Reverse osmosis system with energy recovery devices

A reverse osmosis system comprising:

• a pump operable to pressurize a feed stream;
• a first membrane array configured to receive the feed stream and to generate a first permeate stream and a first brine stream from the feed stream;
• a second membrane array configured to receive the first brine stream and to generate a second permeate stream and a second brine stream from the first brine stream;
• a first hydraulic pressure booster comprising a first shaft, a first pump that is coupled to and rotates with the first shaft, and a first turbine that is coupled to and rotates with the first shaft, the first hydraulic pressure booster configured to receive a first portion of the second brine stream and to use energy from the first portion of the second brine stream to increase the pressure of the first brine stream, wherein the first hydraulic pressure booster includes a first variable area nozzle;
• a second hydraulic pressure booster comprising a second shaft, a second pump that is coupled to and rotates with the second shaft, and a second turbine that is coupled to and rotates with the second shaft, the second hydraulic pressure booster configured to receive a second portion of the second brine stream and to use energy from the second portion of the second brine stream to increase the pressure of the feed stream, wherein the second hydraulic pressure booster includes a second variable area nozzle;
• and
• a motor-generator coupled to the second shaft of the second hydraulic pressure booster, the motor-generator (a) operable to act as a motor to use electric power from a power supply to drive the second shaft of the second hydraulic pressure booster, and (b) operable to (i) convert hydraulic energy from the second portion of the second brine stream into electrical energy and (ii) return the electrical energy to the power supply;
• a controller operatively coupled to the first variable area nozzle, the second variable area nozzle and the motor-generator, said controller controlling the first area variable nozzle based on a flow of the first permeate stream, a flow of the first portion of the second brine stream and a flow of the second portion of the second brine stream, said controller controlling the second variable area nozzle in response to a flow of the second permeate stream, the flow of the second portion of the second brine stream, said controller adjusting torque output of the motor-generator based on the flow of the first permeate stream and the flow of the first portion of the second brine stream.

A method for operating a reverse osmosis system comprising a first hydraulic pressure booster comprising a shaft, a pump that is coupled to and rotates with the shaft, a first variable area nozzle and a turbine that is coupled to and rotates with the shaft, the method comprising:

• directing a pressurized feed stream to a first membrane array to separate the pressurized feed stream into a first permeate stream and a first brine stream;
• directing the first brine stream to a second membrane array to separate the first brine stream into a second permeate stream and a second brine stream;
• receiving, at the first hydraulic pressure booster, at least a first portion of the second brine stream;
• using energy from the first portion of the second brine stream to increase a pressure of at least one of the feed stream and the first brine stream;
• operating an electric motor-generator coupled to the shaft, (a) to act as a motor to use electric power from a power supply to drive the shaft of the first hydraulic pressure booster, and (b) (i) to convert hydraulic energy from the second brine stream into electrical energy and (ii) to return the electrical energy to the power supply;
• measuring a flow of the first permeate stream at a first flow meter;
• measuring a flow of the first portion of the second brine stream at a second flow meter, said flow of the first portion less than a flow of the second brine stream;
• controlling, by a first controller, the first variable area nozzle based on the flow of the first permeate stream and based on the flow of the first portion of the second brine stream;
• and
• controlling, by the first controller, the first variable area nozzle to adjust the flow of the first portion of the second brine stream through the first hydraulic pressure booster and thereby adjust an amount by which the first hydraulic pressure booster increases the pressure of the feed stream.