Secondary Treatment Of Wastewater Involves

Secondary Treatment of Wastewater: A Comprehensive Overview

Wastewater treatment is a critical process in modern society, aimed at removing impurities and contaminants from water before it is released back into natural water bodies or reused. The treatment process is generally divided into three main stages: primary, secondary, and tertiary treatment. Each stage serves a specific function in the purification process, addressing different types of pollutants and ensuring that the treated water meets environmental and health standards. Secondary treatment, in particular, plays a pivotal role in breaking down dissolved organic matter and significantly reducing the biochemical oxygen demand (BOD) and suspended solids in wastewater. Within the broader family of wastewater treatment methods, secondary treatment occupies the position where the bulk of the pollutant load is actually destroyed rather than merely separated — primary treatment removes what settles or floats, and tertiary treatment polishes what remains, but the biological work in between is what converts dissolved organics into biomass and gas. This article delves into the intricacies of secondary treatment, discussing its importance, mechanisms, methodologies, and advancements.

The Importance of Secondary Treatment

Secondary treatment is essential because primary treatment alone is insufficient for meeting the stringent requirements for discharge or reuse. While primary treatment removes large solids and particulate matter through processes like screening and sedimentation, it does not adequately address dissolved organic materials, pathogens, and fine suspended solids. These impurities pose significant environmental hazards, including:

1. 1. Oxygen Depletion: Organic materials decompose in water bodies, consuming dissolved oxygen. This can lead to hypoxic or anoxic conditions, adversely affecting aquatic life.

1. 1. Pathogen Spread: Untreated wastewater can carry harmful microorganisms that cause diseases in humans and animals.

1. 1. Eutrophication: Nutrients in wastewater, such as nitrogen and phosphorus, can promote excessive growth of algae in water bodies, disrupting ecosystems.

1. 1. Aesthetic and Odor Issues: Discharged wastewater with high organic content can be unsightly and produce foul odors.

Secondary treatment tackles these issues by employing biological processes to degrade organic matter and reduce impurities, thus protecting ecosystems and human health.

Subcategory Overview: Core Topics in Secondary Treatment

Secondary treatment is a broad subcategory covering process selection, effluent quality expectations, and the practical limits of biological treatment. The topics below are each treated in depth on their own pages and together define what this stage can and cannot accomplish.

Secondary Treatment Fundamentals

The foundational treatment of the subject covers what the stage is designed to achieve and how performance is measured against permit conditions. A conventional plant performing secondary treatment of wastewater is typically expected to achieve 85 percent or greater removal of both BOD and total suspended solids, with typical secondary effluent falling in the approximate range of 10 to 30 mg/L BOD and 10 to 30 mg/L TSS depending on process type and loading. Those numbers are not arbitrary — they derive from federal secondary treatment standards that define the minimum technology-based effluent limit most publicly owned treatment works must meet before any water-quality-based limits are applied. Understanding this baseline matters because every process decision downstream, from disinfection sizing to whether tertiary filtration is required at all, is made relative to the effluent quality secondary treatment reliably delivers.

What Remains in the Effluent

Biological treatment is highly effective against biodegradable dissolved organics and much less effective against everything else. The substances that remain in wastewater after effective secondary treatment typically include residual nitrogen and phosphorus unless the process has been specifically configured for nutrient removal, dissolved inorganic salts that pass through unchanged, trace organic compounds such as pharmaceutical residues and personal care products, and a surviving pathogen population that requires separate disinfection. Residual soluble microbial products contributed by the biomass itself also persist. Knowing this residual profile is what drives the decision on whether a plant needs nitrification, biological or chemical phosphorus removal, filtration, or advanced oxidation beyond the secondary stage.

The Limits of Conventional Treatment

Closely related but distinct is the question of which constituents no conventional process train will address. The materials that cannot be removed from wastewater by biological means alone include dissolved salts and total dissolved solids, many heavy metals in soluble form, certain recalcitrant synthetic organics including per- and polyfluoroalkyl substances, and radionuclides. These require membrane separation, ion exchange, adsorption, or advanced oxidation rather than biology. For plants receiving significant industrial contributions, the practical consequence is that source control and pretreatment at the point of discharge are usually far more cost-effective than attempting removal at the headworks of a municipal plant.

Treatment Technology Advancements

The technology landscape continues to shift as energy costs, nutrient limits, and footprint constraints tighten. Developments in treatment technologies in wastewater have concentrated on reducing the aeration energy that dominates operating cost, on hybrid fixed-film and suspended-growth configurations that increase treatment capacity within existing tankage, and on process control strategies that use real-time instrumentation to trim dissolved oxygen setpoints without risking permit excursions. For an operating utility, most of these advances arrive not as greenfield plants but as retrofits into concrete that was poured decades ago, which makes retrofit compatibility a more important selection criterion than headline efficiency.

Mechanisms of Secondary Treatment

The primary mechanism of secondary treatment is biological degradation. Microorganisms, such as bacteria, protozoa, and fungi, consume organic matter in the wastewater as a food source. The metabolic processes of these organisms convert complex organic molecules into simpler compounds, primarily water, carbon dioxide, and biomass (sludge).

Secondary treatment is the largest single application of biological wastewater treatment, but the same microbial principles govern anaerobic digestion, nutrient removal, and industrial pretreatment as well. The variables an operator actually controls are few: how much oxygen is supplied, how long the biomass is retained in the system, and how much food is available per unit of biomass. Nearly every performance problem in a secondary process traces back to one of those three being out of balance.

Key Processes in Secondary Treatment

Secondary treatment can be broadly categorized into aerobic and anaerobic processes, depending on the presence or absence of oxygen. Both categories encompass a variety of treatment methodologies, each with its design and operational characteristics. The main aerobic processes include:

1. 1. Activated Sludge Process (ASP)

1. 1. Trickling Filters

1. 1. Rotating Biological Contactors (RBCs)

1. 1. Membrane Bioreactors (MBRs)

1. 1. Sequencing Batch Reactors (SBRs)

1. 1. Oxidation Ponds and Lagoons

1. Activated Sludge Process

Overview

The Activated Sludge Process (ASP) is one of the most widely used methods for secondary treatment. It involves the aeration of wastewater in an aeration tank where microorganisms thrive and form flocs (clusters). These flocs consume organic matter, producing carbon dioxide, water, and new microbial cells.

Components

• • Aeration Tank: Where the biological activity takes place. Air or oxygen is supplied to support microbial metabolism.

• • Clarifier: A sedimentation tank where biomass solids settle out. The settled biomass (activated sludge) is partly recycled back to the aeration tank to maintain microbial population.

• • Return Activated Sludge (RAS): Portions of settled microorganisms are returned to the aeration tank to maintain an adequate concentration of biomass.

• • Waste Activated Sludge (WAS): Excess biomass is removed from the system to prevent overpopulation and is subjected to further treatment and disposal.

Operational Parameters

• • Mixed Liquor Suspended Solids (MLSS): Concentration of suspended solids in the aeration tank, including microorganisms.

• • Sludge Retention Time (SRT): Average time that microorganisms remain in the system.

• • Dissolved Oxygen (DO): Level of oxygen in the aeration tank, crucial for aerobic microbial activity.

2. Trickling Filters

Overview

Trickling filters are fixed-bed systems where wastewater is distributed over a bed of media (rock, gravel, or plastic). Microorganisms grow on the media’s surface, forming a biofilm that degrades the organic material as wastewater trickles through it.

Components

• • Distribution System: Sprays or distributes wastewater evenly over the filter media.

• • Filter Media: The surface on which the biofilm forms.

• • Underdrain System: Collects treated effluent and removes excess biomass.

Operational Considerations

• • Hydraulic Loading Rate: Volume of wastewater applied per unit area of media per unit time.

• • Organic Loading Rate: Amount of organic matter applied to the filter per unit area of media per unit time.

• • Recirculation Ratio: Portion of treated effluent recirculated to the input to enhance treatment efficiency.

3. Rotating Biological Contactors (RBCs)

Overview

RBCs consist of a series of closely spaced, rotating disks mounted on a horizontal shaft. The disks are partially submerged in wastewater. As the disks rotate, microorganisms form biofilms on the disk surface and degrade organic material.

Components

• • Disks: Provide surface area for biofilm growth.

• • Shaft and Motor: Facilitate continuous rotation of the disks.

• • Clarifier: Separates treated effluent from the biomass.

Key Parameters

• • Surface Area per Volume: The amount of disk surface area available for biofilm formation per unit volume of wastewater.

• • Rotation Speed: Affects the oxygen transfer and biofilm thickness.

4. Membrane Bioreactors (MBRs)

Overview

MBRs combine biological treatment with membrane filtration. The biological process occurs in a bioreactor, while membranes separate treated water from suspended solids and biomass.

Components

• • Bioreactor: Houses the microbial population.

• • Membrane Module: Filters out suspended solids and microorganisms, producing high-quality effluent.

Pros and Cons

• • Advantages: Produces high-quality effluent, small footprint, effective pathogen removal.

• • Disadvantages: High operational cost, membrane fouling requires periodic cleaning and replacement.

5. Sequencing Batch Reactors (SBRs)

Overview

SBRs perform biological treatment in batch mode rather than continuous flow. Wastewater is treated in cycles that include filling, aeration, settling, and decanting phases.

Components

• • Reactor Tanks: Where treatment phases occur.

• • Aeration System: Supplies oxygen during the aeration phase.

• • Decanter: Removes treated effluent after settling.

Operational Cycle

1. 1. Fill Phase: Wastewater enters the reactor.

1. 1. Aeration Phase: Microbial degradation of organic matter.

1. 1. Settle Phase: Biomass settles to the bottom.

1. 1. Decant Phase: Treated effluent is removed.

6. Oxidation Ponds and Lagoons

Overview

Oxidation ponds and lagoons are simple, cost-effective systems that use natural processes for wastewater treatment. They typically involve a series of shallow ponds or lagoons where microbial activity, sunlight, and oxygen work together to treat wastewater.

Types

• • Facultative Ponds: Include aerobic upper layers and anaerobic bottom layers.

• • Aerated Lagoons: Use mechanical aerators to supply oxygen.

Key Parameters

• • Detention Time: Time wastewater spends in the pond or lagoon.

• • Pond Depth and Area: Affect oxygen transfer and microbial activity.

Anaerobic Processes

Anaerobic processes, while less common in secondary treatment, can be highly effective for specific applications, particularly in treating high-strength industrial wastewater.

1. UASB Reactors

Overview

Upflow Anaerobic Sludge Blanket (UASB) reactors treat wastewater through anaerobic digestion. Wastewater flows upward through a dense bed of granular sludge, where microorganisms degrade organic matter.

Components

• • Reactor: Vertical tank housing the sludge blanket.

• • Gas-Liquid-Solid Separator: Separates biogas, treated effluent, and sludge.

Advantages

• • Low Energy Consumption: No need for aeration.

• • Biogas Production: Generates methane for energy recovery.

Comparison of Secondary Treatment Processes

The processes described above differ substantially in footprint, energy demand, effluent quality, operator attention, and tolerance for load variation. The table below summarises the practical tradeoffs. All values are typical or approximate and vary with wastewater strength, temperature, and design margin.

Selection and Specification Framework

Matching Process to Site Conditions

Process selection is driven far more by site constraints than by theoretical treatment efficiency. Four questions usually settle the decision. How much land is available? Ponds and lagoons need one to two orders of magnitude more area than an MBR for the same flow. What is the effluent target? If the permit requires reuse-grade water or the receiving stream imposes very low limits, membrane processes or activated sludge with tertiary filtration are the realistic candidates. What is the flow profile? Systems with strong seasonal or diurnal swings favour batch processes such as SBRs, which absorb variability by adjusting cycle time rather than by oversizing tankage. And finally, what operator capability exists on site? A small rural system with a part-time operator will not sustainably run an MBR regardless of how well it performs on paper.

Nutrient Limits Change the Answer

Conventional secondary treatment removes carbonaceous BOD but leaves most nitrogen and phosphorus in the effluent. Where the permit imposes nutrient limits, the process must be configured for nitrification and often denitrification, which requires a longer solids retention time, anoxic zones, and internal recycle. Nitrification is temperature-sensitive and slow: the nitrifying organisms grow far more slowly than the heterotrophs that remove BOD, so the SRT required to sustain them can be two to four times that of a BOD-removal-only plant, and considerably longer again in cold weather. Plants facing ammonia limits should evaluate ammonia treatment in wastewater as an integrated design question rather than a bolt-on, because the SRT and tankage decisions it forces cannot be retrofitted cheaply once the basins are built.

Worked Example: Sizing an Aeration Basin

Consider a plant treating 4 MGD with an influent BOD of 220 mg/L, targeting an effluent BOD of 20 mg/L using a conventional activated sludge process.

1. BOD load: 4 MGD × 220 mg/L × 8.34 = approximately 7,340 lb BOD per day.
2. Select an F/M ratio: Conventional activated sludge typically operates at an F/M of about 0.2–0.4 lb BOD per lb MLVSS per day. Take 0.30 for this example.
3. Required MLVSS inventory: 7,340 ÷ 0.30 = approximately 24,470 lb MLVSS.
4. Select MLSS concentration: Assume 3,000 mg/L MLSS with a volatile fraction of about 0.75, giving roughly 2,250 mg/L MLVSS.
5. Basin volume: 24,470 ÷ (2,250 × 8.34) = approximately 1.30 million gallons, or about 0.33 days of hydraulic retention time at design flow.
6. Oxygen demand: At roughly 1.1 lb O₂ per lb BOD removed for carbonaceous demand alone, the process requires on the order of 8,000 lb O₂ per day before any allowance for nitrification.

If the same plant faced an ammonia limit requiring full nitrification, the governing constraint would shift from F/M to SRT. A design SRT of 10 to 15 days at winter temperature would typically drive basin volume well above the figure calculated here, which is why nutrient limits must be established before basin geometry is fixed rather than after.

Lifecycle Cost, Not Capital Cost

Ranking processes by construction cost alone reliably produces the wrong answer over a twenty-year horizon. Aeration commonly accounts for roughly half of a treatment plant’s total electricity consumption, so a process with a lower capital cost but higher specific energy demand can overtake a more expensive alternative within a decade. Sludge production is the second major lifecycle driver: processes operating at long SRT generate less waste biomass per unit of BOD removed, which reduces downstream thickening, dewatering, hauling, and disposal cost. Because those downstream costs land in a different budget line than the treatment process itself, they are frequently omitted from the comparison that drives the selection decision.

Field Notes

Commissioning and Seeding

A new biological process does not perform on day one. Seeding with waste activated sludge from an established plant treating similar wastewater shortens startup from months to weeks. Expect to run at reduced load while the biomass inventory builds, and expect nitrification to establish considerably later than BOD removal — often four to eight weeks behind, and longer if commissioning occurs in cold weather. Fixed-film processes such as trickling filters and RBCs generally take longer still, because the biofilm has to colonise the media surface before design capacity is available.

Troubleshooting by Symptom

• Poor settling with a clear supernatant: Usually filamentous bulking. Check F/M, dissolved oxygen, and nutrient balance; low DO and low F/M both favour filaments.
• Turbid effluent with pin floc: Often over-oxidised sludge from an excessively long SRT. Increase wasting.
• Rising sludge in the clarifier: Denitrification in the sludge blanket producing nitrogen gas. Reduce clarifier detention time or increase return sludge rate.
• Persistent brown foam: Commonly associated with long SRT and certain filamentous organisms. Selective wasting from the surface is more effective than chemical defoamant.
• Loss of nitrification in winter: Nitrifier growth rate falls sharply with temperature. Increase SRT ahead of the cold season rather than reacting after ammonia breakthrough.

Design Details and Standards

Applicable Standards and Regulatory Basis

Secondary treatment performance in the United States is defined by federal secondary treatment regulations, which establish minimum technology-based effluent limits for publicly owned treatment works, generally expressed as 30-day average limits on BOD and total suspended solids together with a minimum percent removal requirement and a pH range. Design practice for the processes themselves is governed primarily by the Ten States Standards (Recommended Standards for Wastewater Facilities) as adopted or modified by individual states, with supporting guidance from WEF Manual of Practice No. 8 on design of water resource recovery facilities. Equipment specifications commonly reference ASTM material standards for media and ANSI/AWWA standards for associated piping and valves.

Key Design Parameters by Process

• Activated sludge: F/M ratio, SRT, MLSS concentration, dissolved oxygen setpoint, return sludge rate, clarifier surface overflow rate and solids loading rate.
• Trickling filters: hydraulic loading rate, organic loading rate, media specific surface area, recirculation ratio, ventilation.
• Rotating biological contactors: hydraulic loading per unit media area, organic loading per unit media area, rotational speed, submergence fraction, staging.
• Membrane bioreactors: membrane flux, transmembrane pressure, MLSS operating range, air scour rate, cleaning interval and chemistry.
• Sequencing batch reactors: cycle time allocation, fill strategy, decant depth and rate, number of basins for continuous influent.
• Ponds and lagoons: detention time, depth, organic loading per unit area, number of cells in series.
• UASB reactors: upflow velocity, organic loading rate, granule retention, gas-liquid-solid separator design, operating temperature.

Specification Checklist

1. Design flows stated explicitly as average, maximum month, maximum day, and peak hour.
2. Influent characterisation covering BOD, COD, TSS, TKN, ammonia, phosphorus, alkalinity, and temperature range including winter minimum.
3. Effluent limits stated with averaging periods and any seasonal variation.
4. Redundancy requirements defined for blowers, pumps, and basins with the largest unit out of service.
5. Turndown capability specified for start-of-life flows, not only design-year flows.
6. Aeration system specified on standard oxygen transfer efficiency with a stated alpha factor and site elevation correction.
7. Clarifier sized on both surface overflow rate and solids loading rate at peak conditions.
8. Waste sludge production estimated and reconciled with downstream thickening and dewatering capacity.
9. Instrumentation defined for DO, MLSS, flow, and ammonia where nutrient limits apply.
10. Odour control and ventilation addressed for covered or enclosed processes.

Frequently Asked Questions

What percentage of pollutants does secondary treatment remove?

A properly operating secondary process typically removes 85 percent or more of both BOD and total suspended solids, and well-run activated sludge plants commonly achieve 90 to 95 percent. Pathogen reduction is substantial but incomplete, which is why disinfection follows. Nitrogen and phosphorus removal is modest unless the process is specifically configured for it — conventional secondary treatment might remove 20 to 40 percent of influent nitrogen incidentally through biomass synthesis, far short of what a nutrient limit would require.

How do you choose between activated sludge and a membrane bioreactor?

Land availability and effluent target usually decide it. MBRs produce markedly better effluent in a much smaller footprint and eliminate the secondary clarifier entirely, but carry higher energy consumption, membrane replacement cost, and a greater demand on operator skill. Conventional activated sludge remains the default where land is available and the permit does not require reuse-grade effluent. The practical test is whether the effluent quality an MBR delivers is actually needed — paying for it where a conventional process would meet permit is the most common form of over-specification in this decision.

Why do trickling filters still get specified?

Energy and simplicity. A trickling filter is gravity-driven and requires a fraction of the electricity that an equivalent activated sludge process consumes, and it tolerates load variation and operator inattention far better. The tradeoff is weaker effluent and susceptibility to ponding, odour, and filter flies. Many plants use them as roughing filters ahead of activated sludge, capturing the energy advantage on the bulk of the load while relying on the downstream process for final quality.

What happens to the effluent after secondary treatment?

Secondary effluent is normally disinfected and then either discharged to a receiving water under permit, sent to tertiary treatment for further polishing, or routed to reuse. The discharge pathway itself carries its own regulatory and design considerations, and the choice among wastewater disposal strategies influences the effluent quality the secondary process must deliver — a discharge to a sensitive receiving water or a reuse application imposes far tighter constraints than an outfall to a large, well-mixed river.

How much sludge does secondary treatment produce?

Waste activated sludge production typically falls in the approximate range of 0.4 to 0.8 lb of solids per lb of BOD removed, varying with SRT — longer SRT means more of the incoming carbon is respired rather than converted to biomass, so less sludge is wasted. At around 0.6 lb per lb, a plant removing 7,000 lb BOD per day would generate roughly 4,200 lb of dry solids daily, which at typical thickened concentrations translates into a meaningful daily volume that downstream handling must be sized to absorb.

Can secondary treatment handle industrial wastewater?

It can, within limits. Biological processes tolerate high-strength organic loads reasonably well provided nutrients and alkalinity are adequate and the load is reasonably steady. They tolerate toxic shock, extreme pH, and slug loads of inhibitory compounds very poorly, and recovery from a toxicity event can take weeks. This is why pretreatment programs at the industrial discharger, rather than added capacity at the municipal plant, are usually the correct response to problematic industrial contributions.

Challenges and Innovations in Secondary Treatment

Challenges

• • Energy Consumption: Aeration in aerobic processes is energy-intensive.

• • Sludge Management: Handling and disposal of excess biomass.

• • Pathogen Removal: Ensuring complete pathogen elimination.

• • Nutrient Removal: Reducing nitrogen and phosphorus beyond secondary treatment’s capabilities.

Innovations

• • Advanced Aeration Systems: Improve oxygen transfer efficiency, reducing energy use.

• • Integrated Membrane Systems: Combine biological treatment with advanced filtration.

• • Microbial Fuel Cells: Harness microorganisms to generate electricity from organic matter.

• • New Bioreactor Designs: Enhance treatment efficiency and reduce footprint.

Conclusion

Secondary treatment is a cornerstone of wastewater management, offering robust solutions for removing dissolved organic matter and reducing environmental impacts. Through various aerobic and anaerobic processes, it addresses critical issues like oxygen depletion, pathogen spread, and nutrient loading. While challenges persist, ongoing innovations promise to make secondary treatment more efficient, sustainable, and capable of meeting future demands. By understanding and advancing these processes, we can continue to protect water resources and promote public health.