Environmental Health & Safety Compliance Blog

Renewing Wastewater Discharge Permits – Calculating Water Use

Posted by Rebecca McDaniel on Jun 17, 2010 10:57:00 AM

By Sandy Perry, Director of Water Management and Wastewater Services

If you have a wastewater treatment system then you probably (hopefully) have a discharge permit, for which you must have submitted an application to one regulatory agency or another. I’ve found that the section of the application that seems to be the most difficult and takes the most time is where you must list the quantities of wastewater generated from each potential source. This is primarily because few facilities have flow meters on all water use sources.

Ideally, you want to be able to account for the entire volume of water used in a twelve month period. Identifying and quantifying all water uses and the outputs of this water to match the incoming water is typically a challenge. Why? Because it is difficult at best to estimate the amounts evaporated, the boiler blow down quantity, lab sink contributions, or discharges from scrubbers, production operations where some of the water may be consumed in the product or wastes collected for offsite disposal, non-contact cooling (which is clean water and should not go to sewer discharge), RODI reject water, etc.

The best plan is to gather together key people from Facilities, Production, Research, and EH&S to assist in identifying all of the inputs. Piping diagrams are great, but I’ve found that few are kept up to date with piping changes made over the years. Use a white board to begin creating a water mass balance flow diagram and label each box with each water use activity and all of the outputs from each water use source. So for instance, if you have incoming water to your steam boilers, there should be at least two arrows from that box labeled as outputs – evaporation and boiler blow down.

Comparing the total water consumed to the either estimated or metered outputs, you may find that it doesn’t add up. Then you need to review your estimates – what are you missing? What have you double counted? Are your flow meters calibrated? How reliable are they? You may determine the need for additional meters and daily or weekly water use logs. Most regulatory agencies think this is a pretty important aspect of your permit so it’s a good idea to work towards a no more than 1-5% differential between incoming and outgoing water sources. Good luck!

Tags: Wastewater Treatment