As water scarcities become more frequent and cities tighten their treatment requirements, fines increase for polluted water. In residential settings, this isn’t always an issue, but for industrial settings the process becomes a lot more complicated. Industrial wastewater is often high in organics, salts, and other pollutants that require multi-step treatment systems. With a background in innovation and improvement, Kinergetics is a company familiar with many areas of development. We recently found ourselves with the chance to expand to a new field: Wastewater reclamation and repurposing.
In this case, we were asked to treat textile wastewater for process reuse. The reclaimed water needed to meet city water standards for pH and mineral content, along with being visually clear (with all color and textile dyes removed). To start the project, a bare-bones pre and post nano-filter water report provided some basic information: high levels of TSS, BOD, and heavy metals/hardness. Visibly, the water was dense, dark brown and oily, with an odor that hit you in the face from across the room. In essence, the opposite of potable water.
While larger companies have the funds and equipment available to run tests to determine what is in the water and subsequently use this info for picking good filter sizes and types, we’re not a big company. So we had to get creative.
[We wanted to have a treated water with a pH as close to normal water as possible (~7-8), with hardness/minerals also as close to potable water as possible (~130mg/L). With this in mind, we set out on the first task: determining the level of treatment we would need to fit these parameters.]
With limited experience in wastewater treatment, the first thing we did was look at filtration types. We knew that the original nanofilter had taken the color and smell, along with a 65% of the TSS/BOD out. So, we knew that we wanted to use some type of water filtration system like ultrafilter, microfilter, nanofilter or reverse osmosis filtration. However, since we needed to keep the pH ~7.8, we needed to keep the dissolved minerals in the permeate. Because RO removes everything, including the ions and minerals that add “hardness” to the water, RO was ruled out.
The next tightest filtration option was nanofiltration, which only would filter out about 20% of the monovalent ions and essentially unaffect pH. We then moved onto the big two readings we had to determine the next filtration settings for nanofilter pretreatment: Total Suspended Solids (TSS) and Biological Oxygen Demand (BOD). For reference, potable water usually has a BOD ~1, and TSS involves any suspended solid above 2um. Since the two are intertwined, finding a filtration system that would hit TSS would likely bring down the BOD as well.
As for determining the correct filter size: TSS include anything above 2um. The human eye can see around 10um unaided. Since the water had visible solids in it, we knew that whatever we chose would likely take the visible solids out of the water and thus give us a barometer of how tight pretreatment should be.
However, we couldn’t decide on a microfilter size. Most website help suggested doing a particle count, which is something we didn’t have the resources for. We also didn’t have the budget to run multiple tests with multiple microfilter sizes. As a small company, we have to be innovative and save resources as we go.
As we got the initial TSS readings back post-microfilter, we had no visible solids, and our TSS levels dropped from 250mg/L to 2.5mg/L, which was clean enough for the nano. The nanofilter was able to remove all dyes, and also kept the minerals in the permeate.
Checking the pH also revealed that the pH roughly the same (~7.9), but still had higher BOD. However, our microfilter had taken a hit. The filter had significantly dropped in flux and was limping along after only 6 tests with wastewater.
The next steps are using a new microfilter with a wider feedspacing to prevent the high levels of solids from clogging the membrane. Stay tuned for the results!