<prompt> In a slow sand filter, the element responsible for yielding bacteria free water is theRisk of Guinea worm infectionWater is more than depth of 15 meterWhere no water source in a distance of 1.6 km from communityThere is excess of Na, K, F salts  <response>: Level of hardness is expressed in terms of milliequivalents per litre where one milliequivalent is equal to 50mg calcium carbonate in one litre of water.The level of hardness of hard water is equal to 3-6mEqL or 150-300 mgL. 