1 III. Water Treatment Technologies Topic. III. 5. Micro-mesh Screens. Filters: Kinds, Constructions and Operation Micro-mesh Screens I. Purpose §For plankton removal §For colour removal II. Efficiency restriction §Phytoplankton content - up to 10 6 cells/l §Duration of high plankton concentration - more than 1 month §Removal of l % of diatomite algae l % of blue-green algae l 100 % of zooplankton l % of suspended solids

2 Micro-mesh Screens

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4 Filters Kinds: I. According to the filtration rate §Low rate - 0,1 - 0,3 m/h §High rate (15) m/h §Super high rate - 50 m/h II. According to the filter layers §One-layer §Two-layers §Multy-layers III. According to the filter media §Sand (quartz) filters §Coal/sand filters §Ceramic filters §Ceramic/sand filters

5 Filters Kinds: IV. According to construction §Open (non-pressure) §Closed (pressure) l Vertical l Horizontal V. According to impurities removal mechanism §Coagulation/sorption §Ion exchange VI. According to reagents application §Without reagents’ treatment §With preliminary coagulation §With contact coagulation

6 Low Rate Sand Filters Classical structure (Longitudinal Section) 1 - flowrate regulator; 2 - weir; 3 - inlet distribution channel; 4 - drainage pipe; 5 - filtered water pipe

7 Low Rate Sand Filters Particularities: §Capacity limits l Water quantity < 1000 (5000) m 3 /d l Turbidity < 50 mg/l l Colour < grad l Filtration duration d §Sand grains diameter d ef = 0,3 - 1 mm §Sand layer height h = 0,8 - 1 m §Water layer height H = 1,2 - 1,5 m §Filtration rate v = 0,1 - 0,3 m/h

8 Low Rate Sand Filters Particularities: §Filtering total capacity - K tot K tot = K b + K net, g/m 2 K tot = g/m 2 ; K b - biofilm development capacity, g/m 2 ; K b = M.v 0.T b ; K b = g/m 2 ; K net - net filtering capacity, g/m 2 ; K net = M.v 0.T net M - coarse water turbidity, mg/l; v 0 - filtration rate during period T b, m/h; v 0 = 0,05 - 0,1 m/h; T b - biofilm development duration, h; T b = h at M > 100 mg/l; T b = h at M < 100 mg/l;

9 Low Rate Sand Filters Particularities: §Filtering total duration - T tot T tot = T b + T net +T w, h T b - biofilm development duration, h; (time to reach bacteria concentration less than 100 numbers per cm 3 and filtrate turbidity less than 2 mg/l); T net - net filtering duration, h; T net = (K tot - K b )/v; T net = 500 h; T w - filter washing duration, h; T w = h

10 High Rate Sand Filters Scheme of Open Air Structure 1 - coarse water inlet pipe; 2 - washing water inlet pipe; 3 - filtered water pipe; 4 - wash water pipe; 5 - wash water drain channel; 6 - distribution pipes; 7 - supporting layer; 8 - filtering layer; 9 - wash water outflow channel

11 High Rate Sand Filters Scheme of Construction and Operation a - normal operationб - washing 1 - filtering layer; 2 - drainage (support) layer; 3 - distribution channel; 4 - distribution (wasout) channels; 5 - drainage system

12 High Rate Sand Filters Scheme and Operation Mode of Filtering Regulator h 1 - initial filtering head loses; h 2 - operational (decreasing) filtering head loses; H - total filtering head loses (H = h 1 + h 2 = const.); E - constant water level

13 High Rate Sand Filters Structure of Pressure High Rate Sand Filter 1 - coarse water inlet pipe; 2 - filtered water outlet pipe; 3 - wash water inlet pipe; 4, 5, 10 - stop valves; 6 - drainage (support) plate; 7 - filtering layer; 8 - supporting layer; 9 - wash air inlet pipe; 11 - manometer

14 High Rate Sand Filters Particularities: §Capacity limits l Water quantity -unlimited l Turbidity < 25 mg/l l Colour < 25 grad l Filtration duration h §Sand grains diameter d ef = 0,5 - 2 mm §Sand layer height h = 0,8 - 1,2 m §Water layer height H = 1,5 - 2 m §Filtration rate v = (15) m/h

15 High Rate Sand Filters Particularities: §Back-washing parameters l Wash water intensity l/s.m 2 l Wash air intensity l/s.m 2 l First stage water/air washing duration min l Second stage water washing duration min §Post-washing filtration l Duration min l Filtrate wasted to the sewer §Washing water quantity l % of the water treatment plant capacity §Number of spare filtering cells (under reparation) l % of number of the constructed ones