Submerged membranes for tertiary filtration.

1. 1. THE SUPPORTED HOLLOW FIBER IMMERSED MEMBRANE ZeeWeed®
2. ZeeWeed500D® supported hollow fiber membranes are in fact the most widespread submerged organic polymeric membrane technology in the world used in the solid-liquid separation of aerated mixtures from biological plants for the treatment of municipal and industrial wastewater.
3. The supported hollow fiber membrane has a structure and geometry that certainly offers the greatest advantages, low land occupation and reduced operating costs, among the membranes currently available on the market for the treatment of primary water (potabilization) and secondary water (tertiary refinement treatments) characterized by medium-high suspended solids content and for large-sized plants. In fact, filtration devices that use this type of membrane:
4. * can develop a high density of filtering surface per unit of occupied volume;
5. * Can treat water with high suspended solids content;
6. * have a specific energy demand (kWh/m3) among the lowest on the market;
7. * are very reliable and robust, limiting extraordinary maintenance requests to a minimum and have an expected life time among the longest among the products offered on the market;

• They are highly reliable and robust, minimizing the need for extraordinary maintenance, and have one of the longest expected lifespans among the products available on the market.

The ZeeWeed® submerged membrane devices operate in an OUT-IN configuration, meaning the filtration direction goes from the outside of the hollow fiber to the inside. As a result, the suspended solids that are removed are concentrated on the outer surface of the membrane, while only the permeate (clarified water) reaches the inner channel of the fiber (lumen), from where it is collected and directed towards discharge or final use.

The introduction of ZeeWeed® modules into the market, aided by recent technological innovations that ensure a significant reduction in energy consumption, has made it possible to use hollow fiber membranes in complex applications with high suspended solids concentrations, which could not be filtered using membrane technology just a few years ago. The ZeeWeed® hollow fiber is particularly robust, capable of operating with high suspended solids levels, and is “supported,” meaning it consists of a tubular fabric externally coated with a macro-porous polymeric membrane (high void/occupied volume ratio), which acts as the filtering element. The filtration grade of this membrane falls within the ultrafiltration range (0.04 µm nominal), allowing not only the retention of solids but also a substantial reduction in bacterial load, indirectly in viral load, and a good portion of colloids.

The membranes are submerged in the aqueous suspension to be treated, and filtration is carried out by drawing the water into the hollow fibers using a pump that, by creating a slight vacuum, draws clean water (permeate) into the fibers, preventing the passage of suspended material. Once the water permeates into the membrane, it flows freely along the fiber and is pumped to the collection tank, where it is then directed to discharge or reused.

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The ZeeWeed® supported hollow fiber membranes are backwashable: it is possible to reverse the filtration direction and send a pressurized flow of water into the hollow fiber, creating the conditions to remove the solid material that may have accumulated on the surface of the membrane, thus restoring its filtering capacity. ZeeWeed® membranes are capable of withstanding TMP in both directions up to 90 kPa. In practical terms, the ability to perform backwash cycles at the same or higher operating pressures than the filtration cycles is crucial for effective removal of the filtration cake (the material accumulated on the filter), thus ensuring proper control of the membrane permeability and maintaining a constant filtration capacity over time.

To control the natural tendency to clog that occurs during the filtration process, the filtration modules are equipped with an air blow system that ensures, through increased turbulence near the fibers, a minimization of biomass deposition on the fibers themselves.

Composition of filtration devices with ZeeWeed® membranes

The ZeeWeed500D® membrane represents the technological core of the filtration device, which consists of three main elements:

The Module

The module is the component that groups together multiple ZeeWeed® membranes. It can consist of several hundred ultrafiltration membrane fibers joined between two end caps, from which the permeate is extracted. For direct filtration applications (tertiary treatments or primary water treatment), two models are available, distinguished simply by a different filtration surface area.

The Cassette

The cassette is the unit in which the modules are assembled, in a number that varies depending on the type and model selected. The permeate extracted from the modules is collected in a pipe located at the top of the cassette, and from there, through a dedicated connection, it is directed to the permeate header shared by all the cassettes in the same train. Each cassette is also equipped with a connector that distributes the air needed for membrane scouring beneath the modules, through a specific number of air diffusers.

Cassettes are available in three different models: two differ in the maximum number of ZW500D 440 modules they can accommodate, and a third model that can only house ZW500D 350 modules.

All cassettes can be partially populated, meaning they can be equipped with a lower number of modules depending on the specific filtration surface requirements. The minimum number of installable modules is half the maximum capacity.

The Train

The train is the modular unit of the ultrafiltration system that incorporates one or more cassettes. A single train or multiple trains operating in parallel form the membrane filtration unit (MFU). Each train is served by a dedicated pump for permeate extraction and a dedicated blower for membrane scouring. To perform recovery/permeability checks and maintenance activities, each train must be isolatable from the rest of the system. The chemical dosing system required for membrane cleaning procedures is usually shared among the filtration trains. Depending on the size, the tanks that house the cassettes can be built on-site in concrete or constructed using prefabricated metal structures.

Each train containing submerged membranes produces two separate flows: the permeate extracted from the membranes, and a concentrated suspension stream that is discharged to control the concentration of suspended solids in contact with the membranes. The permeate flow is channeled to a main header, then directed to a storage tank, and from there, by overflow, either discharged to the final receiving body or sent for use, reuse, or recovery.

2. ZW500D Submerged Membrane Filtration Systems

Pretreatment

The reliability and performance of a membrane filtration unit (MFU) for tertiary treatment (TT) or drinking water treatment (DW) are strongly influenced by the pretreatment steps and processes the raw water undergoes before reaching the filtration section.

Regarding the requirements for the screening system upstream of the filtration unit, the following guidelines indicate the required filtration grade based on the type of application:

Table 1 – Pretreatment

It is essential to properly size and select the most appropriate pretreatment to be installed upstream of the UF section to ensure optimal performance of the MFU and extend the lifespan of the filtration devices. The presence of a correctly selected screening pretreatment ensures mechanical filtration capable of removing all particulate matter suspended in the fluid to be treated, which could otherwise damage the membranes. In this regard, the maximum allowable filtration grade is 0.5 mm.

Depending on the hydraulic profile and the plant’s capacity, various screening systems may be used, such as pressurized screens, automatic backwashing screens, or rotary drum microscreens. What is crucial for the safety of the filtration process is that 100% of the incoming flow must pass through screening, with no possibility of bypass.

Pollutants in dissolved form or smaller than the membrane pore size can only be removed if they are first transformed into insoluble compounds or into larger particles. The chemical pretreatments that are commonly used upstream of the MFU include:

• Coagulation
• Chemical oxidation
• Addition of powdered activated carbon

Depending on the quality of the feed water entering the MFU, a chemical pretreatment such as clarification-flocculation with FeCl₃ or dosing of PACl with rapid mixing and a contact time of about 10 minutes may be required to promote floc formation before the filtration process. This not only helps remove dissolved pollutants from the feed water, but also reduces the likelihood of fouling phenomena, thereby optimizing membrane performance.

In all cases, the chemical pretreatment must be optimized solely for the removal of solids by membranes and not for achieving the final water quality standards for the filtered water.

The Filtration Process

The core of any tertiary treatment plant or surface water treatment system for drinking or industrial use is the Membrane Filtration Unit (MFU). This unit consists of the filtration devices (membranes and cassettes), auxiliary components (pumps, blowers, instruments, valves), the interconnecting headers and fittings, and a complete electrical system with a control panel for managing and monitoring the filtration process.

The MFU can be installed in custom-built tanks, typically made of reinforced concrete (R.C.), or, if built using metal structures and/or transportable plastic components, the MFU will also include the membrane housing tanks as part of its structure.

The filtration process in the MFUs consists of a sequence of operational cycles that follow a well-defined logic depending on the type of wastewater to be treated and the required performance of the filtered product. These cycles include: permeate extraction (permeation), membrane shaking and scraping with medium-sized air bubbles (aeration), filtration flow reversal (backwash), membrane deconcentration and cleaning (CIP – Cleaning in Place), maintenance cleaning (MC) performed periodically, and recovery cleaning (RC) performed occasionally.

During the filtration process, the suspended solids present in the raw wastewater, retained by the membranes, gradually accumulate in the membrane containment tank until a predetermined concentration threshold is reached. Depending on the selected operating mode, the suspension in the membrane tank can be maintained at a preset suspended solids concentration by means of continuous purge equal to a calculated fraction of the raw water flow fed to the filtration system (Feed and Bleed mode), or it can be deconcentrated through partial or complete emptying of the membrane containment tank (Concentration/Deconcentration).

At frequencies and durations that depend on the characteristics of the raw water to be treated and the required filtration yield, the filtration flow is reversed (backwash) to break the concentration gradient that formed during the previous permeate extraction cycle and to help detach the portion of suspended solids that adhered to or began to penetrate the membrane. The simultaneous injection of air at the base of the filtration device generates the turbulence necessary to remove solids detached from the membrane surface and to bring the aqueous suspension mass into a better-mixed state, breaking the concentration gradient near the filter and thus improving membrane permeability.

These (backwash and continuous or intermittent aeration, depending on the case) permeability maintenance cycles are combined with membrane cleaning cycles aimed at removing substances that adhered (organic substances) or precipitated (inorganic substances) on the membrane during filtration. Again, this involves emptying the membrane tank followed by a backwash cycle. The backwash cycle is enhanced by adding specific reagents designed to remove adhered substances (NaOCl) or precipitated substances (citric acid, possibly supplemented with an inorganic acid to adjust pH), followed by a soaking phase necessary to complete the reaction that removes the fouling agent.

Finally, with a much lower frequency than the MC cycles, more aggressive membrane cleaning cycles, called Recovery Cleaning, are periodically performed. The washing procedure is very similar to that of MC but is carried out using more concentrated reagent solutions and longer soaking times. The purpose is to restore the original permeability levels of the device.

Filtration Modes

Depending on the particular operating mode adopted, the operating criteria by which the MFU for tertiary treatment will function are defined, namely:

• Deposition
• Feed and Bleed
• Full Drain

The choice of criteria is made based on the type of application and the corresponding applicable net flows, the solids load arriving on the membranes, and the desired recovery percentage.

Below are typical values that can be adopted for the MFU depending on the possible types of wastewater to be treated:

Full Drain Mode

In this mode, the MFU operates in dead-end mode without the need to aerate the membranes or perform backflushing to keep solids suspended in the feed. The feed flow to the MFU is equal to the permeate flow produced during filtration. At the end of a production cycle, the train undergoes a sequence of backwash and aeration cycles, after which the membrane tank is drained to remove the mass of solids retained in the tank and send them to discharge or the sludge treatment line.

Feed and Bleed Mode

In this mode, feed and purge occur simultaneously. Aeration is continuously active to keep the solids contained in the feed water in suspension. Flow reversal cycles (backwash) are performed at regular intervals to minimize solids accumulation near the membrane surface. A constant portion of the concentrated solution in the MFU is continuously purged to maintain the solids concentration within the tank within preset limits. Therefore, the feed flow to the MFU (Feed) is always equal to the sum of the permeate flow extracted and the concentrated solution purged (Bleed).

Full Drain Mode

In this operating mode, the MFU is continuously aerated to keep solids in suspension, and backwash cycles are performed at regular intervals. The instantaneous feed flow equals the permeate flow produced, and at the end of each production cycle, the filtration train is backwashed and aerated before being drained.

MFU Feed

Membrane trains are continuously fed during the filtration process. Feed interruption is expected only for a single filtration train during MC and RC cleaning cycles or during deconcentration phases (membrane tank draining).

MFUs can be configured with either gravity-fed or forced-feed systems:

• Gravity-fed: The water exiting the pretreatment section is at a higher elevation than the water level in the membrane tanks, allowing water to flow by gravity.
• Forced-feed: The water exiting the treatment section is at a lower elevation than the water level in the membrane tanks, requiring it to be pumped up.

For both configurations, a common feed/distribution channel shared by all filtration trains constituting the MFU is usually provided. This channel must be designed to ensure uniform distribution to the filtration trains and be sufficiently sized to fill them at the end of the deconcentration phase. Finally, to improve hydraulics in the membrane tanks, it is recommended to configure the system—comprising the feed/distribution channel and membrane tanks—so that the water to be filtered enters at the bottom of the membrane tanks.

Discharge of Concentrates and Spent Solutions

The feed system is complemented by the system for the removal of concentrated aqueous solution/suspension from the membrane tanks and the reagent solutions at the end of the membrane cleaning cycles.

Concentrate Discharge

For MFUs operating in Feed and Bleed mode, a purge channel or piping can be used to return the concentrated solution to the head of the plant or direct it to the sludge storage and thickening section. This channel must be positioned so that the concentrated solutions/suspensions feeding it can exit the membrane tanks from their top. This arrangement optimizes hydrodynamics inside the membrane tanks by fixing the feed water inlet at the bottom of the tanks. Additionally, the purge system must include a tank drainage system to remove concentrates before performing MC and RC cleaning operations. This drainage system must be designed to complete emptying operations in a sufficiently short time.

For MFUs configured to operate in the two dead-end filtration modes (Deposition and Full Drain), a drain at the bottom of the membrane tanks must be provided through which concentrates are sent to the storage and thickening section. This system must also be sized to allow the operation to be completed within a reasonable time.

In both cases, depending on the elevations of the MFU and the concentrate storage/thickening section, transfer between these sections can occur either by gravity or with the help of pumps dedicated for this purpose.

Management of Spent Solutions

Depending on the specifics of the project, two methods for managing spent solutions may be followed:

• Discharge and neutralization of the spent solution in a dedicated tank.
• Mixing with the influent and recirculation to the head of the pretreatment section.

Permeate Extraction

The permeate extraction cycle is carried out by applying a slight vacuum inside the hollow fibers. Both reversible lobe pumps and centrifugal pumps can be used; however, reversible lobe pumps are recommended as they significantly simplify the plant structure and, due to a considerable simplification of automation, provide substantial economic benefits.

Backwash Cycle

As mentioned, flow reversal cycles lasting a few seconds are periodically executed to mitigate the formation of concentration gradients near the membrane and slow down membrane fouling.

Solids accumulating on the membrane surface during repeated permeate production cycles cause an increase in transmembrane pressure (TMP). Depending on the filtration modes adopted, several flow reversal cycles without membrane aeration (BACKPULSE) and some cycles with aeration (BACKWASH) are performed daily to keep TMP within acceptable values, control energy consumption, and minimize the frequency of Maintenance Cleaning (MC).

The backwash phase (BW) aims to detach solids accumulated on the membrane surface and disperse them in the filtration tank, thereby reducing operating TMP. Depending on the influent characteristics and selected operating mode, the backwash cycle may or may not be followed by a membrane tank draining cycle, whereas Backpulse cycles are never followed by draining.

Membrane Aeration

The primary purpose of air injection into the membrane tanks is to control the polarization layer, or more precisely, the solids concentration gradient that forms on the filtration surface during permeate extraction.

Membrane Cleaning Cycles

Depending on the efficiency of previous treatment stages and characteristics of the influent water (alkalinity, hardness, TOC, bacterial load, etc.), both organic fouling and inorganic scaling of membranes must be addressed.

Two strategies have been defined to manage natural membrane fouling: the first aims to limit fouling and reduce the resulting permeability loss; the second is necessary when the first is insufficient and aims to restore the original membrane permeability.

These two procedures are called:

• Maintenance Cleaning (MC): A chemically enhanced backwash (CEB) procedure performed by immersing the cassettes in a low-concentration cleaning solution prepared by dosing the reagent into the CIP circuit and discharging it into the membrane tank. Usually, a backwash with permeate water precedes the MC, and after MC, a rinse cycle is performed before returning the filtration train to production mode to remove any residual chemicals.

MC cleaning uses permeate stored in a dedicated tank (CIP Tank), from which a specially selected pump recirculates the solution for the procedure’s defined duration.

A summary of MC operational parameters is provided below:

• Recovery Cleaning (RC): A cleaning procedure using chemicals to restore membrane permeability. Operationally similar to MC, but performed with higher reagent concentrations and longer soaking times. Like MC, RC is preceded by a permeate backwash and followed by a rinse before returning to production.

A summary of RC operational parameters is provided below: