Split-site plant upgrade meets design challenges.
Challenges during design of the treatment facilities were to provide a high level of ammonia removal, a high level of dissolved oxygen in the effluent, and process selections based on ease of operation for a small staff in a rural community within the following constraints; long but narrow site split by a meandering river, existing facilities that had to be kept in operation throughout the construction period, incorporation of the old facilities where possible, and difficult hydraulic limitations.
Flexibility, ease of operation, and advanced process technology were combined in design to upgrade the existing plant for the Town of Winchester/City of Winsted, Connecticut. To meet the more stringent limitations imposed by the State Department of Environmental Protection (DEP) for discharge to the Still River, the existing trickling filter plant had to be abandoned; some of the facilities were incorporated into the new facilities. After construction was started in 1987, the DEP imposed additional effluent limitations and design requirements. Redesign or modifications to the design were performed during construction to incorporate these mandated changes. The DEP required the addition of dechlorination to reduce chlorine residuals in the plant effluent to low levels and imposed year-round effluent ammonia nitrogen limits in lieu of seasonal limits. Ammonia-nitrogen can cause fish toxicity and can deplete dissolved oxygen levels in the receiving stream to a point that would make the stream uninhabitable for trout and other fish.
The plant design received an Engineering Excellence Award from the Connecticut Section of American Consulting Engineering Council.
Process Design And Selection
The plant is designed to treat an average daily flow of 2.6 mgd and a peak hour flow of 6.5 mgd. Preliminary treatment of raw wastewater is provided by a coarse bar screen, vortex type grit removal tank, comminutor, and fine bar screen. Following primary settling, wastewater is pumped to aeration tanks where single-stage nitrification is achieved by the activated sludge process. Four aeration tanks are provided to meet the projected design flow and loadings and to meet the seasonal variations required due to temperature changes. Lime is added to primary effluent to provide alkalinity, which is destroyed during the nitrification process. Following final settling, secondary effluent is chlorinated, dechlorinated by sulfur dioxide, and post-aerated by cascading steps.
To maintain gravity flow through the existing primary settling tanks and retain the existing facilities, hydraulic limitations were met by selecting new preliminary treatment units with minimum head loss, designing new influent channels and flow splitting arrangement for the primary clarifiers, and paralleling the effluent channel to increase the hydraulic throughput.
A single-sludge nitrification system was selected on the basis of a cost evaluation, ease of process control compared to two-sludge or separate-stage systems, and capability to achieve a high level of ammonia removal. The aeration tanks are laid out as plug flow units that can readily be taken off line to accommodate changes in loadings and temperatures.
To maximize plant performance, the final clarifiers were designed with conservative surface overflow rates, sidewater depth, and baffle system, which contribute to the high level of treatment achieved. The Winsted plant was one of the first plants designed with an effluent trough horizontal baffle arrangement for the final settling tanks. The horizontal ledge is intended to prevent sludge from climbing along the perimeter tank wall and passing over the weir and into the effluent caused by the velocity pattern. The ledge deflects the upcoming velocity currents and redirects the solids back toward the center of the tank without flowing over the weir, thereby increasing the efficiency of solids capture.
The cascade aeration system consists of a series of descending steps following the chlorine contact tanks and dechlorination. While simple in concept, cascade aeration design is based primarily on empirical data. Currently, the cascading steps are providing dissolved oxygen concentrations greater than 8 mg/L, which is well above the permit limit of 5 mg/L.
Return sludge is pumped to a splitter box at the head end of the aeration tank where a portion of sludge is wasted by gravity to the primary settling tanks; the remainder of the sludge is mixed with primary effluent. Waste activated sludge is combined with primary sludge and pumped from the primary settling tanks to a sludge thickener. Following two-stage anaerobic digestion, digested sludge is dewatered by two belt filter presses and discharged directly into a dump truck for hauling to a landfill. Digested sludge can be pumped directly to a tank truck for hauling of liquid sludge, if necessary.
Anaerobic digestion was retained for sludge stabilization because of the existing digesters and benefits associated with methane gas production. With the addition of waste activated sludge to primary sludge, a gravity sludge thickener was added to the system. A septage receiving station and storage tank were included in the design to allow handling of septage solids without passing through the liquid treatment facilities. Septage can be pumped from the storage tank to the primary digester or to the head of the plant.
Another consideration during design was how to allow the contractor to construct the new preliminary and secondary treatment systems without adversely affecting the existing treatment facilities. Once the new activated sludge facilities were on-line, the trickling filters were demolished, the primary settling tanks were taken off-line and refurbished, and the sludge facilities renovated and expanded. During this interim period, the design was such that the activated sludge system could be operated as an extended aeration plant, and one or more of the aeration basins could be used for aerobic sludge digestion and storage.
During the initial wastewater facilities planning stage, the town wanted to realign a portion of the Still River channel running through the existing plant due to the lack of space available for additional facilities. However, environmental concerns and regulatory review and approval procedures associated with realignment of the river channel would have required a rather lengthy process, delaying final design and construction and thereby increasing project costs and jeopardizing grant funding under the USEPA and DEP construction grants program. It was decided not to realign the channel, but to split the new plant with the river running between the two sides. The old plant was located on the south side of the river. The new facilities constructed on the north side of the river consisted of aeration tanks, final settling tanks (Eimco Process Equipment Company, Syracuse, New York), chlorine contact tanks, cascading post-aeration steps, sludge pumping station, and blower building. Three aeration blowers were provided by Lamson Corporation in Syracuse, New York. On the south side, the existing facilities were incorporated into the new facilities consisting of a new pump station, one new primary settling tank, new operations building, new digester building, and new digester.
With the split site concept, several river crossings were required for the pipelines and electrical conduits running between facilities on both sides. The north side was higher in elevation than the south side and with some fill added, the new facilities were naturally protected from the estimated 100-year flood elevation. Rock anchors were drilled into the bedrock at the base slab level of the aeration tanks to protect the tanks from flotation during flooding. On the south side of the river, sheet piling was installed along the river bank and a berm was constructed to protect the site from flooding. The existing wastewater pumps were refurbished and are now stormwater pumps for use during flooding conditions when the river level is too high for storm drainage to flow by gravity.
The split site is connected by a 66-ft span box girder bridge that replaced an older bridge that had acted as a bottleneck for the Still River and increased the flood hazard upstream. With replacement of the old access bridge and stream side slope improvements, peak river levels are now less upstream of the plant.
Modifications during construction were necessary to accommodate several changes mandated by the DEP due to revised regulations and guidelines implemented after design. The effluent limitations were made more stringent by placing monthly limits on ammonia-nitrogen on a year-round basis and a limit on chlorine residual.
It was determined that the aeration tanks had sufficient capacity to achieve nitrification on a year-round basis with no modifications required. The plant was designed to provide single-stage nitrification on a seasonal basis beginning in May. However, to meet the ammonia limit, the aeration tankage was sized such that nitrifying organisms would need to be reestablished in April while wastewater temperatures are still at winter levels. Consequently, the single-stage nitrification system, as designed, was determined to be adequate to achieve nitrification year round.
To meet the limit for chlorine residual, a prepackaged fiberglass housing with sulfonators was installed next to the chlorine contact tanks by change order. A diffuser for sulfur dioxide solution was added to the end of each chlorine contact tank ahead of the effluent weir. With the cascading steps for postaeration following the weirs, adequate turbulence for mixing was readily achieved.
Flexibility in Design And Operation
In general, the facilities were designed with multiple units, and alternate arrangements to allow flexibility in process control and operations. The facilities consist of five primary settling tanks, four influent wastewater pumps, four aeration tanks with standby chlorinator, sulfonator, and sludge pumps. Tanks can be readily taken out of service as the loadings and temperatures warrant at that time of year. A small pump is provided as part of the variable speed wastewater pumping system to handle low flows at night without the need to use the larger pumps, thereby saving wear and tear with no pump cycling plus improved efficiency. Chlorine can be added at several locations to control odors and control sludge bulking.
Both anaerobic digesters were designed with floating covers and piping interconnections to allow either digester to serve as the primary or secondary digester. With one digester out of service, the other digester can provide digestion and gas storage as well.
Combined waste activated sludge and primary sludge can be pumped to the thickener or directly to one of the anaerobic digesters. Should the sludge be poisoned or toxic from some unusual discharge to the plant, the combined sludge can also be pumped to a tank truck to prevent upsets to the biological process. After digestion, the sludge from the secondary digester can be pumped to the belt filter presses for dewatering or to a tank truck for hauling and liquid disposal.
Portions of the liquid side treatment facilities were placed into operation in February 1989. The sludge handling facilities and remaining liquid side treatment facilities were completed and placed into operation by February 1990, shortly before the final effluent limitations went into effect. Since that time the plant has been achieving an excellent effluent quality and has been in compliance with its permit limitations.
From March 1990 to November 1992, the average final effluent concentrations have been as follows:
* |BOD.sub.5~--5 mg/L;
* Suspended Solids--6 mg/L;
* Ammonia-Nitrogen--2.4 mg/L;
* Dissolved Oxygen--8.5 mg/L.
Lower ammonia-nitrogen levels are achieved in summer, when the limits are more stringent.
Completing this project has provided several benefits to the town and surrounding areas. A sewer hookup moratorium has been relieved and has allowed the development of an industrial park and a shopping mall and has permitted the town to pursue new manufacturing opportunities. The plant now serves as a regional septage receiving facility for a rural area that had few other options. An extensive sewer project is planned that will encircle a nearby lake and help protect one of the most important recreational spots in the area. Because of the replacement of the old access bridge and stream side slope improvements, peak flood levels upstream of the plant have been lessened. The plant is a good neighbor to those living nearby; there have been no odor problems, and the plant presents a low profile to passersby. The Still River will become part of the Fisheries Bureau regular spring trout stocking schedule due to the improved water quality.
Municipal Solid Waste Recovery Contract Awarded
Cass County, Texas, has awarded a contract to Rader Companies, Resource Recovery Group, Memphis, Tennessee, to supply and install a municipal solid waste resource recovery facility to be located in Linden, Texas.
The contract calls for a system that will process residential solid waste at the rate of 20 tons per hour. At the beginning of the process, incoming refuse will pass through a bag opener where the plastic bags will be torn open to free their contents. Large objects will then be removed at a picking station before the refuse is introduced onto a two-stage bar screen, which will separate fines, grit, and food waste on the first stage while the second stage separates the medium-sized recyclables from the larger ones. After screening, paper and cardboard, ferrous metal, aluminum, plastic, and glass will be manually removed on picking stations.
The remaining refuse will pass through a grinder for particle size reduction and then to an air knife where inert material will be extracted. The remaining combustibles will be processed in a fuel cuber.
Mr. Storrier is an Associate with Stearns & Wheler Environmental Engineers and Scientists, Cazenovia, New York; Mr. Dufel is a Partner with Stearns & Wheler, Inc., Darien, Connecticut; and Mr. Kemp is Plant Superintendent, Winsted Wastewater Treatment Facility, Winsted, Connecticut.
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|Author:||Storrier, Donald F.; Dufel, Gary A.; Kemp, Richard J.|
|Date:||Jul 1, 1993|
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