Why and how to treat municipal sewage?

The content of suspended matter (SM), which represents particulate matter that can be trapped by a 2µm aperture filter membrane. They are made up of about 25 percent minerals and 75 percent organic matter called volatile substances. Volatile suspended solids are an important component of COD.

Chemical oxygen demand (COD) is the amount of oxygen required to completely oxidize dissolved and granular organic pollution, including biodegradable COD and non-biodegradable COD two parts. This complete oxidation is done in a very acidic environment by using a very strong oxidizing agent (potassium dichromate) and reacting for 2 hours at a temperature of about 150°C [5]. For untreated domestic sewage, about 50% of COD is dissolved and the other 50% is granular.

Five-day biochemical oxygen demand (BOD5) is the amount of oxygen consumed by bacteria as they degrade the biodegradable organic matter in them over a period of five days. The high COD/BOD5 ratio (2 to 5) of municipal wastewater indicates that organic pollutants can be easily removed biologically in wastewater treatment plants.

Kjeldahl nitrogen [6] (NK) is organic nitrogen (including urea, amino acids, proteins...) And ammonia nitrogen (N-NH3).

Total nitrogen (NGL) is the sum of organic nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrite nitrogen. Neither of the latter two forms of nitrogen is present in untreated municipal sewage.

Total phosphorus (Pt) includes organic phosphorus and inorganic phosphorus.

Table 1 shows the average water quality characteristics of municipal sewage at the entrance of the sewage treatment plant and the minimum water quality requirements (maximum allowable concentration or minimum treatment efficiency) of treated sewage as required by regulations.

Table 1. Average composition of untreated municipal sewage and example of discharge standards for large wastewater treatment plants (over 100,000 inhabitants).

Municipal sewage also contains many inorganic and organic compounds in very low concentrations (from a few ng/l to a few µg/l). The main types of micropollutants are cosmetics, pesticide and insecticide residues, solvents, natural and synthetic hormones, drug residues, metals, etc. Such micropollution is the subject of a campaign to control the discharge of hazardous substances to the environment. At present, there is particular concern about the concentration levels of pesticide residues, drugs and endocrine disruptors in the inlet and outlet water of sewage treatment plants.

Municipal sewage also contains high concentrations of fecal microorganisms, especially pathogenic microorganisms, the number and type of which depends on the health status of the population.

2.2 Impact of discharge on water environment

Untreated municipal sewage discharged into surface water can cause visual pollution (floaters), reduce the transparency of the water, and contribute to the siltation of lakes and rivers. The discharge of biodegradable substances will enhance the microbial activity in the water body, resulting in a decrease in the concentration of dissolved oxygen, and even cause other organisms in the water body to suffocate due to lack of oxygen. Nitrogen and phosphorus emissions can lead to eutrophication in water bodies (read Phosphorus and Eutrophication and Nitrates in the Environment).

The discharge of micropollutants can have toxic effects on plants and animals in the water environment. These effects include bioaccumulation of persistent molecules in the food chain, chronic toxicity from very low doses, and changes in endocrine system function that can lead to consequences such as feminization of male fish. Microbial contamination of water can render the water unfit for certain uses.

2.3. Obligations for wastewater treatment

Microorganisms naturally present in surface water can degrade pollutants caused by sewage discharge, but the self-purification capacity of rivers is generally very inadequate. Therefore, sewage must be treated in a treatment plant before being discharged into the natural environment. For the various composite pollution indicators, the maximum allowable concentration of treated sewage must not be exceeded or the minimum purification efficiency stipulated by the provisions of the regulations should be achieved, as shown in Table 1 (see also the French Water Code).

3. How to treat wastewater?

Environmental Encyclopedia - Sewage - Activated sludge floc

Figure 3. Microscopic observation of activated sludge floc (400 times magnification). In the view are a nematode and a rotifer.

Municipal wastewater is mainly treated by biological methods and is combined with liquid/solid separation processes (precipitation, filtration, air flotation) to remove suspended solids and retain the resulting biological matter. The biological material that acts as a purification is basically composed of bacteria (primary producers) that have the property of secreting extracellular polymers [7] and can form settling floc [8] or biofilm in which other microorganisms (protozoa, metazoa) are also constantly multiplying as predators 。

Biological reactions to remove organic matter, nitrogen and phosphorus pollution all require special operating conditions (presence or absence of dissolved oxygen, residence time of biomass in the reactor, etc.). The removal of these contaminants or the purification of the water is accomplished by the cultivation of biological matter suspended in the water or attached to the filler.