Usually factors that contribute to taste and odor in wastewater are mostly due to presence of chemical compounds. For examples, if you have chlorides exceeding 500mg/L or more that will give a salty taste while hydrogen sulfide will cause the rotten eggs foul odor. While this may be true to certain extent, research has proven that the main contributor that degrades the quality of treated water is mostly due to bacterial activity that renders the taste and odor towards unacceptable level of quality.
So far, algae, fungi and protozoa have been blamed to be the culprits that release by-product constituents in particular negatively charged colloidal material mainly mineral oil into the water. When these microorganisms die in large numbers for examples in situation whereby the aeration unit fails or during the period when pH fluctuates on the extreme side, this will lead to sudden buildup of unwanted materials that will heavily pollutes the water.
Usually to counter the effect of unwanted taste and odor left present in treated wastewater, activated carbon is selected and used for this purpose. They are usually introduced prior to the chlorine introduction stage so that the chlorine which is a reducing agent will not ended up oxidizing the carbon instead. Sometimes in different wastewater treatment plants, the addition of powdered or granular activated carbon is performed in the coagulation basin itself and all these depends on how the plant is designed to operate. In certain areas, it is used in the absorption process whereby it is packed into granular media filters.
To determine the right amount of carbon to be used for taste and odor control, usually a Jar Test is conducted. First, different amount of carbon ranging from 10 up to 50 mg/L is measured into a series of Jars. Next, all the test jars will be agitated simultaneously with the time corresponding to the actual time the wastewater will travel through the whole wastewater treatment system. After this is completed the carbon will be filtered out before it is ready for testing (as seen from the attached picture above).
The results are usually measure in a relative standard scale determined by a group of trained panelist who would rate the quality based on the intensity of the residual odor present. The test which is routinely carried out to determine whether the end wastewater quality in terms of taste and odor is acceptable is somehow very subjective as it relies mostly on human factor to quantify whether treatment option is sufficient. However, provided that proper procedures for the testing are being followed (APHA standard) and conditions during the evaluation are met such as the test being carried out in a special isolated room, then everything should be fine. Related topic: Ways to control odor generation from anaerobic processes.