Harnessing Solar Energy for Water Purification

Dated 7th March 2008

Water Purification – A burning and recurring problem

Water in essential for the sustenance of life Water scarcity and shortage is a serious problem currently encountered globally Desalination is a process of transforming salt water to consumable water

Conventional methods of water purification :

i. Reverse osmosis – Require significant energy to move water through the membrane Highly energy intensive process costly and troublesome maintenance Membrane degrades with timeii. Ion exchange process – Generate large volumes of corrosive secondary wastesiii. Electrodialysis – Extensive pretreatment of the feed water Significant consumption of electricity Produce vast amount of gasiv. Evaporation – Energy cost highv. Distillation – Classical distillers are not appropriate Cost of installation, energy consumption and water cost are high Newer, Simper and Energy efficient process for water purification is the need of the hour

Bourouni et al., Desalination 137 (2001) 167Linda Zou et al., Desalination 225 (2008) 329

Solar Energy for Water Purification – A Historical Perspective! Greek navigators used to boil sea water and condense the vapours onsponges to produce fresh water

Arab alchemists practiced solar distillation using polished Damascus concavemirrors – 1551

Della Porta used wide earthen pots exposed to the most intense heat of the solar rays to evaporate water and collect the condensate drop by drop into vases placed underneath

Lavoisier used large glass lenses to concentrate solar energy on to the contentsof distillation flasks

Charles Wilson, 1872, has designed and erected the first conventional solardistillation apparatus near Las Salinars in Northeren Chile to supply freshwater to the workers and animals of the near by nitrate mine. Brackish water wasused as feed and interestingly the plant was in operation for 4o years

Subsequently several attempts from all over the world have been reportedWhere in solar energy, a cost free energy source, is used for the process of desalination

Deiyannis et al., Desalination 50 (1984) 71

A simple, highly reproducible, economical desalination process, where in the unique potential of the solar energy as well as the adsorption capacity of granularactivated carbon have been effectively used, has been developed.

Process Developed for Desalination of water

Salient Features of the Process:

i. Unique combination of solar energy and the adsorption ability of inexpensive activated carbon

ii. Both the solar energy and activated carbon are economically viable optionsfor producing pure and clean drinking water

iii. After several cycles of desalination process the activated carbon bed covered with salt can be washed and the adsorbent can be reused.

iv. In addition, during regeneration of the sorbent, the salts recovered fromactivated carbon serve as a by product of several chemicals.

No sophisticated equipment – No fuel – But pure drinking water from brine solution

Salient Features of the Process (continued):

v. The test solution used for desalination experiments (1 M Brine solution, 58 wt.% salinity) is twice as saline as sea water (38 wt.% salinity). Thus the process developed will be much more effective for sea water and still more efficient for ground water that contains traces of contaminants or impurities.

vi. The apparatus (reactor) is so designed that it is portable, light in weight, easy to fabricate, handle, carry and operate with simple know-how.

vii. The device designed can be deployed in remote rural areas, sea coasts (for sea water desalination) and also in disaster relief camps and will be helpful in situations demanding clean drinking water.

viii. Simplicity in operation as no moving parts are involved, modest costs of installation and operation, use of low temperature and abundantly available and renewable solar energy for the evaporation of saline water makes the process and device appealing.

ix. Unlike other distillation based processes, the current process works at atmospheric pressure.

x. Can cater effectively to the local decentralized needs of drinking water. Suits well to Indian climatic conditions.

Description of the Experimental Set up

The experimental set up comprises of a rectangular shaped base made of aluminum with a length of 12” and breath of 10”. The metal wall is raised to a height of 4.5” at one end of the base (towards the breath direction) and in the exactly oppositedirection the metal wall is raised to a height of 2”. The door made of glass is fixed on top of the based at a height of 4.5” at one end and at a height of 2” at the other end making an angle of 18º towards the opening end of the door. The two opening ends on the length direction of the base are then raised forming a closed reaction vessel made of aluminum metal and comprising of glass door. The reactor (reaction) vessel is made in to two chambers (top chamber and bottom chamber) placing a partition at about 2” length towards the opening end of the reactor.

The top chamber is filled with granular activated carbon and the bottom chamber is meant for collection of purified water obtained by the process of evaporation followed by condensation. 1 M NaCl solution is continuously fed into the reactor at a specified flow rate from a brian solution reservoir (500 mlpolyethylene wash bottle). The test solution reservoir is connected to the inlet of the reactor through rubber tube of suitable diameter and the reservoir is placed at a sufficient height allowing free flow of 1 M NaCl solution into the reaction chamber loaded with activated carbon. The photographs of the solar energy powered reactor packedwith granular activated carbon are shown.

Experimental set up used

Saline WaterReservoir

Water inlet

Reactor withgranular carbonadsorbent

Water outlet/ Air inlet

10”

12”

2”

4.5”

Aerator forair circulationin the reactionchamber

Apparatus for Desalination Process (a) Side View, (b) Front View

(a) (b)

Granular Activated Carbon used for Desalination Process

Preliminary Studies with Adsorbent

i. The apparatus is loaded with 550 g Granular Activated Carbonii. Mesh size of Activated Carbon : 6 x 15; Specific Surface Area ~ 450 m2/giii. 1 M Brine solution is fed into the carbon bed at a flow rate of 1.5 ml/minute

Experimental Conditions:

S. No.

Date and Day of Expt.

Duration of Expt. Specific conditions Yield of water, ml

1 10.1.2008 Thursday

10 AM - 5 PM = 7 h Regulated flow of 1 M NaCl – 1.5 ml/min

26

2 11.1.2008

Friday

9.30 AM - 4 PM = 61/2 h

Regulated flow of 1 M NaCl – 1.5 ml/min

15

3 23.1.2008

Wednesday

10 AM – 6 PM = 8 h The carbon bed is saturated with NaCl

solution

43

4 24.1.2008

Thursday

8 AM – 3 PM = 7 h The carbon bed is saturated with NaCl

solution

55

Observations:

In all the above experiments the water collected is completely free from NaCl Also promising results are obtained when the carbon bed is saturated with test solution rather than using regulated flow of brine solution

Does Air Circulation (dehumidification) aid Evaporation and Condensation Processes

Studies with Air circulation

Experimental Conditions:

i. The apparatus is loaded with 550 g Granular Activated Carbonii. Mesh size of Activated Carbon : 6 x 15; Specific Surface Area ~ 450 m2/giii. Air is circulated through out the reactor using an aeratoriv. Flow rate of air – 5 ml/minutev. 1 M Brine solution is fed into the carbon bed at a flow rate of 1.5 ml/minute(The carbon bed is saturated with 1 M NaCl solution apriori)vi. The reactor is exposed to sun light for 8 h in each of the following expts.

S. No.

Date and Day of Expt. Duration of Expt. Yield of water, ml

Nature of water at the out let

1 25.1.2008 Friday 8 h 30 Pure salt free water

2 26.1.2008 Saturday 8 h 32 Pure salt free water

3 27.1.2008 Sunday 8 h 28 Pure salt free water

The average yield of water when air is circulated through the reactor loadedwith activated carbon is 3 – 4 ml per minute

Air circulation in the reaction chamber decreased the yield and is not necessary

Studies with out AdsorbentIs Activated Carbon Really Needed?

Experimental Conditions:i. Conc. of NaCl solution – 1 Mii. Volume of NaCl solution taken in the solar energy harnessing desalination device - 200 mliii. Duration of exposure of the system (apparatus) to sunlight – 6, 9 and 10 h iv. No air circulation (no dehumidification)

S. No.

Date and Day of Expt. Duration of Expt. Yield of water, ml

Nature of water at the out let

1 28.1.2008 Monday 10 AM – 4 PM = 6 h 26 Water is saline but not as saline as 1 M NaCl

2 29.1.2008 Tuesday 9 AM – 6 PM = 9 h 27 Water is saline but not as saline as 1 M NaCl

3 30.1.2008 Wednesday 9 AM – 7 PM = 10 h 28 Water is saline but not as saline as 1 M NaCl

4 31.1.2008 Thursday 9 AM – 7 PM = 10 h 22 Water is saline but not as saline as 1 M NaCl

On the average the yield with out adsorbent is 3 ml/ h Also with out adsorbent (activated carbon) the water obtained is not free from NaCl

Presence of Activated Carbon as adsorbent is necessary for water desalination

1 M NaCl solution used as test solution, Purified Water with out Activated CarbonAnd Purified Water using Activated Carbon,

Test solution1 M NaCl solution

Water obtained for desalination process with out activated carbon (Saline in nature)

Water obtained after desalination process using activated carbon (completely free from NaCl)

Useful Chemicals obtained as by products

NaCl crystals formed from 200 ml 1 M NaCl solutionafter desalination process (with out adsorbent)

Desalination Processes on New Apparatus Adsorbent carbon used : Granular Activated Carbon

Details of Granular Activated Carbon:

i. Coconut shell based activated carbonii. Granule size – 6 x 15 with a specific surface area of ~ 450 m2/giii. Amount of carbon packed in the reactor – 1500 g

1500 g Activated carbon bed saturated with 2 liters 1 M Brine solution

The reactor is exposed to sun light for 6 h. The yield of water is 150 ml.The water collected at the outlet is tasted and is absolutely free from NaCl.

Yield during preliminary studies: 25 ml/h

Improved version of the apparatus significantly enhanced the yield and furtheroptimization studies can improve the water output. The process can be extrapolated for the removal of heavy elements, dissolved impurities, particulate matter, organic pollutants including toxic chemicals, dyes and pesticides.