Water treatment process

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Water treatment process: Raw water → Pretreatment system → Reverse osmosis system → Post-treatment system → Treated water

Analysis of water quality sources:
Sources of raw water: Tap water (urban and rural areas), surface water, groundwater, rainwater, reclaimed water, wastewater, seawater, brackish water

Precautions: Chemical agents such as cationic polymers, aluminum oxide/iron chloride, and zinc orthophosphate are added in municipal water plants; incompatible chemical agents and oxidants are used. Accuracy and representativeness of sampling are important.

Water quality – Testing parameters:
Ammonium ions (NH4+), Total iron (Fe2+/Fe3+), Nitrite ions (NO2-), Active silica (SiO2), Potassium ions (K+), Manganese ions (Mn2+), Nitrate ions (NO3-), Hardness, Sodium ions (Na+), Copper ions (Cu2+), Chloride ions (Cl-), Carbon dioxide (CO2), Magnesium ions (Mg2+), Zinc ions (Zn2+), Fluoride ions (F-), Free chlorine (Cl), Calcium ions (Ca2+), Aluminum ions (Al3+), Sulfate ions (SO42-), pH value, Barium ions (Ba2+), Alkalinity, Phosphate ions (PO43-), Total dissolved solids (TDS), Strontium ions (Sr2+), Bicarbonate ions (HCO3-), Hydrogen sulfide (H2S), Water temperature, Iron ions (Fe2+), Carbonate ions (CO32-), Colloidal silica (SiO2), Conductivity, Biological oxygen demand (BOD), Chemical oxygen demand (COD), Total organic carbon (TOC), Turbidity (NTV)

Pretreatment system:
Raw water → Pump → Multi-media filter → Activated carbon filter → Pump → Manganese sand filter → Precision filter → Ultrafilter → Water tank
Media: Quartz sand, white coal; Height: 1.0–1.6 m; Flow rate: 10–20 m/h; Backwash pressure difference: 0.03–0.06 Mpa; Water quality requirement: SDI < 5.
Media: Fruit shell charcoal; Height: 1.0–1.6 m; Flow rate: 10–20 m/h; Organic matter removal efficiency: 90%.
Media: Manganese sand; Height: 1.0–1.5 m; Flow rate: 10–20 m/h; Removal efficiency of Fe and Mn: 90%.
Media: Polypropylene; Capacity: >5 times; Precision: 3–10 μm; Media: PSF, PAN; Type: Hollow fiber; Retention molecular weight: 10,000–150,000; COD removal efficiency: 60%; Suspended solids removal efficiency: SDI < 1; Materials: PE, SS; Requirement: No dead corners; Capacity: 0.5 times.

Design basis:
Material and configuration of membrane elements (PA*/CA, spiral-wound */ hollow fiber); Quality of influent water (source and its variations); Influent flow rate (small/large-scale systems); Recovery rate of reverse osmosis (high recovery rates require thorough pretreatment); Post-treatment equipment and requirements (low post-treatment load and high demands require better pretreatment).

Objects and requirements:
Objects: Suspended solids, colloids, organic matter, microorganisms, scale.
Requirements: Suspended solids and colloid turbidity < 1 NTU; SDI < 5 (15 min); Organic matter TOC < 2 ppm; Microorganism colony count < 10,000 cfu/ml; Scale: CaCO3 concentration CSI < 0, SrSO4 Sr < 2 ppm, CaSO4 Ca+SO4 < 250 ppm, BaSO4 Ba < 50 ppb; Mn < 0.05 ppm; Fe (soluble) < 0.3 ppm; Si (soluble) SiO2 < 25 ppm.

Countermeasures:
Full filtration: Multi-media filtering, manganese sand filtering, activated carbon filtering, final polishing filtration.
Cross-flow filtration: Microfiltration, ultrafiltration, mixed-bed filtration.
Ion exchange: Softening, anion and cation exchange resins, lime softening.
Chemical agent addition: Scale inhibitors, dispersants, pH adjustment, chlorination/dechlorination.
Others: UV disinfection, heat exchange, degassing.

Pretreatment design:
Insoluble salts: Ion exchange softening, anion softening, lime softening, addition of chemical scale inhibitors.
Metal oxides: Ion exchange softening, lime softening, manganese sand filtering, addition of chemical dispersants.
Dissolved silicon: Lime softening, heat exchangers, iron removal, silicon dispersants.
Particles and colloids: Clarification, lime softening, sand filtration, multi-media filtering, microfiltration, ultrafiltration.
Natural organic matter: Clarification, lime softening, activated carbon filtering, microfiltration, ultrafiltration.
Microorganisms (growth): Chemical disinfectants, UV disinfection, microfiltration, ultrafiltration, maintaining flow and reducing dead corners.

Reverse osmosis system:
Pretreatment water tank → Booster pump → Final polishing filter → High-pressure pump → Check valve → RO membrane element → Water tank
Material: Stainless steel; Type: Centrifugal pump; Boosting pressure: 0.3 Mpa; Media: Polypropylene; Capacity: 3–5 times; Precision: 1–3 μm.
Material: Stainless steel; Type: Multi-stage centrifugal pump; Boosting pressure: 1.2 Mpa.
Material: Stainless steel; Function: Prevent backpressure on the membrane; Type: Spiral-wound; Material: PA; Dimensions: 8040, 4040; Materials: PE, SS; Requirement: No dead corners; Capacity: 0.5 times.

Configuration methods:
There are generally two types of RO configurations: single-stage multiple sections and multi-stage multiple sections. Increasing the number of sections improves water utilization, while increasing the number of stages enhances water quality. * Returning the concentrated water from the later stages of multiple levels and sections to the raw water in the earlier stages can improve both water utilization efficiency and water quality. Desalination efficiency components: (1) ≥99%, (2) ≥99.7% ； System: (1) ≥95%, (2) ≥98% ； Testing: 2000 ppm NaCl in pure water solution. Operational conditions and their effects:
Pressure: As pressure increases, flow rate rises almost linearly; the desalination rate increases rapidly at first and then rises more slowly.
Temperature: As temperature increases, flow rate rises almost linearly; the desalination rate decreases slowly.
Salt concentration: As salt concentration increases, both water flow rate and desalination rate decrease slowly.
Recovery rate: As the recovery rate increases, water flux and desalination rate decrease slowly to a certain level before dropping sharply.
pH value: Changes in pH within a wide range have no effect on water flow and desalination rate; in extreme cases, the desalination rate decreases.

Causes:
Membrane degradation (hydrolysis, oxidation, mechanical damage)
Deposit formation (calcium carbonate scale, calcium sulfate scale, silica scale)
Colloid deposition (metal oxides, sludge)
Organic matter deposition (natural organic substances, insoluble oils, scale inhibitors, iron deposits…)
Biological contamination (algae, fungi, bacterial growth, biological sludge)

Symptoms (corresponding to these phenomena):
Increased pressure drop between feed water and concentrate water in the system
Changes in the feed water pressure for reverse osmosis
Changes in the standardized product water flow rate
Changes in the standardized salt permeability of the product water

Fault diagnosis:
Possible causes and locations:
Pressure drop between feed water and concentrate water – First stage; Water production volume – All stages; Salt permeability – First stage
Metal oxide contamination – First stage: Normal/Increased; Decreased
Colloid contamination – First stage: Normal/Increased; Decreased
Scaling contamination – Last stage: Increased; Decreased; Increased
Biological contamination – Any stage: Normal/Increased; Decreased; Normal/Increased
Organic contamination – Any stage: Normal; Decreased; Decreased/Increased
Oxidation effects – Most severe in the first stage: Normal/Decreased; Increased; Increased
Wear (from particles) – Most severe in the first stage: Decreased; Increased; Increased
Seal leakage – Randomly distributed: Normal/Increased; Normal/Increased; Increased
Excessively high recovery rate – Any stage: Decreased; Normal/Decreased; Increased

To reduce failures and cleaning needs:
Design the reverse osmosis system based on a comprehensive analysis of water quality. Determine the SDI value of the feed water to the reverse osmosis elements before designing the system. If the quality of the feed water changes, corresponding design adjustments must be made. It is necessary to ensure an adequate level of pre-treatment, select the right membrane elements, choose a conservative water flux, determine an appropriate recovery rate, design sufficient cross-flow and concentrate flow rates, and standardize operational data.

Objects that require cleaning: Scale formed from calcium carbonate, calcium sulfate, barium sulfate, and strontium sulfate ； Hydrated oxide scales of iron, manganese, copper, and nickel; silica scales – organic and inorganic colloidal deposits, natural and man-made organic deposits, as well as biological growths such as bacteria, fungi, and molds. Timing for cleaning (correlated with the object to be cleaned): The standardized water production volume is reduced by 10–15% compared to after the previous cleaning; the quality of the standardized water output is also reduced by 10–15% compared to after the previous cleaning. When the standardized pressure drop increases by 10–15% compared to after the previous cleaning, cleaning should be carried out as part of routine maintenance before the equipment is not used for an extended period