<s>[INST] context: Venturi meter in slow sand filter , is used to measure Bed resistance/ Loss of head. MUST KNOW Slow sand / Biological filters: Standard method in purifying large scales of water 1. Supernatant/ raw water Its level is always kept constant Promotes downward flow of water through sand bed, by maintaining constant head of water Provide waiting period of 3-12 hrs for raw water to undergo paial purification by sedimentation, oxidation & paicle agglomeration 2. Sand bed Most impoant pa. Sand grains should be rounded with effective diameter; free from clay & organic matter. In newly laid filter, this bed merely acts as mechanical stainer & not considered as biological. Soon its surface is covered by slimy, gelatinous substance containing threadlike algae, plankton, diatoms & bacteria k/a Schmutzdecke/ vital/ zoogleal or biological layer. Formation of vital layer is ripening of the filter & is the hea of filter It removes organic matter, holds back bacteria & oxidizes ammoniacal nitrogen into nitrates thus, yielding bacteria-free water 3. Under drainage system Contain porous/ perforated pipes. Provide outlet for filtered water & suppo filter medium. 4. Filter control valves Maintain constant rate of filtration Venturi meter is impoant component of regulatory system. When resistance builds up, operator opens regulating valve & maintains steady rate of filtration. When "loss of head" exceeds 1.3 metre it is uneconomical to run the filter. - Filter box contain supernatant water, sand bed & under-drainage system. [/INST] question: In Biological (Slow sand) filter, which of the following is measured by Venturimeter ? | options: A: Bed resistance B: Thickness of Schmutzdecke/vital layer C: Loss of heat energy D: Level of raw water | answer: A </s>