Discussion on Resource Recovery and Utilization of Reverse Osmosis Concentrate

1. Basic Situation

The wastewater is brine from a primary reverse osmosis desalination unit. The process of preparing pure water using reverse osmosis (RO) technology produces concentrate, which contains various organic and inorganic pollutants. Direct discharge may pollute soil, surface water, and oceans; if discharged into the municipal sewage treatment system, the high total dissolved solids are also very detrimental to the growth of activated sludge. The highly concentrated RO concentrate and chemicals introduced by cleaning agents and antiscalants, if directly discharged into the environment, will inevitably have adverse effects. Therefore, finding a cost-effective method for treating RO concentrate is of great significance for environmental protection.

2. Technical Path

2.1 Overview

Reverse osmosis membrane separation technology has been widely used in many industries in recent years due to its advantages such as no phase change of materials, relatively low energy consumption, good desalination effect, mature and reliable treatment process, simple equipment, high degree of automation, easy operation and management. However, the general design water production rate of current reverse osmosis technology is 75%, and the actual water production rate is even lower, about 30% of brine will be produced. If the raw water is very poor quality underground brackish water or seawater, the concentrate production will be even greater, possibly reaching 50%. Currently, much of the concentrate produced by reverse osmosis processes is discharged directly without treatment, resulting in waste of water resources and energy, and pollution of the surrounding environment.

For reverse osmosis concentrate, current research mainly focuses on three goals: reduction—optimizing reverse osmosis process design to reduce concentrate production; harmlessness—exploring cost-effective treatment methods to mitigate the potential environmental hazards of direct discharge of reverse osmosis concentrate; and resource utilization—exploring ways to reuse reverse osmosis concentrate, turning waste into treasure.

In fact, the reuse of reverse osmosis concentrate needs to consider multiple factors. These three goals are not isolated but need to be considered comprehensively and complement each other.

2.2 Discharge-Oriented

The following are the chemical reaction equations:

※ Mixed with other wastewater for treatment

For the vast majority of production enterprises, in addition to the reverse osmosis concentrate produced by the water production workshop, various other wastewaters will also be produced. For example, production wastewater discharged from the production workshop, factory area domestic sewage, etc. It is often not economical to treat each type of wastewater separately. Therefore, most enterprises choose to separate some special wastewater and treat various wastewaters together after mixing.

The water quality of reverse osmosis concentrate is characterized by high hardness and high salt content, while important pollution indicators such as turbidity and COD are very low. Mixing reverse osmosis concentrate with other wastewater can play a certain dilution and adjustment role, thereby reducing the influent pollution load of the mixed wastewater treatment system.

Some enterprises' production wastewater contains a large amount of alkaline substances such as sodium carbonate and sodium hydroxide, which can react with calcium and magnesium in reverse osmosis concentrate to form hydroxide or carbonate precipitates, reducing water hardness.

For example, a certain aluminum plant's thermal power plant has modified its reverse osmosis concentrate treatment system. By mixing the waste alkaline water produced in the production process with the reverse osmosis concentrate, the calcium and magnesium hardness is removed, and the concentrate is reused in production, reducing the cost of production water by about 1.76 million yuan/year.

2.3 Reduction-Oriented

Reduction is aimed at the reverse osmosis system itself. If the reverse osmosis system is designed reasonably and the water quality of the enterprise's influent is stable, the amount of concentrate produced by the system can be controlled at an optimal ratio. When designing a reverse osmosis system, there are two methods to improve the system's recovery rate, that is, to reduce the output of concentrate. One is to increase the length of the water flow through the reverse osmosis membrane module, and the other is concentrate reflux.

Increase the length of the water flow through the reverse osmosis membrane module.

While the water flows through the reverse osmosis membrane element, fresh water continuously passes through the membrane, achieving separation of concentrate and fresh water. Theoretically, the longer the water flows through the membrane element, the greater the fresh water yield and the higher the recovery rate. However, due to convenience, standardization, and other issues, the length specifications of various membrane elements on the market have been determined, but they can be connected in series into membrane modules according to process requirements. Due to the decrease in flow rate and pressure, the membrane module cannot be too long and needs to be segmented, that is, multiple membrane modules are connected in series.

Therefore, on the premise of ensuring the effluent quality and system stability, in order to reduce the amount of concentrate produced and improve the system recovery rate, the number of stages can be appropriately increased during reverse osmosis design.

However, relatively speaking, the lengthening of the membrane system requires an increase in the driving force of the membrane, that is, the power of the pump needs to be increased or the number of pumps needs to be increased. Therefore, the equipment investment and operating energy consumption cost of the system will increase.

Concentrate reflux:

Concentrate reflux is to return part of the concentrate produced by the reverse osmosis system to the high-pressure pump and mix it with the influent before entering the membrane module again for reverse osmosis treatment. This is also an effective means to improve the recovery rate of the reverse osmosis system. It is particularly suitable for systems with small water production and water flow that cannot flow through a 12m long membrane module.

However, due to concentrate reflux, the pollutant concentration at the inlet will increase, and the risk of scaling in the reverse osmosis system will further increase. Therefore, it is necessary to strengthen the operation control and management of the reverse osmosis system. If the production enterprise uses a reverse osmosis system with stable influent water quality and better than the design value, and the system's processing capacity still has spare capacity, this method can also be considered for system modification.

For example, the influent TDS of a certain chemical company's reverse osmosis system is about 40% less than the design value. In order to improve the recovery rate, it was modified to allow part of the reverse osmosis concentrate to be recycled and mixed with the raw water in a certain proportion before reverse osmosis treatment. In actual operation, by strictly controlling parameters such as influent salinity, system recovery rate, and operating temperature, the discharge of concentrate was greatly reduced while ensuring stable operation.

Some factories have lower salinity in their reverse osmosis concentrate, allowing for partial reflux and reuse in filter backwashing modifications. This significantly reduces concentrate discharge and lowers production costs.

2.4 Reuse as the Goal

Based on the principles of reverse osmosis and numerous domestic and international operational cases, even with optimal reverse osmosis system design and recovery rate, the concentrate will still account for at least 25% of the influent. For large water users in industries like steel and chemicals, concentrate production can reach hundreds of tons per hour. Discharge after wastewater treatment wastes significant energy and water resources. Therefore, finding suitable concentrate reuse pathways and replacing some new water with wastewater is of significant practical and environmental importance.

Domestic and international engineering cases show various reverse osmosis concentrate reuse methods. Most require adapting to the company's production characteristics. Concentrate can be reused in other suitable workshops within the company or replace tap water for factory cleaning and sweeping.

In recent years, responding to national energy conservation and emission reduction initiatives and the call for a circular economy, reverse osmosis concentrate reuse projects have flourished in China, with numerous engineering examples for reference.

A factory uses the pressure of reverse osmosis concentrate to store it in a high-level water tank, then uses it for condenser flushing, ground cleaning, cleaning the outer walls of wine barrels, and workshop cleaning. In three months, they saved 25,000 yuan in water fees and 58,000 yuan in coal fees, recovering investment costs in less than a year.

A steel plant uses reverse osmosis technology to treat cooling water and domestic wastewater from steel production into desalinated water and considers using the concentrate to backwash the multimedia filters in the reverse osmosis pretreatment system. With a one-time investment of 50,000 yuan, the company saves 500,000 m³ of water annually, achieving significant economic benefits.

A thermal power plant uses reverse osmosis concentrate for boiler ash removal, improving ash sedimentation and reducing circulating water replenishment by 110 t/h and chemical addition by 10%.

A brewery uses the reverse osmosis concentrate, originally requiring treatment before discharge, as water for the boiler's water film dust remover, saving 90,000 tons of water annually and 200,000 yuan in water and sewage fees.

A domestic group company recovers and reuses the concentrate from reverse osmosis and upstream ultrafiltration equipment in pure water production as supplementary water for the dilution spray of the exhaust gas scrubbing tower in the process workshop. After the modification, the system saves 25 m³/h of water, saving 210,000 tons of tap water annually, recovering the investment in half a year, and achieving significant energy conservation and emission reduction benefits.

In short, numerous engineering examples show that fully considering the company's situation and digesting reverse osmosis concentrate internally is an effective method for treating reverse osmosis concentrate and rationally utilizing water resources.

3. Process Flow Determination

The choice of water treatment process is crucial to the success of water treatment projects. The rationality of the treatment process directly affects the treatment effect, operational stability, investment, operating costs, and management level of the water treatment facilities. Therefore, the choice of water treatment process should first combine the actual situation of the factory, comprehensively consider various influencing factors within the factory, and carefully select a suitable water treatment process to achieve the best treatment effect and the best economic, social, and environmental benefits of the water treatment facilities.

4. Process Selection Approach

Based on the actual situation of the factory, the selected process flow should meet the treatment requirements while adapting to the factory's actual needs. Currently, the factory discharges level-1 concentrate, which is scaling water. Due to 4-fold concentration, it has high salinity, high hardness, stable water quality, and basically no significant pH fluctuations, belonging to high-mineralization brine. Based on the existing RO process operation and management experience, the level-1 RO concentrate recovery process is still determined as the RO process, i.e., the concentrate reverse osmosis process treats the level-1 reverse osmosis discharge concentrate.

The raw water temperature of this project fluctuates significantly with the seasons. To ensure the long-term stable operation of the concentrate treatment reverse osmosis device, a raw water heating system—a plate heat exchanger—is installed before the reverse osmosis device. Heat source: low-pressure steam.

The main process adopts level-1 concentrate reverse osmosis + concentrate reverse osmosis + three-effect evaporation process (separate design).