Socially and economically acceptable drinking water supply from
rooftop harvested rainwater and improved solar disinfection
M. T. Amin1*, K. Y. Kim2, M. Y. Han2
1
Alamoudi Water Chair, King Saud University, P. O. Box 2460, Riyadh 11451, kingdom of Saudi Arabia
2
Civil and Environmental Engineering Department, Seoul National University, Shinrimdong, Kwanak Gu, Seoul,
151-742, Republic of Korea
* Corresponding Author: Asst. Professor, Tel. +966-14673737, Fax +96614673739, E-mail: [email protected]
Abstract
A comprehensive experimental research has been conducted to improve the efficiency of solar based disinfection of
harvested rainwater identifying the contribution of ultraviolet radiations and heating (thermal) effects towards
disinfection. Several microbiological quality parameters including total and fecal coliform, Escherichia coli, and
Heterotrophic Plate Count were examined using several 2L PET bottles filled with stored rainwater under different
weather conditions and reaction pH and turbidity values. At first step, the efficiency of SODIS was enhanced by
simple techniques including different backing surfaces of PET bottles and by lowering the initial turbidity and pH
values. Further improvement was made by simply concentrating the sunlight by using a wooden box with open wings
covered with aluminum foil for reflecting the sunlight radiations to the base of the box where several PET bottles were
kept with stored rainwater. New modifications were named as solar collector disinfection (SOCO-DIS) which
disinfected the stored rainwater at moderate weather conditions. To achieve the complete disinfection under weak
weather conditions, SOCO-DIS system was further improved by wrapping the PET bottles with locally available heat
resistant plastic bags to enhance the thermal effects and by adding inexpensive food preservatives such as lemon
and vinegar in stored rainwater. Both lemon and vinegar acted as catalysts and were used in acceptable quantities
for potable purposes. A complete disinfection of stored rainwater was achieved at moderate and even at weak
weather conditions when SOCO-DIS system was used in combination with heat resistant plastic bags and
lemon/vinegar, respectively.
Key words: Lemon, Microbial, Potable, Plastic bag, Rainwater, Solar disinfection, Vinegar
Introduction
Rainwater harvesting (RWH) is gaining interest as a safe drinking water supply option as an
alternative source of potable and non-potable water supplies (Kim et al., 2005; Meera and Ahammed,
2006; Ghisi and Ferreira, 2007; Amin and Han, 2009a; Sturm et al., 2009; Lee et al., 2010; Nazer et al.,
2010). The lack of scientific and engineering knowledge, such as uncertainty of microbial quality,
unavailability of proper end of point treatment in developing countries, however, often prohibit the use of
rainwater. Although, simple Solar Disinfection (SODIS) by using a polyethylene terephthalate (PET)
bottle was recommended by World Health Organization (WHO), there is a limitation of this technology
mainly due to incomplete disinfection at moderate and weak weather conditions and there have been no
detailed studies for the evaluation of SODIS for rainwater disinfection in the context of supplying potable
water in rural/semi-urban areas of developing countries.
SODIS is shown to be an effective method in the treatment of contaminated water at the
household level for treating small volumes of drinking water (Gelover et al., 2006; Ubomba-Jaswa et al.,
2009). Effectiveness of simple SODIS was evaluated by exposing rooftop harvested rainwater (Amin and
Han, 2009b) to direct sunlight at different weather conditions depending upon sunlight radiations.
Rainwater with different initial pH, turbidity and dissolved oxygen (DO) values was used in simple and
commonly available and an incomplete disinfection of rainwater, even under strong weather conditions
and for an exposure time of about 8-9 hours, led to the idea of enhancing the thermal and optical effects
of sunlight by the use of a simple wooden box with aluminum foil wrapped on open wings, termed solar
collector disinfection (SOCODIS) (Amin and Han, 2009c).
In the SOCODIS system, the optical and thermal effects are enhanced because of the
concentration of radiation after reflection by aluminum foil. The overall disinfection efficiency of the
SOCODIS system was 30 to 40% higher when compared with that of SODIS. Rainwater was disinfected
completely under strong weather conditions and, also, in moderate weather but at low pH. The main
problem with the SOCODIS system, however, was that of incomplete disinfection under weak weather
conditions and also to some extent under moderate weather conditions.
To overcome the problems of incomplete disinfection under weak weather conditions, some
simple techniques i.e. the addition of commonly available and cheap food products/preservatives to
increase SOCO-DIS efficiency by decreasing pH to a minimum acceptable level was used. Also, the
efficiency of this system was further enhanced by finding the ways to improve the thermal effects of
SODIS and hence the synergistic effects of ultraviolet (UV) and infrared (IR) radiations by obtaining the
desired temperature and this was achieved by using simple techniques, like wrapping the PET bottles with
locally available heat resistant plastic bags to enhance the thermal and synergistic effects by increasing
the temperature and thus to overcome the problems of incomplete disinfection under moderate weather
conditions.
Materials and methods
The water samples were taken from the underground storage tanks of a rainwater facility,
installed in two buildings on campus at Seoul National University in Seoul, South Korea. The rough
schematic diagram of the RWH system and a detailed description has already been published (Han and
Mun, 2008). Basic physicochemical parameters, including pH and turbidity, were analyzed together with
bacteriological parameters, while the discussion is focused mainly on microbial inactivation during
analysis. The water quality analysis was carried out in accordance with the guidelines described in the
Standard Methods (APHA, 1999). Turbidity was measured using a Turbidimeter (Hach 2100, USA), pH
and water temperature were measured using a pH meter (Hach Sension 1, USA), while DO and EC were
measured using the DO meter (Sension 378 – Hach comp. USA). Sunlight radiation was monitored onsite with a SP-110 Pyranometer (Apogee Instruments Inc., Logan, USA) connected to a datalogger (DT80
Series 2) recording 1 minute averages in Watt/m2 (W/m2).
Stored rainwater was exposed to direct sunlight under different weather conditions in 2L PET
bottles and the removal of TFC, E. coli and HPC were monitored by taking the samples at regular time
intervals of 1 hour. Non-treated controls were maintained in the same environmental conditions but
shielded from sunlight by covering the PET bottle with aluminum foil kept under room conditions. In
case of simple SODIS, one used commercially available 2-L PET bottle containing a 1.7-L rainwater
sample and with reflective backing i.e. with an aluminum foil backing was exposed to direct sunlight at
the rooftop (Amin and Han, 2009b). In a SOCO-DIS system, a simple box made of five wooden pieces,
four covered with aluminum foil as side wings and one as a base (Fig. 1) containing 4 PET bottles were
exposed to direct sunlight with each PET bottle containing 1.7-L of stored rainwater (Amin and Han,
2009c).
Figure 1 Development of socially and economically acceptable Solar based disinfection methods using rooftop
harvested rainwater
Locally available UV blocking sheets were used to determine the effects of different wavelengths
on rainwater disinfection. Simple heat-resistant plastic bags, normally used to wrap new shirts, were used
to enhance the thermal and synergistic effects effect by increasing the temperature of the water inside
PET bottles. Both lemon and vinegar were used as commonly available food products/preservatives to
enhance the disinfection efficiency by decreasing the pH to around 3.
Results and discussions
Effects of sunlight radiations on microbial inactivation
Weather was categorized into three types, depending on low, medium and high sunlight
radiations. Weak weather represents sunlight radiation of 200–450 W/m2, with an average value of about
300 W/m2 , moderate weather represents sunlight radiation of 450–700 W/m2, with an average value of
about 580 W/m2, and strong weather is represented by sunlight radiation of 650–1000 W/m2, with an
average value of about 880 W/m2 for about four months from May to August.
Fig. 2 shows the microbial inactivation in simple SODIS based on sunlight intensities. The pH of
the rainwater samples was neutral and initial turbidities were low (<5 NTU). Almost all experiments were
repeated about five times. Results are presented based on the mean average values of each point. Time 0 h
corresponds to 9 am, when the irradiation of rainwater samples commenced; irradiation ended at 6 pm,
corresponding to 9 h.
1000
400
FC (CFU/100ml)
500
TC (CFU/100ml)
1200
800
300
600
200
400
100
200
0
0
0
2
4
6
2
4
6
8
4
6
Time (h)
8
2000
250
200
1500
HPC (CFU/ml)
E. coli (CFU/100ml)
0
8
150
Control
Weak Sunlight
Moderate Sunlight
Strong Sunlight
Drinking Guideline
1000
100
50
0
500
0
0
2
4
6
Time (h)
8
0
2
Figure 2 Microbial inactivation under different weather conditions using only reflective containers
Initial lag period showed persistent nature of microorganisms against sunlight effects for about
couple of hours at weak and moderate weather conditions. Radiations effects were critical during middle
stage at peak radiations for mild or cold weathers but for hot weather, these effects were even effective in
afternoon periods except for E. coli. A direct correlation of radiation and inactivation was observed and
TC removal increased from 50 to 80% with a threefold increase in sunlight radiation. Furthermore,
inactivation difference of TC and FC or E. coli decreased from about 10% under weak sunlight to about
3% for strong sunlight intensities. Simple SODIS remained ineffective for complete disinfection even at
strong sunlight radiations and microbial inactivation did not meet the potable guideline values i.e.
0CFU/100ml for TC, FC and E. coli and 10CFU/ml for HPC, however, the relative removal of indicator
microorganism was HPC < TC < FC/ E. coli.
The disinfection efficiency of the SOCO-DIS system was also determined under different weather
conditions, as shown in Fig. 3 depending on the irradiance range of natural sunlight available on different
days during the experiments. Disinfection exhibited three stages of treatment depending upon the sunlight
intensity; the middle stage was critical. Microbial inactivation could be due to two mechanisms of
treatment—thermal or pasteurization—and UV radiations and the synergistic effects of both can be seen
under strong weather conditions, during which rainwater is disinfected completely, as shown in Fig. 3.
The SOCO-DIS system proved to be effective at strong weather conditions while remained ineffective
under weak weather conditions and, to some extent, at moderate whether where both TC and HPC were
not disinfected completely and this could be due to the absence of the synergistic effects of radiation and
temperature.
1000
400
800
FC (CFU/100ml)
500
TC (CFU/100ml)
1200
R2=0.97, n=4
600
400
R2=1, n=4
200
200
R2=1, n=4
R2=0.93, n=3
100
0
250
2000
HPC (CFU/ml)
E. coli (CFU/100ml)
R2=1, n=6
0
200
R2=0.99, n=6
1500
R2=0.95, n=5
150
Control
Weak Weather
Moderate Weather
Strong Weather
300
1000
100 R2=1, n=4
50
R2=0.96, n=4
R2=0.98, n=4
500
R2=0.89, n=3
0
0
2
4
6
Time (h)
R2=1, n=6
8
0
0
2
4
6
Time (h)
8
Figure 3 Microbial inactivation in SOCO-DIS system under different weather conditions with low turbidity (5NTU)
and neutral pH values
Microbial inactivation is directly related to sunlight intensity. All the results showed a similar
tendency, signifying a close relationship between sunlight intensity and the time required to inactivate
microorganisms. The temperature measured inside the bottles indicated that it is not a predominant factor
in the elimination of microorganismsit is mainly the radiation that determines the efficiency of
disinfection. Table 1 compares the microbial inactivation of the SOCO-DIS system with the SODIS
system for all four microbial parameters based on microbial decay rate constants at neutral pH and low
turbidity values (<5 NTU) under all weather conditions.
Table 1 Comparison of kmax (1/min) between SODIS and SOCO-DIS system at different weather conditions with
low turbidity (5NTU) and neutral pH values
Microbial
parameters
Weak weather
Moderate weather
Strong weather
SODIS
SOCO-DIS
SODIS
SOCO-DIS
SODIS
SOCO-DIS
TC
0.02
0.20
0.10
0.58
0.18
1.24
FC
0.08
0.20
0.12
1.29
0.21
0.96
E. coli
0.05
0.18
0.09
1.02
0.14
0.77
HPC
0.03
0.14
0.10
0.32
0.15
1.66
SODIS proved to be ineffective for achieving complete disinfection even under strong sunlight
radiation; however, a direct correlation between radiation and inactivation was observed, as for the
SOCO-DIS system. In the case of SODIS, no parameter, under any weather conditions, led to the
achievement of potable water guideline values, namely 0 CFU/100 ml for TC, FC and E. coli and 10
CFU/ml for HPC. The difference in disinfection efficiency between the two systems can be summarized
as follows: the SOCO-DIS system is about 20–30% more efficient than the SODIS system. The main
reason for this could be the enhanced effects of concentrated radiation and the synergistic effects of
temperature, mainly under strong and moderate weather conditions.
Effects of Lemon and vinegar on Disinfection Efficiency
The PET bottles in simple SODIS and SOCO-DIS system were exposed to whole day sunlight
under weak weather conditions and different concentrations of lemon and vinegar were used to adjust
three different initial pH values as described in Table 2 (Amin and Han, 2011).
The results of TC and E. coli inactivation under weak weather conditions are shown in Fig. 4 for
different values lemon. The comparisons were performed among three different lemon concentrations and
between the SODIS and SOCO-DIS systems. There was an almost linear relationship between pH and
disinfection efficiency i.e. a constant decrease in microbial concentrations was observed with a linear
increase in pH value (Fig. 4). Disinfection efficiency increased by decreasing initial pH values and there
was almost complete TC and E. coli removal in the SOCO-DIS system at lowest pH of around 3, while
about 90% final inactivation was achieved for E. coli in SODIS by lowering the pH value to around 3. TC
and E. coli inactivation increased by about 60% and 80%, respectively in SODIS at the lowest adjusted
pH of 3 when compared to the sample without any lemon concentration. The synergetic effects of lemon
with low pH in both SODIS and SOCO-DIS system accelerated the reaction process and, hence, enhanced
the disinfection efficiency.
Table 2 Different lemon and vinegar concentrations for adjusting three initial pH values
For 1L rainwater
S. no.
pH
System
Lemon/Vinegar (ml)
% Volume
1
≈8
2
7
0.4-0.7/0.15-0.4
≤0.07/≤0.04
3
5
1.5-2.1/0.7-1.3
≤0.2/≤0.13
4
3
6.3-8/3.3-4
≤0.8/≤0.4
5
7
0.4-0.8/0.2-0.4
≤0.08/≤0.04
6
5
1.5-2.5/0.7-1.5
≤0.25/≤0.15
7
3
7-8.5/3.5-4.2
≤0.85/≤0.4
0/0
0/ 0
Parent
rainwater
SODIS
SOCO-DIS
Disinfection was completed in terms of TC and E. coli inactivation in the SOCO-DIS system only
at lowest pH value of around 3, corresponding to the final lemon concentration of approximately 8 ml per
liter of rainwater. This high concentration of lemon, around 0.8 percent by volume, may cause some odor
or taste problems which were, however, overcome by using both lemon and vinegar in several
combinations aiming at the same disinfection efficiency with low concentrations of both lemon and
vinegar (results not shown). The microbial inactivation efficiency was almost comparable when using the
lemon as twice as the concentration of vinegar (results not shown). In case of vinegar, the disinfection
efficiency increased by 40% by decreasing the initial pH values from almost 8 to nearly 3 in the SODIS
and SOCO-DIS systems for a final concentration of about 4 ml per liter of rainwater (0.8 percent by
volume) i.e. almost half of the lemon concentration.
1200
TC (CFU/100ml)
1000
800
600
Control
Natural
pH=7
pH=5
pH=3
400
200
0
E. coli (CFU/100ml)
300
250
200
150
100
50
0
0
2
4
6
Time (h)
8
0
2
4
6
Time (h)
8
Figure 4 TC and E. coli inactivation with different lemon concentrations in; (a) SODIS, (b) SOCO-DIS system
Lemon, and lime juice concentrates possess intrinsic antimicrobial properties to eliminate E. coli
and other bacterial pathogens in the event of postconcentration recontamination during the production of
thermally concentrated fruit juices at high temperatures (Nogueira et al., 2003). Low pH values may
increase inactivation rates by presenting significant additional stress to the cells that may reduce the rate
at which energy-consuming proteins in cells can scavenge reactive oxygen species that damage the
external structures of microorganisms (Foegeding et al., 1996). Finally, in order to avoid the odor and
taste problems and possible microbial re-growth due to the presence of nutrients in these food products, it
is advisable to wash the used PET bottles on regular basis or to replace them with new ones since these
are easily available.
Effects of Plastic Wrapping on Disinfection Efficiency
Simple heat-resistant plastic bags were used to enhance the thermal effect by increasing the
temperature in both SODIS and the SOCODIS system. The analysis was performed in both weak and
moderate weather conditions for complete disinfection, which was not achieved in the SOCODIS
systems, especially for TC and HPC at moderate weather conditions (Amin and Han, 2009c). The
contribution of the enhanced thermal effect due to the wrapping of PET bottles with heat-resistant plastic
bags, for both SODIS and the SOCODIS system are shown in Fig. 5 in weak and moderate weather
conditions.
1000
TC (CFU/100ml)
800
600
400
200
SODIS
W-SODIS
SOCODIS
W-SOCODIS
0
250
E. coli (CFU/100ml)
200
150
100
50
0
09:00 10:30 12:00 13:30 15:00 18:00
09:00 10:30 12:00 13:30 15:00 18:00
Time (hh:mm)
Time (hh:mm)
(a)
(b)
Figure 5. Effects of plastic bag's wrappings on TC and E. coli inactivation in SODIS and SOCODIS system at; (a)
weak and (b) moderate weather conditions
The maximum temperature difference between samples wrapped in plastic bags and an
unwrapped sample was about 60C and 70C in SODIS and the SOCODIS system, respectively under weak
weather conditions (results not shown). The difference of microbial inactivation, however, was negligible
in both cases for all microbial parameters, including TC, FC, E. coli and HPC (results not shown for FC
and HPC). One reason could be the maximum temperature, which was only about 36 0C and 410C for
SODIS and SOCO-DIS system, respectively, due to weak weather conditions, even after wrapping plastic
bags. This temperature was not enough to inactivate the microorganisms. Thus, weak or even moderate
weather conditions may not be fit for the synergistic effect of radiation and temperature or thermal effects
alone, since the water temperature remained below the critical value of 500C (Simon et al., 2007). In this
research, a temperature of about 450C, however, can be considered as a critical temperature beyond which
either thermal or synergistic effects play an important role in disinfecting microbes (Amin and Han,
2011).
Under moderate weather conditions, the temperature difference generated using plastic bags was
about 4 0C and 70C in SODIS and SOCODIS system, respectively, thus not much different than the
temperature increase at weak weather (results not shown). Disinfection efficiency improved due to
temperature increase in both SODIS and SOCODIS system and the microbial inactivation increased by
5% and 7-8% for TC and E. coli, respectively. This also supports the earlier finding of E. coli being more
affected by IR (heating effects) than UV radiations. Usually, higher disinfection efficiency in SODODIS
system was observed after wrapping with plastic bags, mainly because of a greater temperature increase
in SOCODIS system compared with SODIS. Disinfection was not complete in SODIS, however, both TC
and E. coli were disinfected completely in the case of SOCODIS through wrapping with plastic bags.
HPC was not disinfected completely (results not shown), however, the final concentration was less than
the drinking guideline of 100CFU/ml in case of SOCODIS system at moderate weather after using the
plastic bags.
Conclusions
Emerging water issues owing to increasing population and climate change in developing world
demand economical and feasible solution that can be applied by using avialable local resources. The
RWH systems can be constructed easily with minimal human resources and technical skills. The
widespread use of RWH systems, however, is limited mostly due to the fact that the quality of harvested
and stored rainwater is not so good to be used for potalbe purposes. The SODIS method was used for the
treatment of stored rainwater due to its simplicity and cost-effectiveness. This method despite being
recommended by WHO has few limitations. The innovativeness of this research is addressing the roof
harvested rainwater quality problems and improvement of disinfection efficiency of SODIS system by
simple, low-energy and cost-effective locally available materials. By identifying the role of UV radiations
and thermal effects of sunlight on disinfection, it was possible to get a better performance by
concentrating the sunlight and by catalysing the reaction efficiency after using the lemon/vinegar in order
to achieve the complete disinfection at all weathers.
At first, disinfection efficiency of simple SODIS was evaluated for rainwater disinfection for
possible applications in developing countries. The effects of two most important and critical parameters
i.e. radiation and temperature were evaluated under different weather conditions. At weak sunlight
conditions, disinfection efficiency was about 50-60% which increased to about 80% under strong
sunlight, most probably due to the synergistic effects of thermal and optical inactivation. Simple SODIS,
however, remained ineffective for complete disinfection under the investigated experimental conditions
even at strong weather conditions.
The process efficiency of the SOCO-DIS system was compared with that of a SODIS system
under different weather conditions. Overall, a 20–30% increase in the disinfection efficiency of the
SOCO-DIS system was achieved, mainly due to the concentrated effects of sunlight radiation and the
synergistic effects of thermal and optical inactivation. The inefficiency of SODIS has thus been
significantly improved upon by the SOCO-DIS system, and rainwater was completely disinfected under
strong and even moderate weather conditions. Disinfection did not improve significantly by changing
initial pH and turbidity values in case of simple SODIS, however, the disinfection efficiency of the
SOCO-DIS system was also significantly improved by changing the initial pH and turbidity values of the
water. Rainwater was completely disinfected only at low pH values under moderate weather conditions,
thus improving the process efficiency by 10–20% as the initial pH values changed from basic to acidic
values.
The inefficiency of SODIS for rainwater disinfection in all weather and that of the SOCODIS
system, at weak and moderate weather conditions, led to the idea of simple modifications to solar-based
disinfection for complete disinfection. For this, low-cost and commonly available food preservatives like
Lemon and vinegar were used keeping in mind the better disinfection efficiency at low pH values. It
increased the disinfection efficiency by around 40% in SODIS, however, complete inactivation was not
observed. In SOCO-DIS system, on the other hand, completely disinfected rainwater was obtained under
weak weather conditions, expect for the HPC inactivation when using lemon. The amount of vinegar
required for the same disinfection efficiency was almost half that of the corresponding amount of lemon,
which highlighted the proper selection and choice of catalyst for disinfecting rainwater using sunlight.
Further, optimum combinations of both food products were tried and satisfactory results were obtained at
the lowest pH values of around 3, without any of the problems of taste or odor associated with high
concentrations of lemon or vinegar alone.
Another simple technique to improve the efficiency of simple SODIS and SOCO-DIS system was
to wrap PET bottles with simple, cost-effective and commonly available heat-resistant plastic bags. This
resulted in increased water temperature due to the retaining of heat and increased air temperature inside
plastic bags. The maximum temperature increase due to wrapping with plastic bags under moderate
weather conditions was about 4-70C in SODIS and SOCODIS system, which enhanced the disinfection
efficiency by about 5-8%. Rainwater was disinfected completely, in the case of the SOCODIS system,
possibly due to the synergistic effects of thermal and UV radiations or due to the thermal effects alone as
a result of increased temperature beyond 450C.
The practical benefit of using simple heat-resistant plastic bags and low-cost food preservatives
like lemon and vinegar in SOCO-DIS system will be the application of solar-based systems for the
complete disinfection of stored rainwater under moderate and weak sunlight conditions. Since all the
materials and techniques that are used to improve the efficiency of SODIS are simple, cost-effective and
socially, this innovative approach can be easily applied at household and community scale in any many
parts of the developing world, especially in remote areas and where the centralized water supply system is
not feasible, and thus contribute to achieve the MDGs.
Acknowledgement
This work was supported by National Research Foundation of Korea grant funded by the Korean
government (No. 0415- 20110098) and by Integrated Research Institute of Construction and Environmental
Engineering, Seoul National University Research Program funded by Ministry of Education & Human Resources
Development. This work was also financially supported and is a part of the “Projects & Research” axis of the
Alamoudi Water Chair (AWC) at King Saud University, Riyadh, Saudi Arabia.
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