The City’s Isle La Plume wastewater treatment plant (WWTP) serves as a regional facility, currently accepting and treating waste not only from the City of La Crosse, but also several areas adjacent to the City, including the City of Onalaska; the City of La Crescent, MN; the Town of Campbell; and Sanitary Districts 1 and 2 in the Town of Shelby.
The first treatment plant was constructed on the Isle La plume site in 1936. The extent of treatment was very basic and provided only primary treatment; sewage was pumped through some settling tanks and disinfected with chlorine before being discharged to the Mississippi River. The primary plant was upgraded and expanded in the late-1950s.
In the early-1970s, the wastewater plant underwent major expansion that included systems and processes to both increase hydraulic capacity and provide secondary/biological treatment capability. The upgrade to secondary treatment resulted in much-improved effluent (water discharged) quality to the Mississippi River. The cost of the plant improvements in the 1970s was 80% funded by federal and state grants.
Significant upgrades and additions to the Isle La Plume facility, many of them mandated by changes to regulations, have been accomplished since the upgrade to secondary treatment in the 1970s. Some (but not all) of the major projects have included:
- Installation of ultraviolet disinfection (replaced chlorination system);
- Implementation of biological phosphorus removal;
- Construction of biosolids storage facilities;
- Upgrading screening and grit removal systems;
- Replacement of aeration system; and
- Widespread rehabilitation of plant tankage and structures.
The current wastewater treatment plant on Isle La Plume has a design flow capacity of 20 million gallons per day (MGD). The daily flow to the plant in 2014 averaged approximately 11 MGD as compared to about 16 MGD in 1990 before Sanitary Sewer Utility user fees were initiated. Reductions were due to industrial water conservation, City efforts to eliminate clear water inflow and infiltration into the system, and reduced production from City Brewery. The treatment facility currently has excess treatment capacity.
Unit processes at the City’s wastewater treatment plant include:
- Preliminary treatment: Raw sewage sampling & flow measurement
- Screening & comminutors
- Grit removal – Two Pistagrit systems
- Raw sewage pumping – Five raw sewage pumps
- Primary treatment: Primary settling/clarifiers – Five clarifiers
- Primary effluent pumping – Four primary effluent pumps
- Secondary treatment: Biological system – Aeration and biological nutrient removal
- Secondary settling/clarifiers – Four secondary clarifiers
- Tertiary treatment: Ultraviolet disinfection (seasonal) – Three channel system
- Effluent sampling and flow measurement
- Biosolids system: Anaerobic digestion – Four digesters
- Dewatering – One gravity belt thickener system (room for 2nd unit)
- Liquid biosolids storage – two 3.1 million gallon tanks
- Cake biosolids storage
Facilities planning was completed in 2010 and reviewed the majority of all WWTP infrastructure and processes and made recommendations related to other upgrades and changes to-be-planned, budgeted and scheduled as capital projects. These projects are part of the Sanitary Sewer Utility’s ongoing Capital Improvement Program.
The Utility is also currently conducting facility planning specifically related to compliance with an anticipated, much lower limit for effluent phosphorus concentration. Compliance with the new limit may require a significant and expensive expansion to the existing WWTP.
The Isle La Plume WWTP operates efficiently and discharges very high-quality effluent, well within permit limits, to the Mississippi River. The following summary shows average, annual data for typical compliance criteria over the past five years. Additional, detailed discharge information is available in Compliance Maintenance Annual Reports (CMARs).
There are extensive emergency measures in-place at both the wastewater treatment plant and major lift stations to provide backup power to critical equipment in the event of power outages. Since flow in the sewers and to the wastewater plant never stops, it is imperative that this equipment remains operational during these emergency situations. Standby generators are installed on-site at the WWTP and at major lift stations, and are programmed to start automatically, run over the duration of the power outage, and shut-down when utility power is re-established. All equipment and processes at the wastewater plant remain operational during a power outage. On-site generators at major lift stations are designed to support the facilities’ full pumping capacities
Maintenance of the wastewater treatment plant and lift stations is a primary, operational goal which includes an extensive inventory of spare parts, well-trained maintenance personnel, and a formal, comprehensive, scheduled preventative maintenance program which is specifically designed to avoid emergencies or catastrophic failure of equipment.
Even though the wastewater treatment plant and lift stations incorporate extensive automation for normal operations, skilled employees with experience and knowledge in the fields of chemistry, microbiology, hydraulics, electronics, pumps & mechanical equipment, computers, electronics and controls systems are on-staff to assure proper operation and maintenance of the system. On-call personnel are available 24/7/365 to respond to wastewater plant and sewer system emergencies.