Secondary treatment of wastewater gets rid of over 90% of suspended solids that primary treatment misses. This phase protects our environment and water resources effectively.
The biological processes come into play after primary treatment to clean the water further. These processes target biodegradable organic pollutants and lower biological oxygen demand (BOD). The treatment also handles high nitrogen and phosphorus levels that can harm natural water bodies through eutrophication. Aquatic ecosystems suffer damage when excessive nutrients cause algae to grow out of control without proper sewage treatment.
Let’s get into the biological methods used in secondary treatment – activated sludge, trickling filters, and rotating biological contactors. These processes work together to reduce wastewater’s effect on our natural water systems by a lot before discharge. Engineers and environmental science students need to understand these biological processes to design wastewater management systems that work.
Why Secondary Wastewater Treatment Is Critical
Secondary wastewater treatment connects raw sewage to environmentally safe water. The biological processes used in this stage turn harmful wastewater into a resource that can be safely discharged or reused. This makes it essential to protect our environment and manage water resources.
Environmental impact of untreated sewage
Untreated or poorly treated sewage creates pollution hotspots worldwide that harm biodiversity and ecosystem health. These contaminated areas overlap with important natural habitats like coral reefs, salt marshes, and fish-rich river systems. The effects are severe and widespread:
- Nutrient overload leads to eutrophication, where too much algae growth uses up oxygen and creates deadly “dead zones” for aquatic life
- High levels of pathogens, endocrine disruptors, heavy metals, and pharmaceuticals harm natural ecosystems
- Farmers use untreated wastewater to irrigate about 30 million hectares globally—50% more than earlier estimates
This agricultural use of contaminated water puts 885 million urban consumers, farmers, and food vendors at serious health risks. Rivers in developing countries show severe pathogenic pollution in up to one-third of their length, which directly affects health. Secondary treatment of wastewater proves essential as it removes over 85% of the biochemical oxygen demand (BOD) and suspended solids from domestic sewage.
Regulatory standards for effluent discharge
Regulatory frameworks worldwide set minimum performance requirements for secondary wastewater treatment facilities.
- Monthly average should be less than 30 mg/L BOD in effluent
- Monthly average should be less than 30 mg/L suspended solids
- Facilities should remove at least 85% of BOD and suspended solids from domestic sewage
Effluent guidelines enforce these standards through national regulatory requirements for wastewater discharged to surface waters and municipal sewage treatment plants. Different countries adapt these standards based on their local environmental conditions and water challenges.
Role in water reuse and sustainability
Water shortage has become a pressing global issue, especially in arid regions. Secondary sewage treatment helps address this challenge through the circular water economy concept. This approach maximizes water reuse and minimizes waste.
Secondary wastewater treatment helps achieve 11 out of 17 Sustainable Development Goals (SDGs). The benefits include:
- More water for agriculture (SDGs 2 and 6)
- Better human health worldwide (SDG 3)
- New income sources for smallholders (SDGs 1 and 8)
- Waste-to-energy conversion through biogas recovery (SDGs 7 and 9)
- Lower environmental effects (SDGs 11, 12, 13, and 14)
Many regions now use treated wastewater for non-potable purposes instead of releasing it into natural bodies. These uses include industrial cooling, landscape irrigation, farming, and toilet flushing. Countries like Singapore, the Netherlands, and Israel have made secondary sewage treatment part of their national water strategies. Singapore produces reclaimed water (NEWater) through advanced secondary and tertiary treatment for both industrial and drinking purposes.
Secondary wastewater treatment protects our environment and turns waste into valuable resources. This makes it the life-blood of sustainable water management.
Aerobic Biological Processes in Secondary Treatment
Secondary wastewater treatment relies heavily on aerobic biological processes. These processes need oxygen-dependent microorganisms that break down organic pollutants. Natural biological activity works in controlled environments to create cleaner effluent with much lower contaminant levels.
Activated Sludge Process and Clarification
Activated sludge stands out as one of the most accessible secondary treatment methods around the world. This oxygen-dependent biological process turns soluble organic matter into solid biomass that settles or filters out. The process happens in two main units:
- Aeration basin: Microorganisms contact wastewater in oxygen-rich conditions
- Sedimentation basin: Biological flocs settle, separating sludge from treated water
The microbes in activated sludge each have their own job—Dechloromonas helps with denitrification, Nitrospira takes care of nitrification, and Candidatus Accumulibacter deals with phosphorus. Most systems need 15–48 hours to work, though newer ones usually take longer.
These systems work differently based on their design and operation. They usually remove 60–85% of COD, 85–98% of BOD, 40–65% of AOX, and 20–50% of nitrogen. While they clean water well, they can be unstable and sensitive to disruptions.
Trickling Filters and Biofilm Formation
Trickling filters work differently from activated sludge. They use fixed surfaces where microorganisms grow and create biofilms. When wastewater flows over these surfaces, the attached microbial communities break down organic pollutants.
Filter media comes in many forms—rock, gravel, plastic, and polyurethane foam—giving microbes plenty of space to attach. Biofilms can grow several millimeters thick and house many different microbes, including bacteria, protozoa, annelids, and insect larvae.
Biofilms contain both aerobic and anaerobic zones, which lets various biological processes happen at once. These tiny ecosystems remove pollutants through biosorption, bioaccumulation, and biomineralization. The microbes in biofilms handle toxic compounds better, making them great for cleaning industrial wastewater.
Membrane Bioreactors (MBR) for High-Quality Effluent
MBRs blend activated sludge treatment with membrane filtration to create an advanced way to clean water. They use hollow fiber bundles or plate membranes to separate microorganisms from cleaned water.
MBRs shine because they produce cleaner water, need less space, and run more automatically. They handle more waste and work faster than traditional systems. The water they produce has very few bacteria, suspended solids, BOD, and nutrients.
These systems work great but face some challenges. Membrane fouling can be a problem, and they cost more to build and run. They also need more energy because air must constantly clean the membranes.
Sequencing Batch Reactors (SBR) in Batch Operations
SBRs do everything in one tank through timed operations, unlike systems that flow continuously. The process goes through five steps: fill, react, settle, draw, and idle.
These reactors adapt easily to different needs through internal balancing and controlled biological reactions. They remove nutrients well by handling nitrification-denitrification in a single tank during different treatment phases.
Research shows SBRs often work better than other methods. They remove 98.1% of nitrate compared to 89.7% in regular activated sludge, and clean up 84.1% of total nitrogen versus 79.7% in activated sludge plants.
Aerated Lagoons for Municipal Wastewater
Aerated lagoons offer a simpler way to clean wastewater. They’re just ponds with added air to help biological oxidation. These systems clean 80–90% of BOD in 1–10 days.
Surface aerators in these lagoons do two jobs: they add oxygen for biological reactions and mix everything together. Low-speed surface aerators provide 2–2.5 kg O₂/kWh, making them an affordable choice for bigger treatment facilities.
The lagoons work well between 4°C and 32°C, and biological reactions speed up as temperatures rise within this range. Their simple design and easy maintenance make them perfect for rural areas and small towns.
Anaerobic Treatment Systems and Their Applications
Anaerobic treatment systems make use of oxygen-free biological processes to clean high-strength wastewater and generate valuable biogas as a byproduct. These technologies work alongside traditional aerobic methods in secondary treatment. They use less energy and produce less sludge than conventional systems.
Upflow Anaerobic Sludge Blanket (UASB) Reactors
UASB reactors, which Dutch scientists developed in the 1970s, now have over 1,000 installations worldwide, mostly in tropical countries. These systems’ most important feature is their granular sludge bed that keeps highly active biomass with excellent settling properties. The wastewater flows upward from the bottom through a sludge blanket, which creates good mixing between microorganisms and pollutants.
UASB reactors work best with height-to-diameter ratios of 0.2-0.5 and upflow velocities of 0.5-1.0 m/h. They maintain high microbial concentrations without mechanical mixing, which makes them effective. The systems can remove 50-70% of COD from high-strength industrial wastewaters. They need only 10-30% of reactor volume for the original inoculation.
Anaerobic Lagoons for High-Strength Wastewater
Anaerobic lagoons are man-made earthen basins that pretreat high-strength industrial and agricultural wastewater. These systems work at depths of 8-15 feet without aeration or mixing. They depend on anaerobic digestion processes.
Solids in anaerobic lagoons separate into distinct layers – floating materials rise to the top while sludge settles at the bottom. Manure slurry breaks down through anaerobic respiration and converts volatile organic compounds into carbon dioxide and methane. The systems need 20-150 days of hydraulic retention time and work best above 15°C.
Anaerobic Filters and Biogas Recovery
Anaerobic filters use support materials where microbial populations grow as biofilms. The filter media – crushed rock, plastic, ceramic, or similar materials – provides extensive surface area for biomass to attach. The average specific surface areas reach 100 m²/m³. These reactors can operate in upflow or downflow modes based on wastewater characteristics.
Anaerobic digestion creates biogas that contains mostly methane and carbon dioxide. People can use this renewable energy source to generate electricity, provide heating, or fuel vehicles. The recovered biogas contains 75-85% methane and produces 400-500 liters per kilogram of COD destroyed.
Operational constraints: pH, temperature, and loading
pH levels greatly affect how well anaerobic processes work. The best range falls between 6.8-7.2. Changes outside this range affect microbial activity, especially methanogens, which react strongly to environmental changes.
Temperature plays a crucial role in treatment effectiveness. Most installations run under mesophilic conditions (30-40°C). Recent studies show significant methane production even at temperatures as low as 2.5°C, though efficiency drops.
Systems must control organic loading rates (OLR) carefully based on system type and wastewater characteristics. Anaerobic lagoons can handle organic loads of 54-3,000 pounds BOD₅ per acre daily. UASB reactors can process higher loading rates of 4.5-12.5 kg COD/m³/day and still maintain removal efficiencies above 80%.
Hybrid and Advanced Secondary Treatment Technologies
Recent advances in secondary treatment technologies blend conventional system principles to create more efficient and compact solutions for modern wastewater challenges.
Moving Bed Biofilm Reactor (MBBR) Design
MBBR technology uses plastic carriers that provide surface area for biofilm growth and maximize treatment capacity in smaller footprints. These free-floating carriers move throughout the aeration tank and enable contact between waste and biofilm. The design has aeration grids at the tank bottom that keep carriers moving, along with sieves to prevent their escape. MBBR systems can treat the same water volume as much larger conventional tanks effectively, with hydraulic retention times of just 3-4 hours.
BETT System: Microbial Fuel Cell Integration
BioElectrochemical Treatment Technology (BETT) brings a new approach to secondary treatment that:
- Reduces carbon emissions by up to 90%
- Consumes only 0.1-0.2 kWh per kg of BOD treated
- Produces direct electricity during treatment
- Operates with no methane production
BETT works as a hybrid of anaerobic and aerobic degradation combined with electrochemical reactions. The system converts organic pollutants to dissolved carbon dioxide, direct electricity, and water. While not replacing conventional technologies, BETT serves as effective pre-treatment for high-strength wastewaters.
Aerobic Granular Sludge for Compact Footprint
Aerobic Granular Sludge (AGS) technology brings remarkable benefits through spontaneous microorganism aggregation under aerobic conditions. AGS plants need 25-75% less space and use 20-50% less energy than conventional activated sludge systems. The granules form dense structures naturally resistant to sludge bulking, with settling rates 2-3 times higher than conventional systems.
Combining Aerobic and Anaerobic Units
Hybrid systems blend anaerobic pre-treatment with aerobic polishing to create better results. This setup typically uses anaerobic units like UASB reactors to reduce bulk organic load, followed by aerobic systems to achieve discharge standards. These combinations improve micropollutant removal while minimizing energy consumption.
Design Considerations and Operational Challenges
Engineers must balance performance with operational realities to successfully implement secondary treatment systems.
Energy consumption in aeration systems
Aeration processes consume the most energy in wastewater treatment. These systems account for 45-75% of total energy costs. Fine bubble aeration systems transfer oxygen efficiently and deliver up to 12 pounds of oxygen per horsepower-hour. Studies show that facilities can reduce energy consumption by 30% when they use advanced aeration methods compared to standard practices. Plants can improve effluent quality and reduce power usage by controlling dissolved oxygen levels effectively.
Sludge handling and disposal strategies
The main goals of sludge management are volume reduction and organic material stabilization. Dewatered sludge acts like a solid material despite containing 70% water. Plants can dispose of sludge through land application, landfilling, or incineration. The incineration process reduces volume by 80-90%. Anaerobic digestion creates biogas with 75-85% methane content and produces 400-500 liters per kilogram of COD destroyed.
Monitoring BOD, COD, and nutrient removal
Live BOD/COD monitoring has taken the place of traditional time-consuming testing methods. Modern multiparameter water quality sondes measure several parameters at once. This allows operators to adjust processes right away. The continuous data helps maintain compliance with discharge limits of 30 mg/L for BOD/TSS.
Space and infrastructure requirements
The choice of treatment technology affects space requirements significantly. Rural locations work well for sludge drying beds because they need large land areas. Advanced technologies use sophisticated control systems like PLC, SCADA, and HMI interfaces. These upgrades help facilities improve without expanding their physical footprint.
Conclusion
Secondary wastewater treatment is the life-blood of modern environmental engineering. It removes over 90% of remaining suspended solids and reduces biological oxygen demand substantially. This piece explores the biological processes that are the foundations of this vital treatment phase. Aerobic systems like activated sludge, trickling filters, and membrane bioreactors provide unique advantages based on treatment needs and facility limitations.
Anaerobic processes are great alternatives for treating high-strength wastewater. They generate valuable biogas as a resource too. UASB reactors, anaerobic lagoons, and anaerobic filters show how facilities can tackle pollution and recover energy simultaneously. These technologies work together to create powerful hybrid systems that optimize treatment efficiency and minimize resource use.
New technologies have altered the map of wastewater treatment in recent decades. Moving bed biofilm reactors, BETT systems, and aerobic granular sludge processes help engineers design compact, energy-efficient facilities without compromising performance. These breakthroughs address growing global issues related to water shortage, energy costs, and space constraints.
Successful implementation depends on several operational factors. Engineers need to focus on energy use during aeration, sludge management strategies, monitoring systems, and infrastructure requirements. Secondary treatment facility design needs a complete approach that balances performance goals with real-world operations.
Secondary treatment’s environmental impact is remarkable. This key process shields ecosystems from eutrophication, stops pathogen spread, and enables water reuse worldwide. It contributes to multiple Sustainable Development Goals, making it more than just pollution control.
Without doubt, secondary wastewater treatment will keep evolving as technology advances and regulations get stricter. Digital monitoring, energy efficiency improvements, and resource recovery techniques are becoming more integrated. These changes promise better solutions for future wastewater management challenges. Engineers who become skilled at these biological processes can design systems that protect our environment and turn waste into valuable resources for a greener world.
Key Takeaways
Secondary wastewater treatment is essential for environmental protection and sustainable water management, using biological processes to remove over 90% of remaining pollutants while creating opportunities for resource recovery.
• Secondary treatment removes 85-90% of BOD and suspended solids from wastewater, preventing eutrophication and protecting aquatic ecosystems from harmful nutrient overload.
• Aerobic processes like activated sludge and MBRs offer high treatment efficiency but consume 45-75% of facility energy, while anaerobic systems reduce energy costs and generate valuable biogas.
• Hybrid technologies like MBBR and aerobic granular sludge require 25-75% less space and 20-50% less energy than conventional systems, making them ideal for modern facilities.
• Real-time monitoring of BOD, COD, and nutrients has replaced lengthy traditional testing, enabling immediate process adjustments and better compliance with discharge standards.
• Secondary treatment supports 11 of 17 UN Sustainable Development Goals by enabling water reuse, energy recovery, and pollution prevention—transforming waste into valuable resources for a circular economy.
The future of wastewater treatment lies in integrating energy-efficient biological processes with advanced monitoring systems, creating facilities that not only protect our environment but also contribute to sustainable resource management and energy independence.
Frequently Asked Questions
Q1. What is the main purpose of secondary wastewater treatment?
Secondary wastewater treatment aims to remove over 90% of remaining suspended solids and significantly reduce biological oxygen demand in wastewater. It uses biological processes to further purify the effluent after primary treatment, targeting biodegradable organic pollutants and excess nutrients like nitrogen and phosphorus.
Q2. How does the activated sludge process work in secondary treatment?
The activated sludge process uses oxygen-dependent microorganisms to break down organic pollutants in wastewater. It occurs in two main units: an aeration basin where microorganisms contact wastewater in oxygen-rich conditions, and a sedimentation basin where biological flocs settle, separating sludge from treated water.
Q3. What are the advantages of using membrane bioreactors (MBRs) in wastewater treatment?
Membrane bioreactors combine activated sludge treatment with membrane filtration, offering better effluent quality, smaller space requirements, and easier automation. They operate at higher volumetric loading rates and lower hydraulic retention times compared to conventional systems, producing exceptionally clean effluent with low concentrations of bacteria, suspended solids, and nutrients.
Q4. How do anaerobic treatment systems differ from aerobic processes?
Anaerobic treatment systems use oxygen-free biological processes to treat high-strength wastewater while generating biogas as a byproduct. They offer superior energy efficiency and lower sludge production compared to aerobic methods. Examples include Upflow Anaerobic Sludge Blanket (UASB) reactors and anaerobic lagoons.
Q5. What are some emerging technologies in secondary wastewater treatment?
Emerging technologies include Moving Bed Biofilm Reactors (MBBR), which use plastic carriers for biofilm growth; BioElectrochemical Treatment Technology (BETT), which integrates microbial fuel cells; and Aerobic Granular Sludge (AGS) systems, which offer compact footprints and energy savings. These innovations combine principles of conventional systems to create more efficient, compact solutions for modern wastewater challenges.
