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**How to Design the Sewage Treatment Plant for commercial Complex?**

**Sewage Treatment Plant** (STP) is an important component of sewage management, particularly for commercial complexes that generate large volumes of sewage. STP is a facility that is designed to remove contaminants from sewage and make it suitable for reuse or discharge into the environment.

Here we will discuss how to design an STP for commercial complexes.

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**Design Basis: **

The design basis of STP for commercial complexes includes various factors that are considered while designing the plant. Some of the critical design factors are listed below:

**1. Flow rate:** The flow rate of sewage is an important parameter to be considered in designing an STP. The flow rate determines the size of the STP and the capacity of the treatment process. The flow rate is usually calculated based on the water demand of the commercial complex.

**2. Sewage Characteristics:** The characteristics of sewage are important in determining the type of treatment process required. The parameters that need to be considered are pH, BOD, COD, TSS, and nutrient levels.

**3. Discharge standards:** The discharge standards determine the level of treatment required to meet the environmental regulations. The discharge standards vary from region to region and depend on the receiving water bodies.

**4. Land availability:** The availability of land is an important factor in designing an STP. The size of the STP and the treatment process depend on the available land area.

**5. Cost:** The cost of construction and operation of the STP is an important factor to be considered. The cost of the STP depends on the treatment process, size, and land availability.

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**Design Calculations: **

The design of an STP involves various calculations that determine the size of the plant and the capacity of the treatment process. The critical calculations involved in designing an STP are:

**• Flow rate calculation: **The flow rate of sewage is calculated based on the water demand of the commercial complex. The flow rate is calculated using the formula:

** Q = V x F **

Where

Q is the flow rate in MLD (million liters per day),

V is the volume of water used per capita per day, and

F is the population of the commercial complex.

**• Design of primary treatment process: **The primary treatment process involves the removal of large solids and organic matter from sewage. The design of the primary treatment process is based on the flow rate and the detention time required for settling of solids. The detention time is calculated using the formula:

**t = V/Q**

Where

t is the detention time in hours,

V is the volume of the primary settling tank, and

Q is the flow rate.

**• Design of secondary treatment process: **The secondary treatment process involves the removal of dissolved organic matter and nutrients from sewage. The design of the secondary treatment process is based on the flow rate, organic load, and the desired level of treatment. The organic load is calculated using the formula:

**OL = Q x BOD **

Where

OL is the organic load in kg/day,

Q is the flow rate, and

BOD is the biochemical oxygen demand of the sewage.

**• Design of tertiary treatment process:** The tertiary treatment process involves the removal of remaining contaminants from sewage. The design of the tertiary treatment process is based on the effluent discharge standards. The type of tertiary treatment process depends on the contaminants present in the sewage.

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**Example of STP Design: **

Let us consider an example of designing an STP for a commercial complex with a population of 5000 people. The water demand per capita per day is 150 liters. The effluent discharge standards are BOD < 30 mg/L, COD < 250 mg/L, and TSS < 30 mg/L.

**• Flow rate calculation: **

Q = V x F

= 150 x 5000/1000000

= 0.75 MLD

Where V is in liters and F is in people.

**• Design of primary treatment process:**

Assuming a detention time of 2 hours, the volume of the primary settling tank can be calculated as:

t = V/Q

V = Q x t

= 0.75 x 2 x 1000

= 1500 m3

**• Design of secondary treatment process: **

The organic load can be calculated as:

OL = Q x BOD

= 0.75 x 300

= 225 kg/day

Assuming an MLSS (Mixed Liquor Suspended Solids) concentration of 4000 mg/L and an F/M (Food/Microorganism) ratio of 0.4, the volume of the aeration tank can be calculated as:

**VAT = (OL/F) x (MLSS) / (µ x (S - MLSS)) **

Where

VAT is the volume of the aeration tank in m3,

F is the F/M ratio,

MLSS is the mixed liquor suspended solids,

µ is the specific growth rate of microorganisms, and

S is the substrate concentration.

Assuming a specific growth rate of 0.6/day, the volume of the aeration tank can be calculated as:

VAT = (225/0.4) x (4000) / (0.6 x (250 - 4000)) = 2835 m3

**• Design of tertiary treatment process: **

Based on the effluent discharge standards, a sand filter can be used as a tertiary treatment process. The sand filter should have a surface area of 200 m2 and a depth of 1.5 m.

**Layout: **

The layout and drawing of the STP for the commercial complex can be designed as follows:

**1. Inlet chamber:** The inlet chamber receives the sewage from the commercial complex and allows for the removal of large solids.

**2. Primary settling tank:** The primary settling tank allows for the settling of solids and organic matter from sewage.

**3. Aeration tank: **The aeration tank provides oxygen to microorganisms to remove dissolved organic matter and nutrients from sewage.

**4. Secondary settling tank:** The secondary settling tank allows for the settling of biomass from the aeration tank.

**5. Sand filter:** The sand filter removes remaining contaminants from the sewage.

**6. Disinfection chamber: **The disinfection chamber disinfects the effluent using UV radiation or chlorine.

**7. Effluent discharge:** The treated effluent is discharged into the environment or reused for non-potable purposes.

**Summary. **

Designing an STP for a commercial complex requires the consideration of various factors, including flow rate, sewage characteristics, discharge standards, land availability, and cost. The design calculations involve determining the size of the plant and the capacity of the treatment process. The efficient functioning of the STP is necessary to minimize the environmental impact of sewage and to ensure its safe discharge into the environment.

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