Recycling Treated Municipal Wastewater for Industrial Water Use

TM3: Recycled Wastewater System Components and Costs
Recycling Treated Municipal Wastewater for Industrial Water Use
quality effluent. Membrane bioreactors are included in Figure 3, recognizing it as a
future technology for Minnesota.
The type of phosphorus removal is not identified in the Base WWTP. A facility could
use chemical or biological phosphorus removal to achieve a 1 mg/L total phosphorus
limit. Certain technologies could be selected because of benefits to the phosphorus
removal mechanism. For example, if a facility uses chemical phosphorus removal, the
use of a chemical softening process with coagulation, flocculation, and sedimentation
could provide a more optimum use of chemicals for phosphorus removal, provide
flexibility in chemical addition points, and reduce suspended solids and hardness. For
example, the City of Mankato considered these benefits in the selection of their
processes to produce reclaimed water for the cooling towers of the Mankato Energy
Center.
Additional Disinfection Requirements
The base WWTP assumes the facility has disinfection equipment and structures to
meet Minnesota’s seasonal fecal coliform limits. The majority of Minnesota’s WWTPS
are permitted to disinfect from April through October and is the assumed period of
disinfection for this study. There are three disinfection system improvements required
to produce a reclaimed supply from WWTPs in Minnesota for a non-contact industrial
water use:
Year-round disinfection
Higher levels of disinfection to meet reclaimed water pathogen limits
Transmission system residual disinfection
In Minnesota, disinfection for pathogens in WWTP effluent is required only from
April through October. For a reclaimed supply, disinfection must occur year-round
or any time during which reclaimed water is delivered to users. While the existing
facilities will be adequate to disinfect the WWTP effluent year-round to the levels
required by the NPDES permit limit (typically a fecal coliform limit of 200/100 ml)
there will be additional O&M costs associated with the extra five months of
disinfection a year.
The California Water Recycling Criteria include a total coliform limit of 23/100 ml for
the base system reclaimed supply. Most Minnesota WWTPs use chlorination or UV
radiation for disinfection. In the case of chlorination systems, higher levels of
disinfection can be achieved by increasing the concentration of chlorine in the effluent
while using the same contact tanks. With UV radiation, the need to upgrade or add
equipment will depend on the peaking factor criteria used to design the system and
other flexibilities designed in the system.
The other disinfection requirement is the presence of a residual disinfectant in the
transmission system. The most common practice is the use of sodium hypochlorite
which is the practice assumed for this study. While some systems may have adequate
capacity to add a residual with their existing chlorination system, those with UV
radiation processes will need new facilities and equipment.
20 Craddock Consulting Engineers
In Association with CDM & James Crook
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