Emerging Technologies in Wastewater Collection Systems

Transcription

1 Keville Enterprises Emerging Technologies in Wastewater Collection Systems Douglas McCutchen, CCM Brian Karmasin, P.E., BCEE CMAA Annual Conference Oct 2008 San Francisco, California Today s s Presentation Outline Overview of Technologies Design considerations Cost implications Advantages/disadvantages Case Study: Scott Mill Hill, Jacksonville, FL Engineering Feasibility Study Design Implementation Procurement Construction

2 Today s s Presentation Outline Overview of Technologies Design considerations Cost implications Advantages/disadvantages Case Study: Scott Mill Hill, Jacksonville, FL Engineering Feasibility Study Design Implementation Procurement Construction Overview of Technologies Conventional gravity systems Shallow gravity systems Vacuum sewer systems Low pressure systems Small diameter gravity sewer systems

3 Overview of Technologies Conventional gravity systems Shallow gravity systems Vacuum sewer systems Low pressure systems Small diameter gravity systems The Gravity Sewer System Includes Trunk Sewers, Manholes, Collection Lines, and Private Laterals

4 Why consider conventional gravity sewers? Tried and true for decades System works passively (except for lift stations) Flexibility to expand Design Considerations Size and slope of pipes determined by minimum scouring velocity requirements Installation can require deep excavations to maintain grade Potential for I/I problems due to leaky manholes, pipe joints Operation & maintenance costs and requirements are well known Requires lift stations must have available properties

5 Cost Implications Most cost effective for areas with topographic relief Roadway rebuild typically required due to large open cut trenches Minimal relief = deep sewers = more money Wider open cut trenches Dewatering Impacts to surrounding areas O&M costs only for pumps at lift stations Advantages Familiarity Maintenance Operating cost Storage Future Expansion Peak flow shaving Familiarity with the operation and maintenance of gravity sewer systems and lift stations. There are no mechanical components to gravity connections and therefore no routine maintenance is associated with it. Operational costs for the gravity sewer systems are only for the pumps at the various lift stations. Gravity sewer systems typically have large amounts of storage volume. Conventional gravity sewers are typically oversized in design and are generally accommodating to future flow. Can use a SCADA system to set lift station cycle times to shave peak flows at their WWTPs.

6 Disadvantages Construction cost Impact to natural environment Dewatering Multiple site requirements Infiltration Depending on the depth of construction, amount of dewatering, and amount of replacement done, cost can run high. Gravity sewers in flat areas can tend to run deep. Deep installation can lead to wide trenches and impact to natural flora and fauna in the area. This alternative can require extensive dewatering due to high groundwater conditions and deep trenches; therefore the construction cost can be higher. Typically, several lift stations are required for a sewered area. Ownership of the land and/or easements would be required to facilitate the installation of the stations. Based on the depth of sewers, the groundwater table and the number of manholes, the groundwater infiltration in the collection system can be high. Installation of Conventional Gravity Sewer System

7 Overview of Technologies Conventional gravity systems Shallow gravity systems Vacuum sewer systems Low pressure systems Small diameter gravity systems What is a Shallow Gravity Sewer? Same concept as conventional gravity sewer except: Cleanouts instead of manholes Packaged low flow lift stations Maximum depth of pipe before pumping is 8 to 10 feet

8 Why consider shallow gravity sewers? Same technology as the tried and true gravity system Pipes installed at minimal cover means easier and quicker construction Decreased aesthetic impacts during construction Utility can preclude hiring engineer for lift station design since lift stations are packaged Design Considerations Size and slope of pipes determined by minimum scouring velocity requirements Traffic loading in right of way concerns Decreased flexibility when line is plugged due to lack of manholes Requires additional, but smaller, lift stations compared to conventional gravity Additional lift stations yields additional maintenance requirements

9 Cost Implications Most cost effective for areas with topographic relief Roadway rebuild typically required due to large open cut trenches Minimal relief = deep sewers = more money Wider open cut trenches Dewatering Impacts to surrounding areas O&M costs only for pumps at lift stations Advantages Familiarity Maintenance Storage Peak flow shaving Installation cost Future Expansion Familiarity with the operation and maintenance of gravity sewer systems and lift stations. There are no mechanical components to a gravity connection and therefore no routine maintenance associated with it. Gravity sewer systems typically have large amounts of storage volume. Utilities can use a SCADA system to set lift station cycle times to shave peak flows at their WWTFs. Minimal excavation and backfill along with dewatering. Expandable in developments. Closed system Infiltration from the collection system is minimized.

10 Disadvantages Multiple Site Requirements Typically, several lift stations are required for a sewered area. Ownership of the land and/or easements is required to facilitate the installation of the stations. Solids Deposition Placing pipe in the ground at shallow depths could translate into a minimal slope, causing solids to settle out in the piping. Traffic Loading Pipes can possibly be damaged by traffic loading. O&M Costs A large number of pump station systems can increase O&M costs due to the power consumption and maintenance required on the units. Overview of Technologies Conventional gravity systems Shallow gravity systems Vacuum sewer systems Low pressure systems Small diameter gravity systems

11 Why consider vacuum sewers? Poor site conditions (bad soils, high groundwater) Neighborhood retrofit Site issues for multiple lift stations What is a Vacuum Sewer System? Collection chambers Sit in ROW and collect sewage from homes/businesses Vacuum mains Convey sewage from the collection chamber to the vacuum pump station Vacuum pump station Vacuum pumps and collection tank provide negative pressure on the collection system Sewage pumps draw out of collection tank and pump into force main for further conveyance

12 A Vacuum Sewer System Relies on a Central Vacuum Station to Suck Wastewater from Each Valve Pit Collection Chamber

13 Design Considerations Collection chamber Provide 1 chamber serves up to 4 households Accommodate H-20 H traffic load Set at elevation to allow gravity flow from the home Pipeline SDR 21 PVC -Transmission mains and service laterals Minimum slope of 0.2% between lifts Ductile iron pipe above grade Pipeline installed in a sawtooth profile Sawtooth Profile

14 Design Considerations Vacuum Station Locate in low spot in the collection area Two sets of pumps vacuum and sewage Odor control system needed Standby diesel generator required Careful selection of sewage pumps is crucial for proper facility operation Cost Implications Best suited for areas with poor soils and/or high groundwater with some topographic relief Shallow construction yields reduced roadway repair requirements Vacuum station houses two sets of pumps which will increase O&M costs Increased field time for operations staff due to maintenance needs for collection chambers

15 Advantages Single site requirement Pipeline Impacts to surrounding environment Closed system System integrity Typically, only one vacuum and pumping station is required. The station can typically pull vacuum at the vacuum main at a distance of up to 10,000 linear feet. Small pipe diameters are sufficient if station is properly located. Smaller piping and shallow depths tend to lead to smaller impacts to existing flora and fauna, homeowner landscaping, and driveways. Infiltration from the collection system is minimized, therefore reducing the operating and pumping costs at the wastewater treatment facility. The integrity of the system (from collection chambers to the vacuum station) can be monitored by the operators from a control panel located at the vacuum station Disadvantages Operators will be required to respond to low Maintenance Calls vacuum alarms in a timely fashion. Utility will need to purchase and maintain a spare Spare Parts Inventory parts inventory for the vacuum system. Vacuum sewers are not as flexible as conventional Future Expansion gravity sewers if future growth is expected. Storage Storage in the collection system does not compare to gravity sewer systems. Stations typically run in a batch cycle, meaning Peak flows/loadings to WWTFs will see unsteady flows if the majority of WWTFs the service area is on vacuum. Vacuum system chambers are located in front Aesthetics yards of each home and can require a 4 to 6 inch vent system in customer s yards.

16 Overview of Technologies Conventional gravity systems Shallow gravity systems Vacuum sewer systems Low pressure systems Small diameter gravity systems What is a Low Pressure Sewer System? Common force main Individual grinder pump station

17 Grinder Pumps May Be Installed at the Right-of of-way or Near the Home Grinder Pump Unit Service Lateral Force Main Home Sewer Right-of-Way Force Main Why consider pressure sewers? Difficult site conditions Topography High groundwater level Topography Economics Infiltration / Inflow

18 Design Considerations Grinder Pump One unit per home Plastic tank and controls Positive Displacement vs. Centrifugal Design Considerations Pressure Mains Small Diameter 1 ½-in to 4-in4 Material PVC or HDPE Operating Pressure < 70 psi Service Connection Service pipe 1 ¼-in Shut off / check valve at property line

19 Design Considerations Manholes / Appurtenances Used for isolation, cleanouts and air release A STEP System Removes Solids with a Septic Tank Prior to Pumping

20 Advantages Constructability Construction cost Infiltration Safety Trenching is reduced due to small diameter piping. Installation can also be done utilizing trenchless technologies. Shallow excavation and backfill, along with small diameter piping reduces construction costs. The lack of manholes in a pressure system, along with the use of gasketed PVC or buttwelded HDPE, reduces or eliminates infiltration. There are no manholes or wetwells to be entered by the operation and maintenance personnel. Disadvantages O&M Costs Maintenance cost Maintenance knowledge Reliability Storage Public perception The homeowner is required to provide power and maintain the system. Annual maintenance is generally required. Homeowner will have to learn how to maintain the pumps. Power outages can potentially cause overflows into homes. Storage in the system is only in the gravity line and/or septic tank. Shifting O&M costs to the homeowner may cause customer complaints to the utility provider.

21 Overview of Technologies Conventional gravity systems Shallow gravity systems Vacuum sewer systems Low pressure systems Small diameter gravity systems What is a Small Diameter Gravity Sewer? Building sewer Interception tank Service lateral Collection main Lift station

22 A Small Diameter Gravity Sewer System Removes Solids to Reduce Required Pipe Size Why consider small diameter gravity sewers? Poor site conditions (bad soils, high groundwater) Provides gravity service through the system Use of septic tank provides safeguard for sewage storage

23 Design Considerations Hydraulic design should allow for infiltration and inflow in pipeline Interceptor tanks must be watertight Provide means to flush collector mains Lift station needs to consider wastewater will be septic and corrosive Cost Implications Reduced capital costs Small pipe size Shallow construction (24 to 30 inches) Use of cleanouts minimizes manhole costs Installed in ROW

24 Advantages Familiarity Maintenance Operating cost Storage Peak flow shaving Construction cost Closed system O&M staff is familiar with the operation and maintenance of a gravity sewer system. There are no mechanical components to a gravity connection. Operational costs are only the pumps at the various lift stations. Gravity sewer systems have large amounts of storage volume. The septic tank also has storage volume. Use SCADA system to set lift station cycle times to shave peak flows at their WWTFs. Less excavation and backfill than conventional sewers and lack of manholes and smaller pipe sizes decreases capital costs. Infiltration from the collection system is minimized. Disadvantages Multiple site Requirements Septic Tanks Limited experience Typically, several lift stations are required for a sewered area. Ownership of the land and/or easements is required to facilitate the installation of the stations. Solids from the septic tanks must be pumped out periodically by the homeowner. SDGS systems are not widespread, however several hundred have been installed in the Odors Odors may result from sewage going septic in the stations.

25 Today s s Presentation Outline Overview of technologies Design considerations Cost implications Advantages/disadvantages Case study: Scott Mill Hill, Jacksonville, FL Scott Mill Hill, Jacksonville FL

26 Background Information Existing neighborhood that includes over 400 homes and small businesses Identified as part of JEA s s Septic Tank Phase Out program Located in proximity to the St. Johns River Very high groundwater level Geotechnical investigations indicated some areas had extremely poor soil Owner Concerns Difficulty finding three separate sites for gravity sewer lift stations Construction impacts to residents Roadway closures Tree removal Construction costs Acquire property or easements for lift stations Installing gravity sewer in high groundwater/flat area near St Johns River

27 Engineering Feasibility Study Evaluated conventional gravity sewer versus vacuum sewer Owner did not want low pressure system 20 year lifecycle cost indicated vacuum sewer was 18% cheaper than conventional gravity sewer system JEA decided to install vacuum system Design Implementation Preliminary investigation Soft digs Geotechnical borings Survey Routing analysis for vacuum mains Homeowner survey for service connection

28 Design Implementation Line sizing and layout Roadway impacts Traffic impacts Vacuum station sizing Line layout and design

29 Vacuum Station Collection Tank Incoming vacuum mains Sewage pumps Isolation valves Vacuum Station Vacuum Pumps Suction piping Discharge piping Odor control required

30 Value Engineering Opportunity Initial bids were high VE Team Areas of Discussion Changes made and savings achieved Construction QA inspection oversight Daily vacuum Testing Final 4-hour 4 Test Testing of vacuum chambers

31 Construction Backfill and compaction Use of dedicated air intake lines Construction Changes during construction Use of lifts to avoid utility conflicts

32 Questions & Answers