Explain why when measuring bod that the sample is kept in darkness

Biochemical oxygen demand, or BOD, is a chemical procedure for determining the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. So, how is BOD measured? It is not a precise quantitative test, although it is widely used as an indication of the organic quality of water.

Biological oxygen demand directly affects the amount of dissolved oxygen in rivers and streams. The rate of oxygen consumption is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water. The greater the value, the more rapidly oxygen is depleted in the stream.

This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die. Sources of biochemical oxygen demand include topsoil, leaves and woody debris; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.

Biolgoical oxygen demand is affected by the same factors that affect dissolved oxygen. Measuring biochemical oxygen demand requires taking two measurements. One is measured immediately for dissolved oxygen initialand the second is incubated in the lab for 5 days and then tested for the amount of dissolved oxygen remaining final.

This represents the amount of oxygen consumed by microorganisms to break down the organic matter present in the sample during the incubation period.

When encountering a problem with your values, most operators blame the instrument or probe first. There can be many factors leading to the problem from dirty bottles, DI water, bad seed, bubbles, non-linearity and so on. Place the probe back in this bottle when not in use. Make sure this bottle is kept clean as shown in the bottle on the right of the image.

The bottle on the left is not a clean environment and could affect the calibration. Optical sensors will need an occasional visual check of the paint layer on the sensor cap. Improper warm-up time can easily lead to data drifting due to an inaccurate calibration.

Optical probes do not require a warm-up period. A variety of strategies are employed to deal with specific sample types.

These include varying dilutions and diluent seeding.

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It is often desirable to distinguish between carbonaceous and nitrogenous demand, in which case a nitrification inhibitor is used. Toxic and chlorinated samples also need special handling.

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The operator must be familiar with standard methods and with the technical literautre on the subject. When properly used, the test provides a reliable characterization of wastewater. It can be expected to be a standard for regulatory agencies for many years even though its use as a control tool is limited by the 3 or 5 day wait required for the test and sometimes 20 days!

The Importance of Biochemical Oxygen Demand (BOD) in the Water Analysis Sector

Various methods based on short-term monitoring and extrapolation of quickly estimating the probable results of the test on a sample have been devised and the interested reader is advised to consult appropriate literature but a 'true' test requires time and incubation. Part 1 of 4. Self-stirring BOD Probe. BOD Measurement Problems When encountering a problem with your values, most operators blame the instrument or probe first.

Get your copy of The Dissolved Oxygen Handbook.These measures include test-defined limits for the blank, standard, and seed, as well as limits on dissolved oxygen DO residuals at the end of the analysis. This article will cover additional techniques and hints to get accurate and valid BOD results. As with any biological system, pH affects the efficiency of the bacteria breaking down organic matter in the sample. Adjust the pH of all samples for BOD analysis to between 6.

Any sample that has been chlorinated, even if no chlorine residual is left, must be seeded with viable bacteria so that the organic strength, or BOD, of the sample can be measured.

Samples that show chlorine residual must also be dechlorinated using sodium sulfite see Standard Methods for the recipe. But be careful, excess sodium sulfite in the sample will exert an oxygen demand giving false high BOD readings. It is not the same chemical. These probes usually calibrate to an air setting rather than DO saturated water.

BOD Measurement Problems

If your probe is an air calibration type, calibrate to the barometric pressure in your lab rather than to mm sea level or to a calculated air pressure based on your topographic elevation which is commonly done. Air pressure often changes daily and sometimes hourly. This will be important when measuring the BOD blank.

Since the DO change of the blank should not exceed 0. Algae in a BOD sample and left out on a lab bench exposed to sunlight can be a source of bubbles.

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Always put the BOD bottle in a dark incubator soon after the initial DO is measured and the bottle sealed. But a more common source of bubbles is from dirty glassware. Even though we should try to fill BOD bottles with sample and dilution water as bubble free as possible, there seems to always be tiny bubbles generated. Another source of bubbles can come from aerated dilution water or from samples that are at a lower temperature than 20 degrees C. Since cold water will hold more dissolved air, aerating cold dilution water will give a higher oxygen content than if the dilution water was aerated at 20 C.

After placing the samples in an incubator at 20 C, the water will warm and not be able to hold as much DO.You cannot count BOD molecules. BOD is the amount of oxygen consumed by decomposition of the sample during the incubation period. The intent is to measure what affect the sample will have on oxygen available to living organisms in the waters into which the waste is discharged.

If the BOD of a waste is high enough, the microbial population will quickly deoxygenate the water and render it unsuitable for other forms of marine life. This can cause dead zones in a river or other body of water.

There is an additional subset of BOD that is required in certain areas. This measures along the same basic principle as BOD, except that an inhibitor is added to exclude the oxygen consumption by nitrogen fixing bacteria.

This number is related to the incubation period required for the standard analysis. Other versions of the test are possible and are distinguished by the proper numerical qualifier which is equal to the number of days of incubation. This can go all the way up to ultimate BOD; incubating the sample and reading the depletion until nothing else happens.

His fellow viewers got annoyed and put the poor guy in the hospital where he remained for five days. Once he got back to his lab, instead of redoing his oxygen demand experiment with overnight values he wrote 5 days into his procedure and left it as is. You decide which version is more likely. Approved Methods — While there are a few different methods approved for BOD, one of them is used overwhelmingly by the analytical community, Standard Methods B. Method Summary — A known volume of sample has its initial dissolved oxygen content recorded.

The BOD value is calculated from the depletion and the amount of sample used. What You Should Know — The actual procedure is so much more involved than the summary given above. The biggest challenge in the BOD test is that of time. The holding time for a BOD sample is 48 hours from collection.

The test itself requires a five day incubation period. Doing the math shows that by the time you find out if your test is valid, it is too late to do anything about potential problems.

Therefore you must give the utmost care to each step of your procedure to avoid the possibility of having invalid data. Source water can be the trickiest part of beginning your BOD analysis. Your water will eventually be conditioned by the addition of certain trace nutrients so that the bacteria population can survive.Apr 08 Read Apr 02 Read Apr 09 Read Apr 06 Read Apr 01 Read Sep 02 Read Jul 19 Read Mar 13 Read Feb 26 Read Apr 03 Read Mar 31 Read May 01 Read Times.

A high standard of living involves a high demand for water and, at the same time, causes much greater pollution of this essential element for life. The resultant interference in the natural cycle can often overwhelm natural processes of recovery, so that, in addition to products arising from the decomposition of natural substances e.

They may also consume such large quantities of oxygen that water resources become fouled.

Biochemical oxygen demand

To prevent the threat of possible danger to health, or the very existence of certain species, it is essential to determine the quality of a water source before water is drawn off for consumption. Water quality is dependent on its constituents, which can be divided into organic and inorganic substances. Given the wide range of natural and synthetic compounds which exist, an analysis of the individual constituents is not a practical proposition and, in any case, a complex affair.

In practice, in order to arrive at significant milestones which will permit an evaluation of water quality, rapid methods of analysis are used which consider groups of parameters, rather than the individual parameters themselves. In this connection, BOD is employed to determine the aerobic destructibility of organic substances.

As long ago asFrankland carried out the first BOD measurements, which were very similar to the dilution method in use today see below. It represents the quantity of oxygen which is consumed in the course of aerobic processes of decomposition of organic materials, caused by microorganisms.

The BOD therefore provides information on the biologically-convertible proportion of the organic content of a sample of water. This leads to the consideration of these materials in terms of their susceptibility to oxidation by the use of oxygen.

They feed on salts and organic compounds such as sugar, cellulose and convertible synthetic substances, which they consume in the presence of oxygen O 2 - that is, biochemical oxidation occurs and the products are partially or completely broken down.

The conditions for determining BOD are so defined that the quantity alone of the organic materials contained in a sample limits the growth of microorganisms - i. Determining BOD values after five days BOD 5 has been adopted as a compromise between a short test-period and the detection of a practically complete biological breakdown of organic materials. This kinetic represents a first-order reaction. The advantages of taking measurements after five days are, therefore, on the one hand, the short amount of time required for the analysis and, on the other, the fact that extrapolation can be carried out.

However, in some countries a testing period of 7 days is performed.Search Search. Menu Sections. I n scientific circles BOD stands for Biochemical Oxygen Demand and is widely used as an indicator of water quality as far as organic pollution is concerned. Imagine a crystal-clear stream tumbling under a road bridge. A person leans against the parapet of the bridge eating an apple and admiring the pristine water as it spills along on its journey to the sea.

As the apple is eaten and the few moments of admiration come to an end, the core is tossed into the flowing water without much consideration for what is about to unfold. What unfolds underwater is that bacteria and other microorganisms living in the stream start to feed on the unexpected bounty. In so doing they breed faster and use up some of the stream's reserves of oxygen. The arrival of the apple core puts an additional demand for oxygen on the stream and that additional need is the Biochemical Oxygen Demand or BOD on the water.

Oxygen gas from the air dissolves in the water naturally so the tumbling stream should be well able to cope with a lone apple core. However, a problem would obviously arise in the unlikely event of someone tipping a lorry load of rotting apples into a small stream. A pollution incident would arise and water samples would be collected for analysis.

A BOD result in the range mg dissolved oxygen per litre indicates pristine water quality.

explain why when measuring bod that the sample is kept in darkness

Results in the range indicate doubtful quality; suggest pollution. Efficiently treated sewage effluent ranges while raw domestic sewage is around Vegetables and washings from food processing are in the high hundreds to low thousands.

Wastes and run-offs from agriculture are particularly polluting: cattle slurry has a BOD of 12, pig slurry 30, and silage effluent 60, Milk is worst of all. All of these alarmingly high figures for agricultural wastes evidence the need for extreme care in dealing with spillages and washings and especially runoff from farmyards following periods of prolonged heavy rain.

explain why when measuring bod that the sample is kept in darkness

Facebook Twitter Email Whatsapp.As a precaution against drawing air into the BOD bottle during the 5-day incubation, you must have a water tight seal. What type of BOD bottles should I use? Most importantly, the BOD test is required by law. The BOD test is used to determine how much oxygen is being used by aerobic microorganisms in the water to decompose organic matter.

If these aerobic bacteria use too much of the dissolved oxygen in the water, there will not be enough left over for the fish, insects, and other organisms that rely on the oxygen to live. In this situation, the rich diversity of life in a healthy river is reduced to a low diversity but sometimes high volume of pollution-tolerant organisms. You want to keep a tight time fram when you read your final DO.

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Remember, temperature directly affects the metabolic rate of the bacteria. Below are the dilutions we recommend. Please note that these dilutions may be adjusted to achieve a SCF between 0.

explain why when measuring bod that the sample is kept in darkness

You do not want to add additional oxygen to your samples. Therefore, no visible bubbles should be present during preparation, or any time during the test. Aeration is very important when rehydrating PolySeed. The bacteria in PolySeed require a sufficient amount of air i. Many labs aerate PolySeed by pulling a vortex the tornado-like funnel that occurs when a liquid is being stirred fast.

When this method is used, the vortex sucks outside air into the solution. We do not recommend this method due to the stress it puts on the bacteria. When performing the BOD test, what are the acceptable ranges? Why is it important to have a water tight seal on my BOD bottles? Why is it important to run the BOD test? What temperature should I store my BOD bottles during the 5-day incubation period? Why is it important to store my BOD bottles in dark during the 5-day incubation period?

What dilutions should I use when performing the BOD test? Why is it important not to have any bubbles present in any of the BOD bottles? Why should I use an air pump to aerate my seed soluton instead of using the vortex method?

The Dilution Water Blank i. SCF, seeded blank, or seed check must fall between 0. Having a water tight seal will prevent the following: Possible contamination A change in the volume of solution in the bottle The bottle stopper from becoming stuck 3.

Why is it important to store my BOD bottles in the dark during the 5-day incubation period? Excluding all light prevents the possibility of photosynthetic algae production of DO. Why is it important not to have any bubbles present in any the BOD bottles? Why should I use an air pump to aerate my seed solution instead of using the vortex method?Biochemical oxygen demand BOD is the amount of dissolved oxygen needed i.

BOD reduction is used as a gauge of the effectiveness of wastewater treatment plants. BOD of wastewater effluents is used to indicate the short-term impact on the oxygen levels of the receiving water.

BOD analysis is similar in function to chemical oxygen demand COD analysis, in that both measure the amount of organic compounds in water. However, COD analysis is less specific, since it measures everything that can be chemically oxidized, rather than just levels of biologically oxidized organic matter. Most natural waters contain small quantities of organic compounds. Aquatic microorganisms have evolved to use some of these compounds as food.

9.1 The Winkler method (SL)

Microorganisms living in oxygenated waters use dissolved oxygen to oxidatively degrade the organic compounds, releasing energy which is used for growth and reproduction. Populations of these microorganisms tend to increase in proportion to the amount of food available.

This microbial metabolism creates an oxygen demand proportional to the amount of organic compounds useful as food. Under some circumstances, microbial metabolism can consume dissolved oxygen faster than atmospheric oxygen can dissolve into the water or the autotrophic community algae, cyanobacteria and macrophytes can produce. Fish and aquatic insects may die when oxygen is depleted by microbial metabolism. Biochemical oxygen demand is the amount of oxygen required for microbial metabolism of organic compounds in water.

This demand occurs over some variable period of time depending on temperature, nutrient concentrations, and the enzymes available to indigenous microbial populations.

The amount of oxygen required to completely oxidize the organic compounds to carbon dioxide and water through generations of microbial growth, death, decay, and cannibalism is total biochemical oxygen demand total BOD. Total BOD is of more significance to food webs than to water quality. Dissolved oxygen depletion is most likely to become evident during the initial aquatic microbial population explosion in response to a large amount of organic material.

If the microbial population deoxygenates the water, however, that lack of oxygen imposes a limit on population growth of aerobic aquatic microbial organisms resulting in a longer term food surplus and oxygen deficit. A standard temperature at which BOD testing should be carried out was first proposed by the Royal Commission on Sewage Disposal in its eighth report in This general standard should be prescribed either by Statute or by order of the Central Authority, and should be subject to modifications by that Authority after an interval of not less than ten years.

This temperature may be significantly different from the temperature of the natural environment of the water being tested. Although the Royal Commission on Sewage Disposal proposed 5 days as an adequate test period for rivers of the United Kingdom of Great Britain and Irelandlonger periods were investigated for North American rivers.


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