Ways to reduce chemical use in a DAF system

7 Ways to Reduce Chemical Use in a DAF System

It’s no secret that the greatest operating expense of an industrial wastewater treatment system is in chemical usage. Naturally, finding a way to reduce chemical use would be a good place to start finding savings. The electrical cost of running a PCL-60 DAF system at full capacity (660 gpm) for 24 hours is roughly $30. That figure will fluctuate with up and downtime, but is pretty close to constant. Chemical costs, on the other hand, are directly related to flow rate and wastewater composition (i.e. solids concentration, oil content, pH) and can vary widely from application to application. Where operating a DAF system for one year might cost $11,000 in electricity, chemical costs can easily be five or six times that much.

To paraphrase Einstein, “In every challenge lies an opportunity.” Our challenge here is to reduce chemical use with the opportunity to save thousands on operational expenses. So here we go…

1. Equalize and Mix Flow

A simple way, yet effective way to reduce chemical use is to collect and mix all plant wastewater, rain, and yard runoff into a homogeneous solution. Many plants operate multiple shifts with some generating wastewater with more solids than the others. Rather than treating that heavy load with extra chemicals, blend it in with the lighter loads to dilute the solids concentration. Then you can flow pace your chemical dosing as opposed to dosing based on peaks and valleys in solids loading.

2. Use Pre-Screening Equipment

This externally fed rotary drum screen reduces TSS from 3500 mg/L to 1200 mg/L.

This externally fed rotary drum screen reduces TSS from 3500 mg/L to 1200 mg/L.

The idea is simple – the more solid contaminants you can remove before using chemicals, the better. Install floor drains with tighter screens, run process wastewater through a rotary drum screen, allow heavy solids to settle in a holding tank.

Simple static and mechanical screening can dramatically reduce the volume of solids sent to wastewater. Then when chemicals are dosed they go after the solids that are harder to separate instead of the ones that should have been removed mechanically.

3. Calibrate Dosing Pumps

Liquid chemical feed pumps come in three main forms: peristaltic, motor driven, and electric metering, each with its own application-specific advantages. Regardless of the type of pump your plant employs, liquid feed pumps have to be calibrated and re-calibrated.

Use a graduated cylinder to measure the volume of water moved through the pump in one minute. Repeat the minute-test on four or five different settings between 10%-90% capacity. If your pump moves 35 ml/min when you set it to 30 ml/min, it’s overdosing by 15%. That could amount to thousands of dollars in no time. Calibrate your pumps.

4. Adjust pH with Correct Chemicals

We’ve already had a good discussion about why using coagulants to adjust pH is wrong, but it’s worth reiterating a couple points. Metal-based coagulants used in wastewater treatment are effective only within neutral pH ranges. When wastewater has a high pH, acidulation is required to bring it to neutral. Aluminum and iron based coagulants are acidic in nature and can achieve this neutralizing effect, but that is not their intended purpose. Instead, using less-expensive pH reagents (i.e. sulfuric acid, hydrochloric acid) to achieve neutralization is the right way to go about it. Not only does it make economic sense, but it’s better for the overall wastewater treatment process. Again, read the aforementioned post.

5. Disperse Chemicals Effectively

Flocculator chemical dispersion

FRC’s flocculators disperse chemicals in mixing zones that increase flow-through velocity

In DAF systems, chemical dosing pumps inject a stream of liquid chemical into a mixing tank or pipe flocculator, what happens to the chemical thereafter depends on the design of the equipment. Chemical reaction tanks use mechanical agitators to blend the chemical with incoming wastewater and do so quite well, barring adequate tank size and mixing velocity. Flocculators rely on shear forces to disperse chemicals as water moves through the serpentine structure. Both systems can work in place of the other, though flocculators are much less-expensive than the alternative.

Chemical reaction tanks can better disperse chemicals by slowing the feed flow rate. Pipe flocculators are designed to maintain a specific flow velocity to disperse and mix chemicals. Whichever method your system employs, make sure you’re following the specified operational procedures.

6. Treat to Permit Requirements

If your permit limit for TSS is <250 mg/L, you’re equally as free to discharge at 200 mg/L as you are at 20 mg/L. Some chemical suppliers may show you a jar of clear water where they’ve reduced TSS to <10 mg/L. The thing is, you don’t need to treat to <10 mg/L, you only need to beat 250 mg/L. We’re not advocating skirting right up to 249 mg/L for TSS, we’re saying reduce chemical use to what you need to comply with your permit. The general axiom is, “use the least amount of chemical necessary to meet the treatment requirements.”

7. Jar Test, Jar Test, Jar Test

The easiest and most immediate way to reduce chemical use is to draw wastewater samples and test dosing rates. Grab 100 ml of wastewater, use a pipette to drop in 1 ml of coagulant, and give it a stir. If you see adequate coagulation, you’re good to go. If DAF effluent quality starts creeping too close to the limit, jar test again and adjust your dosing rate.

If you’re already doing all of these things, well done! If not, then hopefully we’ve helped you identify a couple ways to save a buck on operations.

One Final Note…

There are many cases where attempting to save money on operations by eliminating chemistry altogether is absolutely the wrong choice. Many suspended solids will remain suspended without the addition of coagulants or flocculants to bind them into larger, floatable flocs. DAF systems can still remove a significant portion of these solids without added chemistry, however the float sludge is often very watery and the effluent quality is not as good as it could be. This is simply shifting the cost of chemistry in the DAF system to the operations of the sludge management and biological treatment processes.

In other cases the value in using chemistry is realized in the recovered product from the DAF system. For example, a rendering facility might employ a DAF system to recover solids for reprocessing. Without chemistry they recover a certain percentage of the solids which add to their bottom line. With chemistry they recover 5x as many solids and add even more to their bottom line. The extra cost for the chemistry easily pays for itself in the increase in recovered product.

All this being said, it’s important to weigh the options before heading too far down one path. Any questions, shoot us an email to info@frcsystems.com.

 

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