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"Conductivity of Rinse Water"

An ongoing discussion beginning back in 2003 ...


Q. What are the essential characteristics of De-ionised water for use as rinsing water for aluminium strip after it has been cleaned by electrolytic process using sulphuric acid (20% conc. at 90 deg. C)? We need this data for designing the features of a new DI plant we will install for aluminium strip degreasing using acid electrolyte.

Siddhartha Bhattacharya
- Mumbai, Maharashtra, India


A. Some typical specifications are < 5 or < 2 mg/L of dissolved solids. These are common in the automotive industry. It is more customary to use conductivity as the specification since it is easily measured. <5 micro-Siemens is a common specification.

However, it is not the conductivity or dissolved solids in the DI water that is most important, the most important thing is the quality of the last water on your product. Since you are rinsing off acid, there will be some elevated level of contamination in the water, and since DI water has no buffering capacity, it will take a lot of rinsing before the pH is near neutral.

Lyle Kirman
consultant - Cleveland Heights, Ohio


Q. Thanks for the response to my query! Based on your info, can the design of process be so made that water will have higher pH so as to counteract the acid? What can be the adverse result to the above? Also, apart from the factors you have indicated, what others should be considered (e.g., silica content).

Regards and thanks again.

Siddhartha Bhattacharya [returning]
- Mumbai, Maharashtra, India


Q. I want to ask that how water is used in India? For example that how water is wasted and how water is stored? and also about the contetey in india?

OK bye

Maham Nawaz
water - Lahore, Pakistan

Conductivity of Rinse Water

July 19, 2011

Q. Hi I'm an intern and I work with an anodizing and hard coat anodizing line. I'm working on a project to try and reduce our use of water in the rinse water tanks without loosing the quality. Currently we have conductivity meters on each rinse tank but they are set so that the water runs through them all the time. We would like to set them so that they keep the rinse water pure by keeping the conductivity within a certain range but I'm having a really hard time finding out what the conductivity should be in each rinse tank. Can you help me figure out what conductivity would be a good set point or range? We have an acid etch rinse, an alkaline cleaner rinse, a caustic etch rinse, a deox rinse, 2 anodize rinses, a nitric rinse, and a couple dye rinses.

Neysia Wimmer
Intern - Alexandria, Minnesota, US

July 20, 2011

A. Hi, Neysia.

The cleanest water gives the best rinsing, but clean water is expensive from a cost and wastewater volume standpoint. These facts make it very difficult to convince anyone who thinks the rinse rate should be higher, or anyone who thinks it should be lower to change their mind -- there simply isn't any scientifically determinable answer :-)

The best path may be to go back to the very old studies which showed that a good empirical starting point was a dilution rate of 500:1 - 1000:1 in rinses. So take a bit of each solution, dilute it 500:1 and measure its conductivity, and use that as a setting until someone can present a convincing case that it should be raised or lowered. You should have Kushner's "Water and Waste Control for the Plating Shop" in hand. Good luck.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

simultaneous July 21, 2011

A. I was called upon about a year ago to work on this very problem.

Conductivity will help you with the etch, acid, chromate, and cleaner rinses. It is, however, next to useless for seal and dye rinses. The main electrolyte in those baths is acetic acid; it's a weak acid that's very poorly ionized at the pH values you're dealing with.

I did extensive monitoring of the conductivity of the rinses following the Ni acetate seal and the chromating baths, as well as doing a series of dilutions of the baths, and trying to correlate conductivity vs. total dissolved solids. I got excellent correlation with the chromate study; terrible with the seal study.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

July 22, 2011

A. There may be another method to look at.

If you already know what the conductivity of your rinse water is and because you know what the contaminants are, maybe you could look at using your rinse overflows as the feed water for your DI plant.
I will hazard a guess that your rinse water is probably purer than the raw water being supplied to your DI plant currently.
You might need to add some slight pre-treatment but this would depend on which rinse tanks you have and if you intend to use all of them as recycled feed.

You could investigate both and cost out which one provides the greatest saving after whatever investment is required.
You may even end up going for a combination of both.
Managers often like to be provided with more than one option.
They also like it when you tell them which one to choose based on costings and figures rather than opinion.
My advice would also be to monitor water usage after and prove to them that what you said would happen did.

Ciaron Murphy
Aerospace - South Wales, UK

August 5, 2011

A. Hi Neysia,

Which quality has the feed water from your rinses at starting point and from which source does it come from? Also, what happens with your rinse water, how does it get treated before entering the public sewer? When you already have your conductivity monitoring available then why not using these data by setting a certain set-point by collecting as much data as necessary such as flow rates, surface of parts, rinsing technique etc and measuring in a high frequency the conductivity. Secondly recycle as much as you can and this is with today's technologies absolutely possible and an excellent controllable process to meet always specifications as required.
Water savings starts with Investments and upon requirements (& provided consultancy) payback rate can be hold by a low level.

Try it, it's worthwhile & interesting

Kind regards

Dominik Michalek
- Melbourne, Australia

May 1, 2012

Q. I am working on a similar project and I am stuck with calculating rinse water levels for dye, deoxidizer and sealer rinse tanks. I have been using conductivity and TDS based data to calculate optimum water levels for our tanks but have hard time finding any good suggested conductivity levels data for dye, sealer and deox rinse solutions. Can you suggest where I could find any industry average data for suggested conductivity levels for these tanks.

Venkat Reddy
- New York, USA

Rinse tank operating guidelines for galvanizing plant

October 19, 2020

Q. I work for a UK galvanising plant. The reasons for disposal of our rinse water tanks are based solely on age, once per 6 months, but I've now been asked to base the frequency of our water wash disposal on a test result. (For example, maybe this might be how much acid or caustic g/l is in the wash or maybe the pH, etc?).
Can you recommend any operating specs or other factors to base a rinse tank disposal on please?

Thanks in advance for your comments.

Philip Rudland
- Scarborough UK,

October 19, 2020

A. Sometimes water in rinse tanks after plating or anodizing is controlled by water conductivity. Conductivity meters are not expensive and very simple to operate. I'm not familiar with galvanizing line, but probably it may work. You will have to establish your own baseline and limits.

Leon Gusak
- Winnipeg, Canada

October 21, 2020

A. Hi Philip,

I always have problems with measuring for measuring sake. Generating data that benefits no-one is wasted effort and cost.

If you are not having any quality issues and no problems with disposal of waste water, why would you analyse it? What would it benefit you? If you know the pH, total dissolved solids, zinc content or any number of other potential measurements, what are you going to do with the information?

Brian Terry
- Yeovil, Somerset, UK

October 25, 2020

A. Hi Philip!
I agree in theory with rinse tank disposal based upon contamination rather than calendar age, but to set parameters if there isn't anything called out by process specification ... you just need to have a sense of where the first fail point is, and set the dump trigger to be well before that.

Take a look back at old data, and see if Production or QA/QC has any documented failures you can correlate with it that weren't explained by other factors. So for example (and this is just hypothetical) you start to see issues in your product when the rinse tanks are above 1000 ppm TDS. You could say, I will check TDS on a daily basis and when it passes 800 ppm, schedule it to be dumped at the end of the week. There's no point in having it tested for specific contaminants except for environmental compliance, or if you are actively troubleshooting a plating defect... usually TDS is a good indicator of overall rinse health, and it takes only a moment to check. Certainly you could also test pH but I put less trust in that number because the meter will tell you only the raw pH, with no regard as to the total alkalinity or free acid content, and if the overall ionic strength of the solution is quite low... you could find yourself dumping a perfectly good tank that just looks bad to the pH meter! If I were to stick a pH meter in my lab DI water right now, I'll bet it would read about 5.5... pH alone is not nearly so useful to predict end of rinse tank life as TDS/conductivity.

Rachel Mackintosh
Lab Rat - Greenfield, Vermont

October 27, 2020

A. We find that changing rinse tank water in galvanizing plants is based more on convenience than on quality, but of course only changed if the tank is "dirty". We find that a titration for HCl is enough to determine that a tank is really bad. (>1% HCl), but pH is also a good easy measure. (<pH 2)
There are two main methods of disposing of dirty rise water: Use some to make up new acid tanks, and dumping.
Dumping is expensive, as this becomes a waste that in the UK must be carried by licensed contractors in special vehicles, etc. Expensive, almost as much as dumping spent acid. Make up in acid tanks is the best, rather than using any fresh water! If the acid tanks and rinses are the same size, then a typical brew for a new acid tank is 1/3 each of rinse water, 28% HCl and spent acid.
The 28% is to get the HCl conc. up, the spent acid to get the pickling working (if a new acid tank has no iron chloride it pickles very slowly until it develops some of its own), and the old rinse water for dilution.
A typical galv plant has about 5-6 pickle tanks, and that means every time you make up a new acid tank, you'll remove about 1/3 the rise tank contents. Make up the rinse tank with fresh clean water.

Geoff Crowley
Crithwood Ltd.
supporting advertiser
Bathgate, Scotland, UK
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