million US gallons per day
The million US gallons per day is the standard capacity unit for municipal water and wastewater treatment plants in North America. It equals about 0.0438126 cubic metres per second, or about 694.4 US gallons per minute; the ratio is exact as a fraction since the gallon and the day are exactly defined, though the decimal repeats and is rounded here. A plant serving a small city might be rated at 5 to 20 MGD.
Watch out: MGD in the United States means million US gallons per day, but the same abbreviation in British-influenced practice has meant million imperial gallons per day, a 20 percent larger figure. Plant ratings are also quoted as average day, maximum day and peak hour flows, which for the same facility can differ by a factor of two or more.
| 1 MGD | 2,628.7582 L/min |
Municipal engineering in the United States adopted the million gallons per day because plant capacities land on convenient single-digit numbers in it, in the same way that river flows land conveniently in cubic feet per second.
About the million us gallons per day
Municipal water systems are sized in millions of gallons per day because that is the scale at which a city consumes. A rough North American planning figure is 100 to 150 gallons per person per day for all residential, commercial and institutional use combined, so a 10 MGD plant serves on the order of 70,000 to 100,000 people. The metric equivalents worth carrying are that 1 MGD is about 43.8 L/s, about 3785 m³/day, and about 694 gpm.
The number that appears on a plant nameplate is usually the maximum day capacity, and that is not the number the plant runs at. Demand varies through the day, peaking morning and evening, and through the year, peaking in summer where lawn irrigation is common. Typical planning ratios put maximum day at 1.5 to 2 times average day, and peak hour at 2 to 4 times average day, which is why storage reservoirs exist: they let the treatment plant run near a steady average while the distribution system meets the peaks. Wastewater plants face the mirror problem with wet weather inflow, where a single storm can push flow to several times the dry weather average.