High-pressure pumps for reverse osmosis applications

APP pumps belong to the positive-displacement pump family and use axial piston technology, which operates on a fundamentally different principle from centrifugal pumps. In an APP pump, flow is primarily determined by displacement and rotational speed, while pressure is generated according to system demand. This operating principle, combined with low internal leakage and low hydraulic and mechanical losses, enables APP pumps to achieve high pump efficiency 

In addition to achieving higher peak efficiency, APP pumps maintain consistently high efficiency across varying conditions by decoupling pressure and flow. This helps reduce off-design energy losses and sustain lower SEC and operating cost over time.

In centrifugal pumps, pressure and flow are linked through the pump curve, and maximum efficiency is achieved only around the Best Efficiency Point (BEP).

Pumps are often selected and compared at a single specified operating point, typically at or near the Best Efficiency Point (BEP). However, RO plants rarely operate continuously at that exact point. Feed salinity, temperature, membrane fouling and aging, and production demand all change the required pressure and flow over time.

Pump performance should therefore be evaluated across the complete expected operating range and annual operating profile. Because APP pumps maintain consistently high efficiency as operating conditions change, their actual energy savings compared with centrifugal pumps can be substantially greater than a single-point comparison would suggest.

The key question is not only “What is the maximum efficiency?” but “How much energy will the pump consume under actual operating conditions throughout the plant’s lifetime?”

Most SWRO plants globally operate below ~50,000 m³/day and experience:

• Greater load variability
• More frequent staging and turndown
• Modular train architectures
• Space-constrained layouts
• Broader operating envelopes

Under these conditions, maintaining consistently high efficiency across varying operating conditions is key to minimizing specific energy consumption (SEC). At the same time, the compact size and installation flexibility of APP pumps simplify their integration into modular systems and space-constrained plant layouts.

APP pumps are optimized for high-pressure membrane processes where stable pressure, high efficiency, and operational flexibility are critical.

Typical applications include:

• Land-based SWRO plants up to 50,000 m3/day
• Marine, containerized and decentralized SWRO plants
• RO systems for water and wastewater reuse, including Zero Liquid Discharge (ZLD) and Minimum Liquid Discharge (MLD) processes
• Brine concentration and valorization
• Multi-stage RO systems

Their ability to maintain high efficiency across a broad operating range makes APP particularly effective in applications where salinity, recovery rates, and production demands vary over time.

Total cost of ownership (TCO) considers both the initial investment and the operating costs over the plant’s typical 20–30-year lifetime.

High-pressure pumps represent only around 4–5% of SWRO plant CAPEX and approximately 1% of its total lifetime TCO. However, they account for around 70% of the plant’s energy consumption which can represent 30–40% of its total TCO.

Even small differences in high-pressure pump efficiency can translate into substantial differences in energy consumption and operating costs over the plant’s lifetime. Selecting the right high-pressure pump technology is therefore critical to minimizing TCO while also supporting decarbonization and ESG targets.

APP pumps use a compact, direct-drive and water-lubricated design, eliminating the need for gearboxes, belts and external oil lubrication systems. Their predictable service requirements and long service intervals support reliable operation and planned maintenance.

Maintenance can be carried out directly at the plant by trained plant personnel, without sending the pump to an external service center. Danfoss can provide the required training, enabling operators to plan maintenance around production needs, control downtime and maximize plant availability.

APP pumps also use standard low-voltage motors, which are widely available and generally easier to service than medium-voltage motors. They require less complex electrical equipment and maintenance procedures, further simplifying plant operation and spare-parts management.

This simplicity helps improve operational reliability, which has particular advantages in remote, decentralized, and mission-critical water treatment applications.

APP pumps combine high power density with a compact, lightweight design. Most models can be installed horizontally or vertically and operated in parallel to meet the flow requirements of larger RO trains Their relatively small footprint simplifies integration wherever space is at a premium:

• Marine desalination systems
• Containerized RO systems
• Mobile and decentralized treatment units
• Space-constrained plant layouts

Their compact size also simplifies transport, handling, installation and on-site service compared with larger centralized pumping equipment.

APP pumps are inherently suited for modular parallel operation. Multiple pumps can operate together within the same train to provide the required flow without relying on one large centralized pump. 

Unlike traditional standby redundancy architectures, APP pump arrays distribute capacity across multiple operating pumps. If one pump is taken offline for maintenance or experiences a fault, the system can continue producing water at reduced capacity rather than shutting down the entire train.

Parallel arrays can also be adapted to different production and redundancy strategies by combining fixed-speed and VFD-controlled pumps or switching individual units on and off as required.

This architecture offers:

• Distributed redundancy and reduced single-point failure risk
• High efficiency across varying load conditions
• Modular turndown without major efficiency penalties
• Continued operation during maintenance or service
• Reduced downtime risk and increased plant availability
• Independent train control
• Simplified maintenance planning
• Easier scalability and future capacity expansion

This architecture is particularly valuable in SWRO, water reuse, and modular membrane systems where operating conditions and production demand vary over time.

No. In a parallel APP configuration, one pump can be operated with a low-voltage VFD, while the remaining pumps operate at fixed speed using soft starters or direct-on-line starting, where permitted.

Depending on the expected operating range, the broad flow-control range of one APP can allow a single VFD-controlled unit to accommodate the required flow variations. If a wider control range is needed, a second VFD can be incorporated. Alternatively, the control system could be designed and configured to stop one fixed-speed APP while the VFD-driven unit adjusts its speed to maintain the required total flow.

Compared with installing a VFD for every pump, this configuration means that only a proportion of the total installed motor power passes continuously through VFDs. This can reduce:

• Installed VFD capacity and initial investment
• Electrical conversion losses
• Heat generation and cooling requirements
• Overall system complexity and operating costs

These benefits become increasingly relevant as the number of APP pumps operating in parallel increases. The final configuration can be adapted to the plant’s required operating range, redundancy philosophy and control strategy.

APP pumps are a mature and widely deployed technology with more than 30,000 pumps installed globally across high-pressure membrane systems. Collectively, these installations are estimated to:

• Produce more than 2.3 billion m³ of water annually
• Save approximately €200 million in energy costs every year
• Avoid approximately 600,000 tons of CO₂ emissions annually compared to conventional high-pressure pumping systems

This installed base provides extensive operational validation across varying feedwater conditions, climates, duty cycles, and operating environments.

For plant owners and EPCs, installed base matters because long-term reliability, maintainability, and lifecycle performance are proven in actual operation – not only under laboratory or pilot conditions.