BOD5 as a Design Number: What the Five-Day Test Measures, and Where It Misleads

Few numbers in water engineering are quoted as often and understood as loosely as BOD5. It is not a measurement of how much organic matter is in a sample. It is the mass of dissolved oxygen a microbial population consumes while breaking that organic matter down under fixed conditions, and everything about the result depends on those conditions being held.

What the Test Counts

Biochemical - or biological - oxygen demand is the dissolved oxygen consumed when organic matter is oxidised and decomposed by microbial biochemical action into inorganic or gaseous forms. Hydrocarbons, proteins, oils, lignin and similar compounds, the stock in trade of domestic sewage and of sugar, food, paper and fibre industry effluent, are all substrates. Because oxygen is consumed in the decomposition, they are classed as oxygen-demanding pollutants. Discharge enough of them and dissolved oxygen in the receiving water is stripped out; the organic matter then putrefies through anaerobic bacterial decomposition, producing foul-smelling gases such as methane, hydrogen sulfide, mercaptans and ammonia, and the water turns septic and stinks.

Complete oxidation and decomposition of the organic matter in sewage takes on the order of 100 days. Nobody waits that long, so the convention - established by the British Royal Commission on Sewage Disposal - is five days at 20°C, reported as mg/L of oxygen and called five-day BOD. For domestic sewage that is approximately 70% of the oxygen consumed for complete oxidation. Total BOD, the oxygen needed for microbial degradation of all organic matter present, is generally estimated from BOD5 using biochemical oxygen-consumption patterns. Both incubation time and temperature affect oxygen consumption, which is why the full notation is BOD5,20°C - though the temperature subscript is usually omitted and the result written simply as BOD5.

Reading the Number Against Standards

Context is everything. The five-day BOD of clean rivers generally does not exceed 2 mg/L, and above 10 mg/L a water body starts to emit a foul odour. Industrial, agricultural and aquaculture water require BOD below 5 mg/L, while drinking water should be below 1 mg/L. Municipal sewage sits around 200 mg/L, and high-concentration organic industrial wastewater can run into thousands or even millions of mg/L. On the regulatory side, China's Integrated Wastewater Discharge Standard stipulates a secondary-standard maximum allowable BOD concentration of 60 mg/L at a factory outfall, and requires surface water BOD at no more than 4 mg/L. For municipal wastewater treatment plants the tiers are 10 mg/L for first-class A, 20 mg/L for first-class B, 30 mg/L for second class and 60 mg/L for third class.

The BOD/COD Ratio Is the Useful Part

COD, chemical oxygen demand, measures chemically the amount of oxygen needed to oxidise reducible matter in a water sample: the oxidant consumed under fixed conditions, converted to milligrams of oxygen per litre. It is another comprehensive indicator of relative organic content, but it counts everything oxidisable, biodegradable or not. The ratio of BOD to COD therefore indicates how much of the organic pollution is difficult for microorganisms to decompose - and organic pollutants that microorganisms struggle with cause greater environmental harm. The working rule is that a wastewater ratio above 0.3 is suitable for biological treatment. Below that, biology is fighting a losing battle and chemical oxidation or physical separation belongs somewhere in the flowsheet.

Dilution, Calculation and Sample Handling

Classically, a sample or diluted sample is stored and incubated for a period, and the difference in dissolved oxygen before and after incubation is its BOD. The calculation is BOD (mg/L) = (D1 - D2)/P, where D1 is the initial dissolved oxygen of the diluted sample, D2 is the dissolved oxygen after five-day incubation in a 20°C constant-temperature incubator, and P is sample volume divided by final diluted volume. Getting this right is harder than it looks: sample storage, dilution and inoculation all have to follow standard methods, toxic industrial wastewater often needs special equipment, and sometimes measurement is impossible altogether.

If a sample cannot be analysed within 2 hours of collection it should be preserved at 0-4°C and analysed within 6 days. If it cannot be analysed within 6 hours, storage time and temperature should be reported with the results. Under no circumstances should storage exceed 24 hours. Glassware and plastic containers must be cleaned carefully - no toxic or biodegradable compounds may remain on container walls - and contamination has to be prevented throughout handling.

The Microbial Sensor Route

A faster alternative pairs an oxygen electrode with a microbial membrane. Sample saturated with dissolved oxygen enters the flow cell and contacts the sensor; dissolved biodegradable organic matter is acted on by the strains held in the membrane, consuming oxygen and reducing the mass diffusing to the electrode surface. Once the diffusion rate of biodegradable organic matter from sample to membrane becomes constant, the oxygen mass reaching the electrode is constant too and a constant current results. Because a quantitative relationship exists between the constant-current difference and the oxygen reduction, BOD follows directly.

The method applies to surface water, domestic sewage and industrial wastewater that contains nothing markedly toxic to the membrane organisms. Maximum allowable amounts that do not significantly interfere are CO2+ 5 mg/L, Mn2+ 5 mg/L, Zn2+ 4 mg/L, Fe2+ 5 mg/L, Cu2+ 2 mg/L, Hg2+ 5 mg/L, Pb2+ 5 mg/L, Cd2+ 5 mg/L, Cr6+ 0.5 mg/L, CN- 0.05 mg/L and suspended solids 250 mg/L. Samples containing free or combined chlorine are dosed with 1.575 g/L sodium sulfite solution to deactivate it, avoiding excess. Wastewater high in bactericides or pesticides that are toxic to the membrane strains is not suitable at all.

Keeping the Sensor Alive

Reagents are analytical grade, and the distilled water should be boiled for about 2-5 minutes and cooled to room temperature before use. The 0.5 mol/L phosphate stock dissolves 68 g of potassium dihydrogen phosphate (KH2PO4) and 134 g of disodium hydrogen phosphate (Na2HPO4.7H2O) and dilutes to 1000 mL, giving pH around 7; the working solution, which doubles as cleaning solution, is 0.005 mol/L. Hydrochloric acid is 0.5 mol/L, sodium hydroxide 20 g/L, and sodium sulfite 1.575 g/L - the last is unstable and must be prepared before use.

The glucose-glutamic acid standard weighs 1.705 g each of anhydrous glucose (C6H12O6) and glutamic acid (HOOC-CH2-CH2-CHNH2-COOH), both dried at 103°C for 1 hour and cooled, dissolves them in phosphate buffer working solution and dilutes to 1000 mL, giving a 250 mg/L BOD standard solution. The working standard, also prepared before use, is 10.00 mL of that solution made up to 250 mL, at 100 mg/L.

Operationally, the buffer keeps the microbial membrane in contact with solution, regulates sample pH, cleans and maintains the sensor for normal operation, and precipitates heavy-metal ions. A stable temperature field is essential because the microbial electrode response depends on temperature. Strains in the membrane should be uniform and as consistent as possible between membranes, can be preserved wet or dry at room temperature, and should give continuous service life beyond 30 days. New membranes are activated by soaking in 0.005 mol/L phosphate buffer working solution for over 48 hours before installation.