PLANTS FOR PRIMARY WATER

p>With "primary water" we means those natural waters (rivers, lakes, sea or ground water), which contain, in addition to the natural components, also varying amounts of dissolved and/or suspended substances. In order to be used as water for human use or for industrial use (water technology), they must be undergone to treatments of mechanical, physical or chemical nature, to make them meet the requirements.
Here are some examples of treatments:

  • Quartz filter (QF) or disc filter (DF), for the removal of suspended solids and turbidity;
  • Softener (SF) to reduce the water hardness; this system can be replaced by a RO 1st stage, which, in addition to removing calcium and magnesium, also reduces TDS, silica and organic components;
  • Ultrafiltrazione (UF) + RO 1° stadio (come addolcimento);
  • Ultrafiltration (UF) + RO 1st stage (as softening);
  • Decarbonantore (DT) per la rimozione dei bicarbonati;
  • Iron removal (DF) for the removal of the iron;
  • Sistemi di disinfezione tramite dosaggio chimico, ozono o UV;
  • Decarbonated filter (DT) for the removal of bicarbonates;
  • Activated Carbon Filter (CF), for the removal of chemical elements such as chlorine, solvents, oils, pesticides, etc;
  • Disinfection systems through chemical dosing, ozone or UV;
  • Demineralization mixed bed resins;
  • Ultrafiltration (UF) + RO double pass (as demineralization);
  • EDI (electrodeionization);

In particular, the latest solutions are used to produce ultrapure water, which contains only H20, ions H+ and OH- in equilibrium, and it is characterized by very low conductivity. The processes, required to obtain demineralized water, are:

  • Chemical-physical (the so-called “calcium-sodium” treatment);
  • Through selective extraction of the ions of the salts, (ion exchange filters, mixed bed);
  • Osmotic

Distillation is now used only in very specific areas (such as pharmaceuticals and other similar), given the high-energy cost required. In the other industrial areas, only the reverse osmosis or ion exchange resins are used.
primarie1Basing on the data obtained from existing plants, reverse osmosis is the cheaper solution and this is the reason why today the market no longer requires systems with resins exchange, but only the reverse osmosis.
With this technology, the water produced may reach a purity of up to 3-5 mg/l. To obtain higher quality water, with values less than 1 mg/l, up to 0,055 micro Siemens, it is necessary a double stage installation: a reverse osmosis system, and a subsequent EDI system (Electrodeionization) with ion exchange resins with mixed bed, regenerated by electric current, or in the traditional way with exchange resins regenerated with acid and soda.
In addition to savings of operating costs, the reverse osmosis allows to abolish the use of acid and soda, very dangerous products in their use, transport, handling and storage.

 

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ZERO LIQUID DISCHARGE PLANT

The “Zero Liquid Discharge” (ZLD) plants are constituted by a series of technologies that do not include liquid discharges. The result of the total recycling, which is often necessary for the request of authorities or law, is mainly obtained by:

  • Recovery and recycling of all reusable waste water;
  • Removal by evaporation of the aqueous component (rejections) of not reusable wastewaters (obtained in special evaporators and crystallizers) with consequent recycling of condensate water. In this way, the plant produces a solid waste, which can be disposed in a landfill.

Based on our experience, the system, that now has shown greater stability on the results, combined with a low operation cost, involves the following steps: biological (with or without denitrification), disk filter (DF), UF or MBR , softener (SF), degaser (DT), RO, NF, forced wind evaporator (WE), weather permitting, and multi effect evaporator (MEE), with an eventual recycling of Glauber Salt through chiller.

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scarico zero2In the total cycle, the most critical points are the RO and MEE systems. If the feeding water is not properly treated, the RO can present problems of clogging of the membrane with consequent loss of flow rate, raising of the pressure, consequently raising of the absorbed energy and premature replacement of the membranes. The MEE has high operating costs and frequent chemical-mechanical cleaning those limit the overall system performance.
It is clear that there is no ZLD plant without the use of MEE, but it is possible to design the total plant in order to feed the MEE with the least volume of water to treat. With an appropriate managing of the systems before the RO, it is possible to get a water quality such as to permit the recycling up to 92-93% and ensure the useful life of the membranes for over than 4 years. This allows keeping operating costs to very constant values.
A subsequent step on NF, after reduction of the residual hardness and silica, enables to recover up to 50% of NaCl brine, which can be reused in dyeing or to soften the regenerating water. At this point, the volume conveyed to the MEE will be only 4-5% of the initial volume, with a significantly lowering of evaporator costs. With MEE technology, it is possible recover up to 85-90% of the condensate and also recover the sodium sulfate (Na2SO4 · 10H2O) through crystallization with a chiller, if the concentration of Glauber Salt is high..
So the total recycled water with reverse osmosis, nanofiltration and MEE exceeds the 98,5 - 99%. The remaining water is lost in the biological sludge extracted, in the final residue of salt after the MEE and for natural evaporation process. Currently, the management costs of these so advanced plants fluctuate from 0.75 to 1.2 €/m³, depending on the type of effluent to be treated; in particular they are affected by the initial concentration of COD, TDS, silica and hardness.

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Biological plants

The majority of wastewater containing biodegradable compounds can be treated by biological way, provided that the most suitable environmental conditions are created, verified and guaranteed. This type of system, if properly designed and operated, can remove BOD5, COD, surfactants, colloids, TKN, oil and grease and suspended solids for over 90%, without the use of chemical reagents: it is by far the more efficient and economical available process for these types of wastewater. Furthermore, the amount of sludge generated is as low as possible and, being an organic product, it can be mixed with other organic elements to produce compost. biologici1The basic process consists in the transformation (Oxidation) of the biodegradable and particular soluble constituents in final products that include carbon dioxide, nitrates and water. The responsible for this conversion are a mixed colony of living microorganisms, which proliferate in certain environmental conditions; so it is very important that the following parameters are checked and promptly corrected:

  • PH
  • Temperature
  • Dissolved oxygen
  • Hydraulic time retention
  • Concentration of MLSS
  • Organic charge

The biological wastewater treatment plant (WWTP) includes two interconnected lines: the water line and the sludge line.

 

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WATER LINE:

 

SLUDGE LINE:

Partial Recycling plants

The partial recycling plants include a variety of additional treatments, required for the removal of pollutants in suspended, colloidal or dissolved form, still present in the effluent downstream of the biological treatment (or secondary treatments).

  • Efficiency of removal of organic substance and suspended solids, higher than those obtained with the biological plant, in order to meet more stringent limits to the discharge or to the reuse of the final effluent;
  • Removal of residual suspended solids to improve the efficiency of disinfection;
  • Removal of specific pollutants of organic or inorganic nature (heavy metals, silica) in order to meet more stringent limits to the discharge or for the recycling of the effluent in industrial applications (process water).

The systems of advanced treatmnet are:

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  • Disc filters (DF)
  • Quartz filters (QF)
  • Ultrafiltration (UF)
  • MBR
  • Decoloring resing filters (RF)
  • Softners (SF)
  • Degasers (DT)
  • Reverse Osmosis (RO)
  • Nanofiltration (NF)
  • Saturators (SAT)

A studied combination of previous treatments allows producing water with a quality that it can be used in various industrial processes, respecting the needs of the customer.

 

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Following there are some combinations of advanced treatments to install downstream of a biological plant:

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