At those times they did not have rotating drum composts, which process the manure in 7 to 10 days to soil.When horse manure posed a public health hazard: https://cityroom.blogs.nytimes.com/2008/06/09/when-horses-posed-a-public-health-hazard/
For today, roughly estimation would then be that 4000 tonnes of 50/50% manure/dryer composition garden soil could be made in day. Translates to roughly 200,000$ on sales so collection work cost might be bit limiting on the profits. Maybe doable with a electric pavement sweeper with overhead lines.
Or better yet, send it down the sewage pipes to treatment plant where the composting could be easily centralized.
If it fails to make profit, impose "Oh ****" tax for horse owners to cover the difference.
P.S. Extra bonuses: When dried it can be burned in CHP plant! If not dried it can be turned into Biogas and then burned in CHP plant.