Wastewater Collection Systems

1. Wastewater Collection Systems On completion of this module you should be able to: • Discuss the sources of wastewater • Describe wastewater characteristics • Understand the relevant sections of the legislation relating to sewer collection systems and wastewater • Design a wastewater collection system

2. Wastewater Collection Systems Sources of wastewater • Domestic flows • Industrial and trade wastes • Urban stormwater • Infiltration/inflow

3. Wastewater Collection Systems Materials in wastewater Impurities 0.01% Chemical Biological Physical form inorganic suspended organic dissolved living dead bacteria, fungi, protozoa, algae

4. Wastewater Collection Systems • Hydraulic loading (ML/d) • Organic loading (kg/m3.d) • Concept of equivalent population (ep) for design Wastewater presents a unique design problem

5. Wastewater Collection Systems Organic measurements • There are many different sources, types and complexities of aqueous organic matter • It is not possible to quantitatively measure or determine every organic constituents • Use of concept of biodegradability and non-biodegradability through oxygen demand exerted by microorganisms or chemicals

6. Biochemical Oxygen Demand (BOD5) Defined as the oxygen demand for a mixed population of aerobic heterotrophs bacteria in oxidising biodegradable organic carbon present in a sample in 5 days at 20o C • Free oxygen is used as the terminal electron acceptor • [(C,H,O), N,P,S] + O2 CO2 + H2O + NH4+ + S2- + PO43-+ energy • Reproducibility is  20% but reflects actual biodegradation • There are some limitations • In addition to organic carbon, reduced nitrogen I.e. ammonia can also be oxidised by nitrification

7. Chemical Oxygen Demand (COD) The test uses a strong oxidising chemical agent to completely oxidise organics • The oxygen equivalent of the organic matter is determined by the amount of K2Cr2O7 used in boiling acid at 150o C in the presence of a catalyst (silver sulfate) • The test takes 2 – 3 hours and reproducibility is  10% • The test cannot discern between biodegradable and non-biodegradable carbon, consequently COD values will be higher than BOD5

8. Solids in wastewater • Total Solids, mg/L • Total Suspended Solids or Non-Filterable Residue (TSS or NFR), mg/L • Total Dissolved Solids, mg/L

9. Wastewater Collection Systems Relevant legislations • Sewerage and Water Supply Act is replaced with the Plumbing and Drainage Act 2002 and the Standard Plumbing & Drainage Regulation 2003 • Environmental Protection Act 1994that relates to quantity and quality of flows into the environment

10. Plumbing and Drainage Act 2002 The act and its supporting Standard Plumbing & Drainage Regulation 2003 generally deals with • Assessing plumbing and drainage work • Product certification • Installation on premises

11. Environmental Protection Act 1994 The act relates to quantity and quality of flows into the environment • All discharges are subject to the Environmental Protection Act 1994 • Administered by the Environmental Protection Agency • Generally licence conditions of BOD5 < 20 mg/L; NFR < 30 mg/L; DO > 2 mg/L

12. Wastewater Collection Systems Types of wastewater systems • Separate versus combined systems • Gravity and pumped flows • Small collection systems using pressure or vacuum

13. Wastewater Collection Systems Pressure system

14. Wastewater Collection Systems Vacuum system

15. Wastewater Collection Systems Sewer installation • Sewer alignment • Depth of sewer • House connection • Location of manholes • Testing of sewers and house-drains

16. Wastewater Collection Systems Sewer installation

17. Factors that control the depth of sewers • Self-cleansing velocity ie. minimum slope • Minimum cover to protect the sewer • Required depth to drain properties serviced • Sufficient depth to avoid other services

18. House drain and connection • House drain is the property sewer pipe that adjoins council’s sewer • Minimum house drain dia. is 100 mm with a min. slope of 1:60 allowing 0.5 invert depth at the head • House drains must be vented at the head • House connection is made at the lowest point

19. Wastewater Collection Systems Sewer installation Typical sewer and housedrain connection

20. Wastewater Collection Systems Typical manhole configuration

21. Testing of Sewers and House-drains To ensure the integrity of connections • Water test at a pressure equal to 2 m head at the highest point. Loss of water less than 1 L/m dia. m length in 30 minutes • Air test at 30 kPa for 3 minutes. Pressure drop to be monitored • Manholes and inspection chambers also to be tested for water tightness

22. Locations of Manholes • Not more than 90 m apart for sewers 375 mm and less • Not more than 150 mm apart for sewers > 375 mm • Manholes to be provided at end of each sewer and for changes in direction, grade and at intersections

23. Design of gravity sewers • System must drain all points of the catchment • Peak wet weather flow capacity • Self-cleansing flow velocity • gravity flow at minimum slope • Pressure mains where necessary Design parameters

24. Design for maximum flow Queensland Planning Guidelines for Water Supply and Sewerage • Average dry weather flow (ADWF) 275 L/c.d • Peak dry weather flow, PDWF = C1 x ADWF • Peak wet weather flow, PWWF = C1 .ADWF + I/I • Maximum flow at 3/4 pipe depth

25. Peaking factors for maximum flows

26. Proportional velocity and discharge

27. Effects of detention times in rising mains

28. Wastewater Collection Systems Finally do we question: • Is it still environmentally responsible to use 50 - 80 kg/day of drinking water to transport 1 - 1.5 kg/d of human waste to a treatment plant? • Do we have to continue improving the wrong solution or do we have the intelligence for new solutions?