User Guide to Thresholds and Classification - Environmental ...

370User Guide for Thresholds and ClassificationsRedox potentialOne of the most important environment related factors influencing the degradability is probably the presenceof oxygen. The oxygen content and the related redox potential determines the presence of different types ofmicro-organisms in aquatic environments with aerobic organisms present in the water phase, in the upperlayer of sediments and in parts of sewage treatment plants, and anaerobic organisms present in sedimentsand parts of sewage treatment plants. In most parts of the water phase, aerobic conditions are prevailing andthe prediction of the biodegradability should be based on results from aerobic tests. However, in someaquatic environments the oxygen content may be very low in periods of the year due to eutrophication andthe following decay of produced organic matter. In these periods, aerobic organisms will not be able todegrade the chemical, but anaerobic processes may take over if the chemical is degradable under anaerobicconditions.TemperatureAnother important parameter is the temperature. Most laboratory tests are performed at 20–25°C (standardaerobic ready biodegradability tests), but anaerobic tests may be performed at 35°C as this better mimics theconditions in a sludge reactor. Microbial activity is found in the environment at temperatures ranging frombelow 0 °C to 100 °C. However, optimum temperatures are probably in the range from 10°C to 30°C androughly, the degradation rate doubles for every 10 °C increase of temperature in this range (de Henau,1993). Outside this optimum range the activity of the degraders is reduced drastically although somespecialised species (thermo- and psycro-philic bacteria) may thrive. When extrapolating from laboratoryconditions, it should be considered that some aquatic environments are covered by ice in substantial periodsof the year and that only minor or even no degradation can be expected during the winter season.pHActive micro-organisms are found in the entire pH range found in the environment. However, for bacteria asa group, slightly alkaline conditions favour the activity and the optimum pH range is 6–8. At a pH lower than5, the metabolic activity in bacteria is significantly decreased. For fungi as a group, slightly acidic conditionsfavour the activity with an optimum pH range of 5–6 (Scow, 1982). Thus, an optimum for the degradingactivity of micro-organisms will probably be within the pH range of 5–8, which is the range most oftenprevailing in the aquatic environment.Presence of nutrientsThe presence of inorganic nutrients (nitrogen and phosphorus) is often required for microbial growth.However, these are only seldom the activity limiting factors in the aquatic environment where growth ofmicro-organisms is often substrate limited. However, the presence of nutrient influences the growth ofprimary producers and then again the availability of readily mineralised exudates.January 2012 EPA0109