Breaking Through the Fluoride-Removal Bottleneck in MVR Zero‑Discharge Wastewater: A Calcium‑ and Aluminum‑Free High‑Efficiency Defluorination Agent

      As environmental regulations grow increasingly stringent, corporate requirements for wastewater treatment have shifted from “compliant discharge” to “zero discharge.” MVR (mechanical vapor recompression) technology—thanks to its high efficiency and energy savings—has become the core option for zero‑discharge projects. In the MVR zero‑discharge process, the concentrate produced after two‑stage RO (reverse osmosis) often has a high fluoride content. If this concentrate is fed directly into the evaporation system, the purity of the resulting salts and nitrates will fail to meet specifications. Consequently, “fluoride removal from the RO concentrate” becomes a critical front‑end step to ensure the overall zero‑discharge scheme is viable.

      At present, the most commonly used fluoride‑removal solution in the industry is the calcium chloride process. Its principle is to combine calcium ions with fluoride ions to form precipitated calcium fluoride, thereby achieving fluoride removal. However, the process has a “fatal contradiction”: to guarantee the defluorination effect (e.g., controlling the effluent fluoride at 40–60 mg/L), one must overdose calcium chloride. This directly drives the residual hardness in the effluent up to 800–1000 mg/L. When such high‑hardness water enters downstream heat exchangers and evaporators, it readily forms scale and clogs equipment; as a result, an additional “de‑hardening” step becomes necessary. 

      The conventional de‑hardening measure is the addition of sodium carbonate, yet the associated cost and operational burden are non‑trivial: taking an average residual calcium hardness of 800 mg/L as the basis, approximately 1000 mg/L of sodium carbonate must be dosed per ton of water. At a market price of CNY 2/kg for sodium carbonate, this single chemical contributes about CNY 2 per ton of water. Moreover, soda‑ash softening typically requires first raising the pH to 11–11.5 with caustic, and then dosing sulfuric acid to readjust the effluent pH to neutral. The added consumption of acid/alkali and labor further increases cost. More problematically, the front‑end unit operations may already have removed hardness via ion‑exchange resin; dosing calcium chloride effectively “re‑introduces hardness,” which then has to be removed again with sodium carbonate and resin—an inefficient “pollute first, treat later” loop that many practitioners criticize as both labor‑intensive and uneconomical.

      To circumvent the shortcomings of the calcium chloride route, some operators have tried PAC (polyaluminum chloride) for defluorination, but this approach has its own issues: first, PAC itself contains calcium; after dosing, one still needs extra de‑hardening, and the unit price of PAC is much higher than that of calcium chloride, making overall costs harder to control. Second, PAC‑based defluorination typically requires large doses, which can leave elevated residual aluminum in the effluent. These aluminum ions readily combine with silica and iron in water to form colloids that directly foul downstream RO membranes—an issue that has surfaced in multiple real‑world projects, causing equipment damage and O&M headaches.

      Is fluoride removal really “bound” to hardness removal, with no better solution available? The answer is no.

      Targeting these industry pain points and market needs, Wuhan Qiangsheng Technology Co., Ltd. carried out dedicated R&D and, after sustained effort, successfully launched a high‑efficiency defluorination agent (code Q630‑3) that contains no aluminum, iron, calcium, or magnesium—fundamentally breaking through the limitations of traditional processes. The new defluorination agent excels across three dimensions: superior performance, simplified process, and reduced cost.

      Defluorination efficiency that far exceeds conventional methods: with an influent fluoride concentration of 100 mg/L, dosing only 1500 mg/L of the high‑efficiency agent can reduce effluent fluoride to below 20 mg/L. With higher doses, fluoride can be driven down further—significantly better than the 40–60 mg/L typically achieved by the calcium‑chloride route—thereby meeting stricter environmental and product‑purity requirements.

       Complete freedom from “de‑hardening dependence”: because it contains no calcium or magnesium, Q630‑3 does not introduce additional hardness after dosing. There is no need for downstream soda‑ash softening, eliminating the associated acid/alkali adjustment and secondary softening costs and operations.

       Broad pH tolerance and strong membrane compatibility: Q630‑3 stock solution is acidic, and its defluorination efficacy is not affected by the water’s pH—no extra base is required. If an RO membrane system follows, the mildly acidic effluent can even enhance membrane stability and extend membrane lifespan.

       Simplified operations and easy O&M: the entire defluorination workflow requires only three steps—dose the high‑efficiency defluorination agent → allow sufficient reaction → dose flocculant and settle. No complex equipment modifications are needed. Settling is rapid; the supernatant is clear and bright with a distinct sludge–water interface, and sludge yield is low, markedly reducing sludge‑handling costs and operational complexity.

Case Study: Zero‑Discharge Project for Coking Wastewater

      The original process employed a multi‑stage reaction plus precipitation sequence, specifically:

      RO concentrate → calcium chloride dosing/reaction → PFS coagulation → PAM flocculation → clarification → lime dosing/reaction → soda‑ash dosing/reaction → PFS coagulation → PAM flocculation → clarification (effluent F = 40–80 mg/L; pH = 11.5) → acid dosing to readjust pH → resin softening → nanofiltration salt‑splitting → evaporation and crystallization.

      This flowsheet had three core drawbacks: (1) as many as 12 unit steps, resulting in high operational complexity and difficult troubleshooting; (2) six different chemicals—calcium chloride, lime, sodium carbonate, PFS, and PAM among them—driving both specific consumption and procurement costs; and (3) large sludge volumes from multi‑stage coagulation and sedimentation, keeping the per‑ton operating cost stubbornly high.

      After introducing Wuhan Qiangsheng’s Q630‑3 high‑efficiency defluorination agent, the process was streamlined as follows:

RO concentrate → Q630‑3 dosing/reaction → PAM flocculation → clarification → resin softening → nanofiltration salt‑splitting → evaporation and crystallization.

      The benefits were immediate and tangible: unit steps were cut by 50%; the number of chemicals dropped from six to two; sludge production decreased, lowering the cost per ton of water. In addition, the effluent is mildly acidic, removing the need for acid readjustment and directly meeting the nanofiltration feedwater requirements.