BOD5 removal in wastewater treatment

The abbreviation BOD₅ stands for “biological oxygen demand”. This is a key figure for oxygen in mg/l that microorganisms need to break down the substances dissolved in the wastewater – the so-called BOD₅ removal. BOD₅ is a unit of measurement for the sum of all biodegradable substances in wastewater.

What distinguishes BOD₅ from COD?

Substances are dissolved in almost every type of water. These can be minerals that are beneficial to the body in drinking water, but also nitrogen or phosphorus that are harmful to the environment above a certain concentration. The dissolved substances are divided into two categories: Biologically, aerobically degradable substances, which are respired by microorganisms, and persistent or poorly degradable substances, which remain in the wastewater. The BOD₅ indicates the amount of biodegradable substances, the COD the total of all dissolved substances.

The ratio between BOD₅ and COD is therefore decisive in determining the type of dissolved substances. If the proportion of BOD₅ is above 50 per cent, the wastewater is considered to have good biodegradability. This is usually the case in municipal or sanitary wastewater. If the substances that are difficult to degrade predominate, biological treatment processes are usually not suitable. This is the case with industrial wastewater, which comes from textile processing or plastics recycling, for example.

How is BOD₅ determined?

BOD₅ is always determined over a fixed period. These periods vary from country to country. Five days are usually established, but seven days can also be used for the determination. The amount of oxygen required by the microorganisms to break down the organic substances during the specified period is measured. The key figures BOD₅ or BOD₇ indicate the specified period in days.

The wastewater sample is mixed with oxygen-saturated water containing bacteria. The oxygen content is then determined. The sample is then sealed and stored at a constant 20°C. During this time, the microorganisms build up. During this time, the microorganisms break down some of the organic substances. In the process, they consume oxygen, which is measured after the time has elapsed. The oxygen consumed after five days is the BOD₅ in mg O₂/l.

What are the advantages of BOD₅ removal?

A high biological oxygen demand can come from different sources. These can be sanitary wastewater on the one hand, and wastewater from the food and beverage industry on the other, which often adds sugar or other organic substances to the wastewater. If this wastewater is released untreated into the environment, it can cause severe pollution. Microorganisms, which occur naturally in wastewater, see the pollution as a natural food source and thus the potential to multiply. This can lead to an excessive load of microorganisms in public waters.

Since microorganisms need oxygen to respire the organic pollutants and to multiply, they withdraw it from the water. This can lead to oxygen depletion in the water body, which has a negative impact on living organisms. For these reasons, it is crucial to reduce BOD₅ to a prescribed level before discharging wastewater into the environment.

What are the best methods for BOD₅ removal?

The best way to reduce BOD₅ in wastewater is to selectively grow microorganisms to break down the organic load. With the demand determined in advance, a targeted supply of oxygen to the wastewater is easy to determine. The determined pollution load of a source usually hardly fluctuates which is why the treatment plant’s purification capacity hardly needs to be adjusted.

One possibility is the fixed-bed reactor. This essentially consists of a growth material and an aeration system. The growth material is a plastic that is particularly suitable for the settlement of microorganisms. It has a specific, enlarged growth area of between 100 m²/m³ and 300 m²/m³. Depending on the growth surface, more or less microorganisms have the opportunity to settle on the material. This means that exactly the amount of microorganisms needed for BOD₅ removal is used.

A second possibility is the SBR system. This is also a biological treatment process that favours the growth of microorganisms. Aeration systems supply them with oxygen. Pressure sensors and control technologies adjust the aeration times and aeration intervals to the inflows. Thus, SBR systems are flexible towards fluctuating wastewater inflows.

PPU will be happy to advise you on how to remove the BOD₅ from your wastewater. You can rely on our many years of experience.