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Cooling tower deck 2

Introduction

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The increasing demand for water in industrial and power generation sectors, coupled with the growing scarcity of freshwater resources, has led to the exploration of alternative water sources such as treated effluent from sewage treatment plants (STP). This paper discusses the technical feasibility of using STP effluent as makeup water for cooling systems. It addresses the key water quality parameters, treatment processes, and chemical additives required to ensure the successful integration of STP effluent into cooling systems. Emphasis is placed on controlling corrosion, scaling, and biological growth through targeted treatment methods and continuous monitoring.

Cooling water : 
Utilizing Effluent STP as Makeup Water for Cooling Systems

Water scarcity is a critical challenge in many regions, driving the need for sustainable water management practices. The reuse of treated effluent from STPs as makeup water in industrial cooling systems offers a promising solution. This approach not only conserves freshwater but also provides a reliable source of water for cooling processes. However, the reuse of STP effluent poses challenges related to water quality, necessitating advanced treatment and monitoring strategies.

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Feasibility of Using STP Effluent in Cooling Systems
Technical Feasibility
 

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The reuse of STP effluent as makeup water in cooling systems is technically feasible. Studies have demonstrated that treated effluent, when properly conditioned, can perform similarly to potable water in cooling applications. For instance, a pilot plant study in Australia using chlorinated secondary effluent showed comparable biofilm growth rates to those observed with potable water, especially when supplementary chlorine or bromine chloride treatment was applied [(Wijesinghe et al., 1996)This highlights the potential for successful reuse with appropriate treatment and monitoring.

Water Quality Control for STP Effluent
1. Key Parameters

 

  • Total Suspended Solids (TSS) : Ideally, TSS should be reduced to below 10 mg/L to prevent fouling and scaling in cooling systems.

  • Chemical Oxygen Demand (COD) :Effluent with COD levels below 50 mg/L is preferred to minimize biofouling.

  • Nutrients :Effluent should contain low levels of total nitrogen (• pH: The effluent pH should be maintained between 6.5 and 8.5 to prevent corrosion and scaling.

  • Microbiological Quality : Effluent should have low levels of pathogenic bacteria to ensure safe reuse.

2. Treatment Methods

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  • Primary Treatment: Involves screening and sedimentation to remove large solids.

  • Secondary Treatment: Biological processes such as activated sludge or trickling filters reduce organic matter and nutrients.

  • Tertiary Treatment: Advanced methods like sand filtration and nitrification further reduce TSS, COD, and nutrient levels.

  • Disinfection: Chlorination or UV treatment is essential to reduce microbial load in the effluent, making it safer for cooling applications.

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Water Quality Control for Cooling Systems
1. Key Parameters

  • Total Dissolved Solids (TDS) : TDS levels should be maintained below 1,000 mg/L to prevent scaling and corrosion.

  • Alkalinity : Alkalinity should be controlled between 100-150 mg/L as CaCO3 to minimize scaling.

  • Hardness : Hardness levels below 200 mg/L as CaCO3 are ideal to prevent calcium carbonate and magnesium silicate scaling.

  • Chloride and Sulfate : Concentrations should be kept below 300 mg/L to reduce corrosion risk.

  • Silica : Silica levels should be maintained below 150 mg/L to prevent silica scaling.

  • Microbial Load : Regular monitoring and control of microbial populations are essential to prevent biofouling.

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​2. Treatment Methods

  • Filtration: Physical filtration removes particulates and reduces TSS.

  • Chemical Treatment:

  • Scale Inhibitors: Phosphonate-based or polymeric inhibitors prevent scale formation.

  • Corrosion Inhibitors: Phosphate, zinc, or molybdate-based inhibitors protect metal surfaces.

  • Biocides: Chlorine, bromine, and quaternary ammonium compounds control microbial growth.

  • pH Control : Acid or alkali dosing maintains pH within the desired range.

  • Blowdown Control: Regular blowdown controls the concentration of dissolved solids in the system.

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3. Monitoring and Control

  • Real-time Monitoring: Online sensors continuously monitor key water quality parameters, such as pH, conductivity, and ORP.

  • Automated Dosing: Integration of automated chemical dosing systems ensures optimal water quality conditions are maintained.

  • Regular Testing: Periodic laboratory analysis of water samples provides a comprehensive assessment of water quality, including microbial counts and ion analysis.

Best Practices for Chemical Additives

1. Corrosion Inhibitors

  • To form a protective layer on metal surfaces, reducing the risk of corrosion.

  • Polymer additives enhance the effectiveness of corrosion inhibitors by dispersing suspended solids.

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2. Scale Inhibitors

  • Inhibitors are effective in preventing calcium carbonate scaling.

  • Polymer-based additives disperse suspended solids and prevent deposition on heat exchanger surfaces.

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3. Biocides

  • Monochloramine is preferred for controlling biological growth due to its longer residual time and consistent performance.

  • Chlorine and Bromine are effective in controlling biological growth when used with pH control and scaling inhibitors.

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The reuse of treated STP effluent as makeup water for cooling systems is a viable and sustainable solution for reducing freshwater consumption in industrial applications. By implementing advanced treatment processes, applying appropriate chemical additives, and maintaining continuous monitoring, the challenges associated with corrosion, scaling, and biological growth can be effectively managed. This approach not only ensures the efficient operation of cooling systems but also contributes to sustainable water management practices.

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