⟵ 10 Suspended Solids (Problem Set) · Contents · 11 The Pillars of Water Treatment ⟶
Suspended Solids: Answer Key
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Dissolved solids have been dismantled at the molecular level and are governed by chemistry (equilibrium, solubility, reaction kinetics) - they move wherever the water moves because they are part of the solution. Suspended solids are intact particles governed by physics (gravity, flow velocity, inertia) - they settle wherever the system’s hydraulics allow them to stop moving.
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The bulk water chemistry is excellent - but it is only measuring the water column. The 3 inches of settled silt have created deposits on horizontal pipe surfaces. Beneath those deposits, oxygen is consumed but cannot be replenished, creating oxygen differential cells that drive under-deposit corrosion. The corrosion inhibitor, pH, and biocide measured in the bulk water are not reaching the metal surface beneath the deposits. The coupons, which are typically placed in the flowing water column on clean holders, are measuring the chemistry the pipes are not actually receiving. Excellent bulk chemistry and aggressive pitting are entirely consistent when deposits are present.
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Dust settles in low-flow zones and on pipe surfaces, creating deposits. The deposits create oxygen differential cells that drive under-deposit corrosion (Corrosion pillar). Corrosion products (iron oxide) generate additional suspended solids, accelerating the cycle. The deposits also provide shelter for biological attachment - prefabricated homes for biofilm formation that are shielded from biocides in the water column (Biology pillar). The biofilm produces EPS that traps additional particles, further stabilizing the deposit. Meanwhile, the deposit surfaces provide heterogeneous nucleation sites that lower the kinetic barrier for scale formation, particularly on heat-exchange surfaces where temperature is highest and hydration is most stressed (Scale pillar). A single windstorm, left unaddressed, cascades through all three pillars.
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In a closed system that has been operating for two years, 45 NTU is almost certainly a corrosion problem, not an external solids problem. Closed systems are not exposed to atmospheric dust loading. The brown, turbid water is likely iron oxide from active corrosion of carbon steel piping. Investigate first: (a) check the corrosion inhibitor residual - is the treatment program being fed consistently? (b) filter a sample and examine the particulate - brown/red suggests iron oxide, black suggests magnetite or SRB activity, green suggests copper corrosion products. (c) check whether the system was properly flushed after construction - construction debris from two years ago can continue circulating indefinitely.
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(a) Required throughput = 3,000 gal ÷ 4 hr = 750 gal/hr = 12.5 GPM. (b) 12.5 GPM ÷ 800 GPM = 1.56% of the recirculation rate. (c) Yes. Near an active construction site, solids loading will be significantly higher than normal. Increasing the sidestream flow rate to turn over the system volume every 2–3 hours (18.75–25 GPM, or roughly 2.3–3.1% of the recirc rate) would be prudent. Additionally, check the filter type - construction dust may include fine particles that require a sand or multimedia filter rather than a screen or disk filter.
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Stokes’ Law shows that settling velocity depends on the square of the particle diameter and the density difference between the particle and water. Rust particles have a very high density difference (ρp − ρw ≈ 5,200 − 998 = 4,202 kg/m³) and a moderate diameter (50 µm). The hydrocyclone generates centrifugal force that accelerates settling - heavy, dense particles are thrown to the outer wall and separated. Biological debris has a density barely above water (ρp − ρw ≈ 1,050 - 998 = 52 kg/m³) - roughly 80× less density difference - and a smaller diameter (20 µm). The centrifugal force is not sufficient to separate particles with such a small density difference from the water. They simply recirculate with the flow.
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Standard corrosion coupons are mounted in the flowing water column on clean, smooth holders. They measure uniform corrosion in the bulk water environment. Under-deposit corrosion is localized - it occurs beneath silt deposits on horizontal pipe surfaces where the metal is isolated from the treatment program. The coupon never sees this condition because it has no deposit on it. To detect under-deposit corrosion, the monitoring program should be supplemented with: (a) visual inspection of horizontal pipe runs during shutdowns, looking for deposits and localized pitting beneath them; (b) ultrasonic thickness testing on deposit-prone sections; (c) placing some coupons horizontally with intentional deposits or in spool pieces that mimic pipe conditions rather than in clean, vertical holders.
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(a) Energy penalty = 3°F × 1.5%/°F = 4.5%. (b) Baseline energy: 1,000 tons × 0.65 kW/ton = 650 kW. Annual energy: 650 kW × 4,000 hr = 2,600,000 kWh. Annual cost: 2,600,000 kWh × 312,000. Energy penalty cost: 14,040/year. (c) Simple payback: 14,040/yr = 0.57 years, or approximately 7 months. The filter pays for itself well within the first cooling season - and this calculation does not include reduced chemical cost, lower maintenance labor, or extended equipment life.
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The dispersant is working. The turbidity spike from 20 to 60 NTU indicates that the dispersant is lifting deposits from heat-exchange surfaces and piping into the water column. The 1°F improvement in condenser approach temperature confirms this - the insulating deposit layer on the condenser tubes has been partially removed, restoring heat transfer. The “worse” turbidity reading is actually showing material that was already in the system but hidden on surfaces, invisible to bulk water monitoring. This is a success. The correct next steps are to increase blowdown temporarily or ensure sidestream filtration is running to capture the suspended material, and to follow with a biocide treatment to kill organisms that were sheltered beneath the deposits and are now exposed.
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Adjusting chemical feed rates alone will not solve this problem because the suspended solids have created deposits that isolate surfaces from whatever chemistry is in the water. Increasing corrosion inhibitor concentration does not help when the inhibitor cannot reach the metal beneath the silt deposits - under-deposit corrosion will continue regardless of bulk inhibitor residual. Increasing biocide dose does not help when the biocide is consumed by EPS and organic debris in the deposits before it can reach the organisms sheltered within them. The deposits must be physically removed first: the basin needs to be cleaned, sidestream filtration must be installed to prevent reaccumulation, and a dispersant program should be used to lift existing deposits off surfaces. Only then will the chemistry operate in the environment it was designed for. The customer is asking to fix a mechanical problem with a chemical solution - and the amplifier effect guarantees that approach will fail.
⟵ 10 Suspended Solids (Problem Set) · Contents · 11 The Pillars of Water Treatment ⟶