Pollution index in water treatment plants
In order to characterise urban wastewater pollution as well as the efficiency of treatment plants, four main elements are analysed at the entrance and the exit of each plant:
- The biological and chemical oxygen demands (BOD5 and COD), expressing the amount of oxygen in the water used, after the insertion of wastewater in the receiving environment.
- Nitrogen, mainly coming from the human metabolism.
- Phosphates, coming from the metabolism and present in various detergents.
- Suspended solids, which shows the content in small water particles; they essentially come from human excreta.
An increase in one of those four elements affects to a greater or lesser extent wildlife in the aquatic environment. It requires the setting up of expensive treatments to make the water comply with the uses it needs to serve.
BOD5 and COD
What do BOD5 and COD mean?
BOD5, or biological oxygen demand in 5 days, is the amount of dissolved oxygen necessary to decompose the organic matter present in the effluent by biological means.
COD, or chemical oxygen demand, corresponds with the amount of dissolved oxygen used to oxidise both the organic matter and the oxydisable mineral matter present in water.
Why does an increase in BOD5 and COD damage our environment?
If the amount of organic matter is significant, their degradation by microorganisms will cause an increase in the oxygen consumption. If the concentration of oxygen in water goes under a critical level, the environment may become dangerous for susceptible animal species (trout, salmon, etc.).
A significant amount of organic matter in water also risks to render it unfit for its usual uses.
How are BOD5 and COD measured?
Measures of BOD5 and COD are carried out in laboratories with chemical and biochemical analyses. They are uttered in mg of oxygen per litre of water. BOD5 is measured at the end of 5 days at 20°C without light. COD is instantaneously measured.
Total nitrogen (TN)
What does “total nitrogen” mean?
Any living matter is made up of nitrogen, found in different chemical states (ammoniacal nitrogen, nitrites, nitrates). Nitrogen is also to be found in human excreta (organic nitrogen). Water coming from household use (disposal from health facilities) thus contains a lot of it. The term “total” nitrogen is used when all forms of nitrogen present in the water are taken into account in the measure.
Why does an increase in nitrogen damage our environment?
A certain amount of nitrogen is vital for the development of the aquatic wildlife. However, when a significant amount of nitrogen is discharged in a stream, under favourable conditions (especially in the presence of phosphorus), a phenomenon of eutrophication may appear (see below).
How is nitrogen measured?
The nitrogen contained in the wastewater going in and out of the treatment plants is measured by laboratory chemical analyses. The measure is uttered in mg of nitrogen per litre of water.
Total phosphate (TP)
What does “total phosphate” mean?
Phosphates are chemical elements, especially found in some detergents. Therefore, they end up in the wastewater discharged in the sewers. The term “total” phosphate is used when all forms of phosphates present in the water are taken into account in the measure.
Why does an increase in phosphates damage our environment?
Phosphates are vital nutrients for plants. Under favourable conditions, an increase in the phosphate content in the stream means a bigger growth of aquatic plants. Such a growth will mean in turn a bigger production of oxygen during the day (photosynthesis), as well as a bigger consumption of oxygen (breathing), particularly critical during the night. This substantial consumption of oxygen may end up in the death of animals, especially the frailest fish, like the trout. This phenomenon is called eutrophication. In this process, nitrogen is essential, but it is generally accepted that phosphorus is the limiting element.
How is phosphate measured?
The phosphate contained in the wastewater going in and out of the treatment plants is measured by laboratory chemical analyses. The measure is uttered in mg of phosphates per litre of water.
Suspended solids (SS)
What does “suspended solids” mean?
Suspended solids are particles found in water and retained to a great extent by the screens and the settlers set up in the treatment plants. These particles have two main origins:
- human excreta;
- water running off various surfaces (roofs, roads, etc.) to the sewerage.
Why do suspended solids damage our environment?
Suspended solids damage our environment for two main reasons:
- They make water cloudy and prevent a good growth of aquatic plants. Now, aquatic plants are not only an important nutrient source, but also the producers of part of the dissolved oxygen present in water (photosynthesis). The slightest development may mean a reduction of the amount of oxygen in water (another source of dissolved oxygen is the atmosphere, by gas exchange) and lead to the death of various animals.
- They settle in the form of smudge at the bottom of streams, near bridges and dams. These sediments hinder boat traffic, reduce the water flow at some points, and therefore hasten it in other places, causing erosions and floods. If the river is in spate, those floods may cause flooding such as regularly occur on our territory. The accumulation of sediments at the bottom of our streams requires costly dredging which is slow to undertake. Moreover, they cause another problem as they become sludge.
How are suspended solids measured?
To measure them, a sample of known volume is passed through a membrane with smaller pores than most of the microorganisms usually present in the wastewater (0.47 micron). The dry weight of the gathered matter is called suspended solids, or SS, and is uttered in milligram per litre, mg/l.