Saving Money with Your Wastestream
Publicy Owned Treatment Works (POTW)
Many industries aren’t aware that there are simple, cost effective solutions to lower the surcharges they are paying to the Publicly Owned Treatment Works (POTW’s). When the POTW is treating wastewater from an industrial source, they will charge the industry a surcharge to cover the additional cost of treating the industrial waste. Surcharging industries is a common practice since most industries cannot afford the costs of a full primary and secondary wastewater treatment system.
As an industrial plant manager or managing person responsible for the pretreatment of the industrial wastewater, it is often frustrating trying to find a reliable solution to decrease the costs of the surcharges. Equipment vendors will provide equipment solutions, the chemical companies will provide chemical solutions, and consulting engineers will provide studies and engineering recommendations. The responsibility falls on the industrial plant manager to decipher the information available and select the best treatment solution. When attempting to lower surcharges it is imperative to take an overall view of your pretreatment system. This is the key in lowering surcharges. How is a change in chemistry going to effect your equipment, if you change internal plant processes, are you still using the best chemistry, etc. It is possible to save money in wastewater costs by following a simple procedure in evaluating your entire wastewater process, and then optimizing your existing system, thus lowering your surcharges.
The POTW’s will most often monitor industries for Biological Oxygen Demand (BOD), Total Suspended Solids (TSS) and the flow of the wastestream. The POTW’s then have a specific rate structure which determines the total surcharge billed to the industry. By lowering any of these constituents, industries can lower their surcharge.
The following step by step process offers a starting point for evaluating and optimizing your entire waste water system and ultimately lowering your surcharges. It’s important to understand that relatively minor changes in your process can result in substantial annual savings.
VERIFYING SURCHARGES
It is very important to verify the surcharges presented by the POTW. Quite often industries are paying higher surcharges than their wastestream dictates. By sampling and having your own analysis done, you get a good education on where the numbers come from and what you can do to lower them.
The first step is to identify the regulated surcharge constituents from the local POTW. These often include BOD, TSS, and occasionally COD (chemical oxygen demand) and FOG (Fats Oils and Grease), and are evaluated on a total lbs /day produced. Check your last surcharge bill for the regulated constituents. Also, contact the POTW to insure the surcharge rate structure is correct and the formulas are current.
After identifying the regulated constituents in the wastestream, determine the sampling method and procedure the POTW uses. Using the same methods (normally a 24 hour composite) take a sample of your waste water and have it analyzed for the regulated constituents, following the same EPA methods the POTW uses. The local analytical lab is invaluable and will readily assist in sampling procedures and sample storage. Be sure the samples are kept in a refrigerator until they are delivered to a lab. Often local labs will come to your facility, deliver sample bottles, explain the sampling operation and, pickup the samples as part of their customer service.
The wastewater flow is the last component needed to determine the surcharges. The wastewater flow can be estimated by using the plant water data from the wells or water meter. The wastewater flow should be equal to the influent minus internal plant operation water losses (cooling towers, evaporation, boiler steam losses, cookers, domestic usage, etc.). There are often internal processes that use a substantial amount of water and accurate water consumption rates are difficult to estimate. The POTW has standard estimated water consumption rates for some equipment and domestic usage is usually at 20 gal/day per worker. The POTW will tend to error to the favor of the POTW, so it is best to perform some level of study to accurately determine the wastewater discharged. For larger plants, it is usually economical to install a wastewater flowmeter and keep a daily log. This documents the wastewater discharged directly and leaves little room for error. An alternative method is to rent a ultrasonic strap-on flowmeter that will show the actual plant wastewater flow. Whatever the method for flow rate determination, obtain the approval of the POTW since they will need to accept the new measuring device.
When the analysis results are received from the analytical lab, you now have everything necessary to calculate your surcharges. If you don’t know the surcharge rate structure, contact the local POTW, they should provide you with the rate structure, and the calculations they used in evaluating your surcharges. Below is an example of a typical wastewater analysis and the steps to calculating the surcharges.
Surcharge calculations:
PLANT FLOW: 100,000 gal/day, and lab data shows the BOD: 1,200 ppm and TSS: 750 ppm SURCHARGE RATE STRUCTURE: Flow @ $0.35/Ccf, BOD @ $0.25/lb., TSS @ $0.20/lb.
SURCHARGES: Flow: 100,000 gpd / 748 Ccf x $0.35 Ccf = $46.79/day BOD: 1,200 ppm/1,000,000 x 100,000 gpd x 8.34 lb./gal x $0.25/lb. = $250.20/day TSS: 750 ppm/1,000,000 x 100,000 gpd x 8.34 x $0.20/lb. = 125.10/day
TOTAL SURCHARGE= $422.09/day times the number of operating days per month equals the monthly surcharge.
There are often other miscellaneous charges on your bill, so weed out the actual surcharges. After calculating the monthly surcharges, compare to the billings received from the POTW. If there is a discrepancy call the local POTW. Work with them to re-evaluate your surcharges, most POTW’s are very willing to work with industries in this manner.
EVALUATING YOUR EXISTING SYSTEM
Now that the surcharges are fully documented and verified, there is a procedure to decrease them. The next step is to evaluate the existing pretreatment system, in other words, investigate the removal efficiency of the equipment. Most plants will have some kind of liquid/solids separation equipment, (i.e. Screening, DAF, Clarifier, Gravity Separator, etc.). Test for the regulated constituents before and after each piece of equipment. Document the removal efficiencies for individual components. Examine the equipment operations, functions, and removal efficiencies as compared to the original equipment manufacturers specifications.
If the amount of solids exceeds the equipment specifications you should consider an engineering or equipment solution. Do you have the best equipment for your type of waste? If your waste wants to float, some kind of skimming equipment will work best, if it wants to sink some kind of settling or clarifier equipment would work best. This can be determined by performing jartests which model your system. If you decide to use outside help shop around and find a firm or representative that is knowledgeable in the overall wastewater industry (someone who understands the chemistry, equipment and engineering for treating wastewater). Again this is key in optimizing your system. There are companies knowledgeable in all areas, seek them out.
EVALUATION AND OPTIMIZATION OF CHEMISTRY
One of the largest expenditures for pretreatment systems is the chemical expense. Question if the chemistry used for your wastestream is the best to achieve the goals for maximum surcharge reduction. To remove TSS and BOD from the wastewater, chemicals are used to change the solubility of the wastestream’s constituents so they can be removed.
Industries that discharge to POTW’s, benefit most by meeting the zeta potential (colloidal or emulsified charge required to coagulate the wastestream). When the zeta potential is met, the wastestream can be adequately coagulated (particulate formed), flocculated (several coagulated particles are pulled together) and good liquid/solid separation takes place by settling or floatation. The key to maximizing the surcharge savings is to meet the zeta potential of the wastestream in an efficient manner. Meeting the zeta potential is almost a hit and miss operation that requires pH adjustment up or down and the injection of coagulants and/or flocculants into the waste stream. This hit and miss procedure is best determined by jar test before testing on your actual process.
Are you using the most cost effective combination of flocculants and coagulants? This is not an easy question to answer in that many combinations of coagulants and/or flocculants are available and many will work. Chemicals come in varying ratios of charge densities ( the #of charge sites per molecule), concentration and percent activities. As an example, you may be able to cut overall chemical costs by buying a higher concentration of the same chemical and feeding less. These are all chemical aspects you can discuss with your chemical supplier or experiment with using jar tests.
Talk to your chemical supplier about the specifics of the chemicals you are using. Review the chemical Material Safety Data Sheet Understanding all the chemicals available is not necessary. However by knowing the characteristics of your wastestream and how the above three criteria may effect it will give you a the foundation to make a more educated decision in what chemistry will work best for your process
The pH is another chemical aspect which needs close attention. The wastewater pH will greatly effect the performance of the chemicals and therefore the liquid/solid separation. To obtain a true measure of the pH swings, measure the pH of the wastewater over the operating day. Measuring the pH can be performed by using a pH controller and chart recorder (if one is available) or use litmus papers and measure the pH every 1/2 hour and record. If you find your plant has severe pH swings, You should consider how to stabilize the pH for optimal system efficiency.
Most complaints about wastewater treatment is the annual cost of the chemicals. Again, the best procedure to reviewing the wastewater chemistries is to perform jartests to model the treatment chemicals and wastewater system. This service is typically performed by your chemical supplier, if not call around, most chemical suppliers are very helpful in this area. Request that the chemical supplier perform a matrix of jartests with dosage versus percent removal for a variety of chemicals. A further step is to request chemical samples and have the plant staff perform the jartests, avoiding any biased opinions. Most often optimization of the wastewater chemistry results in decreased chemical usage and substantial annual savings.
SYSTEM OPTIMIZATION
One of the simplest things to look at are the chemical injection points on your system.. Simply, chemicals need to have sufficient mixing energy (contact frequency with the wastestream) to work.
The typical chemical addition process is to pH adjust, add a coagulant, and then add a flocculant to wrap up or sweep the floc particles together. If their is inadequate mixing of the wastestream after any of these chemical additions you will not have optimum system performance. As an example, if there is not adequate mixing after pH adjusting (with acid or caustic), then the pH at the point of coagulant addition will be unstable and out of the proper pH range. This means whatever coagulants and flocculants you use, they will not be working efficiently, therefore you will have to overdose them which leads to higher overall chemical costs. So in this scenario, simply changing the point of pH adjustment higher upstream, where it has more time and mixing energy will ultimately lower your overall chemical costs.
The above example illustrates another point to be looked at, the concept of under and overdosing chemicals. If the coagulant is underdosed, then there will be few coagulated particles and poor liquid/solid separation. An alternative scenario is to have the coagulant dosage too high (overdose) which will then cause the particles to act as dispersants causing poor results. Again, you want to use the results from the jar tests to estimate the chemical usage. If there are drastic differences between the actual amount of chemicals used versus the jar test results, you will really want to consider the points outlined in this article.
Adjust the pH, coagulate, then flocculate and verify there is sufficient mixing energy after each chemical addition point. The complete chemical addition process needs to be closely setup and monitored. Mixing energy and chemical addition should be modeled in the jartests, and then the system changed based on the results. The changes should be evaluated based on the real system.
Any changes made need to be closely monitored and documented. Make sure sufficient time is allotted to properly evaluate the impact of the change on your system. When you change chemical dosages allow at least twice the system detention time before making any further system changes.
SUMMARY
In summary, it is possible to lower surcharges assessed by the local POTW. First, by understanding the surcharges and how they are assessed. Secondly by optimizing your facilities entire wastewater system including chemistry, equipment and system operation. Overall savings can show up as lower chemical costs and/or lower surcharge costs. By simply taking the time to properly evaluate your wastestream, and optimizing any existing pretreatment system already in place, you can create substantial savings for your facility.
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