Select the incorrect statement$(s)$ from the following.

• A
  It is difficult to remove solid particles from polluted water.
• B
  It is easy to remove dissolved salts such as nitrates,phosphates,other nutrients,toxic metal ions,and organic compounds from polluted water.
• C
  Domestic sewage primarily contains biodegradable inorganic substances,which do not decompose easily.
• D
  All of the above.