Many human activities generate emissions, known as wastewater, which require purification treatment before being released into the environment. This is due to the presence of pollutants in quantities exceeding the self-purification capacity of soil, seas, rivers and lakes.
Therefore, it is essential to employ wastewater purification systems that accelerate natural biological processes through the use of advanced technologies.
The urban waste water purification process consists of several consecutive stages, during which pollutants are removed from the water and concentrated in the form of sludge. This process produces a final effluent of such quality that it is compatible with the self-purifying capacity of the receiving environment (soil, lake, river or sea through underwater or shoreline pipelines), without causing damage.
The waste water purification system is based on a combination of chemical, physical and biological processes. However, the sludge produced during the purification cycle may be contaminated with toxic substances and also needs specific treatment to make it suitable for disposal, such as disposal in special landfills, or reuse in agriculture, either as such or after composting.
Bellin progressing cavity and lobe pumps are successfully used in many of the steps involved in the waste water treatment process.
Waste water from the various sewerage systems is collected and transported to the treatment plant through collector systems. In many cases, it is necessary to lift the collected sewage from the collectors to transfer it to the next treatment steps.
The pumps lift the sewage, which is then screened to remove bulky material (such as plastic, wood, stones and paper), thus preventing clogging of pipes and pumps. The screened material is then washed, pressed and sent to landfill. Afterwards, the sewage is pumped to the desander, where the sands are separated through natural sedimentation. At the same time, the separation and rising of oils and fats to the surface is facilitated by the insufflation of air, which generates controlled turbulence, preventing the sedimentation of organic substances.
Finally, the surface oil is removed using an overhead crane or other mechanical device, while the settled sand is removed using a dredger.
Primary sedimentation is based on gravity separation of settleable solids. By the end of this phase, mechanical treatments will have removed about one third of the organic load.
The next stage is secondary sedimentation, a process that exploits the metabolic action of microorganisms, such as bacteria, that utilise the organic substances and dissolved oxygen in the slurry for their activity and reproduction. In this way, flocs composed of bacterial colonies are formed that can be easily eliminated in the subsequent sedimentation phase. During secondary sedimentation, three processes usually occur: denitration, second clarification and third oxidation.
The last stage is final sedimentation, where the sludge flocs are separated from the aerated mixture by sedimentation in the final sedimentation tank.
The use of flocculants, such as ferric chloride, is essential for phosphorous removal. Moreover, these agents are added to the mud before dehydration. Flocculants, generally composed of organic polyelectrolytes, promote the aggregation of clots into larger flakes, thus improving the effectiveness of dehydration.
The precipitation of hydroxides is mainly done by the use of lime milk or caustic soda. Although lime milk can generate larger quantities of sludge, it has the advantage of increasing sedimentation efficiency, reducing the sludge volumes produced and enhancing filterability. The use of milk of lime becomes indispensable when it is necessary to remove phosphates (found in phosphating and degreasing baths), fluorides (associated with treatments on aluminium alloys) and sulphates (found in some pickling baths).
In addition to mechanical and biological processes, further treatments are required to reduce nutrients such as nitrogen and phosphorous in the final effluent, which can cause eutrophication in rivers and lakes. Nitrogen removal is achieved by biological processes through the action of specialised bacteria in the oxidation tanks.
In the final filtration, the bacteria are exposed to UV light to inactivate them, and then, using progressive cavity pumps, the purified water is discharged into the river.
Sludge from primary and secondary sedimentation is pumped into the thickener unit, where the solids concentration is increased and the sludge volume reduced accordingly. The goal is to achieve a dry solids content of 6% to 11% in the pumped fluid.
Then from the thickener sludge is sent to the fermenter, where it remains for about 20 days in an anoxic environment at a temperature of 45°C. Specialised bacteria reduce the organic matter and partially convert it into inorganic substances, producing a gas with a high methane content (biogas) as a result of their metabolism. The gas produced is stored in the gasometer and used as an energy source for heating the digester.
Biomasses represent a renewable energy source for the future and have an intermediate consistency between liquid and solid products. Inside a digester, thanks to the action of microorganisms, methane, carbon dioxide and other gases are produced from the biomass. Depending on the process, the biomass inside the digester must be continuously stirred to ensure efficient biogas production.
The thin sludge, with a significantly reduced odour, is pumped into the post-thickener to further reduce its moisture content to between 1% and 4% dry matter. Subsequently, the sludge is transferred to centrifugal separators, where it is mixed with polyelectrolyte to achieve further thickening of the excess sludge (secondary thickening of excess sludge).
For the continuous feeding and agitation of sludge within the tanks, progressing cavity and/or lobe pumps are used. The digested sludge is then pumped into a thickening tank, where the separation of solids and water takes place.
Through mechanical dewatering and conditioning treatments, an initial volume reduction of between 65% and 80% is achieved. The result is a crumbly, compact and manageable product. Dewatered sludge takes on a semi-solid consistency that facilitates its use in agriculture, composting or landfilling.
