Biological treatment plant

biological wastewater treatment

Zero liquid Discharge System

The biological section consists in a process in which are present in suspension bacteria, activated sludge (friendly bacteria, protozoa, amoebae, rotifers and other micro-organisms), usually in the form of flakes.
These muds are the basis of the biological oxidation with activated sludge. The role of this biomass in the purification process is to use the biodegradable organic substances present in the wastewater for their nourishment and reproduction and to degrade the contaminants in smaller compounds and less dangerous.
The biological treatment with active sludge is carried out in tanks, which reproduce in artificial environment the same biological mechanisms that occur in nature for the purification of water polluted by biodegradable organic substances. Indeed, by mixing an effluent with activated sludge in which is present a high bacterial charge, it has the same process of self-purification that occurs in nature, but in a faster way and in less space.
The advantage of the treatment of active sludge than the natural purification is that the microbial flora used to treat the wastewater, rather than remain dispersed in the effluent treated tends to agglomerate, forming flakes (organic matter and bacteria) that, when placed in conditions of quiet,
tend to sediment and can be easily separated with ease from the purified water r that remain on the surface (supernatant).

Therefore, the organic matter present in the wastewater is partly mineralized with formation of gaseous products such as CO2, H2O and energy that is harmless to the environment. Another part, highly putrescible, before its disposal must undergo a series of operations that constitute the sludge line of purification plants.
biological wastewater treatmentEssential for all these reactions is the presence of oxygen in the biological tank that allows bacteria to breath, growing and reproducing themselves.

The best way of insufflating oxygen in oxidation tanks is through micro-bubble air diffusing system, this because it allows a better and more uniform distribution of oxygen in all the volume of the basin.
Air diffusers are fixed on a pipeline net located on the bottom of the tank. An oxygen-meter will measure the quantity of oxygen contained in water and accordingly will regulate through an analogical signal the working of turbine compressors.

The advantages of 100% biological treatment than the other traditional technologies are:
– Less operational costs (approx. 0,10 €/m3), due to the very reduced use of chemicals
– Easy in maintenance of the plant
– very low production of sludge
– the sludge produced is not toxic and can be even used as fertilizer in agriculture

MBR (Membranes Biological Reactor) TECHNOLOGY

The MBR technology solution employed in this specific project is based on the use of hollow fibre membranes. This technology presents several substantial advantages that we try to summerize in the

1. The hollow fibre membrane is the most developed technology in the world therefore the most tested at present state of art. So it has be considered as the most reliable in the course of time.

MBR_Technology_012. the hollow fibre membrane technology occupies less space and has operational costs which are lower when compared to flat type membranes technology (operational costs are mostly connected to the quantity of air to provide to keep membranes clean)

3. Wastewater coming from textile processes contains encrusting agents for membranes such as: hardness, salinity etc. A flat type membrane is more subject to the forming of a film on its surface that would reduce the permeability in the course of time.
While the membrane hollow fibre type thanks to its continuous moving in water and to its capillary geometry is substantially reducing such risk

Description of MBR process

Biological treatment by activated sludge is among the most common processes used to eliminate the biodegradable substances from waste waters.
It is based on the biological activity of some bacterial agglomerates which use pollutants present in waste waters to guarantee both their maintenance and their growth. This biological process takes advantage of the tendency of some bacterial associations to aggregate in flocks with good settling characteristics. This allows the separation of the biomass in the sedimentation tanks and its subsequent re-cycle into the reactors, where waste water treatment takes place.

It is important to underline that the main characteristic determining the performance of wastewater treatment, is the capacity of bacteria to agglomerate and therefore to easily settle by allowing an optimal separation from water.
As a consequence, all micro organisms unable to form solid particles are more or less washed away with the clarified effluent, independently of their ability to degrade pollutants present in the waste water. In other words, in conventional plants, micro organism selection is not based on the ability to degrade pollutants present in waste water, but only on their ability to aggregate into flocks large enough to settle and be kept at the bottom of the sedimentation tanks. Such bacteria having size less than 10 Mm are washed away independently of their treatment characteristics.
Selection based only on settling characteristics can create problems well known to active sludge plant operators, such as the growth of sludge rich in filamentous micro organisms which impedes biomass settling, even if they could be excellent pollutant degraders.

This phenomenon known as filamentous bulking prevents a correct functioning of the sedimentation tanks and can cause unacceptable final effluent suspended solids values. Aggregation potential therefore plays a key role in bio-purification processes using suspended biomass, and even a temporary loss of this characteristic can lead to very serious consequences for the plant such as an important reduction of active biomass in the bioreactor due to washed away micro-organisms that leads to a significant decrease in treatment efficiency, and a momentary solid overload in the secondary clarifiers.

Another problem generated by waste water biological treatment is excess sludge production, concentration of pollutants (bacterial biomass, metabolic by-products, resistant pollutants, etc) removed from the effluent by gravity separation, whose treatment and disposal cost can reach even 60% of operational costs.
These economic considerations have increased efforts aimed to: reduce surplus sludge production, obtain a good quality of treated waste-water, substantially reduce by-products, and therefore reducing disposal costs. Recently, a new and innovating treatment technology has been introduced on the market aimed to increase treatment process reliability in relation to the problems described above: Membrane BioReactors (MBR).
A hybrid process born from the simple and clever intuition of substituting the secondary sedimentation step for biomass separation from the treated effluent, with a porous membrane filtration step. These membranes can be placed inside the bioreactors allowing an important save of space. As explained below, in this process, the suspended biomass produced by a “conventional” reactor is retained on specific filtration surfaces which allow the separation of both sludge (consisting of aggregates and/or single cells) and small sized substances.
There are multiple advantages offered by such a change: first, the physical separation by filtration is much more efficient compared to that by clarification, producing a purified effluent often compatible with re-use, with superior qualitative characteristics compared to that coming from traditional plants. In addition, the absence of the secondary sedimentation step means a gain in space for the plants which adopt membrane technology.
With the employment of MBR it is also possible to maintain, inside the bioreactor, elevated biomass concentrations of 10÷12 g SST/l. Values much higher than those obtained with traditional activated sludge technology due to the limits imposed by the sludge sedimentability. The increase of solids retention time so obtained means a higher treatment efficiency of pollutants (even organic macromolecules hard to biodegrade will have the time to be degraded) and in addition, facilitates the development of bacteria with slower growth rates such as, for example, nitrificating bacteria.


Zero liquid discharge plant is composed by means four main treatment:

1 Disc-Filtration Plant (Micro-filtration)

ZERO_LIQUID_DISCHARGE_01The 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 ZERO_LIQUID_DISCHARGE_02even 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 ZERO_LIQUID_DISCHARGE_03declared 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 :
Coloured water
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.
Zero liquid Discharge System

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 module’s 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 operator’s 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.


High pH values in the waste water coming from work processing of many factories are the result in using of alkaline caustic soda and hydrated lime.
The waste water of the cotton textile industry, of the leather industry as well as of many others have mean pH value much higher than 10 – 11, so neutralization process is required before the waste water is treated or discharged into sewerage.
The discharge into sewerage as well as into surface water bodies is presently allowed by the rules being the pH values closed to neutrality or however abiding by local rules. In order to do that an acid must be dosed so that the pH achieved the required value.
The above result is obtained by dosing of sulfuric acid or hydrochloric acid , but the use both of them shows as consequence the increase of conductivity as well as of dissolved solids since these strong minerai acids are releasing into the water their saline components (i.e. sulfates and chlorides) The liquid carbon dioxide (CO2) could be used as alternative to strong acids use; by dissolving it in the water carbonic acid forms. The residual salinity doesn’t increase since by reaction with caustic soda carbonates and bicarbonates form as follows:

1° fase 2NaOH + CO2 = NaHCO3 + H2O
2° fase Na2CO3 + CO2 = 2NaHCO3

However either in case of sulfuric or hydrochloric acid as well as liquid CO2 , commercialized products must be used whose cost has high incidence in the operating cost of the treatment plant.


The use of smoke isn’t dangerous at all and it avoids the handling of dangerous chemicals like sulfuric or hydrochloridric acid. Moreover no overdosing risk can occur with the smoke since being organic acid the overdosing doesn’t reduce pH more than neutrality limit (pH 7- 7.5) soda.

In conformity of fuel type and excess of oxygen which is used to burn the fuel in the boiler, the
quantity of CO2 change. It is in the volumetric range between 6 and 12%.

Most usual concentrations are the following:
NATURAL GAS 120-160 gr of CO2 each Nm3
GAS OIL 180 – 200 gr of CO2 each Nm3
DIESEL OIL (BUNKER) 200-220 gr of CO2 each Nm3

The gurgling of smoke in the alkaline water makes it possible the CO2 and particles removal so that the quality of discharge in the atmosphere is much higher and this is a contribution to environment protection.
The neutralized water which contains smoke particles, oil traces and other soluble substances will enter the waste water treatment plant where the treating cycle will be completed.


The advantages from the use of chimney smoke when compared to sulfuric acid are the following:
• EuroMec have developed a technology which is able to neutralize the alkaline waste water by means of the smoke coming from boilers or cogeneration plants, which contain high quantity of C02. The consequences are remarkable savings and, contemporary, much cleaner smoke discharge in the atmosphere.
• Obtainment of carbon credits and green certificated according to Kyoto Agreements.
• Free of charge” use of CO2 from exhaust smoke and usually available as quantity more than the required one for neutralization.
• Consumption less than H2SO4 as percentage for neutralization of the same quantity of soda.
• No one risk of acidification more than 7 – 7.5 pH value, so that troubles in oxidation section are avoided.
• S04 and Cl don’t increase in the discharged water; this means the possibility to increase the water recovery percentage downstream of biological plant.


The neutralization with smoke chimney can be performed in two ways, in conformity of alkali quantity in the water. When pH is up to 12 or slightly higher the neutralization can take place in the storage tank of the treatment plant.


The smoke are sucked from chimney through a Stainless Steel AISI 304 or AISI 316 L pipe which is connected to sucking units like a pump – ejector or a positive displacement blower. The ejector or diffusers in the storage tank provide for smoke diffusion in the water volume.
The thermal dispersion of the beat of smoke is performed by installation of fins on the pipe or, alternatively, by injection of small nebulized water quantities, whose vaporization provides for smoke cooling.
Smoke with CO2 are mixed with storage tank waste water, so providing for a progressive
neutralization as per above described reactions.
After the neutralization the residual caustic soda is 2 – 3 gr./m 3 which corresponds to a neutralization efficiency higher than 90%.
For any emergency, like stop of boiler or cogeneration plant, the acid dosing will act as emergency
unit providing for neutralization.