Nanofiltration systems and applications in wastewater treatment: Review article

1. Existing situation of wastewater in Egypt

Egypt population has reached around 96 million according to 2017 census. Most of the inhabitants reside in the small area of the Nile valley and delta. The available water resources are Nile river 55 × 109 m3/y, rainfall 1.3 × 109 m3/y, fossil groundwater extraction 2.2 × 109 m3/y, desalination 0.2 × 109 m3/y, extracting groundwater from renewable resources 6.2 × 109 m3/y, wastewater 3 × 109 m3/y, and reuse of the drainage of agricultural water 13 × 109 m3/y. Egypt, with around 670 m3/year/capita in 2017, still is a country under conditions of water stress (1000 m3/year/capita) [1].

Villages and rural areas suffer from low or almost disappearance of effective wastewater management system, it is important to expand the system of wastewater management throughout Egypt especially in villages and rural areas. Different systems and networks have been installed over the last decade throughout Egypt, a comparison of the different networks used in Egypt are shown in Table 1, Table 2, Table 3, Table 4 [2].

Table 1. Networks of Wastewater Management in Egypt, 2015.

Year	2005	2015
Service region	12 governorates	27 governorates
Subsidiary companies	14 companies	25 companies
Water service coverage – millions	2.5 No coverage 7.5 Rotation system 15 Unacceptable Service	98%
Wastewater service coverage	40%	50% 85% Urban 10% Rural
Water production – annual average	18 m m3/day	27 million m3/day
Water treatment plants	1005 plants	2845 plants
Wastewater treatment plants	149 plants	395 plants
Water distribution networks	74,000 km	167,000 km
Wastewater collection networks	28,000 km	48,000 km

Source: Holding company for water and wastewater in Egypt, 2015.

Table 2. Basic figures of water/wastewater services in Egypt 2016.

Item	Data
Current Population	92.5 million capita
No of WWTP	412
No of affiliated companies	25
Total produced water (million m3/day)	24.9
Average water coverage %	97%
Average water capita/person/day	277 L
Total treated wastewater (million m3/day)	Design capacity :14.1 Actual discharge:10.5
Average Wastewater Coverage % in urban cities	83%
Average Wastewater Coverage % in rural villages	15%
Average Wastewater Coverage %	56%

Table 3. WWTP classification and technologies.

Treatment technologies	No
Activated sludge	99
Extended Aeration	122
Up-flow Aerobic Sludge Blanket (UASB)	5
Trickling filter	29
Rotating biological contactor (RBC)	24
Sequential batch reactor (SBR)	13
Oxidation ponds	85
Primary treatment	2
Tertiary treatment (by UF membrane)	4
Wetland	3
Aerated Lagoon	3
Others	12
Total	412

Table 4. Number and distribution of WWTP in each governorate.

Governorate	No of WWTP	Governorate	No of WWTP
Assiut	5	Matrouh	2
Behira	24	Menufya	19
Cairo	12	Minia	19
Damietta	27	New Valley	8
Garbia	34	Port Said	6
Ismailia	6	Qalubiya	13
Alexandria	17	Qena	5
Aswan	15	Red Sea	1
Beni-Suef	15	Sharqya	29
Dakahelya	44	Sinai	12
Fayoum	25	Sohag	6
Giza	7	Suez	1
Kafr El-Sheikh	22	Luxor	5

In Table 1, Table 2, Table 3, Table 4, a list of developed projects; the currently ongoing projects is shown below. The following projects are funded by international donations or funded by the Egyptian government [3]:

•
To improve the access to clean water, wastewater management, and health services for around 1 M Egyptians at the Nile delta, the World Bank has funded a program to improve the life quality by a $550 M.
•
The Sinai Peninsula has attracted interest to find more water resources through drilling wells; Arab Fund for Economic and Social Development (AFESD) has funded this project, $200 M has been given as a loan to the Egyptian Government.
•
Two projects are currently taking place in southern Egypt. Two irrigation projects are funded by The OPEC Fund for International Development (OFID) and a drinking water sanitation project is funded by the French Development Agency (AFD).
•
Amount of $110 M is provided by the Islamic Development Bank to improve the water and irrigation treatment in Egypt.
•
Upper Egypt drinking water and wastewater management are under development by the support of Switzerland and German development bank KFW through a $250 M grant which represents the second phase of an extended project.
•
USAID funded a project to improve water infrastructure in northern Sinai governorate in 2017 by $50 M.
•
Improving infrastructure throughout Egypt was funded by a loan from Germany by $225 M; a $65.5 M is directed to Assuit Barrage irrigation project.
•
Amount of $2 Billion projects is undergoing to improve sanitation in rural areas funded in Egypt.

The NWRP project started in 1998 (Framework and Guidelines Egypt State of the Water Reporting). The NWRP technique applicable in Egypt, that is currently applicable, has a timeframe till the end of 2017. This NP is employed as an outline for the sectarian tactics and plans on resources and supply of water, WWT and recycle, agricultural expansion, national growth, security of the water supply etc. Regarding the national strategy for water, the document for the “2050 National Strategy for the Development and Management of Water Resources” has considered six political pillars of this strategy [1]:

i.
Development of water resources
ii.
Water usage justification
iii.
Pollution control of existing water resources
iv.
Water irrigation and resource systems restoration
v.
Weather changes adaptation
vi.
Better management of water

Pillars of I, iii and vi have a direct effect on the wastewater reuse, considering the following important objectives:

•
Increasing water awareness through media and communication
•
Controlling the main drains pollution source
•
Spreading the benefits of better water management
•
Water legalization and IWRM techniques enhancement
•
Developing national plans to be applied on governorate level
•
Imposing the industrial buildings to develop wastewater treatment units
•
Spreading the units of water treatment in villages

Before membrane filtration process, wastewater was pretreated by suitable techniques to remove most of the suspended or un-dissolved ingredients like suspended solid, inorganic and organic compounds to protect the membrane from damage due to its high cost (recommendation all of the manufacturing membranes). Residual contaminants are mainly dissolved heavy metals salts, so in the treatment technique, we try to increase the molecular size of the pollutants then selected the suitable membrane filtration procedure for pollutants separation.

There are a plethora of very effective technologies available to reduce the conc. of contaminants capable of fouling NF membrane. They include filters, coagulation and precipitation processes, oil/water separators, adsorbing resins and many others. Two relatively new entrants in the field of pretreatmenttechnologies are MF and UF; MF designed for removal of suspended solids, while UF is designed to remove dissolved macromolecules (organic). These technologies are available in a wide variety of pore sizes and materials of construction, as described earlier [4].

The basic science of the membrane processes can be explained by the heavy metals formation of cationic forms which are initially complexes by a bonding agent which will increase the molecular weight of the bonded cations and to increase the size of the molecule to a size greater than the pores of the membrane which is used for separation. The membrane filtration is distinguished by the following advantages compared to the other conventional separation technologies: low-energy requirements, high selectivity of separation, and very fast reaction kinetics [3], [4], [5], [6], [7].

2. Overview of membrane separation technology

The membrane filtration has two aspects which discriminate membrane filtration compared to other conventional filtration techniques. The first aspect is, membranes are asymmetric and the feed is faced by the pore small side which reduces the pressure drop across the membrane and eliminates membrane plugging tendency. The second aspect is, a strong cross flow over the membrane surface is necessary to operate membrane systems. The cross-flow eliminates the possibility of filter cake build-up. Usually, the filter cake or the concentration polarization in membranes is limited to few microns [4].

2.1. Brief history of membrane filtration

At the beginning of the twentieth century, the recent membrane filtration technology was considered, the membrane was fabricated similarly to the artificial polymeric membrane which is well known today, after the Second World War, the need for membrane filtration has increased and played a crucial role in the drug industry, medical applications, and microbiology field. Later on, the reverse osmosis membrane has been developed and applied through different stages: considered and produced initially in the 1950s, research, and development in the 1960s, and finally, the RO was used on a commercial level in the 1970s.

The RO was developed initially considering desalination of seawater and brackish water for a rural area to get drinking water. After that, the UF has been established and industrialized to cover the gap between reverse osmosis depending on salt rejection and MF based on particle retaining and salt passing technique. To approach an economical operating mode, the cross-flow mode should be employed for RO and UF. The cross-flow mode may results in a processing obligation in the certain operating situation; however, the RO and UF technologies represent a major improvement.

2.2. Types of membrane separation and scope of application

According to the pressure gradient across the membrane, membrane techniques can be divided into MF, UF, NF, and RO. Both RO and NF are classified under the main umbrella of membrane separation by which treated water is pressurized and forced at the face of the semi-permeable membrane. As a result desalted water pass through membrane pores. Fig. 1 represents the filtration spectrum for each type and the applicable range for each type. Correlation of membrane features with ranges of separation is illustrated in Table 5.

Table 5. Correlation of membrane features with ranges of separation.

Types	Reverse Osmosis	Nanofiltration	Ultrafiltration	Microfiltration
Membrane	Asymmetrical	Asymmetrical	Asymmetrical	Asymmetrical Asymmetrical
Thickness Surface film	150 µm 1 µm	150 µm 1 µm	150–250 µm 1 µm	10–150 µm
Pore Size	<0.002 µm	<0.002 µm	0.02 – 0.2 µm	0.2 – 5 µm
Rejects	HMWC, LMWC, sodium chloride, glucose, amino acids, proteins	HMWC, mono, di – and oligo – saccharides, polyvatent anions	Macromoie, cutes, proteins, polysac – charides, viruses	Particulates, clay, bacteria
Membrane material (s)	CA: thin film	CA: thin film	Ceramic, PSO, CA, PVDF, thin film	Ceramic, PP, PSO, PVDF
Membrane Module	Tubular, spiral – wound, plate and frame	Tubular, spiral – wound, plate and frame	Tubular, hollow, fiber, spiral, wound, plate and frame	Tubular, hollow fiber, plate and frame
Pressure	15–150 bars	5–35 bars	1–10 bar	2 bars<

*CA-cellulose, acetate; PSO-polyaulfone; PVDF, polyvinylidene fluoride; PP-polypropylene; HMWC (high molecular weight compounds): 100.000–1 millions mole/g, LMWC (low molecular-weight compounds): 1.000 – 100.000 mol/gm, macromolecules: 1 million mole/gm.

2.2.1. Nanofiltration: the up-and-coming membrane process

After the comparison mentioned in the above section, it was found that NF has the special attraction in different applications such as water reuse, industrial wastewater treatment, and drinking water sectors. So the nanofiltration process through historical development is shown in Table 6.

Table 6. A general overview of historical development of nanofiltration membranes.

Membrane	Persons/Manufacturer	Year
Porous CA membranes (integrally asymmetric)	Reid, Breton, Leob-Sourirajan	1959
CA NF membranes (integrally asymmetric)	Leob-Sourirajan, Cohen	1970
Composite RO membranes	Rozelle, Cadotte, Riley	1970
Composite NF membranes	Rozelle, Cadotte	1976
Polypiperazineamide NF membranes (99% MgSO4 < 60% NaCl rejection)	Cadotte, Steuck, Petersen	1981
Fully aromatic cross linked polyamide NF membranes	Filmtec Co.	1985
Polyethyleneimine NF membranes	Linder, Aviv, Perry, Katraro	1988
Acid/base stable NF membranes	Linder, Perry, Aviv	1988
Chlorine resistant NF membrane	MeCray, Petersen	1989
NF membrane modified from RO membranes by acid, base, oxidant treatment	Strantz, Brehrn, Cadotte	1989
Solvent resistant NF membrane	Black, Shavit Perry, Yacubowiez, Linder	1990

2.2.2. Why nanofiltration technology is needed?

RO membranes have been developed and a class of membranes has been fabricated to be able for retaining all dissolved salt ions and even the organic solutes with no charges. In addition, UF membranes with special pore size can reject any molecular weight higher than 10,000 gm-moles and can be used efficiently for various industrial purposes. What is really needed to separate the solute from the solution for molecular weight range from 500 to 10,000 gm-moles?

2.2.3. The start of new technology and membrane classification

Dr. Peter Eriksson named the new class of membranes in market application NF membranes at 1984. The term NF is related to the estimated pore size in a membrane characterized by MW removal. The new membrane technology has initiated hat we can call the fourth class of membranes operating under pressure driven operation. NF is distinguished by the ability to separate small solutes from solution by two mechanisms. The first mechanism, which is well admitted in the science community, is separating molecules based on their charge in water which is known ionic separation of NF. The second mechanism is sieving according to the molecular weight of uncharged solutes.

2.2.4. Current situation on nanofiltration

While RO and UF usage in water and wastewater treatment is increasing gradually, NF applications are increasing exponentially in water and wastewater treatment, other applications in the industry like separation of solute or chemical from solution, producing bio-materials, drug industry, and flavors. In addition to producing different chemicals using NF, a recovery of fine chemicals from outlet streams is widely used in industrial applications like in medical applications and feed additives. With all such applications, NF is a major player in the separation technology in current commercial applications. Also, NF membranes are used now to replace RO in different applications like drinking water and extracting fine and expensive materials to gain profits and reduce energy expenses.

2.2.5. Future of nanofiltration

RO and UF have been used widely for different applications but their applications are still limited and hard to be extended further, NF applications are expanding and replacing other membrane filtration techniques. NF membrane is composed of different materials and its preparation is flexible either by employing RO membrane polymers such as cellulose acetate and polyamide polymers in addition to other chemically resistant polymers. Currently, NF membranes are also made of ceramic materials to withstand high temperature. The flexibility of preparation and the variety of raw materials for NF preparation will increase and spread its application in different processes. With such flexible raw material selection and easiness to be modified for different applications, NF will be soon the major and most used membrane filtration technology which requires that research community should focus more on NF development.

2.3. Types of nanofiltration membrane

Types of the membrane are classified according to the membrane structure and pore shape into isotropic micro-porous, nonporous, dense, electrically charged, asymmetric, ceramic, and liquid membranes [8], [9], [10], [11].

2.3.1. Typical flow configurations

Usually, two flow configurations are distinguished in membrane systems as shown in Fig. 2:

•
Cross-flow with concentrate recycle (CFCR); and
•
Flow system with a dead-end.

Cross-flow (also defined as tangential flow) filtration is conducted by employing a high-pressure feed water flow across the membrane. The solution is divided into two parts, a part passes through the membrane or filtered which is called permeate and the remaining just flow marginally with the membrane surface without separation or filtration which is called the reject or concentrate. The concentrate composed of all rejected salts and it is usually concentrated with all undesired materials.

The flow system that contains dead-end unit is operated by accumulating reject until backwashing is required. The backwashing process flushes and disposes of all the accumulated concentrate using a washing liquid volume of 2–5% of the total inlet solution. The cross-flow helps to preserve the uniform flow rate of permeate and help to keep a longer membrane life by eliminating irreversible membrane fouling.

2.3.2. Nanofiltration membrane material and configurations

The NF membranes are characterized essentially by chemical and physical compatibility with process liquors, pore size distribution, surface chemistry, porosity, and cost. The membrane functionality depends on three layers: an active layer, porous supporting layer, and macroporous structure underneath. The active layer properties determine the permeability of a certain component and hence the selectivity of a certain membrane for a separation process. The supporting layer helps to modify the mechanical properties. And the last layer is a macroporous layer below the medium layer.

2.3.3. Configuration of nanofiltration membrane elements

The membrane surface working area per unit of membrane element volume range for different membrane configuration, Table 7 Characteristics of the principal module designs [12], [13]:

•
Plate and frame module [60–300 m2/m3];
•
Tubular membrane module [60–200 m2/m3];
•
Spiral wound module [300–800 m2/m3]; and
•
Hollow fiber membrane module [20 000–30 000 m2/m3].

Table 7. Characteristics of the principal module designs.

Characteristics	Spiral Wound	Hollow Fiber	Tubular	Plate and Frame
Packing Density (m2/m3)	800	6000	70	500
Required Feed Flow (m3/m2-s)	0.25–0.50	∼0.005	1.0–5.0	0.25–0.50
Feed Pressure (psi)	43–85	1.4–4.3	28–43	43–85
Membrane Fouling Propensity	High	High	Low	Moderate
Ease of Cleaning	Poor to good	Poor	Excellent	Good
Feed Stream Filtration Requires	10–25 µ filtration	5–10 µ filtration	Not required	10–25 µ filtration