cell density (growth), cell viabili

cell density (growth), cell viability, floc morphology, and enzymatic
activity including adenosine triphosphate (ATP) and dehydrogenase.
In addition to the assays with activated sludge or anaerobic
sludge as the target, toxicity can also be estimated using indicator
organisms. This is considered an indirect measurement because the
indicator organisms are exogenous to the wastewater community.
The bioluminescence assay is the most well-known of such methods,
and recently biosensors based on enzymes or whole bacterial
cells have been studied extensively. Sometimes a biosensor based
on a large scale full performance reactor has also been used [20];
using this type of biosensor is a fairly good mimic of a direct measurement,
but there will always be some difference between the
microbial community in reactors and the one in the biosensors. For
this reason, they will still be considered as indirect methods in this
paper. In the following two sections, we shall further examine each
assay in greater detail.