EQUAL DISTRIBUTION OF WASTEWATER USING LOW-PRESSURE DISTRIBUTION Larry D. Stephens, P.E. *

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1 EQUAL DISTRIBUTION OF WASTEWATER USING LOW-PRESSURE DISTRIBUTION Larry D. Stephens, P.E. * INTRODUCTION Experience with onsite systems has proven that equal application of wastewater over the entire soil absorption area will provide better treatment, will enhance aerobic conditions in the soil, and will provide greater longevity of the soil absorption system. This can be accomplished by the use of low-pressure dosing techniques using pump(s) and a network of small diameter PVC, pressure-rated pipe with small diameter orifices drilled at equal spacing along the laterals. The size and length of the laterals and the size and spacing of the orifices can be specifically chosen to achieve equal distribution for most site conditions. The designer must keep several principles in mind with regard to the design of pressure distribution systems: 1. At any given system pressure the discharge exiting an orifice will vary significantly with the hole size. The number and size of the holes will affect the lateral and forcemain sizes and the size of the pump required to dose the system. 2. The size of the pipe used in the system will affect the head that the chosen pump is required to meet for any given number and size of orifices. The smaller the forcemain and manifold pipe diameter --- the higher the head-loss at any given flow rate. And, the smaller the lateral diameter, the greater the head difference from one end of the lateral to the other, leading to a greater flow differential between the first and last hole in a lateral. A large head-loss across a lateral will mean less than equal distribution. 3. On the other hand, smaller laterals will have higher scouring velocities that tend to prevent the deposition of solids that may clog orifices. With all this in mind, the designer s job is to balance the hole size and spacing, the pipe sizes chosen, and the pump specifications to achieve efficient and sufficient equal distribution. If good effluent screens are used on the septic tank outlets, or if sufficient additional treatment is provided beyond the septic tank proper, smaller diameter orifices can be used in the distribution pipe network to keep the pump size to a minimum. It is common to use 1/8 to 3/16 orifices in such cases. However, if screening or filtering is not provided, hole sizes should be kept larger in the range of 1/4 to 5/16 diameter. Hole sizes larger than this usually result in excessive pump size requirements. Orifice spacing along the laterals usually vary between 2 feet on center to as much as 6 feet on center, with 3 to 5 feet being the most common. This is a designer s choice, however, the closer the orifice spacing the more infiltrative surface will be used from the first day of system use. * Larry D. Stephens, P.E., President, Stephens Consulting Services, P.C., P.O. Box 708, Haslett, Michigan, Phone: (517) , scscons@yahoo.com 1

2 In some cases it may be beneficial to use an automatic hydraulic alternating valve to dose zones of the soil absorption system in sequence. In this way, the pump only has to be sized to dose one zone at a time. If sufficient slope is available through the tanks, and between the tanks and the soil absorption system, siphons may be used in lieu of a pump. The use of a siphon will normally require several feet of fall to work properly. If a siphon is used, care must be taken to install the device according to the manufacturer s recommendations. It is also recommended that larger orifices be used in the distribution piping network if a siphon is used (3/8 to 1/2"), because a siphon does not develop higher pressure like a pump if the holes begin to clog. With regard to the orientation of the holes in the laterals, some argue that placing the holes up in the 12 o clock position, or sideways at the 3 or 9 o clock positions will provide longer service life between maintenance events. Others argue they have not observed this benefit, and that positioning the holes down in the 6 o clock position allows the laterals to drain between pump cycles and prevents freezing in cold climates. If the holes are oriented in the up position, the feed lines from the dosing tank, the manifold and the laterals should be laid to drain back to the tank between pump cycles where freezing conditions may occur. It has also been proven that aerobic conditions and better treatment are encouraged in the soil absorption system by micro-dosing of the system. This is accomplished through the use of a programmable timer in the pump control box. The adjustable timer controls the pump cycle frequency and the pump run time such that small, measured doses can be delivered to the soil absorption system throughout the day. In this way, flow to the soil absorption system is equalized at the pump tank to a great extent, and peak flow events are spread over longer periods of time. When timers are used, the doses are kept as small as practical. The time of each dose must consider the time it takes to prime the forcemain and the distribution laterals during each pump cycle. Pressure Distribution Design Procedure 1. Select the number and length of the laterals to be used. These choices will generally depend upon the shape and size of the soil absorption system. The closer the laterals are together, the better the distribution that can be expected, but the greater the pump size necessary to deliver the required flow (generally speaking). Lateral spacing typically ranges from 8 to 12 feet for trench-type systems, and 3 to 5 feet for bed-type systems. Laterals usually start and end 1 to 3 feet from the end of the stone, depending upon the orifice spacing chosen, with the first and last orifices near the ends. 2. Select the orifice diameter and orifice spacing for the laterals. Orifice spacing in bed-type systems usually approximates the lateral spacing to provide a grid of application points that is uniform in both directions. The greater the number of orifices, the larger the pump required for any given orifice size. The larger the orifice size, the greater the pump size 2

3 required to maintain design pressure. Orifice diameters of 1/8 to 1/4, and an orifice spacing of 3 to 6 feet is the most common. 3. Calculate the lateral discharge rate. To accomplish this, you must first choose a residual head that you wish to maintain in the distribution system. The residual head is the head (in feet of water) that you wish to maintain at the end of each of the laterals. Some designers call this the squirt height, or the height that water will squirt out of an upward facing orifice in the far end of the lateral. The squirt height is usually slightly less than the actual driving head inside the lateral, and will decrease slightly from the upstream to the downstream end of the lateral due to friction loss in the pipe. Residual heads for systems with larger orifices are usually chosen to be in the 2 to 4 foot range. When smaller orifices are used in the laterals, a larger residual head of 5 feet or more can be used for design. The lateral discharge rate is determined by multiplying the discharge per orifice by the number of orifices in each lateral. The orifice discharge rate can be found in Table 1 for a wide range of head pressures. The designer must keep in mind that a greater pressure head must be maintained at the upstream end of the lateral to maintain a minimum residual pressure at the downstream end. This does result in a little greater discharge out of the upstream orifices than those near the end of laterals. The difference should be less than 10% with a good design. This additional flow must be considered in choosing the pump for the job. A simple and conservative way of accommodating the difference in lateral flow caused by the head-losses in the lateral as described above is to add 10% to the lateral flow. 4. Determine the total required flow. This number is determined by multiplying the lateral discharge rate calculated in #3 by the number of laterals you have chosen. This will be the flow the pump will be required to deliver. As you consider the pump size required at this point, decide if it is acceptable. If not, go back to step #1 or #2, make the required changes in the number of laterals, or the number, size and/or spacing of orifices, and recalculate until you have an acceptable result for pump size. 5. Determine the lateral diameter. Smaller laterals have more friction loss in the pipe from end to end (tending to reduce equality in distribution), but have higher velocities which tend to do a better job of scouring the inside of the pipe and keeping it cleaner. A compromise between these objectives must be chosen. A generally accepted goal is to allow no more than a 10% difference in flow rate between the first and last orifice in a lateral. Table 2 shows the calculated flow velocity and head loss for a variety of pipe sizes and flows. This particular table is based upon actual inside pipe diameters of Schedule 40 PVC pipe. If another type of pipe is used, be sure to check and use the actual inside diameter for friction loss calculations. For longer lengths of pipe, it can make a significant difference in the performance of the system. 6. Choose a manifold size and configuration. Use of a center manifold with laterals feeding in two different directions usually provides better distribution. However, on smaller systems, or due to specific site circumstances, it may be desirable or necessary to use an end manifold. This will usually result in longer laterals that will have more pressure loss from one end to the other. The goal of the manifold is to equally distribute the wastewater to all of the laterals. Feeding the manifold with the forcemain connection from the dosing tank at 3

4 the center is usually best. For individual homes with only 2 to 6 laterals, a manifold the same size as the laterals may be acceptable. But for larger systems, a manifold 1 to 3 pipe sizes larger may be needed. Table 2 will give you an idea of how much head loss you can expect in the various sizes of pipe. To determine the amount of head loss, estimate how much flow will be going through a section of pipe, and then multiply the head loss per foot found in the table for the size pipe used by the number of feet of pipe. Compare this with the loss in the next section, etc., to decide if it is within acceptable limits. 7. Calculate the pump size in discharge rate (GPM) and total design head (TDH). The required discharge rate was calculated in #4 above, and is the sum total of all of the lateral discharge rates. The total design head (TDH) is the sum of the following: The number of feet that the pump must lift the wastewater from the low water elevation in the dosing tank to the lateral elevation in the soil absorption system (Static ); plus The number of feet of required pressure at the manifold to achieve equal distribution in the laterals (Residual, plus the head-loss in the lateral); plus The friction loss in the forcemain, fittings, and valves between the dosing tank and the soil absorption system (Friction Losses). Static : This will require that you know the elevation of the low water level expected in the dosing tank, and the elevation of the proposed soil absorption system laterals. Subtract one from the other to obtain the Static. (Note: If this number is a negative number, meaning you are pumping downhill, you will need to take extra precautions to prevent siphoning of the dosing tank contents after a pump cycle. A vacuum breaker or vent hole at a high point in the pump discharge piping may do the trick.) Residual : This number will match the head you chose in #3 above as a driving head to pressurize the system of laterals. Friction Losses: Friction losses are the result of resistance to the movement of liquid through pipes due to the friction of the liquid along the pipe walls. This resistance is greater with increases in velocity caused by smaller pipes, and conversely less with lower velocities. It does vary with different materials, but PVC is the most commonly used material in wastewater applications, and has very good flow characteristics. Table 2 gives the estimated friction loss per foot of pipe for various pipe sizes and a wide range of flow rates. The flow rate was determined in #4 above. Choose a forcemain pipe size that will not result in an unreasonable amount of head loss for the required flow rate. To calculate the head loss for the forcemain, measure the length of the forcemain required, chose a size, find the head-loss per foot from Table 2, and multiply the length by the number from the table. (Note that this table is again based upon Schedule 40 PVC pipe. If any other pipe material or specification is used, these numbers must be modified accordingly for the inside pipe diameter for the 4

5 material chosen). All installations also have a number of valves and fittings that add head loss to the pipe losses. The procedure for calculating these is somewhat complicated, but experience has shown that adding 10% to 20% to the forcemain losses is usually sufficient. If an automatic hydraulic sequencing valve is used in the design, a substantial amount of additional head loss through the valve will need to be accommodated in the pump size. Refer to the manufacturer s literature for the valve chosen to determine how much additional head loss to add. If this result requires excessive design head, choose a larger pipe size and repeat the calculation. The larger these numbers, the larger the pump required in horsepower. With regard to forcemain size, it is also desirable to keep velocities between 2 and 8 feet per second. Slower velocities may result in the accumulation of solids in the pipe, and higher velocities can actually result in pipe wall scouring. Now, add the Static, the Residual, and the Friction Losses, to get the Total Design required for the pump. Remember --- both the required flow and the TDH are used to choose a pump. These two criteria will allow the designer and supplier to choose the correct pump for the job. 8. Evaluate the design for efficiency and adequacy of distribution. Step back at this point and look at the results you obtained. Does the number and spacing of laterals and orifices provide sufficiently equal distribution? Is the pump size reasonable for the application? If necessary, make some appropriate changes and repeat the necessary steps to arrive at an optimum design. EQUATIONS USED Flow through an orifice is determined using the orifice equation as follows: Q = CA(2gH) 1/2 Where: Q = Discharge (ft 3 /s) C = Discharge Coefficient (Assumed to be 0.6 in this calculation) A = Orifice Area (ft 2 ) g = Gravitational acceleration (32.2 ft./s 2 ) H = on Center line of Orifice (ft) loss in pipe is calculated by using the flow, the pipe diameter and a friction factor. The Hazen Williams formula allows the designer to calculate headloss as follows: h = kq x 5

6 Where: h = head loss (ft.) Q = discharge (ft 3 /s) x = reciprocal of the exponent of the hydraulic slope in the friction formula (for Hazen Williams, x = 1.85) And where k is the constant calculated from the following equation: k = (1594/C) 1.85 *(l/d 4.87 ) Where: C = Hazen Williams coefficient for the pipe material used (C = 150 used here for PVC pipe) l = pipe length (ft) d = diameter (in) SPREADSHEET APPLICATION With a little effort, the design engineer can develop a spreadsheet using Microsoft Excel, or similar, software to perform these calculations expeditiously. Such tools will make the designer s job easier and more enjoyable, and will reduce the opportunity for error. This writer has created such a spreadsheet, and is willing to share it with those interested for a nominal cost. Contact the writer at the address listed on page 1. Table 1. Orifice Discharge Table Given in G.P.M. 6

7 Pressure (psi) Pressure (ft) Hole Diameter (inches) 1/8" 3/16" 1/4" 9/32" 5/16" 11/32" 3/8" Conversions 1 psi = 2.3 ft. of water 1 ft. of water = 0.43 psi Table 2. Forcemain Loss Calculations 7

8 Flow (gpm) Loss per foot of pipe (ft.) Schedule 40, PVC Pipe 1" Pipe 1-1/4" Pipe 1-1/2" Pipe 2" Pipe Fluid Velocity (ft/sec.) Loss per foot of pipe (ft.) Fluid Velocity (ft/sec.) Loss per foot of pipe (ft.) Fluid Velocity (ft/sec.) Loss per foot of pipe (ft.) Fluid Velocity (ft/sec.) Conversions 1 psi = 2.3 ft. of water 1 ft. of water = 0.43 psi 8