Electrocrystallization + EC-Floc: Low-Energy Removal of Hardness and Silica from RO Concentrate (ROC)
kunyapak
Sep 2
4 min read
The problem we actually face on the production floor
If you run a plant, you already know that reverse osmosis concentrate (ROC) is unavoidable. It also tends to carry two stubborn passengers' hardness and silica. Together they seed scale in cooling towers and heat exchangers, quietly shaving off efficiency and forcing more downtime. When we try to push higher water reuse or aim for ZLD, those two numbers must be under control. Classic fixes (dosing chemicals or relying on membranes alone) often run into fouling and fragile operations once the water gets “real” and complex.
A 2024 paper in Desalination offers a simpler, better-timed approach: let electricity set the chemistry up front split “acid–alkali” inside the crystallization tank itself then use electricity again to make solids fall out fast. The goal isn’t just pretty numbers on a report; it’s steadier water and a continuous process that holds up on the floor.
How it works: split acid–alkali at the source, then sweep solids down fast
At the heart of the system is a membrane-free electrochemical crystallizer. By gently pulling H⁺ out of the anodic boundary layer under slight vacuum, the tank produces two distinct outlets an acidic stream and an alkaline stream without a separator in the middle. On the alkaline side, abundant OH⁻ pushes HCO₃⁻ → CO₃²⁻, so CaCO₃ crystallizes more readily. Meanwhile, silica at high pH turns into silicate, which can co-precipitate with Ca²⁺/Mg²⁺ (e.g., CaSiO₃, MgSiO₃), giving you real solids to remove.
From there, water flows to an electrocoagulation (EC) tank with iron anodes. The EC step generates lots of Fe (OH)₃ flocs that play a double role seed crystal that help the nascent solids grow and sweep flocs that capture what’s still floating. Add a small dose of PAM and you’ll see clean, cohesive flocs that settle in minutes, not hours. Compared with gravity alone, this is a different world, and it fits a continuous production rhythm.
A bonus: because the core is membrane-free, the system tolerates real-world waters better les drama from suspended solids and organics that usually trip membranes up.
What the researchers saw
They ran the crystallizer at about 13 mA/cm² (current density), with ~200 mL/min feed flow and ~40 mL/min acid extraction. Result: two clear outlets — acid pH ≈ 1.8 and alkaline pH ≈ 11.5.If they didn’t extract H⁺, pH only climbed to ~9.x, which isn’t strong enough to drive crystallization well.
The results After adding electrocoagulation (iron anodes) + a small dose of PAM, the system delivered:
~90% total hardness removal
~76% Mg-hardness removal
~82% silica removalEnergy use averaged ~2.4 kWh per m³, which the authors translate to about $0.26/m³ for power.Settling sped up dramatically: gravity alone needed >90 min to hit the turbidity target; with EC + floc it dropped to <5 min.
Why this happens Normally Mg²⁺ makes CaCO₃ harder to crystallize. But when silica is present, Mg²⁺ is drawn into magnesium silicates / sepiolite-like phases. Less free Mg²⁺ = fewer interruptions = CaCO₃ crystals form more easily.
From lab to pilot
The starting point
Grab real ROC (not synthetic). Measure hardness (split Ca/Mg), silica, alkalinity, TSS, conductivity. Have basic controls ready: a DC supply, flow control, pH meters on both outlets.
The quick batch trial
Screen current density at ~10–15 mA/cm².Give enough time to actually see crystals form. Then adjust acid extraction until the two outlets separate in pH (acid low, alkali high) instead of neutralizing.
Dial #2: Time to turbidity target — how fast the water clears.
If #1 looks good but #2 is slow, add EC + a small PAM dose to speed settling.
Move to continuous (and add recovery if needed)
When batch looks stable, go continuous.
Add DAF or filtration for solids handling.
If you want acid/base or salt recovery, bolt on ED/EDBM after polishing.
The YASA ET loop (how we’d build it)
PREDEST® (EC/EO) + DAF — pretreat & polish; make solids drop fast.
DESALT® (ED/EDBM) — recover acids/bases/salts you can reuse.
EVADEST® + SOLIDEST® — evaporation & crystallization for ZLD tail.
Result: waste → resources, with sensible cost and footprint.
Practical limits and the questions to ask on day one
This setup appreciates operational steadiness current, flow, and acid extraction should be held within a reasonable band. If the feed swings widely, add a buffer or feedback control so you stay near the sweet spot. Think early about sludge management from EC/floc: in many cases it dewaters easily, but plan for purge frequency, filtrate recycle, and handling so the “back end” doesn’t become a new bottleneck.
Smart early questions include:
– How high is the organic load?
– Any heavy metals or species that might disrupt crystallization or weaken flocs?
– Do you want the acid/base back in your process, or will you discharge?
– Is the endgame only scale control, or also salt/chemicals recovery?
Those answers shape the equipment map and the site-specific economics.
A short wrap-up
This isn’t about chasing flashy tech. It’s about staging the chemistry, so nature does the heavy lifting bringing hardness and silica down reliably, at low energy, and in a format that fits continuous production. If you’re ready to start, we’re ready to help tune the cadence to your actual water.
Wang, X., Sun, X., Liu, Q., Liu, Y., Li, Y., Wang, W., Feng, Z., Song, W., Jiang, B. (2024). Coupled electrochemical crystallization–electrocoagulation–flocculation process for efficient removal of hardness and silica from reverse osmosis concentrate. Desalination, 580, 117549. https://doi.org/10.1016/j.desal.2024.117549
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