Biodegradability describes the ability of organic compounds to be broken down by the metabolic processes of microorganisms such as bacteria, fungi or algae. In industrial water and wastewater treatment, this property is of crucial importance for assessing the environmental compatibility of substances and developing suitable treatment processes.
Table of contents
Technical basics
Biodegradability is divided into aerobic and anaerobic processes.
- Aerobic degradability takes place in the presence of oxygen, whereby the microorganisms convert organic compounds into carbon dioxide (CO₂), water (H₂O) and biomass. This process is widely used in biological wastewater treatment plants, particularly in activated sludge or biofilm processes.
- Anaerobic degradability takes place in the absence of oxygen, producing methane (CH₄), carbon dioxide and hydrogen (H₂) as end products. Anaerobic processes are used in digestion towers or biogas reactors in particular to break down organic substances and simultaneously generate energy in the form of biogas.
Valuation methods
In industrial wastewater treatment, the biodegradability of a substance is often determined using tests such as biochemical oxygen demand (BOD₅) or chemical oxygen demand (COD). The BOD₅ measures the oxygen consumption of microorganisms over a period of 5 days and provides an indication of the amount of biodegradable organic substances in the wastewater. A high BOD₅/CSB ratio indicates good biodegradability, while a low ratio indicates that a significant proportion of the organic compounds are difficult or impossible to biodegrade.
Photo: Photo of our biologically activated filtration, a combination process of mechanical cleaning and biodegradation(ALMA BHU BAF)
Role of macronutrients
In order for microorganisms to efficiently break down organic substances, they not only need the organic compounds to be treated (carbon sources), but also a balanced supply of macronutrients, in particular nitrogen (N) and phosphorus (P), as well as oxygen (O) in aerobic processes. These nutrients play a central role in the cell structure and energy production of microorganisms:
Carbon (C): The organic compounds in wastewater are the main source of carbon for microorganisms and serve both as a source of energy and for the build-up of biomass.
Nitrogen (N): Is mainly required in the form of ammonium (NH₄⁺) or nitrate (NO₃-). Nitrogen is essential for the formation of proteins, nucleic acids and enzymes, which are required for the growth and function of microorganisms.
Phosphorus (P): Mostly available in the form of phosphates, phosphorus is necessary for the synthesis of nucleic acids and adenosine triphosphate (ATP), a central molecule for energy transfer in cells.
Optimal nutrient ratios
In practice, the C:N ratio (carbon:nitrogen) has established itself as a decisive parameter for assessing the degradation processes in biological plants. A frequently recommended ratio for efficient biodegradation in industrial wastewater treatment is around 100:5:1, which means that there should be around 5 parts nitrogen and 1 part phosphorus for every 100 parts organic carbon. Deviations from this ratio can have a negative impact on the degradation process:
- A lack of nitrogen means that microorganisms cannot build up sufficient biomass, which leads to a drop in biological activity.
- Phosphorus deficiency limits energy transfer and cell division, which also leads to inefficient degradation.
In industrial practice, nutrient dosing is therefore often carried out to ensure the optimum nutrient ratio in wastewater treatment plants. This is particularly relevant in the case of highly organically contaminated wastewater from the food industry or wastewater that only contains small amounts of nitrogen and phosphorus due to production processes.
Trace elements in biodegradability
In addition to macronutrients, trace elements are necessary in low concentrations to support the enzymatic processes of the microorganisms. These elements often play a catalytic role in various biochemical reactions. The most important trace elements include
Iron (Fe): Is required as a component of many enzymes, particularly in the electron transport chain and in oxygen transfer.
Zinc (Zn): Is contained in many enzymes involved in the synthesis of proteins and DNA. Zinc also plays a role in regulating the pH value in cells.
Copper (Cu): Also has a catalytic function and is involved in redox reactions that are important for the energy production of microorganisms.
Manganese (Mn): Acts as a cofactor for enzymes involved in the decomposition of organic substances.
Cobalt (Co): Is a component of vitamin B12, which plays a key role in the metabolic pathways of bacteria, especially in the synthesis of amino acids.
In many industrial applications, the concentration of these trace elements is sufficient. In cases where these elements are missing or greatly reduced, this can lead to an inhibition of the biological degradation processes. In such cases, it is often necessary to add these trace elements to the wastewater in order to optimize degradation.
Photo: Aeration tank for the biodegradation of our ALMA BHU BIO process
Valuation methods
In industrial wastewater treatment, the biodegradability of a substance is often determined using tests such as biochemical oxygen demand (BOD₅) or chemical oxygen demand (COD). The BOD₅ measures the oxygen consumption of microorganisms over a period of 5 days and provides an indication of the amount of biodegradable organic substances in the wastewater. A high BOD₅/CSB ratio indicates good biodegradability, while a low ratio indicates that a significant proportion of the organic compounds are difficult or impossible to biodegrade.
Relevance in practice
In industrial wastewater treatment, knowledge of the biodegradability of substances is crucial for the choice of treatment process. Substances that are easily biodegradable can be treated efficiently in conventional biological plants, such as activated sludge processes or fixed-bed reactors. On the other hand, substances that are difficult to biodegrade or toxic, such as certain heavy metal complexes or chlorinated hydrocarbons, often require additional processes such as adsorption, chemical oxidation (e.g. Fenton process) or membrane filtration in order to achieve a sufficient reduction of pollutants.
The biodegradability of chemicals used in cooling water treatment or in the cleaning of membranes in reverse osmosis systems is also important. Products such as biocides or antiscalants must be selected in such a way that they are not only effective in use, but can also be degraded or safely removed after use to minimize environmental impact.
Conclusion
Biodegradability depends not only on the type of organic compounds, but also on a balanced nutrient distribution. A sufficient supply of macronutrients such as nitrogen and phosphorus as well as the availability of essential trace elements is crucial for the efficient degradation of organic compounds in industrial wastewater treatment plants. Optimal nutrient ratios and the correct addition of nutrients are therefore important factors in ensuring the efficiency of biological treatment processes and minimizing environmental pollution.
