Recycling Treated Municipal Wastewater for Industrial Water Use

TM3: Recycled Wastewater System Components and Costs
Recycling Treated Municipal Wastewater for Industrial Water Use
Table 4. Comparison of Microfiltration/Ultrafiltration to Depth/Surface Filtration 1
Advantages
Better microorganism removal: removes protozoans, cysts, oocysts, and helminthes;
partial removal of bacteria and viruses; could result in lower costs for disinfection.
Smaller footprint for equipment; typically 50-80 percent less.
As an MBR, can be cost-competitive to conventional secondary treatment processes.
Disadvantages
Higher O&M costs associated with:
o Energy
o Membrane replacement (approximately every 5 yrs)
o Monitoring for performance (membrane integrity testing)
o Residuals handling and disposal of concentrate (for some facilities)
Pretreatment may be required to prevent fouling, adding to footprint and overall costs.
Scale formation can lead to problems.
Less flow variation capability.
1Adapted from Metcalf & Eddy, 2007
The DAF process relies on the formation of microbubbles released after air dissolved
under pressure in the water is brought to standard conditions. The bubbles surround
slow-settling particles and float them to the surface. The float layer accumulates solids
and thickens and is removed by mechanical skimming systems. Clarified water is
removed from below the surface.
3.2.4 Residual Dissolved Solids Removal (Demineralization/ Softening)
Reclaimed supplies from areas with traditionally hard source waters and high
dissolved salts may require some type of softening or demineralization process to
meet the requirements of certain industrial water uses. Most of Minnesota’s water
supplies are of medium to high hardness and are higher in dissolved salts. However,
some waters may have an adequate balance of anions and cations and depending on
the use of the water, could be of adequate quality without the need for additional
treatment. A complete analysis of the secondary effluent is required to assess the
additional treatment needs. The applicable technologies addressed in this study are
limited to membrane processes and VRTX, a hydrodynamic cavitation process, since
lime softening was summarized in Section 3.2.2. Reuse applications include recycled
cooling water uses and electronics production.
There are two basic membrane separation processes: pressure driven and electrically
driven. Nanofiltration (NF) and reverse osmosis (RO) are pressure driven processes
and require hydrostatic pressure to overcome the osmotic pressure of the feed stream.
Reverse osmosis provides the most complete removal of constituents of concern for
industrial water use applications, such as TDS, hardness, nitrate, and dissolved
organic compounds. The removal rate of these constituents is between 90-98 percent
for osmosis and half that for nanofiltration (Wong, 2003). Electrodialysis has removal
rates of 50-95 percent for multivalent ions and does not remove smaller organic
compounds. Microorganism removal (bacteria, protozoa, viruses) is considered to be
4-7 log removal for RO and 3-6 log removal for NF. No log removal credit is given for
Craddock Consulting Engineers 13
In Association with CDM & James Crook
TM3-Component&Costs_0707