BOD5 Biochemical Oxygen Demand, 5-Day

One of the most commonly measured constituents of wastewater is the biochemical oxygen demand, or BOD. Wastewater is composed of a variety of inorganic and organic substances. Organic substances refer to molecules that are based on carbon and include fecal matter as well as detergents, soaps, fats, greases and food particles (especially where garbage grinders are used). These large organic molecules are easily decomposed by bacteria in the septic system. However, oxygen is required for this process of breaking large molecules into smaller molecules and eventually into carbon dioxide and water. The amount of oxygen required for this process is known as the biochemical oxygen demand or BOD. The Five-day BOD, or BOD5, is measured by the quantity of oxygen consumed by microorganisms during a five-day period, and is the most common measure of the amount of biodegradable organic material in, or strength of, sewage.

BOD has traditionally been used to measure of the strength of effluent released from conventional sewage treatment plants to surface waters or streams. This is because sewage high in BOD can deplete oxygen in receiving waters, causing fish kills and ecosystem changes. Based on criteria for surface water discharge, the secondary treatment standard for BOD has been set at 30 mg BOD/L (i.e. 30 mg of O₂ are consumed per liter of water over 5 days to break down the waste).

However, BOD content of sewage is also important for septic systems. Sewage treatment in the septic tank is an anaerobic (without oxygen) process; in fact, it is anaerobic because sewage entering the tank is so high in BOD that any oxygen present in the sewage is rapidly consumed. Some BOD is removed in the septic tank by anaerobic digestion and by solids which settle to the bottom of the septic tank, but much of the BOD present in sewage (especially detergents and oils) flows to the leaching field. Because BOD serves as a food source for microbes, BOD supports the growth of the microbial biomat which forms under the leaching field. This is both good and bad. On the one hand, a healthy biomat is desired because it is capable of removing many of the bacteria and viruses in the sewage so that they do not pass to the groundwater. The bacteria in a healthy biomat also digest most of the remaining BOD in the sewage. Too much BOD, however, can cause excessive growth of bacteria in the biomat. If the BOD is so high that all available oxygen is consumed (or if the leaching field is poorly aerated, as can be the case in an unvented leaching field located under pavement or deeply buried) the biomat can go anaerobic. This causes the desirable bacteria and protozoans in the biomat to die, resulting in diminished treatment of the sewage. Low oxygen in the biomat also encourages the growth of anaerobic bacteria (bacteria which do not require oxygen for growth). Many anaerobic bacteria produce a mucilaginous coating which can quickly clog the leaching field. Thus, excess BOD in sewage can cause a leaching field to function poorly and even to fail prematurely.

Many of the enhanced treatment technologies discussed later in this document were designed specifically to reduce BOD in treated sewage. BOD removal can be especially important where sewage effluent flows to a leaching field in tight soils. Tight soils are usually composed of silts and clays (particle size < 0.05 millimeter). These small soil particles are tightly packed and the pore space between them is small. Reducing BOD means that the sewage will support the growth of less bacteria and therefore the effluent will be better able to infiltrate tight soils. Many enhanced treatment technologies that remove BOD were designed specifically to enhance disposal of effluent in tight silt or clay soils.

BOD is fairly easy to remove from sewage by providing a supply of oxygen during the treatment process; the oxygen supports bacterial growth which breaks down the organic BOD. Most enhanced treatment units described incorporate some type of unit which actively oxygenates the sewage to reduce BOD. This unit is often located between the septic tank and the leach field. Or, it can be located within the septic tank in a specific area where oxygen is supplied. Reduction of BOD is a relatively easy and efficient process, and results in sewage of low BOD flowing to the leaching field. It is important to note, however, that low BOD in sewage may result in a less effective biomat forming under the leaching field.

It is also important to note that BOD serves as the food source for the denitrifying bacteria which are needed in systems where bacterially-mediated nitrogen removal takes place. In these situations BOD is desired, as the nitrification/denitrification process cannot operate efficiently without sufficient BOD to support the growth of the bacteria which accomplish the process.