10.1 Secondary Process Fundamentals

10.1 Secondary Process Fundamentals

Quick Answer: Secondary treatment uses microorganisms to remove dissolved and colloidal organic matter after preliminary/primary steps. Activated sludge is a suspended-growth process: an aeration tank grows mixed liquor (MLSS/MLVSS) and a secondary clarifier separates solids; RAS returns biomass and WAS wastes excess. Control uses F/M, SRT/MCRT, DO, and settleability. Typical secondary effluent targets roughly ≤30 mg/L BOD and ≤30 mg/L TSS (classic 85% removal framework), with Florida plants often going further toward reuse or AWT.

Primary clarification removes settleable solids and some BOD, but most of the dissolved and fine colloidal organic load still remains. Secondary treatment is the biological engine that converts that biodegradable organic matter into settleable biomass, carbon dioxide, water, and residual solids. On FDEP wastewater Class C (and higher) exams, secondary process items test whether you understand what grows where, which solids inventories matter, and how wasting and recycle set sludge age—not architectural trivia about tank paint colors.

Goal of Secondary Treatment

Secondary treatment aims to:

1. Oxidize biodegradable organics measured as BOD (and related COD fractions).
2. Convert dissolved organics into biomass that can be settled and wasted.
3. Produce a clarified secondary effluent suitable for disinfection and disposal or further tertiary/AWT polishing.
4. Maintain a stable microbial culture under changing flow, load, temperature, and toxicity.

Federal secondary-treatment technology-based expectations historically framed municipal plants around roughly 85% BOD and TSS removal and effluent concentrations on the order of 30 mg/L BOD and 30 mg/L TSS (30-day averages in classic framing). Florida permits may be tighter—especially for surface discharge near sensitive waters, nutrient limits, or reclaimed water under FAC 62-610. For the exam: secondary is the biological organics/solids step; advanced nutrient and pathogen barriers are layered on top.

Suspended Growth vs Fixed Growth

Biological secondary processes fall into two broad families:

Suspended growth keeps microorganisms suspended in the liquid so every cell has access to substrate and oxygen if mixing and aeration are adequate. Fixed-film systems grow a biofilm on rock, plastic media, or rotating disks; wastewater flows past the film. Many modern plants are hybrids (e.g., TF followed by activated sludge polishing). Exam questions often ask which family a unit belongs to and which control knobs apply.

Activated Sludge Overview: Aeration Tank + Secondary Clarifier

The classic activated sludge flowsheet is deceptively simple:

1. Primary effluent (or raw after preliminary treatment at some package plants) enters the aeration tank.
2. Air or pure oxygen supplies dissolved oxygen (DO) while mixers keep mixed liquor suspended.
3. Mixed liquor flows to the secondary clarifier (final clarifier).
4. Settled sludge is split: Return Activated Sludge (RAS) recycles biomass to the aeration tank; Waste Activated Sludge (WAS) removes excess solids from the system.
5. Clarified secondary effluent goes to disinfection, filtration, reuse, or discharge.

Without a working clarifier, you do not have activated sludge—you have a dirty aeration tank. Without RAS, biomass washes out and MLSS collapses. Without WAS, solids accumulate until the clarifier is overloaded and effluent TSS rises.

Mixed Liquor and Solids Terminology

MLSS includes inert and organic suspended solids in the aeration basin. MLVSS is the volatile (mostly organic) portion and is preferred when estimating “bugs.” Typical conventional activated sludge MLSS might run roughly 1,500–3,500 mg/L, while extended aeration package plants often run higher (e.g., 3,000–6,000+ mg/L ranges are common textbook bands—always follow plant design and permit targets). Know the concept and relative ranges, not a single universal number.

Food-to-Microorganism Ratio (F/M)

F/M compares the organic food applied per day to the microorganism mass under aeration:

• Food is typically mass of BOD (or COD) applied per day (lb/day or kg/day).
• Microorganisms are typically mass of MLVSS (or MLSS) in the aeration tank(s).

High F/M means relatively more food and fewer bugs—growth is fast, sludge may be young, and effluent quality can suffer if the culture cannot keep up. Low F/M means older, more endogenous sludge typical of extended aeration—more complete oxidation, lower sludge production rate per BOD removed, but larger tank volume and aeration energy.

Exam framing:

Operators rarely “set F/M” with a single dial; they waste (WAS) and manage MLSS inventory so F/M lands in the target band for the mode they run.

Solids Retention Time (SRT) / Mean Cell Residence Time (MCRT)

SRT (also called MCRT or sludge age) is the average time a solid particle remains in the activated sludge system. Conceptually:

SRT ≈ (total mass of solids in system) / (mass of solids leaving system per day)

Solids leave primarily in WAS and secondarily in effluent TSS. If you waste too little, SRT rises, sludge ages, endogenous decay increases, and you may see nitrification (if DO, alkalinity, and temperature allow) plus potential settling or foaming changes. If you waste too much, SRT falls, MLSS drops, treatment capacity falls, and pin floc or high effluent BOD can appear.

Key control relationship for exams:

• WAS rate is the main long-term control of SRT and MLSS inventory.
• RAS rate mainly manages clarifier solids and return of biomass—not a substitute for proper wasting.

Florida package extended aeration plants intentionally run long SRT (often on the order of 20–30+ days in textbook descriptions) for process simplicity and sludge stability. Conventional plants may run shorter SRT (often roughly 5–15 days depending on design and whether nitrification is required). Use plant-specific targets on the job; use relative long-vs-short reasoning on the exam.

RAS and WAS Roles

Return Activated Sludge (RAS)

RAS recycles settled biomass from the secondary clarifier underflow to the aeration tank inlet (or designated return point). Purposes:

• Maintain desired MLSS inventory under aeration.
• Control sludge blanket depth in the clarifier.
• Provide continuous seed so the process does not wash out during hydraulic peaks.

RAS is often expressed as a percent of influent flow (e.g., 50–100% for many conventional systems; package plants vary). Too little RAS can raise the blanket and risk solids washout; too much RAS can dilute the aeration tank unnecessarily, increase clarifier hydraulic load from recycle, and mask wasting problems.

Waste Activated Sludge (WAS)

WAS removes solids from the system so net biomass growth does not accumulate forever. Wasting may be from the RAS line or from a separate WAS pump, depending on plant design. Consistent, measured wasting based on SRT or MLSS targets beats random “pull some sludge when it looks thick.”

Typical Effluent Quality After Secondary

After well-operated secondary treatment (before or after disinfection, depending on the sampling point):

Secondary effluent is not finished water for unrestricted reuse without additional barriers. Florida reuse and AWT standards (see later chapters) build on secondary biology with filtration, higher disinfection, and nutrient limits.

How Secondary Fits the Full Plant Train

Think of the train as a story:

1. Preliminary — protect equipment (screens, grit).
2. Primary — settle what gravity can easily remove.
3. Secondary — biologically remove dissolved organics and convert to settleable biomass.
4. Tertiary / AWT / reuse — nutrients, filtration, advanced disinfection as required.
5. Solids handling — thicken, digest, dewater WAS and primary sludge.

If secondary fails, disinfection and disposal cannot fully compensate—high TSS and BOD will dominate effluent violations, chlorine demand will spike, and filters (if any) will blind.

Exam Anchors for Section 10.1

Memorize the goal of secondary (biological organics removal), suspended vs fixed growth, the aeration + clarifier pair, definitions of MLSS/MLVSS, F/M, SRT/MCRT, and the distinct roles of RAS vs WAS. Know that classic secondary effluent is framed around roughly 30/30 BOD/TSS and ~85% removal, while Florida permits and reuse often demand more. That cluster covers most “fundamentals of secondary” items before process-control and troubleshooting details.