Abstract: We describe a floating respirometer, in particular for monitoring an activated sludge vessel of a sewage treatment plant. The device comprises: a buoyancy device to allow the respirometer to float in an aqueous liquid; a respirometer chamber, supported by the buoyancy device and arranged such that, when the respirometer is floating in said aqueous liquid, said chamber is partially filled with said aqueous liquid and defines an enclosed head space above said aqueous liquid; and a gas sensor in gaseous communication with said head space.

Abstract: We describe a method of closed-loop control of a waste water treatment plant, the method comprising: obtaining a fluid sample from a fluid of said plant; providing said fluid sample to a sealed chamber such that said fluid sample incompletely fills said sealed chamber leaving a headspace; incubating said fluid sample in said sealed chamber; determining a change in pressure in said headspace during said incubating; and controlling a degree of aeration of said waste water treatment plant responsive to said change in pressure. We also describe a method of measuring one or both of the food content and the biomass content of a fluid of a waste water treatment plant, the method comprising determining a value for one or both of food content and biomass content from a change in pres sure.