Relationship between olfactory response and major odorants from livestock wastes

Determination of the human olfactory response measured as odour concentration to mixtures of the four of the major odorants from livestock manure livestock manure was performed. Once defined onsite measurements of the odorants present can be used to estimate and simplify onsite olfactory response, which would reduce costs, improve assessment both spatially, and in terms of reducing sample deterioration possible with odour panel measurement. A requirement for this advance would be an understanding of the relationship between the chemical or instrumentally determined concentration of odorants and the odour concentration. We used different concentrations of hydrogen sulphide, 4-methyl phenol, ammonia and acetic acid to simulate livestock waste odour determined in a second order uniform precision rotatable central composite design to produce an accurate and precise model. Olfactory response was determined by using a dilution to odour threshold from which an odour concentration can be calculated. A multiple linear regression model described the odour concentration accounting for 68% of the variance. A decreasing order of influence was found as hydrogen sulphide > 4-methyl phenol > acetic acid > ammonia within the bounds of the concentrations used. Surprisingly 4-methyl phenol concentration produced a negative effect on odour concentration. A better fit of the complex olfactory response was achieved using a radial neural network model that accounted for 88% of the variance, again with the greatest weighting for sulphide. Both models were assessed using emissions from pig manure with the multiple regression model producing a good fit with predictions 20% less than those observed which might be accounted for by the presence of additional lesser odorants.