Effluent Conveyance. Paul Trotta, P.E., Ph.D. Justin Ramsey, P.E. Chad Cooper

Transcription

1 Effluent Conveyance Paul Trotta, P.E., Ph.D. Justin Ramsey, P.E. Chad Cooper University Curriculum Development for Decentralized Wastewater Management 1

2 NDWRCDP Disclaimer This work was supported by the National Decentralized Water Resources Capacity Development Project (NDWRCDP) with funding provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR ) 0) with Washington University in St. Louis. These materials have not been reviewed by the U.S. Environmental Protection Agency. These materials have been reviewed by representatives of the NDWRCDP. The contents of these materials do not necessarily reflect the views and policies of the NDWRCDP, Washington University, or the U.S. Environmental Protection Agency, nor does the mention of trade names or commercial products constitute their endorsement or recommendation for use. 2

3 CIDWT/University Disclaimer These materials are the collective effort of individuals from academic, regulatory, and private sectors of the onsite/decentralized wastewater industry. These materials have been peer-reviewed reviewed and represent the current state of knowledge/science in this field. They were developed through a series of writing and review meetings with the goal of formulating a consensus on the materials presented. These materials do not necessarily reflect the views and policies of University of Arkansas, and/or the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT). The mention of trade names or commercial products does not constitute an endorsement or recommendation for use from these individuals or entities, nor does it constitute criticism for similar ones not mentioned. 3

4 Citation Trotta, P.D., and J.O. Ramsey Effluent Conveyance - PowerPoint Presentation. in (M.A. Gross and N.E. Deal, eds.) University Curriculum Development for Decentralized Wastewater Management. National Decentralized Water Resources Capacity Development Project. University of Arkansas, Fayetteville, AR. 4

5 Conveyance Overview 5

6 Section Objectives: Provide introduction to Conventional Sewer Systems Introduce design limitations of Conventional Sewer Systems Discuss Vacuum Sewers Discuss Grinder Pumps Compare and contrast Pressure and Gravity Flow Effluent Sewer Systems 6

7 Overview of Onsite and Decentralized All aspects of Onsite and Decentralized wastewater treatment and dispersal involve the movement of effluents of varying qualities. This includes transfers from: Individual homes to cluster or community collection systems. Individual homes to onsite treatment facilities. Onsite treatment facilities to onsite disposal facilities 7

8 Design Constraints of Large Scale Systems Large-scale municipal gravity flow sewer systems are generally designed with the following constraints in mind: Flow velocities Average, low, or high flows Inverted siphons Minimum sewer diameters Minimum and maximum sewer depths Access facilities (man-holes) holes)minimum horizontal and vertical separations 8

9 Additional Design Issues of Large Scale Systems Large-scale municipal gravity flow sewer systems are generally designed with the following constraints in mind: Lift Stations Cleanouts Air Relief Valves 9

10 Decentralized Sewer Systems The onsite counterparts or alternatives to municipal sewer system include: Septic Tank Effluent Pump Enhanced-Flow STEP systems Low Pressure Pipe/Low Pressure Distribution Septic Tank Effluent Gravity Grinder Pump Vacuum Systems 10

11 STEP and GP Systems Compared below are the basic features of STEP and GP systems. 11

12 Pressure Flow vs. Gravity Flow Pressure Flow hydraulics most often makes use of a pump to provide the energy necessary to overcome friction, provide velocity, and/or change elevation. Gravity Flow hydraulics always make use of gravity as the source of force necessary to overcome friction and provide velocity. There are, however, situations in which the distinction can be blurred. 12

13 Wastewater Design Flows 13

14 Section Objectives: Cover community sewer systems Design Flows and Peak Factors Introduce STEP and STEG Design Flows Design process for STEP Design Flow and Peak Flow Estimation 14

15 Community Sewer System Design Flows and Peaking Factors The design of all wastewater collection systems, weather they are centralized, decentralized or individual onsite systems, are affected by the daily variation of the wastewater or treated effluent that they are designed to carry. 15

16 Design Flows for STEP & STEG Systems Shown is a typical diurnal flow pattern for a single residence with gravity flow discharges, the average per house discharge from five homes and the average per house discharge from 61 homes 16

17 Flow Modulation from Multiple Contributors to a STEP system There are several additional hydraulic features of the pump systems used within STEP systems that can either increase or decrease the peak flows that reach it. The factors to be considered include: The discharge characteristics of the pump chosen The tank dimensions The control floats set points 17

18 STEP Diurnal Flow Stream from a single home Shown is a simulated typical STEP diurnal flow pattern for a single residence 18

19 Design Flow and Peak Flow Estimation for STEP Systems It should be apparent that joining the discharges from several homes and developing a reasonable design flow presents a challenging problem. Two fundamental approaches to developing a design flow have been identified: Probability Method Rational Method 19

20 Pressure Distribution 20

21 Section Objectives: Understand and calculate Friction Losses Calculate Minor losses with Equivalent Lengths and Loss Coefficients Compute the total head required to pump water from a tank to a disposal field for a given flow rate. Interpret pump performance curves Compare pump performance curves to system requirements and select the appropriate pump for a system. 21

22 Types of Pressure Distribution Pressure delivery to a distribution box for subsequent gravity flow to individual disposal trenches Pressure delivery to the laterals within the individual disposal trenches, which is often referred to as Low Pressure Pipe or LPP. Pressure delivery to the common community pressure line of a STEP system Pressure delivery to the common community gravity line of a STEG system 22

23 Friction Losses Friction Losses are generated as the flow slides along the pipe and bumps into obstacles (turns, bends, expansions and contractions) and the energy that is used up in the water as it slides around itself in turbulence. Generally losses are related to these variables: Pipe Length Water Velocity Pipe Diameter Water Viscosity Pipe Material 23

24 Head Loss Equations Darcy Weisbach H = f * L/D * V 2 /2g l Hazen Williams Head loss/100 ft of pipe = 100 * (Q/(0.285 * C * D 2.63 )) 1.85 Q = flow in gallons per minute D = pipe diameter in inches C = smoothness coefficient 24

25 Head Loss Table If it is desired to use the Hazen Williams Formula directly, use a C value of 140 (for plastic pipe). If it is desired to use the Darcy Wesibach H l = f *(L/D)*V 2 /(2g) be sure to use the actual inside pipe diameter and a friction factor f equal to

26 Minor Losses Impacts of Minor Losses: losses resulting from changes in direction, changes in flow area and changes in friction due to fittings. Equivalent lengths or loss coefficients are used to calculate minor losses. 26

27 Equivalent Length and Loss Coefficients Equivalent lengths (L e ) assume each fitting or flow variation produces a head loss that is equal to the losses caused by an equivalent length of the pipe. For example, a 2-inch 2 gate valve may produce the same amount of friction as 1.5-feet of 2-inch 2 pipe. Therefore the equivalent length of the gate valve is 1.5-feet. Each fitting has a loss coefficient, K, associated with it. This coefficient is multiplied by the kinetic energy to get the associated loss 27

28 Head Loss: Continuous versus Perforated Hl multiple orifices along a pipe = 1/3 * Hl total pipe carrying the total flow 28

29 Pump & Pipe System A simple diagram of a sample Pump and Pipe System 29

30 System Curve Components The system curve is defined as the total of the static lift (the change in elevation) plus the friction loss in the piping system. 30

31 Hydraulic Machine For each pound of water lifted one foot, one foot-pound of work is done. If 550 foot-pounds of work are done per second we call that 1 HP {0.746 kw}. 31

32 Ideal versus Actual Pump Curve Pumps have efficiencies somewhat less than 100% and exhibit different efficiencies at different flow rates. This results in the typical pump curve departing more and more from the ideal curve as the flow departs more and more from its optimum design point. 32

33 Family Of Pump Curves Multiple pump curves from a manufacturer are needed prior to selecting a pump best suited for a design. 33

34 System Curve & Pump Curve Illustrated is a system curve superimposed upon a series of pump curves that will determine which operating point is used for a design. 34

35 Determining Flows Along The Line The first step in the design of a community STEP system is the application of pump hydraulic considerations to the common line serving the entire community. 35

36 Establishing the Hydraulic Grade Line The design starts with the required elevation and exit pressure or head and adds the head losses in each section of pipe based upon the computed design flow for that section of pipe 36

37 From Septic Tank To Common Line The second step in the design of a STEP system is the application of pump hydraulic considerations to the pressure line from the individual pump to the common transport system. 37

38 From Septic Tank To Dispersal System One of the more common applications of pump hydraulics in the onsite/decentralized arena is the use of pumps to deliver effluent to a dispersal field. 38

39 Gravity Conveyance in Onsite & Decentralized 39

40 Section Objectives: Understand the basics of gravity flow and its uses in Onsite design Understand and be able to use Manning s equation in Onsite design Become familiar with the basics in Gravity Sewer design Understand some basic designs and examples for wastewater distribution 40

41 Overview of Gravity Flow in Individual Onsite Systems The primary system components of a Onsite Gravity Flow system are as follows: Conveyance Treatment Distribution Dispersal 41

42 Gravity Collection and Conveyance in Decentralized Systems Decentralizes systems tend to serve smaller communities or groups of homes and/or businesses. 42

43 STEG/VGS Vs. Conventional Sewage Collection Systems STEG/VGS sewer systems, while still relying upon gravity to move the effluent along are different in several significant ways from the more conventional municipal sewer system. Shallow Depth Diameter Inverted Siphons Lack of Solids Velocities Scour 43

44 STEG/VGS Vs. STEP Systems STEG/VGS systems may be a more cost effective solution to effluent transfer in areas where a predominant downhill path can be developed. STEG/VGS systems differ from STEP systems in the following ways: Pumps Grades Diameter Flow Equalization 44

45 Gravity Conveyance Hydraulics In Onsite & Decentralized As the sewer pipe s s grade and elevation follow the natural contours there are sections of the pipe which flow full under slight hydrostatic pressure and there are areas which flow with a free water surface as open channels. 45

46 Important hydraulic considerations for gravity flow In gravity sewer design there are four major design factors to be considered: Slope Diameter Roughness Velocity {Min/Max} 46

47 Manning s Equation V = R 2 3 n S 1 2 A Q = Flow or discharge in cubic feet per second n = Coefficient of roughness A = cross-sectional sectional area of flow in square feet R = Hydraulic radius in feet S = Slope of the hydraulic gradient in feet per foot 47

48 Flow Depth versus Pipe Diameter. The hydraulic radius is the cross-sectional sectional area divided by the wetted perimeter. Determining the hydraulic radius for circular pipes flowing partially full can be extremely difficult. This figure can be used to solve a variety of unknowns based on the ratio of the flow depth to the pipe diameter. 48

49 Design Process for Gravity Sewer Systems Gravity flow can be used where there is a sufficient elevation difference between the treatment outlet and the disposal plumbing. Gravity flow systems are simple, passive and inexpensive, but are the least efficient method of distribution. Distribution is very uneven over the infiltration media Maximum and minimum flow variations are necessary factors in properly sizing and designing system components. 49

50 Design Limitations For Gravity Flow Sewers Sewer lines must be placed at a sufficient depth to prevent freezing and to receive wastewater from the lowest fixture unit location. 50

51 Distribution Gravity dispersal systems are used to disperse treated wastewater back into the environment For percolating systems the gravity distribution system is located in permeable, unsaturated natural soil or imported fill material Perforated pipe is installed to distribute the wastewater into the distribution system 51

52 Typical Soil Absorption Trench Though both the sidewalls and the bottom of the trench may act as infiltrative surfaces, most design guidelines call for the area of the drain field to be based only on the area of the bottom of the trench. 52

53 Uneven Flow Even with careful attention paid to keeping the distribution pipes level or with a constant grade, the effluent distribution will concentrate at the beginning of the pipe. Unfortunately, often due to less than perfect installation, the actual discharges between orifices is more unpredictable. 53

54 Serial Loading Trench Instead of dividing the flow equally among the trenches as in a system using D boxes, the highest trench is loaded until completely flooded before the next (lower) trench receives effluent and so on down slope. 54

55 Application of a diversion valve With diversion valves individual lines or entire sections of drain fields can be rested as considered necessary. 55

56 Distribution Example 1 Given: A four bedroom residence generating 600 gallons per day wastewater from typical family activities is discharged after treatment to a disposal field. The system will discharge into soils with a soil application rate of 0.4 gpd/sf. Due to soil conditions at deeper levels and regulatory constraints the trenches can only be 48 deep. Local regulations allow trenches with a maximum width of 24 and a minimum cover of 12.. The regulations also allow absorption area to include the bottom of the trench and the sidewalls up to the invert of the 4 4 dispersal pipe. Find: Total length of trenches. 56