Researchers Bernardo João Francisco Companhia and Paulo Sérgio Scalize of the Federal University of Goiás in Brazil set out to solve a problem that has dogged electrocoagulation, an emerging water treatment technology, for years: how do you take results from a small laboratory batch reactor and faithfully translate them into a full-scale, continuously flowing treatment plant? Electrocoagulation works by passing an electric current through water between sacrificial electrodes, typically made of aluminum. The anode dissolves electrochemically, releasing aluminum ions that hydrolyze in solution to form aluminum hydroxide flocs. These gelatinous, highly adsorptive particles sweep fluoride ions out of the water through adsorption and coprecipitation, while gas bubbles generated at the cathode float much of the sludge to the surface. The process needs no added coagulant chemicals, which is a major sustainability advantage over conventional chemical treatment, but its performance depends on a delicate interplay of current density, contact time, electrode geometry, and, crucially, the way water actually moves through the reactor.

The team built two cylindrical reactors with concentric aluminum electrodes, one operating in batch mode and one in continuous flow, and challenged both with synthetic water containing 10 milligrams of fluoride per liter, nearly seven times the WHO limit. In the batch phase, they tested three current densities: 9.79, 13.06, and 16.32 amperes per square meter. All three eventually drove fluoride below the guideline value within ten minutes, but the energy economics told a more nuanced story. The lowest current density achieved 95.6 percent removal for just 3.27 kilowatt-hours per cubic meter of water treated. Raising the current to 13.06 amperes per square meter lifted removal to 98.25 percent but nearly doubled the energy bill to 5.97 kilowatt-hours per cubic meter. Pushing further to 16.32 amperes per square meter added a mere 0.45 percentage points of removal while consuming 9.04 kilowatt-hours per cubic meter. The sweet spot, the researchers concluded, was the middle condition, which balanced near-complete defluoridation against a tolerable energy demand.

With the optimal batch parameters in hand, the team transposed them to the continuous flow reactor and varied the hydraulic retention time, the average time water spends inside the reactor, from three to seven minutes. The results were dramatic. At retention times of six and seven minutes, the continuous system removed more than 98 percent of fluoride, stabilizing residual concentrations at just 0.15 and 0.12 milligrams per liter respectively, far below the WHO guideline. Shorter retention times performed progressively worse: five minutes yielded about 81 percent removal, four minutes 60 percent, and three minutes only around 45 percent. But here is the headline finding: at every single retention time tested, the continuous reactor outperformed the batch system that had been used to calibrate it. The improvement ranged from 9.7 percentage points at seven minutes to a remarkable 17.7 points at three minutes, and paired statistical tests confirmed the differences were highly significant, with p values below 0.001.

The energy story was equally compelling. Specific energy consumption in continuous flow was lower than in batch mode across all conditions, with savings reaching up to 40 percent. At the six-minute retention time, the continuous system consumed roughly 1.40 to 1.45 kilowatt-hours per cubic meter, a fraction of the 5.97 kilowatt-hours per cubic meter recorded in the optimized batch run. The researchers attribute this advantage to the hydrodynamic regime of the continuous reactor. Under near plug flow conditions, water moves through the vessel in an orderly fashion, and freshly generated aluminum hydroxide coagulant is distributed uniformly along the reactor’s length. Each parcel of water encounters newly formed flocs as it travels, maximizing the contact between adsorbent and contaminant. In a batch vessel, by contrast, mixing is temporal rather than spatial, and the effective use of electrochemically generated coagulant may be limited by the way flocs and fluoride interact within a single, well-stirred volume.

To understand exactly how water behaved inside their continuous reactor, the team performed a residence time distribution analysis, a classic chemical engineering technique in which a pulse of tracer, in this case methylene blue, is injected at the inlet and its concentration tracked at the outlet over time. The tracer curve peaked between two and three minutes, with a long asymmetric tail stretching to roughly fourteen minutes, a signature of moderate axial dispersion. Quantitatively, the axial dispersion number came out at 0.154, indicating a predominantly plug flow regime with limited longitudinal mixing. The real hydraulic retention time measured by the tracer test closely matched the theoretical value calculated from reactor volume and flow rate, meaning the reactor delivered the contact time its designers intended, a precondition for any trustworthy scale-up. The slight deviation from ideal plug flow, the authors suggest, may actually help, enhancing contact between coagulants and contaminants without compromising overall performance.

Perhaps the most practically valuable contribution of the study is a quantitative bridge between the two operating modes. The researchers defined conversion factors, ratios relating removal efficiency and energy consumption in batch mode to those in continuous mode, and found that both factors decrease steadily as hydraulic retention time increases. The efficiency conversion factor fell from about 1.64 at three minutes to 1.11 at seven minutes, while the energy conversion factor dropped from 1.7 to 1.3. Both relationships fit second-order polynomial models with coefficients of determination above 0.98, meaning the convergence is strong and predictable. In plain terms, the longer the water stays in the reactor, the more the performance gap between batch and continuous operation closes, and the more reliably batch-derived laboratory data can predict full-scale continuous behavior. This gives engineers a mathematical tool for designing continuous electrocoagulation plants directly from bench-scale experiments, rather than relying on trial and error.

The chemistry underlying the process proved robust throughout. In batch experiments, the pH of the initially acidic water, which started at 4.32, spiked toward 8.2 in the first five minutes as hydroxide ions accumulated at the cathode, then settled into a stable range between 5.5 and 6.2, conditions known to favor fluoride adsorption onto aluminum hydroxide. In continuous operation, pH barely moved at all, hovering between roughly 5.3 and 6.8 regardless of retention time, a buffering effect arising from the balance between hydroxide generation at the cathode and hydroxide consumption during aluminum hydrolysis. Electrical conductivity declined modestly as ions were scavenged by flocs but otherwise remained stable, indicating that the treatment removes fluoride without substantially altering the water’s overall ionic character. The researchers note that they deliberately avoided adding supporting electrolytes or adjusting pH, testing the process under restrictive, realistic conditions rather than chemically favorable ones.

The implications extend well beyond fluoride. Electrocoagulation has demonstrated effectiveness against phosphorus, nitrates, organic matter, turbidity, dyes, and even resistant microorganisms, and the framework developed here, coupling electrochemical optimization with rigorous hydrodynamic characterization, offers a template for scaling any of these applications. The authors are careful about the limits of their work: sludge production was observed visually but not quantified, electrode wear was not measured gravimetrically, and the low conductivity of their test water may have inflated energy consumption relative to what real, more conductive groundwaters would demand. Future studies, they write, should address long-term operation, pumping energy, electrode consumption, and residue management. Still, the core message stands. Continuous flow electrocoagulation, properly designed with an eye on how water actually flows, can beat the batch systems used to develop it, remove nearly 99 percent of a dangerous contaminant, and do so while cutting energy use by up to 40 percent. For communities sitting atop fluoride-laced aquifers, that is not just an engineering curiosity. It is a credible path to safe water, powered by nothing more exotic than aluminum, electricity, and a well-shaped pipe.

Subject of Research: Batch-to-continuous scale-up of electrocoagulation reactors for fluoride removal from drinking water

Article Title: From batch to continuous electrocoagulation: integrating hydrodynamics and process parameters for efficient fluoride removal and reactor scale-up

Article References: Companhia, B. J. F., & Scalize, P. S. (2026). From batch to continuous electrocoagulation: integrating hydrodynamics and process parameters for efficient fluoride removal and reactor scale-up. Environmental Science and Pollution Research, 33(30), 15329-15351. https://doi.org/10.1007/s11356-026-38203-y

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38203-y

Keywords: electrocoagulation, fluoride removal, water treatment, continuous flow reactor, hydraulic retention time, residence time distribution, axial dispersion, aluminum electrodes, plug flow, energy consumption, reactor scale-up, drinking water