Rinse-water carryover

Identity and terminology

For a uniform transferred liquid, constituent mass equals concentration × carried volume. At 50 g/L and 2 mL per rack, the load is 100 mg per rack. The volume must include the retained liquid that actually crosses the boundary, such as droplets, trapped pools and carrier retention. A clear bulk rinse sample leaves trapped-liquid cleanliness unresolved. EPA, Guides to pollution prevention: The metal finishing industry, October 1992, page 5 and Figure 2

The same transfer is dragout from the source and drag-in to the receiver. EPA distinguishes liquid reaching the rinse from liquid lost to the floor, spent bath, and vapor or mist. EPA 1992, page 12, "Drag-in reduction"

A dragout tank is a named rinse configuration, not a unit of carryover. EPA describes dead or static dragout tanks as rinses without continuous feed-water flow. Their chemical concentration changes as work passes through them. EPA 1992, pages 14–15.

Surface films, droplets, and trapped liquid

The following distinctions synthesize EPA's descriptions of surface films, edge retention, grooves, cavities, and carrier geometry. They describe different contributions to retained volume, rather than separate chemical substances. EPA, Development document for the metal finishing point source category, June 1983, pages VII-246–VII-247; EPA 1992, pages 13–14.

EPA identifies geometry, viscosity, surface tension, withdrawal history, drainage, and racking as factors in dragout. Its 1983 discussion links increased kinematic viscosity with greater retention during withdrawal and slower drainage. Surface tension and the liquid's wetting relationship with the solid, represented in part by contact angle, affect film removal and edge retention. These are relationships under the described metal-finishing conditions, not a universal prediction for every liquid and geometry. EPA 1983, page VII-246.

Temperature can change viscosity, evaporation, and the condition of retained liquid. EPA's 1992 discussion also notes dry-on and bath-chemistry limitations. Consequently, a temperature change cannot be treated as an unconditional carryover reduction. EPA 1992, page 13. No quantitative film law or universal reduction factor is established by this reference.

Quantities and reporting bases

The table is dimensional synthesis. Each denominator identifies a different quantity, even when reports use the same word, "carryover."

A per-rack volume becomes a time-based rate only with a compatible rack frequency and representative loading basis. A calendar-hour average and an active-transfer-hour average have different time boundaries. Incoming rinse-water supply flow is a different stream from liquid carried by parts.

For a uniform film over a planar wetted area, Vfilm = A × h, with area A in m² and thickness h in m measured perpendicular to that plane, gives volume in m³. This planar geometric identity does not predict thickness. It omits separate droplets, trapped pools, and carrier retention unless their volumes are included explicitly. For nonuniform thickness over that planar area, the corresponding exact expression is Vfilm = ∫A h(x, y) dA, with thickness measured perpendicular to the plane. Neither expression is an exact identity for a film of finite thickness on a curved surface. These are explanatory deductions from geometry, not plant measurements.

Conditional constituent balances

EPA states that characterizing dragout requires both concentration and volume. Its simplified static-rinse expression is Vd = Cr × Vr / Cp, where Vr is rinse volume and Cp and Cr are the process-bath and rinse concentrations. EPA 1992, page 6, displayed equation and definitions.

The relationships below are conservation-based synthesis. They make the collection and composition assumptions explicit rather than extending that simplified expression to every rinse.

For a transferred liquid with representative constituent mass concentration cd:

mtransferred = cd × Vd

With cd in mg/L and Vd in L, the result is mg. For a steady representative transfer rate, ṁtransferred = cd × Qd gives mg/h when Qd is in L/h. If concentration varies during the transfer, the mass is the integral of concentration over transferred volume, not an arbitrary single sample multiplied by total volume.

For mass-fraction data, the corresponding identity is:

mtransferred = wd × ρd × Vd

Here wd is in kg/kg, liquid density ρd is in kg/L, and Vd is in L. The result is kg of constituent. Thus mg/kg is not numerically interchangeable with mg/L without a density basis. Density and volume refer to compatible temperatures and liquid composition.

For an ideal closed collection with initial liquid volume V0 and concentration c0, addition of volume Vd at concentration cd gives:

cf × (V0 + Vd) = c0 × V0 + cd × Vd

This assumes additive volumes, complete mixing, complete transfer into the collection, and a conserved constituent. No other inflow, outflow, evaporation, reaction, precipitation, or adsorption occurs. If cd ≠ cf, rearrangement gives:

Vd = V0 × (cf − c0) / (cd − cf)

For initially constituent-free liquid, c0 = 0. When cf is much smaller than cd, the expression approaches Vd ≈ V0 × cf / cd. This explains a restricted dilute-collection interpretation of EPA's simpler equation. The approximation is not a universal inverse measurement of carryover.

A fixed-volume receiving rinse with overflow has a different balance. In a well-mixed model with a conserved constituent:

d(V × c)/dt = Σ(Qin × cin) − Σ(Qout × cout)

The terms require the same constituent and consistent volume and time bases. Workpiece carryover, feed water, liquid returns, and overflow are separate terms. A constant concentration does not by itself mean zero incoming mass. At steady state, incoming and outgoing constituent mass rates can be equal.

Sample boundary and interpretation limits

The following are explanatory synthesis based on the distinct streams in EPA's process diagram and its concentration-and-volume characterization. EPA 1992, pages 5–6.

A constituent balance applies only to the constituent it tracks. A metal assay, total formulation concentration, and concentration expressed as a particular compound have different mass bases. Conversion between an element and its compound requires the chemical identity and stoichiometric basis. A low receiving-rinse concentration can result from dilution and does not alone prove low transferred mass or low surface residue.

Source versions and coverage

This reference uses EPA's June 1983 development document, specifically pages VII-245–VII-248, and its October 1992 guide EPA/625/R-92/011, pages 5–6 and 12–15. The relevant process diagram, displayed equation, and scanned table headings were inspected alongside the prose. The 1983 tables distinguish theoretical rinse arrangements from sampled-plant comparisons. They do not establish universal carryover values, and no table flow or reduction value is adopted here. EPA 1983, Tables 7-96 and 7-97; EPA 1992 guide.

These historical metal-finishing sources support terminology and qualitative relationships. They do not supply a validated film model for arbitrary parts, a specific line's carryover inventory, or current product specifications. Reactions, suspended material, multiphase liquids, drying, and inaccessible cavities can invalidate the simple balances above. No operating setting, rinse sequence, reduction guarantee, acceptance criterion, discharge permission, or customer capability follows from this article.

Surface-finishing processes. Related conductivity reference.