Purportedly, his figures are based on the records of Frontinus, but I don't possibly see how that could be: Grimal's cumulative total of the nine aqueducts for which Frontinus supplied measurements is fully double the total of other authors, including Hodge's figures in his collaborative work with Blackman. Almost all estimates are based on the figured supplied by Frontinus, and tend to be in the range of 12,000 to 15,000 quinariae. Grimal is hovering around 25,000.
The generally accepted conversion from a quinaria to some unit of volume over time appears to be about 40 m³/day. As always, estimates vary; some seem to have a more conservative figure of 20 m³/day, and I saw a few estimates above 40 as well. To get a sense of what 40 cubic meters of water actually looks like, think of it as being a cube of water ~3.41 meters on all sides, or about ~10,000 gallons of water. A swimming pool 24 feet round and four feet deep has about 12,000 gallons in it.
Knowing this, Grimal's total for the water supply of Rome is 24,805 quinaria per day, which Hodge seems to happily accept and convert to cubic meters using the 40 m³ estimate: 992,200 m³ per day. To get a sense of how much water this is, this would be a cube of water 99.74 meters on all three sides. This is 262 million gallons of water. Per day. Assuming Rome contained a million inhabitants (which seems to be the generally accepted estimate), this means that the water supplied 262 gallons of water for each person in Rome, per day.
Personally, I find Grimal's figures wildly suspect. For starters, Grimal arrives at totals double of other estimates. An "alternative estimate," according to Hodge, was H. Fahlbusch's proposed range of 520,000 to 635,000 m³. Other estimates seem to fall in this range as well.
Second, looking at what typical US cities used, it seems really unlikely to me. In a paper by Morgan, he gives the following table for U.S. cities in 1900:
...assuming the above figures are correct (they seem reasonable, but I haven't verified them myself), Hodge/Grimal are stating that Rome supplied more water per person than most major cities in the U.S. did by the start of the 20th century. I find that a very hard pill to swallow. I'm not alone on this: in Morgan's paper, his estimates are based on Frontinus' figures, and he arrives at a more typical ~14,000 quinaria, which using the 40 m³ conversion would end up being 560,000 m³ of water per day. However, Morgan settles on a more conservative figure of 6000 gallons/day per quinaria (22.7 m³/day), which makes his estimate 318,000 m³ per day. This is fully one-third of Hodge/Grimal's estimates.
Worth note is that when the Aqua Traiana and Aqua Alexandrina are added in (which Grimal estimates at 113,920 m³ and 21,160 m³ per day, respectively, although one what basis these estimates were made is unknown), it pushes the Rome total to 1.127 million m³ per day. Considering most other authors don't even bother supplying figures for the Traiana and Alexandrina, I wonder A) where Grimal got this information, and B) on the basis of what estimations. It certainly wasn't Frontinus' estimates.
What do modern aqueducts supply? The Catskill Aqueduct, which begins at the Ashokan Reservoir in Olivebridge, New York, in Ulster County and runs 120 miles to New York City, has an operational capacity of about 580 million gallons (219,240 m³) per day. It typically carries less than this, with flows averaging around 350 - 400 million gallons (132,200 - 151, 200 m³) per day.
Another example--the Colorado River Aqueduct--runs 242 miles from the Arizona-California border. Its capacity is 1.3 million acre-feet (1.6 km³) per year, or 1.6 billion cubic meters per year, or ~4.4 million cubic meters per day. Average flow is probably less than this, but it's a staggering number to think about. The construction project lasted eight years, and employed a total of 30,000 people.
I won't even list the figures for the California Aqueduct. It'd make your head explode. :)
Perhaps the most appropriate comparison, however, is with another Roman Aqueduct: Pont Du Gard. The water conduit, which is 1.8 meters (6 feet) high and 1.2 meters (4 feet) wide and has an average gradient of 0.4 percent, supplied the Roman city of Nemausus (now Nîmes) with an estimated 20,000 cubic meters of water per day (note: this is an uncited figure from wikipedia; I'll have to look into how that number was arrived at). This is also 500 quinaria, using the typical 40 cubic meter conversion rate.
Using the cube analogy, Pont Du Gard supplied Nemausus with a daily 27.1 meter cube, or 5.3 million gallons a day. Estimates for the population of Nemausus were around 50,000 inhabitants, so that would be roughly 106 gallons per person, per day, which is a pretty far cry from Hodge/Grimal's estimate of 262 gallons person/day in Rome.