Primary and Secondary Wastewater Treatment

Why Wastewater Must Be Treated

Wastewater leaving homes, businesses, and industries carries suspended solids, dissolved organic matter, nutrients, and pathogens. If discharged untreated, these contaminants deplete oxygen in receiving waters, harm aquatic life, and threaten public health. Treatment plants use a sequence of physical, biological, and chemical steps to reduce pollutants to permitted levels. This lesson covers the first two major steps: primary treatment (physical removal of settleable solids) and secondary treatment (biological reduction of dissolved and colloidal organic matter).

Primary Treatment: Screening, Grit Removal, and Primary Sedimentation

Wastewater enters the plant through bar screens and fine screens that catch rags, wipes, and large debris. Removed screenings are typically hauled to a landfill. Next, the flow passes through a grit chamber, where velocity is reduced enough for sand and grit to settle without capturing lighter organic solids. Removing grit protects downstream pumps and equipment from abrasion.

The flow then enters the primary clarifier, a large settling basin where retention time—commonly 1.5 to 2.5 hours—allows heavier organic solids (settleable solids) to sink to the bottom as primary sludge, while grease and scum float to the surface and are skimmed off. The clarified liquid flowing over the effluent weir is called primary effluent.

Primary sedimentation typically removes roughly 50–70% of total suspended solids (TSS) and 25–40% of five-day biochemical oxygen demand (BOD₅). These percentages vary with wastewater characteristics and clarifier design. Although significant, these removals are not enough to meet most National Pollutant Discharge Elimination System (NPDES) permit limits for BOD and TSS, which is why biological secondary treatment is required.

Secondary Treatment: Biological Processes

Secondary treatment uses microorganisms—primarily bacteria—to consume dissolved and colloidal organic matter. The bacteria use the organics as food, converting them into carbon dioxide, water, and new cell mass (biomass). Two common secondary processes are the activated-sludge system and the trickling filter.

Activated-Sludge Process

In an activated-sludge system, primary effluent enters an aeration basin where it mixes with a dense, living suspension of microorganisms called mixed liquor. Air blowers or mechanical aerators supply oxygen, which the bacteria need for aerobic metabolism. The concentration of microorganisms in the basin is measured as mixed-liquor suspended solids (MLSS), commonly maintained in the range of 1,500–4,000 mg/L in conventional systems.

From the aeration basin, mixed liquor flows to a secondary clarifier. There, the biomass settles, producing a clear secondary effluent that goes to disinfection and discharge. The settled biological solids are called return activated sludge (RAS). Most of the RAS is pumped back to the aeration basin to maintain the MLSS concentration—this recycled population of microorganisms is what makes the sludge "activated." A portion is removed daily as waste activated sludge (WAS) to control the population size and process performance.

Sludge retention time (SRT)—also called mean cell residence time or sludge age—is the average number of days microorganisms remain in the system. SRT is controlled by how much WAS is wasted. A longer SRT produces a more stable, nitrifying community; a shorter SRT keeps a younger, faster-growing population. Matching SRT to treatment goals is one of the operator's most important process control decisions.

Insufficient dissolved oxygen (DO) in the aeration basin is a common operational problem. When DO drops below about 1–2 mg/L, aerobic bacteria are stressed, BOD removal efficiency falls, and filamentous bacteria may proliferate, causing bulking sludge that settles poorly in the secondary clarifier. Operators respond by increasing aeration, checking blower operation, and verifying DO meter calibration.

Trickling Filters

A trickling filter is a bed of media (rock, plastic, or other material) over which primary effluent is distributed by rotating spray arms. A biofilm of microorganisms grows on the media surface and absorbs and metabolizes organics as wastewater trickles past. Oxygen is supplied by natural ventilation or forced air. Effluent from the filter passes to a secondary clarifier where sloughed biofilm settles out. Trickling filters are simpler to operate than activated sludge but generally achieve somewhat lower and less consistent BOD removal at high organic loads.

Safety Points

• Confined spaces: Primary and secondary clarifiers, wet wells, and digesters are permit-required confined spaces. Follow your facility's confined-space entry program: test the atmosphere (oxygen, flammable gas, toxic gas), use continuous monitoring, and ensure an attendant and rescue plan are in place before entry.
• Slip, trip, and fall hazards: Wet grating around clarifier walkways is extremely slippery. Wear slip-resistant boots and use handrails.
• Biological hazards: Wastewater contains pathogens. Wear appropriate PPE—gloves, safety glasses or a face shield, and waterproof apron—when collecting samples or handling sludge. Wash hands thoroughly before eating or drinking.
• Lockout/tagout (LOTO): Before any maintenance on pumps, blowers, or clarifier drives, follow your facility's LOTO procedure to isolate and de-energize equipment per OSHA 29 CFR 1910.147.
• Chemical handling: Polymers used for sludge conditioning can be slippery and irritating. Consult the Safety Data Sheet (SDS) before handling.