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Sanitary Sewer &amp; Septic System
AED Design Requirements Sanitary Sewer &amp; Septic Systems
TABLE OF CONTENTS AED DESIGN REQUIREMENTS FOR SANITARY SEWERS &amp; SEPTIC TANKS VARIOUS LOCATIONS, AFGHANISTAN
Appendix Appendix A - Example Gravity Sewer Calculation Appendix B ­ Drawing Details Septic Tank Details Absorption Bed and Trench Details Dosing system Layout Leaching Chambers Option 1A and 1B (Traffic Loading) Leaching Chambers Option 2
AED Design Requirements Sanitary Sewer &amp; Septic Systems (1) Sewers shall be located no closer than 15 meters measured horizontally to water wells or earthen reservoirs that are used for potable water supplies. (2) Sewers shall be located no closer than 3 meters measured horizontally to potable water lines; where the bottom of the water line will be at least 300 mm above the top of the sewer line, the horizontal space shall be at a minimum of 1.83 meters. (3) Sewer lines crossing above potable water lines shall be constructed of suitable pressure pipe or fully encased in concrete for a distance of 3 meters measured horizontally on each side of the crossing. If concrete encasement is used, the sewer line shall be encased with a minimum of 150 mm of cover all the way around the pipe. Pressure pipe will be as required for force mains in TM 5-814-2/AFM 88-11, Chapter 2, and shall have no joint closer than 1 meter horizontally to the crossing, unless it is fully encased in concrete. c) Quantity of Wastewater. The design of the wastewater system shall be based on two factors: the average daily flow and the peak diurnal flow (PDF). (1) Average Daily Flow (ADF). The Contractor shall verify the average daily flow considering both resident (full occupancy) and non-resident (8hr per day) population. The average daily flow will represent the total waste volume generated over a 24-hour period, and is defined as 80% of the product of the total population of the facility (c), the per capita water usage rate per day (ADD) , and the applicable capacity factor (CF) (0.8* c*ADD*CF). The capacity factor for installations with populations less than 5,000 residents is 1.5. Capacity factors for larger installations shall be determined using Chapter 4 Basic Design Considerations, UFC 3-240-09FA Domestic Wastewater Treatment, Table 4-1. For example, the average daily flow at a compound with a population of 500 personnel, would be calculated by multiplying the population (500) by the water usage rate (190 lpcd) by the capacity factor (1.5) by 80% resulting in a flow of 114,000 liters/day (30,160 gallons per day). (2) Peak diurnal rates (PDF) of flow occur on a daily basis and must be considered. The sewer shall be designed with adequate capacity to handle these peak diurnal flow rates. The peak diurnal flow rate is computed by the following equation: PDF= __Q_C___ 2Q0.167 Where PDF = Peak diurnal flowrate Q = Average daily flow in gallons per day (including the capacity factor) C = 38.2 for gallons per day So for the same compound with a population of 500 personnel, the peak diurnal flow rate would be the 114,000 liters/day (calculated above multiplied by 38.2 divided by 2 times (30,160)0.167 which equals 389,027.3 liters per day.
AED Design Requirements Sanitary Sewer &amp; Septic Systems diameter pipe may be used throughout the installation. This shall be the minimum size. Larger pipe diameters may be used, but should be used only if flows require a larger diameter. (2) For installations with populations greater than or equal to 450 personnel the sanitary sewer shall be designed to meet the following conditions. If, based on the following conditions, it is determined that a lift station is necessary to meet the flow and pipe size requirements than the designer should contact AED immediately. (A) Peak Diurnal Flow (PDF). Piping shall be designed to provide a minimum velocity of 0.6 meters per second (mps) or (2.0 feet per second (fps) and shall NOT flow at greater than 80% full or at a velocity greater than 3.0 mps (10 fps). It is required that all the pipes are designed to achieve a scouring velocity of 0.6 mps at the PDF. (B) Average Daily Flow (ADF). When possible, piping shall be designed to provide a minimum scouring velocity of 0.6 mps ( 2.0 fps) at the ADF, and shall NOT flow at greater than 80% full or at a velocity greater than 3.0 mps (10 fps) in every segment of the sewer system. It is preferred that the scouring velocity be achieved by the ADF however it is not a requirement (C) Flow Allocation. Flows in laterals, mains and trunk lines shall be based on allocating the proportion of the average daily and peak diurnal flow to each building or facility on the basis of the drain fixture unit flow developed for the plumbing design. [For example, consider a lateral receiving flow first from building A, then from building B and then from building C prior to emptying into a main. These buildings have drain fixture units of 10, 25, and 5, respectively. The entire facility has a total of 6 building and a total of 80 drain fixture units. The flows used to design the lateral receiving flow from building A would be 10/80 times the ADF and the PDF. The flows used to design the lateral after receiving flow from buildings A and B would be (10+25)/80 times the ADF and the PDF. Finally, the flows used to design the lateral after receiving flows from buildings A, B, and C would be (10+25+5)/80 times the ADF and the PDF.] (D) Minimum Pipe Slopes. Table 1 defines the minimum pipe slopes allowed in the sewer system. These shall be the minimum provided, regardless of the calculated flow velocities to prevent settlement of solids suspended in the wastewater. Table 1 does not apply to building connections. (E) Building connection. Sewer lines from buildings will be designed to provide a minimum velocity of 0.6 meters per second or 2.0 feet per second at the drain fixture unit flow for that building. The building connection is the pipe from the building to a manhole or pipe that has more than one pipe entering it, see Figure 2. The minimum slope of building connection shall be 1% regardless the size of the installation. (F) Minimum Pipe Diameter. The minimum pipe diameter used in the sewer system (after the building plumbing connection) shall be 150mm. These sizes shall be provided regardless of flows being received. Larger pipe diameters shall be provided in the sewer system based on flow and velocity requirements. Unless otherwise indicated (see Paragraph 3 (g) Building Connections and Service Lines below), gravity sewer pipe shall be installed in straight and true runs in between manholes with constant slope and direction. Pipe slopes shall be sufficient to provide the required minimum velocities and depths of cover on the pipe. Table 1 below provides the minimum allowable slopes for various diameter pipes. Table 1 does not apply to installations with populations less than 450 persons. The minimum slope for 150mm piping at these installations is to be 1%.
This table does not state that pipes are designed at this slope regardless of flow depth and velocity. Other criteria listed above shall be used to determine the slopes necessary to meet the conditions previously listed above. The word &quot;minimum&quot; is defined as &quot;the least quantity or amount possible, assignable, allowable, or the like&quot;. Greater slopes shall be used as needed to achieve the design requirements previously listed.
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AED Design Requirements Sanitary Sewer &amp; Septic Systems The grease interceptor shall either be a gravity type or hydro-mechanical type. If the designer selects a gravity type, the grease interceptor shall be of reinforced cast-in-place concrete, reinforced precast concrete or equivalent capacity commercially available steel, with removable three-section, 9.5 mm checker-plate cover, and shall be installed outside the building. Concrete shall have 21MPa minimum compressive strength at 28 days. Steel grease interceptors shall be installed in a concrete pit and shall be epoxy-coated to resist corrosion as recommended by the manufacturer. For sizing of the grease interceptor, follow the guidance provided in the AED Design Guide which is based on the EPA document 625/1-80-012 Onsite Wastewater Treatment and Disposal Systems. If the designer selects a hydro-mechanical type, the grease interceptor shall be sized and tested in accordance with Standard PDI- G101, Testing and Rating Procedure for Type I HydroMechanical Grease interceptors with Appendix of Installation and Maintenance. Drainage to grease interceptors shall be separate and distinct from other sanitary sewer lines. Wastes that do not required treatment or separation shall not be discharged into any interceptor or separator, per ICC IPC 2007 Section 1003.2 Approval. j) Oil Water Separators. Design and install oil water separators per the AED Design Guide which is based on the ICC IPC 2007 Section 1003.4.2 Oil Separator Design. k) Field Tests and Inspections. Prior to burying the sewer lines, field inspections and testing shall be done to ensure the lines were properly installed and free of leaks. When conducting tests and inspections the following steps shall be conducted: (1) Check each straight run of pipeline for gross deficiencies by holding a light in a manhole; it shall show practically a full circle of light through the pipeline when viewed from the adjoining end of the line. When pressure piping is used in a non-pressure line for nonpressure use, test this piping as specified for non-pressure pipe. (2) Test lines for leakage by either infiltration tests or exfiltration tests. (3) Deflection testing will not be required however; field quality control shall ensure that all piping is installed in accordance with deflection requirements established by the manufacturer. 4. Septic System a) General. When determining an appropriate septic tank location, the Contractor shall provide protection for the septic system by ensuring that vehicles, material storage and future expansion shall be kept away from the area. Signage or other prevention methods (i.e., pipe bollards) shall be used to provide this protection. The finished grade for the site shall ensure that storm water runoff shall drain away from the site to prevent ponding, inflow and infiltration. Once an appropriate site is located, the Contractor shall conduct soil investigations for the site to determine ground water levels, soil conditions and the percolation rate. b) Septic Tank. Septic tanks are buried, watertight receptacles designed and constructed to receive and partially treat wastewater. The tank separates solids from the liquid, provides limited digestion of organic matter, stores solids, and allows the clarified liquid to discharge for further treatment and disposal. Settleable solids and partially decomposed sludge accumulate at the bottom of the tank, while scum rises to the top of the tank's liquid level. The partially clarified liquid is allowed to flow through an outlet opening position below the floating scum layer. The clarified liquid will be disposed of to the absorption field for further treatment and disposal.
AED Design Requirements Sanitary Sewer &amp; Septic Systems Factors to be considered in the design of a septic tank include tank geometry, hydraulic loading, inlet and outlet configurations, number of compartments and temperature. If a septic tank is hydraulically overloaded, retention time may become too short and solids may not settle properly. For Afghanistan, a baffled multi-compartment or dual chamber design shall be utilized. Refer to Attachment A for further details. The septic tank shall be designed with a length-to-width ratio of 2:1 to 3:1 and the liquid depth should be between 1.2 meters and 1.8 meters. This depth is determined by the outlet pipe invert elevation. If not specified in the contract, the septic tank shall be sized based on the ADF, an additional 100% for sludge storage capacity and peak flows (0.8*c*ADD*CF*2). The tank shall be constructed of reinforced, cast-in-place concrete, with a minimum compressive strength of 21MPa at 28 days. Wastewater influent and effluent shall enter and exit on the short sides of the tank, which will allow the wastewater longer detention and settling time. The baffled tank shall have two compartments, with the first compartment (influent entry point) having 2/3 thirds the volume capacity of the tank. The tank shall have a minimum earth backfill cover of 300 mm. Access shall be provided at the entry (influent) and exit (effluent) points of the tank by installing reinforced concrete risers, with steel access hatches, that will rise 50 mm above the finished grade. The following is an example of how to determine the volume and dimensions of the septic tank: Example 2: Size a Septic Tank - Size a septic tank for a design population of 120 individuals. -Assume that tank volume and dimensions are not specified in the contract documents. V = ADD*0.8*c*2*CF =190 (liters/capita/day)*0.8*120(capita)*2(sludge retention)*1.5(capacity factor) = 54,720 liters (54.72 m3) Where, ADD = Average Daily Demand (Water Flow) per Person (liters/capita/day) 0.8 = conversion of water use to sewage flow c = design population (capita) 2 = represents an additional 100% storage for sludge and peak surges CF= Capacity Factor from UFC 3-240-09FA Domestic Wastewater Treatment V = Volume (cubic meters) -Assume 1.8 meter liquid depth and a length-to-width ratio of 2:1. A = V/1.8 meters (liquid depth) = 54.72 (m3)/1.8 (meters) = 30.4 m2 LW = A 2W*(W) = 30.4 (m2) W2 = 30.4 (m2)/2 W = (15.2 m2)1/2 = 3.90 meters (3900 mm*) L = 2*W = 2*3.90 meters = 7.80 meters (7800 mm*) Inside dimensions of tank = 7800 mm X 3900 mm X 1800 mm (liquid depth) where, A = Area L = Length (meters) = 2*W W = Width (meters) *Always round up to the nearest 100 mm for final septic tank dimensions.
AED Design Requirements Sanitary Sewer &amp; Septic Systems c) Absorption Field. Absorption fields (also termed &quot;leach fields&quot;) are used, in conjunction with septic tank treatment, as the final treatment and disposal process for the septic system. Absorption fields normally consist of perforated distribution pipe laid in trenches or beds that are filled with rock. Refer to Attachments B or C for minimum perforation requirements. The septic tank effluent is distributed by the perforated pipe and allowed to percolate through the ground, where it is filtered and treated by naturally occurring bacteria and oxygen. Maximum depth for leach field percolation pipe lines shall be one (1) meter to allow for air exchange with the surface. Once effluent is released from the septic tank, it travels by gravity through a solid PVC pipe, at a minimum 1.0% slope, to the distribution box. The distribution box is a reinforced concrete structure that distributes the septic tank effluent evenly throughout the absorption field through several 100 mm diameter perforated pipes. Distribution piping and laterals shall be placed at a depth between 650 mm to 1500 mm. Because of the desire for the effluent to be distributed evenly over the absorption trenches or beds, the perforated pipe shall have a maximum slope of 0.5% and shall be capped at the end of each pipe. Generally, distribution piping is spaced from one meter to 1.8 meters apart and is no longer than 30 meters. Absorption trenches are a minimum 610 mm wide but can be widened to shorten the length of the trench. A bed can be as wide as needed based on the total area needed for absorption, but maybe limited in size due to available real estate, or by construction constraints. Large absorption beds are susceptible to the bed bottom being compacted during excavation and pipe installation. Compaction of the bed bottom will degrade percolation and may lead to failure of the absorption field. The absorption field has three (3) zones: (1) The first zone is the absorption zone, which is the layer of in-situ material that filters and treats the effluent. This zone is determined to be suitable material for wastewater treatment based on the percolation test results, with a minimum thickness of 600 mm. Below the absorption zone, the material is considered unsuitable soil or bed rock or the seasonal water table is too high. If percolation tests determine that there isn't a minimum 600 mm of suitable soil, the Contractor can remove the unsuitable soil to the desired depth and replace it with material determined to be suitable; however, the Contractor must get approval from the COR before attempting this. (2) The second zone is the drainage zone, which is a 300 mm thick layer of rock fill, where the distribution pipe network lies. The bottom of this zone is filled with a minimum 150 mm of 19 mm to 38 mm diameter rock. The perforated distribution pipe is laid on top of the rock. A minimum of 50 mm of rock is placed carefully over the pipe network, and then a semipermeable membrane (geotextile fabric) is placed over the rock to prevent fine-grained backfill from clogging it. (3) The final zone is the backfill zone. This is the upper most part of the absorption field, where backfill material is placed and is a minimum 500 mm thick. The backfill material protects the lower lying zones from storm water infiltration and freezing. The Contractor shall leave a mound of backfill material above the desired finished grade to allow for settlement. Table 2 lists percolation rates and the corresponding sizing factor (m2/liters/day). The sizing factors are used, in conjunction with average daily flow (ADF), to determine the size of the absorption field. The following is an example of how to calculate the absorption field size for trenches and beds:
AED Design Requirements Sanitary Sewer &amp; Septic Systems Example 3: Size of Absorption Field - Size an absorption field (trench type) for a facility with an average daily flow of 27,360 liters/day and a percolation rate of 50 minutes. A=Average Daily Flow*Water Absorption of Soil =27,360 liters/day*0.054 m2/liters/day= 1,477.44 m2 where, A = Area footprint needed for the absorption field (m2) Average Daily Flow (liters/day) Water Absorption of Soil = By looking below, at Table 2, a percolation rate of 50 minutes falls in the 46 to 60 row and the correlating sizing factor is determined to be 0.054 m2/liters/day. Dimensions for trenches: -Assume a 0.9144 meter wide trench bottom. -Assume maximum trench length to be 30 meters. *NT = A/(Tw*TL) = 1,477.44 m2/(0.9144 m*30 m) = 53.86 say: 54 Trenches (0.9144 meters X 30 meters) where, NT = Number of Trenches Tw = Trench width (meters) TL = Trench Length (meters) *Note: Trench bottom area can be reduced by 20 percent, if 305 mm of rock is placed below the distribution pipe. The area can be reduced by 34 percent for 457 mm of rock being placed below the pipe and by 40% for the maximum rock depth of 610 mm. Keep in mind that the additional rock added below the distribution pipe adds additional thickness required for the drainage zone. For example, where normally 150 mm of rock is placed below the pipe for a total 300 mm thickness for the drainage zone. Placing 305 mm of rock is placed below the pipe increase total thickness for the drainage zone to 455 mm of rock, (305 mm below the pipe; 100 mm around the pipe; and 50 mm above the pipe). Dimensions for bed: (Absorption Beds are not recommended for large systems due to the difficulty of constructing the bed bottom without compacting or disturbing it, and relative inability to function over terrain at various elevations.) Absorption Bed Design Population of Twelve: Average Daily Flow = 190 lpdc*0.8*12*1.5 = 2,736 lpd Area Required = 2,736 lpd * 0.054 m2/liter/day = 147.74 m2 Absorption Bed Dimensions = A1/2 = (147.74 m2)1/2 = 12.15 meters, say: 13 meters per side Absorption Bed Dimensions = 13 meters X 13 meters Refer to Attachments B and C for further design details of absorption fields.
AED Design Requirements Sanitary Sewer &amp; Septic Systems 3) Pump Screen - All pressure distribution pumps should be surrounded by a plastic mesh screen with 3.175mm (0.125 in) diameter holes. The screen should have a sufficient surface area so that the velocity of the sewage effluent passing through the screen does not allow the screen to become plugged with solids. 4) Pump Chamber - A pump chamber separate from a septic tank is required for PDS. The pump chamber shall be sized to allow a set volume of sewage. 5) Control Panel - All pumps shall be connected to a control panel. The panel should have an alarm in case the pump fails to operate properly. 6) Transport Pipe - The transport pipe is the pipe that connects the sewage effluent pump to the manifold pipe. The diameter of this pipe is determined by friction losses caused by the flow of sewage effluent through the pipe and by the desire to have a cleansing velocity where possible of 0.6 m/s (2 ft/s) passing through the pipe during operation. There should be a flexible connection between the pump and the transport pipe to allow for the possible settlement of the pump chamber or the septic tank after installation. The PVC pipe shall conform to ASTM D2241 PVC Pressure-Rated Pipe (SDR series) and have a maximum SDR of 35. 7) Manifold Pipe - The manifold pipe is located between the transport pipe and the laterals in the leach field. This pipe is sized so that there is no more than a 15% variation in the rates of discharge between the first and last orifices in the network. The PVC pipe shall conform to ASTM D2241 PVC Pressure-Rated Pipe (SDR series) and have a maximum SDR of 35. 8) Lateral Pipe - Laterals in the PDS are used to distribute the sewage effluent to the soil. Their length configuration and number are determined by soil condition, percolation rate and leach field geometry. The diameter of a lateral should be the smallest diameter that achieves nearly uniform pressure along the entire length of the lateral. The PVC pipe shall conform to ASTM D2241 PVC Pressure-Rated Pipe (SDR series) and have a maximum SDR of 35. 9) Orifice - Orifice shields are placed over the orifices (holes) in the laterals to prevent sewage effluent from being forced under pressure to the surface of the drain field when the orifices are in the 12 o'clock position (the crown of the lateral). They also prevent the orifices from becoming blocked by drain rock in the leach field. The shields will be asphalt building paper; minimum of 0.73 kg/m2 or geotextile material. Clean outs - Clean outs shall be placed at the ends of the laterals, a minimum of 0.5 meter above ground. They should have treaded caps at their ends to allow for inspection and cleaning of the laterals. In cold climates they should be insulated to prevent the laterals from freezing.
AED Design Requirements Sanitary Sewer &amp; Septic Systems Figure 3. Pressure Distribution System (Source: Washington State Department of Health.)
While there are additional operational costs associated (valve cleaning and occasional replacement), the advantages for sloping sites include:      Lower cost of leveling the site Avoiding the destruction of the natural soil horizon and the microbes that promote the biological treatment of the wastewater effluent Ground water recharge of the aquifer where water is being drawn Use previously unusable sites where the alternative (holding tanks or package WWTP) are more costly More frequent smaller doses assure unsaturated flow through the soil and reduce the potential for clogging and destroying the treatment capacity of the site.
b) PDS SYSTEM DESIGN EXAMPLE STEPS The following design example is based on information prepared by Professor James Converse, dated 2000 (reference 3). The designer is recommended to review all reference material listed at the end of 4 (d) as well as apply his/her engineering judgment. Units are shown as imperial, however can be easily converted to &quot;soft&quot; metric. Design is a two part process: PART 1 consists of sizing the distribution network which distributes the effluent in the aggregate and consists of the laterals, perforations (orifice) and manifold. PART 2 Consists of sizing the force main, pressurization unit and the doze chamber and selecting controls. Within each part, there are several steps associated with design. Example 4: Size a pressure distribution network with the following given information: (units are in imperial units) Absorption area ­ 113ft long by 4 ft wide Force Main ­ 125 ft long Elevation Difference ­ 9 ft Number of elbows - 3 Part 1. Design of the distribution network. Step 1. Configuration of the network. This is a narrow absorption unit on a sloping site. Determine the lateral length. Use a center feed (meaning the manifold for the line from the dosing tank is in the center of the absorption field), the lateral length is: Lateral length = (B/2) ­ 0.5 ft Where B = absorption length. (113/2) ­ 0.5 = 56 ft Note: Recommend to have the manifold down the center of the absorption field and then have laterals going out from the center. Step 3. Determine the perforation spacing and size. Each perforation will cover 6ft2/orifice (note: this value is a based on rule of thumb for 30-36&quot; orifice spacing). This value is the area/orifice parameter. This means, each orifice will discharge an equivalent effluent on a 2ft x 3 ft area. Will use two laterals on each side of the center feed. Spacing = (area/orifice x number of laterals / (absorption area width). Spacing = (6ft2 x 2/(4ft)) = 12/4 = 3 ft
AED Design Requirements Sanitary Sewer &amp; Septic Systems Size ­ use either a 3/16&quot; or ¼&quot; diameter. Recommend using a 3/16&quot; diameter orifice. Whichever diameter is used, it requires placement of an effluent filter (screen, such as wire mesh at the outlet pipe) in the septic tank to eliminate carryover of large particles. Designer can choose to use larger or smaller diameter, (refer to reference 3 for other sizes). Step 4. Determine the lateral diameter. Refer to the following figure, &quot;Orifice Spacing in Feet&quot; for a 3/16&quot; diameter orifice.
Using lateral length of 56 feet and orifice spacing of 3, the point of intersect is between the 1-1/4&quot; and 1-1/2&quot; lines. Round up to the next value, therefore it will be 1-1/2&quot;. In this design example, the lateral diameter is 1.5&quot; Step 5. Determine number of perforations per lateral and number of perforations. Using 3 feet spacing in 56 feet length yields: N = (p/x) + 0.5 = (56/3) + 0.5 = 19 perforations/lateral Number of perforations = 4 laterals x 19 perforations/lateral = 76. Check: Maximum of 6 ft^2/perforation Number of perforations = (113 ft x 4 ft)/6ft^2 = 75 , so OK.
Note: Perforation is the same as orifice. The 4 laterals correspond to two laterals on each side of the center manifold. Step 6. Determine lateral discharge rate (LDR). Using network pressure (distal) pressure of 3.5 ft and 3/16&quot; diameter perforations, using the table below, gives a discharge rate of 0.78 gallons per minute (gpm) regardless of the number of laterals. Table A -1 Orifice Diameter
Note: The value 3.5 ft for distal pressure corresponds with 3/16&quot; diameter orifice. LDR = 0.78 gpm/perforation x 19 perforations = 14.8 gpm Step 7. Determine the number of laterals. This is already completed in steps 3 and 4. Total of four laterals, two on each side of the manifold. The spacing between laterals to be at 2 ft apart. Step 8. Calculate the manifold size. This is the same size as the force main, the line from the dosing tanking out to the absorption field. Since the diameter of the laterals are 1.5 &quot;(see step 4), the force main and the manifold should be at least half size larger, say 2&quot; diameter. Determine network discharge rate (NDR). NDR = 4 laterals x 14.8 gpm/lateral = 59.2 gpm. Round up to 60 gpm. This is the value that will be utilized in Part 2 when sizing for a pump. Step 10. Provide for flushing of laterals. Provide cleanouts at the end of each lateral line as well as at the end of the manifold. This will allow for ease of maintenance.
AED Design Requirements Sanitary Sewer &amp; Septic Systems Part 2. Design of Force Main, Pressurization Unit, Dose Chamber and Controls. Total Dynamic Head (TDH) The total dynamic head is the sum of the following: System head + Elevation head + Head Loss = TDH System Head = 1.3 x distal head (ft) SH = 1.3 (3.5) = 4.55 ft. (Note: distal head of 3.5 comes from Step 6 Part 1. This value is a rule of thumb). Elevation Head = (pump shut off to network elevation) this is also the static head. Will depend on the design invert elevations. Head Loss due to friction = This is the sum of losses due to fittings and pipe run. For fittings and friction loss, use the two tables below. For 2&quot; diameter fittings the friction loss for the 90 degree elbow is 9. Since we have three elbows (in the problem statement) the total head loss due friction from fittings is 27ft. Below is the friction loss table to use for our problem. In our example, the flow rate is 60 gpm and the nominal pipe size of the manifold and the force main is 2-inches in diameter. Therefore, the friction loss per 100 feet of pipe is 7. This value will depend on the size and type of pipe used in design. Refer to a standard fluids/hydraulics references for values other than given below.
AED Design Requirements Sanitary Sewer &amp; Septic Systems The total head loss due to friction is as follows: 7 (125 ft + 27 ft)/100 ft = 10.6 ft Total Dynamic Head (TDH) = 4.5ft + 9 ft + 10.6 ft = 24.1 ft Step 2. Pump Summary. Pump must discharge 60 gpm against a head of 24.1 feet with 2&quot; force main. These are the calculated flow and head values. The actual flow and head will be determined by the pump selection. A system performance curve plotted against the pump performance curve will give a better estimate of the flow rate and total dynamic head the system will operate under. Step 3. Determine the dose volume. For dose volume, use 5 times the lateral void volume. Void volume is selected from the table listed below.
For 1.5&quot; lateral line, the void volume is 0.092 gal/ft. For 2&quot; force main line, the void volume is 0.163 gal/ft Dose rate in laterals = 5 x 56ft x 4 laterals x 0.092 gal/ft = 103 gal/dose Dose rate in main line = 125 feet x 0.163 gal/ft = 20.4 gal/dose Dose rate total = 103 gal + 20 gal = 123 gallons/dose There will be 5 doses at 123 gallons per dose throughout a 24 hour period. Step 4. Size the dose chamber. Based on the dose volume, storage volume and room for a block beneath the pump and control space, 500- 700 gallon chamber will suffice.
AED Design Requirements Sanitary Sewer &amp; Septic Systems Step 5. Select controls and alarms. This example is for demand dosing. The controls will include on-off float and alarm float. The designer will set the on float based on the volume of the dose (123 gallons). The pump will dose five times through out the day.
Example 5: Size of a Seep Pit ­ Based on a population of ten (10) and a percolation rate of 6 min/cm. ADF = 10 capita*190 liters/capita/day*0.8*1.5 capacity factor = 2,280 liters/day Area Required = 2,280 lpd * 0.031 m2/liter/day = 70.68 m2 = 761 ft2 Seep Pit Size equals approximately 2 pits (A= 754 ft2) 3.66 m in diameter and 3 m deep below the inlet pipe. The seepage pits should be separated by at least 2 diameters spacing between the sides of the pits. This example shows that seepage pits when properly sized are not viable for larger wastewater flows. 5. Design Submittal Information a) Gravity Sewer. A preliminary sewer flow depths can be calculated assuming normal flow regime. This is a simplification of actual flow regime because there will be energy losses at the entrance of other side laterals and manhole energy losses that will make the water profile will not be uniform. If sewers are at flat grades for long pipe runs, these losses shall be considered because they will over the length of the project become a significant design factor. For short pipe runs and normal slopes (greater than minimum
AED Design Requirements Sanitary Sewer &amp; Septic Systems slope values) the uniform flow assumption can be used. Design documentation for gravity sewer design using the uniform flow assumption shall include the following:  Sewer pipe line number  Pipe diameter  Pipe length  Pipe slope  Incremental wastewater inflow  Total cumulative wastewater flow  Pipe roughness (n value)  Full flow area of pipe  Hydraulic radius and full flow velocity  Calculation of ratio of actual design flow to full flow in the pipe  Calculation of the actual design flow velocity for sewer pipe length Flow velocities shall be compared to design standards and profiles or pipe size adjusted accordingly. An example analysis is shown in Appendix A. c) Pressure Distribution. A series of design examples follows this section that illustrates the design submittal information required for pressure distribution systems (PDS) for effluent disposal in leach fields. d) Septic Tanks and Leach Fields. Required design calculations to be submitted fro projects are shown in Examples 1 through 3 previously described. 6. As-Builts Upon completion of installing the sanitary sewer and septic systems, the Contractor shall submit editable CAD format As-Built drawings. The drawings shall show the final product as it was constructed in the field, with the exact dimensions, locations, materials used and any changes made to the original design. Refer to Contract Sections 01335 and 01780A of the specific project for additional details. Reference ­ Dosing tank: 1) Unified Facility Criteria 3-240-9a Domestic Water Treatment, January 2004 http://www.wbdg.org/ccb/DOD/UFC/ufc_3_240_09fa.pdf 2) Environmental Protection Agency (EPA) 625 R-00 008, February 2002 http://www.epa.gov/safewater/uic/class5/pdf/techguide_uicclass5_2002_onsite_wwt_sys_man.pdf 3) Pressure Distribution Network Design by James C. Converse January 2000 http://www.iowadnr.com/water/wastewater/files/dg_app_b_pdd.pdf 4) Washington State Department of Health, publication # 337-022, July 2007 http://www.doh.wa.gov/ehp/ts/WW/pres-dist-rsg-7-1-2007.pdf
Example Gravity Sewer Calculation The objectives of the analysis for the system include: 1 Verifying the minimum diameter (without flowing more than 80% full) for the collection piping is large enough to convey flow throughout the system such that the technical criterion for minimum velocity is achieved. This is to be done with the necessary slope greater than the minimum grade that reduces settling of suspended solids and provides economical construction depth, without the need for excessive number of lift stations. 2 Verifying whether sewer drop structures are needed to achieve minimum cover at acceptable sewer grade on long pipe runs. 3 Verifying if there is a need for lift stations over long pipe runs. 4 Verifying the heights assumed for manholes are acceptable; generally less than 5 meters depth. The project site plan is used to obtain pipe information for the calculations. See the attached site plan example. The analysis can be set up in an electronic spreadsheet format. 1 Organize a numbering system from upstream to downstream a. Enter hydraulic information for each pipe run i. Pipe diameter ii. Pipe length iii. Pipe slope iv. Pipe roughness properties ­ Manning's n value b. Determine the flow added at each intersection that represents the downstream pipe discharge. Add flows in the downstream direction and enter into a table containing the pipe information Calculate the full flow capacity and velocity of the pipe runs a. Use Manning's equation to calculate flow velocity b. Calculate flow base on total flow area and velocity Use a nomograph or flow properties chart (see attached) to calculate the proportional flow and velocity for the actual flow rate in each pipe based on the design flow Check if the flow depth is less than 80% and the flow velocity exceeds the minimum required. If not, try again Tabulate the design information on the construction drawing sewer site plan in a pipe schedule
Qs/Qf =0.25 START HERE
QS/QF OR VS/VF
Appendix B ­ Drawing Detail
Microsoft Word - AED Design Requirements - Sanitary Sewer and Septic Systems - Mar10
Microsoft Word - ARE-Ch9 Plumbing Systems.doc