Health Stream Literature Summary - Issue 56 - December 2009

Development of a mobile water maker, a sustainable way to produce safe drinking water in developing countries.
Groendijk L and de Vries HE. (2009) Desalination, 248(1-3); 106-113.

This paper reports on the development and laboratory testing of a solar powered mobile water treatment unit with a production capacity of approximately 500 L/day for use in developing countries. The unit consists of a feed water tank, a membrane module with tubular ceramic membranes and an anodic oxidation disinfection unit. A feed water storage tank is situated 3 metres above the membrane filtration unit to provide a constant Trans Membrane Pressure (TMP) of 0.3 bar. Feed water is transferred to the tank using a hand pump. A control box controls the charging of the solar power panel and also controls the disinfection level of the filtered water. Prior to raw feed water being pumped into the water supply tank it passed through a raw filter nylon screen of 0.13 mm to remove large particles which could clog the membrane inlet tubes. Ceramic tubular ultra filtration (UF) membranes with a total area of 0.8 sq m and pore size of 40 nm are used. The permeate flows to a disinfection unit powered by a 6 V battery which produces a low voltage electrical field to generate hypochlorous acid from the chloride naturally present in water. This inactivates microorganisms and also provides a post-disinfection residual of free chlorine.

In the laboratory tests, the filtration unit was operated for 10 hours a day and the filtration tests were conducted for about 52 weeks. Sewage effluent was used as the feed water. Each morning the module was first backwashed with permeate using compressed air from a container which is initially filled using a bicycle pump. A forward flush of feedwater in combination with a few pulses of compressed air on the permeate side of the membrane is then done. Water production then commenced with continuous filtration all day or sometimes start-stop filtration, simulating use by a person several times a day. When production ceased the tap water valve was closed for a relaxation period until the next morning. These tests showed that the water production rate declined slowly during 10 hours of operation as the membrane became fouled. Failure to perform the daily backwash/ forward flush cycle caused a decrease in water production. Using surface water as the feed water, satisfactory production rates could be maintained for at least two months using this cleaning regime, indicating that membrane fouling was reversible and easily removed. Water quality tests using surface water showed a reduction in turbidity from 100 NTU to less than 0.2 NTU. Colony forming units (22 degrees C) were reduced from more than 1000/mL to 23/mL. Other organisms were reduced to less than 1/mL from initial counts of more than 275/mL (coliforms at 37C), more than 600/mL (E. coli), more than 300/mL (Clostridia), and more than 600/mL (Aeromonas). No bacteriophages were detected in raw water or processed water. The free chlorine concentration in the permeate was 0.2 mg/L. The long-term tests in the laboratory showed no decrease of flow rate (60 L/hour) during 1 year of operation, even with some breaks of 3-4 weeks without production. The solar panel was big enough to provide sufficient back up energy for 3 days of water production. In over 2 years no parts have been replaced or repaired, making the system very reliable.

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