Organic compounds are an extensive class of chemical substances consisting of carbon (C) and usually hydrogen (H), often combined with oxygen (O), nitrogen (N), sulphur (S), phosphorus (P) or halogens. They play a central role in industrial water and wastewater treatment, as they occur in the form of pollutants, nutrients, energy sources and even operating materials. Understanding their chemical properties, their behavior in water and their interactions with treatment processes is crucial in order to develop efficient water and wastewater treatment systems.

Properties and classification of organic compounds

Organic compounds can be divided into different categories depending on their chemical structure, origin or function:

1. hydrocarbons

• Consist exclusively of carbon and hydrogen atoms.
• Subdivision into:
  • Aliphatic hydrocarbons (e.g. methane, hexane): Single chains or branched chains.
  • Aromatic hydrocarbons (e.g. benzene, toluene): Ring structures with delocalized electrons.
• Relevance: Common components of industrial wastewater from the petrochemical industry.

2. halogenated organic compounds

• Hydrocarbons containing halogens (Cl, Br, F).
• Examples: Trichloroethylene, dichloromethane.
• Relevance: Highly persistent and toxic, poorly biodegradable.

3. organic acids

• Carboxyl groups (-COOH) give these compounds acidic properties.
• Examples: Acetic acid, citric acid.
• Relevance: Influence the pH value and the precipitation processes in water treatment systems.

4. organic nitrogen compounds

• Contain amino groups (-NH₂) or other nitrogen compounds.
• Examples: Amines, urea.
• Relevance: Can increase nitrogen inputs into water bodies and contribute to eutrophication.

5. organic phosphorus compounds

• Contain phosphorus in organic binding.
• Examples: Pesticides, phospholipids.
• Relevance: Contribute to nutrient pollution.

6. biopolymers and natural substances

• Carbohydrates, fats, proteins, lignin and humic substances.
• Relevance: Common in municipal and agricultural wastewater.

7. synthetic organic compounds

• Plastics, surfactants, pharmaceutical residues, plasticizers (e.g. BPA, PAH).
• Relevance: Increasing problem in water treatment due to their persistence and toxic effect.

Sources of organic compounds in water and wastewater

1. Industrial processes

   • Petrochemical industry: hydrocarbons, aromatics, solvents.
   • Chemical industry: Halogenated compounds, surfactants, plastics.
   • Food industry: organic acids, fats, proteins.

2. Household wastewater

   • Cleaning agents, personal care products, pharmaceutical residues.
   • Organic compounds such as surfactants and fragrances enter wastewater treatment plants via wastewater.

3. Agriculture

   • Input from pesticides, herbicides and animal excrement.
   • Degradation products of organic substances accumulate in surface and groundwater.

4. Natural sources

   • Decomposition of organic matter such as plant residues and humic substances.
   • Dissolved organic carbon (DOC) from natural decomposition processes.

Relevance of organic compounds in water and wastewater treatment

1. pollutants and environmental pollution

Many organic compounds are toxic, carcinogenic or mutagenic and difficult to biodegrade. These include

• Halogenated compounds such as trichloroethylene and PCBs, which are persistent and bioaccumulative.

  • Danger: Toxicity to aquatic organisms, potential carcinogenicity.
  • Grenzwert: AOX im Trinkwasser < 0,1 mg/L (Deutschland).

• PAHs (polycyclic aromatic hydrocarbons), e.g. benzo[a]pyrene, which originate from combustion processes.

  • Danger: Carcinogenicity and damage to ecosystems.
  • Grenzwert: PAK (Benzo[a]pyren) im Trinkwasser < 0,01 µg/L (EU).

• Drug residues, e.g. diclofenac and antibiotics, which are biologically active and difficult to break down.

  • Danger: Development of resistance in microorganisms, disruption of hormone systems.
  • Richtwert: Empfohlen < 100 ng/L (keine gesetzlichen Grenzwerte).

Influences on ecosystems:

• Even low concentrations can have a toxic effect on fish and amphibians, impairing reproduction and growth.
• Persistent substances accumulate in sediments and remain active for decades.

Challenges and requirements:

• Conventional wastewater treatment plants cannot completely remove many of these compounds. Persistent substances require specialized processes such as adsorption or oxidation.
• Grenzwertbeispiele: EU-Wasserrahmenrichtlinie: Benzo[a]pyren in Oberflächengewässern < 0,03 µg/L.

2. nutrients and eutrophication

Organic compounds containing nitrogen and phosphorus are important nutrients. However, in high concentrations they promote eutrophication, which significantly disturbs the balance of water bodies.

• Nitrogen compounds (e.g. ammonium, urea):

  • Properties: Rapidly bioavailable, promotes the growth of algae.
  • Consequences: Algal blooms (phytoplankton) lead to oxygen deficiency (hypoxia) and damage aquatic organisms.
  • Grenzwert: Ammonium im Trinkwasser < 0,5 mg/L (Deutschland).

• Phosphorus compounds (e.g. orthophosphates, phospholipids):

  • Properties: Even small quantities can trigger eutrophying effects.
  • Consequences: Formation of toxic blue-green algae and deterioration in water quality.
  • Grenzwerte: Gesamtphosphor in Kläranlagen < 1 mg/L (EU), Phosphat im Trinkwasser < 6,7 mg/L (Deutschland).

Influences on wastewater treatment:

• High concentrations of organic nutrients, e.g. from agriculture or industry, overwhelm biological treatment processes.
• Excess nutrients are removed by nitrification/denitrification (nitrogen) or chemical precipitation (phosphorus).

Process for removing organic compounds 

The removal of organic compounds from water and wastewater is a key aspect of industrial water treatment. Due to the different properties of organic compounds - from easily degradable to highly persistent - various physical, chemical and biological processes are used. These are often combined to ensure maximum efficiency.

1. biological processes

Principle: Biological processes use microorganisms to break down or convert organic compounds. The microorganisms oxidize organic substances to CO₂ and water or reduce them to methane and other intermediate products.

Examples of biological processes:

• Aerobic wastewater treatment:

  • Microorganisms oxidize organic substances under aerobic conditions.
  • Application: Wastewater treatment plants for municipal and industrial wastewater.
  • Process: Oxygen is introduced into the aeration tank by aerators to promote the activity of the bacteria.
  • Typical decomposition products: CO₂ and biomass (sewage sludge).

• Anaerobic wastewater treatment:

  • In oxygen-free reactors, microorganisms convert organic compounds into biogas (methane and CO₂).
  • Application: Suitable for highly contaminated wastewater (e.g. from the food industry).
  • Process: Organic substrates are broken down in stages by hydrolytic, acidogenic, acetogenic and methanogenic microorganisms.

Advantages of biological processes:

• Cost-effective, as no expensive chemicals are required.
• Sustainable, especially through biogas production.

Disadvantages of biological processes:

• Not suitable for poorly degradable (refractory) organic compounds such as PAHs or halogenated hydrocarbons.
• Sensitive to temperature fluctuations and toxic substances.