Intensive processes used in large wastewater treatment plants

When more than 2000-4000 population equivalent sewage can be centrally collected [9], its treatment is generally carried out in a centralized sewage treatment plant, where the microorganisms used for sewage purification are either suspended in water (called activated sludge) or fixed to fillers (submerged biofilters). The advantage of these sewage treatment plants is their small footprint. However, these treatment plants consume a lot of energy (about 60-90 KWH/person/year), especially for providing oxygen to bacteria by means of agitation and aeration (Figure 6). They produce a large amount of residual sludge (20-22 kg dry matter/inhabitant/year), mainly composed of non-biodegradable suspended matter and biological matter produced in bioreactors.

Environmental Encyclopedia - Sewage - Sewage treatment processes include:

Pretreatment steps to remove large debris (grille unit), sand (sanding unit) and grease (degreasing unit)

Activated sludge reactor

Settling tank

It may also include tertiary treatment (such as UV irradiation disinfection) before release into the natural environment.

Activated sludge treatment plants usually consist of two or three bioreactors in series, whose layout and operating conditions are chosen to optimize the removal efficiency of organic, nitrogen and phosphorus pollution. In the clarifier, the bacterial floc formed in the activated sludge tank occurs liquid/solid separation, i.e. the treated sewage overflows from the upper part of the clarifier tank, while the sludge settles at the bottom of the clarifier tank (Figure 8). Some of the sludge is returned to the bioreactor, while the remaining sludge is sent to the sludge treatment and resource/energy recovery systems. This type of sewage treatment plant was invented in 1914 and is currently used to treat 90% of municipal sewage in France.

Environmental Encyclopedia - Sewage - Activated sludge reactor samples

Figure 8. Biological material sample just taken from the activated sludge reactor (left sample) and 30 minutes after settling (right sample). The supernatant represents treated water. [©Joseph De Laat]

The use of porous membrane filtration instead of clarifier can achieve higher biomass concentrations in bioreactors (membrane bioreactors). Compared to precipitation, membrane filtration ensures that the treated water does not contain suspended solids and is better able to remove bacteria [10].

Biological filtration wastewater treatment plant. This process, developed in the 1980s, removes organic and nitrogen pollution through attached growth biological matter formed in an immersed filter. The medium in the filter (particle size: 4-6 mm) is both a filler for biofilm growth and a filter medium. The treated water at the outlet of the biofilter can be discharged directly into the receiving water environment (no clarification tank is required). In order to avoid the biological filter being clogged too quickly, the suspended solids in the sewage should be thoroughly removed before biological filtration. The biofilter is cleaned daily to remove the suspended solids and biomass trapped in the filtration process.

3.3. Extensive processes used in small wastewater treatment plants

The natural lagoon method refers to the circulation of sewage to be treated in three connected, shallow (1-1.4 m), non-leaking pools (lagoons) for several days.

Encyclopedia of the Environment - Sewage - Natural Lagoon Law

Figure 10. Natural lagoon method for removal of organic, nitrogen and phosphorus pollution. [©Joseph De Laat]

Natural lagoon wastewater treatment is mainly achieved through biological reactions occurring in the upper water layer and sedimentation of sedimentable substances occurring at the bottom of the pool.

Upper water layer: Under aerobic conditions, bacteria oxidize to degrade biodegradable organic matter (especially in the first pool) and nitrogenous organic matter (nitrification occurs mainly in the second and third pools). Dissolved oxygen is provided through the exchange between water and the atmosphere, in which microalgae (photosynthesis) play an important role. In the process of growth and reproduction, microalgae assimilate and absorb part of nitrogen and phosphorus in sewage.

Pool floor: Since there is no dissolved oxygen (no photosynthesis) at the bottom of the lagoon, partial denitrification of nitrated nitrogen can occur as well as possibly leading to an anaerobic fermentation reaction (producing hydrogen sulfide).

The sewage purification performance of the lagoon method changes with the change of season, sunshine and water temperature. The ponds need to be cleaned every 10 to 12 years because of sediment buildup.

Encyclopedia of the Environment - Sewage - filter for growing reeds

FIG. 11. Filter for planting reeds.

The reed filter represents an artificial version of a natural marsh (Figure 11). The conventional method is to filter large particles of impurities through two cascaded filters that have been previously removed (through the grid). Each filter consists of a 60-90 cm thick gravel layer planted with reeds. Biological purification is achieved by trapping suspended solids on the surface of the filter and by the biomass of purification attached to the filter material and reed roots. The basic function of the reed is to achieve mechanical cleaning of the filter under the action of wind. The footprint required to grow the reed filter is five to six times smaller than the natural lagoon method and fits in well with the surrounding landscape (see emphasis: Reed filter).