Biomonitoring testing problems - why you can fail when no one common problem compound is above toxic range

Daphnia sp. commonly used in biomonitoring tests. From http://marmorkrebs.blogspot.com

Biomontoring is an expensive test that is complex to run when compared to most other effluent monitoring. Usually, tests involve a vertebrate (fish) species tested for survival and weight gain. The other component uses invertebrates that are evaluated for survival and reproduction. 

The most common offending compounds in biomonitoring tests include:

• Ammonia
• Nitrite
• Surfactants
• Phenol & other aromatic compounds
• PAH (Polycyclic aromatic hydrocarbons)
• Cyanide (CN) and derivatives
• Sulfides

What can cause failure when none of the above are near toxic ranges? There are two potential answers, and I have seen both cause testing failure.

• Combined compounds increase or sum up to toxicity 
  With no compound above critical range, you need to consider potential for a blend of toxic/inhibitory compounds having additive inhibition potentials. Often you see this in the chronic  toxicity portion of the test - weight gain or reproduction.
  
  
•  Soluble intermediate biological products
  This phenomenon happens when biological unit organisms partially degrade or transform influent compounds into more toxic intermediates. If the system does not have sufficient time for complete biodegradation, soluble intermediates can cause problems with biomonitoring test compliance.

Suggested tests - look at the following options

• Activated carbon post biological treatment
• Oxidation post biological treatment (peroxide, permanganate, etc)
• If you have multiple influent sources, do source stream toxicity testing. I personally use Aster Bio's Tox-Bac and Tox-N tests that take 30 min to 2 hours. Find which are most inhitory to bacteria including nitrifiers.
• Test what a longer biological contact time or lower F/M has on effluent testing - this is a more complex test, but may be necessary.