Rinse peas, remove bad ones and soak in 2L water with salt for 12 hours.Bring the peas with the soaking water to the boil and simmer for about 2 hours until tender and cooked. Add the bacon and pork trotter after 1 hour.After the 2 hours, take out the meat and pour of the cooking liquid. Use a masher to mash the peas, I prefer them fine but you can also leave some chunks.Pour the cooking liquid back and put the meat back in. Also add in the potatoes, celeriac, leeks and celery.Season with salt and pepper, and add some more water if the soup is too thick.While stirring, bring soup back to a simmer and cook until vegetables are tender.Meanwhile heat the smoked sausage (follow instructions on packaging) and slice.Take out the bacon and slice. The sausage and bacon can be served on the side, or can be put back in the soup.Take out the pork trotter, serve on the side, or slice off the meat and put back in the soup.If preparing in advance: refrigerate everything and prepare smoked sausage when serving.Serve in big bowls with slices of rye bread, that can be topped with the bacon or the sausage, and some mustard.