13.3 Wastewater treatment and water quality management

Wastewater Treatment Stages

Wastewater treatment removes pollutants from used water through physical, biological, and chemical processes, producing water clean enough to return to the environment or reuse. Water quality management keeps lakes, rivers, and oceans healthy by setting standards, monitoring conditions, and regulating pollution sources. Together, these systems prevent contamination and maintain water suitable for drinking, recreation, and aquatic life.

Primary Treatment

Primary treatment is all about physical removal. No biology, no chemistry, just separating out the stuff that shouldn't be there based on size and weight.

• Screening removes large debris (rags, sticks, plastic) using bar screens or mesh screens
• Grit removal settles out sand, gravel, and other heavy inorganic materials in grit chambers
• Primary sedimentation allows remaining suspended solids to settle in large tanks called clarifiers, forming primary sludge (also called biosolids)

Primary treatment alone removes about 50–60% of suspended solids and 25–35% of biochemical oxygen demand (BOD). That's a decent start, but most of the dissolved organic pollution is still in the water, which is why secondary treatment is needed.

Secondary Treatment

Secondary treatment tackles the dissolved and suspended organic matter that primary treatment can't catch. It relies on biological processes, specifically aerobic bacteria that consume organic matter and convert it into $CO_2$, water, and more microorganisms.

Common methods include:

• Activated sludge process — Wastewater is mixed with microorganism-rich sludge in aeration tanks where oxygen is pumped in. After the bacteria break down organic matter, the mixture flows to secondary clarifiers where treated water separates from the microbial flocs. Some sludge gets recycled back to the aeration tank to keep the microbial population active.
• Trickling filters — Wastewater trickles over a bed of media (rocks or plastic) coated with a biofilm of microorganisms that absorb and digest organic pollutants.
• Rotating biological contactors — Discs partially submerged in wastewater rotate slowly, exposing biofilm-coated surfaces alternately to wastewater and air.

Secondary treatment removes up to 85–90% of BOD and suspended solids, a major improvement over primary treatment alone.

Tertiary Treatment and Disinfection

Tertiary treatment is the polishing step. It targets contaminants that survive primary and secondary treatment, including nutrients, pathogens, heavy metals, and micropollutants like pharmaceuticals and pesticides.

Tertiary processes include:

• Filtration (sand, membrane)
• Adsorption (activated carbon to capture dissolved chemicals)
• Ion exchange (swapping harmful ions for harmless ones)
• Membrane technologies like reverse osmosis and ultrafiltration

Disinfection then inactivates any remaining pathogenic microorganisms. The three main methods are:

• Chlorination — effective and widely used, though it can produce disinfection byproducts
• UV irradiation — damages microbial DNA without adding chemicals
• Ozonation — a strong oxidizer that kills pathogens and breaks down some organic compounds

The result is high-quality effluent suitable for discharge into sensitive water bodies or even direct reuse.

Wastewater Treatment Processes

Biochemical Oxygen Demand (BOD)

BOD measures the amount of dissolved oxygen that microorganisms need to decompose organic matter in a water sample. It's the go-to indicator for how much organic pollution is present and how well treatment is working.

The standard BOD test works like this:

1. Collect a wastewater sample
2. Incubate it for 5 days at 20°C (this is why you'll sometimes see it written as $BOD_5$)
3. Measure how much dissolved oxygen the microorganisms consumed during that period

High BOD values mean heavy organic pollution. When water with high BOD enters a river or lake, microorganisms decomposing that organic matter consume so much oxygen that dissolved oxygen levels plummet. This can suffocate fish and other aquatic organisms, a condition that can lead to dead zones.

Activated Sludge Process

This is the most widely used secondary treatment method, so it's worth understanding the steps clearly:

1. Wastewater enters an aeration tank where it mixes with activated sludge, which is clumps (flocs) of aerobic microorganisms
2. Air or pure oxygen is pumped in continuously to keep conditions aerobic
3. Microorganisms consume dissolved organic matter, using it for energy and to build new cells, releasing $CO_2$ as a byproduct
4. The mixture flows into a secondary clarifier, where the heavier microbial flocs settle to the bottom
5. Clarified water (effluent) exits from the top
6. A portion of the settled sludge is returned to the aeration tank (return activated sludge) to maintain the microbial population; excess sludge is removed for disposal or further treatment

The result is effluent with low BOD and low suspended solids.

Filtration in Wastewater Treatment

Filtration is a tertiary treatment step that removes remaining suspended solids, fine particulates, and microorganisms before disinfection.

• Granular media filtration passes wastewater through beds of sand, anthracite, or a combination. Particles get trapped in the pore spaces between grains.
• Membrane filtration uses semi-permeable membranes to physically exclude particles by size. Microfiltration membranes have pores small enough to remove bacteria, while ultrafiltration membranes have even smaller pores that can remove viruses.
• Disc filters use rotating filter media to screen out fine solids in a compact footprint.

Filtration significantly reduces turbidity and pathogen levels, which makes the downstream disinfection step more effective.

Water Quality Management

Water Quality Standards and Regulations

Standards set legal limits on pollutant concentrations and other parameters to protect water bodies for their designated uses, whether that's drinking water supply, recreation, or aquatic habitat.

In the U.S., the Clean Water Act gives the Environmental Protection Agency (EPA) authority to establish these standards. They cover physical, chemical, and biological characteristics including temperature, pH, dissolved oxygen, toxins, and pathogens.

Two key regulatory tools to know:

• Effluent standards — Wastewater treatment plants must meet discharge limits tailored to the quality and designated uses of the receiving water body. A plant discharging into a drinking water source faces stricter limits than one discharging into an industrial waterway.
• NPDES permits (National Pollutant Discharge Elimination System) — These regulate point source discharges specifically. Each permit sets effluent limits, monitoring requirements, and reporting schedules for the facility.

Water Quality Monitoring and Assessment

Monitoring involves regular sampling and analysis of water bodies to check compliance with standards and track down pollution sources.

Common monitoring parameters include temperature, pH, dissolved oxygen, nutrient concentrations (nitrogen, phosphorus), bacterial counts, and specific contaminants relevant to the area.

Monitoring happens at multiple scales:

• Treatment plant level — Operators monitor both influent (incoming) and effluent (outgoing) water quality to optimize processes and verify permit compliance
• Watershed scale — Agencies assess the overall health of rivers, lakes, and coastal waters to guide broader management decisions
• Biomonitoring — Uses aquatic organisms like macroinvertebrates and fish as indicators of long-term water quality. A stream with diverse mayfly, stonefly, and caddisfly populations, for example, is generally in good ecological health. A stream dominated by pollution-tolerant worms and midges signals problems.

Water quality data feeds into total maximum daily loads (TMDLs), which calculate the maximum amount of a pollutant a water body can receive and still meet its quality standards. TMDLs then drive pollution control strategies and restoration efforts for impaired waters.