FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS

Transcription

1 FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS NOVEMBER 2005 DAYTON & KNIGHT LTD. Consulting Engineers

2 FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS TABLE OF CONTENTS 1.0 BACKGROUND AND INTRODUCTION DESCRIPTION OF ONSITE SYTEMS Page 2.1 Location of Systems Potential Problem Areas for Ground Disposal DESIGN OF SEWAGE COLLECTION SYSTEMS Design Criteria Concept Designs and Costs Increased Loads to Existing Treatment Facilities CONCLUSIONS AND RECOMMENDATIONS APPENDIX A Design Details and Cost Estimates TABLES 2-1 Description of Areas Using Onsite Systems 3-1 Estimated Existing Wastewater Flows 3-2 Estimated Future (Design) Wastewater Flows 3-3 Summary of Cost for Concept Design 3-4 Increased Loads to Existing Treatment Facilities FIGURE 2-1 Concept Designs for Providing Sewer Service to Lots Using Onsite Systems Dayton & Knight Ltd. Page i

3 FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS 1.0 BACKGROUND AND INTRODUCTION The City of Powell River is currently undertaking the preparation of a Liquid Waste Management Plan (LWMP). One of the recommendations developed by the LWMP Advisory Committees for inclusion in the draft Stage 2 Report was that the City undertake a study to determine the cost of eliminating the use of onsite sewage disposal systems within the municipality, by providing sewer service to all properties that currently rely on onsite systems. The City subsequently commissioned this study to develop concept designs and costs for sewer service to the properties in question, and to summarize available information regarding the probable performance of existing and future onsite systems within the municipality. The objectives of the study were as follows: 1. Identify existing and future areas served by onsite (septic tank) systems within the City, together with known problems associated with ground disposal of sewage in these areas. 2. Develop concept designs for servicing of the areas within the City that currently rely on ground disposal of sewage. The concept designs will include Class D cost estimates for collection of domestic wastewater and conveyance to the nearest municipal treatment facility. 3. Produce a letter report summarizing the study findings, including recommendations on whether or not the sewer collection system should be extended to service specific areas that currently rely on ground disposal Dayton & Knight Ltd. Page 1-1

4 FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS 2.0 DESCRIPTION OF ONSITE SYSTEMS 2.1 Location of Systems The locations of the areas within the City of Powell River that currently have onsite sewage disposal systems was provided by City staff; these areas are illustrated on Figure 2-1. The individual areas were coded by number and then divided into sub-areas for convenience in developing concept designs and costs. As shown on Figure 2-1, Areas 1 through 4 are located within the service area for the Wildwood Lagoon, Areas 5 through 7 are located within the service area for the Townsite Treatment Plant, and Areas 8 through 12 are located within the service area for the Westview Treatment Plant. All occupied properties within the onsite areas were assumed to rely on onsite systems for sewage treatment and disposal. The number of lots, occupied residences, zoning, area and maximum population at build-out according to the City zoning for the areas that have onsite systems are summarized in Table 2-1. As shown on Figure 2-1, the southern portion of Area 7 is currently being serviced by the City. In the Wildwood area, the hillside below Areas 3 and 4 has been cleared and is slated for development, which will include the installation of sanitary sewers Dayton & Knight Ltd. Page 2-1

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6 Area TABLE 2-1 DESCRIPTION OF AREAS USING ONSITE SYSTEMS No. of Lots No. of Residences on Septic 1 Future OCP Zoning 1 Maximum Density 1 (Unit/ha) Area (ha) Maximum Population 2 Rural-Agricultural Resources Small Holdings Small Holdings Small Holdings Low Density Residential Small Holdings Small Holdings Rural-Agricultural Small Holdings Low Density Residential Resource Resource Low Density Residential Low Density Residential TOTAL Note: 1 Information from City 2 Assuming 2.13 people/unit 2.2 Potential Problem Areas for Ground Disposal Soils mapping of the study area was not available from Maps B.C. or from other sources. It appears that soils mapping in the Powell River area has not been undertaken. A summary of known or suspected problems associated with onsite systems in the study area was requested from the local Health Unit; these areas are identified on Figure 2-1. Two of the three areas identified (Yukon Avenue/Covey Street identified as Area 7 and Smarge Avenue/Fraser Street at the far north end of Wildwood identified as Area 1) have high water table with slow perking soils. High fecal coliform counts are reported in surface water drainage in the Yukon Avenue area (Area 7). The third area identified by the local Health Unit is at the far south end of the City, above Thunder Bay Street (identified as Area 12); although known problems were not identified in this area, it is reported to be relatively close to several private water wells and at least two commercial drinking water wells Dayton & Knight Ltd. Page 2-2

7 FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS 3.0 DESIGN OF SEWAGE COLLECTION SYSTEMS 3.1 Design Criteria For the purposes of developing concept designs for wastewater collection systems to service the areas currently relying on onsite systems, the system designs were based on the assumption that all properties within each of the areas would eventually be occupied. Land use according to the City Official Community Plan was used in estimating future service populations as follows: average number of people per residence = 2.13 people/unit rural-agricultural and resource land use = 1 unit/ha small holdings land use = 2.5 units/ha low density residential land use = units/ha Average day wastewater generation was assumed to be 410 litres/capita/day, and infiltration was assumed to be 0.1 litres/second/hectare in accordance with the City s Servicing Bylaw. The design flow for sewers and pumping stations was based on peak total flow, which was estimated as the sum of peak sewage flow plus infiltration (where peak sewage flow was assumed to be 5 times the average day wastewater generation). Design of gravity sewers was based on pipes flowing at 70% of total capacity at peak total flow. Pipe grades were estimated from topographical mapping of the study area where possible. PVC pipe with a roughness coefficient of 0.11 was assumed. Minimum and maximum design flow velocities were 0.6 m/s and 2.5 m/s, respectively for gravity Dayton & Knight Ltd. Page 3-1

8 sewers. Minimum and maximum velocities for forcemains were 0.9 m/s and 3.5 m/s, respectively. The estimated wastewater flows and infiltration for each area using onsite systems are summarized in Table 3-1. Unit cost criteria were developed from recent similar construction projects and the experience of the project team. The ENR cost index was used to update costs from past projects. Unit costs developed for construction of collector gravity sewers were $1/mm dia/m-length. Unit costs for construction of forcemains were $120/m for 100 mm diameter pipe, $150/m for 150 mm diameter pipe, and $200/m for 200 mm diameter pipe. 3.2 Concept Designs and Costs Where possible, the concept designs were based on gravity connections to collector sewers serving newly constructed pumping stations located in strategic low-lying areas, with conveyance via forcemains to the nearest existing trunk sewer or existing pumping station. Gravity sewers are assumed to follow natural grade unless the minimum velocity could not be met (in a few cases). In isolated cases where gravity connections to the new sewer collection system would not be possible due to topography, it was assumed that individual on-lot pumping systems would be used to connect to the nearest existing or new forcemain. Capital cost for individual on-lot pumping systems was assumed to be $5,000 each. The number of individual lots was based on existing zoning information. It was assumed that the City would provide the on-lot pump system but homeowners would bear the O&M cost. A site visit was conducted on May 3, 2005 by Al Gibb of Dayton & Knight Ltd. and Frank D Angio of the City of Powell River to review the areas included in this study. The schematic diagram for the concept designs is illustrated on Figure 2-1. Design details (pipe lengths and slopes, service population) and costs for each of the sub-areas shown on Figure 2-1 are contained in Appendix A. The design capacity of the collector systems was based on future projected flows assuming that all lots would eventually be Dayton & Knight Ltd. Page 3-2

9 occupied. A summary of the estimated existing and future (design) flows are shown in Tables 3-1 and 3-2 respectively. As shown on Figure 2-1, the two northern lots in Area 1 would flow by gravity to a small pump station with a forcemain along Smarge Avenue connecting to the existing sewer at the west end of Taku Street. The southern lot in Area 1 would require a single-home pumped connection to the new forcemain. The lots (to the north of Taku Street) in Area 2 would be serviced by pumped connections to a pressure sewer along Sutherland Street leading from a new pump station at the south end of Sutherland to discharge into the existing system at the east end of Taku Street. A gravity system for Area 2 was also considered; this would require a gravity collector running to the north along Sutherland which would discharge to the new pump station at the north end of Sutherland with a new forcemain to the connection at Taku Street. The capital cost of the gravity system for Area 2 (north) was estimated at $266,000 compared to $148,900 for the pressure option; the pressure option was therefore selected. Lots in Area 2 to the south of Taku Street would be serviced by pumped connections to a gravity sewer along Sutherland Street from Taku Street to discharge into the existing trunk main on Nass Street. Areas 3 and 4 in the Wildwood service area lie immediately above a large area that has been cleared and is reported to be slated for development (see Figure 2-1). For servicing Areas 3 and 4, it was assumed that a gravity collector sewer along Atlin Avenue could be connected to the sanitary sewers that would be installed as part of the new development, and that the wastewater from the entire area would then be routed to Pump Station No. 12 (Figure 2-1). Design of the sewer system servicing the new development would have to account for the additional flows from Areas 3 and 4. The northern most lot on Atlin Avenue in Area 3 would be serviced by individual on-lot pumping to the new development (Figure 2-1) Dayton & Knight Ltd. Page 3-3

10 Area 5 would be serviced by a gravity sewer along Grenville Avenue connecting to the existing trunk on Warner Street. Area 6 would also be serviced by gravity, with a new collector sewer along Tanner Avenue connecting to the existing sanitary system at Allen Avenue (an easement would be required see Figure 2-1). The southern portion of Area 7 is currently being serviced by the City. The remainder of Area 7 would be serviced by a gravity sewer along Yukon Avenue leading to a new pump station and forcemain connecting to the system currently being constructed at Cassiar Street (see Figure 2-1). Area 8 would be serviced by a gravity sewer along Tanner Avenue connecting to the existing system at Barnet Street. Area 9 would be serviced by a pressure sewer with pumped connections along Myrtle Avenue leading from a new pump station at the south end of Myrtle via a forcemain connecting to existing Pump Station Number 6. Two creek crossings would be required in Area 9, one to connect the northernmost lot to the new system, and the other for the forcemain (see Figure 2-1). As with Area 2 described earlier, a gravity collector along Myrtle Avenue leading to the new pump station would be significantly more costly to construct than the pressure system. The northern section of Area 10 would be serviced by a new gravity collector along Manson Avenue connecting to the existing trunk at Westview Avenue. The southern part of Area 10 and Area 11 would be serviced by a new gravity collector along Nootka Street connecting to the existing trunk at Ontario Avenue. Servicing of Area 12 would require a new gravity sewer connecting to the existing trunk at Thunder Bay Road Dayton & Knight Ltd. Page 3-4

11 TABLE 3-1 ESTIMATED EXISTING WASTEWATER FLOWS Sewer Collection System W ildwood Townsite Westview Area No. of Residences on Septic Area (ha) Existing Population Average Dry Weather Flow (ADWF) 1 (m 3 /d) Peak Flow (PF) 2 (m 3 /d) Infiltration (I/I) 3 (m 3 /d) ADWF + I/I (m 3 /d) PF + I/I (m 3 /d) Sub-total Sub-total Sub-total TOTAL Note: 1 Assumes 410 Litres/capita/day (per City's Engineering Design Standard) 2 Assumes Peak Wastewater Flow = 5 x ADWF (per City's Engineering Design Standard) 3 Assumes Infiltration = 0.1 litres/hectare/s (per City's Engineering Design Standard) TABLE 3-2 ESTIMATED FUTURE (DESIGN) WASTEWATER FLOWS Sewer Collection System W ildwood Townsite Westview Average Dry Area Maximum Weather Flow Peak Flow (PF) Infiltration ADWF + I/I PF + I/I Population 2 Area (ha) (ADWF) 1 2 (m 3 /d) (I/I) 3 (m 3 /d) (m 3 /d) (m 3 /d) (m 3 /d) Sub-total Sub-total Sub-total TOTAL Note: 1 Assumes 410 Litres/capita/day (per City's Engineering Design Standard) 2 Assumes Peak Wastewater Flow = 5 x ADWF (per City's Engineering Design Standard) 3 Assumes Infiltration = 0.1 litres/hectare/s (per City's Engineering Design Standard) The costs for providing each of the areas shown on Figure 2-1 with collector sewers are summarized in Table 3-3. Details of the cost estimates are included in Appendix A. The capital costs shown in Table 3-3 include pipelines, pump stations, and individual on-lot pump systems. The costs for service connections and works on private property other than individual on-lot pump systems are not included. Capital costs include an allowance of 30% of construction cost to cover engineering and contingencies. Annual operating Dayton & Knight Ltd. Page 3-5

12 costs include electricity cost for collector sewer system pump stations and labour cost for maintenance. An estimate of $6,000 per year was assumed for labour cost based on the City s O&M cost for pump stations. As shown in Table 3-3, the estimated total capital cost for servicing all of the areas currently using onsite systems within the City of Powell River would be $1,833,000. The capital cost for servicing the two areas identified by the local Health Unit as having high water table with low perking soils (Area 1 and the remainder of Area 7 on Figure 2-1) would be about $286,000 (note that part of Area 1 would be serviced by the pump station in Area 2). The capital cost for servicing Area 12 (near drinking water walls) would be $66,300. TABLE 3-3 SUMMARY OF COST FOR CONCEPT DESIGN Sewer Collection System Area Construction Capital Cost Engineering & Contingencies 4 Total Annual Operating Cost Wildwood Townsite Westview Note: 1 $118,000 $35,400 $153,400 $7,000 2 $286,000 $85,800 $371,800 $7,000 3 $107,000 $32,100 $139,100 $0 4 $124,000 $37,200 $161,200 $0 Sub-total $635,000 $190,500 $825,500 $14,000 5 $27,000 $8,100 $35,100 $0 6 $61,000 $18,300 $79,300 $0 7 $102,000 $30,600 $132,600 $7,000 Sub-total $190,000 $57,000 $247,000 $7,000 8 $160,000 $48,000 $208,000 $0 9 $155,000 $46,500 $201,500 $7, $191,000 $57,300 $248,300 $0 11 $28,000 $8,400 $36,400 $0 12 $51,000 $15,300 $66,300 $0 Sub-total $585,000 $175,500 $760,500 $7,000 TOTAL $1,410,000 $423,000 $1,833,000 $28,000 1 Assumes 410 Litres/capita/day (per City's Engineering Design Standard) 2 Assumes Peak Wastewater Flow = 5 x ADWF (per City's Engineering Design Standard) 3 Assumes Infiltration = 0.1 litres/hectare/s (per City's Engineering Design Standard) 4 Assumes Engineering & Contingencies = 30% of Construction Cost 3.3 Increased Loads to Existing Treatment Facilities The increased loads to each of the City s three wastewater treatment facilities that would result from servicing of the areas that currently rely on ground disposal (based on existing residences) are summarized in Table 3-4. The additional population in Townsite includes the population in Area 7 in which the lots are being connected to the sewer system at the moment. As shown in Table 3-4, connection of the areas currently using onsite systems should not significantly increase the service population of the Townsite and Westview facilities. Servicing of lots in the Wildwood area would increase the service population Dayton & Knight Ltd. Page 3-6

13 of the Wildwood Lagoon by about 5%. The analysis conducted in the draft Stage 2 LWMP showed that the lagoon system has the capacity to handle at least a 10% increase in service population, provided that adequate aeration and mixing are provided (the City recently upgraded the aeration and mixing in the lagoon). Servicing of the onsite systems in the Wildwood area would not exceed the design capacity of the treatment system, although it would reduce the amount of future development that the existing treatment system could accept. TABLE 3-4 INCREASED LOADS TO EXISTING TREATMENT FACILITIES Additional Service Population if Existing Sewer Collection Area Current Service Population Onsite Systems are Connected Population % Increase Wildwood 1, Townsite 4, Westview 9, Dayton & Knight Ltd. Page 3-7

14 FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS 4.0 CONCLUSIONS AND RECOMMENDATIONS The following conclusions and recommendations are based on the findings of this study. 1. The cost for providing sanitary sewer service to all of the areas currently relying on, onsite ground disposal systems is estimated at $1,833,000. These improvements should be undertaken on a priority basis where there are concerns that on site sewage disposal in specific areas poses a risk to human health or the environment. 2. The three areas where onsite sewage disposal is most likely to be problematic appear to be Yukon Avenue/Covey Street, Smarge Avenue/Fraser Street, and Thunder Bay Street, identified as Areas 1, 7 and 12 respectively on Figure 2-1. Efforts should initially be focused on these three areas. The City should meet with representatives of the local Health Unit to identify a monitoring program that will help to set priorities for servicing lots currently using onsite systems. 3. The cost estimates and recommendations in this study are based on the existing zoning information provided by the City. Should the zoning density change in future, this conceptual design will need to be re-evaluated Dayton & Knight Ltd. Page 4-1

15 FEASIBILITY STUDY FOR SEWER SERVICING OF LOTS USING ONSITE SYSTEMS APPENDIX A DESIGN DETAILS AND COST ESTIMATES

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