Wastewater Collection

Types of Sewer Systems Combined Separate In wet weather, both wastewater and stormwater flow in same system If flow is too high, untreated water is discharged into river (or channels in tehran) Separate Wastewater and stormwater flow in separate sewer systems

Sewer systems

Sewer Basics Collection and transport of wastewater from each home/building to the point where treatment occurs. Wastewater Characterization Solids Liquids Pipe System Plastic Ductile iron Concrete/lined Bricks

Satellite Wastewater Management Also called “decentralized” or “distributed” Undertaken by utilities for a variety of reasons: Economics To reuse water locally To avoid expanding “centralized facilities As communities expand, the distances from the new developments to existing wastewater treatment facilities becomes so great as to not be economically feasible Sustainable Cost Effective, good for the public and the environment

Collection System Alternatives Conventional Gravity sewers Septic Tank Effluent Gravity (STEG) Septic Tank Effluent Pump (STEP) Pressure Sewers with Grinder Pumps Vacuum Sewers

Gravity Flow Least expensive method of operation Gravity sewers are never full More vulnerable to H2S corrosion

Conventional Gravity Sewer Large pipe (200 minimum), manholes spaced 90-150 m Designed to transport solids Minimum velocity > 0.6 m/s (to avoid the deposition of solids) Max velocity = 4.5 m/s Infiltration and Inflow (I & I) Uniform Slope between manholes Sewer size Minimum slope (m/100 m) 8-inch (200) 0.40 10-inch (250) 0.28 12-inch 0.22 14-inch 0.17 16-inch 0.14 18-inch 0.12 24-inch 0.08

Conventional Gravity Sewer

Conventional Gravity Sewer Alignment: 24-inch sewers -600 (or smaller) should be laid with straight alignment between manholes. Changes in pipe size: When a smaller sewer joins a larger one (at a manhole), the invert (bottom) of the larger sewer should be lowered sufficiently to overcome head losses. An approximate method is to place the 0.8 depth point of both sewers at the same elevation. Sewer Materials: many types, materials and bedding shall prevent damage from external loads, and joints shall prevent leakage 0.8 depths are aligned

Manholes * Located at changes in sewer size, direction, or slope * Or every 90 – 150 m * Provides access for maintenance (cleanout, etc.) * Problematic because of Infiltration and Inflow (I&I)

Small Diameter Gravity Sewer Systems Wastewater flows from home to interceptor (septic) tank where settleable solids and grease are removed Wastewater flows by gravity to central collector pipe Central collector pipe can flow full in certain areas Pipe sizes are typically 4 and 6-inch diameter (100 – 150 mm)

Small Diameter Sewer Layout Pipe will flow full, under pressure in these areas

Interceptor Tank (same as Septic Tank)

Most Sewers Rely on Gravity Flow Most sewers are designed to flow by gravity (water flows down-hill) Includes sewer pipe from home to septic tank or to a municipal collector pipe Gravity sewers must follow the topography of the land Where gravity flow is not possible, pumps are used Individual unit pump Large municipal lift (pump) station

Hydraulics of Sewers Most sewers designed to flow at a velocity of 0.6 m/sec when flowing half-full Do not want pipe to flow full at peak flows Do not want pipe flow velocity too low—can’t transport solids Most designs are complex, but use basic hydraulic equations for computing necessary size and slope

Manning Equation for Pipe Flow V = velocity (m/sec) n = coefficient of roughness (dependent upon pipe material/condition) R = hydraulic radius = area/wetted perimeter (m) S = hydraulic slope (assumed to be slope of pipe) (m/m)

Basic Sewer Design Collector pipes (pipe in street) is minimum 8 inches (200) diameter (to allow cleaning) Service pipes (home or building to collector is 4 to 6 inches (100-150 mm)diameter Gravity sewer pipes have no bends, manholes used to make transitions in direction and pipe size Pipe sections between manholes are at a constant grade or slope (S)

Proportional velocity and discharge

Proportional geometry elements 0.015 0.4911 28 5.48648E-05 0.0368 0.1164 0.00036 0.016 0.5073 29 6.04235E-05 0.0380 0.1215 0.00042 0.017 0.5230 30 6.61557E-05 0.0392 0.1265 0.00047 d/D  radian  Ad Pd v/V q/Q

Conventional Gravity Sewer Bedding: specified by an engineer for pipe type and anticipated loads

Conventional Gravity Sewer Backfill: suitable material, free of debris, stones, etc. DO NOT DISTURB SEWER ALIGNMENT Separation from Water Lines: 3 m horizontal Water line 0.5 m above sewer

Crown Corrosion H2S + H2O    H2SO4 H2SO4 H2S

Pressure Flow Wastewater is actively pumped Used when gravity flow will not work, due to terrain or other factors Construction and operation usually more expensive than gravity flow

Pressure Sewers Each home/building has individual pump Wastewater pumped to a central treatment location Pumps “grind” sewage solids

Pressure Systems Can Pump Wastewater Treated by Septic Tank – Used for Homes Previously on Septic Systems

Wet Well Design

Conventional Gravity vs. Pressure Sewers

Sewer Alternatives and Characteristics

Septic Tank Effluent Gravity STEG Small diameter plastic pipe Conveys effluent from a septic tank with an effluent filter No solids to transport (no minimum velocity required) Can be installed at variable (flat) grades No manholes Air-release valves needed at high points

Septic Tank Effluent Gravity STEG

STEG Sewer Components

Pipe Size and Velocity

Septic Tank Effluent Pump STEP High-head turbine pump used to pump screened septic tank effluent into a pressurized collection system. Small diameter, plastic pipe (50 mm) No Solids transport (no minimum velocities required) Installed shallow Can follow terrain Air-release valves incorporated

STEP Components Building sewer (from house to septic tank) Septic Tank (or interceptor tank) Vaults/pump basins (effluent filter and pump) Pumps (submersible, high-head, turbine) Service lateral (1.25-inch typical) Check Valves (at pump outlet and at edge of property)

Septic Tank Effluent Pump STEP

STEP System Design Data

STEP System Interceptor Tank

Pressure Sewer with Grinder Pumps Discharge pump with chopper blades in a small pump basin Small diameter, pressure line, installed shallow Solids and greases are transported Relatively simple installation Somewhat higher O&M No I&I

Pressure Sewer Grinder Pump basin

Grinder Pump Basin

Vacuum Flow Not very common, but use is gradually increasing Vacuum pumps used to move waste through sewer Same advantages and disadvantages as pressure sewers Less water needed to transport waste

Vacuum Sewers Wastewater flows by gravity to a central collector well (up to four homes per well) When well fills a vacuum lines pulls wastewater to a central vacuum tank Wastewater pumped from central vacuum tank to treatment or a gravity sewer

Vacuum System Components Could also come from existing septic tank

Vacuum Sewer Central vacuum source maintains a vacuum on a small diameter sewer Pulls wastewater to a central location Ideal application: Flat terrain High water table 70-100 connections to be economical

Vacuum Sewer

Vacuum Sewer Components

Vacuum Station

Estimating Flow rate Equivalent Dwelling Unit (EDU) A residence with a given number of people (say 4) Represents the average household flowrate Design Peak Flowrate (DPF) Flowrate expected in the collection system, assuming a given number of EDUs are discharging at the same time Typical values for systems with >50 EDUs is 1.3 to 1.9 L/min-EDU Total DPF QDP = 0.5 N

How to Obtain Quantity (lpd) numbers Estimated design numbers in codes These are generally higher than actual use They provide a factor of safety Metered flow Add a factor of safety Average flows from similar facilities Peak flows Design

Estimated Wastewater Flows

How much wastewater do we produce each day? These values are rough estimates only and vary greatly by locale. Wastewater Characteristics

Typical data on unit loading factor and expected wastewater contaminant loads for individual residences. (Taken from Wisc. Comm. 83 code)

Daily flow pattern for a home Peak flow Flow – gallons/hr/person Average flow 6 pm MN 6 AM Noon MN Time of day

What are the wastewater flows? Domestic (sanitary) Industrial Infiltration/inflow Stormwater

Wastewater flow rates Rule of thumb: Iran domestic is 250 lpd/capita (ref. Tbl. 3-1 M&E) Developing countries: 20 to 200 lpd/capita ( Tbl. 3-9 M&E) Other: depends on facility (industry, commercial, etc.) I/I can be significant (Fig. 3.2 M&E)

Factors affecting flow rates Geographical location & socioeconomic conditions Type of development Season Time of Day Climate (rain or dry)

“Diurnal variations” in domestic wastewater flows

Wastewater flows Flows are either normally distributed or log-normal (log of flows are normally distributed) Statistical procedures based on flow history used to determine average (dry weather, wet weather, annual daily), peak (instantaneous, hour), maximum (day, month), minimum (hour, day, month) (Table 3-11 M&E)

Wastewater loads Treatment is to decrease chemical constituents, but quantity to be treated depends on: Concentration (typ. mg/L) Flow rate (typ. lpd or MLD) Mass loading: Concentration x Flow = mass /time

Weekly flow Peak flow Average flow Flow – gallons/day M T Day S W T F

Commercial Sources Many sources Variable from site to site High organic matter High fat, oil and grease Nitrogen Phosphorus Cleaning agents Heat (high water temperature)

Influences on Wastewater Characteristics Organic matter BOD and TSS Nitrogen Organic nitrogen, ammonia, nitrate Pathogens Bacterial –use fecal coliform as indicator organism Viral – coliphage as indicator organisms Fats, oils and greases Phosphorus Cleaning agents/antibacterial/VOC Excessive laundry wastes Medications What else????

Many styles of low Flow toilets today. All toilets sold today has 1.6 gal./flush.

Manholes Needed for: Spaced every 100 to 120 m along the sewer Change in direction Change in pipe size To route sewer under roads, rivers, etc. Clean-out and inspection Spaced every 100 to 120 m along the sewer

Manhole Construction Brick was once common Most manholes are now concrete Smaller sizes are pre-cast concrete Large manholes are constructed in-place

Typical Manhole Fig 5.1, p 134

Problems in Sewers Blockage Corrosion in sewers made of Inflow Concrete Iron Inflow Infiltration Leakage

Blockage in Sewers Caused by sand, rocks, tree roots, various wastes Can cause flooding In streets In basements

Corrosion in Sewers Concrete is made of cement, which is basic H2S gas formed by anaerobic bacteria H2S converted to sulfuric acid (H2SO4), chemically or by bacteria Sulfuric acid reacts with cement (neutralization) Result: cement is dissolved by sulfuric acid

Effects of long detention times

Sources of Inflow Water from roof and cellar drains Water leaking around manhole covers Cause excess hydraulic load on treatment plant

Sources of Infiltration Seepage of groundwater into pipe joints and manholes Controlled by Good construction materials and practices Avoiding construction in saturated soil, if possible

Leakage from Sewers Caused by Cracked, broken, or corroded pipe Bad connections at joints Can cause groundwater contamination, and damage street and building foundations

Pump Stations Pump (lift) sewage from low to higher elevation, generally from end of one gravity sewer section to another, higher section Consist of a wet well and pumps Wet well forms a place for wastewater to collect and be pumped from

Large Pump Station Source: Metcalf & Eddy, Inc. Wastewater Engineering: Collection, Treatment and Disposal. McGraw-Hill:New York, 1972.

Small Pump Station

Pump Station

Pump Station Details

Main Pump Station