🚰Advanced Wastewater Treatment Unit 7 – Tertiary Treatment Design Principles

Key Concepts and Objectives

• Understand the purpose and goals of tertiary treatment in wastewater treatment plants to achieve higher effluent quality standards
• Identify the key pollutants targeted by tertiary treatment processes such as nutrients (nitrogen and phosphorus), suspended solids, and pathogens
• Recognize the importance of tertiary treatment in protecting receiving water bodies and meeting stringent discharge regulations
• Differentiate between the various types of tertiary treatment processes based on their removal mechanisms and target pollutants
  • Physical processes (filtration, membrane separation)
  • Chemical processes (chemical precipitation, advanced oxidation)
  • Biological processes (nutrient removal, constructed wetlands)
• Comprehend the design principles and criteria for selecting and sizing tertiary treatment units based on influent characteristics and desired effluent quality
• Evaluate the performance of tertiary treatment processes using key indicators and monitoring techniques to ensure consistent and reliable operation

Types of Tertiary Treatment Processes

• Filtration processes remove suspended solids and particulate matter from secondary effluent using granular media (sand, anthracite) or cloth filters
  • Rapid sand filtration utilizes high filtration rates and frequent backwashing cycles
  • Slow sand filtration employs lower filtration rates and biological activity for additional treatment
• Membrane processes use semi-permeable membranes to separate pollutants based on size exclusion and pressure-driven flow
  • Microfiltration (MF) removes larger particles and bacteria
  • Ultrafiltration (UF) removes viruses and colloids
  • Nanofiltration (NF) and reverse osmosis (RO) remove dissolved solids and ions
• Chemical precipitation removes dissolved phosphorus by adding metal salts (alum, ferric chloride) to form insoluble precipitates
• Advanced oxidation processes (AOPs) degrade recalcitrant organic compounds using highly reactive hydroxyl radicals generated by combining oxidants (ozone, hydrogen peroxide) with catalysts or UV light
• Biological nutrient removal (BNR) processes remove nitrogen and phosphorus through sequential nitrification-denitrification and enhanced biological phosphorus removal (EBPR)
  • Nitrification converts ammonia to nitrate under aerobic conditions using nitrifying bacteria
  • Denitrification reduces nitrate to nitrogen gas under anoxic conditions using denitrifying bacteria and organic carbon source
• Constructed wetlands provide additional treatment and polishing of secondary effluent through natural processes involving vegetation, soil, and microbial communities

Removal Mechanisms and Principles

• Physical removal mechanisms in tertiary treatment rely on size exclusion, straining, and adsorption to remove suspended solids and particulate matter
  • Filtration processes capture particles larger than the pore size of the filter media or membrane
  • Adsorption involves the attachment of pollutants to the surface of filter media or adsorbents (activated carbon)
• Chemical removal mechanisms involve the transformation or destruction of pollutants through chemical reactions and oxidation
  • Chemical precipitation converts dissolved phosphorus into insoluble precipitates using metal salts (alum, ferric chloride) that can be settled or filtered out
  • Advanced oxidation processes generate highly reactive hydroxyl radicals ($\cdot$OH) that oxidize and mineralize recalcitrant organic compounds
• Biological removal mechanisms harness the metabolic activities of microorganisms to convert and remove nutrients (nitrogen and phosphorus) from wastewater
  • Nitrification is an aerobic process where ammonia-oxidizing bacteria (AOB) convert ammonia ($\text{NH}_4^+$) to nitrite ($\text{NO}_2^-$), followed by nitrite-oxidizing bacteria (NOB) converting nitrite to nitrate ($\text{NO}_3^-$)
  • Denitrification is an anoxic process where denitrifying bacteria reduce nitrate ($\text{NO}_3^-$) to nitrogen gas ($\text{N}_2$) using organic carbon as an electron donor
  • Enhanced biological phosphorus removal (EBPR) alternates between anaerobic and aerobic conditions to promote the growth of phosphate-accumulating organisms (PAOs) that store excess phosphorus in their cells

Design Parameters and Calculations

• Filtration process design considers key parameters such as filter media characteristics (size, uniformity coefficient), filtration rate, backwash frequency and duration, and head loss
  • Filter media size and uniformity affect particle removal efficiency and head loss development
  • Filtration rate ($\text{m}^3/\text{m}^2\cdot\text{h}$) determines the hydraulic loading and required filter surface area
  • Backwash frequency and duration are optimized to maintain filter performance and prevent clogging
• Membrane process design involves selecting appropriate membrane material, pore size, and module configuration based on target pollutants and required permeate quality
  • Membrane flux ($\text{L}/\text{m}^2\cdot\text{h}$) and transmembrane pressure (TMP) are key operating parameters affecting permeate production and fouling propensity
  • Membrane fouling control strategies include pretreatment, chemical cleaning, and backwashing
• Chemical precipitation design requires determining optimal chemical dosage, mixing conditions, and solids separation method
  • Jar tests are conducted to determine the optimal coagulant dose and pH for effective phosphorus removal
  • Rapid mixing ensures proper dispersion of chemicals, while flocculation promotes particle aggregation
  • Clarification or filtration is used to separate the precipitated solids from the treated effluent
• Biological nutrient removal (BNR) process design involves sizing the aerobic, anoxic, and anaerobic zones based on influent characteristics and target effluent quality
  • Solids retention time (SRT) and hydraulic retention time (HRT) are critical parameters affecting the growth and performance of nitrifying and denitrifying bacteria
  • Internal recycle flows (nitrate recycle, mixed liquor recycle) are designed to optimize nutrient removal efficiency
  • Carbon-to-nitrogen (C/N) ratio is balanced to ensure sufficient organic carbon for denitrification

Equipment and Technology

• Filtration systems employ various types of granular media filters (rapid sand filters, dual-media filters) or cloth media filters (disk filters, drum filters) for solid-liquid separation
  • Granular media filters consist of layers of sand, anthracite, or other materials with specific size gradation and depth
  • Cloth media filters use woven or non-woven fabric with small pore sizes to capture suspended solids
• Membrane systems utilize different membrane modules and configurations for efficient and compact treatment
  • Hollow fiber modules consist of bundles of thin, tubular membranes with high packing density
  • Spiral-wound modules arrange flat sheet membranes and spacers around a central permeate collection tube
  • Plate-and-frame modules stack flat sheet membranes with support plates for mechanical stability
• Chemical feed systems include storage tanks, metering pumps, and injection points for accurate dosing of coagulants, polymers, and other chemicals
  • Gravimetric or volumetric feeders ensure precise chemical dosing based on flow rate or demand
  • Static mixers or rapid mix basins provide efficient mixing of chemicals with the wastewater
• Advanced oxidation processes employ specialized reactors and equipment for generating and applying oxidants and catalysts
  • Ozone generators produce ozone gas from air or oxygen using corona discharge or electrolysis
  • UV reactors use high-intensity lamps (low-pressure or medium-pressure) to irradiate the wastewater and activate photocatalysts
• Biological nutrient removal systems incorporate aeration equipment, mixers, and pumps for maintaining the required environmental conditions in each zone
  • Fine bubble diffusers or membrane aerators provide efficient oxygen transfer in the aerobic zone
  • Submersible mixers or jet mixers ensure complete mixing and suspension of biomass in the anoxic and anaerobic zones
  • Internal recycle pumps transfer nitrate-rich mixed liquor from the aerobic to the anoxic zone for denitrification

Performance Evaluation and Monitoring

• Effluent quality parameters are regularly monitored to assess the performance of tertiary treatment processes and compliance with discharge standards
  • Total suspended solids (TSS) and turbidity indicate the effectiveness of solids removal by filtration or membrane processes
  • Nutrient concentrations (total nitrogen, total phosphorus) are measured to evaluate the efficiency of biological nutrient removal or chemical precipitation
  • Pathogen indicators (E. coli, fecal coliforms) are monitored to ensure adequate disinfection and public health protection
• Online instrumentation and sensors provide real-time monitoring and control of critical process parameters
  • Turbidity meters and particle counters detect breakthrough or deterioration of filter performance
  • Dissolved oxygen (DO) and oxidation-reduction potential (ORP) sensors optimize aeration and maintain the desired conditions in biological treatment zones
  • pH and conductivity meters monitor chemical dosing and membrane system performance
• Membrane integrity testing is performed regularly to detect and locate any breaches or defects in the membrane modules
  • Pressure decay tests monitor the rate of pressure loss in a closed membrane system to indicate integrity issues
  • Bubble point tests use air or gas to determine the maximum pore size and identify any leaks or defects
• Bioassays and toxicity tests assess the potential ecological impacts of the treated effluent on aquatic organisms
  • Whole effluent toxicity (WET) tests expose sensitive species to the effluent and measure acute or chronic responses
  • Biomonitoring programs evaluate the long-term effects of the discharged effluent on the receiving water body's ecosystem

Case Studies and Real-World Applications

• The Chesapeake Bay nutrient removal program implemented tertiary treatment at wastewater treatment plants to reduce nitrogen and phosphorus loads and combat eutrophication
  • Enhanced nutrient removal (ENR) processes, such as 4-stage Bardenpho and modified Ludzack-Ettinger (MLE), were adopted to achieve stringent effluent limits
  • Biological aerated filters (BAFs) and denitrifying filters provided additional polishing and ensured consistent performance
• The Orange County Water District's Groundwater Replenishment System (GWRS) in California utilizes advanced treatment technologies to produce high-quality water for aquifer recharge and indirect potable reuse
  • Microfiltration, reverse osmosis, and advanced oxidation processes (UV/H2O2) are used in series to remove contaminants and ensure public health protection
  • The GWRS has become a model for sustainable water management and drought resilience in water-stressed regions
• The Ulu Pandan Water Reclamation Plant in Singapore employs membrane bioreactor (MBR) technology for tertiary treatment and production of high-grade reclaimed water (NEWater)
  • MBR combines activated sludge treatment with membrane filtration in a compact footprint, achieving excellent effluent quality suitable for industrial and indirect potable use
  • NEWater is a key component of Singapore's diversified water supply portfolio and contributes to the country's water self-sufficiency goals
• The Stickney Water Reclamation Plant in Chicago, Illinois, one of the largest wastewater treatment facilities in the world, implemented a phosphorus recovery system to address nutrient pollution in the Mississippi River Basin
  • The Ostara Pearl® process precipitates struvite (magnesium ammonium phosphate) from the digester centrate, reducing the phosphorus load in the final effluent
  • The recovered struvite is processed into a slow-release fertilizer, creating a valuable byproduct and promoting a circular economy approach

Emerging Trends and Future Directions

• Intensification of tertiary treatment processes aims to reduce the footprint, energy consumption, and operational costs while maintaining or improving effluent quality
  • Hybrid processes combining different treatment mechanisms (e.g., membrane aerated biofilm reactors, electrochemical membrane bioreactors) offer synergistic benefits and enhanced performance
  • Modular and decentralized treatment systems enable flexible and scalable implementation, especially in urban or remote settings
• Resource recovery and circular economy principles are driving the development of innovative technologies for nutrient and energy recovery from tertiary treatment processes
  • Phosphorus recovery through struvite precipitation or adsorption generates a valuable fertilizer product and reduces the dependence on finite mineral resources
  • Bioelectrochemical systems (microbial fuel cells, microbial electrolysis cells) convert organic matter into electricity or hydrogen while simultaneously treating wastewater
• Nature-based solutions and green infrastructure are gaining attention as sustainable and cost-effective alternatives or complements to conventional tertiary treatment
  • Constructed wetlands, algal ponds, and treatment marshes harness natural processes for nutrient removal, pathogen reduction, and habitat creation
  • Integration of nature-based solutions with urban landscapes provides additional benefits such as stormwater management, biodiversity enhancement, and recreational opportunities
• Digital technologies and advanced analytics are transforming the operation, monitoring, and optimization of tertiary treatment processes
  • Sensors, internet of things (IoT) devices, and real-time data acquisition enable remote monitoring and predictive maintenance of treatment equipment
  • Machine learning algorithms and artificial intelligence (AI) assist in process control, anomaly detection, and decision support for operators and managers
• Water reuse and fit-for-purpose treatment are becoming increasingly important in the face of water scarcity, population growth, and climate change
  • Tailoring tertiary treatment processes to meet specific water quality requirements for different end uses (irrigation, industrial, potable) maximizes the value and efficiency of water resources
  • Decentralized wastewater treatment and reuse systems close the water loop and minimize the energy and infrastructure costs associated with long-distance water conveyance