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Post-treatment calculator

UV Disinfection Sizing & Chlorination Contact-Time Calculator

Use two separate preliminary design calculators for MBR permeate disinfection: a vendor-validated UV-reactor sizing calculation and a chlorination CT/contact-basin mass balance.

Preliminary design tool only. Do not use illustrative UV intensity × time arithmetic to claim a validated dose or pathogen-log credit. Do not use a generic CT target to establish chlorination compliance. Select UV reactor duty points, CT targets, chlorine residuals, and operating envelopes from the applicable permit, water/reuse objective, site testing, and validated equipment documentation.
Used only to label chlorine-product cost outputs; no currency conversion is applied.
1

UV Disinfection Sizing

Size duty and standby reactors at a specified vendor-validated capacity, target dose, and minimum UV transmittance.

UV sizing inputs

The validated capacity must be from the selected reactor at the entered dose and UVT condition.

Flow and design envelope
m³/d annual-average permeate flow
peak ÷ average flow
mJ/cm²; set by target/reuse basis
% UVT at 254 nm; use design minimum
Validated reactor duty point
m³/h at entered UVT and dose
installed N+standby arrangement
kW; vendor input for preliminary energy estimate
days/year
Illustrative fluence arithmetic only
mW/cm²; not a validation input
seconds; not a validation input
% multiplier for the illustration only; validated manufacturer curves govern design.

UV sizing results

Primary capacity results use the vendor-validated reactor duty point.

Peak design flow
62.5m³/h
1,500 m³/d
Duty reactors
2units
60.0 m³/h each
Installed reactors
3units
2 duty + 1 standby
Duty train capacity
120.0m³/h
52.1% peak utilization
Minimum UVT documented
65.0%
Entered design condition
Target validated dose
40.0mJ/cm²
Permit/reuse basis input
Illustrative fluence
43.2mJ/cm²
Not a validation result
Illustrative dose margin
3.2mJ/cm²
Arithmetic screen passes
Preliminary electrical energy
52,560kWh/y
2 duty × 3.0 kW
UV design boundary: The illustrative arithmetic is above the entered target dose, but it is not a validation result.
2

Chlorination Contact-Time & Chemical-Dosing Screen

Screen chlorine product demand and the contact basin using peak flow, outlet residual, and a transparent T10/baffling-factor approach.

Chlorination and CT inputs

Use the outlet residual and a permit- or target-specific CT requirement; do not substitute the default screening values for compliance.

Flow, dose, and CT target
m³/d; used for average chemical demand
used for contact-time screen
mg/L as available Cl₂; site-test input
mg/L as available Cl₂ at basin outlet
mg·min/L; set by permit/pathogen/pH/temperature basis
Contact basin hydraulics
m³ at minimum operating level
use tracer-tested factor when available
Commercial chlorine product
% active by mass; editable product input
kg/L; use supplied data sheet
days/year
days of average product demand
percent above coverage demand
currency/kg commercial product

Chlorination contact-time results

The contact-time test uses peak hydraulic flow and the selected outlet residual.

Total chlorine dose
5.00mg/L
4.00 demand + 1.00 residual
Active chlorine demand
5.00kg/d
Average flow basis
Product volume
33.3L/d
12.5% available chlorine
Peak product pump screen
2.08L/h
Continuous peak-flow equivalent
Theoretical detention time
96.0min
100.0 m³ at peak flow
Effective T10
48.0min
T × 0.50 baffling factor
CT achieved
48.0mg·min/L
At/above screening target
Required contact volume
62.5m³
30.0 mg·min/L target CT
Contact-volume margin
37.5m³
Available − required
Nominal product storage
1,150L
30 days + 15%
Annual product volume
12,167L/y
365 operating days
Annual product cost
USD 14,600
Commercial product only
CT screening result: The entered contact volume, peak flow, residual, and baffling factor meet the entered screening CT target.

Governing formulas

UV validated-reactor sizing

Peak flow = Average permeate flow × Peak-flow factor
Duty reactors = ceiling(Peak flow, m³/h ÷ Validated capacity per reactor)
Installed reactors = Duty reactors + Standby reactors
Illustrative fluence = Effective intensity × Exposure time × Availability factor

The illustrative fluence is shown only to make the intensity/time relationship transparent. Reactor capacity is sized from the entered validated operating point; a simple intensity × time estimate cannot replace reactor validation.

Chlorination dose, T10, and CT

Chlorine dose = Chlorine demand + Outlet residual
Theoretical detention time, T = Contact volume ÷ Peak flow
T10 = T × Baffling factor
CT achieved = Outlet residual × T10
Required contact volume = Peak flow × [Target CT ÷ Outlet residual ÷ Baffling factor]

Daily product demand uses average flow; the displayed peak product flow is a continuous equivalent pump-rate screen. Confirm actual chemical-feed turndown, residual decay, mixing, and dosing control with plant data and the supplier.

UV and chlorination calculation basis

Documentation revision: 2026-10-02.

Decision-use boundary: Defaults are illustrative examples, not recommended design or compliance values. UV credit requires the selected OEM validation curve/certificate at the entered flow, dose and minimum UVT, with lamp ageing, fouling, sensor calibration, hydraulics, alarms and standby conditions documented. CT is screening only: confirm disinfectant species, organism/log target, pH, temperature, residual decay, contact segments, tracer/T10 evidence and the applicable authority's tables.

Unit basis: 1 mW/cm² × 1 second = 1 mJ/cm²; the fluence display is arithmetic only, not validated RED or pathogen credit. For chemical demand, mg/L × m³/day ÷ 1,000 = kg/day; available-chlorine percentage is a mass fraction and product litres require verified density and assay.

This page documents the calculation basis so an engineer can trace the inputs, units, assumptions, and review needs. Results are preliminary screening estimates; they are not a permit determination, OEM guarantee, or construction/procurement approval.

Method and source basis

UV duty and standby counts use peak flow divided by vendor-validated reactor capacity at the entered dose and UVT. The intensity-time fluence is deliberately illustrative. Chlorination uses dose = demand + outlet residual, CT = residual multiplied by T10, and T10 = theoretical detention time multiplied by the entered baffling factor.

The sources below provide technical context or analytical/design methodology. They do not endorse this calculator's defaults, and any jurisdiction-specific requirement applies only where that authority has jurisdiction.

  1. U.S. EPA, Ultraviolet Disinfection Guidance Manual (2006)
  2. U.S. EPA, Wastewater Technology Fact Sheet: Ultraviolet Disinfection
  3. U.S. EPA, Disinfection Profiling and Benchmarking: Technical Guidance
  4. Pennsylvania DEP, Tracer Studies and Contact Time (jurisdictional example)

Assumptions, limitations, and review actions

Professional review checkpoint

Before using the result for specification or operation, reconcile it with representative site data, the applicable permit and design standard, the selected equipment or membrane supplier's current data, and a qualified wastewater/process engineer. Record the input basis, date, units, and any pilot, bench-test, or comparable full-scale evidence used to select the assumptions.