Reduction of the biological oxygen demand (BOD) in wastewater treatment processes

• 1. Reduction of biological oxygen demand
• 2. What is BOD?
• 3. Removal of BOD from wastewater.
• 4. Our technologies for wastewater BOD reduction.
• 5. Comparative table of technologies used for BOD reduction.
• 6. Summary.

1. Reduction of biological oxygen demand

Biological oxygen demand (BOD) is a measure of the amount of dissolved oxygen needed by aerobic microorganisms to break down organic matter in water. It is a parameter used to evaluate water quality according to its ability to support aquatic life.

BOD is used as an indicator of water quality and the level of organic contamination in water. Elevated BOD levels in water can indicate the presence of organic pollutants, which can lead to oxygen depletion and have negative impacts on aquatic ecosystems. Authorities can monitor water quality by measuring BOD levels and take measures to mitigate the effects of organic pollution.

2 What is biological oxygen demand (BOD)?

Known by its initials (BOD), it is identified with the biodegradable fraction of organic matter contained inwater. It is part of the chemical oxygen demand (COD) and is expressed inmg _O2/l. It is used as a measure of the organic pollution of a discharge.

Any industry that generates wastewater containing organic matter or other pollutants may require BOD removal to protect the environment and meet regulatory requirements.

Some industries that typically require BOD removal from their wastewater are:

• Food and beverage industry: The food and beverage industry generates large quantities of wastewater containing organic matter such as oils, greases and lubricants.
• Paper industry: the pulp and paper industry generates wastewater containing high levels of organic matter, lignin and cellulose.
• Chemical industry: The chemical industry generates wastewater containing various organic compounds and other pollutants.
• Textile industry: The textile industry generates wastewater containing organic compounds, dyes and other pollutants.
• Pharmaceutical industry: The pharmaceutical industry generates wastewater containing organic matter, chemicals and other pollutants.

Industrial, urban and agricultural wastewater often contains high levels of organic matter that can be broken down by microorganisms. This process consumes dissolved oxygen in the water which can have negative repercussions on aquatic life if oxygen levels become too low.

For this reason, in aerobic oxidation treatments, oxygen must be added if the process is to be promoted. This is done by insufflating air by means of suitable equipment (blowers, aeration turbines, etc.) or by adding pure oxygen. There are calculation systems to establish the amount of _O2 necessary to carry out a correct biological oxidation. The approximate consumption is in the order of 1.5 - 2 Kg _O2/Kg BOD. The residual level of_dissolved O2 that is usually maintained in aerobic biodegradation processes is about 2 ppm, and is controlled by means of oxymeters and redoximeters.

We can say that a wastewater is biodegradable when the BOD/COD ratio ≥ 0.6 is met.

BOD is usually measured by taking a water sample and incubating it in the dark at a controlled temperature for a certain period of time (usually 5 days). During this time, microorganisms in the water consume the available organic matter and deplete the dissolved oxygen in the water. The BOD is then calculated by measuring the difference in dissolved oxygen levels between the beginning and the end of the period.

The unit used to measure BOD is _BOD5, which indicates the amount of oxygen that bacteria and other microorganisms in water consume over a period of 5 days at a temperature of 20ºC for the aerobic degradation of organic matter.

The decomposition cycle of biodegradable organic matter is established in 21 days, but in 5 days it is usual to be able to reduce this contamination by approximately 75%.

TOC(total organic carbon) has no relationship with the oxygen demand of a discharge, although it can be used to establish correlations with BOD and COD, which will vary in each case.

3. Removal of BOD from wastewater

To determine the removal efficiency of BOD in wastewater, its concentration is measured before (BODi₎ and after treatment (_BODf), and the formula is applied:

BOD removal efficiency (%) = [(BODi - BODf) / BODi] x 100 %.

There are several treatment methods used to remove BOD from wastewater, including physical, chemical and biological processes:

• Physical processes such as screening, sedimentation and filtration remove large particles and suspended solids from wastewater.
• Chemical processes such as coagulation, flocculation and oxidation use chemicals to remove suspended solids and organic matter from water.
• Biological processes, such as activated sludge treatment, use microorganisms to break down and consume organic matter in wastewater. In this process, wastewater is mixed with a combination of bacteria and other microorganisms in an aeration tank (mixed liquor). The microorganisms feed on the organic matter and convert it into carbon dioxide and water, while consuming dissolved oxygen in the process.

The choice of BOD removal process depends on a number of factors, including wastewater characterization, volume, required effluent quality, current regulations, available resources, etc. It is common to use a combination of different processes to achieve the desired level of BOD removal.

PROCESSES	ADVANTAGES	DISADVANTAGES
Physical (Sieves, grids, sedimentators, floats, filtration...)	- Separates large solids - Low cost - Simple process	- Does not affect suspended or dissolved matter. - High maintenance and cleanliness
Chemicals (coagulation/flocculation, neutralization, oxidation...)	- Facilitate removal of suspended solids. - May reduce dissolved solids and M.O. - May improve biodegradability.	- They are expensive and generate hazardous waste. - They require specialized technical supervision.
Biological (aeration basins, biodiscs...)	- They can remove dissolved and suspended organic matter. - Low cost. - Environmentally friendly.	- Requires large footprint and high volume tanks. - Sensitive to pH, temperature and other factors. - Sludge production.

Aeration turbines are a classic example of a mechanical system for supplying air to effluents for biological treatment, but they generate bioarosols that are hazardous to the environment and to health.

4. Our technologies for wastewater BOD reduction

The content of organic pollutants in wastewater varies according to its origin. An important part of organic matter is usually present in suspension in the water. It is common to find that more than 30% of these substances are in colloidal form, which gives us an idea of the importance of having a primary treatment system that separates them efficiently, since dissolved organic matter cannot be separated by physicochemical means.

4.1. Dissolved air flotation (DAF)

Dissolved air flotation (DAF) is a highly efficient technology for BOD removal, especially in industrial wastewater treatment.especially in the treatment of industrial wastewater. Dissolved air flotation is a water treatment process that uses microbubbles to attach to contaminants and float them to the water surface, where they can be easily removed.

A DAF system is effective in removing suspended solids including organic matter. It is especially effective in removing fats, oils, grease and lubricants that can contribute to elevated BOD levels in wastewater. In addition, a DAF can be used as a pretreatment step for other BOD removal processes, such as biological treatment. This will result in simplification of the downstream installation and better treatment performance.

Some of the advantages of dissolved air flotation (DAF) are as follows:

• High removal efficiency of organic matter and other impurities.
• It can be used for a wide range of wastewater types and concentrations.
• It requires less space compared to other physical treatment processes.

In addition to DAF systems, we also offer other solutions for BOD removal and reduction, including our technologies for biological wastewater treatment.

4.2. MBR(Membrane Bioreactor) Systems

MBR (membrane bioreactor) systems are a type of wastewater treatment that combines biological processes with membrane filtration. The biological process consists of using microorganisms suspended in the wastewater effluent (biomass) to break down the organic matter in the wastewater, while the membrane filtration stage separates the treated water from the solids and microorganisms.

MBRs are effective in removing both suspended and dissolved solids and can achieve high levels of BOD removal. MBR systems are efficient in removing BOD from wastewater due to the following reasons:

• MBR systems have a high treatment efficiency and can remove up to 99% of BOD from wastewater. MBR systems can achieve higher levels of BOD removal compared to conventional treatment systems due to the use of membrane filtration, which can remove smaller particles and suspended solids more effectively than other methods. Additionally, they allow higher biomass concentrations within the reactor, between 6000 and 12000 mg/L MLSS.
• They use ultrafiltration membranes, which have a higher separation efficiency than conventional methods. Ultrafiltration membranes have a pore size of 0.1 micron or less, which allows better removal of suspended solids, bacteria and viruses.
• MBR systems are less sensitive to variations in influent water quality and can maintain consistent performance over time and generate effluent of consistent quality.
• MBR systems produce high quality treated water that can be reused, for example, for irrigation or industrial purposes.
• MBR systems produce less sludge than conventional treatment systems, which can reduce disposal costs.
• They are versatile equipment, which can be used as a stand-alone treatment system or integrated with other treatment processes, depending on the specific needs of the application.
• MBR systems require less space than conventional treatment systems because they do not require a sedimentation basin or clarifier for solids separation.

4.3. MBBR REACTORS

MBBR (moving bed biofilm reactor) systems are another type of wastewater treatment that are effective for BOD removal. are another type of wastewater treatment that are effective for BOD removal.

MBBR reactors use a moving bed biofilm reactor process that involves the presence of attached biomass (biofilm), which grows on a carrier medium, and moves in the reactor tank. The microorganisms in the biofilm degrade the organic matter in the wastewater, thereby reducing BOD.

These reactors use a combination of biological and physical processes to remove organic matter from wastewater. They are effective in reducing BOD, and also require less space and energy compared to traditional biological treatment systems. However, MBBRs can be sensitive to temperature and pH variations.

MBBR systems are effective for BOD removal for the following reasons:

• The carrier media of MBBR systems have a large surface area, which allows the growth of a high density of microorganisms and their adhesion to the media. The result is a high organic matter removal rate and efficient BOD removal, which occurs due to the high biomass concentration of up to 4000 - 8000 mg/L MLSS.
• MBBR reactors are modular and can be designed to suit different wastewater treatment applications. The system can be easily expanded or modified to accommodate changes in flow rate or wastewater characteristics.
• They take up less space than conventional treatment systems, since they do not require a sedimentation basin or a separate clarifier for solids separation.
• An MBBR reactor requires less energy to operate compared to conventional treatment systems, as there is no need for mechanical aeration and mixing.
• MBBR systems are robust and can handle variations in influent wastewater quality and flow rate. They can maintain consistent performance over time, making it a reliable treatment.

5. Comparative table of technologies used for BOD reduction

Based on an example of a typical biodegradable effluent with the following pollutant concentrations:

• COD = 1,000 mg _O2/l
• BOD₅ = 600 mg O₂/l
• TSS = 500 mg/l
• Oils and fats = 30 mg/l

Technique	COD mg/l	BOD mg/l	TSS mg/l	Oils/Fats	Surface.	Energy	Maintain.	Sludge
FQ(Decant.)	700	400	≤100	≤15	BIG	BAJA	UNDER	ALTA
FQ(DAF)	500	300	≤50	≤5	MEDIA	MEDIA	UNDER	MEDIA
Aeration + Decant. 2º	≤150	≤50	≤40	≤5	VERY LARGE	BAJA	UNDER	VERY HIGH
Aeration + DAF	≤120	≤40	≤30	≤2	BIG	LOW - MEDIUM	UNDER	ALTA
Aeration + MBR	≤100	≤25	≤10	≤1	MEDIA	MEDIA	MEDIO	ALTO
MBBR	≤150	≤40	≤40	≤5	MEDIA	BAJA	MEDIUM - LOW	BAJA
Tertiary (UF)	≤50	≤10	≤1	≤0,5	BAJA	MADIA - ALTA	MEDIUM - HIGH	BAJA

The expected purification yields and the comparative between processes shown in the table are indicative.

6. Summary

Reducing the BOD of a wastewater is essential for its suitability for final discharge. It is perhaps one of the most frequent pollutants, but at the same time, one of those that can have the greatest environmental impact.

At SIGMADAF, we are committed to providing customized solutions that meet the unique needs of our customers. That's why we offer a range of treatment options and work closely with our customers to develop solutions that are tailored to their specific needs. Whether you are in the food and beverage industry, chemical manufacturing or any other industrial activity, we have the experience and resources to provide you with effective BOD removal solutions.

Contact us today to learn more about how we can help you achieve your wastewater treatment goals and ensure the health and safety of our environment.