For circular clarifiers, Weir Length = π × Diameter
Please enter valid positive numbers for Flow Rate and Weir Length.
Weir Overflow Rate (Imperial):–
Weir Overflow Rate (Metric):–
function calculateWeirRate() {
// 1. Get Input Elements
var flowInput = document.getElementById("flowRate");
var flowUnit = document.getElementById("flowUnit").value;
var lengthInput = document.getElementById("weirLength");
var lengthUnit = document.getElementById("lengthUnit").value;
var errorDisplay = document.getElementById("errorDisplay");
var resultBox = document.getElementById("resultBox");
var resultImperial = document.getElementById("resultImperial");
var resultMetric = document.getElementById("resultMetric");
// 2. Parse Values
var flow = parseFloat(flowInput.value);
var length = parseFloat(lengthInput.value);
// 3. Validation
if (isNaN(flow) || isNaN(length) || flow <= 0 || length <= 0) {
errorDisplay.style.display = "block";
resultBox.style.display = "none";
return;
}
errorDisplay.style.display = "none";
// 4. Normalize inputs to Standard Units (GPD and Feet) for calculation
var flowGPD = 0;
var lengthFeet = 0;
// Convert Flow to GPD
if (flowUnit === "gpd") {
flowGPD = flow;
} else if (flowUnit === "mgd") {
flowGPD = flow * 1000000;
} else if (flowUnit === "m3d") {
flowGPD = flow * 264.172; // 1 m3 = 264.172 gallons
} else if (flowUnit === "lps") {
flowGPD = flow * 22824.465; // 1 L/s = ~22,824.465 GPD
}
// Convert Length to Feet
if (lengthUnit === "ft") {
lengthFeet = length;
} else if (lengthUnit === "m") {
lengthFeet = length * 3.28084;
}
// 5. Calculate WOR (Imperial: gpd/ft)
var worImperial = flowGPD / lengthFeet;
// 6. Calculate WOR (Metric: m3/m/day)
// Convert normalized GPD back to m3/day and Feet back to Meters
var flowM3D = flowGPD / 264.172;
var lengthMeters = lengthFeet / 3.28084;
var worMetric = flowM3D / lengthMeters;
// 7. Format and Display Results
resultImperial.innerHTML = worImperial.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 2}) + " gpd/ft";
resultMetric.innerHTML = worMetric.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 2}) + " m³/m/day";
resultBox.style.display = "block";
}
Understanding Weir Overflow Rate
The Weir Overflow Rate (WOR), also known as the Weir Loading Rate, is a critical hydraulic parameter used in the design and operation of wastewater treatment clarifiers and sedimentation tanks. It measures the volume of liquid flowing over a specific length of the weir per day.
Proper management of the weir overflow rate is essential to prevent the resuspension of settled solids. If the velocity of the water flowing over the weir is too high (indicated by a high WOR), it creates turbulence that can pull solids from the bottom of the tank or the sludge blanket up and out of the clarifier, leading to poor effluent quality.
The Formula
The calculation is straightforward. It divides the total daily flow by the total length of the weir:
WOR = Q / L
Where:
WOR = Weir Overflow Rate (e.g., gpd/ft or m³/m/day)
Q = Hydraulic Flow Rate (e.g., Gallons per day or m³/day)
L = Total Length of the Weir (e.g., Feet or Meters)
Calculating Weir Length
For rectangular tanks, the weir length is simply the total linear length of the overflow launder. However, for circular clarifiers, which are very common, the weir is typically located along the perimeter of the tank. In this case, the length is the circumference:
L = π × Diameter
Typical Design Guidelines
Regulatory agencies and engineering standards provide maximum weir overflow rates to ensure efficient settling. Values vary depending on the type of clarifier (primary vs. secondary) and the size of the plant.
Treatment Process
Typical Range (gpd/ft)
Typical Range (m³/m/day)
Primary Clarifiers
10,000 – 15,000
125 – 185
Secondary Clarifiers (Activated Sludge)
10,000 – 20,000
125 – 250
Small Plants (< 1 MGD)
< 10,000
< 125
Example Calculation
Imagine a wastewater treatment plant with a flow of 2.5 MGD (Million Gallons per Day). The plant utilizes a circular clarifier with a diameter of 60 feet.
Convert Flow: 2.5 MGD = 2,500,000 gpd.
Calculate Length: L = π × 60 ft ≈ 188.5 ft.
Calculate WOR: 2,500,000 gpd ÷ 188.5 ft ≈ 13,262 gpd/ft.
This result is within the typical range for secondary clarifiers, suggesting the design is adequate for hydraulic loading.
Why It Matters
Short-circuiting: Uneven weir levels can cause high localized overflow rates, leading to short-circuiting where water bypasses the settling zone.
Floc Shearing: Excessive velocities over the weir can shear delicate biological flocs, increasing the total suspended solids (TSS) in the effluent.