TS 518 is a standard for the biodegradability of active substances in Turkey. The purpose of this standard is to determine how rapidly and completely biodegrades a surfactant or other chemical when released into the environment. Biodegradability refers to how quickly a substance is broken down by natural microorganisms. This is a critical factor in assessing the environmental impact of a substance.

The following steps are usually followed when testing the biodegradability of a surfactant or other chemical:

1. **Sample Preparation**: The substance to be tested is prepared at a specific concentration in a test solution.
2. **Microorganism Culture**: Biodegradation is based on the activity of microorganisms, therefore, natural sources of microorganisms such as activated sludge or natural water samples are often used.
3. **Incubation**: The prepared sample is incubated with microorganisms at a certain temperature and for a certain time. During this process, data is collected on how quickly microorganisms break down the chemical.
4. **Analysis**: It is used to determine how much of it is biodegraded by analyzing the amount of chemical remaining in the sample at the end of the incubation period. Usually, this analysis is performed by gas chromatography, spectrophotometry or other analytical methods.
5. **Evaluation of Results**: Based on the data obtained, the percent biodegradability of the chemical is calculated.

TS 518 specifies specific protocols and requirements for this type of test, so you can refer to this standard for more detailed information on how the test should be conducted. Such tests are crucial for environmental impact assessments and can help determine whether a chemical is environmentally acceptable.