A+E Wins ACEC 2017 Engineering Excellence Award for Bellows Falls WWTF Upgrade Project
By Amanda Gosselin | August 14, 2018 | Featured News
The American Council of Engineering Companies – Vermont Chapter (ACEC-VT) chose A+E has their 2017 Grand Award winner in the Environmental Category of their Engineering Excellence Awards for Bellows Falls Wastewater Treatment Facility. You can find our submission here. Below is more information about the project.
The Bellows Falls Wastewater Treatment Facility (WWTF) was built in 1987 and refurbished once in the early 2000s. Several years prior, the Headworks and rotating biological contactors (RBCs) were upgraded in separate projects, phased to spread out the cost of each project over time to reduce impacts to sewer rates for debt service retirement. What remained were age-related needs primarily for solids handling, treatment and digestion. Typically, the focus is to simply replace aged equipment in-kind to extend the useful life of the affected processes, however, Bellows Falls wanted to make sure they were truly setting themselves up to serve their Village wastewater customers for the next twenty plus years, with reliable, cost-effective, and energy efficient equipment that will not only reduce capital costs, but also stabilize operating costs over the life of the refurbishments.
A particular focus was placed on anaerobic sludge digestion and sludge dewatering as both systems were well past their useful life and were the largest consumers of energy and labor. Finding efficiencies in energy usage, operating and maintenance costs, and sludge disposal was a priority to ensure that the improvements achieved their stated goal of improving long-term reliability while reducing costs and meeting the long-term needs for facility performance. To achieve each of these goals, several innovative technologies, never before installed in a Vermont wastewater treatment facility were integrated into the project.
The first innovation was the use of a linear mixer retrofitted to the refurbished floating primary digester cover. Mixing the sludge in the primary digester to create a homogeneous mix is critical to optimize digestion for solids reduction and treatment. Since this is a large mass of semi-dense liquid, that needs to be completely mixed, rotating blade mixers are relatively large and consume substantial energy to operate. Linear (plunger) mixers have been used in somewhat similar applications, but not in sludge digestion at wastewater treatment facilities in New England. With a vortex blade that plunges up and down in the suspended solids, the blade forces adequate mixing of the sludge with a simple up and down motion to mix with a 70% reduction in energy consumption over more conventional mixing systems. This is the first time a linear mixer was placed into operation in a Vermont WWTF and one of the first cases where it was retrofitted to an existing steel floating cover (that was sandblasted and painted, instead of being completely replaced, with accommodations for the mixer).
The second innovation was replacement of a failed steel cover on one of the two secondary digesters with a membrane “floating cover” that would rise and fall with both sludge and methane gas levels in the secondary digester. Not only was this an economical solution to a new steel digester cover, the membrane cover provides additional storage of methane gas (generated from the digestion process), which can be used as boiler fuel (reducing the consumption of LP gas) to heat the digester sludge and optimize anaerobic digestion.
One of the most significant costs to wastewater treatment facilities similar to Bellows Falls is sludge dewatering and disposal. Several different technologies were piloted to replace their old belt filter press, which has been a “tried and true” way to dewater sludge, but has also proven to be a high labor and operating and maintenance expense, with less than optimal performance. Several newer technologies have been installed at other facilities throughout Vermont, but not a volute dewatering press, which substantially reduces energy and water consumption, and substantially reduces labor costs as it is simple to operate and maintain. This type of sludge dewatering system has been successful at some similar style facilities in the US and Europe, but has never been placed into operation in Vermont.
Several different dewatering technologies were pilot tested for a “side-by-side” comparison of performance to determine which system was best suited to Bellows Falls. Utlimately the volute dewatering system was judged to be the optimal application for Bellows Falls. While pilot testing demonstrated the suitability of this technology for sludge dewatering, the final installation performed even better than the pilot tests, producing a dewatered sludge cake at a much higher solids content than their old belt filter press, and the other piloted technologies. This technology has proven to be reliable, energy and labor efficient and high-performing, thereby reducing the sludge disposal costs, one of several new technologies that have demonstrated their suitability for Vermont wastewater treatment facilities.