This might be a very wide question to answer because of two main reasons: first the very different nature and forms in which Organics might be present in the water, and secondly the purpose of the wastewater treatment system to be designed.
A simplified way to answer the question is by aligning the two reasons mentioned above. In practice, making the lab water analysis output and the wastewater treatment technologist speaking the same language. This is specially necessary since water treatment equipment in most of the cases performs based on Removal Efficiencies, hence the way to get released a Site Acceptance Test (SAT) of the water treatment facility is by measuring the organics on the same design basis in/out around the system. Of course the equipment performance shall be maintained during a period of time agreed between End-user and Contractor, but this will be a matter of other discussions.
When talking about hydrocarbons, as usual is necessary to collect data about the nature (source) of the oil, concentration, viscosity, specific gravity, volatility among other parameters, however is also relevant to know how oil is actually present in the water. Oil and Grease as contaminant can be present in water in different and combined forms, the most typical are: dissolved, liquid free, emulsified, colloidal, solid particulate. These, as the reader may know, have a critical impact in the selection of the oily water treatment technology.
There are several different applications where to deepen about this subject. In order to illustrate the potential issues between what to be measured/set as design basis versus what the treatment equipment is capable to do, a case related to the Oil & Gas industry has been chosen. API and CPI separators are typically used as primary oil/water deoiling treatment stage. Their main purpose is to skim most of the oil that floats naturally (Stoke’s law) to the liquid surface, but also catching sand and suspended solids that may settle down. The floating oil is usually called Free Oil, and in order to differentiate it from other oil forms, a practical rule suggest to set the limit between free and emulsified oil at 20 microns droplet size. This premise shall be taken into account for the separator sizing.
The Standard Method 5520 “Oil & Grease” could be fair enough to measure the oil content to be managed in API or CPI separators as much as the total hydrocarbons are about floatable free oil, nonetheless this is not always the case. Some amount of the oil could be disperse in smaller droplets that are not able to float, or forming emulsions, or in colloidal state or even disolved. Such oil forms could be measured as part of the reported “Oil & Grease” value but however not removed in an oily water separator, hence someone could jump into the unfairly conclusion that the separator is not performing well.
The truth is that API and CPI are not designed to deal with oil emulsions, colloid matter, dissolve hydrocarbons but only free oil that can float or settle. When evaluating such equipment performance for instance the Standard Method 2530 “Floatables” can provide a better and fair understanding of what the separator is doing.
Similar cases can be found in the industry where there is a misalignment between what the water treatment equipment is capable to manage and what is measured to verify performance. Actually there are other methods that provide useful information about hydrocarbons, for instance Total Organic Carbon (TOC - SM 5310) or Chemical Oxygen Demand (COD - SM 5220), however each method shall be compatible with the purpose of the water treatment process when used to verify performance. A recommendation to End-users and Water Treatment Contractors is to set a feasible SAT protocole during the project detail engineering stage.
