“Biblical scholars have difficulty identifying the bittern. … The bittern [in this case] is a symbol of a desolate and forsaken land, of an area destroyed by the wrath of the Lord” (McConkie and Parry, Guide to Scriptural Symbols, 21).
A besom is a broom (see Isaiah 14:23nb). “Just as one sweeps his or her house to eliminate dust and dirt, so will God sweep Babylon of her foul matter so that none remains” (Parry et al., Understanding Isaiah, 152).