North Americans produce approximately 250 litres of wastewater (a.k.a. sewage) per person per day. The actual amount varies from place to place, of course, but it is generally higher than in European countries (by several times in some cases), and much higher than in most other parts of the world. That liquid waste comes from toilets, showers and baths, laundry, and kitchens. Because North Americans use so much water, our wastewater is relatively dilute, with dissolved and suspended solids content in the range of 1000 to 2000 mg/L. In other words, each of us sends 250 to 500 g (dry weight) of solid matter down the sewer each day.

The solids in our waste include feces, food particles, toilet paper, grease, oil, soaps, dissolved salts and metals, and mineral matter (sand, clay etc.). Those components give wastewater chemical characteristics similar to those summarized in Table 14.5.1.^[1]

Table 14.5.1 Typical Concentrations of the Major Components of Domestic Wastewater, in mg/L. (See Table 14.3.1 for an explanation of BOD5 and COD.  “Volatile suspended solids” is the fraction of the suspended matter that will evaporate or burn on heating above 550° C.)

BOD5	COD	Organic Carbon	Total Suspended Solids	Volatile Suspended Solids	Carbonates	N	P	Fats, Oils & Grease
305	740	250	450	320	37	80	23	100

Not listed in table 14.5.1 are the microorganisms and trace elements that are present in our liquid wastes. Some of the microorganisms are as follows:^[2]

Most of the microorganisms in sewage are harmless to us, but some can lead to illnesses if wastewater is not treated or dealt with adequately. Unfortunately, inadequate treatment is common in most parts of the world.  The problem is especially serious if wastewater mixes with drinking water supplies.

According to Erik Peterson, Resources Analyst at the World Health Organization, more than 1.5 million people (including over 400,000 children) died in 2019 as a result of diseases contracted from water, and “at any given time, close to half the population of the developing world is suffering from waterborne diseases associated with inadequate provision of water and sanitation service.”^[3]

Pathogens from wastewater are a health problem.  On the other hand, some other components of wastewater, including oxygen-demand and trace elements, are geological problems because they impact the quality and physical properties of water bodies, and aquatic ecosystems. Because of its high levels of nitrogen and phosphorous, wastewater can promote significant algal growth, which has undesirable aesthetic and ecosystem implications. In surface water some of the suspended components will gradually settle, but the smaller particles and the microorganisms will likely stay in suspension, and dissolved components will tend to stay in solution. Such contaminants can travel a significant distance.

Wastewater that is dispersed into an aquifer represents a very different problem than that which gets into a stream, or a lake or the ocean. In an aquifer the wastewater will interact with the minerals that surround it. Dissolved components will tend to become attached to surfaces, especially clay mineral surfaces, and so they may not be dispersed for more than tens to hundreds of metres. In general, aquifer materials act like filters, so suspended contaminants (including microorganisms) won’t get very far: typically metres to tens of metres depending on the nature and grain size of the aquifer. Because the supply of oxygen is limited at depth, there will be minimal oxidation of the organic constituents of the waste.

Organic matter and other components that make up the COD and BOD will consume oxygen from the water, and that will have negative implications for aquatic life. A condition called eutrophication can also arise from excessive nitrogen and phosphorus (from sewage) entering a body of water. Nitrogen and phosphorus are used in fertilizer for a very good reason: they occur naturally in low concentrations, and that restricts plant growth. When nitrogen and phosphorus are added to surface water, they contribute to rapid plant growth and algae blooms. Excessive algae can clog fish gills and block sunlight, resulting in dead organic matter. Compounding this problem, the algae will also eventually die and then it too becomes oxygen-demanding waste.

Treatment of wastewater is typically divided into three levels: primary, secondary and tertiary. These can be summarized as follows^[4]:

Wastewater sludge, which is rich in plant nutrients, is widely used as a supplement for agricultural and forest soils.^[5] Risks of microorganism contamination can be minimized by heating above 55° C for at least 4 hours, but potentially toxic metals cannot be easily removed from sludge. The metals that are most likely to be problematic to food crops are zinc, copper, nickel, cadmium, lead and mercury. These are most likely to be seriously elevated in the sludge from treatment plants that accept a significant component of industrial wastewater.

Most residents of rural areas are not connected to sewage collection piping networks and so are required to deal with their own wastewater (or else have it taken to a treatment plant in a tanker). The most common solution is a septic tank with a drainage field.  The septic tank is typically a plastic or concrete tank with two chambers that is designed so that most of the solids can settle and the scum that develops on top of the wastewater is constrained.  The liquid is allowed to flow to a drainage field similar to the one shown on Figure 14.5.2. The key features to note are that the plastic pipes have perforations to allow the liquid to drain out, and that a layer of permeable gravel has been placed in order to allow the liquid to drain slowly and enter the underlying soil or permeable rock. The premise behind an installation of this type is that as the water seeps slowly through the drainage medium and then the underlying natural soil and/or rock it will become sufficiently clean so that it won’t seriously contaminate the nearest body of surface water. For that reason, it is important to assess permeability and porosity of the natural materials present in the area where a drainage field is to be constructed.  If the permeability is too high the wastewater may flow through too quickly to be effectively treated. If the permeability is too low the wastewater may not flow away at all, and instead will pool on the surface.

Before it is released to the natural environment effluent from a wastewater treatment plant, runoff from a road, or “grey water” from a kitchen or shower can be further decontaminated in a wetland.  A constructed wetland is situated within a natural or excavated basin that has an impermeable liner. It has a perforated plumbing system at the base embedded in permeable gravel which is covered with finer material (Figure 14.5.3). Wetland plants (a.k.a. macrophytes) are grown within the wetland, but their primary function is to provide a substrate (roots, stems and leaves) and appropriate chemical conditions for a population of microorganisms (algae, bacteria etc.) called a periphyton. The periphyton is responsible for removal of about 90% of the pollutants. Well-designed constructed wetlands are effective in removing nitrogen, phosphorous and trace metals and in reducing BOD and COD levels.

Media Attributions

• Figure 14.5.1 Marlborough East Wastewater Treatment Plant by Nick Allen, 2015, CC BY SA 4.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Marlborough_East_Wastewater_Treatment_Plant_Aerial.JPG
• Figure 14.5.2 Drain Field in Progress by Nonztp, 2019, CC BY SA 4.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Drain_field_in_progress.jpg
• Figure 14.5.3 NZ0963: Waste water treatment works and transport depot, Low Prudhoe by Andrew Curtis, CC BY-SA-2.0, via Geograph, https://www.geograph.org.uk/photo/2039001
• Figure 14.5.4 Steven Earle, CC BY 4.0, after Tilley, E, et al. (2014). Compendium of sanitation systems and technologies (2nd edition). Swiss Federal Institute of Aquatic Science and Technology, via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Tilley_et_al_2014_Schematic_of_the_Vertical_Flow_Constructed_Wetland.jpg