Jae K. (Jim) Park, Professor Dept. of Civil and Environmental Engineering University of Wisconsin-Madison 1

Grit Removal Grit: sand, dust, cinder, bone chips, coffee grounds, seeds, eggshells, and other materials in wastewater that are nonputrescible and are heavier than organic matter. Reason for grit removal To protect moving mechanical equipment and pumps from unnecessary wear and abrasion To prevent clogging in pipes, heavy deposits in channels To prevent cementing effects on the bottom of sludge digesters and primary sedimentation tanks To reduce accumulation of inert material in aeration basins and sludge digesters which would result in loss of usable volume Specific gravity Grit: 1.5~2.7 Organic matter in wastewater:

Location of Grit Removal Facility LocationAdvantagesDisadvantages Ahead of lift Max. protection of Frequently deep in the stationpumping equipmentground, high construction cost, not easily accessible, and difficult to raise the the grit to ground level After pumpingGround level structure -Some abnormal wear to stationeasy to access andpumps operate Degritter inUsually low capital andPumping equipment not conjunctionoperation and maintenanceadequately protected with primarycosts, cleaner and drier sludgegrit 3

Velocity-Controlled Grit Channel  A long narrow sedimentation basin with better control of flow through velocity - used for small plants Design Factors  Detention time: 60 sec.  Horizontal velocity: 0.3 m/sec  Settling velocity for a 65-mesh (0.23 mm) material: 1.15 m/min  Headloss: 30~40% of the max. water depth in the channel  Grit removal: manual or mechanical 4

Grit Channel 5 Recommended for small to medium size plants. This requires high maintenance and grit removal is not easy. In this plant, a crane is used by an operator, which is an labor-intensive operation.

Grit Removal System 6 Grit removal by grab bucket crane.

Grit Removal System 7 Grab bucket crane

Grit Hopper 8

Grit Loading System 9

Pista ® Grit Removal System  Operates on the vortex principle.  Headloss: max inch  Removal efficiency 95% of the 50 mesh size grit 83% of the 80 mesh size grit 73% of the 140 mesh size grit  Capacity: 1~70 MGD  Can be installed above or below ground  Lower power usage  Supplied in steel for easy installation and/or attachment to a concrete channel  Installed in multiples 10

Pista ® Grit Removal System 11

12 Pista ® Grit Removal System

Grit Removal System Impeller mixer 13

Grit King ® Dynamic Separator Has no moving parts Requires no external power source Virtually maintenance free Highly efficient w/ min. headloss Recovers clean grit Self-cleansing Designed to operate over a wide range of flows Compact, requiring minimal space Simple to install and operate Easily linked with new or existing plant Economical, reduces long term expenditure 14

Grit Washing/Sludge Degritting System Degritting dilute primary or secondary sewage sludges The SCS™ uses a very strong free vortex and an accelerated boundary layer to separate abrasives as small as 50 µm sand from organic solids and water and concentrate these abrasives in a slurry stream. Sand is then separated from the slurry stream and dewatered by the total particle capture GRIT SNAIL™. Washing (classifying) and dewatering grit abrasives removed by a headworks grit chamber 15

Grit Separation and Washing Unit 16

Grit Separation and Washing Unit 17

Aerated Grit Chamber l Widely used for selective removal of grits l Create a spiral current within the basin using diffused compressed air l Designed to remove grit particles having a specific gravity of 2.5 and retained over a 65-mesh (0.21-mm  ) screen l Used for medium to large treatment plants 18

Aerated Grit Chamber 19

20 Aerated Grit Chamber

21 Aerated Grit Chamber

Aerated Grit Chamber - continued Advantages l Can be used for chemical addition, mixing, and flocculation ahead of primary treatment l Fresh wastewater, thus reduce odors and remove BOD 5 l Minimal headloss l Grease removal by providing a skimming device l Remove low putrescible organic matter by air supply l Remove any desired size by controlling the air supply Volatile organic compound (VOC) and odor emission l Due to a health risk, covers may be required or nonaerated type grit chambers may be used. 22

Aerated Grit Chamber - continued Design Factors l Depth: 2~5 m; length: 7.5~20 m; width: 2.5~7 m; width/depth ratio: 1:1~5:1; length/width ratio: 2.5:1~5:1 l Transverse velocity at surface: 0.6~0.8 m/sec l Detention time at peak flow: 2~5 min l Air supply: 4.6~12.4 L/sec·m of tank length (3~8 cfm/ft) - Higher air rate should be used for wider and deeper tanks; provision for air flow control l Inlet structure: Inlet to the chamber should introduce the influent into circulation pattern. > 0.3 m/sec under all flow conditions l Outlet structure: Outlet should be at a right angle to the inlet. > 0.3 m/sec under all flow conditions l Baffles: longitudinal and transverse baffles l Chamber geometry: consider location of air diffusers, sloping tank bottom, grit hopper, and accommodation of grit collection and removal equipment 23

Aerated Grit Chamber Design Checklist l Design average, peak, and low initial flows l Information on existing facility in case of expansion, site plan, and topographic maps l Type of grit removal facility to be provided l Influent pipe data, and static head, force main, hydraulic grade line if grit removal is preceded by a pumping station l Headloss constraints for grit removal facility l Treatment plant design criteria l Equipment manufacturers and equipment selection guides 24

Design Criteria Used in Example l Two grit chambers with spiral circulation l Typically designed for max. flow delivered by the pumping station: 1.56 m 3 /sec (  24,700 gpm) l Design peak flow: m 3 /sec; due to friction headloss and installation of variable-drive pumps, use design peak flow l Detention time: 4 min at Q max l Air supply rate: 7.8 L/sec·m of tank length l Provide nozzle diffusers with coarse bubbles. Provisions for 150% air capacity for peaking purposes l Inlet and outlet min. velocity: 0.3 m/sec l Chamber width: 3.5 m l Screw conveyer to move the grit to the hopper and grab buckets for grit removal 25

Design Example A. Geometry of Grit Chamber 1.Q max through each chamber: m 3 /sec Volume: m 3 /sec  4 min  60 sec/min = m 3 Average water depth at midwidth: 3.65 m Freeboard: 0.8 m Total depth: 3.65 m m = 4.45 m Surface area: m 3 /3.65 m = 43.5 m 2 Length: 43.5 m 2 /3.5 m = 12.5 m  13 m Design surface area: 3.5 m  13 m = 45.5 m 2 2. Diffuser arrangement: along the length of the chamber on one side and place them 0.6 m above the bottom 3.Actual detention time at Q max = (3.5 m  13 m  3.65 m)  (0.661 m 3 /sec  60 sec/min) = 4.2 min When only one chamber is in operation, HRT = 2.1 min 26

Design Example - continued B. Design of Air Supply System 1.Air requirements Air required = 7.8 L/sec·m  13 m = L/sec Total capacity of diffusers: 1.5  L/sec = L/sec per chamber Blower capacity: 2  L/sec = 18.3 standard m 3 /min Provide two 20 m 3 /min blowers (one standby unit) at the operating pressure of 27.6 kN/m 2 (4 psig) at the outlet. Air piping shall deliver a min. of 0.15 m 3 /sec air to each chamber. Provide control valves and flow meters on all branch lines to balance the air flow. 2. Diffusers and blowers Provide coarse diffusers with air pipe headers and hanger feed pipes having swing joint assembly. 27

Design Example - continued 28

Design Example - continued 29

30 Design Example - continued

C. Surface Rise Rate 1.Overflow rate when both chambers are in operation Overflow rate: (0.661 m 3 /sec  86,400 sec/day)  (3.5 m  13 m) = 1,255.2 m 3 /m 2 ·day (30,805 gpd/ft 2 ) 2. Overflow rate when one chamber is in out of service Overflow rate: 2  1,255.2 = 2,510.4 m 3 /m 2 ·day D. Influent Structure 1.Arrangement of influent structure Provide 1-m wide submerged influent channel with two 1 m  1 m orifices. Provide a baffle at the influent to divert the flow transversally to follow the circulation pattern. Provide sluice gates to remove one chamber from service for maintenance purposes. 31

Design Example - continued 2.Headloss calculation through the influent structure  z = z 1 - z 2 = difference in elevation of free water surface into the channel and the chamber (m). h L = h L (into influent channel) + h L (exit loss thru port) negligible 0 32

Design Example - continued whereA = orifice area, m 2 and C d = discharge coef. = square-edged entrance 33 If ≠ 0.24, go back to Slide #32 and repeat the calculation until z 1 - z 2 ≈ h L. z1z1 z2z2

Design Example - continued E. Effluent Structure 1.Arrangement of effluent structure Provide a 2.5-m long rectangular weir, an effluent trough (2.5 m long  1.5 m wide), an effluent box (2.3 m  1.5 m), and an outlet pipe. Provide removable gates at the effluent box to drain the effluent trough when one chamber is removed from service. 2.Head over the effluent weir whereQ= flow over weir, m 3 /sec; H = head over weir, m; C d = discharge coef. = 0.624; and L’ = L H (L = length of weir). 34

Design Example - continued At peak design flow when both chambers are in operation, Q = m 3 /sec. Calculate H by trial & error Assume L’ = 2.44 m  H = 0.28 m  L’ = ·0.28 = 2.44 m (same - ok); thus, H = 0.28 m 3.Height of the weir crest above the bottom of the chamber Height of weir crest = 3.65 m m = 3.37 m 4.Head over the effluent weir at Q max when one chamber is out of service Assume L’ = 2.41 m  H = 0.45 m  L’ = ·0.45 = 2.41 m (same - ok); thus, H = 0.45 m 5.Water depth in the chamber at Q max when one chamber is out of service = 3.37 m m = 3.82 m 35

Design Example - continued 6.Depth of the effluent trough Flow varies in a free falling weir discharge. For uniform velocity distribution, the drop in the water surface elevation between two sections is expressed as follows: where  y ’ = drop in water surface elevation between sections 1 and 2, m;  x = horizontal distance between sections 1 & 2, m; y 1 and y 2 = depth of flow at sections 1 and 2, m; q 1 and q 2 = discharge at sections 1 and 2, m 3 /sec·m; v 1 and v 2 = velocity at sections 1 and 2, m/sec; and (S E ) ave = average slope of the energy line, m/m. 36

Design Example - continued R ave = (R 1 + R 2 )/2 wheren = roughness coefficient and R = hydraulic mean radius, m. 37

Design Example - continued Depth of flow in the trough at the upstream section wherey 1 = water depth at the upstream end, m; y 2 = water depth in the trough at a distance L from the upstream end, m; q ’ = discharge per unit length of the weir, m 3 /sec·m; b = width of the trough, m; and N = number of sides the weir receives flow (1 or 2). 38

Design Example - continued Assume water depth in the effluent box at the exit point (center of the effluent pipe) is 1.5 m; thus, the water depth in the trough at the effluent box, y 2, is also 1.5 m. Allow 12% additional depth to account for friction losses, and add 15 cm to ensure a free fall. Thus, Total depth of trough = 1.54 m  m = 1.88 m F. Headloss through the Grit Chamber Total headloss = h L at the effluent structure + h L at the influent structure + h L in the basin + h L due to baffles 0 39

Design Example - continued 40

Design Example - continued Headloss due to influent and effluent baffles wherev 2 = velocity through the chamber; A b = vertical projection of the area of the baffle; and C D = drag coef. = 1.9 for flat plates. v 2 = m 3 /sec  [(3.5 m width)  (3.82 m water depth)] = m/sec The headloss is small; so it can be neglected. Similarly, the headloss due to effluent baffle can also be ignored. G. Quantity of Grit Grit produced = 30 m 3 /10 6 m 3  0.44 m 3 /sec  sec/day = 1.14 m 3 /day Combined system: 10~30 ft 3 /Mil gal; Separate system: 2~10 ft 3 /Mil gal 41

Operation and Maintenance l Requires well-trained operators l Maximize grit removal efficiency l Adjust the air flow to allow grit to settle but prevent organic material from settling l Use swing type diffusers for easy maintenance Trouble Shooting Guide l Rotten-egg odor, corrosion or wear on equipment: increase air supply and inspect the walls, channels, and the chamber for debris l Low recovery of grit: reduce air supply l Grit chamber overflow: adjust pump controls l Reduced surface turbulence: clean diffusers l High volatile matter content in grit: reduce air supply l Grey in color, smelly, greasy: increase air supply 42