Recycling Treated Municipal Wastewater for Industrial Water Use

TM3: Recycled Wastewater System Components and Costs
Recycling Treated Municipal Wastewater for Industrial Water Use
Sodium hypochlorite feed systems provide a unit process that can meet the three
disinfection system improvement requirements: to meet a year-round disinfectant,
provide a higher level of disinfection, and maintain a residual in the transmission
system. A conservative assumption is made that all WWTPs will require new
equipment for application of sodium hypochlorite. It is assumed that the chlorine
dose can be elevated sufficiently to meet the disinfection requirements without the
need for additional detention time (new contact tanks).
Chlorine doses were assumed as follows for the two annual operating practices and
residual disinfection:
April-October months with disinfection practiced by Minnesota WWTPs, where
chlorination provides incremental disinfection from the NPDES pathogen limit to
the reclaimed water pathogen limit.
November-March months with no disinfection practiced by Minnesota WWTPs to
provide disinfection to the reclaimed water pathogen limit.
A chlorine dose of 2.5 mg/L was selected to achieve adequate residual through
the transmission system. This is a dose commonly used by reclaimed systems
across the country.
Chlorination practices at MCES facilities and facilities with reuse systems were
reviewed to identify chlorine doses for existing systems to meet NPDES permit limits
and to meet a variety of state reclaimed water criteria. For MCES facilities, average
chlorine doses to meet NPDES discharge limits range from 2-4 mg/L with peak
demands requiring 6 mg/L of chlorine. A reclaimed system in Cary, North Carolina
reported the use of an 8 mg/L dose to meet pathogen kill and residual disinfection
requirements. Use of the Refined Collins-Selleck Model to estimate chlorine dosages
for disinfection of a nitrified secondary effluent (White, 1999) to meet a 23/100 ml
total coliform limit indicates that for a contact time of 15 minutes (the contact tank
design criteria typically used in Minnesota), a dose of 4-15 mg/L is required
depending on the nitrification process (ammonia at concentrations from 0.5 – 2
mg/L).
For this study, it is assumed that a chlorine dose of 6.5 mg/L applied to an advanced
secondary treatment system effluent is required to meet a total coliform limit of
23/100 ml. For the disinfection season months of April through October it is assumed
that WWTPs have a disinfection process equivalent to a chlorination system with an
average dose of 3 mg/L. Therefore, an additional 3.5 mg/L chlorine is required to
meet the more stringent reclaimed water pathogen limit from April to October. When
the 2.5 mg/L chlorine dose for disinfection residual is included, the chlorine doses are
as follows:
April-October (7 months): 3.5 mg/L + 2.5 mg/L = 6 mg/L
November-March (5 months): 6.5 mg/L + 3.5 mg/L = 9 mg/L
Average Annual (based on weighted average, rounded) = 7 mg/L
Craddock Consulting Engineers 21
In Association with CDM & James Crook
TM3-Component&Costs_0707