This has arisen for a number of reasons. S US and S UP are soluble and particulate, respectively, while S BS and X S are readily and slowly biodegradable, respectively. However, the slowly biodegradable material is both colloidal and particulate, and not only particulate. When colloidal slowly biodegradable material is combined with activated sludge the colloidal matter likely is adsorbed onto the flocc rapidly, and removed from the liquid phase. Therefore, within the process the fate of the colloidal matter essentially is associated with the particulate phase.
Analyses conducted on influent wastewater samples often involve different filtration techniques.
Such a filter medium may well retain solids, but will allow a large portion of the colloidal matter to pass through the filter. In other instances the filtration may use a 0.45 micron filter. In this case only a portion of the colloidal matter may pass through the filter. In each case the same amount of readily biodegradable and unbiodegradable soluble COD should be found in the filtrate, but the amount of colloidal matter likely will differ. The comments above are best illustrated by an example.