Connecticut Department of Energy & Environmental Protection

Industrial Wastewater Compliance Assistance

Flow Proportioned Sample Collection Procedures

The Regulations require flow proportioning for all daily composite samples. See RCSA Section 22a-430-3(a)(3) for more information. This requirement is often a major area of non-compliance for permittees. You can call 860-424-3018 for questions and assistance regarding sampling procedures.

Permittees that have a constant discharge rate as demonstrated by recorded flow data can collect and combine equal volume aliquot samples taken at equal intervals over a full operating day.

Permittees that have a fluctuating discharge rate, however, must collect a series of aliquot samples over a full operating day and combine them proportionally to the flow rate at the time each aliquot is collected or by the total flow discharged between each hourly aliquot.

Sample Collection Methods

Listed below are four widely used methods for collecting flow proportioned samples. (Please remember that prior to using any of the following methods, preliminary calculations should be performed to ensure that an adequate final sample volume will be obtained.)

Constant Aliquot Sample Volume
Proposed Reissuance with Modifications of the Gen

Constant aliquot sample volume, variable time interval (maximum allowable time between each aliquot is one hour unless otherwise approved by the commissioner), samples collected based on flow rate. When using this method you need to know four things:

Example of Constant Aliquot Sample Volume
 Minimum Daily Flow  22,000 gallons
 Hours of Discharge  10
 Required Sample Volume  10,000 ml
10,000 ml divided by 22,000 gallons equals 0.45ml/gallon (round 0.5)
If sample period is set at 1,000 gallon intervals 0.5ml/gal x 1,000 gal equals 500 ml aliquot sample volume

After each 1000 gallons of discharge, collect a 500 ml aliquot sample. This will give you the required sample volume plus a safety of 1000 ml. (Always use a safety factor to guarantee that enough samples are collected.)

This method can be used for manual collection or collection by an automatic sampler that is interfaced with a flow meter.

Variable Aliquot Sample Volume

Variable aliquot sample volume, time interval between samples remains constant (maximum allowable time between each aliquot is one hour unless otherwise approved by the Department), based on the total flow between aliquot sample collection. When using this method you need to know four things:

Example of Variable Aliquot Sample Volume
 Minimum Daily Flow  22,000 gallons
 Required Sample Volume  10,000 ml
10,000 ml divided by 22,000 gallons equals 0.45ml/gallon (use 0.5)
Constant Time Interval

Constant time interval, variable aliquot sample volume based on instantaneous flow at the time of aliquot sample collection or the total flow per hour. When using this type of sampling you need to know the following items:

  1. Range of instantaneous flow
  2. Average instantaneous flow based on historical data (or total hourly flow)
  3. Sample volume required by the laboratory performing the analysis
  4. Number of aliquot samples to be taken (at least one per hour)
  5. Sample of volume required divided by the number of aliquots
  6. Factor for flow proportioning (number 5 above divided by 2 above)
Example:
  1. Range of instantaneous flow: 10 gpm to 100 gpm 
  2. Average instantaneous flow: 34 gpm 
  3. Sample volume required: 4000 ml  
  4. Number of aliquots: 5    
  5. Volume required divided by the number of samples: 4000 ml/5 samples=800 ml 
  6. Factor: 800 ml/34 gpm = 23.5 ml/gpm (round to 24)
Example of Constant Time Interval
Time
Flow Rate, gpm
Avg. Instantaneous Flow Rate, gpm
Volume Required = Sample Volume/Number of Aliquots
Factor =
Volume Required/Avg. Instantaneous Flow Rate
Aliquot
Volume, ml = Factor * Flow Rate
07:00
25
34
800
24
600
08:00
100
34
800
24
2400
09:00
10
34
800
24
240
10:00
15
34
800
24
360
11:00
20
34
800
24
480
Total Volume=4080 ml
Continuous Composite Sample

This type of composite sample is collected by having a continuous drip through a small tap valve that flows only while a discharge is occurring. Flow rate is controlled by pressure in the discharge line from either pumping or gravity. The samples generated tend to be larger than needed. When properly installed, this method requires very little manual effort.

Learn more about common violations.

How to Maintain Compliance with Your Individual Discharge Permit

Content last updated April 2020