In order to preserve the planet and respect the environment, several laws and regulations have been written to force certain companies to act better towards our planet. Generally speaking, the discharge of industrial wastewater is governed by these laws since the discharges can be dangerous in many ways.
In addition to improving your environmental footprint, proper wastewater management is often a financial benefit as it often allows you to avoid unnecessary costs such as fines or penalties. For example, the biological oxygen demand and chemical oxygen demand (BOD / COD) must not exceed a certain threshold or else management fees may be imposed in cases of discharge to sewers or fines for spills in nature.
From the food industry through pulp and paper to the cosmetics industry, BOD and COD are a challenge in many cases. That's why, in this article, we'll take the time to define "What is Biological Oxygen Demand/Chemical Oxygen Demand?", "What are the problems caused by BOD/COD?", "What are the permissible discharge limits?"
What is biological oxygen demand (BOD)
Biological oxygen demand is a measure to identify the amount of oxygen required to oxidize organic and inorganic matter biologically in any aqueous sample.
To measure biological oxygen demand, an effluent must be incubated at 20°C for 5 days in the dark. At the time of incubation and after 5 days, the dissolved oxygen concentration is measured electrometrically. The dissolved oxygen differential, i.e. the amount of oxygen consumed, represents the concentration of oxidizable matter and is expressed in mg/L.
- The method for calculating BOD is known and expressed as BOD5.
It should be noted that interference may occur if the aqueous sample contains large quantities of metals (chromium, copper, mercury, nickel, lead and zinc), bactericides (phenols, formaldehyde, chlorine and cyanide) or residual chlorine. If so, the results would represent an underestimate of BOD5.
What is chemical oxygen demand?
While biological oxygen demand represents only biologically degradable contaminants, chemical oxygen demand is concerned with anything that can be oxidized in water. Moreover, it should be noted that the presence of COD undoubtedly means the presence of BOD.
In fact, unlike BOD, COD is calculated with the addition of a powerful oxidizing agent, potassium dichromate (K2CR2O7).
As with anything, the use of potassium dichromate for COD identification can run into problems if K2CR2O7 comes into contact with aliphatic hydrocarbons like pyridine and its derivatives.
Since the identification of chemical oxygen demand is done with potassium dichromate, the results are immediate. Therefore, it is easier to monitor COD than BOD.
As an aside, in Quebec, for the taking of samples, standards and procedures must be respected. You can find them on this page:https://www.ceaeq.gouv.qc.ca/documents/publications/echantillonnage.html
These two analyses allow us to understand the effect of bacteria and other microorganisms on the amount of dissolved oxygen during the decomposition of organic and inorganic matter in an aerobic environment.
- Importantly, dissolved oxygen is the amount of oxygen (O2) that has been absorbed into the water through the absorption principle or photosynthesis of algae.
Next, BOD and COD are parameters used to control organic and inorganic pollutants from various industrial effluents. In general, the industries producing the highest concentration of BOD and COD are the metal refineries, the food industry and the pulp and paper industry.
Because these parameters are used to control pollutants from industrial effluents, the determination of biological oxygen demand and chemical oxygen demand is required by law.
The following are some of the regulations requiring BOD/COD monitoring in Quebec:
- Regulation respecting industrial wastewater treatment certificates;
Regulation respecting pulp and paper mills.
What problem can result from a high BOD/COD concentration?
Since the biological oxygen demand and chemical oxygen demand of water represent the amount of dissolved oxygen required by organisms in water for the oxidation of organic and inorganic matter, several problems can occur when it is too high.
Before discussing the benefits of lowering biological oxygen demand and chemical oxygen demand, we will address the problems that arise from too high a concentration of these.
When an effluent that is too concentrated in BOD or COD is discharged into a lake, river or other water source, the bacteria in the effluent consume all the dissolved oxygen in the water source. Although it may seem irrelevant, but the oxygen dissolved in the water is used for the survival of aquatic life such as fish, algae or any other type of underwater life. In other words, when a high BOD/COD discharge is made, aquatic life is at risk of dying or being forced to migrate.
Although the primary problems related to these water parameters are related to the survival of aquatic life, it should not be forgotten that a high concentration of organic matter, ergo BOD/COD, is a precursor to color or odor in the water.
What are the maximum concentrations of BOD/COD allowed?
Generally speaking, if you are located in Canada, the BOD concentration limit in your wastewater is 25mg/L. This regulation comes from the Wastewater Systems Effluent Regulation.
Of course, depending on your geographic location, other laws or regulations may apply to your situation - these regulations and laws may be more or less severe.
- For example, the city of Saint-Hyacinthe, Quebec, allows discharges into its sewer system of concentrations up to 144mg/L in biological oxygen demand. This type of exception is explained by the fact that a city uses organic matter in its wastewater to its advantage. Saint-Hyacinthe uses the biological material in its sewage to produce methane gas through a complete biomethanization plant.
It should be noted that the standard of 25mg/L is a strict minimum and for the good of the planet and its living beings, nothing prevents a company from being eco-responsible and aiming for more.
What can you do to reduce the BOD/COD concentration?
Several options are available to better manage the biological oxygen demand in the effluent. In some cases, such as the City of Saint-Hyacinthe, BOD can be used for methane production.
- It should be noted that for biomethanization, the quantities of water and infrastructure required are immense. In other words, it is extremely rare that this solution is financially viable.
Not being in a utopia, most companies find themselves in the obligation to decrease the concentration of BOD/COD in their effluent.
BOD/COD reduction techniques
To begin with, it is recognized that the concentration of suspended solids (SS) present in water is intimately related to the concentration of BOD/COD. It is therefore important to understand that the removal of suspended solids allows the reduction of the biological oxygen demand and the chemical oxygen demand. Thus, coagulation/flocculation followed by a filtration step are effective methods for BOD/COD reduction.
In some cases, the use of hydrogen peroxide may be an option. Among the situations where the use of H2O2 is prioritized is the pre-digestion of wastewater containing moderate/high levels of toxic, inhibiting or recalcitrant compounds to biological treatment.
- These compounds can be pesticides, plasticizers, resins, coolants or dyes.
The option of using H2O2 is also interesting when one seeks to improve the flotation separation of organic matter.
- It is important to note that hydrogen peroxide can be used alone or with a catalyst. The decision to add a catalyst or not should be made based on the type of BOD/COD present in the water. For example, hydrogen peroxide alone is effective against sulfide and thiosulfates, but will have no effect on phenols, cyanide and amines.
Since the use of H2O2 represents the addition of chemical and is used to combat any toxic compounds, this solution is not always the most environmentally friendly.
Among other solutions for reducing the biological and chemical oxygen demand of water, membrane bioreactors are a technology of choice for many reasons. First developed in the 1960s to combat the exponential growth in the number of people on earth, membrane bioreactors were intended to be used for sewage treatment. The invention of this technology represented a major advance in wastewater treatment and the prioritization of the environment in societal mores.
Membrane bioreactors use filtering membranes (microfiltration or ultrafiltration) to separate treated water from bacterial flocs. There are two types of membrane bioreactors: submerged membrane bioreactors and external membrane bioreactors.
Other than the location of the membranes, both types of bioreactors operate in the same way. Raw water enters an aeration tank where there is a sludge with a high concentration of bacteria. Once treated by the bioreactor, the water passes through a filtration membrane (submerged or external) to be filtered and is ready for reuse or discharge.
The aeration tank
The sludge in the aeration tank is composed of bacteria that accelerate the oxidation of organic and inorganic materials in the wastewater. The addition of oxygen, through aeration, allows the bacteria to have the necessary amount of dissolved oxygen to carry out the complete oxidation of the contaminants. For this reason, the aeration tank is the heart of the treatment capacity of a membrane bioreactor.
The larger the volume of the tank, the higher its BOD/COD treatment capacity. This is because the bacteria responsible for the BOD/COD treatment are located in the tank. Therefore, the larger the tank, the more bacteria are present to reduce the concentration of biological and chemical oxygen demand.
- For information purposes, the treatment capacity of a membrane bioreactor is usually calculated in kilograms/day.
In general, the mixed liquor suspension concentration (MLSS) is one of the key parameters to monitor for proper bioreactor operation. Too high a concentration in MLSS can cause bacterial death and make the treatment ineffective.
- Exceptions: In general, a concentration of 8000 to 9000 ppm MLSS is considered the maximum for treatment effectiveness. However, in some cases, if the contaminants are always the same, the bacteria become accustomed to and can cope with a higher concentration. For example, at Crofter's organic, MLSS concentrations reach impressive levels of over 25,000ppm. Despite this, the treatment is effective since the oxidizable material is consistently sugar carbohydrates. Read this article to learn more about the Crofter's / Durpro partnership, the system they adopted and its performance!
As mentioned above, once the organic and inorganic matter, i.e. the contaminants, have been oxidized, the water passes through the membranes to extract the physical contaminants and the excess sludge and is rejected.
The treated water can then be discharged without concern for the environment or can be reused for pre-determined purposes as in the example of Crofter's which makes the most optimal use of its wastewater.
Moving membrane Bioreactors
Generally speaking, the operating principle of MBBRs is the same as that of MBRs. The difference is that inside the aeration tanks, there is no sludge concentrated in bacteria, but rather a special media is added to the water. Typically, an MBBR aeration tank is filled with approximately 50 to 70% media.
Their variable shapes are optimized to promote the growth of bacteria. Thus, thanks to the same oxidation principle discussed in the membrane bioreactor section, the bacteria on the media will reduce the BOD and COD.
There are several types of media that can be used for bacterial growth in the aeration basins of a MBBR. The most common are sponge, chip, coin and tube media.
MBR vs MBBR
First, while effective in reducing BOD and COD, MBBRs have a weakness against suspended solids, which does not affect MBRs. On the other hand, MBBRs are generally less expensive than traditional MBRs.
The water quality offered by membrane bioreactors is better, and they allow water reuse, whereas MBRs do not. Finally, in terms of facility size, they are both very similar.
What are the benefits of reducing BOD/COD?
Several benefits can come from the reduction of BOD and COD. First, as discussed above, biomethanization is a cost-effective option for equipment and water treatment. However, unless your company is the size of a small town, the return on investment is not worth the cost. It is also important to note that an effluent that is too variable in BOD/COD can be problematic for methane production.
At the environmental level, the impact of the reduction of these contaminants before the wastewater discharge is very positive and allows companies to drastically reduce their ecological footprint. Closely related to this benefit are the costs associated with discharges, risk management and potential government sanctions.
In addition, as in the Crofter's organic example, proper wastewater treatment often allows for the reuse of wastewater for a variety of purposes. In short, reducing BOD / COD can have a positive impact on the environment, your business and your environmental impact.
To briefly summarize, BOD and COD are water parameters that reflect the oxidation capacity of the bacteria in the water. The higher the BOD, the more oxygen must be dissolved in the water in order not to negatively affect the surrounding aquatic life.
Whatever the constraints your company is facing, the different existing techniques for BOD/COD reduction allow the treatment of water, regardless of your situation and your needs.
Finally, whether your motivations are financial, environmental or other, there is a solution to help you achieve your objectives with respect to your water use and treatment.
If you have any questions regarding the treatment of biological oxygen demand and chemical oxygen demand, do not hesitate to contact us or leave a comment below.