The Main Function of the Equalizing Tank in the Wastewater Treatment Plant System

In a wastewater treatment system, the reservoir tank—often called the equalizing tank (EQ tank)—is the first component that determines the overall stability of the process. Without a reservoir tank, wastewater flows directly into the WWTP and causes the system to receive peak loads (shock loading).

Shock loading makes the subsequent units such as coagulation, flotation, and biological processes operate unstably. Therefore, the reservoir tank is not just a storage space but a strategic buffer that regulates the rhythm of the entire WWTP.

2. Industrial Wastewater Flow Fluctuation Patterns

Industries rarely produce wastewater in constant quantities. Changes in factory activities cause fluctuations in both flow rate and pollutant concentration. Examples include:

• Peak discharge occurs at the end of production shifts
• Concentration increases during equipment cleaning processes
• Flow rate drops sharply during breaks or when machines stop
• Certain chemicals enter suddenly during batch changes
   

Such fluctuations are difficult to treat without a flow buffer.

Reservoir Tank as a Process Stabilizer (Equalizing)

The main function of the reservoir tank is to maintain homogeneity:

• Smoothing flow fluctuations → outflow becomes more stable than inflow
• Neutralizing pollutant concentrations → COD, pH, TSS become more uniform
• Preventing chemical spikes → protects downstream processes from disruption
• Providing initial contact time for aeration or chemical mixing
   

With more stable conditions, subsequent units can operate more consistently and efficiently.

Reservoir Tank Capacity Calculation

The reservoir capacity is calculated based on flow patterns and required buffer time. Basic formula:

Capacity (m³) = Peak Flow (m³/hour) × Equalizing Time (hours)

For example:

• Peak flow: 50 m³/hour
• Required equalizing time: 2 hours
• Thus, minimum capacity = 100 m³.

However, in practice, reservoirs are often given a 20-30% margin to anticipate sudden surges.

Important Features of Modern Reservoir Tanks

Modern reservoirs typically include the following supporting components:

Table: Reservoir Tank Features and Their Functions

The presence of these features helps improve the overall system efficiency.

Efficient Inlet-Outlet Design

The flow design inside the reservoir affects the equalizing performance. The main principles are:

• Inlet should be directed to an area that does not directly lead to the outlet to avoid short-circuiting.
• Outlet should be located in the most stable area where water quality is already homogenized.
• Baffle systems are used to direct flow so that residence time is evenly distributed.

Poor design causes water to simply “pass through” without undergoing equalizing.

When a Reservoir Tank Should Be Added

A reservoir tank needs to be added or enlarged if:

• Daily flow rates fluctuate significantly
• Wastewater parameters change suddenly
• Downstream processes (coagulation/biological) frequently overload
• The WWTP fails to meet effluent quality standards
• The industry enters a new, more intensive production phase

Adding a reservoir is one of the most effective ways to improve WWTP stability without replacing the entire system.

The reservoir tank plays a fundamental role in maintaining WWTP stability, especially in industries with high flow variability. With its equalizing, homogenizing, and peak-load protection functions, the reservoir tank ensures downstream processes run more efficiently, stably, and in compliance with regulatory standards.

If you need a reservoir tank design study, equalizing optimization, or reservoir integration into an existing WWTP system, PJL Enviro is ready to provide technical solutions and planning tailored to your industrial process needs.