What does MGD mean?
MGD: Million gallons per day
In short
MGD stands for million gallons per day, the unit used in the United States for the flow a water or wastewater treatment plant handles. 1 MGD equals about 694 gallons per minute, 1.547 cubic feet per second or 3,785 cubic metres per day, which is roughly the wastewater of 10,000 people.
The 11,774 publicly owned plants Klarifi tracks treat a combined 15,264 MGD for about 145 million residents. A small-town lagoon may treat 0.1 MGD, while the largest US plants treat several hundred.
Converting MGD
1 MGD is 694.4 gallons per minute (gpm), 1.547 cubic feet per second (cfs), 3,785 cubic metres per day or 43.8 litres per second. Going the other way, 1 cfs is 0.646 MGD and 1,000 cubic metres per day is 0.264 MGD. Pumps are usually rated in gpm and rivers in cfs, which is why permits and design reports switch between the units.
Pollutant loads follow from flow: pounds per day equals flow in MGD times concentration in mg/L times 8.34. A 10 MGD plant discharging 10 mg/L of suspended solids releases 834 pounds a day.
Design flow, average flow and peak flow
A plant has several flows, and they are easy to confuse. Design flow is the capacity the plant was built and permitted for, usually as an annual average or a maximum-month figure. Average flow is what it actually treated over a period, as reported on its discharge monitoring reports. Peak hourly flow during storms can be several times the average where the sewers let in groundwater and rain.
The ratio of average to design flow shows how much headroom a plant has left. Several states require a utility to start planning an expansion well before the plant is full; Texas, for example, requires planning at 75 percent of permitted flow and construction at 90 percent.
How big is a 1 MGD plant?
At the common planning value of 100 gallons per person per day, 1 MGD serves about 10,000 people. The EPA classes a municipal plant with a design flow of 1 MGD or more as a major discharger, which brings closer permit and compliance oversight. Klarifi shows average and design flow in MGD on every facility page and ranks the largest plants nationally and by state.