“Municipal Wastewater Reuse – The Challenges Ahead”

By Randy Turner, Technical Director, Swan Analytica l USA

Introduction

Water resources are becoming more constrained. Obta ining surface or groundwater withdrawal permits is becoming increasing difficult . Often times deep well total dissolved solids (TDS) is often very high which requires pret reatment to reduce TDS thereby increasing operating cost. Salt water intrusion is occurring in coastal regions. Droughts routinely impact water quality and availability. Mu nicipal waste water is becoming a viable option:

− California – Title 22

− Florida

− Palo Verde Nuclear Station Municipal waste water is being used for: − Cooling water − Agricultural − Scrubber − Boiler Feedwater − Other Waste water quality may be better than available wa ter sources in some cases or may contain more TDS, organics, microbial agents than a vailable surface and ground water. Characterizing the water or waste water source ever y aspect over time is critical. There are benefits and challenges.

Why Reuse Water?

As our population grows and more power plants are b uilt as well as other industries that require water increase the demands on our limited s upply of water increases. Currently 200 billion gallons per day is withdrawn to support pow er generation. This is expected to increase by 50% by 2030. Twenty five watersheds ar e currently stressed by power generation activities. We must use water wisely an d utilize alternate sources such as treated waste water. Table 1 illustrates the amoun t of water withdrawn and consumed for various plant and cooling system type.

Information Resources Regarding Water Reuse

There are a number of sources available to obtain i nformation regarding water reuse. 1. EPA 2. Selected state environmental protection agencies that regulate wastewater discharges. 3. Water Environment Federation (WEF). 4. Water Reuse Association. 5. National Association of Clean Water Agencies (NA CWA). 6. Association of State and Interstate Water Pollut ion Control Administrators (ASIWPCA). 7. Electric Power Research Institute (EPRI). 8. Edison Electric Institute (EEI). 9. Utility Water Act Group (UWAG). 10. American Society of Mechanical Engineers (ASME) Water Management Committee

EPA (2004) includes suggested guidelines for reusin g treated municipal wastewater in various applications. The portions of those guidelines rela ting to using reclaimed water for cooling are listed below. 1. The minimum treatment requirements include secon dary treatment, disinfection, and possible chemical coagulation and filtration, if ne eded. Additional treatment may be performed by the user to prevent scaling, corrosion , biological growth, fouling, and foaming. 2. The reclaimed water should contain <30 mg/L of B OD and TSS, <200 fecal coliform/100 milliliters (ml), a minimum of 1 mg/L residual chlo rine, and a pH range of 6.0–9.0. 3. Windblown spray should not reach areas accessibl e to workers or the public. This requirement can be met by providing a setback dista nce of 90 meters. The setback may be reduced or eliminated if a high level of disinfe ction is provided. 4. Pipes, pumps, and other components used to conve y reclaimed water must be painted bright purple to distinguish them from other types of water. 5. In addition to federal and state regulations the re may be local regulations that must be considered as well.

Issues and Concerns

As with surface and ground water, it is critical to characterize the chemistry of the water over a period of a year preferably. It is critical to i dentify variations in chemistry that occur throughout the year caused by season climate change s, seasonal agricultural and industrial activities impacting runoff and demand, and it is a lso necessary to determine the flow patterns over a year. Flows vary throughout the da y, week, and seasonally. It is strongly recommended to perform a proper “pilot” study to ev aluate the proposed water source for the application(s) intended to minimize the risk of failure and identify potential issues.

Potential Problems

Some of the chemical constituents may cause the fol lowing problems: 1. Mineral scaling from calcium carbonate, calcium phosphate, and other products. 2. Corrosion, pitting, and stress cracking damage t o metal heat transfer surfaces and to structural metal surfaces (e.g., damage to copper, copper alloys, and other “yellow metals” from ammonia). 3. Biofouling of heat transfer surfaces and excessi ve biological growth on cooling tower fill material surfaces from BOD, phosphate, and ammonia. In some cases the contract with the municipality is structured such that if the water does not meet the quality specified in the contract for para meters such as chlorine and phosphate, the customer does not have to pay or pays a reduced rat e for the water. Some of the parameters one should consider routinely monitoring are:

 1. pH 2. Chlorine residual 3. Conductivity 4. Phosphate 5. Ammonia 6. Chloride 7. Sulfate 8. Total iron 9. Manganese 10. Silica 11. BOD 12. COD 13. Scaling index It is critical to do this to provide the design eng ineers and chemists with an accurate picture of the chemistry and flow patterns to determine: 1. If additional treatment process is required. 2. To properly design any required treatment system . 3. Determine if treatment is needed to avoid scalin g or corrosion. a. Develop needed treatment concept. 4. Determine if storage capacity is required to com pensate for flow variations. Due to the variability of some key parameters some end users continuously monitor these parameters to adjust their process accordingly. Som e of the parameters that may be continuously monitored are: 1. pH 2. Conductivity 3. Chlorine residual 4. Phosphate Regulatory Concerns In many cases the water from the plant processes ar e discharged into rivers, other bodies of water, or back to the treatment facility. The dis charge is regulated under the Clean Water Act and requires a National Pollution Discharge Eli mination System Permit. In some cases the regulated contaminants in the waste water can e xceed the regulatory limit due to concentration in processes such as cooling towers. Therefore proper monitoring of the chemistry of the waste water and cycled water is ne cessary to ensure permit violations do not occur.

Conclusion

Water resources are becoming more stressed and dema nd continues to increase. This is forcing industry and regulators to evaluate technol ogies and alternative sources of water for non-potable use. Treated municipal waste water is abundant and is a viable option for use as cooling water, boiler feedwater makeup, FGD make up, cooling pond makeup, and agricultural uses. As the industry sites new plants or expands capacit y at existing sites, it must identify sufficient supplies of water to cool the steam. Re claimed water can help meet that need. California and Florida lead the movement toward was te water reuse.

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