Turning Seaweed Gel Waste Into Resources: Electrodialysis and Enzymes Offer a Greener Path

Every year, the global appetite for seaweed gels—the agar and carrageenan that thicken everything from desserts to toothpaste—leaves behind a staggering trail of industrial waste. A new open-access review published in Blue Biotechnology by researchers at Jimei University and their collaborators takes the most comprehensive look yet at how that waste could be tamed, recycled, and even turned into valuable products. The numbers involved are enormous: according to data cited from the China Fishery Statistical Yearbook, even at its lowest level over the past five years, waste from seaweed gel processing in China still reached nearly eight million tons per year. The review argues that the industry stands at a turning point, with emerging membrane and enzyme technologies offering a realistic route to cutting emissions while recovering the organic riches currently flushed away.

To understand the problem, it helps to understand how seaweed gel is actually made. The dominant extraction method is an alkaline process, in which raw seaweed—most importantly the red alga Gracilaria, which contains roughly 30 percent agar and supplies about 53 percent of global agar market demand—is treated with strong alkali. This treatment serves a precise chemical purpose: it converts L-galactose-6-sulfate into 3,6-dehydrated L-galactose, stripping sulfate groups from the polysaccharide chains that would otherwise weaken the gel. The full production line runs from raw seaweed through alkali treatment, neutral cleaning, acidification, bleaching, gel cooking, filtration, cooling, dehydration, and drying. At every stage, chemistry is working to transform a slippery marine plant into a stable, marketable colloid.

That chemistry comes at a cost. During alkali treatment, the alkali attacks the seaweed cell wall, dissolving pigments, polysaccharides, and proteins into the solution, which turns from transparent to a dark liquid known in the trade as black lye. Factories typically reuse industrial lye two or three times, but once the organic concentration climbs too high and the liquid becomes viscous, it can no longer serve. Roughly 30 percent of the alkali remains trapped in the seaweed itself and must be washed out with repeated rinsing and acidification. Producing a single ton of seaweed gel consumes between 8.08 and 10.5 tons of alkali and generates approximately 460 to 600 tons of low-concentration alkali washing wastewater. In Fujian Province, China’s leading seaweed-processing region, nearly 100,000 tons of high-COD waste lye are produced annually; if alkali is reused more than seven times, the chemical oxygen demand can exceed 100,000, making the process unsustainable.

The solid waste stream is equally troubling. Filtration of the gel liquor is hindered by the colloid’s tendency to form a film, so producers add perlite—a porous volcanic glass with a large specific surface area—as a filter aid. The result is a residue that mixes perlite with a substantial load of organic matter. Each ton of seaweed gel generates roughly 2.5 tons of waste residue on a dry basis, and Fujian Province alone produces more than 35,000 dry tons annually. Left to accumulate, this residue decomposes, releasing methane and sulfides and other pollutants. The residue is rich in resources—about 70 percent perlite, along with trehalose, protein, and polyphenols—yet aside from minimal fertilizer recycling and some use as animal feed, most of it ends up as industrial landfill.

The review systematically evaluates the treatment options available for waste lye, and finds most of them wanting. Neutralization followed by activated sludge degradation at sewage plants, the current industrial default, wastes the active substances dissolved in the lye and requires large quantities of acid. Activated carbon adsorption can clean filtered lye, but saturated carbon cannot be reused, creating new disposal problems. Electrochemical oxidation on Ru-iridium titanium electrodes can rapidly strip alkali from seaweed and recover caustic through electrolytic cell reactions—one study reported continuous alkali recovery over 20 days in a bipolar chamber—but it does not recover the organic components. Even green approaches such as using algae to absorb nitrogen and phosphorus from agar wastewater leave the question of concentrated waste lye unresolved.

The most promising lye technology, the authors argue, is electrodialysis combined with membrane concentration. Electrodialysis uses an electric field to drive charged ions selectively across membranes, separating them from the solution; it is already widely used in food processing, salt production, and wastewater treatment. In studies of seaweed-derived waste lye specifically, electrodialysis achieved a cumulative alkali recovery rate of 37 percent while separating alkali from organic matter with low energy consumption and no phase change. The authors’ own group reported a chemical oxygen demand removal rate of 69.97 percent and an alkali recovery rate of 45.31 percent for electrodialysis-treated lye. Membrane concentration adds another layer of capability: multistage membrane systems have recovered tea polyphenols with retention rates of 70 to 83.8 percent, and nanofiltration membranes have cleaned paper-making black lye to reuse standards. New alkali-stable membranes, including quaternized graphene oxide composites that survived 32 days in 2 mol/L sodium hydroxide at 60 degrees Celsius, are making these processes increasingly robust.

For the solid residue, the review champions a hybrid of enzymatic and chemical degradation over calcination, which is costly and can create air pollution, or acid hydrolysis alone, which generates waste acid. Enzymes are gentle catalysts that cleave the glycosidic bonds of polysaccharide macromolecules, producing low-molecular-weight oligosaccharides without generating additional waste. In a key demonstration, the research team enzymatically treated nearly half of the polysaccharides in the residue—excluding the perlite—and recovered specific active oligosaccharides including neoagarobiose, neoagarotetraose, cellobiose, and cellotriose, which initially cleaned the pores of the perlite. Food-grade hydrochloric acid and hydrogen peroxide then degraded the remaining polysaccharides lodged inside the perlite pores, restoring the filter aid’s structure. The recovered perlite achieved a permeability of 2.61 Darcy, meeting reuse standards, and performed comparably to commercial perlite in subsequent gel extraction.

Yet significant scientific puzzles remain, and the review devotes considerable attention to them. During electrodialysis, charges generated in the process are adsorbed by organic substances, which accumulate on the surface of the permeable membrane and reduce the lye recovery rate. Related studies have shown that organic compounds containing benzene rings adsorb especially strongly onto anion exchange membranes, restricting transmembrane ion migration, and that sodium ions alter the internal molecular structure of organic polysaccharides in ways that change how they foul membranes. On the residue side, the structural activity and recovery of organic polysaccharides depend on the specific structure of the enzymes used, and organic material conversion involves energy interactions during solid-liquid separation that are not yet fully understood. These mechanisms, the authors contend, are the critical barriers to industrial-scale recovery of organic active substances from seaweed gel waste.

To crack those mechanisms, the review proposes a concrete analytical toolkit. Trace-charge analysis combined with scanning electron microscopy can quantify how organic active substances aggregate and how charges adsorb and release on membrane surfaces, while charge diffusion models can simulate charge transport during electrodialysis with membrane concentration. For the residue, microcalorimetry and thermodynamic models can track the energy changes that occur as specific enzymes act on organic matter, and Fourier transform infrared spectroscopy, X-ray fluorescence, and X-ray photoelectron spectroscopy can reveal how elemental composition and chemical bonding change during recovery. Analogous work on lignin and sugarcane bagasse has already demonstrated that pyrolysis behavior and potential energy shifts correlate with structural changes during enzymatic hydrolysis, providing a template the seaweed field can follow.

The stakes extend well beyond one industry. Seaweed aquaculture reached approximately 36.3 million tons globally in 2021—97 percent of it cultivated—and China alone produced 2.72 million tons in 2022, with Fujian Province accounting for nearly 48 percent of that output and 60.36 percent of national seaweed gel production. As the blue industry expands, the review’s framework offers a way to close the loop: recover the alkali, reclaim the perlite, extract the oligosaccharides, and return cleaner water to coastal ecosystems. If electrodialysis and enzyme chemistry can be scaled from laboratory benchmarks to factory floors, the mountains of black lye and perlite-laden sludge that currently shadow the seaweed gel industry could become feedstocks rather than liabilities—turning one of aquaculture’s messiest byproducts into one of its most elegant success stories.

Subject of Research: Waste reduction and resource recovery technologies in seaweed gel (agar and carrageenan) production

Article Title: Frontier review of key reduction technologies and resource utilization of waste during the seaweed gel production process

Article References: Luo, X., Wu, Y., Wang, S., Xiong, Z., Du, X., Zheng, M., Zhu, Y., Jiang, Z., Li, Q., Ni, H., Li, Z., & Chen, Z. (2024). Frontier review of key reduction technologies and resource utilization of waste during the seaweed gel production process. Blue Biotechnology, 1(1), Article 12. https://doi.org/10.1186/s44315-024-00013-7

Image Credits: AI Generated

DOI: 10.1186/s44315-024-00013-7

Keywords: seaweed gel, agar, carrageenan, waste lye, electrodialysis, membrane concentration, perlite recovery, enzymatic degradation, polysaccharides, oligosaccharides, wastewater treatment, blue biotechnology