Wastewater Biological Control
It is generally agreed upon that one of the main ways of measuring the need for wastewater to be treated microbiologically is in terms of three parameters: COD, BOD, and TOC, but is this justified?
Almost all treatment plants are required to utilize at least one of these three parameters as a measure of the pollution value in the water, and to validate the aerobic or anaerobic treatment processes. For microbiological treatment there are two distinct processes that can be utilized, whether individually or in combination: aerobic, which requires oxygen to succeed, and anaerobic, which can occur in the absence of oxygen. Dissolved oxygen must be present within water to support a plethora of naturally occurring microorganisms. Many of these species will convert COD, BOD, TKN, along with chemical and microbiological treatments that are introduced from upstream of the wastewater treatment facility. If the concentration or loading of a particular wastewater is excessive, it may require additional dissolved oxygen to ensure successful treatment. The balance of dissolved oxygen consumption in wastewater treatment facilities is quite delicate and accurate testing is required.
Biological Oxygen Demand (BOD) is a measurement of the amount of dissolved oxygen required by microorganisms to decompose organic matter under aerobic conditions. Essentially the test is a measurement of the amount of oxygen required to remove waste matter from water via decomposition utilizing aerobic bacteria. There are several approved methods for determining BOD but the most widely accepted method is referred to as BOD5. The 5 refers to the standard 5-day incubation period at a set temperature required to produce the most accurate results. As you can see, the largest challenge with the BOD test relates to the time requirement. In addition, the sample needs to be incubated within 48 hours upon collection. By the time results are available, plant conditions are no longer the same and real-time monitoring/control cannot be relied upon solely using BOD.
Chemical Oxygen Demand (COD) is the total measurement of all chemicals in the water that can be oxidized, or the oxygen required to oxidize soluble and particulate organic matter in water. In other words, COD is an important indicator of how much oxygen will be required to treat the incoming waste streams. A COD test measures all organic carbon apart from a few other aromatics such as benzene and toluene. The use of COD monitoring for wastewater compliance is increasing as the test only takes a few hours and can almost be utilized as a real-time adjustment parameter. This test allows for treatment facilities to better trend data to accurately predict and control the wastewater processes. Although COD is comparable to BOD, it actually measures chemically oxidized matter and is not a direct substitute for the BOD test.
Total Organic Carbon (TOC) is a general term referring to any organic material present in the water. This includes FOGs (Fats, Oils & Greases) and residues from chemical processes (cleaning or cooling sprays). There is a corresponding term, Total Inorganic Carbon (TIC) which refers mainly to carbonate species. Total carbon would be the sum of both TOC & TIC. TOC is one of the standard analyses for measuring the concentration of organic impurities in wastewater and is used as a control parameter for judging water quality. An advantage of TOC is the short test completion time and correlation with BOD with constant composition.
Figure 1: Relationship Between the Organic Carbon Fractions in Wastewater
Source: “The Biological Basis of Wastewater Treatment.” Strathkelvin Instruments
The three procedures TOC, BOD and COD described have become the standards used to measure the organic loading of wastewater. Wastewater will often be described in terms of these values, particularly the last two, and a process will be chosen for its ability to reduce them to acceptable levels for recycle or discharge. As a general rule of thumb, I would concur that COD, BOD, and TOC are some of the main methods for understanding whether a wastewater stream should be treated microbiologically.
Source: Hach, “The Science of Chemical Oxygen Demand” Booklet No. 9
Table 1: Advantages/Disadvantages of COD,BOD5 & TOC