High value of BOD (Biochemical Oxygen Demand) indicates that

High value of BOD indicates that water is highly polluted. BOD (biochemical oxygen demand) is the amount of oxygen required by microbes todecompose the organic matter present in the water sample. High BOD value indicates that the water sample contains a higher amount of organic matter, which requires a higher amount of oxygen to decompose,leading to a decrease in dissolved oxygen  in the water body. The decrease in dissolved
oxygen can result in the death of aquatic organisms and can be an indication of water pollution.
(1) A low BOD value indicates that there is less organic matter present in the water sample, which is a good sign indicating that the water is less polluted. 
(2) Microbes decompose but do not consume organic matter in water. High BOD does not necessarily indicate that the consumption of organic matter in the water is higher by the microbes. It depends on the availability of oxygen in water. Moreover, if consumption of organic matter in the water is higher by the microbes, it would lead to a decrease in the amount of organic matter left in the water, leading to a decrease in BOD.
(3) BOD is not a measure of water purity but a measure of organic pollution. Even a low BOD value indicates that there is less organic matter present in the water sample, but it does not necessarily mean that the water is pure.