Recycling Treated Municipal Wastewater for Industrial Water Use

TM2: Sampling Plan and Results
Recycling Treated Municipal Wastewater for Industrial Water Use
3.2 Specific Constituents of Concern
Constituents exceeding the recommended limits for cooling water use or higher water
quality uses were evaluated in more detail. The mean and standard deviation for the
following constituents are summarized in Figures 1-8 at the end of the memorandum:
Alkalinity
Bicarbonate
Calcium
Chloride
Hardness
Silica
Sulfate
TDS
Lower hardness, alkalinity, calcium and bicarbonate concentrations would be
expected for softer source water areas. A large percentage of the Metropolitan
WWTP’s service area is provided lime softened water by Minneapolis and St. Paul.
The City of Bloomington has a lime softened water supply and contributes to a
significant portion of the Seneca WWTP’s influent. The Seneca WWTP also had lower
average values for constituents characterizing hardness, as shown in Figures 1-4.
High WWTP effluent chloride concentrations often occur in areas with hard source
water because of the increased loadings to the WWTP from the brine discharges of
domestic water softeners or the concentrate streams from commercial or industrial
softening systems. Figure 5 indicates this inference applies to the Council facilities
evaluated. The Metro WWTP chloride concentration averaged 240 mg/L and never
exceeded 265 mg/L. The Seneca WWTP had chloride concentrations averaging
around 300 mg/L. The Empire and Blue Lake WWTP chloride concentrations
averaged nearly 400 mg/L and had a wider range of variability.
All four WWTPs exceed chloride concentrations required for cooling water with
systems recycling water more than 2 times. At concentrations greater than 500 mg/L
there is concern with corrosion. This threshold will vary with materials of
construction, and is referenced for 316 stainless steel condenser tubes (personal
communication with D. Taylor, Great River Energy).
Chloride concentration in WWTP effluent has also been linked to the influence of
infiltration and inflow (I&I) and chlorides imparted from road salt. The City of
Oconomowoc, Wisconsin has monitored chlorides for ten years and found evidence of
significant chloride contributions from chloride in soil below roadbeds that
accumulated from road salting (personal communications with Tom Steinbach, City
of Oconomowoc). The longer-term monitoring indicated that the highly variable
loadings of chlorides can be inaccurately characterized with limited sampling (e.g.
one sample each quarter).
8 Craddock Consulting Engineers
In Association with CDM & James Crook
TM2-Sampling_0707.doc