The amount of oxygen utilised by microorganisms present in the pond to metabolize organic solids (food) into biological/cellular material (Biomass). This is called Biological Oxygen Demand and is related to the quantity of organics available to the active organisms in the pond. This BOD is determined by the type/characteristics of the wastewater influent to the pond.If there is a high content of organic solids I.e biodegradable material like human waste the BOD will be high and vice versa.Also BOD is normally in mg/L units.