Trace Organics

Industrial wastes, household chemicals, and pesticides are main contributors to organic chemicals in sludges.  These chemicals are of concern because of their known and unknown hazards to the public health and the environment.  Many of these occur in small concentrations due to modern treatment plant efficiencies.  They do pose some threat due to the long life of most of the common trace organics.  Any accumulation of trace organics in mammalian or plant tissue could be passed along in the food chain and eventually to humans.
Based on the concentrations of trace organics normally found in biosolids and the typical loading rates, the risks from organics when biosolids are applied to agricultural soils appears minimal.  This risk is represented by the Part 503 regulations, which does not include numerical limits for any organics. Trace organics' potential in a treatment system is difficult to quantify due to the variation in concentration in any community waste stream.  These concentrations vary from day to day and, in some cases, hour to hour.

Most trace organics degrade rather slowly in land based waste treatment systems.  The main factors that control the microbial rate of degradation are the number of substitutions, the type of substitution, and the location of the substitutions on the individual organic compound.  The following list are some of the ways organics break down in a waste treatment system environment. (Hagedorn, 96)

• Dealkylation-- usually achieved in an alkyl group attached to nitrogen, oxygen or sulfur atoms.   insertion of a hydroxyl group into the ring structure.

• Oxidation-- this is a common reaction when dealing with cyclic organic compounds and involves the insertion of  a hydroxyl group into the ring.

• Reduction-- this is the reduction of NO₂ groups to NH₂  in soils.

• Ring Cleavage-- Organic ring structures are broken and reform less stable organic compounds.

These are just some of the complex reactions occurring in the waste treatment systems which slowly break down these stable trace organics.  The ease of degradation of some organics follow this sequence. OH > COOH > NH₂ > NH₂ > CH₃ > Cl

Compounds with OH substitutions are easily degraded and the halogen compounds are the hardest to degrade.
 

Some of the types of trace organics are listed in the following table. This is not a complete list but it contains the majority of the different types of organics found in wastewater. (Hagedorn, 96)
 

• PHENOLICS
• PHTHALATE ESTERS
• NAPHTHALENES
• MONOCYCLIC AROMATIC HYDROCARBONS
• POLYCYCLIC AROMATIC HYDROCARBONS
• HALOGENATED ETHERS
• POLYCHLORINATED BIPHENYLS (PCB'S)
• NIROGENOUS ORGANICS
• HALOGENATED ALIPHATIC HYDROCARBONS
• ORGANOCHLORIDE PESTICIDES

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