Bod Definition

The analytical parameter known as biochemical oxygen demand (also known as BOD or biological oxygen demand) quantifies the amount of dissolved oxygen (DO) that aerobic bacteria that are growing on the organic material in a water sample at a certain temperature over a specific time are consuming. The BOD value is frequently used as a proxy for the level of organic water pollution. It is typically stated in milligrams of oxygen utilised per litre of sample during 5 days of incubation at 20 �C.

Reduction in Biochemical Oxygen Demand (BOD) is a measure of how well wastewater treatment facilities are working. The direct effect of wastewater effluents on the oxygen levels of the receiving water is determined by the BOD of the effluents. Both the BOD analysis and the chemical oxygen demand (COD) analysis assess the content of organic molecules in water. COD analysis is less exact since it analyses all organic components that can be chemically oxidised rather than just concentrations of those materials.

Background

Small amounts of organic molecules are present in the majority of natural waters. Some of these substances have been adapted for use as food by aquatic microbes. Dissolved oxygen is used by microorganisms that live in oxygenated waters to oxidatively break down organic molecules and release energy for growth and reproduction. These bacteria' populations tend to grow in direct proportion to the amount of accessible food. An oxygen requirement resulting from this microbial metabolism is proportional to the quantity of organic molecules that can be used as food. In some cases, microbial metabolism can use up dissolved oxygen more quickly than it can enter the water or be produced by the autotrophic community (algae, cyanobacteria, and macrophytes). When microbial metabolism depletes oxygen levels, fish and aquatic insects may perish.

The biochemical oxygen demand is the amount of oxygen required for microbial metabolism of organic compounds in water. Depending on temperature, nutrient concentrations, and the enzymes readily available to local microbial populations, this need happens over a varying amount of time. The amount of oxygen required for the complete oxidation of organic molecules into carbon dioxide and water over several generations of microbial development, death, degradation, and cannibalism is known as total biochemical oxygen demand (total BOD). Water quality is less important to food webs than total BOD. Most likely, evidence of dissolved oxygen depletion will be seen during the initial surge in aquatic microbial population in response to a substantial amount of organic material. However, if the microbial population deoxygenates the water, it will slow down the population growth of aerobic aquatic microbial organisms, leading to a longer-term food surplus and oxygen deficit.

History

The BOD5 test was chosen as the gold standard for determining organic pollution of rivers in 1908 as a result of the 1865 establishment of the Royal Commission on River Pollution and the 1898 establishment of the Royal Commission on Sewage Disposal. Since it is believed that river water in the United Kingdom takes five days to travel from source to estuary, this time frame was chosen for the test. In its sixth report, the Royal Commission recommended that 15 parts per million of water be the standard. However, the stated criteria in the ninth report had been changed by the panel. A quick calculation reveals that dilution with at least 8 volumes of river water taking up 0.2 part would be required if the final mixture was to contain no more than 0.4 part of an effluent carrying up 2-0 parts dissolved oxygen per 100,000. Our experience showed that the volume of river water would be more than eight times that of effluent in the vast majority of cases. The value of 2-0 parts dissolved oxygen per 100,000, which had been shown to be practicable, would be a safe figure to accept for the purposes of a general standard when combined with the condition that the effluent does not contain more than 3-0 parts per 100,000 of suspended particles.

Typical values

A 5-day carbonaceous BOD below 1 mg/L will be present in the majority of pure rivers. In highly contaminated rivers, a BOD value in the 2 to 8 mg/L range may be observed. If BOD readings are greater than 8 mg/L, rivers may be deemed extremely contaminated. A reading of 20 mg/L or below would indicate that municipal sewage has been treated effectively by a three-stage method. Untreated sewage levels can range, but are frequently around 600 mg/L in Europe and as low as 200 mg/L in the United States, especially when there is a lot of groundwater infiltration or surface water intake. Because Americans use a lot more water per person than people in other countries do, U.S. values are generally lower.

Use in sewage treatment

The BOD is used to gauge how much trash is loaded into treatment facilities and to gauge how effectively those facilities remove BOD.

Methods

Since Winkler's publication of the methodology for a straightforward, precise, and direct dissolved oxygen analytical approach in 1888, the examination of water's dissolved oxygen levels has been essential for identifying surface water. Only the Winkler method and Henry's law-based oxygen solubility at saturation calibration procedures are currently utilised to calibrate oxygen electrode metres. Two ways of measuring dissolved oxygen for BOD are acknowledged, while a variety of additional methods are not yet recognised as standard procedures internationally.

Dilution method

The EPA has approved this standard methodology, which is listed as Method 5210B in the Standard Methods for the Examination of Water and Wastewater. BOD must be produced by measuring the concentrations of dissolved oxygen (DO) in a sample before and after the incubation period, and then correcting for those readings using the sample's appropriate dilution factor. In order to avoid DO generation from photosynthesis, the 300 mL incubation bottles used for this investigation are held for 5 days in a dark room at 20 �C while being dosed with seed bacteria and buffered dilution water. In the past, the bottles were constructed of glass, which needed to be cleaned and rinsed between samples. There is a plastic BOD bottle that is disposable and SM 5210B authorised that skips this step. In addition to the different dilutions of BOD samples, this method calls for dilution water blanks, glucose glutamic acid (GGA) controls, and seed controls. Using the dilution water blank, the quality of the dilution water used to dilute the other samples is checked. This is essential because contaminants in the water used for dilution could materially influence the outcomes. A standard solution used to assess the quality of the seed, the GGA control is supposed to have a BOD5 concentration of 198 mg/L to 30.5 mg/L. To measure carbonaceous BOD (cBOD), a nitrification inhibitor is added to the sample after the dilution water. The inhibitor prevents ammonia nitrogen from being oxidised, which is how nitrogenous BOD (nBOD) is produced. It is customary to assess only cBOD during the BOD5 test since nitrogenous demand does not accurately represent the oxygen demand from organic matter. This is due to the fact that cBOD is created by the breakdown of organic molecules, whereas nBOD is produced by the breakdown of proteins.

Manometric method

This technique is only capable of measuring the oxygen consumption brought on solely by carbonaceous oxidation. The oxidation of ammonia is prevented. The sample is stored in a container that is sealed and equipped with a pressure sensor. Above the sample level in the container, a material that absorbs carbon dioxide (usually lithium hydroxide) is introduced. The sample is kept in storage settings that mirror the dilution process. Carbon dioxide is emitted while oxygen is consumed due to the inhibition of ammonia oxidation. Because carbon dioxide is absorbed, the volume of the gas drops and the pressure consequently rises. The sensor electronics calculates and shows the amount of oxygen consumed from the pressure decrease.

Alternative methods

Biosensor

A biosensor is a detection tool for analytes that combines biological and physicochemical detecting elements. The most often used biological sensing components in the creation of biosensors are enzymes. Their use in the creation of biosensors is constrained by the laborious, expensive, and time-consuming enzyme purification processes. Microorganisms are a perfect replacement for these bottlenecks.

Numerous microorganisms that are helpful for measuring BOD are inexpensive to grow, cultivate, and maintain in pure cultures. Additionally, the employment of microorganisms in the field of biosensors has created new opportunities and benefits, including simplicity in handling, preparation, and low device cost. Many people have employed various pure cultures, such as Trichosporon cutaneum, Bacillus cereus, Klebsiella oxytoca, Pseudomonas sp., etc., separately in the creation of BOD biosensors. However, several workers have immobilised activated sludge, or a combination of two or three bacterial species, on different membranes for the BOD biosensor's development. Polyvinyl alcohol and porous hydrophilic membranes were the membranes that were employed the most frequently.

By undertaking a systematic investigation, or pre-testing of chosen microorganisms for use as a seed material in BOD analysis of a wide range of industrial effluents, a specified microbial community can be created. A consortium of this composition can be charged nylon membrane that is a suitable membrane on which to immobilise it. Due to the particular interaction between positively charged nylon membrane and negatively charged bacterial cell, charged nylon membrane is suited for the immobilisation of microbes.

Therefore, the following are some benefits of the nylon membrane over other membranes: Adsorption and entrapment are both involved in the dual binding, which makes the immobilised membrane more stable. Such specialised Microbial consortium-based BOD analytical tools may be highly useful for rapidly monitoring the level of pollutant strength in a variety of industrial waste waters.

By using a quick (about 30 min) BOD substitute and a matching calibration curve approach, biosensors can be utilised to measure BOD indirectly. Thus, biosensors are now commercially available. They do, however, have some disadvantages, including high maintenance costs, short run times because of the requirement for reactivation, and the inability to respond to changing quality characteristics as would typically occur in wastewater treatment streams, such as diffusion processes of the biodegradable organic matter into the membrane and various microbial species' varying responses that lead to problems. The calibration function's uncertainty in converting the BOD substitute into the genuine BOD is another significant constraint.

Fluorescent

A resazurin derivative has been used to create a BOD5 substitute that shows how much oxygen microorganisms are consuming during the mineralization of organic materials. Cross-validation on 109 samples from Europe and the United States revealed strict statistical parity between the two methodologies' findings.

Based on the luminescence emission of a chemical compound that is photoactive and the quenching of that emission by oxygen, an electrode has been created. For dissolved oxygen in a solution, the Stern-Volmer equation describes this quenching photophysics mechanism:

Polargraphic method

In the 1950s, an analytical device that makes use of oxygen's reduction-oxidation (redox) chemistry in the presence of diverse metal electrodes was developed. This redox electrode used an oxygen-permeable membrane to let gas flow into an electrochemical cell, with polarographic or galvanic electrodes used to measure the concentration of the gas. This analytical technique is sensitive and precise down to dissolved oxygen concentrations of 0.1 mg/L. This membrane electrode's redox electrode still needs to be calibrated using the Henry's law table or the Winkler test for dissolved oxygen.

Software sensor

In order to swiftly forecast BOD for use in online process monitoring and control, automation has been recommended. As an example, think of employing computerised machine learning to swiftly infer BOD using easy-to-assess parameters for water quality. Among other characteristics, suspended particles, pH, chemical oxygen demand, ammonia, nitrogen, and flow rate can all be directly and precisely measured using on-line hardware sensors. Using data of these factors collected over a three-year period, together with BOD, a prediction model was created and tested. The method might accommodate some missing data. It said that while this strategy was feasible, there needed to be enough historical data accessible.

Real-time BOD monitoring

BOD has been difficult to monitor in real time due of its complexity up until recently. A top UK institution recently found a connection between several indicators of water quality, including electrical conductivity, turbidity, TLF, and CDOM. Through a combination of established techniques (electrical conductivity via electrodes) and cutting-edge techniques like fluorescence, these factors can all be monitored in real-time. Escherichia coli (E. Coli) has been the focus of much research on the monitoring of tryptophan-like fluorescence (TLF), which has been successfully used as a proxy for biological activity and enumeration. TLF-based monitoring is applicable in a variety of situations, including but not restricted to freshwaters and wastewater treatment facilities. As a result, there has been a noticeable shift towards integrated sensor systems that can track variables and use them to produce a BOD reading of laboratory quality in real-time.

Dissolved oxygen probes: Membrane and luminescence

In the 1950s, an analytical device that makes use of oxygen's reduction-oxidation (redox) chemistry in the presence of diverse metal electrodes was developed. An oxygen-permeable membrane was used in this redox electrode, also known as a dissolved oxygen sensor, to let gas diffusion into an electrochemical cell, with the concentration of the gas being detected by polarographic or galvanic electrodes. This analytical technique is accurate and sensitive to dissolved oxygen concentrations as low as 0.1 mg/L. This membrane electrode's redox electrode still needs to be calibrated using the Henry's law table or the Winkler test for dissolved oxygen.

Test limitations

Variables in the test procedure restrict reproducibility. In tests, observations typically vary by plus or minus ten to twenty percent from the mean.

Toxicity

Some wastes contain elements that can prevent germs from growing or functioning normally. Among the potential sources are industrial wastes, antibiotics in pharmaceutical or medical wastes, sanitizers used in food processing or commercial cleaning facilities, chlorination disinfection used after conventional sewage treatment, and odor-reducers used in sanitary waste holding tanks in passenger cars or portable toilets. The test result will be lower if the microbial community that is oxidising the waste is suppressed.

Appropriate microbial population

The test is based on a microbial ecosystem containing enzymes that can oxidise the available organic material. There are some waste waters that already have a significant population of microorganisms accustomed to the water being examined, such as those from biological secondary sewage treatment. During the holding period before the test procedure starts, a sizable amount of the waste may be put to use. On the other hand, industrial organic wastes could need specialised enzymes. It could take some time for microbial communities from common seed sources to develop those enzymes. In order to represent the conditions of an advanced ecosystem in the receiving waters, specialised seed cultures may be appropriate.