ZERO LIQUID DISCHARGE PLANTS AND WASTE WATER RECOVERY
Zero liquid discharge plant is composed by means four main treatment:
1 Disc-Filtration Plant (Micro-filtration)
The Disc Filter is a high-rate filtration device that utilizes an innovative woven polyester pleated filter panel design. The pleated filter panel provides significantly more filtration area than any other woven media flat panel designs and includes a 20 micron absolute media rating.
The pleated panel configuration is stronger than comparable designs and includes a robust pressure-assisted seal that allows the panel to sustain and operate at higher head losses The filter panels are also housed in a trash tolerant filter panel housing, which assures the unhindered flow of water between panels and rejects plastics, algae clumps or other floatables.
The Disc Filter inside out filtration design allows the water to flow into the center drum and then out through the disc filters capturing solids on the inside surface of the media. This filtration characteristic eliminates the need for a separate system for handling floating material and settling sludge. The captured solids are also backwashed into a reject trough using a one-pass spray cleaning system. A backwash cycle is automatically initiated by a level probe in the influent channel with filtration continuing during backwash.
2. Ultra-Filtration Plant
Principal aim of ultra-filtration is the removal of suspended solids that are still residual in water even after the Micro-filtration. The target is to reach a complete removal of solids in order to allow a higher performance of the following step, the Reverse Osmosis.
Suspended solids and solutes of high molecular weight are retained in the so-called reject, while water and low molecular weight solutes pass through the membrane in the permeate. Ultrafiltration is not fundamentally different from microfiltration. Both of these separate based on size exclusion or particle capture. Ultrafiltration membranes are defined by the molecular weight cut-off (MWCO) of the membrane used.
UF processes are currently preferred over traditional treatment methods for the following reasons:
No chemicals required (apart from cleaning)
Constant product quality regardless of feed quality
Compact plant size
Capable of exceeding regulatory standards of water quality, achieving 98-99% pathogen removal
UF is used for pre filtration to Reverse Osmosis plants to protect the RO. Ultrafiltration is an effective means of reducing the silt density index of water and removing particulates that can foul Reverse Osmosis membranes.
Thereby with a good Ultrafiltration system, we achieve
Higher percentage of water recovery through a 3 stage R.O. (Reverse Osmosis)
Longer preservation of R.O. membranes
Lower operational costs derived from a better preservation of R.O. Membranes
So the Design of the Ultra-filtration system is of fundamental importance for allowing to reach the declared parameters reported in the table at page 5.
The U.F. (Ultra-Filtration) system works by exploiting the weight and molecular cut of the U.F. membranes. This allows a separation between water and Suspended solids up to 0,04 microns.
So by pushing water under specific pressures through the membranes any solid having a dimension bigger or equal to 0,04 microns will be retained in a solution that will be called concentrate or reject, while the water which will result released from such solids will be called permeate and this permeate will be sent to the following section of Reverse Osmosis.
The concentrate solution will be returned to the previous Disc Filtration unit.
The Ultra-filtration system herewith proposed is completely automatic and the membranes washings take place automatically when reaching a determined pressure, which is detected by appropriate sensors.
Ultra-filtered water is then collected in a tank and from here forwarded lifted by a system of pumps and sent to the Reverse osmosis Unit
3. Reverse Osmosis
Reverse Osmosis membranes were originally studied for treating clean water rich in salts, so the only task required to the common membranes present on the market was that to retain salts (application commonly used in the sea water desalination treatment).
WaterNext along with its technological partners have over a period of time developed Reverse Osmosis membranes specific to the Textile Industry. The membranes here employed and the system herewith proposed have really nothing to do with the common R.O. described above.
The membranes herewith proposed have been duly studied and tested in the years by Team WaterNext Team in order to be able to work in conditions of :
High temperature water
Water with colloidal substances
Water with Sequestrants substances
Water with high silica content
Starting therefore from a still difficult water the R.O. membranes here employed are able to allow the separation of 95-99% of salts contained in water, the removal of residual colour, the removal of the residual turbidity, allowing therefore to reach a grade of purity that allow its recycling back in the Textile process.
3.a Technical Aspects of R.O.
The Membranes herewith employed are specific semi-permeable membranes, also called selectively permeable membrane, a type of membrane, that will allow water to pass through it by retaining certain molecules or ions.
Reverse Osmosis, commonly referred to as RO, is a process where we demineralize or deionize water by pushing it under pressure through a semi-permeable Reverse Osmosis Membrane.
Reverse Osmosis works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.
The desalinated water that is demineralized or deionized, is called permeate (or product) water. The water stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject (or concentrate) stream
As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure) the water molecules pass through the semi-permeable membrane and the salts and other contaminants are not allowed to pass and are discharged through the reject stream (also known as the concentrate or brine stream), which is then taken to then next stage of RO.
The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it.
It is important to understand that an RO system employs cross filtration rather than standard filtration where the contaminants are collected within the filter media. With cross filtration, the solution passes through the filter, or crosses the filter, with two outlets: the filtered water goes one way and the contaminated water goes another way. To avoid build up of contaminants, cross flow filtration allows water to sweep away contaminant build up and also allow enough turbulence to keep the membrane surface clean.
The result is that the solute is retained on the pressurized side of the membrane and the pure solvent (water) is allowed to pass to the other side.
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The movement of a pure solvent is driven to reduce the free energy of the system by equalizing solute concentrations on each side of a membrane, generating osmotic pressure. By Applying an external pressure the natural flow of pure solvent will work in a reverse way, thus, is Reverse Osmosis.
The separation generated by such Hydraulic pressure will form two flows:
Permeate flow : Will be the pure water (low salts/contaminants water) obtained by the R.O. treatment
Concentrate or Reject flow: Will be the concentrate of all impurities which have been retained by the membranes
In the specific case of 3 stage Reverse Osmosis the system will work in the following way:
1st R.O. stage : 100% of permeate water coming from Ultra-filtration plant is pushed under pressure in the R.O. system. This will give as a result an output composed of :
70% permeate water – 30% reject water
2nd R.O. stage : Reject water coming from previous stage is pushed under pressure in the 2nd R.O. stage. This will give as a result an output composed of :
50% permeate water – 50% reject water
3rd R.O. stage : Reject water coming from previous stage is pushed under very high pressure in the 3rd R.O. stage. This will give as a result an output composed of :
46,7 % permeate water – 53,3% reject water
It is obvious that the membranes employed in the 3 stages are of different type, since more concentrated is water to treat after each stage, more sophisticated will have to be the membranes..
Consequently more concentrated is water to treat, less will be the recovery %.
With a 3 stage R.O. we will be therefore able to recover up to 92% of total inlet water.
In summary, the permeate flow :
Increases (or decreases) according to temperature as this affects viscosity. The variation factor is different for the various types of modules and is often expressed in a table. If the temperature of the feed solution is subject to frequent changes it is often necessary to adjust operational conditions and can penalize plants under manual regulation.
Increases if the operation pressure increases: it is not always convenient or possible to work with high pressures; the resistance limit of modules to the pressure should be respected.
Decreases if the concentration of the feed solution increases: however this relationship cannot be generalized as in the temperature case.
Decreases over the time, due to the irreversible degradation of membranes; the increase accompanied by the worsening of permeate quality always indicates the existence of corrosion or oxidation of the active layer of the membrane.
The quality of permeate flow depends on membrane rejection which is indicated as the difference between the concentrate flow and the concentration feed. In order to select and check membranes and modules it is best to determine the rejection factor referred to just one element (normally sodium chloride). However in order to check the general running of a plant it is convenient to use rejection factors with own standards which, once verified with rather simple calculations, provides a summary of the modules status.
The rejection flow :
Increases with pressure increase, as this accentuates the velocity rate in the crossing over of water and solids
Slightly decreases with temperature increase
Decreases over the time, due to dirt accumulated on membrane surface
Is affected by pH variations in the feed solution
Is generally not affected by variations in the concentration of feed solution, at least till the increase of this last one does not modify the concentration near the membrane surface
The quality of permeate flow does not depend only on rejection flow : it must be taken into consideration that with a constant rejection flow, permeate concentration increases or decreases directly according to variations of the concentration in the feed solution.
Inside the modules (especially if placed in series) concentration increases gradually from inlet to outlet and there is a progressive worsening of the permeate. It is therefore understandable that the real plant performance from a productivity and quality (concentration) point of view also depends on the recovery factor (or conversion factor) of the system. This factor describes the quantitative relationship between permeate and feed solution. Any deviation from the prescribed recovery factor, possibly due to operators choice or to unpredictable external causes, changes the permeate quality. In the case of constant flow (normalized) an eventual worsening of permeate flow quality (which is revealed in an
increase in the conductivity) can be due to variations in the concentration of the feed solution or from the recovery factor – two causes which can be easily detected and eventually corrected. However it can also depend on dirt or chemical damage to the membranes for which nothing can be done.
3.b Automation of R.O.
The R.O. membranes will gradually increase their dirtiness state, since the retained substances will constantly deposit on their surface. Periodically, and according to the increase of pressure in the system (symptom of membranes obstruction), an automatic washing will start.
Membranes obstruction in addition to pressure increase will also cause a decrease of permeate quality and quantity. When such reduction of performance will be in the measure of 10-15% then a chemical washing will be necessary, and this will be achieved by pushing a button on control panel.
4. Evaporation Unit
The multiple-effect series of evaporators is designed for efficiently treating middle and big-quantities of water-based solutions by using any thermal source as power and recycling the produced steam in a sequence of vessels.
The water-based solution is automatically fed into the evaporation vessels, recirculated in the tube heat exchangers, and evaporated through isenthalpic expansion when newly poured in the vessels.
As said above, the vapour produced is used to heat the sequence vessels and heat exchangers and finally automatically condensed in closed loop and extracted as condensate. We hardly suggest the recycling of the distillate as technical water.
The vacuum evaporator system is composed of:
A multiple-level squared load-bearing skid made in AISI 304.
N.3 vertical boiling vessel in AISI 316, equipped with N.3 tube high efficiency heat exchangers made in AISI 316 for the heating of the product to treat.
Inspection portholes for the fast verification of the evaporation section.
– Sight glass with luminaire for the sight inspection of the boiling area.
The crystallizers are composed of:
Monobloc skid with squared framework complete with vertical boiling vessel made in SAF 2507;
High efficiency external heat exchanger in AISI316L;
steam pollutant interception system;
internal scraping shaft driven by gearmotor. The scraper is designed for a full non-stop cleaning of the heating surface avoiding any stratification of the product.
Manhole through which the user can easily check the machine condition by the lit visual hatch;
Flanged boiling section is to simplify any maintenance;
Piping, fittings and valves made in AISI 316.