**2.5 Odours**

Unpleasant odours in food industry wastewater are usually caused by gases produced by anaerobic decomposition of organic matter. The most common odour causing compound is hydrogen sulphide whose characteristic odour is that of rotten eggs (Metcalf & Eddy, 2003).

Sulphur is required in the synthesis of proteins and is released in to the degradation process. Under anaerobic conditions, sulphate is reduced biologically to sulphide which can further combine with hydrogen forming hydrogen sulphide (H2S). This gas is readily soluble in water, colourless and inflammable, but also toxic. Although hydrogen sulphide is the most common gas formed during the anaerobic decomposition of organic matter when considering odours, other volatile compounds, such as indole, skatole and mercaptans, may cause odours far more unpleasant than H2S (Metcalf & Eddy, 2003).

In recent years, the control of odours has become more important in the designing and operating of wastewater collection, treatment and disposal plants. Odours are the foremost concern of the public in wastewater treatment processes. Quite often, the psychological stress causing by odours is far more important rather than the harm they do to the health of humans (Droste, 1997).

Unpleasant odours are detected by the olfactory system, but which the precise mechanism is not well known. One of the difficulties in developing a global theory has been the insufficient explanation of why compounds with different molecular structures may have similar odours. Nowadays, some agreement has been achieved that the odour of a molecule has to be related to the molecule as a whole. Malodorous compounds in untreated

**Organic compound Rate constant [M-1 s-1]** 

Hydroxyl radical is one of the most active oxidising agents known. It acts very rapidly with most organic molecules with rate constants in the order of 108 – 1011 M-1 s-1 (Table 4) (Munter, 2001). Depending upon the nature of the organic species, generated hydroxyl radical can attack organic radicals by radical addition, hydrogen abstraction, electron

*Radical addition.* Reaction of the hydroxyl radical and unsaturated or aliphatic organic compound produces organic radical which can further oxidise by oxygen or ferrous iron to

*Hydrogen abstraction.* Generated hydroxyl radical can be used to remove hydrogen from an organic compound forming an organic radical and initiating a chain reaction where the organic radical reacts with oxygen. This produces a peroxyl radical, which can react with

→ R·

*Electron transfer.* Electron transfer results in the formation of ions with a higher valence. Oxidation of a monoatomic negative ion will result in the formation of an atom or a free

→ ROH (2)

313

+ H2O (3)

→ Rn-1+ HO- (4)

→ H2O2 (5)

R + HO·

R + HO·

Rn + HO·

HO·

+HO·

Generally, the reaction of hydroxyl radicals and organic compounds will produce water, carbon dioxide and salts (SES, 1994). However, the attack of the HO radical, in the presence of oxygen, generates a complex series of oxidation reactions in which the exact routes of these reactions to complete mineralisation of the organics are still not quite clear. Chlorine containing organic compounds, for example, are oxidised first to intermediates, such as aldehydes and carboxylic acids, and finally to carbon dioxide and water, and to chlorine

A very important point, which has to be considered in the case of natural waters, is the presence of carbonates. Efficient trapping of HO· radicals by bicarbonate (equation 6) and

*Radical combination.* Two radicals form a stable product.

 **O3 HO**  Alcohols 10-2-1 108-109 Aromatics 1-102 108-1010 Chlorinated alkenes 103-104 109-1011 Ketones 1 109-1010 N-containing organics 10-102 108-1010 Phenols 103 109-1010 Table 4. Reaction rate constants for ozone and hydroxyl radical for organic compounds

(Munter, 2001).

radical.

ions (Munter, 2001).

transfer and radical combination.

form stable oxidised end products.

another organic compound, and so on.


wastewater are listed in Table 2. All these compounds can be found or may be developing in wastewaters, depending on ambient conditions (Metcalf & Eddy, 2003).

Table 2. Malodorous compounds in untreated wastewater (Metcalf & Eddy, 2003).

In the complete characterisation of odour, four independent factors can be classified: intensity, character, hedonics and detectability. Odours can be measured by sensory methods and a specific odorant concentration can be measure by instrumental methods, such as GC-MS analysis (Metcalf & Eddy, 2003). In the sensory method, a panel of human subjects is exposed to odour-free air diluted odours and the minimum detectable threshold odour concentration (MDTOC) is noted. This procedure can be performed according to the Standard Method 2150B Threshold Odour Test (APHA, 1998).
