Photo description: PLWSA Wastewater Treatment Plant
The facility plan determined that the most cost effective and environmentally sound long-term plan was to construct a new treatment facility on the Houghton sands site. Construction of this facility was completed and placed into operation in 1993. The treatment plant has a design flow of 3.1 MGD with a capacity to treat 18 MGD during wet periods of snowmelt and rainfall.
The Hancock lift station at the old treatment plant site was remodeled twice since the new plant was built. The latest remodel was to install new pumps and variable frequency controls at the lift station and also to construct a new 16” forcemain, which runs through Ripley, MI, along the snowmobile trail and crosses the Portage Canal to discharge Hancock’s sewage into the gravity interceptor. The Houghton lift station was remodeled and a 24” forcemain was constructed to convey W. Houghton’s sewage to the gravity Interceptor. The Interceptor collects all of the lift station flows and any remaining gravity flows and conveys it to the treatment plant.
Wastewater that arrives at the treatment plan is lifted by screw pumps through the treatment units and to Portage Lake.
Pretreatment consists of screening and grit removal. The mechanically cleaned bar screen removes coarse solids from the flow. The aerated grit chambers remove sand, gravel and heavy grit to protect the downstream treatment units.
The primary clarifiers remove a significant portion of the suspended solids from the wastewater. These tanks allow solids to settle slowly to the bottom of the tank by gravity. Settled solids are collected at the bottom of the tanks and pumped as "sludge" to the anaerobic digestors.
Secondary treatment is a biological process that is used to remove organic material present in the wastewater. The biological process utilized is called activated sludge. Aerobic (oxygen-demanding) bacteria grow in these tanks and consume the organic materials in the wastewater. This process is accomplished in the secondary aeration tanks and the secondary clarifiers. After aeration, the bacteria-laden solids are separated in the secondary clarifiers and a portion is returned to the aeration tank. This arrangment of "return sludge" allows the operator to control the amount of bacteria present in the process. Excess solids are wasted to the primary clarifiers where it is combined with the primary solids.
Ultraviolet (UV) disinfection is the final treatment of the wastewater. Disinfection is accomplished by an innovative process that utilizes high dosages of ultraviolet light to eliminate any potentially harmful pathogenic organisms that could be present in the wastewater. A specific wavelength of ultraviolet light is germicidal and is used to destroy nearly all of the bacteria that are present.
An underground pipeline carries the treated effluent to a 100-foot-long submerged diffuser for discharge into Portage Lake.
Anaerobic digesters process the sludge removed from the clarifiers to produce methane gas and biosolids fertilizer. The Primary Digester is heated and mixed to foster the growth of anaerobic methanogenic bacteria. The Secondary digester is a quiescent basin which allows the biosolids to separate from the liquid as it settles. Anaerobic digestion converts most of the organic material in the sludge to biogas, comprised mainly of methane and carbon dioxide, and produces biosolids that have a significant fertilizer value which can be used as a soil amendment to promote vegetative growth.
The biosolids are dewatered by belt filter presses to a moist state containing about 16% solid material by weight. The dewatering step removes water and reduces the volume to be hauled to land application. A storage facility provides storage of dewatered biosolids between periods of land application.
Land Application of
The processed biosolids are applied as a soil amendment with a fertilizer value to area hay fields to assist in plant growth. The biosolids are also applied to the Mason stamp sands site, one of the area’s mine tailings sites, to help maintain the plant growth. As a former Superfund site, the Mason stamp sands site was capped with a sandy loam soil and seeded with vegetation in 2004. The PLWSA has been applying biosolids onto the Mason site since 1968.
The Portage Lake Water & Sewage Authority
(PLWSA) was established in 1964...
by the cities of Hancock and Houghton for the purpose of constructing and operating a joint use wastewater treatment facility. The facilities that were constructed included a lift station for each city to convey wastewater to a common site and a wastewater treatment plant located on the south shore of Hancock.
Planning was initiated by the PLWSA in 1987 when it became evident that the existing treatment plant was operating at flows in excess of the design flow and was not able to treat all of the excess flow that was being generated by both cities.