Wastewater Treatment Facilities Improvements Project

Wastewater Treatment Facility Plan and Plant Improvement Project

Project Description

July 2020 

The City of Arcata proposes to construct upgrades to the existing Arcata Wastewater Treatment Facility (Treatment Facility, or Facility) to update aging infrastructure and comply with the requirements of the Regional Water Quality Control Board (RWQCB) Order No. 1-2019-0006. Ongoing preventative maintenance has kept the original treatment plant in operation but there has been little replacement of equipment or structures since original construction, and minimal maintenance in the natural treatment system. 

The Treatment Facility provides secondary treatment using natural processes including two oxidation ponds and six wetland treatment marshes. Enhancement to the secondary treated water is provided by three enhancement marshes located in the Arcata Marsh Wildlife Sanctuary (Wildlife Sanctuary). The Treatment Facility currently includes headworks, primary clarifiers, oxidation ponds, treatment wetlands, enhancement wetlands, and chlorine disinfection. Treated effluent is discharged into the Humboldt Bay (Outfall 001) or is circulated into Enhancement Wetlands (Outfall 002) in the Wildlife Sanctuary for further treatment. Solids removed in the primary clarifier are treated in anaerobic digesters and solids drying beds. 

Proposed upgrades include oxidation pond and wetland treatment system improvements, construction of a parallel oxidation ditch treatment system, construction of an Ultraviolet (UV) disinfection system upstream of discharge to the Enhancement Marshes, and an updated facility configuration allowing for discharges from the Wildlife Sanctuary to the recently created Brackish Marsh in order to provide overall improvements to effluent quality discharged to Humboldt Bay. 

The upgraded UV System equipment will be purchased with Community Development Block Grant (CDBG) funds, which requires this environmental assessment. The installation of a 9.8 MGD capacity UV disinfection system, replacing the primary disinfection utilizing chlorine gas, will reduce the number of violations for dichlorobromomethane, a chlorine disinfection by-product. This byproduct has historically been one of the major issues with Arcata’s treated effluent.

The full scope of the upgrades will be undertaken in two separate phases, and will include the following elements:

Phase 1 will focus on rehabilitation of the current system and reconfiguring the effluent flow to a single path. Rehabilitation will be done to the natural system facilities (i.e. ponds and the wetlands) and to the aging infrastructure. It will include replacing and installing new equipment (i.e. pumps, aerators, and monitoring equipment), and increasing the resilience of facilities for treatment and hydraulic capacities. Many of the activities associated with Phases 1 and 2 would be considered routine maintenance of the plant if evaluated as stand-alone activities.

Phase 1 includes:

  • Excavation and contouring for improved water quality and maintenance at the outlet of the Hauser Enhancement Marsh.
  • Laying a new discharge pipe to the new Outfall 003 in the brackish marsh from the existing Hauser pump station.
  • Replacing aged equipment (screw pumps, bar screen, and grit chamber) in the existing headworks structure.
  • Installation of a 9.8 MGD ultra-violet UV disinfection system into one half of the existing chlorine contact basin (CCB), eliminating the use of chlorine gas and sulfur dioxide for disinfection except in an emergency disinfection scenario (flows > 9.8 MGD) or if emergency power was interrupted.
  • Replacement of existing clarifier no.2 rotating center armature/skimmer, various flow configuration improvements throughout the system for automation, and creating a higher quality effluent.
  • Placement of up to 24 new 12.5 Hp mixer aerators to improve treatment within the oxidation ponds.
  • Replacement/upgrading of aboveground vertical pumping stations within the plant and at Hauser Marsh pump station with submersible variable speed pumping systems.
  • Installation of new SCADA and plant control and monitoring system for all AWTF operations to be constructed within the existing centralized control room.
  • Construction of new building for the placement of new electrical services, PGE connection, and switchgear including a diesel-powered emergency generator rated at 0.75megW and removal of an existing natural gas-powered 150 KW generator.
  • Placement of structural fill up to 1.5’ deep to elevate existing 16’ wide roadway between The Hauser Treatment Marsh and Gearheart Treatment Marsh to a minimum height of 8.0’.
  • Rehabilitation of existing footbridge and coating maintenance.
  • Adding additional photovoltaic panels to existing facility roofs. 
  • Raising levees to meet minimum FEMA and City requirements for construction of new/rehabilitated facilities. 

Phase 2 will focus on constructing the additional secondary treatment system, as well as additional site improvements within the AWTF and the City of Arcata Corp Yard. In general Phase 2 builds on the performance of Phase 1 improvements.

Phase 2 includes the construction of all the additional facilities needed for the parallel secondary treatment facility, including:

  • Construction of the new oxidation ditch.
  • Construction of two new secondary clarifiers and pumping for the return activated sludge (RAS) and the waste activated sludge (WAS).
  • If determined necessary, following ongoing Phase 1 performance, an alkalinity feed system may be constructed within the existing developed AWTF.
  • The footprint of the preferred design will require relocation of some of the existing AWTF facilities within the project area.

Related Documents

The City of Arcata has initiated a Wastewater Treatment Facility Plan and plant improvement project. Below are several documents related to this project: