The most common offending compounds in biomonitoring tests include:
- Phenol & other aromatic compounds
- PAH (Polycyclic aromatic hydrocarbons)
- Cyanide (CN) and derivatives
- 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.