CUSTOM SOLUTIONS
There are cases in which the wastewater to be treated must be addressed with a particular approach, using various treatment stages, often combined with each other, among which we list the following types of treatment:
The function of a screen is to retain coarse solids that may be present in the wastewater to be treated, ensuring that the next treatment stage receives wastewater with solid particles smaller than the screen’s free passage. There are many types of screens, varying both in free passage and construction structure. There can be static screens, motorized screens, comb screens, drum screens, brush screens, screw extraction screens, chain screens, etc., etc.
Another physical treatment system for wastewater (the simplest and oldest) is gravimetric solid separation using dedicated tanks: septic tank, grease trap, Imhoff tank (with sedimentation and digestion compartments).
The most advanced system (which also uses coagulating chemical agents) is dissolved air flotation, used for separating suspended solids, oils and greases, fibers, and other low-density materials. Its operation is based on introducing micro air bubbles into a tank, which “attach” themselves to pollutants, causing them to rise to the surface. At the same time, chemicals are introduced into the wastewater feeding the tank through a tortuous pipe to aid flocculation, benefiting the solid/liquid separation that will occur in the tank. From the surface, a motorized skimming system then conveys the floated sludge to a pump, and from there to discharge.
The operation of activated sludge plants is based on the oxidation process: by providing and diffusing intensive and prolonged oxygenation, bacterial colonies are activated in the wastewater. These combine with the organic matter they feed on, aggregating into flocs and degrading the organic matter into smaller, less harmful compounds (part of which are reused by the microorganisms themselves for their own nutrition and reproduction). The activated sludge flocs can then be settled, discharging the clarified effluent.
SBR reactors are simple in construction and management, have the ability to equalize load peaks, offer operational flexibility, do not require sludge recirculation, and allow the selection of floc-forming bacteria thanks to the alternation of process conditions.
Unlike traditional activated sludge processes, SBR systems are fed discontinuously and have an operating cycle that includes a fill phase, one or more reaction phases (aerobic, anoxic, or anaerobic), a sludge settling phase, an effluent discharge phase, a waste sludge removal phase, and an idle phase.
MBR plants, on the other hand, exploit a much more advanced technological setup (ultrafiltration), whose features allow obtaining an effluent completely free of any pollutant. They are intensive plants that require less space compared to other biological treatment techniques; in addition to having plant costs commensurate with the degree of technical complexity, management costs are expected in terms of specialized technicians and mandatory maintenance operations (cartridge washing, cartridge replacement after a few years).
MBBR plants are based on a moving bed biofilm process (Moving Bed Biofilm Reactor, hence the acronym MBBR by which they are commonly known). They consist of tanks partially filled with inert polyethylene supports (“carriers”), kept in motion by appropriate mixing and/or aeration systems. The surface of the supports is colonized by an active biological film, entirely similar to that of biofilters. Mechanical separation between the treated effluent and excess biomass occurs in a subsequent treatment stage.
The MBBR system is a biological system with high organic load removal efficiency, combining the advantages of classic activated sludge systems with suspended biomass with those of attached biomass systems (cultures fixed on an inert support material). In terms of installation space required, the MBBR solution allows for a higher concentration of microorganisms than usual for classic activated sludge treatments and therefore a proportional reduction in footprint.
Filtration is one of the most widespread mechanical treatments applied to water: very often, in fact, it is associated with many other chemical-physical or biological treatment processes (such as desalination/demineralization), which act on the other characteristics of the water.
The water to be treated passes through the filter bed in co-current (from top to bottom) and as solids and/or pollutants deposit in the free spaces, the resistance to water passage progressively increases, and therefore the pressure drop. When the pressure drop (pressure difference between filter inlet and outlet) reaches around 0.7 bar, it is necessary to wash the filter to avoid the possibility of pollutant leaks with a consequent decrease in the flow rate of treated water.
According to a preset program, backwashing of the filter media is carried out to optimize the process and maintain the efficiency of the filter material over time.
The effectiveness of the filtration and/or adsorption process is directly proportional to the speed at which the wastewater passes through the system, and therefore a function of the contact time with the aforementioned filling materials. Furthermore, these times are directly dependent on the type of wastewater to be treated and must take into account the quantity of substances to be retained/adsorbed and the desired result in terms of efficiency and/or performance of the filtration system itself.
Membrane filtration is a process used not only in biological processes, but also, for example, as a pre-treatment before nanofiltration stages (e.g., in desalination plants). Ultrafiltration membranes can be essentially of two types (tubular or flat), and can completely remove suspended solids, colloids, and all types of microorganisms such as bacteria, viruses, protozoa, germs, and larvae. Compared to conventional water treatment systems, ultrafiltration technology offers several advantages:
– it provides a barrier against bacteria and microparticles in general;
– it allows the elimination of any pathogens resistant to chlorination;
– The concentrate generated from the ultrafiltration process consists of water contaminants. The quantity produced for disposal is significantly less than that of sludge from conventional treatments;
– the possibility of designing compact systems allows for limited investment in structures and requires limited space to install the systems themselves.
The reverse osmosis process occurs when a saline solution is brought into contact with a membrane permeable to water (but not permeable to dissolved solids), at a pressure higher than the osmotic pressure of the solution itself.
It is the opposite process of “direct” osmosis, which occurs naturally in the absence of external pressures on the system. When two solutions at different concentrations, contained respectively in two vessels, are brought into contact through a semipermeable membrane, a flow occurs from the lower concentration solution to the higher concentration solution, causing a decrease in concentration in the latter.
Through the membrane, a solution (permeate) with a lower salt content than the starting one passes, while outside the membrane a solution rich in salts (concentrate) is formed.
This process represents the finest water filtration technique: a technique that, today, constitutes the most convenient and safe way for demineralizing water of various origins, progressively replacing other treatment systems, thanks to the excellent combination of outstanding results and contained operating costs (reverse osmosis plants guarantee low energy consumption and limited consumption of chemicals). Furthermore, the parameters of the waste water generally fall within the limits set for discharge, thus reducing the increasingly felt problem of eluate disposal (a problem that exists, for example, for softeners and demineralizers with ion exchange resins).
To correctly design a reverse osmosis process, it is important to determine the most suitable “pre-treatment” based on the characteristics of the water to be treated, in order to ensure maximum operational continuity of the plant and reduce the frequency of cleaning. This prevents membrane clogging due to dirt and/or physical impurities, or due to the precipitation of scaling salts and/or metals, bacterial proliferation, and contact with oxidizing agents.
