Recycling Treated Municipal Wastewater for Industrial Water Use

Section 3: Recycled Wastewater System Components and Costs
Recycling Treated Municipal Wastewater for Industrial Water Use
The regulatory requirements impose two basic treatment process modifications or additions for all
Minnesota WWTPs providing recycled wastewater:
Disinfection – higher levels of disinfection, year-round disinfection (currently required only from
April – October in Minnesota), and for transmission system residual
Filtration and possibly coagulation processes (or membrane processes), for industrial water uses where
worker contact is likely
Recycled Wastewater Quality
Municipal wastewater treatment processes generally include pretreatment, primary and secondary
treatment processes. The secondary treatment processes perform the dissolved organic removal and final
solids removal step in what is typically referred to as a secondary treatment system. In some WWTPs, the
secondary process also removes ammonia, through the process of nitrification. Complete nitrogen
removal, which includes removal of the nitrates produced through nitrification, is less common at
Minnesota WWTPs, but several facilities are equipped for it. Phosphorus removal is also performed at
many facilities in Minnesota. For this study, the term advanced secondary treatment, is used to define a
secondary wastewater treatment plant that removes ammonia and phosphorus.
The historic water quality record of WWTP effluent is extensive for constituents of concern to the
receiving waters. These constituents include carbonaceous or total biochemical oxygen demand (CBOD,
TBOD, or BOD), total suspended solids (TSS), ammonia (NH3), total phosphorus (TP), and fecal
coliform. Many WWTPs also have a historic record of heavy metals and priority pollutant compounds
collected on a less frequent basis. These parameters are also important in characterizing the effluent
quality and applicability for industrial use. However, there are many other constituents of concern for
industrial applications, as discussed previously, and the majority of these are not commonly characterized
in municipal WWTP effluent. Sampling performed for this project, reported in Appendix II-2, and
literature values provide a general basis for establishing wastewater effluent quality assumptions for these
parameters.
Historic records of Minnesota’s municipal WWTP effluent quality (MPCA, 2005) were evaluated for this
project and are summarized in Appendix II-4. The results from the 2005 analysis indicate that larger
facilities (greater than 1 mgd in capacity) produce a high quality effluent with organic, solids, and
microbiological concentrations at levels acceptable for many industrial uses. Over 90% of the larger
WWTPs produced effluent with annual average CBOD and TSS concentrations less than 10 mg/L. Most
smaller WWTPs also produced high quality effluent, with over 250 facilities reporting TSS
concentrations under 10 mg/L and over 400 facilities reporting CBOD concentrations less than 10 mg/L.
Phosphorus was shown to meet a 1 mg/L limit at over 40% of the larger WWTPs, or approximately 30
facilities.
Sampling conducted for this study characterized water quality constituents not routinely analyzed by
WWTPs, many of which are listed in Table 3.2. The results of the monitoring of four of the Met Council
WWTPs, review of Minnesota surface and water supply data, and literature values were used to define a
standard quality of recycled wastewater to assess use of this water by industries. Recognizing that water
quality varies over the state, a broad assumption was made that Minnesota’s waters tend to be harder than
other regions of the country, and higher in dissolved solids in many regions of the state. The constituents
associated with hardness and dissolved solids are generally not removed by advanced secondary treatment
processes.
The identification of treatment technologies and estimation of system costs required that a standard
WWTP effluent water quality be identified. This study uses an advanced secondary treatment WWTP to
define a “base” WWTP effluent quality or a “base” supply. The reason for this selection is based on the
future expectations for WWTP process requirements and the water quality requirements for Minnesota’s
larger industrial water uses.
44 Metropolitan Council Environmental Services