How To Select A Submersible Recirculation Pump For Wastewater Treatment

In mainstream biological nitrogen removal processes in wastewater treatment, such as A²/O, oxidation ditches, and modified SBR processes, the nitrification liquid recirculation pump is a core piece of equipment for maintaining stable total nitrogen (TN) levels. This equipment is characterized by a high flow rate and low head and is primarily responsible for transporting the nitrate-rich mixture from the aerobic zone to the anoxic zone, thereby providing substrate for denitrification.

A common problem encountered in the operation and maintenance of wastewater treatment plants is that, despite precise carbon source dosing, stable dissolved oxygen (DO) control below 0.5 mg/L in the anoxic tank, and otherwise normal process parameters, the effluent TN remains consistently high or fluctuates significantly. Extensive on-site investigations have revealed that many such failures are caused by improper pump selection, non-standard installation, or inadequate maintenance of the nitrification liquid recirculation pump. These issues can result in insufficient return flow and excessive oxygen entrainment in the return liquid, disrupting the anoxic denitrification environment and directly reducing nitrogen removal efficiency.

So, how do you select a suitable submersible recirculation pump?

Biological nitrogen removal consists of two core steps: nitrification and denitrification. The effectiveness of the connection between these two steps directly determines the total nitrogen concentration in the effluent. The nitrification liquid recirculation pump is the essential carrier connecting these two processes.

The first step is aerobic nitrification, in which aerobic microorganisms convert ammonia nitrogen in the wastewater into nitrates and nitrites in the aerobic tank. The second step is anoxic denitrification, in which anoxic microorganisms reduce nitrates to nitrogen gas in the anoxic tank, thereby achieving total nitrogen removal.

The core task of the nitrification liquid recirculation pump is to stably transport the nitrate-rich activated sludge mixture from the aerobic zone to the anoxic zone at a low level of reoxygenation, providing sufficient reaction substrate for denitrifying bacteria. If the equipment is undersized, malfunctions, or the oxygen content of the return liquid is too high, nitrates may accumulate in the aerobic zone, the denitrification reaction may be inhibited in the anoxic zone, and the total nitrogen concentration in the effluent may ultimately exceed the required standard.

The core principles for selecting a nitrification liquid recirculation pump can be summarized as follows: high flow rate, low head, prevention of reoxygenation, and wear resistance. Precise selection requires consideration of five dimensions: flow rate, head, pump type, material, and supporting configurations, while taking into account stability, energy efficiency, and durability.

The return flow rate is determined by the process return ratio. For municipal wastewater, the typical return ratio is 100%–400%. For industrial wastewater, the return ratio should be adjusted according to the influent total nitrogen load and the required effluent discharge standards.

The core calculation formula is:

Return flow rate = Instantaneous influent flow rate × Return ratio

Practical Selection Guidelines: Use the peak influent flow rate as the calculation basis. Selection based solely on the average daily flow rate should be strictly avoided, as this may result in insufficient recirculation under high-load conditions.

A 10%–20% operating margin should be reserved based on the calculated flow rate to compensate for flow losses caused by pipe scaling and biofilm accumulation.

A variable-frequency speed control system is recommended as the standard configuration, allowing stepless speed regulation from 0% to 100%. This enables the recirculation flow rate to be dynamically adjusted according to fluctuations in water quality and quantity, allowing the system to adapt to both low-load and high-load conditions while reducing equipment energy consumption.

Example: For a wastewater treatment plant with a peak influent flow rate of 100 m³/h and a required recirculation ratio of 200% to achieve high-standard denitrification, the selected pump should have a flow rate of at least 220 m³/h, including a 20% operating margin.

Nitrification liquid recirculation is typically a low-head application. The applicable head is generally only 0.8–3 m, and for some large tanks, it does not exceed 5 m. Avoid blindly selecting high-head pumps.

Excessive head can result in reduced effective flow, increased energy consumption, and accelerated impeller wear caused by unfavorable operating conditions. It can also increase the risk of reoxygenation in the return liquid.

Accurate head calculation must include the following three components, together with a 1–2 m safety margin to accommodate long-term operating resistance losses:

Static Head: The actual difference in liquid level between the aerobic and anoxic tanks.

Friction Resistance: The flow resistance of the medium corresponding to the pipe length and diameter.

Local Resistance: The combined resistance caused by components such as wall penetrations, valves, tees, elbows, and coupling bases.

Submersible recirculation pumps (through-wall/internal return pumps) are the preferred pump type for this application. Their core advantages closely match the operational requirements.

They have a stable structure without long-shaft wear and feature an automatic coupling installation design, making lifting and maintenance convenient. They also have low operating costs and wide flow passages that provide strong solids-handling capacity, enabling stable transportation of high-solids activated sludge mixtures while reducing the risk of clogging caused by fibers and sludge impurities.

In addition, excellent sealing performance and stable hydraulic flow help prevent vortex formation, air entrainment, and leakage. This reduces dissolved oxygen in the return liquid at the source, maintains a stable process environment in the anoxic tank, and avoids the excessive reoxygenation associated with conventional pumps.

Nitrification solutions contain nitrates and nitrites and, in some cases, trace amounts of hydrogen sulfide. They therefore exhibit both corrosive properties and abrasive effects caused by suspended solids. The material must be selected according to the specific water quality to extend equipment service life.

For corrosive industrial wastewater, 304/316L stainless steel or duplex stainless steel is recommended as the main body material.

For special conditions where the medium temperature exceeds 40°C, an H-class insulated motor is required to accommodate high-temperature operating conditions.

Configuration is fundamental to stable equipment operation and should not be compromised. The core configuration requirements are as follows:

Sealing System: The standard configuration includes dual mechanical seals and an independent oil chamber. The oil chamber provides multiple functions, including lubrication, cooling, and leak prevention, helping prevent wastewater from entering the motor.

Motor Protection: Protection rating ≥ IP68, suitable for long-term submerged operation; insulation class ≥ F; built-in PTC thermistor for real-time monitoring of motor temperature rise; and automatic overload protection.

Equipment Configuration: Strictly follow the N+1 backup principle. Each system should have no fewer than two units: one duty pump and one standby pump, preventing process interruption caused by equipment shutdown or flow interruption.

Safety Protection: The equipment should be equipped with overload, phase-loss, and low-liquid-level dry-run protection to accommodate fluctuations in operating conditions.