Waste-concentrated Seawater Becomes Resource

For every three tonnes of waste concentrated seawater fed into the process, one tonne of refined concentrated seawater with a concentration of 120 g/L and two tonnes of freshwater can be produced. The refined concentrated seawater is delivered directly by pipeline to a soda ash plant 50 kilometers away, replacing externally purchased raw salt.

This sounds unbelievable, but the concentrated seawater disproportionation treatment and deep utilization technology jointly developed by Tangshan Sanyou Lanhai Technology Co., Ltd., and Chinese universities and research institutions has made it happen. The first phase of the world's first project to utilize waste concentrated seawater as a resource recently entered trial production.

Trapping impurities precisely

In the pretreatment workshop of Lanhai Technology, concentrated seawater passes through multiple layers of filter media, which trap and remove insoluble solids and other impurities step by step.

This is only the first step, Zhao Lin, leader of the company's R&D team, said. The real challenge lies in divalent ions such as calcium, magnesium and sulfate ions. In ordinary seawater, these ions are harmless. But once seawater is concentrated to a high level, they rapidly precipitate, much like scale forming in a kettle, clogging pipelines and damaging membrane equipment. They are also more difficult to remove than scale.

The separation of calcium and magnesium ions from sodium chloride in concentrated seawater is already technically difficult, and doing so in an economically feasible manner on an industrial scale is even harder.

The team started with nanofiltration, using membranes to separate divalent ions from monovalent ions. They tried more than 10 kinds of membrane products and each test took at least 15 days in three phases. However, smooth operation of the first two phases could not guarantee the third phase.

The team figured out the root cause: the functional layer of the membrane, made of polyamide, had been compacted. And once it becomes compacted, its density increases sharply, causing a drastic decline in its separation performance.

"Ideally, the membrane structure should be like a spring: even after being compressed to a certain extent, it should be able to spring back," Li Xiaoguang, member of the R&D team, said.

The team finally found a solution from three membrane-making techniques.

Innovating concentration techs

After removing impurities, the team also needed to raise the concentration density of the waste concentrated seawater to 120 g/L from 40-50 g/L, another challenging task.

Using conventional methods to do that would require equipment with high pressure resistance and consume too much electricity for ordinary enterprises to bear, Li said.

Traditional reverse osmosis technology faces a dilemma: the higher the concentration, the greater the osmotic pressure and the higher the pressure on the equipment. As the pressure rises, the membrane undergoes physical deformation, while electricity consumption soars.

To tackle this problem, the team adopted a "four-stage, two-pass" reverse osmosis concentration technique, the first of its kind. The "four-stage" were four concentration stages connected in series, while the "two passes" were a two-pass reverse osmosis system.

The team moved beyond the conventional approach of applying pressure in a single stage. The lengthy concentration process was broken down into four stages, Zhao said. "It's like turning a 10,000-meter race into a four-leg relay. Each stage squeezes out fresh water and keeps salt, but each has different characteristics and therefore requires different membrane elements."

Membrane selection is the key, but how the membrane elements are connected in series or in parallel is even more important, Zhao said. To this end, they innovatively introduced the concept of "circulating reverse osmosis," allowing part of the concentrated seawater to flow back to an upstream stage for further treatment, much like the way capillaries and major arteries work together. Ultimately, the system achieved the target concentration of 120 g/L.

Connecting the industrial chain

Once the process route had been determined, how to put it into practice became another major hurdle for the team.

The most difficult part was selecting the right membrane elements, Zhao said. Parameters of membranes from different manufacturers vary, and even membranes from different batches produced by the same manufacturer fluctuate in performance. Moreover, a membrane that performs exceptionally well in the laboratory may not perform as well in an industrial setting.

In 2017, Tangshan Sanyou Chemical Industry Co., Ltd., a company that Lanhai Technology is subordinate to, built a pilot line combining nanofiltration and electrodialysis and advanced a nanofiltration-reverse osmosis concentration process. The team tested products from leading membrane manufacturers at home and abroad and finally succeeded in putting the entire production line into operation.

"Using refined concentrated seawater directly as a raw material for the salt chemical industry is a key technical pathway for addressing the potential ecological risks associated with discharging concentrated brine into the sea at the source," said Liu Lan, a senior engineer at the Institute of Marine Development of Ocean University of China. The technology provides a viable pathway for addressing the global challenge of discharging concentrated seawater into the sea.

"There is currently no precedent for the resource utilization of concentrated seawater on such a large scale," Liu said. The resource utilization of waste concentrated seawater has, for the first time, enabled a closed-loop industrial-scale process covering the entire chain.

Source: Science and Technology Daily