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.
In 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.