Chapter 5 Existing Wastewater Facilities

Transcription

1 Chapter 5 Existing Wastewater Facilities \\ \TOC.doc

2 CHAPTER 5 Existing Wastewater Facilities 5.1 INTRODUCTION Lacey s wastewater utility serves over 21,000 acres (33 square miles) of residential and commercial customers within the City s Urban Growth Area (UGA) boundary. The system is classified as a separate system, meaning that storm water connections are not allowed. The wastewater system consists of gravity sewers, pump stations, force mains, septic tank effluent pump (STEP) systems and community on-site septic systems. A sizeable portion of the service area east of the City limits is currently developed with single-family homes served by individual on-site septic systems, which are not maintained by the City. The original wastewater collection system was constructed and brought on-line in the late 1960s and early 1970s. The system, both inside the City limits and within the UGA outside incorporated Lacey, has grown incrementally since that time as additional development occurred and wastewater facilities were constructed. The system has experienced substantial growth during the later part of the 1990s to the present. The system collects and transports wastewater to the regional LOTT Treatment Facility. The wastewater utility is responsible for operating and maintaining approximately 160 miles of gravity, STEP and force main piping sized 1-inch through 30-inches in diameter, 24 conventional lift stations, 10 STEP system lift stations serving approximately 2,500 residential and community Septic Tank Effluent Pump (STEP) systems. Emergency generation facilities are installed at 19 of the pump stations and there are eight odor control facilities. All wastewater collected by the City flows to the LOTT Regional Wastewater Treatment Facility adjacent to Budd Inlet in the City of Olympia. The treatment plant is operated by a joint intergovernmental agreement developed and signed by the Cities of Lacey, Olympia, Tumwater and Thurston County. 5.2 WASTEWATER SYSTEM MAPPING The City currently maintains mapping of the sanitary sewer system in AutoCAD drawing files. Historically, city staff updates the drawing files to reflect newly constructed facilities after as-built information are received. The City has future plans to convert mapping from the AutoCAD format to a Graphical Information System (GIS) format. The AutoCAD mapping provided by the City was converted into GIS format and forms the basis for figures included in this report. The existing wastewater system is shown in Figure GRAVITY SEWERS 5-1

3 The gravity sewers transport wastewater to one of the pump stations operated by the City where it is ultimately conveyed to the LOTT interceptor system. The gravity sewers are 4-inches through 30-inches in diameter with the vast majority of the system being 8-inch collector sewers. The City s collection system is relatively new. The older parts of the system were constructed only years ago and most of the system is in good condition. Gravity lines are constructed out of PVC and concrete, however the majority is PVC. Table 5.1 presents the City s inventory of gravity pipe as of early Table 5.1 Existing Gravity Sewer Inventory* Pipe size Length (ft.) Length (mi.) 4 ** , , , , , , , , , , Total 536, * Data from filename: Sewer Footages 05.xls ** Does not include sides sewers 5-2

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5 5.4 STEP SYSTEMS There are two styles of STEP (septic tank effluent pump) systems in use. One of these is the traditional STEP system where wastewater generated at an individual residence discharges to an on site septic tank in which a STEP pump is installed. The system is similar to a septic tank on-site treatment system in that side sewer piping conveys wastewater from the residence directly to the septic tank where grit and organic solids are allowed to settle out. However, rather than discharging the effluent to a drain field, it is pumped into dedicated STEP system piping. Within these STEP neighborhoods, the individual STEP pumps discharge into a common small diameter pipe that increases in size as more individual STEP flows are added. The wastewater is ultimately conveyed to a gravity sewer or lift station. The other STEP system style is currently being used at most of the new developments now being constructed which are too distant from Lacey s existing gravity system to be economically connected. In this system, individual residences within the development discharge into local gravity sewers, which in turn discharge into a neighborhood STEP/submersible lift station. Prior to entering the lift station, the wastewater enters a debris tank where solids are allowed to settle out. The lift station effluent is discharged into dedicated STEP system force main piping, which is ultimately conveyed to the gravity system. Over time, solids build up in both the individual septic tanks and the debris tanks at the STEP/submersible lift stations and must be periodically pumped out. The existing STEP piping varies in size from 1-inch through 16-inches in diameter. Table 5.2 displays the City s STEP piping inventory by size and length. Table 5.2 Existing STEP System Pipe Inventory* Pipe size Length (ft.) Length (mi.) 1 4, , , , , , , , , Total 242, * Data from filename: Sewer Footages 05.xls 5-4

6 The City initially decided to allow STEP systems because they offer a relatively low cost method of providing sewer service to outlying portions of the service area. However maintenance costs have proven to be higher than anticipated. The large diameter STEP forcemains were evaluated to assess the potential impacts of replacing the existing STEP pumps with grinder pumps. Refer to Chapter 6 for further discussion. Refer to Chapter 9 for a detailed discussion on maintenance and performance of the STEP systems. Approximately 2,464 STEP systems are currently in service, served by over 46 miles of pressure lines. Almost all of the installations have been in new developments; however new STEP systems continue to be constructed and may be used to replace existing onsite septic systems in the future. The systems have been utilized in new housing developments and in areas where conventional gravity and pump systems are considered cost prohibitive. Overall, the systems are in good condition; however maintenance on these systems causes a substantial financial burden on the wastewater utility. Much of the maintenance cost is due to STEP tank pump outs. Odors and some additional maintenance due to corrosion, which results from the introduction of the STEP effluent into the conventional gravity collection system, are also a problem. 5.5 ODOR CONTROL Residents in areas served by STEP systems have historically registered odor complaints, which the utility takes very seriously. The odor complaints are concentrated near manholes where STEP effluent is injected into the gravity system. Odors are typically caused by the formation of hydrogen sulfide in the STEP tanks and force mains. Hydrogen sulfide is formed when wastewater is deprived of oxygen for an extended period of time, as is often the case with STEP systems where wastewater can remain in the lines for long periods of time. When released into the atmosphere above the wastewater level in partially full sewer lines, the hydrogen sulfide tends to accumulate at the crown of the pipe where it can oxidize into sulfuric acid, which is corrosive to concrete. Hydrogen sulfide is a colorless, inflammable compound with the characteristic odor of rotten eggs. In addition to the odor problems, it can and does cause corrosion problems in the collection system downstream of the STEP system discharges. Concrete facilities and metal appurtenances are primarily impacted. Since most of the newer sewer piping is constructed of PVC, generally only concrete manhole structures are showing signs of degradation. The 1999 Plan listed a partial inventory of manholes that were evaluated for corrosion problems caused by low dissolved oxygen levels and the formation of Hydrogen sulfide. Field investigations of manholes were not conducted for the 2005 Plan Update. The reader is referred to the inventory of manhole corrosion presented in Table 5-5 of the 1999 Plan. To date the City has installed eight odor control facilities to mitigate odor complaints and reduce corrosion associated with STEP systems. Bioxide is being injected directly into STEP force mains at five locations. There are two active soil biofilter air-scrubbing systems installed and three additional systems that are inactive at this time. 5-5

7 Lastly, there is a single site where air is injected into a 1,500 gallon STEP force main receiving tank to increase dissolved oxygen levels. The City has had success with the Bioxide systems and anticipates adding more of these injection systems in the future. 5.6 SEWAGE LIFT STATIONS The City operates and maintains 24 conventional sewage lift stations, 10 STEP system lift station and approximately 12 miles of force main. A physical inspection of each lift station was performed during the study and a description of each station is provided in Appendix D. A summary of lift station data is presented in Table 5.3 and Figure 5-2 shows a schematic relationship of the existing lift stations. 5-6

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9 L.S. NO GPM COMBINED FORCE MAIN L.S. NO GPM L.S. NO GPM GRAVITY TO LOTT ~ 80%OF ~ LACEY VOL. GRAVITY TO LOTT ~ 20%OF ~ LACEY VOL. SLEATER-KINNEY TRUNK L.S. NO GPM L.S. NO GPM CENTRAL LACEY TRUNK LINE L.S. NO GPM L.S. NO GPM CARPENTER RD STEP FORCE MAIN L.S. NO GPM L.S. NO GPM RUDDELL TRUNK L.S. NO. 4 L.S. NO GPM L.S. NO GPM 175 GPM L.S. NO GPM L.S. NO GPM L.S. NO GPM L.S. NO GPM L.S. NO GPM L.S. NO GPM L.S. NO GPM LAC-FIG5-2.DWG 12/22/05 L.S. NO GPM L.S. NO GPM L.S. NO GPM EX

10 5.7 FORCE MAINS An inventory of the City s force mains, sorted by pipe diameter, is presented in Table 5.4. Most of the smaller lift stations pump to 4-inch diameter force mains, while the larger lift stations generally pump to 6 and 8-inch diameter force mains. Table 5.4 Inventory of Force Mains Pipe Diameter (inches) Length* (lf.) Length (miles) 18 7, , , , , , Total 62, * From data provided by City personnel Table 5.5 displays a listing of force main characteristics for each lift station. The capacity is calculated using a maximum velocity of eight (8) feet per second. 5-9

11 Table 5.5 Lift Station Force Mains Lift Station 1, 2 Diameter Material 3 Capacity 4 Length (lf.) Force Main (inches) (GPM) L.S. No. 2 4 AC 3, L.S. No AC 3,000 1,960 L.S. No. 4 6 PVC 1, L.S. No. 5 4 AC L.S. No. 6 4 AC L.S. No. 8 4 PVC L.S. No. 9 6/6/6.8 PVC/PVC/HDPE 1300/1300/ /1285/1375 L.S. No PVC 2,500 1,250 L.S. No PVC L.S. No PVC L.S. No PVC 1, L.S. No PVC L.S. No PVC L.S. No PVC 1, L.S. No PVC 2,640 1,250 L.S. No PVC 1, L.S. No. 21 N/A PVC 50 N/A L.S. No PVC 2, L.S. No PVC N/A 1,250 L.S. No. 24 N/A N/A N/A N/A L.S. No. 25 N/A N/A N/A N/A L.S. No. 26 N/A N/A N/A N/A L.S. No. 27 N/A N/A N/A N/A L.S. No. 28 N/A N/A N/A N/A L.S. No. 29 N/A N/A N/A N/A L.S. No. 30 N/A N/A N/A N/A L.S. No. 31 N/A N/A N/A N/A L.S. No. 32 N/A N/A N/A N/A L.S. No. 33 N/A N/A N/A N/A L.S. No. 34 N/A N/A N/A N/A L.S. No. 35 N/A N/A N/A N/A L.S. No. 36 N/A N/A N/A N/A L.S. No. 37 N/A N/A N/A N/A L.S. No. 38 N/A N/A N/A N/A 1. All information was provided by City maintenance personnel. 2. The location of each lift station is shown in Figure PVC = Polyvinyl Chloride; AC = Asbestos Concrete; HDPE = High Density Polyethylene. 4. Capacity based on velocity of eight feet per second (8fps). 5-10