We had a hog lagoon that we hired a floating agitator a couple of years.  The people that pumped our lagoon also had and used a big agitator every year.  We closed the lagoon 4 years ago, and we had 3 feet of solids on the bottom that we hired a dozer and payloader in January to clean out. They loaded on side dumps. A lot of loads of very high phosphorus solids.  We stocked piled for 2 years and it really fermented and became like dry compost.  We spread on the areas of our fields that needed it.  So very hard to get that bottom into  suspension.  You could hire a long reach excavator to dig out the solids after you plump all that you can out to get your capacity of your lagoon back.  There is a lot of value in the laying in the bottom of your lagoon.  It would be well worth the money to dig it out.