What is the BOD limit in an STP plant?

One of the key parameters that are monitored to assess the quality of treated sewage is the BOD. It shows how much oxygen is needed to break down the organic wastes in wastewater. When the BOD level is high, a considerable amount of biodegradable material still exists in sewage. This is water of the type that can cause a depletion of dissolved oxygen in rivers and other waters. This can affect the water quality and can increase pollution. BOD is important if the STP is used for residential buildings, hotels, hospitals, institutions, industries, commercial buildings, and big housing estates. The growth of residential and commercial properties in cities such as Gurgaon has been on the rise. The expansion has also resulted in the need for sufficient sewage treatment. A sewage treatment plant manufacturer designs the treatment process so as to get the required outlet quality of the treated water and the purpose of the treated water.

Depending on the discharge standard, the specific BOD requirement could differ. The value could also be a function of the condition(s) offered by the pollution control authority and the plant location.

What is the meaning of BOD in STP?

BOD indicates the amount of organic matter present in sewage that can be degraded. It measures the amount of oxygen that microorganisms can use to break down this organic material. Hence, BOD gives a good measure of the strength of effluent pollution.

Typically, BOD is reported in terms of milligrams per litre. The BOD concentration in raw domestic effluents may be much higher than in treated effluents from domestic sewage. The majority of the biodegradable material is removed during the treatment process before the water leaves the end of the pipe. The biological treatment is found to reduce the organic load, from the lower BOD at the outlet.

1. Why BOD Matters

The significance of BOD is that untreated sewage may result in a high demand for oxygen in natural water streams. If lots of organic matter enters a river or pond, the microorganisms use up the dissolved oxygen in the water as they break down the organic material. This reduces the oxygen supply for fish and other aquatic life.

This organic load is removed using biological treatment in an STP. In controlled treatment units, microorganisms break down organic matter. It then filters out the biological solids from the treated water. BOD can be reduced to the outlet requirement with effective treatment.

The BOD value is also used to gauge plant performance. If the BOD increases rapidly at outlet B, it may be due to poor aeration. Regular testing, therefore, enables operators to detect issues before they develop, before they become significant.

2. BOD and Treated Water Reuse

BOD also plays an important role when reused effluent from the treatment process is discharged. Treated wastewater is being used in many buildings for gardening, toilet flushing, ecosystems, landscaping, and other appropriate non-potable uses. This reduces the amount of organics, and the treated water is more suitable for these applications.

But the necessary quality is dependent on the re-use’s purpose and the conditions. BOD cannot be used as the sole criterion; therefore, TSS, fecal coliform, and other parameters such as nitrogen may also be important. It is important that a sewage treatment plant manufacturer take into account what the treated water will be used for when designing a sewage treatment plant.

What is the BOD limit in STP in India?

The BOD limit is not always uniform throughout all the STPs in India. There are specific STP discharge standards in the Environment Protection Rules. The BOD limit specified in the notification dated 16.12.2017 issued under G.S.R. 1265(E) specifies the BOD concentration as not more than 20 mg/L for Metro Cities/Listed State Capital Cities. It defines the concentration of 30 mg/L for all areas and regions not in that category.

That implies that the BOD requirement for STPs, which is always talked about, may be either 20 mg/L or 30 mg/L within that notified framework. More severe requirements may also be imposed by the conditions of consent for a project. Higher norms can be set under local conditions by the pollution control authorities.

1. The acceptable BOD level for the water is 20 mg/L

The current STP standards for 2017 have defined the BOD limit of 20 mg/L for metro cities and some of the state capitals. Delhi has been added to the notified “metro cities.” In the case where the notified standard applies, this limit comes into play in many STP projects in the Delhi region.

A stable biological treatment is required for an STP designed for such a requirement. Adequate oxygen must be supplied to the plant. The biological population should be kept healthy, and the sludge system should work properly. Appropriate screening and preliminary treatment further reduce loads to the biological treatment stage.

2. The BOD of the water sample was determined to be 30 mg/L

Areas and regions outside of the 20 mg/L category have a maximum BOD of 30 mg/L as specified in the same notified standard.

This does not mean that an STP should aim only for 30 mg/L. When the treatment process and operating conditions are conducive to achieving a significantly lower BOD, a well-designed system can often achieve this. The target should be consistent with the actual consent requirement and the purpose of re-use/discharge.

3. There is a 10 mg/L limit for which you may hear

A figure that is seen in the discussions of STP performance in India is also present. The CPCB materials refer to an order dated 30 April 2019 issued by NGT, which lays down standards including a BOD of 10 mg/L in relation to NGT compliance. These values are still being used by CPCB in assessing some of the STPs.

This can cause some confusion, as the 2017 central notification and subsequent compliance requirements for NGT are mentioned in many technical reports together. The target for a particular project should therefore be aimed at from the latest applicable legal direction and conditions issued by the competent authority.

How Does an STP Reduce BOD?

Proper biological treatment is needed to reduce the BOD. The process provides a favorable habitat for microorganisms to feed on biodegradable organic matter. The design should give a satisfactory retention time. There should also be adequate oxygen transfer and sludge management to ensure stable biological activity.

For a sewage treatment plant manufacturer, the treatment process is generally decided based on the characteristics of the sewage, plant capacity, available land, and quality and re-use requirements at outlets. Let’s explore the primary steps that aid in reducing the BOD.

1. Preliminary and Primary Treatment

The first stages remove large solids and heavier suspended matter from sewage. Screens capture plastic, cloth, hair, and other larger materials. Grit removal takes out sand and other dense particles.

Primary treatment can reduce part of the organic load before the wastewater enters the biological stage. This step helps protect downstream equipment and improves process stability. When the initial treatment works properly, the biological system receives a more manageable load.

2. Biological Treatment

Biological treatment performs most of the BOD reduction in a conventional STP. Processes such as MBBR, SBR, ASP, and other biological systems encourage microorganisms to consume dissolved and suspended organic matter.

In an MBBR system, microorganisms grow on specially designed media. Aeration supplies oxygen and keeps the reactor mixed. In an SBR system, treatment occurs in timed stages inside the same reactor. The correct process depends on project requirements.

3. Secondary Clarification and Tertiary Treatment

After biological treatment, the plant needs to separate treated water from biological solids. Clarifiers help settle the biomass. The clarified water then moves toward disinfection or further polishing.

Some projects require tertiary treatment to achieve tighter water quality targets. Filters, ultrafiltration, and other advanced treatment steps can improve the final quality when the application demands it. A lower BOD target can therefore require a more controlled and carefully operated treatment train.

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Conclusion

BOD gives a clear indication of how much biodegradable organic matter remains in treated sewage. Under the 2017 national STP discharge standards, the specified BOD limit is 20 mg/L for the listed metro cities and state capitals, while the limit is 30 mg/L for other areas. At the same time, some projects operate under stricter requirements. CPCB materials referencing the NGT order of 30 April 2019 use a BOD target of 10 mg/L for the relevant compliance framework.

The right target should therefore come from the applicable regulations and the project’s consent conditions. Effective sludge management and regular testing all help maintain treated water quality.

Netsol Water is a leading sewage treatment plant manufacturer that can support projects with suitable STP solutions based on capacity and treatment requirements. Contact Netsol Water to discuss your wastewater needs and request a consultation for the right treatment system.

Contact Netsol Water at:

Phone: +91-9650608473

Email: enquiry@netsolwater.com