Analysis of the impact of the industrial fat, oil and grease (FOG) on a municipal wastewater treatment plant by means Respirometry and microscopic Bioindication

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Courtesy of SURCIS, S.L.

The main impact that the industrial fat, oil and grease (FOG) could make in a municipal wastewater treatment plant is mainly coming from its slowly biodegradable COD (sbCOD) The wastewater entering in a municipal WWTP tends to include a percentage of slowly biodegradable COD in the range of 35 to 65% of the total COD; but when this percentage surpasses the habitual range, normally it is due to the presence of any industrial waste that can adversely affect the process and which is usually called recalcitrant COD. Just in the case of a high slowly biodegradable COD coming from FOG wastes, the process become into a specific bulking and foaming phenomena accompanied by specific filamentous bacteria, a deteriorated sludge under the state of dispersed flocculation and certain degree of bioactivity reduction. Often those effects are erroneously classified as toxicity, when in fact chances are it is not. BM Respirometry and microscopic Bioindication stand as an effective combination to analyze this situation accurately and figure out some conclusions that could afford to take the corrective actions that can reduce the harmful effects coming from this situation.


The hydrolysis process of a high percentage of slowly biodegradable COD in the total COD has an important impact in the biomass and, simply said, this is mainly coming from the accused impoverishment of the food (organic matter) quality for the microorganisms.

It must taken into account that the bacteria responsible of the substrate removal are in need of the soluble readily biodegradable COD for their growth and, when the COD profile includes the presence of a high slowly biodegradable COD fraction, they may not reach to satisfy their metabolic needs; in this way, its reproduction and flocculation abilities are significantly depleted.

In the case the treatment plant includes the biological process of nitrification-denitrification, injury tends to be more accused in the anoxic denitrification process where most probable the concentration of readily biodegradable COD may be too small for its normal performance.

As we had mentioned, one of the most effective methods to analyze this complex situation is by means the Respirometry and microscopic Bioindication. However, is also important to stress that the effectiveness of the respirometric tests, results and conclusions will largely depend on the type of respirometry in use, since the respirometer must have the ability to accurately determine the stoichiometric yield coefficient, automatic measurements of the COD biodegradable fractions within relatively short time, as well as the specific COD utilization rate.

This paper includes the description of a protocol based on a set of respirometric tests which can be performed within a relative short time and that can figure out the concentration of any slowly biodegradable COD and the its consequences in the activated sludge. It will also include a real case of study composed by the Respirometry followed by the Bioindication part where somehow complemented and confirmed some of the most important conclusions.

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