Sedimenting echoes in urinary bladder:

Sediment producing echogenic debris in the urinary bladder can be caused by a number of factors.
Excessive amounts of Phosphate crystals in the urine is one cause. It can also be due to pyuria or pyogenic material in the urine secondary to urinary tract infection; sometimes it may be caused by hematuria (blood in urine) or chyluria following filariasis. Other causes include uricosuria (uric acid excess) or increase in oxalate crystals. This ultrasound image shows debris in the distended urinary bladder, gravitating to the dependent part.
This ultrasound image is courtesy of Dr. Ravi Kadasne, UAE.
For more sonographic images of the urinary bladder visit: http://www.ultrasound-images.com/urinary-bladder.htm
Case-2:
Here is a nice real time B-mode ultrasound video clip showing freely mobile particles in the urinary bladder:

                                                                                  

This patient has active urinary tract infection with cystitis. The particulate matter in the urinary bladder are the result of debris- both pyogenic material and phosphate crystals floating within the urine.

This is a still image of the same case (as the video above):                                                                                

 Fine particles are seen throughout the urine in the bladder.