Every recirculating wet scrubber answers two questions with one number: how much fresh water to add, and how much water leaves the loop as liquid waste. That waste stream is the wet scrubber wastewater you have to handle, and getting its volume right starts with a water balance. This guide shows you how to set up that balance from data you can collect — exhaust gas flow, inlet temperature and humidity, your dissolved solids or solids limit, and your mist eliminator’s stated performance. You will learn the four streams (makeup water, evaporation, drift and entrainment, and blowdown), how to estimate each one, and how to convert the result into gallons per day and cubic meters per hour. A 10,000 CFM worked example ties the method to a checkable ledger.
Key Takeaways
- One equation governs everything. A wet scrubber water balance is one equation: makeup water must equal the sum of evaporation loss, drift and entrainment, blowdown, and any other water that leaves the loop. That ledger defines your wet scrubber wastewater volume.
- Evaporation usually dominates. It grows with inlet gas temperature and humidity and often sets the whole water demand (EPA Cost Manual §6 Ch.2).
- Blowdown is a design decision, not a fixed 5% or 10%. It follows from your TDS or solids limit through cycles of concentration — a method borrowed from cooling-tower management, applied with your own limit.
- Two blowdown conventions exist. The EPA solids-based equation and the TDS-based manufacturer convention answer different limits; use them consistently.
- There is no generic drift number. No credible published value applies; drift is a mist-eliminator vendor input or an explicit conservative assumption.
- The balance ends at the wastewater route. The volume you calculate here feeds treatment selection and blowdown management.
Wet Scrubber Wastewater: Where the Water Goes
Every wet scrubber that recirculates its scrubbing liquid runs a four-stream water ledger: makeup water in, and evaporation, drift and entrainment, and blowdown out. Evaporation carries water out as vapor — the exhaust gas leaves the tower essentially saturated. Drift and entrainment carry droplets past the mist eliminator and out of the stack. Blowdown is the deliberate purge that removes material captured from the gas. Most wet scrubber systems recirculate (EPA Cost Manual §6 Ch.2), and the captured mass — particulates, absorbed gases, reaction products — accumulates in the liquid until the loop is purged. Between the purge, overflow, leaks, and maintenance drainage, the liquid that leaves the loop is the wet scrubber wastewater the site must manage; evaporation, by contrast, returns to the atmosphere.
Each stream has a failure mode if the loop is not balanced. Skip or under-size blowdown and the solids content of the recirculating water climbs until salts precipitate — scaling on packing, plugged nozzles, and salt entrainment in the gas; the EPA acid-gas chapter treats the 10–15% solids range as the zone where purging is triggered to avoid that carryover (EPA Cost Manual §5.2 Ch.1). Add makeup faster than water leaves and the sump overflows, the level controller throttles back, and you pay for water you did not need while creating unplanned discharge. Let makeup fall behind and the sump drains, the recirculation pump loses suction, the liquid-to-gas ratio collapses, and removal efficiency drops with it. The failure modes are mechanical, but the cause is arithmetic: the level controller balances the tank, while the chemistry limits are what force blowdown.
This article stops at the math — how much water each stream carries. What is actually in the blowdown, how the purge is controlled and monitored, and which treatment route the water takes are separate decisions that this guide hands off rather than repeats.
The Water Balance Equation: Four Streams
In symbols, the wet scrubber water balance is:
M = E + D + B + L
Makeup water (M) equals evaporation (E) plus drift and entrainment (D) plus blowdown (B) plus other losses (L). At steady state — level holding, chemistry steady — this is an identity, not an estimate: whatever leaves the loop must be replaced. The EPA’s total-water equation for wet scrubbers writes the same requirement as evaporation plus bleed water, Q_T(H2O) = Q_wv(evap) + Q_bleed (Cost Manual §6 Ch.2, eq 2.38). This guide carries drift and entrainment and other losses as explicit terms because the manual’s consumables scope stops at evaporation and bleed; in the field those streams are real, so the ledger below includes them.
| Stream | Symbol | Direction | Set by | Estimated in |
|---|---|---|---|---|
| Makeup water | M | into loop | sump level control; at steady state = sum of losses | Makeup Water section |
| Evaporation | E | out, as vapor | inlet gas temperature and humidity; outlet saturation | Evaporation section |
| Drift and entrainment | D | out, as droplets | mist eliminator performance (vendor input) | Drift and Entrainment section |
| Blowdown | B | out, deliberate purge | captured load; TDS/solids design limit | Blowdown section |
| Other losses | L | out, leaks/spills/drainage | housekeeping and maintenance practice | counted into makeup |
Work every term in the same unit before adding. The conversion table below gives the factors engineers reach for, and one kilogram of water is one liter at scrubber temperatures (density ≈ 1,000 kg/m³), which keeps mass-based evaporation estimates easy to convert.
| Convert from | to | multiply by |
|---|---|---|
| gpm | GPD | 1,440 |
| gpm | m³/h | 0.2271 |
| gpm | L/h | 227.1 |
| m³/h | gpm | 4.403 |
| m³/h | GPD | 264.2 |
| kg/h of water | m³/h | ≈ 0.001 |
Two configurations change how the ledger reads. Recirculating systems — the norm, and the case behind the EPA equations — concentrate captured material in the loop and therefore need blowdown; makeup only replaces the losses, so demand is modest. Once-through systems draw fresh water continuously and discharge continuously; there is no concentration problem, but consumption scales directly with gas flow. Research on waste-to-energy wet scrubbers models the same structure — drain flow plus evaporation defines makeup demand (Dal Pozzo & Cozzani, Chemical Engineering Transactions vol. 86, 2021) — confirming that the ledger holds across scrubber types, though that study’s numbers are specific to waste-to-energy service.
Makeup Water: How Much Fresh Water You Need
Ask how much makeup water does a scrubber need and the short answer is: as much as it loses. At steady state the makeup flow equals the sum of the streams you have already identified. The EPA frames total water demand the same way — evaporation plus bleed water (Cost Manual §6 Ch.2, eq 2.38) — and once drift and other losses are added, makeup is simply that total. This is why a makeup meter doubles as a balance check: over a stable operating period, metered makeup should match the sum of the outbound streams. When it does not, one of your inputs is wrong.
Continuous makeup holds the sump level: a level controller trims the makeup valve as water leaves the loop. Batch makeup covers events rather than steady operation — refilling the sump after maintenance, or topping up after a heavy campaign. Design for the continuous case first, then confirm the water supply can deliver the batch fill rate — usually far above the steady rate but lasting only minutes.
Makeup quality sets the starting line for concentration. If the fresh water already carries dissolved solids, the loop reaches its design limit after fewer cycles and needs more blowdown for the same evaporation. Makeup TDS or conductivity is therefore a design input to the balance, not a water-treatment afterthought: cleaner makeup means less water purged.
As a rough sanity check on magnitude, a general engineering reference (grade Q — a compilation figure, not a design guarantee) lists water consumption of about 0.45–0.55 L per m³ of treated air for nozzle-type spray scrubbers, with the value depending on process gas temperature. Treat that as an indicator only — it is a general reference figure rather than your process data, and it says nothing about how the consumption splits between evaporation, drift, and blowdown. If your calculated makeup lands far outside such a range, re-examine the gas temperature and humidity inputs before trusting the result.
The makeup stream also has an equipment side: a pump sized for the makeup rate and for the water’s chemistry. With aggressive scrubbing liquors, wetted materials matter as much as flow, and the PP centrifugal chemical pump page covers the options for this duty.
Estimating Evaporation Loss from Gas Temperature and Humidity
Evaporation is the biggest stream in most packed scrubber balances, and the EPA is explicit about what drives it: evaporation is determined by the inlet gas temperature and humidity, and high evaporation raises the liquid-to-gas ratio the system must maintain (Cost Manual §6 Ch.2). In acid-gas service the same physics shows up as solvent loss — the acid-gas chapter warns that excessive gas temperature causes significant scrubbing-liquid loss through evaporation and can justify a precooler (Cost Manual §5.2 Ch.1) — and the packed-bed fact sheet puts practical absorption temperatures at 4–38 °C (40–100 °F), with high temperatures driving significant scrubbing-liquid loss (EPA-452/F-03-015).
Mechanically the tower behaves like a saturator: the exhaust gas enters dry or humid, exchanges heat and mass with the recirculating liquid, and leaves saturated at close to the liquid temperature. The water it carries out is the humidity it leaves with minus the humidity it came in with, multiplied by the dry gas flow:
E [kg/h] = (W_out − W_in) × m_dry_air
where W is the absolute humidity ratio (kg water per kg dry air) and m_dry_air is the dry gas mass flow. Outlet W is taken as saturation at the expected outlet temperature — in practice close to adiabatic saturation of the sump liquid; inlet W comes from a psychrometric measurement or your process data.
Here is the scrubber evaporation rate calculation on example data; every value is an assumption for this illustration. Exhaust at 10,000 CFM (≈283 m³/min) enters at 140 °F (60 °C) with a humidity ratio of about 0.012 kg/kg (roughly 10% relative humidity). The gas leaves saturated at about 100 °F (38 °C), where saturation humidity is about 0.043 kg/kg. Dry gas flow is about 300 kg/min — 283 m³/min at about 1.06 kg/m³ — or 18,000 kg/h. Evaporation is then about 18,000 × (0.043 − 0.012) ≈ 560 kg/h ≈ 2.5 gpm ≈ 0.56 m³/h. These numbers are illustrative, not a property of your scrubber; the full four-stream balance built on them appears in the Worked Example section.
Precooling changes this estimate directly. Cooling the inlet gas before the packed section lowers the saturation temperature, cuts the water vapor carried out, and protects the scrubber-liquid inventory at high inlet temperatures (EPA-452/F-03-015). If your process gas sits well above the absorption window, bring a precooler into the design conversation before locking in makeup capacity; choosing between a direct-contact quench and a heat-exchange precooler belongs to the sizing review.
Blowdown and Cycles of Concentration
Blowdown exists because recirculation concentrates. Particles and absorbed species captured from the gas stay in the liquid, and every kilogram that enters must eventually leave; the purge is how it leaves in a controlled way. The EPA describes the mechanism plainly: most wet scrubber systems recirculate, part of the liquid is bled to decrease its solids content, and fresh water is added (Cost Manual §6 Ch.2). Your purge rate is a design decision set by two limits — how much captured mass the loop must shed, and how concentrated you allow the liquid to become.
The EPA’s cost manual builds the solids-based convention from three quantities. The mass of particulate captured into the liquid (eq 2.36):
m_PM = η · L_PM · Q_i
with η the overall collection efficiency, L_PM the inlet particulate loading, and Q_i the inlet waste gas flow. The bleed rate needed to hold the design solids fraction (eq 2.37):
Q_bleed = m_PM / (f_solids · ρ_H2O)
with f_solids the design mass fraction of solids in the recirculating water and ρ_H2O water density (1.0 kg/L). Total water demand is then evaporation plus bleed (eq 2.38), and annual volume is flow times operating time (eq 2.39):
Q_T(H2O) = Q_wv(evap) + Q_bleed
V_T(H2O) = Q_T(H2O) · t
All four equations come from the EPA Air Pollution Control Cost Manual §6 Ch.2 (grade A). The choice of f_solids is a design value: the manual cites peak solids concentrations of 20–30% for venturi scrubbers, and the acid-gas chapter triggers purging around 10–15% solids to avoid salt entrainment — the number you adopt comes from your process and materials, not from a universal default.
The second convention attacks the dissolved-solids ceiling instead of the captured mass. It comes from cooling-tower management, where cycles of concentration (COC) relates the circulating concentration to the makeup concentration (US DOE FEMP Best Management Practice #10, grade A). Given evaporation E and a target COC, the purge is:
B = E / (COC − 1)
The formula transfers cleanly to scrubbing loops with one explicit boundary: published COC values come from cooling towers, not wet scrubbers, so you apply the method with your own TDS or solids design limit and never import a tower’s number.
The same dissolved-limit logic is often written directly in manufacturer and design-office practice — the TDS-based convention:
B = E × C_i / (C_max − C_i)
where C_i is the makeup concentration and C_max your accepted operating maximum (grade M — a manufacturer convention). This is the form used on this site’s scrubber blowdown management guide, whose material-specific saturation values are design inputs for that page’s context, not universal standards. The solids equation purges captured mass; the COC and TDS equations cap dissolved concentration. One purge stream must satisfy both, so the larger required rate governs. Adopt one convention and apply it consistently — mixing the solids equation and the TDS formula in the same estimate double-counts the purge.
Drift and Entrainment Losses
Drift and entrainment is the water that rides out of the stack as liquid droplets rather than vapor. The mist eliminator — a mesh pad or chevron bank at the top of the tower — is designed to strip nearly all droplets from the gas; what gets past it is the scrubber entrainment loss, joined by droplets that re-entrain from wall film or bypass the eliminator through gaps or damage. This stream is usually small next to evaporation, but it is real, and it is the one term in the ledger that carries liquid-phase material straight out of the system.
For the mist eliminator drift loss part of the balance we will not give you a default, because there is no credible default to give: no public source publishes a generic drift percentage for wet scrubbers, so no universal number applies. Treat drift as a design input with two legitimate sources — the mist eliminator vendor’s guaranteed carryover at the design velocity, or an explicit conservative assumption you state in the calculation and defend in review. With a vendor number, drift = recirculation rate × guaranteed carryover fraction. Without one, adopt a conservative figure, label it as assumed, and confirm the real value with the vendor before the number leaves the office.
Drift stops being minor when conditions defeat the eliminator: gas velocity above its rating, a damaged or bypassed pad, operation near flood, or startup transients with abnormal liquid hold-up. During those periods entrainment can exceed blowdown for hours. A persistent makeup rise with no change in purge or evaporation should send you back to the eliminator before you touch the blowdown setting — the balance turns a mechanical fault into arithmetic you can see.
Turning the Balance into Annual Water Consumption
Flow rates are the working currency of the balance; annual consumption is what procurement, permits, and the wastewater budget actually talk about. The EPA’s eq 2.39 does the conversion — annual volume equals flow rate times operating time, V_T(H2O) = Q_T(H2O) · t. Apply it to the makeup rate for total wet scrubber water consumption and to the blowdown rate for the wastewater volume that leaves the plant. As a preview of the worked example in the next section, a makeup rate near 4 gpm at 6,000 operating hours per year is roughly 1.4 million gallons — about 5,500 m³ — annually; the example section computes the streams that produce it.
Do not annualize a single snapshot. Inlet gas temperature and humidity move with the season and evaporation follows them; process load drives captured mass and therefore blowdown. If your site runs warm-weather peaks, build the annual number month by month and check the peak month’s makeup demand separately, because the peak month sizes the water system — not the annual average.
The water-cost conversation starts here and is deliberately left unquantified: makeup is priced by local water-supply tariffs and the discharge stream by local sewer or wastewater charges, both site-specific. What this balance delivers is the volume input those quotes need — not a savings promise and not a total-cost-of-ownership claim. A large blowdown stream moves the economics toward the treatment route handed off in the final section.
Worked Example: 10,000 CFM Scrubber
This example runs the full four-stream ledger on one consistent set of stated assumptions. Nothing here is a property of your scrubber: the gas conditions, captured load, solids fraction, and drift allowance are all example values chosen to make the arithmetic checkable. Substitute your own inputs before using any number.
Worked Example: 10,000 CFM Scrubber
| Example input | Value | Used for |
|---|---|---|
| Exhaust gas flow | 10,000 CFM (≈283 m³/min) | dry gas mass flow |
| Inlet gas temperature | 140 °F (60 °C) | evaporation |
| Inlet humidity ratio | ≈0.012 kg/kg (≈10% RH) | evaporation |
| Outlet condition | saturated at 100 °F (38 °C), W ≈ 0.043 kg/kg | evaporation |
| Captured load into the liquid | 30 kg/h | blowdown, eq 2.36/2.37 |
| Design solids fraction | 0.10 (10% by mass) | blowdown |
| Recirculation rate | 500 gpm | drift basis |
| Mist eliminator drift allowance | 0.05% of recirculation (assumed for this example) | drift |
| Other losses (leaks, spillage) | 0.05 gpm (assumed) | other losses |
| Operating hours | 6,000 h/yr | annual volume |
Step 1 — evaporation. Dry gas mass flow is about 283 m³/min × 1.06 kg/m³ ≈ 300 kg/min, or 18,000 kg/h. Evaporation is the humidity difference times that flow: 18,000 × (0.043 − 0.012) ≈ 560 kg/h.
Step 2 — blowdown (EPA solids convention, eq 2.37). Q_bleed = m_PM / (f_solids · ρ_H2O) = 30 kg/h ÷ (0.10 × 1,000 kg/m³) = 0.30 m³/h.
Step 3 — drift. 500 gpm × 0.0005 = 0.25 gpm.
Step 4 — other losses. 0.05 gpm, carried as assumed.
Step 5 — makeup closes the ledger. M = E + D + B + L = 2.5 + 1.3 + 0.25 + 0.05 ≈ 4.1 gpm.
| Stream | m³/h | L/h | gpm | GPD |
|---|---|---|---|---|
| Evaporation (E) | 0.56 | 560 | 2.5 | 3,530 |
| Blowdown (B) | 0.30 | 300 | 1.3 | 1,900 |
| Drift and entrainment (D) | 0.057 | 57 | 0.25 | 360 |
| Other losses (L) | 0.011 | 11 | 0.05 | 70 |
| Makeup (M = sum) | 0.93 | 930 | 4.1 | 5,870 |
Cross-check with the TDS convention. If an assumed design allowance of 8 cycles of concentration applies, B = E / (COC − 1) = 560 ÷ 7 ≈ 80 kg/h ≈ 0.08 m³/h — far below the solids-based 0.30 m³/h, so in this example the captured-solids limit governs the purge and the TDS cap is not binding.
Round-trip sanity check. Makeup equals the sum of the four losses: 2.45 + 1.32 + 0.25 + 0.05 = 4.07 gpm, and the table values are the same numbers rounded for display, so the displayed columns do not add to exactly the displayed total. Over a year at 6,000 operating hours, 4.07 gpm is about 1.47 million gallons (≈5,550 m³) of makeup water, while blowdown alone accounts for roughly 1,900 GPD (≈1,800 m³/yr) of wastewater leaving the loop. If a makeup meter disagreed with that figure over a stable month, the disagreement is the finding — one input is wrong, or water is leaving the loop somewhere the ledger does not yet name.
Design Inputs Checklist Before You Commit to a Number
Every number in this guide traces back to the inputs below. If you can fill the table, you can defend the balance; if an input is missing, that is what you collect before committing to a result.
Wet Scrubber Water Balance Input Checklist
| Input | Needed for | Where the value comes from |
|---|---|---|
| Gas flow rate (state actual/standard and dry basis) | dry gas mass flow; eq 2.36 | fan curve, duct survey, or process data |
| Inlet gas temperature | evaporation | process measurement or design data |
| Inlet humidity or dew point | evaporation | psychrometric measurement, site climate data |
| Outlet saturation assumption | evaporation | assume saturated at outlet liquid temperature |
| Captured load L_PM and collection efficiency η | captured mass m_PM, eq 2.36 | inlet loading analysis and removal duty |
| Design solids fraction f_solids or TDS limit C_max | blowdown, eq 2.37 / COC | process design; manual cites venturi peaks of 20–30% and a purge trigger near 10–15% |
| Makeup TDS / conductivity C_i | COC and TDS conventions | water-supply analysis |
| Recirculation rate | drift basis, L/G context | pump curve or sizing calculation |
| Mist eliminator carryover guarantee | drift | vendor datasheet |
| Operating hours and seasonal profile | annual volume | production schedule |
| Blowdown control setpoint | purge rate and purge control | design office and control philosophy |
Fill the table in this order: gas-side inputs, then the chemistry limits, then the mechanical inputs, then the operating calendar. Three entries are decisions rather than measurements — the design solids or TDS limit, the blowdown setpoint, and the drift allowance — so keep them visible as stated assumptions a reviewer can challenge.
Gas velocity, packing depth, and liquid-to-gas ratio are absent from this list on purpose: they are inputs and outputs of the tower’s sizing calculation, which this site covers in the scrubber sizing calculation guide. The water balance takes the gas flow from that work and hands the resulting makeup and wastewater volumes to the treatment decision; purge-control instrumentation and monitoring sit with blowdown management, and the disposal route follows in the final section.
What to Check Before You Trust the Numbers
The first check is the one this guide keeps returning to: does makeup equal the sum of the losses? Add the streams in one unit and compare against the makeup rate you would set. In the worked example, 2.45 + 1.32 + 0.25 + 0.05 = 4.07 gpm. If your total is off by more than rounding, find the missing term before refining any single number — an unbalanced ledger usually means a stream was left out, not that a coefficient needs tuning.
The errors that break water balances are mostly bookkeeping rather than physics. Mixing gas-flow bases is the most common: actual cubic feet per minute, standard cubic feet per minute, and dry gas flow are three different quantities, and the humidity calculation needs dry gas. Unit drift across the balance is next — gpm, GPD, L/h, and m³/h appearing in the same column. Then the assumptions: omitting drift entirely, setting the solids or TDS limit from a rule of thumb instead of the process, importing a cooling-tower COC value, or annualizing one mild-weather snapshot. Each of those shifts the answer without leaving a trace.
Keep the boundary between this calculation and daily operation clear. The balance is a design and audit estimate; what the plant meters, samples, and controls continuously is a separate discipline, covered by the blowdown-management page. When the two disagree, the balance is the faster diagnostic — it tells you whether to look at the gas side, the chemistry limits, or the mist eliminator first, and it keeps your wet scrubber wastewater estimate honest.
Wet Scrubber Wastewater FAQ
Is 5% blowdown a normal setpoint? There is no normal percentage to copy. Blowdown follows from the captured load and from the solids or TDS limit you adopt, so the same tower can purge very different volumes in different services. A fixed percentage silently assumes a fixed load and a fixed limit — and both change.
Does a mist eliminator remove all drift? No. It removes most droplets at its design velocity, and some liquid always escapes as carryover. Treat the residual as an input from the eliminator vendor rather than as a constant.
Can blowdown be reused? Sometimes, after treatment appropriate to its composition — a question for the wastewater route rather than for the water balance. The volume computed here is what that decision needs.
Why does makeup rise in hot weather? Warmer inlet gas carries more water vapor away, so evaporation grows and makeup follows. That is a physical effect, not a leak.
Do I need a flow meter to run this balance? No, but a makeup meter turns the balance into a repeatable check that validates the wet scrubber wastewater volume your balance estimates.
Your Next Step: From Water Balance to Wastewater Route
With the four streams estimated, you hold two numbers that drive the next decisions: the makeup rate the water supply must deliver, and the blowdown rate that becomes wastewater. The second number is the input to the treatment decision — volume sets the scale of the equipment, and the character of the stream sets the route. The wastewater treatment guide covers how those routes are selected, and the blowdown-management page covers how the purge is controlled from day to day; both are handoffs from this calculation rather than repeats of it.
On the equipment side, the balance tells you what the tower and its auxiliaries must do: hold the liquid-to-gas ratio under peak gas flow, keep the mist eliminator inside its rated velocity so drift stays near the value you assumed, and move the makeup and recirculation flows through materials suited to the liquor. If you want the numbers checked against a specific configuration, send the completed input checklist — gas flow, inlet temperature and humidity, captured load, solids or TDS limit, and mist eliminator performance — and we will run the same balance against a packed bed scrubber design.
Two cautions before you file the result: keep the assumptions next to the answer so the next reader can retest them, and re-run the balance whenever gas flow, process temperature, or a product change moves the captured load — and with it your wet scrubber wastewater volume.
