Yep,It's bumble foot and you should get some attention to it right away.
Will be a lot of work but if you follow Amiga's advice you'll get through it.
Keep on it...soak clean,wrap,change soak clean.
The big bumbles do require surgery..which sounds insane to most but is not that bad and your duck can be saved...just be organized have everything by your side-no distractions.....work steadily..allow for breaks for water for the duck (and you)ducks tend to get hot when they are worked on-it is stressful even if they appear totally calm..keep clean etc etc
Your bond will be greater after all is healed...i did this one time and this hen is so trusting and friendly now..as if she knows I helped her somehow grateful.It was scary at first...but i was determined to help...and was so proud of myself for doing this..it's truly amazing what a person will do for an animal sometimes!
You have to get the nastiness out or this infection will progress and lameness will be the start of other many other issues...like lack of eating drinking and being outed by the flock. All the best.
epson soaks,antiseptic (bactine or even better is Dettol solution for cleaning during surgery) and iodine or polysporin in gauze wraps during healing. I gauzed her bumble woulds heavily with polysporin or iodine and wrapped with that sport wrap (sticks to itself. Wrap just as the duck would stand on a normal foot...toes spread flat...not squished together. They pull at it ...so tape with bandage tape with the edge at the back or wrap around a few times...if you see their feet are not staying flat you can cut a square or circle of cardboard and place it after gauze to stiffen shape before sport bandage wrap goes on. Lots of rest---- greeeeeens! high protein duck feed and clean water and duck gets alone alone time...other ducks will bother the injured and wear them out...isolate and visit often. they will walk funny at first but then get used to it. do not put bandaged ducks back outside or in messy conditions. And for the others...time to make sure they are on dry ground at least during nightime hours...ducks can take wet of course but not 24hours...their winter callusses swell and crack and bacteria get in and voila..bumble foot...most often seen in the spring...when we all struggle to keep things tidy (especially here in Ontario Canada) I've almost perfected my duck management (pat on back

...they are alot of work! Ducks should never live in mud...at least put strw down...layered..and it can be hosed off...so its' wet but at least cleaner than shavings or mud.
check and clean those healing feet at least 2 times a day after surgery..may need a couple hours of bumble removal.... preferably all in the same afternoon so the worst is done all at once and then the bird can rest up,be spoiled and heal up...at the same time you are cleaning and changing bandages all the time. tough to say 3 weeks...once scabs form...clean gently and use leess polysporin or iodine and start to allow for more air time to dry out a bit beetween wraps on dry towels...once the scabs drop...you're golden. best of luck,Tara
I wish dducks pooed waaay less...thank goodness they are so cute and entertaining!