The coastal city of Warrenton, Ore., was at a crossroads in the mid-1990s.
Population growth and an overloaded sewer system had overwhelmed the city's 45-year-old, 25-acre facultative lagoon system, leading to National Pollutant Discharge Elimination System (NPDES) permit violations—just when the city of 4400 residents was placing additional emphasis on conserving and protecting the environment.
In 2004, the Oregon Department of Environmental Quality (DEQ) announced it would not approve any sanitary sewer line extensions until the city came into compliance with current NPDES discharge requirements. This essentially halted the city's growth by keeping sewer lines from being built for new subdivisions as well as new industry.
A happy medium had to be found.
The solution: Warrenton signed a mutual agreement and order with the DEQ outlining the schedule and steps it would take to study, plan, design, and construct a new wastewater treatment plant that includes three sequencing batch reactor (SBR) basins and 1 mile of outfall pipe into the Columbia River.
With these additions, the public works department can prepare for anticipated residential and commercial population growth of 60% over the next 20 years while keeping the environment at the forefront of the city's social and economic awareness.
A Team Approach
Warrenton officials selected local engineering firm HLB Otak Inc. to complete its plant improvements. When the city and DEQ established the need for a new plant, HLB Otak turned to other design professionals to join the team. Ledby the city of Warrenton, the team consisted of H.R. Esvelt Engineering as design engineer, Richard Sample Engineering for electrical design, Structural Research Co. for structural design, and HLB Otak for site civil engineeringand general oversight-construction management.
In February 2005, the city and DEQ began the final plan review for the new plant. Funding for the $7.9 million project came from the Oregon Clean Water State Revolving Fund, which is supported by annual grants from theEPA. The plans for the treatment plant consisted of three concrete-lined SBR basins; new site piping; influent force mains; and an influent, effluent, and operations building.
Warrenton officials opted for an environmentally and economically prudent activated sludge process and UV light disinfection system that uses no chemicals, produces no toxic residuals, and has no adverse impact on the environment, the Columbia River, or the multiple fish and marine species nearby.
Plant plans included a unique feature that enables operators to reach out to the public and teach the importance of caring for the environment: a 200-gallon aquarium that is continually filled with treated effluent. This aquarium, home to indigenous wildlife such as bass, blue gill, freshwater clams, and a Red Eared Slider turtle, provides a tangible display for the public to learn about the importance of caring for the symbiotic environment.
Construction started in late 2005. Five months in, a horizontal directional drilling (HDD) company was hired to install the new outfall by boring 80 feet below the Columbia River bottom and horizontally drilling and pushing 4407 linearfeet of pipe from the plant site to the Columbia River.
Design engineer firm Cosmopolitan Engineering Group handled this separate contract. Normally, HDD involves creating a bore hole and pulling pipe to anexit hole. However, The HDD Co. Inc., Orangevale, Calif., proposed two changes: forward-drilling the 18-inch bore hole from the land side to under the surface of the Columbia River shipping channel, then pushing a steel pipe with a treated Permalock coating. Once reaching the exit hole, divers wouldbe shuttled by barge to install the 40-foot diffuser. This process created the treatment plant outfall.
These innovative construction methods served two purposes. First, pushing rather than pulling the pipe enabled the crew to complete most of the installation from land, thus avoiding interrupting the crucial shipping lanes in theColumbia River. Second, this strategy helped mitigate the effects of the harsh winter weather and water currents, which would make handling the pipe extremely challenging in light of the Oregon Fish and Wildlife Department-enforced in-water work period, which limits construction in the river to four months: Nov. 1 to Feb. 28.