Sand fliter vs Carbon fliter vs Ultrafiltration vs Reverse Osmosis — Best Uses Explained

Executive Summary
Sand filtration — stabilizes TSS/NTU, protects downstream units.
Activated carbon — adsorbs chlorine, odor, DOC; finite capacity.
Ultrafiltration (UF) — removes colloids & bacteria, stabilizes SDI.
Reverse osmosis (RO) — desalination; concentrate pathway is critICal.

Why "one-size-fits-all" fails

The use of sand filter, activated carbon filter, ultrafiltration, and reverse osmosis must match contaminant size. A single technology cannot span six orders of magnitude.

Pollutant morphology and scale mapping

Sand filters remove 10–100 µm particles. UF screens 0.01–0.1 µm colloids. GAC adsorbs molecules below 2 nm. RO rejects ions under 1 nm. Treating 5 µm particles with RO is wasteful. Using sand to remove sodium ions defies physICs.

Misalignment causes most underperformance. At Deiiang™, RO membranes fouled in 90 days because upstream floc carryover bypassed the sand filter. A properly sized sand filter would have solved it at one-tenth the cost.

Jason.peng insight: In Yangtze River basin lithium battery projects, RO membranes failed prematurely. The root cause was manual backwash valves on sand filters. Operators skipped night‑shift backwash. Turbid water hit UF and RO directly. Automatic valves would have saved tens of thousands in membrane replacement.

Key water quality indicators

Fractionate COD into particulate, colloidal, and dissolved phases. Map turbidity, UV₂₅₄, conductivity, hardness, and SDI₁₅. A 120 mg/L COD may be 80% particulate (sand filter), 15% colloidal (UF), 5% dissolved (GAC).

Pretreatment failure chain — how upstream upsets cascade downstream

Treatment trains are only as strong as their weakest link. A small disturbance triggers a domino effect that damages expensive downstream assets.

Two failure chains dominate field calls: (1) turbidity escape from a poorly backwashed sand filter forces UF into accelerated CEB, shortens membrane life, and risks fiber breakage. (2) Exhausted GAC lets free chlorine oxidize RO polymer, causing irreversible salt rejection loss within 200–1,000 ppm‑hours.

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3‑step selection method

1. Identify dominant phase: particulate, colloidal, dissolved organic, or ionic.

2. Set numeric targets: NTU <0.1, SDI₁₅ <3, conductivity <10 µS/cm.

3. Match technology: particulate → sand; colloidal + microbial → UF; dissolved organics + chlorine → GAC; ionic → RO.

Sand Filtration

The use of sand filter is essential for reliable particulate removal. A properly designed bed delivers effluent below 1 NTU from 15–30 NTU feed, ACHieving 85–95% TSS removal. Deiiang™ projects report 48–72 h between backwash cycles.

Scenarios & design

Post‑clarification floc capture, GAC/UF pre‑filter, and final polishing. Design at 5–12 m/h with d₁₀ 0.35–0.55 mm, UC <1.6. Backwash at 30–50 m/h uses 2–4% of daily flow. Fine floc (<50 µm) shortens filter runs.

Deiiang™ Sand Filter Data

• TSS removal: 88–96%

• Backwash water: 2.2–4.5% of throughput

• Effluent turbidity: 0.5–2.0 NTU

Fig.1 — Sand filter depth filtration.

Granular Activated Carbon (GAC)

The use of activated carbon filter solves dissolved organics and chlorine. With 800–1,200 m²/g surface, it adsorbs molecules that pass physical barriers. Capacity is finite — a 2 m³ vessel at 10 min EBCT may last 15,000–25,000 bed volumes before chlorine breakthrough. NOM pre‑loading cuts target adsorption by 30–50%.

Scenarios & lifecycle

Dechlorination (1–3 mg/L to <0.1 mg/L), DOC polishing (40–70% UV₂₅₄ reduction), odor removal, biological activated carbon. EBCT 8–15 min for chlorine, 15–30 min for DOC. Thermal reactivation restores 85–95% capacity with 5–10% mass loss per cycle.

Monitoring risks

Track ΔP and online UV₂₅₄. Excessive biofilm causes channeling and sloughing. HPC >10,000 CFU/mL demands investigation.

Ultrafiltration (UF)

The use of ultrafiltration (UF) bridges granular filtration and RO. A 0.02 µm membrane achieves >4‑log bacteria removal, effluent turbidity<0.1 NTU, and SDI₁₅ <2.5. Deiiang™ Electronics reuse applications show SDI₁₅ averaging 1.8–2.2.

Removal spectrum & fouling control

UF rejects colloids, bacteria, macromolecules >100 kDa. It passes salts, dissolved silica, and small organics. Operate at 40–70 LMH, TMP 0.3–1.5 bar. Backwash every 25–45 min; CEB every 12–48 h. CIP when TMP exceeds 1.5× baseline.

SDI stability matters more than average. A fluctuation between 1.5 and 3.8 damages RO more than a steady 2.8.

Deiiang™ UF Snapshot

• Effluent turbidity: <0.08 NTU

• SDI₁₅: 1.5–2.5

• TMP ramp: 0.02–0.06 bar/week

• CIP interval: 45–90 days

Reverse Osmosis (RO)

The use of reverse osmosis (RO) targets ionic separation. Polyamide membranes reject 99.0–99.7% NaCl. Deiiang™ two‑stage systems reduce conductivity from 1,200 µS/cm to 18–35 µS/cm at 75% recovery. RO generates concentrate at 4× feed salinity — 25 m³ per 100 m³ feed.

Critical conditions & concentrate

Pretreatment: SDI₁₅ <2.5, chlorine <0.1 mg/L, iron <0.05 mg/L. Antiscalant 2–5 mg/L. Recovery limited by least soluble salt. Concentrate options: sewer discharge, recycling, ZLD ($3–8/m³), resource recovery. Without a viable pathway, RO is not feasible. For downstream polishing, EDI is often required.

Deiiang™ RO Data

• Recovery: 72–78%

• Permeate: 12–35 µS/cm

• Flow decline: <8%/year

• CIP recovery: 92–98%

Selection Framework

Target‑first approach

Discharge TSS 30 mg/L needs only sand. Cooling tower <500 µS/cm may need RO. Boiler <10 µS/cm demands RO. Map each parameter to its numeric threshold.

Typical process trains

Four long‑term costs

Water losses (3–8%), energy (0.8–1.5 kWh/m³ brackish RO), membrane/media replacement (RO every 3–5 yr), concentrate disposal (up to $5/m³). CAPEX is only 25–35% of 10‑year TCO.

OPEX Comparison — Cost per 1,000 m³ Treated

Operating expenditure drives 65–75% of lifecycle cost.

When NOT to install RO

Data & Visualization

Performance bars

Scale mapping & pretreatment thresholds

Pull Quote

"Select by contaminant morphology, not price. A well‑designed sand filter protecting membranes delivers more value than a misapplied RO system with unmanageable concentrate."
— Jason.peng, Deiiang™

Deiiang™ Product Data & Methodology

Test standards

Turbidity iso 7027; SDI ASTM D4189; UV₂₅₄ SM 5910B; RO rejection ASTM D4516; GAC breakthrough ASTM D6586.

Deiiang™ Case Studies

Case 1: Lithium battery reuse (Yangtze River basin)

120 m³/h, target <5 µS/cm. Raw: TDS 850 mg/L, NTU 8–25. Solution by Jason.peng: Coag→SF→GAC→UF→RO→EDI. Results: RO flow decline <6%/yr, SDI₁₅ 1.6–2.1, permeate 2–4 µS/cm. Concentrate to ZLD ponds.

Case 2: Municipal reuse polishing (East China)

200 m³/h to Class A. Rainy season turbidity spikes 8→55 NTU in 2 h. Installed flow‑paced coag + dual‑media SF + pressurized UF. UF effluent <0.06 NTU, SDI₁₅ 1.5–2.0 year‑round.

FAQs

Can UF ｒｅｐｌａｃｅ sand + carbon?

UF replaces sand for turbidity, but not GAC for dissolved organics or dechlorination.

Is carbon always before RO?

Only if oxidants present. Dechlorination is mandatory.

Safe SDI for RO?

SDI₁₅ <3 minimum, <2.5 recommended.

GAC breakthrough planning?

Pilot test, online UV₂₅₄, budget 5–10% makeup carbon.

No concentrate pathway?

RO is infeasible. Explore sewer, high‑recovery, ZLD.

References

• AWWA B100

• ASTM D4189

• ASTM D4516

• ASTM D6586

• ISO 7027

• WHO Guidelines

• GB/T 19249

© 2026 Deiiang™ | Designer: Jason.peng