RO System Calculators: TDS Rejection, Flow Rate & Membrane Life | AMPAC USA

function calcMembrane() {
var source = document.getElementById(‘mem_source’).value;
var carbon = document.getElementById(‘mem_carbon’).value;
var flush = document.getElementById(‘mem_flush’).value;
var count = parseInt(document.getElementById(‘mem_count’).value);
var cost = parseFloat(document.getElementById(‘mem_cost’).value);

if (!count || count <= 0) { alert('Enter number of membranes.'); return; }
if (!cost || cost <= 0) { alert('Enter cost per membrane.'); return; }

// Base life by source water
var baseLife = {
'muni': 5.0,
'well_soft': 6.0,
'well_hard': 3.5,
'well_iron': 1.5,
'surface': 3.0,
'industrial': 2.0
}[source];

// Carbon filter maintenance modifier
var carbonMod = { 'good': 0, 'late': -0.75, 'none': -1.5 }[carbon];

// Flushing modifier
var flushMod = { 'auto': 0.5, 'manual': 0, 'none': -0.5 }[flush];

var estLife = Math.max(0.5, baseLife + carbonMod + flushMod);
estLife = Math.round(estLife * 10) / 10;

var annualCost = (count * cost) / estLife;
var tenYrCost = annualCost * 10;

document.getElementById('res_mem_life').textContent = estLife.toFixed(1);
document.getElementById('res_mem_annual').textContent = '$' + Math.round(annualCost).toLocaleString();
document.getElementById('res_mem_10yr').textContent = '$' + Math.round(tenYrCost).toLocaleString();

var notes = [];
if (source === 'well_iron') notes.push('⚠ Iron above 0.05 ppm will foul TFC membranes rapidly. An iron pre-filter is required — membrane life estimate assumes it is eventually installed. Without it, membranes may fail within months.‘);
if (source === ‘well_hard’) notes.push(‘⚠ Hard well water without a softener accelerates calcium carbonate scaling on the membrane. Adding a softener pre-stage could extend membrane life to 5–6 years and reduce 10-year cost by 40–50%.‘);
if (carbon === ‘none’) notes.push(‘⚠ Failure to replace carbon pre-filters allows chlorine or chloramines to reach the TFC membrane, causing irreversible oxidative damage. This is one of the most preventable causes of early membrane failure.‘);
if (flush === ‘none’) notes.push(‘⚠ Without periodic flushing, concentrate sits on the membrane surface during standby, accelerating biofouling and scaling. Auto-flush timers cost $50–200 and can add 1–2 years to membrane life.‘);
if (estLife = 5.5) notes.push(‘✓ Good maintenance practices — expected membrane life is above average for commercial applications.‘);

document.getElementById(‘mem_note’).innerHTML = notes.join(‘

‘);
document.getElementById(‘mem_result’).classList.add(‘show’);
}

About These Calculators

These calculators use industry-standard formulas and FILMTEC™ membrane performance data. Results are estimates based on standard operating assumptions:

• TDS rejection calculator: Assumes new membrane at rated rejection; single-pass RO; 75% system recovery for concentrate TDS estimate.
• Flow rate converter: Temperature derating uses the standard TCF (Temperature Correction Factor) approximation of ~3% output change per degree Fahrenheit from 77°F. Actual TCF varies by membrane model.
• Membrane life estimator: Based on typical commercial installation data. Actual life depends on specific water chemistry, operating pressure, and maintenance program — a formal FILMTEC™ system design using the ROSA software tool provides more precise projections for critical applications.

Need a system sized for your specific application? These calculators give you ballpark numbers — a complete system specification requires a water test and engineering review. Contact AMPAC USA with your daily water demand, feed water source, and target permeate quality. We provide a complete system specification and quote within one business day.

Related: Commercial RO Buyer’s Guide | RO Maintenance Guide | Commercial RO Systems

Related Articles

• RO Recovery Rate Explained: What It Is, What Limits It, and How to Optimize | AMPAC USA
• RO Membrane Types: BW30, SW30, High-Rejection & Fouling-Resistant | AMPAC USA
• AMPAC USA Commercial RO System Specifications: APRO300 to APRO6000 | AMPAC USA
• How to Choose a Commercial RO System: Complete Buyer's Guide | AMPAC USA

Frequently Asked Questions

What does the RO Permeate TDS Estimator calculate?
It estimates the TDS of RO product water based on your feed water TDS and the membrane rejection rate, helping verify whether a proposed system will meet your target permeate quality before purchase.

Do I need to log in to use AMPAC’s RO calculators?
No. The calculators are free to use directly on the page with no login required.

How accurate are membrane rejection rate estimates for aged membranes?
The calculator includes rejection rate presets for aged membranes at 2-3 years (98.5 percent) and 4-5 years (97.0 percent), reflecting typical performance decline, though actual rejection depends on your specific feed water and maintenance history.