinoptilolite zeolite influence on nitrate leaching and nitrogen use efficiency in simulated sand based golf greens. J. Environ. Qual. 23:1190–1194.[Abstract/Free Full Text]
• Husted, S., L.S. Jensen, and S.S. Jorgensen. 1991. Reducing ammonia loss from cattle slurry by the use of acidifying additives: The role of the buffer system. J. Sci. Food Agric. 57:335–349.
• Johnston, N.L., C.L. Quarles, D.J. Fagerberg, and D.D. Caveny. 1981. Evaluation of yucca saponin on performance and ammonia suppression. Poul. Sci. 60:2289–2295.
• Jongebreur, A. 1977. Odour problems and odour control in intensive livestock husbandry farms in the Netherlands. Agric. Environ. 3:259–265.
• Jutras, P.J., C. Weil, D. Martin, and R.D. Richard. 1980. Progress in the control of swine odours with ozone. Paper no. 80-412. Am. Soc. Agric. Eng., St. Joseph, MI.
• Kemme, P.A., A.W. Jongloed, B.M. Dellaert, and F. Krol-Kramer. 1993. The use of Yucca schidigera extract as a `urease inhibitor' in pig slurry. p. 330–335. In Proc. of the 1st Int. Symp. on Nitrogen Flow in Pig Production and Environ. Consequenses, Wageningen, the Netherlands. 8–11 June 1993. EAAP Publ. no. 69. PUDOC, Wageningen, the Netherlands.
• Kibble, W.H., C.W. Raleigh, and J.A. Sheperd. 1972. Hydrogen peroxide for industrial pollution control. In Proc. of the 27th Purdue Ind. Waste Conf., Purdue Univ., Lafayette, IN. Purdue Univ. Publ., Lafayette, IN.
• Koelikker, J.K., M.L. Hellickson, J.R. Miner, and H.S. Nakane. 1978. Improving poultry house environments with zeolite. Paper no. 78-4044. Am. Soc. Agric. Eng., St. Joseph, MI.
• Komarowski, S., and Q. Yu. 1997. Ammonium ion removal from wastewater using Australian natural zeolite: Batch equilibrium and kinetic studies. Environ. Technol. 18:1085–1097.
• Krieger, R., J. Hartung, and A. Pfeiffer. 1993. Experiments with feed additives to reduce ammonia emmisions from pig fattening houses—Preliminary results. p. 295–300. In Proc. of the 1st Int. Symp. on Nitrogen Flow in Pig Production and Environ. Consequenses, Wageningen, the Netherlands. 8–11 June 1993. EAAP Publ. no. 69. PUDOC, Wageningen, the Netherlands.
• Kroodsma, W., and N.W.M. Ogink. 1997. Volatile emissions from cow cubicle houses and its reduction by immersion of the slats with acidified slurry. p. 475–483. In Proc. of the Int. Symp. on Ammonia and Odour Emissions from Animal Production, Vinkeloord, the Netherlands. 6–10 Oct. 1997. NVTL, Rosmalen, the Netherlands.
• Lauer, D.A., D.R. Bouldin, and S.D. Klaususner. 1976. Ammonia volatilization from dairy manure spread on the soil surface. J. Environ. Qual. 5:131–141.[Abstract/Free Full Text]
• Liao, C.M., and D.S. Bundy. 1994. Bacteria additives to the changes in gaseous mass-transfer from stored swine manure. J. Environ. Sci. Health. 296:1219–1249.
• MacKenzie, A.F., and J.S. Tomar. 1987. Effect of added monocalcium phosphate monohydrate and aeration on nitrogen retention by liquid hog manure. Can. J. Soil. Sci. 67:687–692.
• Mackie, R.I. 1994. Microbial production of odor components. p. 18–19. In Proc. of Int. Round Table on Swine Odor Control, Ames, IA. 13–15 June 1994. Iowa State Univ. Publ., Ames.
• Mackie, R.T., P.G. Stroot, and V.H. Varel. 1998. Biochemical identification and biological origin of key odor components in livestock waste. J. Anim. Sci. 76:1331–1342.[Abstract/Free Full Text]
• Mader, J.R., and M.C. Brumm. 1987. Effect of feeding sarsaponin in cattle and swine diets. J. Anim. Sci. 65:9–15.
• Martinez, J., J. Jolivent, F. Guiziou, and G. Langeoire. 1997. Ammonia emissions from pig slurries. Evaluation of acidification and the use of additives to reduce losses. p. 475–483. In Proc. of the Int. Symp. on Ammonia and Odour Emissions from Animal Production, Vinkeloord, the Netherlands. 6–10 Oct. 1997. NVTL, Rosmalen, the Netherlands.
• Miner, J.R. 1984. Use of natural zeolites in the treatment of animal wastes. p. 257–262. In W.G. Pond and F.A. Mumpton (ed.) Zeo-agriculture: Use of natural zeolites in agric. and aquaculture. Westview Press, Boulder, CO.
• Miner, J.R., S.N. Raja, and W. McGregor. 1997. Finely ground zeolite as an odour control additive immediately prior to sprinkler application of liquid dairy manure. p. 717–720. In Proc. of the Int. Symp. on Ammonia and Odour Emissions from Animal Production, Vinkeloord, the Netherlands. 6–10 Oct. 1997. NVTL, Rosmalen, the Netherlands.
• Miner, J.R., and R.S. Stroh. 1976. Controlling feedlot surface emmision rates by application of commercial products. Trans. ASAE 19:533–538.
• Misselbrook, T.H., C.R. Clarkson, and B.F. Pain. 1993. Relationship between concentration and intensity of odours for pig slurry and broiler houses. J. Agric. Eng. Res. 55:163–169.
• Molloy, S.P., and H. Tunney. 1983. A laboratory study of ammonia volatilisation from cattle and pig slurry. Irish J. Agric. Res. 22:37–45.
• Moore, P.A., Jr., T.C. Daniel, D.R. Edwards, and D.M. Miller. 1995. Effects of chemical amendments on ammonia volatilization from poultry litter. J. Environ. Qual. 24:293–300.[Abstract/Free Full Text]
• Muck, R.E., and B.K. Richards. 1980. Losses of manurial N in free-stall barns. Agric. Manure 7:65–93.
• Muck, R.E., and T.S. Steenhuis. 1982. Nitrogen losses from manure storage. Agric. Manure 4:41–54.
• Mumpton, F.A., and P.H. Fishman. 1977. The application of natural zeolites in animal science and aquacluture. J. Anim. Sci. 45:1188–1203.[Abstract/Free Full Text]
• Nakaue, H.S., J.K. Koelliker, and M.L. Pierson. 1981. Studies with clinoptilolite in poultry: II. Effect of feeding broilers and the direct application of clinoptilolite zeolite on clean and reused broiler litter on broiler performance and house environment. Poul. Sci. 60:1221–1228.
• Ogink, N.W.M., and W. Kroodsma. 1996. Reduction of ammonia emissions from a cow cubical house by flushing with water or a formalin solution. J. Agric. Eng. Res. 53:23–50.
• O'Halloran, L.P., and A. Sigrest. 1993. Influence of incubating monocalcium phosphate with liquid hog manure on inorganic phosphorous and phosphourous availability in two Quebec soils. Can. J. Soil. Sci. 73:371–379.
• O'Neill, D.H., and V.R. Phillips. 1991. A review of the control of odour nuisance from livestock buildings. Part 1: Influence of the techniques for managing waste within the building. J. Agric. Eng. Res. 50:1–10.
• O'Neill, D.H., and V.R. Phillips. 1992. A review of the control of odour nuisance from livestock buildings. Part 3: Properties of the odorous substances which have been identified in livestock wastes or in the air around them. J. Agric. Eng. Res. 53:23–50.
• Pain, B.F., R.B. Thompson, L.C.N. De La Cremer, and L. Ten Holte. 1987. The use of additives in livestock slurries to improve their flow properties, conserve nitrogen and reduce odours. p. 229–246. In Van Der Meer et al. (ed.) Development in plant soil sciences. Animal manure on grassland and fodder crops. Fertiliser or waste? Martius Nijhoff Publ., Dordrecht, the Netherlands.
• Pain, B.F., R.B. Thompson, Y.J. Rees, and J.H. Skinner. 1990. Reducing nitrogen losses from cattle slurry applied to grassland by the use of additives. J. Anim. Sci. 45:1188–1203.
• Pain, B.F., T.J. Van der Weeden, B.J. Chambers, V.T. Phillips, and S.C. Jarvis. 1998. A new inventory for ammonia emissions from U.K. agriculture. Atmos. Environ. 32:309–313.
• Patni, N.K. 1992. Effectiveness of manure additives. Report for Ontario pork producers. The Centre for Food and Animal Res., Research Branch, Agriculture Canada, Central Exp. Farm, Ottawa, ON, Canada.
• Patni, N.K., and P.Y. Jui. 1991. Nitrogen concentrations in dairy-cattle slurry stored in farm tanks. Trans. ASAE 342:609–615.
• Peltola, I. 1986. Use of peat as a litter for milking cows. p. 181–187. In Odour prevention and control of organic sludge and livestock farming. Elsevier Appl. Sci. Publ., London.
• Ritter, W.F. 1981. Chemical and biochemical odour control of livestock wastes: A review. Can. Agric. Eng. 23:1–4.
• Ritter, W.F. 1989. Odour control of livestock manure: State-of-the-art in North America. J. Agric. Eng. Res. 42:51–62.
• Ritter, W.F., N.E. Collins, and R.P. Eastburn. 1975. Chemical treatment of liquid dairy manure to reduce malodours. p. 381–384. In Managing Livestock Manure, Proc. 3rd Int. Symp. on Livestock Manure. Publ. PROC-275. Am. Soc. Agric. Eng., St. Joseph, MI.
• Safley, L.M., D.W. Nelson, and P.W. Westerman. 1983. Conserving manurial nitrogen. Trans. ASAE 26:1166–1170.
• Schaefer, J. 1977. Sampling, characterisation and analysis of malodours. Agric. Eng. Res. 3:121–127.
• Smith, K., A. Drysdale, and D. Saville. 1980. An investigation into the effectiveness of some odour control treatments in stored pig manure. Project Rep. 24. New Zealand Agric. Eng. Inst., Lincon College, Canterbury, New Zealand.
• Spoelstra, S.F. 1980. Origin of objectionable odorous compounds in piggery manure and the possibility of applying indicator components for studying odour development. Agric. Environ. 5:241–260.
• Stevens, R.J., R.J. Laughlin, and J.P. Frost. 1989. Effect of acidification with sulphuric acid on the volatilisation of ammonia from cow and pig slurries. J. Agric. Sci. 113:389–395.
• Subair, S. 1995. Reducing ammonia volatilisation from liquid hog manure by using organic amendments. M.Sc. thesis. McGill Univ., Montreal, QC, Canada.
• Sutton, M.A., C.J. Place, M. Eager, D. Fowler, and R.I. Smith. 1995. Assessment of the magnitude of ammonia emissions in the United Kingdom. Atmos. Environ. 29:1393–1411.
• Turner, C., and C.H. Burton. 1997. The inactivation of viruses in pig slurries: A review. Bioresour. Technol. 61:9–20.
• Ulich, W.F., and J.P. Ford. 1975. Malodour reduction in beef cattle feedlots. p. 369–371. In Managing Livestock Manure, Proc. 3rd Int. Symp. on Livestock Manure. Publ. PROC-275. Am. Soc. Agric. Eng., St. Joseph, MI.
• Vandre, R., and J. Clemens. 1996. Studies on the relationship between slurry pH, volatilisation processes and the influence of acidifying additives. Nutr. Cycl. Agroecosyst. 47:157–165.
• Varel, V., H. Nieenaber, and B. Byrnes. 1997. Urease Inhibitors reduce ammonia emmssion from cattle manure. p. 721–728. In Proc. of the Int. Symp. on Ammonia and Odour Emissions from Animal Production, Vinkeloord, the Netherlands. 6–10 Oct. 1997. NVTL, Rosmalen, the Netherlands.
• Warburton, D.J., J.N. Scarborough, D.L. Day, A.J. Muehling, S.E. Curtis, and A.H. Jensen. 1980. Evaluation of commercial products for odour control and solids reduction of liquid swine manure. p. 309–313. In Livestock waste: A renewable resource. Am. Soc. Agric. Eng., St. Joseph, MI.
• Watkins, B.D., S.M. Hengemuehle, H.L. Person, M. Yokoyama, and S.J. Masten. 1997. Ozonation of swine manure wastes to control odors and reduce the concentrations of pathogens and toxic fermentation metabolites. Ozone Sci. Eng. 19:425–437.
• Williams, A.G. 1983. Organic acids, biochemical oxygen demand and chemical oxygen demand in the soluble fraction of piggery slurry. J. Sci. Food. Agric. 34:212–220.[Medline]
• Williams, A.G. 1984. Indicators of piggery slurry odour offensiveness. Agric. Manure 10:15–36.
• Witter, E. 1991. Use of ClCa₂ to decrease ammonia volatilisation after application of fresh and anaerobic chicken slurry to soil. J. Soil Sci. 423:369–380.
• Witter, E., and H. Kirchmann. 1989. Peat, zeolite and basalt as adsorbents of ammoniacal nitrogen during manure decomposition. Plant Soil 115:4.
• Woestyne, M.V., and W. Verstraete. 1995. Biotechnology in the treatment of animal manure. p. 311–327. In Biotechnology in animal feeds and animal feeding. VCH Verlagsgesellschaft mbH, Weinhiem, Germany.
• Wu, J.J., S.H. Park, S.M. Hengemuehle, M. Yokoyama, H.L. Person, J.B. Gerrish, and S.J. Masten. 1999. The use of ozone to reduce the concentration of malodorous metabolites in swine manure slurry. J. Agric. Eng. Res. 72:317–327.
• Wu, J.J., S.H. Park, S.M. Hengemuehle, M. Yokoyama, H.L. Person, and S.J. Masten. 1998. The effect of storage and ozonation on the physical, chemical, and biological characteristics of swine manure slurries. Ozone Sci. Eng. 20:35–50.
• Yasuhura, A., K. Fuwa, and M. Jimbu. 1984. Identification of odorous compounds in fresh and rotten swine manure. Agric. Biol. Biochem. 48:3001–3010.
• Yu, J.C., C.E. Issac, R.N. Colemen, J.J.R. Feddes, and B.S. West. 1991. Odourous compounds from treated pig manure. Can. Agric. Eng. 33:131–136.
• Zhu, J., D.S. Bundy, L. Xiwei, and N. Rashid. 1997a. The hindrance in the development of pit additive products for swine manure odor control—A review. J. Environ. Sci. Health. A 32:2429–2448.
• Zhu, J., D.S. Bundy, L. Xiwei, and N. Rashid. 1997b. Controlling odor and volatile substances in liquid hog manure by amendment. J. Environ. Qual. 26:740–743.[Abstract/Free Full Text]
• Zhu, J., D.S. Bundy, L. Xiwei, and N. Rashid. 1997c. A procedure and its application in evaluating pit additives for odor control. Can. Agric. Eng. 39:207–214.
• Zhu, J., and L.D. Jacobson. 1999. Correlating microbes to major odorous compounds in swine manure. J. Environ. Qual. 28:737–744.[Abstract/Free Full Text]
• Zhu, J., G.L. Riskowski, and M. Torremorell. 1999. Volatile fatty acids as odour indicators in swine manure—A critical review. Trans. ASAE 42:175–182.

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