Water is one of the most important resources for human existence, whether it be for human consumption, agriculture and industry. Ensuring access to safe drinking water is emerging as one of the important challenges of the present century because of steady decrease in the available water resources, industrialization, urbanization, etc. Water in several pockets of the country contains high salinity, and/or harmful contaminants. Desalination of brackish and sea water and water refinement are the major requirements for solving this challenge.
Membrane separation processes such as reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF) have emerged as an effective solution to produce safe drinking water. CSMCRI has pioneered the membrane research and development in the country for desalination of brackish and sea water, and for water refinement i.e., removal of harmful contaminants like pathogens, hardness, arsenic, fluoride, etc., present in water.
Thin film composite (TFC) membrane is the state of the art membrane used worldwide for desalination of brackish water and sea water. CSMCRI has developed technology for producing polyamide TFC RO membrane of size 1m wide x 100m long in a single batch using continuous casting and coating machines. TFC RO membrane is a three layer composite structure of non-woven fabric (100−120 microns), polysulphone (30-35 microns) and polyamide (150-200 nanometers). TFC membrane preparation consists of two stages:
Stage- I: Preparation of a reinforced polysulfone (PS) ultrafiltration membrane having molecular weight cut off (MWCO) value of about 200 KDa by solution casting process and phase inversion to form polysulphone layer over nonwoven fabric.
Stage II: Synthesis of polyamide skin layer of about 150−200 nm thickness on the surface of polysulfone ultrafiltration support by in situ interfacial polycondensation of multifunctional diamine namely m-phenylenediamine with a multifunctional acid chloride such as trimesoyl chloride under optimized conditions.
The TFC membrane thus developed exhibits salt rejection of 94-96% with about 65 liters/m2.hour product water rate under standard brackish water test conditions. TFC membrane of 1 m wide x 500 m long in a batch could also be prepared in a similar way. CSMCRI also has expertise and the necessary facilities for making of spiral modules of the flat sheet membranes.
UF membranes are useful for the treatment of drinking water for the removal of harmful pathogens, suspended solids and colloidal materials, etc.
CSMCRI has developed a process for the preparation of polysulphone, polyethersulphone and polyacrylonitrile based fouling resistant flat sheet ultrafiltration membranes having different molecular weight cut off values (20, 50, 90, 150 KDa) using microprocessor controlled mechanical casting unit. The process involves the continuous coating of polymer solution of desired composition on a non-woven fabric and effecting the phase inversion. This results in the production of fabric reinforced UF membrane with about 30-35 micron thick polymer layer on nonwoven fabric. The membrane cut off value depends on the polymer composition and other membrane casting conditions. The relationships between membrane preparation conditions and the cut off values have been extensively studied by permeating different molecular weight polymers and determining their rejection values by size exclusion chromatography.
Part of the infrastructure given in Table 1 can be utilized for making 1 m wide x 500 m long ultrafiltration membranes at the rate of 4─10 meters/min depending on the specific need.
The institute has developed a process for making polysulphone hollow fiber ultrafiltration membranes having about 100KDa cut off value based on the rejection of 90-95% for polyethylene glycol of 100 KDa. The hollow fibers typically have about 1 mm OD and 0.6-0.7mm ID. Hollow fiber cartridges of different size have been made and extensively characterized for their performance in terms of rejection, water permeability and stability (Figure 2). The major advantage of the HF cartridges is that they can be utilized as either out-to-in or in-to-out permeate mode thus making them easily washable to minimize fouling.
Using the indigenous TFC membranes, CSMCRI has installed brackish water desalination plants to produce 500-5000 Liters/hour potable water from feed water having salinity in the range of 2000 to 7000 ppm in different places of the country (Figure 3). The units operate at 250-300 psi and produce safe and clean drinking water having TDS of less than 500 ppm, which meets the WHO specification for drinking water. Apart from salinity, the units also remove hardness, fluoride, pathogens, pesticides, organics, etc., present in water.
TFC membrane can be utilized for the production of safe drinking water from water containing excess salinity as well as fluoride. The institute has recently licensed to TATA Projects Limited the technology for an integrated RO system for desalination of brackish water containing excess fluoride followed by the chemical treatment of RO reject to remove fluoride before disposal. The institute has designed a mobile RO unit and demonstrated its utility in Nalgonda district of Andhra Pradesh and Amreli district of Gujarat (Figure 5) for the treatment of brackish water containing excess fluoride. The feed and product water analysis data given in Tables 3 shows the versatility of RO for simultaneous desalination and defluoridation of brackish water to obtain safe drinking water.
|RO Feed water||RO Product water|
|TDS (ppm)||Fluoride (ppm)||TDS (ppm)||Fluoride (ppm)|
|Barmer Air force||4288||1.8||570||0.2|
Power supply in many villages is irregular and limited in duration. To overcome this problem, it is important to use non-conventional energy sources like solar-power, oxen, camel, etc., for operating RO plants. These units would be attractive, or rather would serve the purpose, in rural areas, even though they are somewhat costly. The units would also be useful while dealing/confronting with natural calamities. CSMCRI, in collaboration with the Barefoot College (Tilonia, Rajasthan) has set up the first community scale solar-powered brackish water desalination plant with output of ca. 700 LPH (Figure 6). The electricity to power the unit is generated by 2.5 kW solar panels. CSMCRI also installed RO desalination plants which are powered by oxen (West Bengal) or camel (Rajasthan) and produces 500-700 liters/hour of product water from brackish water containing salinity of about 3000-5000 ppm total dissolved salts (Figure ).
Seawater RO desalination can be carried out in a single step using high salt rejection (>99%) membrane operating at a high pressure (800-900 psi) to produce large volume of potable water. Sea water desalination can also be carried out in 2-stages using brackish water desalination membranes (salt rejection 90-95%) to produce small volumes and is highly suitable for coastal villages which usually have small population.
CSMCRI has installed 1000 LPH 2-stage sea water RO desalination plants at Mullimunai, Nelamdur, Karankudu and Thirupallaikudi villages in Ramanathpuram District, TN during 2005-2006 (Figures 8). In all these plants, after preliminary treatment with alum to remove turbidity, sea water is pumped through the RO uni at 600 psi to obtain product water (first stage) containing 6000-8000 ppm TDS, with 35-40% recovery. The first stage product water is again treated with the same or another RO unit at 300-400 psi to obtain potable water containing around 500 ppm TDS (Table 4), with 40-50% recovery. The second stage RO reject water is recycled along with the first stage feed water. The day to day operation and maintenance of the RO plants is being carried out by the local persons who have been trained at CSMCRI. After monitoring the plants for 2 years the plants have been handed over to the local authorities. The institute is providing the technical assistance whenever required.