By Robert A. Governal, Ph.D.

Summary: Water treatment technology costs can drive a customer toward or away from your door. In this paper, the impact of varying water contamination levels and flow rates on innovative wastewater treatment system designs, capital costs, operational costs and area requirements is presented.


U.S. environmental laws and regulations during the past 10-to-15 years have increasingly required more organic and inorganic contaminant levels in treated wastewater to meet parts per billion (ppb) limits. In the organic chemical industry,1 treated wastewater effluent is expected to meet ppb discharge limits for 63 constituents, including 56 organics, six metals and cyanide. To simplify the list, researchers categorized the list of contaminants into four main groups—anions, cations, metals and organic contaminants.

With limited funding
To meet permit requirements based on these increased effluent limitations, improve performance and reduce costs,2 many activated sludge wastewater treatment plants are now implementing biological nutrient removal (BNR) strategies as part of the base design. Design teams can retrofit existing plants for BNR by adding recycle streams (return activated sludge, or RAS) to the liquid process flow and creating anaerobic and anoxic—oxygen deprived—zones within existing tanks. In order to obtain effective BNR, the plant design should ensure no oxygen is introduced into the anoxic environment as well as allowing sufficient nutrient conversion time.

BNR is an important requirement to small municipalities where funding is scarce. In some high-altitude communities including small municipalities in Alaska,3 for example, peak flow demands and contamination levels may be greater than those experienced by larger municipalities; therefore, storage systems as well as wastewater treatment plants may be more expensive to purchase, operate and maintain per gallon of water treated than technologically equivalent systems for larger municipalities.

Since common wastewater system design requirements continued to resurface in the case studies observed, minimum key design parameters such as system flow rate and wastewater contamination levels at both the influent and effluent should be fixed as closely as possible to create successful designs and applications toward present and future wastewater systems.

Plant design criteria
The sequential oxidation process has been presented previously to the reader (see Governal, R.A., “Advanced Wastewater Treatment System Design: Recycling Water for Reuse Using POU/POE Technology,” WC&P, February 2000) to provide fundamental understanding and opportunities for design of advanced wastewater treatment systems.4

For this study, BOD/TSS/NH3—biological oxygen demand, total suspended solids and ammonia—levels were fixed at 250/250/40, 300/300/60 and 350/350/80 milligrams per liter (mg/L), with design flow rates at nine points, ranging from 25,000 gallons per day (gpd)—0.025 million gallons per day (mgd)—to 2 mgd. Only four of these points are shown in charts included here. Based on experience, effluent BOD/TSS/NH3 were fixed at 10/15/1.0 mg/L for all system designs. Additional study design parameters are shown in Table 1. It’s important to note these parameters aren’t all inclusive and can easily vary from one municipality to another—they’re meant as representative examples for discussion purposes only.

This approach resulted in a matrix of designs from which capital costs, operational costs and surface area or footprint requirements were determined. The reference case of BOD/TSS/NH3 levels of 250/250/40 mg/L and a system flow rate of 25,000 gpd were assigned values of 1.00 for each requirement. All other designs were compared to this base.

Results
Figure 1 shows the effect of increasing flow rate and contaminant level on a wastewater treatment system’s capital cost. As shown for each contamination level, the capital cost ratio increases at a rate slower than the equivalent increase in flow rate ratio. Increasing the flow rate by a factor of 10 (25,000 gpd to 250,000 gpd) results in a system that may increase in capital cost by a factor of approximately 3.5. Further increasing flow rate to a factor of 80 over the base can result in a system that’s only 20 times more expensive. The cost increase in higher flow rate is generally due to additional requirements for equipment and services—but the decrease in cost per gallon of water treated with a higher flow rate is due primarily to obtaining bulk discounts from vendors and lower up-front engineering and manufacturing costs through use of “modular” designs.

Above results are also influenced by influent contamination levels. As shown, when the contamination level is increased to 350/350/80, a required flow rate of 25,000 gpd results in a system that’s approximately 4 percent more expensive than the base. At this higher loading, increasing the flow rate to 250,000 gpd and 2 mgd results in capital cost ratio jumps to four and 24 times the base, respectively. The increase in cost is primarily due to additional oxygen requirements to remove contaminants to effluent levels specified. This translates to more blower horsepower requirements, larger blowers and more diffusers within the tanks.

Figure 2 displays the effect of increasing flow rate and contaminant level on operating cost of a sequential oxidation wastewater treatment system. Again, the base design is set with influent and effluent contaminant levels of 250/250/40 and 10/15/1.0 and a system flow rate of 25,000 gpd—with an operating cost factor of 1.00. As shown in the first set of bars, operating cost decreases with increasing flow rate—but increases as wastewater influent contamination level increases.

For the 250/250/40 case, increasing the flow rate by an order of magnitude (10X) results in a reduction in the operational cost ratio of the system to about 38 percent of the base value, while an increase in system flow by a factor of 80—to 2 mgd—decreases the operational cost ratio of the system to roughly 23 percent of the base value. Small municipalities, therefore, may expect to pay more per gallon of water recovered than equivalently designed larger systems.

As contamination levels increased to 350/350/80, for example, operating cost factors of the 25,000 gpd, 250,000 gpd and 2 mgd systems increased to 1.21, 0.50 and 0.30, respectively, when compared to the base case—and by 21 percent, 32 percent, and 30 percent, respectively, over lower contamination cases. Small facilities with high loadings were consistently found to require water treatment plants with the greatest operational costs per gallon of water treated.

Figure 3 shows the effect of increasing flow rate and contaminant level on the surface area or footprint requirement for a sequential oxidation wastewater treatment system. Again, the base design is set with influent and effluent contaminant levels of 250/250/40 and 10/15/1.0 and a system flow rate of 25,000 gpd. This resulted in a surface area factor of 1.000. Area requirement was found to increase both with higher flow rate and contamination levels.

When the required flow rate increased by factors of 10 and 80—250,000 gpd and 2 mgd, the surface area required increased by factors of 7 and 42, respectively. Notice larger systems can require less operational area per gallon than those utilized by smaller municipalities. Larger systems may also include the option of utilizing deeper tanks than those typically specified in smaller systems. Tank depths of 16 feet or 18 feet aren’t uncommon in larger systems (>100,000 gpd), and would result in a further decrease in area requirement and resulting footprint.

Above results are also influenced by influent contamination levels. As shown in Figure 3, when these increased to 350/350/80, a required flow rate of 25,000 gpd results in a system that can require an additional 12 percent of treatment area over the base scenario. Increasing flow demands by factors of 10 and 80 can result in systems requiring nine and 58 times the area requirement over the base scenario. Larger systems require about 25 percent and 38 percent more space than low contamination systems. This result is most relevant to low flow/high contamination systems typically generated by prisons, resorts or municipalities that also handle commercial wastes.

Conclusion
Wastewater treatment plant design should be performed on a case-by-case basis, with particular emphasis on understanding the client’s needs for low capital cost, low operating cost and minimized area requirements. Key design parameters to determine the above factors should include—but not be limited to—system flow rate, contamination requirements at the influent and effluent and peak flow factors. Up to this point, hourly and sustained peak flow factors were set to default values of 4.0 and 2.5. In the real world environment, however, these values are application dependent and their effects on system capital costs, operating costs and surface area requirements will be the subject of future work.

References

  1. Schroy, J., M. Garrett, J. Budzinsli, R. Manross, S. Nutt and W. Schuk, “Sensing Future Opportunities,” Industrial Wastewater, 4(3), pp. 49-53, May/June 1996.
  2. Lynch, D., and P. Cooper, “Teaching an Old Plant New Tricks,” Water Environment Technology, 8(1), pp. 37-42, January 1996.
  3. Fraser, D., C. Davies and R. Jones, “Capital Needs of Small Systems,” Journal AWWA, 87(11), pp. 32-38, November 1995.
  4. Governal, R., “Advanced Wastewater Treatment System Design: Recycling Water for Reuse Using POU/POE Technology,” WC&P, pp. 132-135, February 2000.

About the author
Dr. Robert A. Governal is senior marketing manager with CUNO Inc. in Meriden, Conn. Governal earned his doctorate in chemical engineering from the University of Arizona. He has written over 35 technical papers in the field of water purification technologies. This paper was initiated when he was a regional sales manager with Waterlink/Aero-Mod in Manhattan, Kan. Governal can be reached by phone at (203) 238-8869, (203) 238-8962 (fax) or email: rgovernal@cuno.com

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