⟵ 10 Suspended Solids (Narrative) · Contents · 10 Suspended Solids (Problem Set) ⟶
Suspended Solids: Engineering Notes
“If you are reading this straight through, you can skip this section and lose nothing essential to the story. These notes are for the operators, engineers, and technicians who need to do the math.”
Why This Matters
Suspended solids are not a chemistry problem. They are a mechanical problem with chemical consequences. In the field, this means:
- Filtration determines how well your treatment program performs - not just how clean the basin looks.
- Turbidity trending is the fastest way to detect changes in solids loading before deposits form.
- Under-deposit corrosion is invisible to standard coupon programs - if you see pitting during shutdowns that your coupons did not predict, deposits are the likely cause.
- Rising biocide demand without rising biological load usually means deposits are consuming chemistry before it reaches organisms.
- Energy cost from fouled heat exchangers is real and measurable - a 3°F rise in condenser approach temperature costs roughly 4.5% in chiller efficiency (rule of thumb; system dependent).
Measuring Suspended Solids
Because suspended solids are physical particles, we measure them differently than dissolved chemistry.
Total Suspended Solids (TSS):
A known volume of water (typically 1 L) is filtered through a pre-weighed glass fiber filter, dried at 103–105°C, and reweighed. The mass gain is reported as mg/L.1 TSS is direct but typically requires lab equipment and time.
Turbidity:
A turbidity meter measures the scattering of light by particles in the water, reported in Nephelometric Turbidity Units (NTU). Turbidity is fast, field-ready, and trends well over time. It correlates roughly with TSS but is not a direct substitute - particle size, shape, and color all influence the reading.
In cooling towers, trending turbidity regularly is often more useful than occasional TSS lab tests. Trends tell the story long before deposits are visible.
Turbidity Guidelines
| System | Target (NTU) | Notes |
|---|---|---|
| Cooling Tower (basin) | < 25 | Higher values indicate solids accumulation; investigate source and filtration |
| Cooling Tower (post-filter) | < 5 | Sidestream filter effectiveness check |
| Closed Loop | < 5 | Higher values suggest active corrosion or construction debris |
| Boiler Feedwater | < 1 | Pretreatment should deliver near-clear water |
| RO Feed | SDI < 5 | Silt Density Index (SDI) measures membrane fouling potential; not turbidity |
These are general operational targets, not regulatory limits. Actual targets depend on system design and treatment program.2
Common filter types
| Filter Type | Removal Rating | Notes |
|---|---|---|
| Sand / multimedia bed | ~10–20 µm | Low maintenance (auto backwash). Best for general cooling tower use. |
| Disk filters | ~25–100 µm | Compact, auto-flushing. Good for light-to-moderate loading. |
| Cartridge / bag filters | 1–25 µm | Precise but requires manual changeout. Polishing or small systems. |
| Hydrocyclone separators | Dense particles only | Centrifugal force. No consumables. Ineffective for fine silt or bio debris. |
Sidestream Filtration Sizing
The general sizing rule: turn over the system volume through the filter at least once every 4 hours. More frequent turnover for heavily loaded or critical systems.
Example:
- System volume = 3,000 gal
- Turnover target = 4 hr → throughput = 3,000/4 = 750 gal/hr = 12.5 GPM
- If recirculation = 800 GPM, sidestream ≈ 1.6% of recirculation.
Settling Velocity (First-Order Estimate)
Stokes’ Law governs the settling velocity of spherical particles in still fluid:
v = (d² × (ρp − ρw) × g) / (18 × µ)
Where:
- v = settling velocity (m/s)
- d = particle diameter (m)
- ρp = particle density (kg/m³)
- ρw = water density (~998 kg/m³)
- g = gravitational acceleration (9.81 m/s²)
- µ = dynamic viscosity of water (~0.001 Pa·s at 20°C).
Caveat: This is a first-order estimate. Real systems deviate due to turbulence, non-spherical particles, and flocculation.
Practical implication: A 100 µm sand particle (density ~2,650 kg/m³) settles at roughly 8 mm/s in still water. A 10 µm silt particle settles at roughly 0.08 mm/s - 100× slower. At typical cooling tower flow velocities of 3–6 ft/s in piping, the silt particle has no chance of settling. It will remain suspended indefinitely unless mechanically filtered. (Order-of-magnitude estimate; turbulence and non-spherical particles dominate in real systems.)
This is why particle size matters: heavy grit settles in basins; fine silt requires filtration.
The Cost of Dirty
Quantifying solids cost is difficult because damage is indirect, but consequences are commonly observed:
- Heat transfer loss: A thin particulate layer on condenser tubes can reduce heat transfer by 10–20%, translating to measurable increases in chiller energy consumption (use ~1.5% efficiency loss per °F of approach temperature rise).
- Biocide demand: Dirty systems often require 20–50% more oxidizing biocide to achieve the same planktonic kill, because chemistry is consumed by organic deposits before reaching organisms.
- Misleading coupons: Under-deposit corrosion is not captured by standard coupon placement. Clean coupons in dirty systems often show acceptable rates while the actual piping beneath deposits is pitting aggressively.
- Maintenance frequency: Strainer cleaning, heat exchanger rodding, and basin cleaning labor increases significantly in systems without filtration.
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Footnotes
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Standard Methods for the Examination of Water and Wastewater, Method 2540 D, “Total Suspended Solids Dried at 103 to 105 C.” APHA, AWWA, WEF. Confirm the current edition at time of publication. ↩
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ASTM D4189, “Standard Test Method for Silt Density Index (SDI) of Water,” ASTM International. Confirm the current edition at time of publication. ↩