Nationwide multi-industry (thermal power / combined heat and power / iron and st
Pilot to engineering scale (3–30 m³/h per single three-compartment tank unit, ZLDesulfurization Wastewater Triple-Box: Dosing Sequence Pulled by Three Objectives at Once, and Chloride Ions Simply Cannot Be Removed
"Triple Tank" is the most commonly used name in flue gas desulfurization wastewater from thermal power plants, and also the one most easily mistaken for a "generic neutralization tank." It is actually not as simple as three-step chemical dosing: the same pH knob is simultaneously pulled by three things—"let heavy metals settle out," "don't let amphoteric metals re-dissolve," and "don't let pipes scale"—and in the end only a compromise value can be chosen. And the chloride ion that is repeatedly asked about—the indicator with the highest concentration in desulfurization wastewater—passes through essentially unchanged from the Triple Tank influent to effluent. This determines that it is destined to interface with dry ash conditioning, ash yard spraying, or a Zero Liquid Discharge / ZLD system, rather than finishing the job on its own.
I. First, establish its position: which cell is the Triple Tank in the process sequence of DL/T 5046
The "Triple Tank" discussed in domestic engineering does not have its process sequence based on verbal experience, but has a clear source in provisions. The desulfurization wastewater treatment process given in Article 6.2.3 of the Technical Code for Design of Wastewater Treatment in Fossil-Fuel Power Plants (DL/T 5046—2006) is: wastewater tank (basin) → pH adjustment tank → reaction sedimentation tank → flocculation tank → water clarification thickening tank (clarifier) → final neutralization/oxidation tank → effluent tank → sludge dewatering machine → sludge cake trucked off-site for disposal → reuse or discharge, and it explicitly requires complete dosing facilities for calcium hydroxide or lime, acid, coagulant, organic sulfide, oxidant, coagulant aid, and dewatering agent. The same standard also gives another parallel sequence: reaction tank 1—reaction tank 2—coagulation tank—flocculation tank—primary clarification thickening tank—sludge storage tank—dewatering treatment or transfer to the slag yard. It can be seen that the "Triple Tank" is only the most compact tank arrangement among these two sequences: making the three stages of pH adjustment, reaction sedimentation, and flocculation into an integrated three-compartment tank.
It will be clearer if broken down into a chain:
Laying out this chain reveals a counterintuitive but crucial fact: the success or failure of the triple chamber is hardly determined by itself. Whether there is a pre-sedimentation tank upstream, how large the water quality fluctuations are, and whether the downstream is ash yard spraying or evaporative crystallization—these three things determine whether the system can run stably over the long term far more than how much chemical is dosed inside the chambers. In engineering practice, a large number of cases of "unsatisfactory commissioning" of triple chambers can be traced to the two ends of the chain, rather than the three chambers in the middle.
II. Water Quality Characteristics and Three Hard Constraints
The water quality of desulfurization wastewater is "one water per plant," but the typical ranges given in publicly available materials are quite concentrated, sufficient to support magnitude-level judgment before engineering design. Based on typical values from "Zero Liquid Discharge Treatment Process for Wet Flue Gas Desulfurization Wastewater in Coal-Fired Power Plants" (Safety Management Network), "Zero Liquid Discharge Treatment System for Desulfurization Wastewater in Coal-Fired Power Plants" (Wastewater Treatment Engineering Network, 2018), and "Flue Gas Desulfurization Wastewater Treatment in Thermal Power Plants" and other industry materials:
| Parameter | Typical Range (Based on Public Data) | Significance for the Three-Compartment Tank |
|---|---|---|
| pH | 4.0~6.0 (weakly acidic) | Alkali must be added to raise it; also a source of corrosiveness |
| Suspended Solids (SS) | 10000~150000 mg/L; another source gives 20000~50000 mg/L | Determines the pre-sedimentation tank and flocculation load; the main "visible" problem |
| Total Dissolved Solids (TDS) | 25000~60000 mg/L; another source gives 25000~35000 mg/L | Determines whether discharge is possible and the cost of Zero Liquid Discharge (ZLD) |
| Chloride Cl⁻ | 10000~20000 mg/L; up to 30000 mg/L under high-chloride coal conditions | Cannot be removed by the three-compartment tank; determines material selection and final disposal route |
| Fluoride F⁻ | 20~40 mg/L; another source gives 20~200 mg/L | Removed incidentally via CaF₂ formation with lime, but cannot be expected to drop very low |
| Sulfate SO₄²⁻ | 2000~5000 mg/L | Forms supersaturated gypsum with calcium; the main cause of scaling and pipe blockage |
| Sulfite / Dithionate | SO₃²⁻ 0~200 mg/L; S₂O₆²⁻ 500~1000 mg/L | Creates "false COD" and is a source of reducing interference |
| Heavy Metals (as ions) | Fe ≤30, Al ≤50, Zn 5~25, Pb 3~15, Cd 0.5~25, Hg 0.2~5 mg/L | Hg is essentially ineffective via hydroxide precipitation; organic sulfur must be used |
| Water Temperature | Approx. 45 ℃ | Favorable for sodium hypochlorite decomposition; also affects flocculation performance |
There are three hard constraints in the table above. If any one of them is ignored, the three-compartment tank will expose it during operation.
Constraint 1: Among Heavy Metals, There Are "Three Brothers" That Do Not Follow the Hydroxide Logic
Mercury is the most typical. Mercury hydroxide is unstable and basically does not precipitate in the form of hydroxide within the pH window of the neutralization tank. It must rely on organic sulfur (TMT-15 or sodium sulfide) to form sulfides or chelated precipitates with a smaller solubility product. The literature states this very directly: the chemical precipitation method "has a strong removal effect on most metals and suspended solids, but has no removal effect on soluble salts such as chloride ions, and the removal rate of heavy metal ions such as selenium is not high." Optimization Process for FGD Desulfurization Wastewater Treatment also points out that organic sulfur is dosed into the reaction tank to enable "heavy metal ions that cannot precipitate in the form of hydroxides to form sulfide precipitates with a smaller solubility product," and the precipitate volume is small, its chemical stability is good, and it is not prone to re-dissolution.
Arsenic follows another pathway. During lime neutralization, Ca(OH)₂ not only reacts with F⁻ to form CaF₂, but can also react with arsenic to form Ca₃(AsO₃)₂ and Ca₃(AsO₄)₂ precipitates, so arsenic is "incidentally partially removed" in the three-stage tank. But precisely because it depends on calcium, the arsenic removal effect is highly bound to the lime dosage and pH, and therefore also enters the subsequent "pH compromise" problem.
Selenium is the one that the three-stage tank is most powerless to handle. In desulfurization wastewater, selenium mainly exists in the form of oxyanions such as selenate (SeO₄²⁻) and selenite (SeO₃²⁻), with concentrations that can be on the order of 0.5~2.0 mg/L (based on the research scope of Castaldi et al.), and it does not fall within the applicable range of hydroxide precipitation. Research and Application Progress of Desulfurization Wastewater Treatment Technology in Coal-Fired Power Plants summarizes this as follows: the chemical precipitation method "has a low removal rate for heavy metal ions such as selenium." Alternative routes given in publicly available international studies are ferrous ion reduction/coprecipitation (ORP controlled at −650~−750 mV, pH 8.8~9.2, ferrous dosage >40 mmol/L), or hybrid zero-valent iron (hZVI) technology—but the latter "is still at the industrial trial stage and has not yet been put into use." This means that any design relying only on the three-stage tank has no hard guarantee on the selenium item.
Constraint 2: Chloride Ions 10000~20000 mg/L, This Is Not a "Treatment" Indicator, but a "Destination" Indicator
The behavior of chloride ions in the three-stage tank can be summarized in one sentence: whatever comes in, basically the same amount goes out. Multiple publicly available engineering materials express this in highly consistent terms—"this pretreatment process wastewater can only treat suspended solids, but cannot treat chloride ions in desulfurization wastewater, and cannot be directly discharged," and "the removal effect of Cl⁻ and SO₄²⁻ is very limited, affecting the recovery and utilization of treated desulfurization wastewater."
The reason chloride ions must be discharged is that they continuously accumulate in the absorber tower: more than 90% of the chlorine in coal enters the absorber tower in the form of HCl gas and is washed down by the slurry. On the one hand, an excessively high chloride ion concentration lowers the pH of the absorber tower, leading to a decrease in desulfurization efficiency and an increased tendency for calcium sulfate scaling; on the other hand, it affects gypsum quality. The industry-standard control line is that Cl⁻ in the absorber tower slurry is generally controlled at 10000~20000 mg/L (depending on equipment material), and wastewater only needs to be discharged after this is exceeded. Therefore, the discharge volume of desulfurization wastewater itself is "back-calculated" from chloride ions: it is not a matter of discharging as much as one wants, but of discharging until chloride is pressed back to the control line.
This constraint directly determines materials and outlets. Article 3.2.10 of the Design Code for Wastewater Treatment in Power Plants (DL/T 5046—2018) clearly stipulates: treated desulfurization wastewater that meets standards can be used for dry ash conditioning, ash yard spraying, and wet slag removal systems. Meanwhile, Article 6.2.7 of DL/T 5046—2006 requires that the material of the desulfurization wastewater transfer pump "should be corrosion-resistant and wear-resistant, especially resistant to corrosion by high-concentration chloride ions," and Article 6.2.4 requires anti-corrosion protection for the inner walls of wastewater pools (tanks). In other words, although chloride ions are not "treated," at the standards level three disposal pathways have already been designated for them: dry ash conditioning, ash yard spraying, and wet slag removal.
Constraint 3: Salinity and water volume fluctuations determine that "compliance" and "zero liquid discharge" are engineering efforts of two different orders of magnitude
TDS is on the order of 25000~60000 mg/L, which means the osmotic pressure and scaling tendency of this water far exceed those of conventional industrial wastewater. At the same time, its flow rate is unstable and discontinuous, fluctuating with coal quality, limestone composition, desulfurization system operating mode, and unit load. A very typical engineering phenomenon is: a power plant's triple tank was originally designed with a treatment capacity of 3 m³/h and a design solids content of 1%. After the desulfurization ultra-low emission retrofit, the actual wastewater flow rate rose to 15 m³/h, and the actual solids content rose to 5%~8%. The result was bending of the sludge transfer pump rods, blockage of the pump inlet and outlet piping, damage to the neutralization tank agitator, and burnout of the clarification/thickening tank agitator motor, forcing the entire system to shut down. This is not a case of water quality exceeding the capability of the triple tank, but rather "simultaneous increase in both flow rate and solids content" exceeding the equipment capacity designed according to old parameters.
III. Mechanisms and Conflicts: One pH Knob, Three Hands Pulling
The chemical dosing sequence of the triple tank has been almost standardized in engineering practice. According to the sequence in DL/T 5046 and consolidated from multiple public sources, the order is as follows:
| No. | Dosing Location | Chemical | Objective to Be Achieved |
|---|---|---|---|
| 1 | Neutralization Tank | Lime milk (Ca(OH)₂) or NaOH | Raise pH; heavy metals form hydroxide precipitates; F⁻ forms CaF₂; lime has a flocculating effect on impurities |
| 2 | Settling Tank (Reaction Tank) | Organic sulfur TMT-15 or Na₂S; coagulant dosed simultaneously | Convert heavy metals such as Hg, Pb, Cd that cannot precipitate as hydroxides into sulfides/chelates |
| 3 | Flocculation Tank | Flocculant FeClSO₄ / Polymeric Ferric Sulfate Chloride / PAC | Allow micro-flocs to bridge and grow into settleable flocs |
| 4 | Flocculation Tank Outlet Pipe | Coagulant Aid PAM | Further enlarge flocs and increase the settling velocity in the clarifier |
| 5 | Effluent Tank (Clear Water Tank) | Hydrochloric acid (30% diluted to 5%~6%) | Adjust pH from >9 back to 6~9 to meet discharge/reuse requirements |
| 6 | Effluent Tank/Front End | Oxidant (sodium hypochlorite, etc.) | Oxidize redox-state inorganic ions to reduce the measured COD value |
This sequence seems perfectly justified, but it conceals a fact: the pH value at step 1 must simultaneously satisfy three mutually conflicting objectives.
The first hand comes from the absorber.The limestone–gypsum wet desulfurization system itself has a strict preference for pH: the ideal pH for sulfite oxidation and gypsum crystallization is around 4.5~4.7, and during operation the absorber slurry pH is generally controlled at 5.0~5.8 (other sources give 5.2~5.6, 5.5~5.7). When pH is above 5.8, limestone dissolution slows down, SO₃²⁻ oxidation is inhibited, and scaling and blockage readily occur. Yet the three-compartment tank, in order to precipitate heavy metals, must raise pH to 8.8~9.5.The same plant, the same water quality, one end needs around 5, the other end needs above 9—this is the physical reason why "desulfurization wastewater must be treated separately and cannot simply be recycled."
The second hand comes from the precipitation objective. The pH of the neutralization tank is the parameter with the most divergent values across all sources. The publicly available ranges we retrieved include: 9.0~9.5, 9.2~9.5, 8.8~9.5, 9.2~9.8, 9~10, and 8.0~9.0 for the precipitation tank. These figures appear contradictory, but in fact each corresponds to different water quality and objectives:
- 9.0~9.5: The most mainstream engineering range, balancing heavy metal hydroxide precipitation and fluoride removal.
- Around 9.2: The "relatively ideal operating condition" on site. Public operation and maintenance documentation explicitly states: when the lime milk dosage is excessive and the pH is too high, some amphoteric metals form complexes and the precipitate re-dissolves, which both wastes chemicals and increases the solids content in the wastewater.
- 9.2~9.8 / 9~10: Higher ranges, mostly occurring in cases where Suspended Solids and structure operation are prioritized, or where the heavy metal load in the water is relatively high.
- 8.0~9.0 (precipitation tank): A formulation that separates "precipitation chemical pH" from "neutralization pH," at which point the solubility of heavy metal sulfides is already extremely low.
The third hand comes from discharge and reuse. The neutralization tank pH is above 9, while the outlet tank requires 6~9, so acid must be added in between for adjustment. This brings about an easily overlooked cycle: add alkali → precipitate → consume acid again for neutralization. Alkali and acid are consumed in large quantities within the same system, and what is gained is mainly the act of "moving heavy metals into the sludge," while salinity actually increases further due to the introduction of Ca²⁺ and Cl⁻ (hydrochloric acid). This is also why, when the water treated by the three-compartment tank is subsequently sent for evaporation and crystallization, the softening burden is heavier than expected.
IV. Key Parameter Window: A Checklist for Direct Reference
The table below consolidates the verifiable key parameters of the three-compartment tank from public sources. It must be emphasized that: these are "engineering ranges," not "design values" — a bench-scale test using this plant's water quality must be conducted before selecting values.
| Stage | Parameter | Publicly Available Reference Range | Source |
|---|---|---|---|
| Neutralization Tank | pH Control | 8.8~9.5; actual engineering recommendation around 9.2 | Literature [8][12] |
| Lime Milk Preparation | Preparation Concentration | Thick lime milk 20%~30% → diluted in metering tank to 5%~6%; some plants change to 2%~3% to reduce deposition and pump wear | Literature [12][25] |
| Lime Milk | Dosage | Optimal in a pilot test of a 1,000 MW unit: 2.5~2.8 g/L (corresponding to effluent pH 9.1~9.3) | Literature [13] |
| Lime | Defluoridation Dosage | At 900 mg/L, F⁻ can be reduced to below 10 mg/L; the lime-only process generally can only reduce it to 15 mg/L, requiring secondary coagulation for defluoridation | Literature [14][19] |
| Settling Tank | Organic Sulfur (TMT-15) | 30 mg/L (optimal in tests); another patent specifies mercury removal agent in settling tank at 30 ppm | Literature [13][18] |
| Flocculation Tank | Coagulant (Polymeric Iron/PAC) | 13 mg/L (optimal in tests); another lower limit specification of ≥20 ppm | Literature [13][18] |
| Flocculation Tank Outlet | Coagulant Aid (PAM) | 20 mg/L (optimal in tests); solution preparation at 0.5:100 (add 500 g per 100 kg of water); another lower limit specification of ≥1 ppm | Literature [13][18][25] |
| Three-Tank Retention Time | Hydraulic Retention | pH adjustment tank ≥30 min; reaction settling tank ≥30 min; flocculation tank ≥30 min | Literature [2] (DL/T 5046—2006 Clauses 6.2.8~6.2.10) |
| Pre-settling Tank | Settling Time / Volume | Settling for more than 120 min can reduce SS by more than 99%, with actual solids content reduced by more than 50%; when influent is 30 m³/h, a pre-settling tank of more than 500 m³ should be provided | Literature [12] |
| Aeration for COD Reduction | Aeration Time / Air-Water Ratio | 6~8 h, air-water ratio 2:1; COD removal rate only 8%~10% | Literature [14] |
| Pre-oxidation (Optional) | Sodium Hypochlorite / Aeration | 100~500 ppm, aerate for 2 h after dosing for 30 min, control water temperature at 30~40 ℃, allowing residual oxidant to decompose to avoid damaging the mercury removal agent | Literature [18] |
| Piping and Materials | Anti-clogging and Anti-corrosion | Lime milk pipeline flow velocity should not be less than 2.5 m/s, gravity pipe slope should not be less than 5%; transfer pumps must withstand high-concentration chloride corrosion; wastewater tank inner wall anti-corrosion | Literature [2][13] |
V. Real Ledger: Triple-Box Comparison and Zero Liquid Discharge Cost Comparison of 11 Sets of Public Data
The table below is entirely sourced from public literature, appendix data of industry standards, and public engineering reports. It is only compiled and aligned without extrapolation. Any fields not provided in public materials are marked as "No public basis available."
| Project / Scale | Key Influent Indicators | Key Effluent Indicators / Performance | Technical Highlights | Source |
|---|---|---|---|---|
| A coal-fired power plant in Zhejiang, 2×300MW, desulfurization wastewater approx. 15 m³/h | pH weakly acidic; Cl⁻ >10000 mg/L; TDS 25000~35000 mg/L; total calcium and magnesium >3000 mg/L | After conventional pretreatment, only pH 6~9, SS ≤70, COD ≤150 are met; Cl⁻ not reduced | The triple-chamber tank + clarification cannot remove chloride; softening + membrane distillation + evaporative crystallization must be added; public reports give a capital cost basis of 70~200 万 CNY/(t·h) and a treatment cost of 130~220 元/t | Reference [11] |
| Desulfurization wastewater retrofit project at a coal-fired power plant in Shandong, integrated high-efficiency clarification replacing the original triple-chamber tank | SS 13650 mg/L, CODCr 350 mg/L, heavy metal ion concentrations exceeding standards | Effluent SS 1.5 mg/L, CODCr 132.2 mg/L; total lead 0.14, total cadmium 0.01, total mercury 0.01, total chromium 1.04, total zinc 1.24 mg/L, meeting DL/T 997—2020 | First determine the baseline chemical dosage through laboratory bench tests, then optimize parameters on site and reduce equipment damage rate; after 168 h of continuous commissioning and operation | Reference [6] (Industrial Water Treatment 2025, 45(1): 179-185) |
| A power plant 5×330MW, triple-chamber tank unit capacity 15 m³/h | Untreated desulfurization wastewater quality measured during the environmental impact assessment period (original attached table) | Reused within the plant after treatment, not discharged externally | Designed according to the desulfurization unit wastewater treatment capacity of 125%; sludge transported to the dry ash yard; all pumps 100% standby; each tank provided with a bypass | Reference [15] (Construction Project Environmental Impact Report Form) |
| A power plant triple-chamber tank original design 3 m³/h, solids content 1% | After ultra-low emission retrofit, actual wastewater 15 m³/h, solids content 5%~8% | System forced to shut down, wastewater discharged directly without treatment to the acid-alkali wastewater tank | Equipment damage concentrated in sludge transfer pump rod bending, pump inlet and outlet pipeline blockage, and damage to two agitators; this is "insufficient design margin" rather than water quality exceeding process capability | Reference [13] |
| Initial design values of the chemical dosing system for desulfurization wastewater 20 m³/h at a power plant | Neutralization system pH adjusted to 9.3~9.5 | No publicly available effluent water quality basis | PAM metering pump 300 L/h (0.1% solution), polyaluminum chloride 80 L/h (10% solution), organic sulfur 8 L/h (10% solution), automatically adjusted in proportion to influent flow rate | Reference [14] |
| Optimized design of the desulfurization wastewater treatment system for a 1,000 MW unit | Dosage tests conducted targeting effluent turbidity and heavy metals | Recommended to control lime milk dosage based on effluent pH 9.1~9.3; system operated continuously for nearly 3 months, effluent quality meeting standards | Optimal lime dosage 2.5~2.8 g/L; coagulant 13 mg/L; coagulant aid 20 mg/L; organic sulfur 30 mg/L | Reference [13] |
| A power plant triple-chamber tank system 22.8 t/h (shared by two wet desulfurization units) | Influent is gypsum cyclone overflow, density approx. 1040 kg/m³, solids content 3.7% | After optimization, stable daily discharge of qualified water 400 m³ or more; treated water sent out 6 km for dust suppression | Initial chemical cost 12 元/t; after optimization lime 1.4 kg/t, coagulant 0.1 kg/t, flocculant 0.23 kg/t, treatment cost reduced to 5.4 元/t, annual chemical cost savings of approximately 94.8 万 CNY | Reference [10] (public engineering case analysis) |
| Investment comparison between triple-chamber tank and integrated treatment (calculated on the same basis) | — | — | Triple-chamber tank total investment 1989.2231 万 CNY, operating cost 25 元/t; integrated total investment 964.7336 万 CNY, operating cost 9.75 元/t; of which chemical cost 12 元/t vs 8 元/t, sludge treatment cost 4 元/t vs 0 元/t, labor 1.5 元/t vs 0.5 元/t, power consumption 2.5 元/t vs 1.0 元/t, spare parts 5 元/t vs 0.25 元/t | Reference [5] |
| A 2×350MW cogeneration plant in Shanxi, desulfurization wastewater advanced treatment 480 t/d | Pretreated and softened concentrated brine, solutes mainly sodium chloride, concentrated brine volume approx. 20 t/h | Distilled water discharged meeting standards; product is bagged sodium chloride with purity >97.5%, meeting GB/T 5462—2003 refined industrial salt grade II | "MVC horizontal-tube falling-film evaporation + MED-TC negative-pressure low-temperature two-effect forced circulation evaporation" salt separation crystallization; mother liquor returned to raw liquid tank; crystals dried and automatically bagged | Reference [9] |
| Investment and operating cost comparison of three zero liquid discharge solidification routes | — | All target achieving zero liquid discharge | Evaporative crystallization: investment approx. 250~350 万 CNY/(t·h), operating cost approx. 50~80 元/t; main flue duct evaporation: investment approx. 50~100 万 CNY/(t·h), operating cost approx. 10~20 元/t; bypass hot flue gas evaporation: investment approx. 100~250 万 CNY/(t·h), operating cost approx. 20~30 元/t | Reference [4] (Electric Power Technology and Environmental Protection 2022, 38(3): 224-231) |
| Desulfurization wastewater high-temperature flue gas bypass drying (unit adaptation basis) | — | Achieves zero liquid discharge of desulfurization wastewater; after drying, chloride-containing salts enter the fly ash, requiring blending to ensure chloride content meets standards | 300MW desulfurization wastewater volume per single unit should be <3 m³/h; 600MW should be <5 m³/h; 1000MW should be <8 m³/h. Investment per ton of water 140~300 万 CNY/(t·h), operating cost (including boiler efficiency impact) 80~120 元/t | Reference [20] |
Reading this table horizontally reveals a very consistent pattern: the money spent on the triple-chamber tank stage (total investment in the tens of millions of CNY, operating cost 5~25 元/t) does not account for a high proportion of the entire treatment chain; the real costs are concentrated in the downstream concentration, volume reduction, and solidification stages (investment in the tens of thousands of CNY/(t·h) range, operating costs of tens to over a hundred CNY/t). This also explains a common budget misallocation: piling money into chemical dosage optimization for the triple-chamber tank yields very little marginal benefit; fully providing design margin for water volume and solids content, and ensuring the reliability of softening pretreatment, are instead more cost-effective investments.
VI. Process Comparison: The Position of the Three-Compartment Tank and Four Alternative/Extension Routes
| Route | Core Actions | What Can Be Removed | What Cannot Be Removed | Cost Magnitude (Public Basis) | Applicable Scenarios |
|---|---|---|---|---|---|
| Three-Stage Tank (Neutralization—Sedimentation—Flocculation) + Clarification | Lime slurry pH adjustment, organosulfur mercury precipitation, flocculation and coagulation aid | SS, most heavy metals, part of F⁻, part of reduced COD | Cl⁻, TDS, ammonia nitrogen; low selenium removal rate | Total investment in the tens of millions of RMB (comparison basis 1989 万 yuan); operation 25 元/t (another basis 5.4 元/t) | Compliant discharge or as pretreatment for zero liquid discharge; new projects with large water quality fluctuations and ample site space |
| Integrated High-Efficiency Clarification (Hydrocyclone Classification + High-Shear Reaction + Hydrocyclone Clarification) | Single-agent dry powder dosing, high-shear reaction, underflow with high solids content directly connected to dewatering | SS and heavy metals at the same magnitude as the three-stage tank; effluent can reach DL/T 997—2020 | Likewise Cl⁻ and TDS; more sensitive to the upper limit of influent solids content | Investment approximately 964.7 万 yuan; operation 9.75 元/t; footprint approximately 1/10 of the three-stage tank | Retrofit projects, space-constrained sites, and occasions with relatively stable influent flow |
| Three-Stage Tank + Softening + Membrane Concentration (NF / RO / DTRO) | Lime—soda ash softening, Ultrafiltration, high-pressure Reverse Osmosis or Disc Tube Reverse Osmosis | Intercepts salts, reduces concentrate volume to 1/3 or even lower; permeate can be reused | Salts do not disappear, they only enter the concentrate; membranes still face scaling and fouling | Single-stage DTRO recovery typically 60%~75%, two-stage series can reach above 80%; softening chemicals cost is relatively high | Projects requiring water recovery and having a downstream outlet for concentrate (crystallization or flue evaporation) |
| Concentration and Volume Reduction + Evaporative Crystallization (MVR / MED) | Multi-effect or MVR evaporation, terminal forced circulation crystallization, centrifugal drying | Complete separation of water and salt; industrial salt or mixed salt can be obtained | High investment and energy consumption; most mixed salts still need to be disposed of as solid waste/hazardous waste | Investment 250~350 万 yuan/(t·h), operation 50~80 元/t; salt separation projects can obtain purity >97.5% sodium chloride | Units with low-cost steam/waste heat, a salt outlet, and high water reuse quality requirements |
| Concentration and Volume Reduction + High-Temperature Bypass Flue Evaporation | Concentrate is sprayed into high-temperature flue gas before the air preheater, salts are captured by the dust collector along with fly ash | Liquid wastewater is reduced to zero; simple system, small footprint | Chlorine and heavy metals are merely transferred to fly ash, increasing the chloride content of fly ash and affecting its resource utilization quality | Investment 100~250 万 yuan/(t·h), operation 20~30 元/t; another basis is investment 140~300 万 yuan/(t·h), operation 80~120 元/t (including boiler efficiency impact) | Units with tight site space, surplus ash yard capacity, and low requirements for fly ash quality |
| Post-Treatment Disposal: Dry Ash Moistening / Ash Yard Spraying / Wet Slag Removal | Does not pursue water quality indicators, but pursues "having an outlet" | Saves investment and operating costs of advanced treatment | Neither volume reduction nor chlorine reduction; need to prevent corrosion of equipment and media, and prevent seepage | No unified public cost basis available | Compliant pathway explicitly specified in DL/T 5046—2018 Article 3.2.10; suitable for power plants with fly ash landfill disposal and permitting land conditions |
VII. Three Bases That Need to Be Broken Down on the Spot
As of this round of retrieval, the current standard is DL/T 997—2020 "Water Quality Control Indicators for Limestone-Gypsum Wet Flue Gas Desulfurization Wastewater from Coal-Fired Power Plants", issued on 2020-10-23 and implemented on 2021-02-01, fully replacing DL/T 997—2006 (the 2006 version is in "abolished/void" status). A large number of commercial technical documents, and even the appendix tables of some environmental impact assessment reports, still bear the label "DL/T 997—2006". The core limits of the two versions are highly similar (total mercury 0.05, total cadmium 0.1, total chromium 1.5, total arsenic 0.5, total lead 1.0, total nickel 1.0, total zinc 2.0, SS 70, COD 150, fluoride 30, sulfide 1.0, pH 6~9, all based on daily average values). The 2020 version newly adds ammonia nitrogen 25 mg/L, and explicitly states that COD 150 mg/L "shall deduct the portion introduced with the process water make-up water". The standard name has also been narrowed from "thermal power plant" to "coal-fired power plant", and it does not apply to wastewater generated by seawater desulfurization, ammonia-based desulfurization, or circulating fluidized bed semi-dry desulfurization.
The more critical layer is its positioning: DL/T 997 belongs to the category of industry standards for "water quality control indicators", specifying the water quality control requirements that desulfurization wastewater should meet when treated within the plant, and is not equivalent to an emission standard for the external environment. If desulfurization wastewater is ultimately discharged to the environment, it must still comply with emission standards such as GB 8978—1996 and local requirements. The two have different positioning and cannot be conflated in engineering practice.
This round of retrieval found multiple contradictory statements: some sources claim that flue evaporation "achieves true zero liquid discharge of desulfurization wastewater", while other sources clearly point out that spraying desulfurization wastewater into the flue for evaporation "not only disrupts the original desulfurization wastewater balance because the wastewater enters the desulfurization system, but also causes chloride ions to concentrate"; spraying wastewater onto the coal yard, on the other hand, "causes chlorine elements to volatilize during combustion, increasing the HCl gas concentration in flue gas and raising the corrosion risk of the boiler tail heating surfaces and flue", and "sodium salts tend to cause slagging in the furnace under high-temperature conditions".
Putting these together, the conclusion is clear: the triple tank does not reduce chlorine; flue evaporation and ash yard spraying also do not reduce chlorine—they merely move chlorine from water into fly ash or ash slag.The only way to truly remove chlorine from the plant's water system is to turn it into solid salt and transport it off-site (evaporative crystallization or drying). Therefore, to judge whether a zero liquid discharge route is "truly zero discharge", do not look at whether water droplets come out at the end—look at who the final host of the chlorine is, and whether this host can be disposed of compliantly. When utilizing fly ash as a resource, the blending ratio usually must be controlled to within 20% for high-chlorine fly ash, so that the chloride ion content of the finished cement can meet the limit requirement of GB 175—2007 ≤0.06%, and this must be calculated based on measured chlorine content.
A common formulation in commercial copy is to treat "chloride ion removal rate" as an optimizable performance indicator, as if the right chemicals could bring it down. But in the chemical reaction set of the triple tank, only the pathway of "forming sparingly soluble solid phases" can carry substances away: CaF₂ carries away fluoride, hydroxides and sulfides carry away heavy metals, and flocs carry away suspended solids. However, Na⁺, K⁺, and Cl⁻ do not form any sparingly soluble salts within this pH range, so they inevitably all remain in the water. Therefore, the indicator "chloride ion removal rate" simply does not exist for the triple tank; what can be discussed is only "at what level of chloride ion concentration should wastewater be discharged" and "where does the discharged chlorine go". Likewise, Cl⁻ in desulfurization wastewater has a positive interference on COD measurement (when chloride content exceeds 2000 mg/L, it causes great error in COD analysis), so when seeing data showing "a significant decrease in COD between the influent and effluent of the triple tank", one should first confirm whether the method used includes a chloride ion masking/extraction step.
VIII. Engineering Truths
Truth 1: The core value of the triple tank is not "meeting standards", but "making the water transferable".It separates water from solids and moves heavy metals from the liquid phase to the sludge phase, so that downstream softening, membrane concentration, evaporative crystallization, or ash yard spraying has material to work with. Evaluating the triple tank by "how much chloride ion is in the effluent" is evaluating the wrong object.
Truth 2: Most old systems are overwhelmed by "getting bigger", not by water quality.For a design of 3 m³/h and solids content 1%, when encountering actual operating conditions of 15 m³/h and solids content 5%~8%, the first things to fail are always the sludge transfer pumps and agitators. Leaving margin at the design stage according to "wastewater treatment volume ×1.25" is more cost-effective than replacing them afterward with more wear-resistant pumps.
Truth 3: The online pH meter is the single point of failure in this system. The electrode surface is highly susceptible to being surrounded by calcium sulfate and calcium hydroxide scaling. If the pH reading is too high, dosing is insufficient; if too low, dosing is excessive—and both deviations will be amplified all the way to the clarifier effluent. The value of regular calibration and regular cleaning exceeds that of any chemical substitution trial. In some projects, the lime milk dosing pipeline was changed from manual flushing to automatic flushing during shutdown, the dosing concentration was reduced from 5% to 2%~3%, and lime powder with a calcium hydroxide content above 95% was selected, resulting in significantly reduced pipeline deposition and pump wear.
Truth 4: If pre-oxidation is performed, it must be done "before entering the tank," and residual oxidant must be thoroughly removed. Reducing metal ions and dithionate in desulfurization wastewater will lock the mercury speciation. The publicly disclosed technical approach is to dose 100~500 ppm sodium hypochlorite in the pre-sedimentation tank, aerate for 2 h after dosing for 30 min, and control the water temperature at 30~40 ℃ to promote the decomposition of excess sodium hypochlorite. The purpose is precisely to "prevent sodium hypochlorite from damaging the mercury removal agent." Reversing the sequence (adding organic sulfur first, then having residual chlorine) will directly cause mercury removal efficiency to drop.
Truth 5: The disposal outlet for sludge and crystallized salt should preferably be determined at the scheme stage, not left until the acceptance stage. The sludge cake from the triple tank is conventionally sent to a qualified unit for disposal; the mixed salt at the zero liquid discharge end "can only be disposed of as solid waste for landfill, which not only fails to generate added value, but instead requires substantial cost and land for disposal," whereas salt separation can yield industrial salt with purity >97.5% (up to GB/T 5462—2003 refined industrial salt Grade II). This single difference is enough to make the operating cost of projects of the same scale differ by an order of magnitude—and it depends on the front-end softening and salt separation design, not on the triple tank.
Truth 6: The three compliant outlets for chloride ions differ greatly in cost. Dry ash humidification/ash yard spraying/wet slag removal (permitted by DL/T 5046—2018 Article 3.2.10) adds almost no treatment investment, at the cost of fly ash quality and equipment corrosion; bypass flue evaporation investment is 100~250 万 yuan/(t·h), operating cost 20~30 元/t; evaporation crystallization investment is 250~350 万 yuan/(t·h), operating cost 50~80 元/t. Which one to choose is essentially choosing "where to put the chloride," not choosing "how to treat the chloride."
IX. References
- DL/T 997—2020 "Water Quality Control Indicators for Limestone-Gypsum Wet Flue Gas Desulfurization Wastewater in Coal-Fired Power Plants", National Energy Administration, issued on 2020-10-23, implemented on 2021-02-01 (fully replaces DL/T 997—2006).
- DL/T 5046—2006 "Technical Code for Design of Wastewater Treatment in Thermal Power Plants" and DL/T 5046—2018 "Design Code for Wastewater Treatment in Power Plants", National Development and Reform Commission / National Energy Administration (process sequence, tank retention time, material and anti-corrosion requirements, Clause 3.2.10 reuse pathways).
- GB 8978—1996 "Integrated Wastewater Discharge Standard", Clause 4.2.1.1 sampling location provisions for Category I pollutants, State Environmental Protection Administration.
- Chen Haijie, Li Fei, Yang Shuying, Gu Xiaobing, Che Guangmin, Yang Linjun. Comparison of Zero Liquid Discharge Treatment Technologies for Desulfurization Wastewater in Coal-Fired Power Plants [J]. Electric Power Technology and Environmental Protection, 2022, 38(3): 224-231. DOI: 10.19944/j.etep.1674-8069.2022.03.009.
- Feasibility Comparison of Two Limestone-Gypsum Wet Flue Gas Desulfurization Wastewater Treatment Processes [J]. 2019(31): 167-168 (Article No. 2096-4390(2019)31-0167-02), Table 1 investment cost comparison, Table 2 operating cost comparison.
- Commissioning and Analysis of a Novel Desulfurization Wastewater Treatment System in a Coal-Fired Power Plant [J]. Industrial Water Treatment, 2025, 45(1): 179-185 (integrated high-efficiency clarifier replacing triple-box, 168 h commissioning data).
- Two industry references: Zero Liquid Discharge Treatment Process for Desulfurization Wastewater in Coal-Fired Power Plants (Safety Management Network) and Zero Liquid Discharge Treatment System for Desulfurization Wastewater in Coal-Fired Power Plants (Wastewater Treatment Engineering Network, 2018): typical water quality range of desulfurization wastewater, evaluation of triple-box removal capability, measured water quality of a 2×300MW unit in Zhejiang.
- Optimized FGD Desulfurization Wastewater Treatment Process (including Cl⁻ 8000~18800 mg/L, neutralization pH 8.8~9.5, precipitation chemicals and heavy metal sulfide stability), Wastewater Treatment Engineering Network, 2022.
- Application of MVR Evaporator in Salt Separation and Crystallization of Power Plant Desulfurization Wastewater (a 2×350MW project in Shanxi, 480 t/d, NaCl purity >97.5%, GB/T 5462—2003), public technical report.
- Solutions for High Turbidity in Wastewater from WFGD Systems (a power plant triple-box 22.8 t/h optimization case: lime 1.4 kg/t, coagulant 0.1 kg/t, flocculant 0.23 kg/t, treatment cost 5.4 元/t), public engineering case.
- Desulfurization Wastewater Treatment Device and Its Control System and Control Method (triple-box + clarification process, chloride ion cannot be treated, deep treatment investment and operating cost basis), Wastewater Treatment Engineering Network, 2017.
- Process Flow of Desulfurization Wastewater Treatment in Thermal Power Plants, Optimization and Pre-Sedimentation of Desulfurization Wastewater (including 120 min sedimentation, pre-sedimentation tank 500 m³, pH 9.2 ideal operating conditions, lime milk system management), public technical reference.
- Research on Optimal Design of Desulfurization Wastewater Treatment System (a million-kW unit: lime 2.5~2.8 g/L, coagulant 13 mg/L, coagulant aid 20 mg/L, organic sulfur 30 mg/L, effluent pH 9.1~9.3), public engineering design literature.
- Flue Gas Desulfurization Wastewater Treatment in Thermal Power Plants (including lime 900 mg/L defluorination to below 10 mg/L; COD aeration 6~8 h, gas-water ratio 2:1, removal rate 8%~10%), industry training materials.
- Construction Project Environmental Impact Report Form (a 5×330MW unit, triple-box capacity 15 m³/h, designed according to desulfurization system wastewater treatment capacity 125%, sludge transported to dry ash yard), government public document.
- Power Plant Desulfurization Wastewater Treatment Process (a power plant triple-box original design 3 m³/h, solids content 1%, after technical modification 15 m³/h, solids content 5%~8%, equipment damage list), public engineering analysis.
- Wastewater Chemical Dosing Optimization Method (a power plant in Anhui 20 m³/h: PAM 300 L/h @0.1%, polyaluminum chloride 80 L/h @10%, organic sulfur 8 L/h @10%), Wastewater Treatment Engineering Network, 2018.
- An Oxidation Treatment Process for Desulfurization Wastewater Triple-Box Device (sodium hypochlorite 100~500 ppm, aeration 2 h, water temperature 30~40 ℃, neutralization tank pH 9, mercury removal agent 30 ppm, coagulant ≥20 ppm, coagulant aid ≥1 ppm), public patent and technical literature.
- Process Method for Removing Fluorine from High-Concentration Fluorine-Containing Wastewater in Thermal Power Plants (lime-only method generally can only reduce fluorine to 15 mg/L, secondary coagulation defluorination required), Wastewater Treatment Engineering Network, 2018.
- High-Temperature Flue Gas Bypass Drying Technology for Desulfurization Wastewater (300MW <3 m³/h, 600MW <5 m³/h, 1000MW <8 m³/h; investment per ton of water 140~300 万 yuan/(t·h), operating cost 80~120 元/t), power industry technical compilation.
- T/CWDPA 150—2026 "Technical Specification for Zero Liquid Discharge of Desulfurization Wastewater in Thermal Power Plants", China Western Development Promotion Association, implemented on 2026-05-25; specifies water quality requirements, process selection principles, pretreatment units, concentration and reduction units, solidification treatment/reuse unit design and other requirements.
- Castaldi F J, Behrens G P, Hargrove O W. Removal of Selenium from FGD Scrubber Purge Water. URS Corporation (selenium exists as selenate/selenite 0.5~2.0 mg/L; ferrous reduction/coprecipitation ORP −650~−750 mV, pH 8.8~9.2, ferrous >40 mmol/L; HADS inhibition effect).
- EPRI. Selenium Removal by Iron Cementation from a Coal-Fired Power Plant FGD Wastewater in a Continuous Flow System — A Pilot Study. EPRI Report 1017956 (selenate reduction slower than selenite, pilot test did not stably achieve <0.050 mg/L target).
- Two articles: Factors Influencing Gypsum Slurry Crystallization in Desulfurization Absorption Towers (Industrial By-Product Gypsum Resource Utilization Engineering Technology Center: gypsum crystallization pH 5.0~5.8, process water Cl⁻ <20000 mg/L) and Effect of pH on Wet Flue Gas Desulfurization (China Environmental Protection Network: optimal absorption tower slurry pH 5.2~5.6).
- Detailed Discussion on Operation and Maintenance Techniques of Desulfurization Wastewater Triple-Box System (Ministry of Ecology and Environment Industrial By-Product Gypsum Engineering Technology Center); Research and Application Progress of Desulfurization Wastewater Treatment Technology in Coal-Fired Power Plants (chemical precipitation method has no removal effect on chloride ions, low removal rate for selenium, hZVI still in industrial pilot stage); concentration-crystallization process and flue gas evaporation process will be the focus of zero liquid discharge technology development for desulfurization wastewater (International Energy Conservation and Environmental Protection Network, including chlorine migration analysis of ash yard spraying and coal yard spraying).
