It’s well known that chemical usage is the largest operating expense in an industrial wastewater treatment system. The electrical cost for running a typical (660 gpm) industrial DAF system at full capacity is approximately $30 per day (24 hours). Finding a way to reduce chemical use will lead to uncovering savings. Of course there is uptime and downtime that will affect this figure, but it remains fairly consistent.
Flow rate and wastewater composition (i.e., solids concentration, oil content, pH) are directly related to chemical costs and can vary dramatically among applications. 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. Outlined are several different ways you can reduce chemical use for the opportunity to save thousands on operational expenses.
Collect and Mix the Flow
Collecting and mixing all plant wastewater, rain and yard runoff into a homogeneous solution is a simple yet effective way to reduce chemical use. Because plants operate 24 hours a day, a plant can operate multiple shifts resulting in a variety of flow volume. A day shift might generate wastewater with more solids or vice versa — rather than treating that heavy load with extra chemicals, blend it in with the lighter loads to dilute the solids concentration. Then you can pace your chemical dosing, as opposed to dosing based on peaks and valleys in solids loading.
Remove as much Solid Contaminants as Possible by Pre-Screening Equipment
It’s a simple concept — the more solid contaminants you can remove before using chemicals, the better. Installing floor drains with tighter screens, running process wastewater through a rotary drum screen, and allowing heavy solids to settle in a holding tank are all simple strategies that will help reduce solids.
Screen selection is very specific to the flow rates of the process and as wells as the types of solids that need to be screened out. In some situations a simple static or mechanical screen can substantially reduce the volume of solids sent to the wastewater flow while a more sophisticated rotary drum screen may be required to achieve the optimum solids removal. When pre-screening is done right and the chemicals are dosed, they go after the solids that are harder to separate instead of the ones that could have been removed by screening.
Calibrate and Recalibrate
Peristaltic, motor driven and electric metering are the three main liquid chemical feed pumps, each with its own application and specific advantages. Regardless of the pumping technology, all liquid feed pumps must be calibrated and recalibrated.
When calibrating or recalibrating, use a graduated cylinder to measure the volume of water moved through the pump in one minute. Then repeat the test on four or five different settings between 10 percent and 90 percent capacity. If your pump moves 35 ml/min when you set it to 30 ml/min, it’s overdosing by 15 percent. This alone could amount to thousands of dollars in no time. It’s easy and fast — calibrate your pumps.
Adjust and control pH Levels
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.
Distribute Chemicals Sufficiently
To distribute chemicals effectively in a DAF system, a chemical dosing pump will inject a stream of liquid chemical into a mixing tank or pipe flocculator. Chemical reaction tanks use mechanical agitators to blend the chemical with incoming wastewater, assuming adequate tank size and correct mixing velocity. Flocculators rely on shear forces to disperse chemicals as water moves through the serpentine structure. While both reaction tanks and flocculators can be used interchangeably flocculators are much less expensive option.
Chemical reaction tanks 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.
Check Permits and Jar Test
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. While going right up to 249 mg/L for TSS is not advisable, you may have the opportunity to reduce chemical use to what’s needed to comply with your permit. The general axiom is to use the least amount of chemical necessary to meet the treatment requirements.
The easiest and most immediate way to reduce chemical use is to draw wastewater samples and test dosing rates. Take 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.
Don’t Eliminate Chemistry
When companies are attempting to save money on operations by eliminating chemistry altogether, it 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. A well designed DAF system 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.
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 adds to their bottom line. With chemistry, they recover five times 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.
It’s important to weigh the options before heading too far down one path. Reaching out to a company that is prepared to help optimize your system is a great place to start.