PROJECT EXPERIENCE
CLIENT OR OWNER NAME – ARIAL NARROW SIZE 9
BACK RIVER WWTP EXPANSION
Baltimore, Maryland
Black & Veatch has designed two waste activated sludge flotation
thickening facilities and additional anaerobic digester facilities for
the 180 mgd Back River Waste Water Treatment Plant operated
by the City of Baltimore, Maryland. The new digesters, once
constructed, will be the largest egg-shaped digesters in North
America.
Sludge Flotation Thickening Facilities The initial flotation
thickening facility was designed by Black & Veatch to thicken
waste sludge from an existing 60 mgd activated sludge plant to a
minimum solids concentration of 4 percent and a capture rate of
95 percent. The facility is a unique design consisting of two 50foot-diameter circular thickeners and a centrally located Process
Control Building. The facility is designed for maximum
operational flexibility and for ease of future expansion. Process
flexibility is provided by varying both the recycle rate and the
recycle pressure to achieve optimum thickening performance.
Key Team Members
Doug Brinkman, PE
Keith Pronske, PE
R. Parker, PE
Dr. J.R. Stukenburg, PE
Dr. Terry Johnson, PE
Project Duration
1988-1997
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Cost
$25.46 million
Engineering:
Initial: $1.16 Million
Final: $1.16 Million
Construction:
Initial: $17.7 Million
Final: $20.0 Million
Either internal clarified effluent or external auxiliary process water can be used as a
source of saturated recycle. A polymer feed system is provided to feed a variety of
liquid polymer types over a wide range of feed rates. For further operational flexibility,
the facility is provided with both local and remote monitoring instrumentation. For odor
control, the basins are furnished with aluminum geodesic domes and a countercurrent
packed scrubber tower. The system is designed to accommodate a variety of scrubbing
chemicals. In addition, laboratory and office space was provided in the Process Control
Building.
Following construction completion of the initial flotation facility, the City of Baltimore
retained Black & Veatch to design an additional facility with capacity to thicken waste
sludge from a new 120 mgd activated sludge plant then under construction. A feasibility
study was performed to determine the most cost-effective process alternative for the
additional thickening capacity. Several thickening alternatives were evaluated including
Black & Veatch provided
centrifuges, belt filter presses, rotary drum thickeners, and flotation thickeners. Due to
study and design services
the advantage of shared facilities with the existing thickeners, additional flotation
only.
thickeners were determined to be the most cost-effective alternative. The second-phase
Client Reference
flotation thickeners consisted of two 60-foot-diameter basins and an extension of the
Jaswant Dhupar
Chief, Water & Wastewater central Process Control Building. This phase included a second polymer feed system
Division
designed to feed dry polymer to all four basins. In addition, the existing liquid polymer
300 Abel Wolman Mun. Bldg
feed system was modified to feed polymer solution to all four basins. A computer-based
200 N. Holliday Street
Baltimore, MD 21202
process control system was added, with the capability to monitor and control the entire
410-396-3437
facility either locally or remotely. Although consisting of two separate design and
jaswant.dhupar
@baltimorecity.gov
BLACK & VEATCH
Pursuit Name • 1
PROJECT EXPERIENCE
CLIENT OR OWNER NAME – ARIAL NARROW SIZE 9
construction packages, the finished Sludge Flotation Thickening Facility's operations
and appearance reflect a single, unified facility.
Anaerobic Digester Facilities. Black & Veatch recently completed detailed design for
new anaerobic digesters at the Back River Waste Water Treatment Plant. A preliminary
design report was prepared which identified the need for additional digester capacity to
serve the 120 mgd activated sludge plant. Two complete anaerobic digester designs were
prepared for this facility. The first design was completed on a compressed schedule to
ensure EPA/state funding and consisted of conventional circular cast-in-place concrete
digesters and a new digester control building. Four 100-foot-diameter digesters with
fixed steel covers and confined draft tube gas mixing systems were designed. The design
included replacement of an existing sludge heating system serving the existing six
digesters with new centralized boilers and tube-in-tube heat exchangers.
After completion of the conventional design, the City directed Black & Veatch to
proceed with an alternative design consisting of two 3 MG egg-shaped digesters and a
new direct steam injection digester heating system. The anticipated operational
advantages will be the concentration and continuous removal of bottom grit, scum, and
foam, and the elimination of heat exchangers that normally require regular cleaning and
maintenance. Both welded steel and prestressed (post-tensioned) concrete egg-shaped
digesters were specified for competitive bidding. The 90-foot-diameter and the 125-foothigh digesters are served by a central access tower with an elevator and control building.
Digester recirculation pumps and mixing compressors are housed within the vessel
support structure, thereby minimizing the need for a separate equipment building. The
facility was modeled after successful designs in Germany and California, and included
special provisions for sludge recirculation, mixing, and foam handling.
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Operational flexibility is provided for both single-phase and two-phase digestion. In the
two-phase mode, the entire sludge stream is directed to the first egg-shaped vessel (acidforming phase) followed by flow distribution to the remaining new and existing
digesters (methane-forming phase). Two independent mixing systems are provided for
further operational flexibility. The primary mixing system consists of a central
mechanical draft tube mixer to provide 10 volume turnovers per day. An unconfined
perimeter gas mixing system is provided to serve as a supplement or backup to the
mechanical mixer. The pumped recirculation system provides one additional turnover
per day through the steam heating injectors. Two new steam boilers are provided to heat
the new and existing digesters and to replace five existing combination sludge heaters in
an existing building.
Scope of Services. The flotation thickening facilities involved a study of thickening
process alternatives, detailed design of the selected alternative preparation of drawings
and specifications, construction administration services, startup services and operator
training. The anaerobic digester design involved a predesign report, detailed design of
both cylindrical and egg-shaped digesters, preparation of drawings and specifications,
and construction cost estimating.
BLACK & VEATCH
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