Water quality management requires a variety of strategies. Some water quality improvement strategies involve close coordination with treatments to improve air quality (such as sulfur emissions scrubbers for coal-fired power plants) or soil quality (such as monitoring of landfill liners and leachate). Many strategies, particularly those involved with managing non-point sources, require a multipronged approach that involves both technological innovation and changes to management practices.

Increasingly, the USEPA and state environmental agencies recognize the connection between water quality from an environmental protection and a drinking water perspective. Source water protection programs are designed to proactively protect the rivers, lakes, and groundwater that form drinking water supplies from contamination, rather than relying solely on reactive treatment. Many of the same best management practices that are employed to reduce non-point source pollution are also effective source water protection strategies. Source water protection can also include legal and policy tools such as land protection easements, local ordinances preventing certain types of damaging activity near source waters, and the development of emergency response plans.

This overview provides a high-level synopsis of the measures being currently implemented to address the most common forms of water quality pollution.

Wastewater Treatment and Polishing

Traditional wastewater treatment plants follow a five-stage treatment process to meet their NPDES permit requirements [29].

1. Screen for large-scale items and grit

Fine grit and larger items like sticks can damage treatment equipment in the plant and so must be removed early in the process. A bar screen is used to remove large debris, while a grit chamber is used to settle smaller particles out of the solution. At this stage, a skimmer may also be employed to remove floating grease or fat. 

2. Settle out larger organic materials

In the primary clarifier, the water is slowed down to allow larger organics to settle out of suspension. This process removes 25–50 percent of solids in the mixture. The sludge that settles at the bottom of the clarifier is removed from the system—it may be reused as fertilizer after further treatment.

3. Use bacteria to break down organic matter

All wastewater plants use some form of enhanced biological process to provide further treatment to the waste stream. This process is known as secondary treatment. Two alternatives might be used at this stage: a suspended-growth process like an activated sludge aeration chamber, or a fixed-film process like a trickling filter. In some plants, these two processes are used together to achieve even higher rates of BOD and nutrient removal.

a) Air is bubbled through the liquid in an aeration chamber to allow aerobic bacteria to break down organic matter quickly and effectively, thereby reducing BOD. The additional oxygen also helps to remove ammonia by promoting nitrification: the conversion of ammonia into nitrates. 

b) In a trickling filter, water is spread using a distributor arm over a film of aerobic bacteria attached to a permeable medium like rock, slag, or plastic. The spreading of water in a thin layer, and the porosity of the media, allows aeration to take place without the need to pump extra oxygen through the system. As the biofilm grows, excess bacteria ‘sloughs’ off and is routed through a clarifier for removal and disposal.

4. Remove excess material and bacteria

In a suspended-growth process, the amount of bacteria in the waste stream will multiply given the excess oxygen and abundant food. In a secondary clarifier, the water is slowed down again to allow any remaining organic sediments—including excess bacteria—to be removed from the system. Some of this “activated sludge” is returned to the aeration tank to help maintain the microbial population. The rest is discarded.                                                      

5. Disinfect

The final stage of treatment for most wastewater plants is disinfection to remove potential pathogens from the effluent. The cheapest option is typically chlorine, though UV and ozone are also options. If chlorine is used as a disinfectant, the plant must monitor for excess free chlorine before discharging the treated wastewater to the environment.

In some instances, wastewater treatment plants may opt to take treatment a step further. For example, additional treatment may be required when the municipality wants to reuse the water for other purposes. Tertiary treatment (also known as wastewater polishing) can be designed to remove remaining excess BOD, nutrients, dissolved solids, heavy metals, radioactive contaminants, and even micropollutants [29]. Tertiary treatment encompasses a wide variety of processes, depending on the type of pollutants that are being treated.

• Granular activated carbon can be used to adsorb pollutants such as chlorine, radon, and micropollutants.
• Membrane filtration technologies use a membrane with very fine pores to remove contaminants of different sizes from wastewater. Depending on the size of the pores, the treatment might be known as microfiltration, nanofiltration, or ultrafiltration: in both cases, the membranes filter out the smallest solid particles (including bacteria and viruses) suspended in the liquid but allow dissolved solids to pass through. Reverse osmosis is a special type of membrane filtration where a high-energy gradient is applied across a membrane with very fine pores to remove dissolved solids like salts. In all cases, membrane filtration can be expensive, both due to the cost of the membrane and the large amount of energy required.
• Ion exchange resins can be used to remove dissolved ionic species such as nitrates. They can also be used to remove dissolved heavy metals. The type of resin used must be carefully selected to address the particular type of pollutant being addressed. Ion exchange resins may be amended with activated charcoal to remove organic contaminants. This technology is comparatively expensive.
• Treatment wetlands mimic the biological, physical, and chemical processes that occur in natural wetlands to treat BOD and nitrates. The presence of vegetation slows down the flow of water and allows for settlement of solids, microbial breakdown of BOD, transformation of nutrients by bacteria, and plant uptake of nutrients. To be effective, treatment wetlands must be constructed outside of the floodplain of waters of the United States. Certain types of treatment wetland can be developed to provide multiple ecological benefits beyond water quality, including improved biodiversity and carbon sequestration.

Urban Best Management Practices

Urban best management practices (BMPs) provide a range of alternatives to manage urban stormwater runoff.

Nonstructural BMPs

Focus on minimizing contact of pollutants with stormwater in the first place. Most of these nonstructural BMPs relate to behavioral changes, such as picking up after pets, better management of yard waste, and proper maintenance of vehicles to avoid oil leakage. Stormwater management and erosion control for construction sites to reduce sedimentation is also classified as a nonstructural BMP.

Structural BMPs

Are engineered structures designed to capture, store, and treat runoff at its source. Structural BMPs are also known as green infrastructure or low impact development. These practices use natural processes to slow, filter, and infiltrate water. BMPs help to reduce flooding, surface erosion, and the risk of combined sewer overflows, thereby significantly improving water quality. Depending on the design, location, and type of the BMP, there may be considerable reductions in the amount of sediments, nutrients, and heavy metals in stormwater treated by a BMP [43]. Planted BMPs like rain gardens often focus on using native vegetation to reduce maintenance requirements while improving biodiversity co-benefits. At the same time, green infrastructure performance in pollutant reduction can be highly variable. Performance depends on the magnitude and duration of the storm event, the location of the BMP, the size of associated upstream flows, and the type of native soil in the area [43] [44]. The design of the BMP can also affect its performance; for example, certain types of BMPs are much more efficient at nitrate removal than others, depending on the presence of anaerobic conditions for denitrifying bacteria [45]. Infiltration-based BMPs (like rain gardens and bioretention cells) are not advisable in areas with contaminated soils or high water tables. Structural BMPs also function most efficiently when they are dispersed widely throughout an urban watershed, making them a prime target for public-private partnerships [46].

Agricultural Best Management Practices

A range of BMPs can be utilized to reduce the amount of sediments, nutrients, and pesticides entering waterways off farm fields. Many of these practices, such as conservation tillage, can boost farmers’ crop yields as well as provide environmental benefits [47].

Conservation Tillage

Refers to a range of farming practices that limit tilling to maintain a surface crop residue throughout the year. All forms of conservation tillage lead to better quality soil through less erosion, more soil organic matter, improved soil structure, and improved infiltration. These soil improvement benefits translate into improved water quality, while maintaining or even improving crop yields

Nutrient Management

Requires additional upfront investment of time and resources by farmers but can save on long-term fertilizer application costs. Nutrient management strategies aim to target nutrient (fertilizer) application only where it is most needed to prevent waste and associated downstream water quality impacts. A nutrient management strategy begins with tests to assess what nutrients are lacking in the soil, the nutrient status of the crop, and what organic material (like manure) might be added to the crop. With these values, farmers can build a nutrient budget for their crop and a schedule, which can be updated season to season, to determine the optimal rate, time, and amount of fertilizer application.

Conservation Buffers

Are strips of land on the edge of farm fields maintained with permanent vegetation to improve runoff water quality, air quality, biodiversity, and soil quality. There are many different types of buffers, each with different abilities to enhance specific ecological functions to meet stakeholder needs. However, all buffers must be properly installed and maintained to achieve their water quality objectives. Conservation buffers can be part of an effective strategy to manage runoff from CAFOs and can also be used to reduce sedimentation and erosion.

[29] M. Davis, Water and Wastewater Engineering, 1st ed., Columbus, Ohio: McGraw-Hill Education, 2010.

[43] F. Ahammed, “A review of water-sensitive urban design technologies and practices for sustainable stormwater management,” Sustainable Water Resources Management, vol. 3, p. 269–282, 2017.

[44] C. Bell, et al., “Characterizing the Effects of Stormwater Mitigation on Nutrient Export and Stream Concentrations,” Environmental Management, vol. 59, pp. 604–618, 2017.

[45] A. Roy-Poirier, P. Champagne and Y. Fillion, “Review of bioretention system research and design: Past, present, and future,” Journal of Environmental Engineering, vol. 136, no. 9, pp. 878–889, 2010.

[46] R. William, J. Garg and A. Stillwell, “A game theory analysis of green infrastructure stormwater management policies,” Water Resources Research, vol. 53, no. 9, pp. 8003–8019, 2017.

[47] United States Environmental Protection Agency, “Watershed Academy Web: Agricultural Management Practices for Water Quality Protection,” [Online]. Available: https://cfpub.epa.gov/watertrain/moduleframe.cfm?parent_object_id=1362. [Accessed June 6, 2022].