Equipment Selection Guide

Things to Consider

  • The feed water quality that you are starting with – Surface water, well water, suspended solids, dissolved solids
  • The quality that you need – dissolved solids, TOC, turbidity

Reverse Osmosis Equipment

RO will typically give you rejections in the range of 96 – 99+% on dissolved solids. Nanofiltration (NF) will typically give you rejections in the range of 80 – 90% on dissolved solids.  Use the Filtration Spectrum to determine what type of membrane to use if you are interested in something other than dissolved solids. Winflow or Argo Analyzer software can help you determine specific ion by ion rejections for RO or NF membrane elements

EDI Equipment

Deionization and/or polishing is required if you cannot reach the customer requirements with RO alone. 

There are several deionization options:

  • Electrodeionization (EDI) – regenerated continuously with electricity
  • Traditional DI – regenerated with acid and caustic on-site
  • Traditional DI – polishing bottles, regenerated off-site

All of the options from the previous slide will lower the dissolved solids in water to 2 – 18 MegOhm range.

Final Treatment

Now that you have designed the final treatment to meet the customers requirements, you need to meet the feedwater requirements to the piece of equipment you just chose. DI units don’t have specific feedwater requirements in terms of dissolved solids.  However, the more dissolved solids in their feedwater, the more frequently they will require regeneration.

EDI machines require RO quality feedwater – and in some cases, even single pass RO doesn’t meet the needs.  Two pass RO is occasionally required to feed an EDI

Pretreatment

Sand Filtration

Provides reduction of 80-90% on turbidity and total suspended solids without addition of coagulant upstream of filter. Provides reduction of 90-95% on turbidity and total suspended solids with the addition of coagulant upstream of filter. Provides reduction of 50-90% on iron (sometimes higher if Manganese Greensand is used in place of silica sand)

Microfiltration

Microfiltration with hollow fiber membrane technology is required on feedwaters with > 50 ppm total suspended solids or turbidity > 50 NTU.

Chemical Dosing

Several chemicals are commonly dosed in pretreatment applications:

Coagulant – dosed in front of sand filters to improve reduction in turbidity, TSS, and SDI

Anti-scalant – dosed in front of RO machines to prevent precipitation of salts and minimize scaling and fouling

Sodium Bisulfite – dosed in front of RO or NF machines that are chlorine intolerant to reduce free chlorine to chloride ion

Acid – dosed in front of RO or NF machines to lower pH to prevent precipitation of salts and minimize scaling and fouling

Caustic – dosed in front of RO or NF machines to raise pH to convert dissolved CO2 to bicarbonate which is rejected by membranes

Summary

There are many different types of equipment and types of treatment available to us for providing water purification solutions. Often, there is more than one way to meet a customer’s water quality requirements.  Different designs can accomplish the quality goal, but still not meet other customer needs – such as floor space or hazardous chemical constraints. (examples of carbon vs. bisulfite or DI vs. EDI)

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