Closely associated with Level 3 Oxygen Uptake Rate (OUR) testing (testing for inhibition or toxicity) is the testing of individual waste streams to determine which streams have the highest oxygen demand. Take a look at Figure 1. Three influent waste streams combine in the Equalization (EQ) Tank and the flow out of the EQ tank goes to the activated sludge bioreactor. When there is a lot of variability in the chemical oxygen demand (COD) in the wastewater out of the EQ tank it is necessary to know the organic strength of the different wastewater sources in the industrial plant in order to regulate the flow rates from these sources when possible.
In Figure 1, illustrating a refinery, the largest flow contributor is from the desalter and this unit is often suspected of being the main contributor to the organic load entering the wastewater plant. The result is that the crude tank water draws, referred to bottom sediment and water (BS&W), are often overlooked as being a wastewater flow that should be carefully regulated.
Figure 1: Individual Waste Streams Entering the Wastewater Plant
Figure 2: Separation of Oil and Water in a Crude Storage Tank
Failing to account for the impact of BS&W on the total organic loading to the wastewater plant is a common problem. And this problem has only worsened with the increasing use of opportunity crudes in many North American refineries. Opportunity crudes often have high concentrations of alcohols, amines, solvents, sulfur compounds, and surfactants, all of which can add a significant oxygen demand to the bioreactor.
Look at the crude oil sample in the centrifuge tube taken from a crude oil storage tank shown in Figure 2. The water that you see at the bottom (BS&W) of this centrifuge tube is representative of the water that is drained from the crude storage tank and sent directly to the wastewater plant. This is often done simply by opening a valve and either allowing the water to drain by gravity or by pumping. In either case, control of the flow rate is possible but is often not regulated. In gravity drainage situations the drain valve is typically opened-full to shorten the time the plant operator has to stand by the tank monitoring for the first sign of oil. Certain crude oils produce a BS&W with a high soluble COD content that the bacteria readily consume. If this waste stream is allowed to flow to the wastewater plant at too high a rate it can quickly increase the COD concentration in the wastewater out of the EQ tank. Oxygen uptake rate testing can directly inform plant operators about the strength of any given waste stream.
Figure 3 shows the oxygen uptake rate for five different waste streams. The units are in pounds of oxygen consumed per million gallons of flow. In particular, note the very high oxygen demand in the BS&W from Crude Storage Tank A. This tank is storing an opportunity crude with a soluble COD that is easy for the bacteria to utilize. Unregulated, this BS&W flow would begin to cause an increase in the COD concentration leaving the EQ tank, perhaps resulting in an oxygen deficiency in the bioreactor.
Also note how the EQ tank, at least at the time these samples were analyzed, does provide some dilution, helping to initially reduce the high organic load from Tank A. Using OUR testing to evaluate the oxygen demand in the individual waste streams provided guidance to the tank farm operators, letting them know the flow out of Tank A needed to be carefully regulated in order to protect the wastewater plant.
Figure 3: OUR of Individual Waste Streams