Process Puzzler: Tackle Turmoil About Wastewater Treatment

THIS MONTH’S PUZZLER

Our corporate engineering department can’t agree on how to deal with the wastewater at our new beet sugar plant. The flow averages about 1,600 gpm and peaks at about 2,400 gpm. We also have to consider rainwater, namely, a once-in-10-yr flood event of 16-in./hr. Our current plan is to use a flocculant in a clarifier to eliminate the dissolved solids that discolor the water. However, both the flocculant vendor and the clarifier manufacturer say we must ensure the clarifier feed won’t contain any suspended solids.

The production manager insists we need a centrifuge but dismisses bench tests as useless for predicting the operation of full-scale equipment. One centrifuge maker wants to rent several units to us on a trial basis.

However, the project manager and process manager both want to test the samples on a bench scale.

Our problem is becoming urgent. The city water-treatment works where the wastewater goes is tired of our brown tea and sludge. It claims that ultraviolet light can’t sanitize our water because of its cloudiness. They’ve already cited us once. Maybe, we have three months before we are cut off.

Our supplier of boiler chemicals agreed to send several samples to its lab in Houston to evaluate the flocculant. That’s 1,000 miles away! Unfortunately, I discovered the pH had to be neutralized to ship the samples; I think this ruined the tests.

What do you think we should do?

Test On-Site

The politics of this situation are unnerving to most young engineers. We’re always taught that reason is more powerful than position in an organization.

You should test the equipment on-site because the water properties will change, even if stored in a sealed container. The contamination of the samples with base affects the flocculant trials the chemical supplier will run. In addition, any suggestion that someone could duplicate the chemical makeup of the water either by re-acidifying the sample or recreating the water properties is plain silly.

While bench tests offer an optimistic perspective on the unit performance, doing a full-scale test seems suicidal. I’d be wary of this equipment supplier; it seems to be pandering too directly to production.

One effective approach for solving this problem is to treat a side stream of the flow. This allows you to test and reject ideas without disrupting production. In addition, testing can reveal hidden problems you can’t test easily in a lab.

If you can’t readily rent the equipment for side-stream testing, bench testing is a good option for getting a handle on a problem. You can test your flocculant and unit operations in the lab. This will be more expensive than sending samples to Houston but will permit you to bench test other operations in your plant.

The production manager’s insistence on a centrifuge would lead to a costly purchase. A centrifuge capable of handling 1,600–2,400 gpm would be extremely expensive. So, first consider a poor man’s centrifuge, i.e., a hydrocyclone. Finding a hydrocylcone in that range will be difficult but will provide a far less expensive option: it will probably be a bank of units.

Also, a centrifuge would require a prefilter to protect it from large debris, such as pulp, straw and even small stones. A hydrocyclone will effectively remove silt (<1 micron), sand (100 microns) and even other debris that isn’t too large. This could result in too much flushing of the solids to the collection pot.

One option to consider during the bench tests is injecting the flocculant in a mix tank or static mixer upstream of the hydroclone and clarifier. Duplicating residence time in a bench test will be difficult, so you’ll have to make estimates and extrapolate to the production scale.

The greatest danger in bench testing is considering ideas that, when extended to the full-scale, present insurmountable technical challenges or extreme costs. Vendors are little help here. It’s no hole in their wallet if you spend a million dollars when you could have spent a thousand.
Dirk Willard, consultant
Burley, Idaho

January’s Puzzler

We are investigating whether we can increase our refinery capacity by about 15%. I’ve been told not to worry too much about the vent system. However, I have some concerns, especially because all the pressure safety valves (PSVs) are conventional and some of the scenarios I am looking at involve more mixed-phase flow (see Figure 1). A model of the vent shows that all PSVs have backpressure ratios less than 10%, although valve 2 is close at about 9%. Also, some of the steam heat tracing and the insulation on the vent to the knockout drums haven’t been inspected in some time. Should we be concerned?