Sensing methods and systems

A system for determining a food-to-biomass ratio in an aqueous fluid, the system comprising:

• a sensor to determine an amount of food in said aqueous fluid available to said biomass;
• cell polarisability measuring apparatus to determine a measure of viable biomass in said aqueous fluid by measuring polarisability of viable biomass cells in an AC electric field;
• and
• a system to determine a food-to-biomass ratio from said amount of food and said measure of viable biomass.

A system for differentiating between, or providing a selective response to, different types or classes of viable biomass in activated sludge (AS), the system comprising:

• a probe for electrically probing said activated sludge (AS);
• and
• cell polarisability measuring apparatus, couplable to said probe, to determine a measure of viable biomass in said aqueous fluid by measuring polarisability of viable biomass cells in an AC electric field;
• wherein said apparatus is configured to differentiate between said different types or classes of viable biomass organisms by operating at one or more selected frequencies or frequency ranges of said AC electric field.