Activated sludge plants

The active sludge biological process refers to a type of aerobic treatment conducted by more or less prolonged aeration of the wastewater in a biological reactor in the presence of a microbial population (biomass).
In activated sludge plants, with suspended biomass, the bacterial populations responsible for the cleaning treatment are present in the form of flakes kept in suspension through the air blower (if aerated reactors) or by mixer (if anoxic or anaerobic reactors).

The active biomass is constituted by numerous microorganisms (bacteria, protozoa, metazoa, rotifers, etc. ..) and is continuously produced inside the reactor following the biochemical reactions of degradation of organic carbon and nutrient utilization, resulting in synthesis of new cellular material.

Schematically, an activated sludge plant is represented by a reaction tank (aerobic, anoxic or anaerobic) where there is a contact between the bacterial population and unload to be purified. The aerated mixture, in output from the tank, is sent to the sedimentation where the activated sludge flakes are separated from the effluent purified which can be discharged from the surface of the settling tank, while the sludge is partially reintroduced into the system, by means of a circuit recirculation, and in part it is extracted periodically as excess sludge to be disposed.

The wastewater originated from any pre-treatment (screening, primary sedimentation, grit removal, etc..) is conveyed into an aeration tank which is oxygenated mainly for insufflation of air. Within the biological reactor, occurs the process of demolition organic substance by means of bacteria which transform it into simple substances (CO2, H2O) and energy which then use it to grow and multiply.
The bioflocculation is a formation of gelatinous flakes of a few mm of organic material present in suspension. The aggregation of these flocculi is assisted by the surface electric charge of the organic molecules many of which are colloidal in nature and by the presence of filamentous bacterial forms that constitute a real inner armature on which depends the cohesion of the flocculo. The bacterial forms present are varied and selected from time to time and this depends on the type of waste and global conditions present in the tank; The active biomass in the flocculo varies from 10 to 40% on the total of dry matter. The flocculi if well-formed settle out and go to form the so-called activated sludge, which settles to the bottom and is placed in recirculation allowing a progressive degradation of the organic substance present in it.
At the exit from the oxidation tank the slurry is introduced into the final sedimentation tank, in which is obtained the separation of flocs from the purified liquid part that is sent to potential tertiary treatments (mainly filtration, disinfection) to be identified in relation to the final delivery.

The most recent plant solutions also provide the degradation of other nutrients (N and P).
The nitrogen is found in the sewage, mainly in the form of ammonia: initially combined in molecules of various complex organic substances, which are key components of the cells of living organisms; the transformation occurs by oxidation by biological means and degradation of these substances, with consequent solubilization into ammonia. The ammonia nitrogen is subsequently oxidized, always for biological pathway, to "nitrites" and further oxidized to "nitrate", according to the following simplified scheme:
These reactions occur in the oxidation basin by specific strains of bacteria (Nitroso monas and Nitrobacter) using the dissolved oxygen available in the tank, introduced by means of a suitable system.
In the aerobic nitrogen transformation process we are witnessing the conversion of the same from the reduced form of ammonia nitrogen to the oxidized form of nitrate nitrogen. The latter may be present in the final effluent in a limited concentration and therefore will have to undergo further processing.

The purpose is a step of denitrification in the anoxic tank, in which numerous specialized bacterial strains fix the oxygen present in the molecule of the nitrates by using it for endogenous needs about the metabolization of the organic substances; In fact, these strains (heterotrophic) are facultative aerobes, that is, if the surrounding environment there is the presence of dissolved oxygen using it directly, while in case of anoxia can use the oxygen present in the nitrate.
This phase, with the aim of accelerating the reaction kinetics and consequently the necessary volumes, is usually carried out upstream of the compartment oxidation-nitrification, and for this reason is defined pre-denitrification.

The control of phosphorus discharged from municipal and industrial wastewater treatment plants is a key factor in the prevention of eutrophication of surface waters.
The municipal waste waters can contain from 5 to 20 mg / l of total phosphorus, of which 1-5 mg / L are organic and the remainder is inorganic.
The removal of phosphates is currently used mainly by chemical precipitation with the addition of a coagulant, in which the chemicals are fed directly into the aeration tank or before it.
The most commonly used multivalent metal ions are iron and aluminum.
The continuous recirculation of the sludge, together with the adsorption processes and coagulation-flocculation due to activated sludge, allow a reduction in chemical consumption.

Ultimately the result that is obtained with an activated sludge system is the elimination of the biodegradable organic matter by means of transformation into inert material and in a muddy concentrated solution of the organic substance which must be subjected to further treatment before final disposal.
The plant scheme provides for:

• A pretreatment section (if any);
• A denitrification tank;
• A nitrification tank and bio-oxidation of carbon;
• A recirculation circuit (sludge and aerated mixture) and a removal system of excess biomass;
• Adequate mixing and aeration systems;
• A sub-fund for the separation of the sludge from the waters "purified".

Applications

• Domestic and urban wastewater treatment;
• Industrial waste water treatment:
  • Agro-food industries;
  • Paper industries;
  • Textile industries;
  • Tanneries;
  • Chemical and pharmaceutical industries;
  • Petroleum and petrochemical industries;
  • Metallurgical and mining industries;
  • Agricolture animal husbandry.