Cox Creek Water Reclamation Facility, Anne Arundel County,
Maryland
GHD was retained by Anne Arundel County for the Enhanced
Nutrient Removal (ENR) Upgrade of the Cox Creek Water Reclamation
Facility (WRF). The 15.0 mgd (56.7 ML/d) Cox Creek WRF
is the largest of the County’s seven (7) major wastewater treatment
facilities and is also the County’s largest single contributor of
nitrogen and phosphorus to the Chesapeake Bay.
During the facilities planning phase, a comprehensive
examination was made of all plant unit processes and
hydraulics. As part of this project, a decision was made to
expand the peak day hydraulic capacity of the facility from 30 to
45 mgd (113 to 170 ML/d) to accommodate observed I/I derived peak
flows. ENR alternatives were evaluated in a two-step approach
with the first step shortlisting to three alternatives and the
second step selecting the final recommendation based on conceptual
designs and lifecycle cost analysis. A Membrane Bioreactor
(MBR) process was selected to replace the existing MLE process.
MBRs proved to be the right choice for the Cox Creek WRF due to
very tight site constraints, inadequate existing reactor volume to
fully nitrify year-round, inadequate existing secondary
clarification facilities, limited hydraulic head available for
process modifications, and the County’s desire to produce as high
of quality of effluent as possible with the maximum degree of
reliability.
Following a Schematic Design phase, the project moved to
detailed design for three (3) separate phased construction
contracts.
- Phase 1 includes new primary clarifiers and influent flow
measurement
- Phase 2 includes new fine screening, reactor improvements,
membrane tanks, membrane facility, conversion of existing secondary
clarifiers to flow equalization, high flow management facilities,
new gravity thickener, new scum handling, and new SCADA system
- Phase 3 includes headworks upgrade, gravity thickener upgrades,
disinfection system upgrades, and odor control
The MBR system, when complete, will be tied for the largest
capacity ENR-performance MBR in the world. The MBR system
includes eight (8) parallel trains of hollow fiber membranes and
has an attached vertical turbine mixed liquor return pump station
with 80 mgd capacity. The MBR system follows four (4)
two-pass biological reactors configured as a four-stage nutrient
removal configuration with 600% nitrate recycle. Methanol
will be added for supplemental carbon for denitrification and
ferric chloride for chemical phosphorus precipitation.
Centrifugal blowers are used for process aeration and turbo blowers
for air scour. The three-phase project has an estimated
construction cost of almost US $140 million.
A unique feature of the project is the high flow management
system. As a result of lifecycle cost analysis, it was
determined that the capital and more importantly, the operating
cost of the MBR process could be significantly reduced by diverting
flows greater than 30 mgd (302 ML/d) to a separate parallel high
flow management system. Initially, flows will be stored in
flow equalization tanks (converted secondary clarifiers).
When the tanks become filled, a high flow management system will
automatically be initiated. The system uses a combination of
contact stabalization and ballasted flocculating settling to
achieve secondary biological treatment in a compact footprint which
can be started up in less than 30 minutes. The process will
be one of the first of its kind in the world and meets state
regulatory requirements to provide secondary treatment for all
sanitary sewer flows.
The project incorporates many sustainable design features, such
as a “green roof” on the membrane facility and state-of-the-art
stormwater management and quality control facilities. The
process control system is specifically configured to minimize
energy use at the facility through a combination of optimizing
in-service tankage, cycling aeration, turning down VFD-driven
equipment, and using diurnal flow and load equalization.
Design of the first two phases is complete and construction will be
completed in 2015.