1. Introduction

Some water sources contain disease-causing organisms which need to be removed or killed before the water can be used in drinking. Bio-films in drinking water systems can be a source for microbial contamination, leading to a potential health risk for consumers ^{[1, 2]}. Water hygiene and sanitation systems continue to be a burden for developing countries despite the painstaking effort to solve these problems ^[3].

Disinfection through inactivation is usually achieved by the use of chlorine, ozone, chlorine dioxide, or a combination of chlorine and ammonia (chloramines) which prevents the water from containing pathogenic microorganisms ^[4]. Chlorination as a disinfectant can remain even after the disinfecting process to provide protection of drinking water from water source to the consumer ^{[5, 6]}. When chlorine is added to water, some of the chlorine initially reacts with organic materials and metals in the water and is therefore not available for disinfection; this is called the ‘chlorine demand’ of water. The remaining chlorine is defined as ‘Total Chlorine’. Total chlorine is further divided into: 1) the amount of chlorine that reacts with nitrates and is thus unavailable for disinfection (which is called “Combined Chlorine’) and, 2) the ‘Free Chlorine’, which is the chlorine available to inactivate disease-causing organisms. Free chlorine in drinking-water is not particularly toxic to human when its concentration is below the health-based guideline value (GV). The major source of human exposure to chlorine is drinking-water where, GV of residual chlorine (the sum of hypochlorous acid and hypochlorite ions) was established at 5 mg/l ^[7]. Based on the taste and odor threshold of free chlorine, it is doubtful however that consumers would tolerate such a high level of chlorine. Most individuals are able to taste chlorine at concentrations below 5 mg/l, and some at levels as low as 0.3 mg/l. Therefore, the health-based GV for chlorine should not be interpreted as a desirable level of chlorination. Groundwater is the main source of water supply in Gaza Strip where the main pollution is the salinity due to the seawater intrusion ^{[8, 9]}. Accordingly, the Ministry of health adopted the WHO limitations for chlorination to protect the water from further pollution during the distribution process.

In order for the chlorine to function as a disinfectant it must be allowed time to react with the water and the targeted disease-causing organisms. Chlorine dosages must be adequate to exceed the chlorine demand, and leave behind chlorine residual. The longer the contact time the more effective the disinfection will be. A pre-determined chlorine concentration is applied to the water for a specified length of contact time. The chlorine concentration is the lowest continual chlorine residual in the treatment process, while the time is the exposure time for that residual ^[10].

The World Health Organization has estimated that 94% of diarrhea cases can be prevented through environmental interventions, amongst them increasing the availability of safe drinking water ^[11]. Giardiasis, Entamabiasis, Ascariasis, Diarrheal diseases Hepatites A, Salmonilasis and Shegellosis are some of the waterborne diseases registered in Gaza Strip. Also, these diseases can be transmitted to human by food. In Gaza Strip as a whole, a strong positive correlation was found for Giardiasis and diarrhea diseases with Fecal Coliform contamination in water networks. Diarrhea was strongly associated with source of drinking water in Gaza Strip ^[12]. 1996 saw the launch of the Water and Wastewater Services Improvement Project (SIP) for Gaza strip, financed by the World Bank. This project contributes to the proper operation and maintenance of water systems to ensure the provision of microbiologically safe drinking water in order to minimize the disease burden of the human population potentially emanating from water systems. Within this project, The World Bank representative proposed to monitor the microbiological contamination in drinking water. Accordingly, the operator of the SIP started the process of monitoring for 150 testing points all over Gaza Strip. At present, Sodium hypochlorite solution is used to disinfect the water supply from groundwater wells ^[13]. The drinking water supplied in the Gaza Strip is insufficient and is plagued with supply interruption sometimes lasting for several days. With inadequate disinfection, such practices will lead to suitable conditions for biofilm bacteria growth ^[14]. The Water-related diseases are most prevalent in the village of Um Al-Nasser, including the disease meningitis, inflammation of the liver Epidemiological, pneumonia, Alascrias, bedwetting, Gardia and widespread diarrhea among children under the age of three years ^[15].

Therefore, the aim of the current research is to investigate the ongoing chlorination process and the effect of chlorine depletion in the intermitted water supply on the incidence of various water-related diseases amongst the customers of Um Al-Nasser village as a case study.

2. Study Area and Methodology

The village of Um Al-Nasser is located in the northern part of Gaza Strip. The coordinates of Um Al Nasser village are 31°34'50'' north, 33 ° 31'31''east and its rise about 38 m above sea level. The total population is 5000 inhabitants living in a space of approximately 800,000 m^{2 [16]}.

A computer program (Watercad v8i) was used to determine 1) the direction of flow and 2) the travel distance to each individual sampling point in the network from the source. The water distribution network feeds with water from one ground water well, the safe yield of the well is approximately 115 m³/hr. The chlorine is added directly to the main line from the well using a chemical injection pump that continuously pumps the chlorine solution into the water through an in-line venture device to mix the pre-treated water. The distribution network consists of about 16.29 km of water pipe ranging in diameter from 2 to 6 inches as shown on Figure 1. The existing system consists of a variety of pipe types including: UPV, Steel and High Density Polyethylene (HDPE) pipe; the dates for pipe installation vary also. Sampling points are distributed along network representing the whole area as shown in Figure 1.

Figure 1. Um Al Nasser Water Distribution system and the location of sampling points (Source: 18)

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As a general guide, the World Health Organization ^[17] recommends that one sample location per 1000 persons be examined each month for a water supply system that serves up to 100,000 persons. According to this recommendation, the number of residual chlorine testing points for Um Al Nasser district should be 5 considering the total population served by the distribution network is 5000 inhabitants. Yet the actual sampling points taken for this study are nine locations distributed throughout the network. Samples were collected for a four year period (2010- 2013) on a monthly basis from the nine sampling points and analyzed for residual chlorine. The following digital colorimeters where used:

A DPD-1 (free chlorine) or DPD-3 (total chlorine, used in tablet or powder form) is added to a vial of sample water which turns the water pink. The vial is then inserted into a digital meter which emits a wavelength of light to determine the degree of color change. The degree of color change is essentially the amount of free or total chlorine in the sample which is expressed digitally in mg/l

In order to link the occurrence of water borne diseases and the concentration of free chlorine in the drinking water distribution system, information on the number of the residents who were infected, including diagnoses, were gathered from the village medical centre during the same time of network sampling. The data was computer analyzed using Microsoft Excel Program and SPSS.

Figure 2. Free chlorine concentration at the nine sampling points 2010, 2011, 2012 and 2013

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Table 1. Statistical analysis and correlation of chlorine concentration in the distribution network and the distance from chlorination source

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3. Results

The first sampling point is the location of the well where the water is pumped from. As shown in Figure 1, the next sampling point is CT4 at 1290 m from the source followed by another 8 sampling points; CT1, CT6, CT2, CT7, CT3, CT8, CT9 and CT5 at distances 1400, 1400, 1560, 1840, 2100, 2110, 2180 and the farthest sampling point CT5 at 2250 m from the chlorination unit, respectively. The distances of the sampling points were measured using the flow direction of the water distribution network and the WaterCad model. The chlorine concentrations were calculated using monthly results recorded at each sampling point and taking the monthly average (2010-2013). The relationship between the residual chlorine and the distance between the disinfection source and the nine sampling points for Um Al Nasser’s water network during the four continuous years are shown in Figure 2,a,b,c,d and Table 1. The chlorine given at the source has a concentration of 0.45 mg/l. The variation in the correlation factor R² is high in various years from 2010 to 2013 in all sampling locations based on the distance from chlorination source.

A significant correlation between the distance and the decrease of chlorine concentration is shown, R² for 2010 is 0.75, while for 2011, 2012 and 2013 the R² is 0.81, 0.72 and 0.58, respectively. Moreover, ANOVA test indicates clear correlation between the chlorine depletion and the distance from chlorination source (Table 1). As in Figure 2a, b,c,d. there are no samples in the network above or equal to the maximum recommended by the WHO (>1.2 mg/l) because of the chlorine injected at the source is 0.45 mg/l. Also 220 out of the 324 samples over the 4 years period are below the minimum value (<0.2 mg/l).

The concern of the current study is given to some of the most common waterborne diseases such as Pin Worms, Giardiasis, Amoebiasis, and Diarrhea of Um Al Nasser village for the period between January, 2010 to December, 2013. The variation in disease type from January 2010 to December 2013 is very small; the numbers of infected residents during the study period are 557, 578, 557and 1145 with Pin Worms, Amoebiasis, Giardiasis and Diarrhea, respectively.

This study attempts to measure the impact of residual chlorine depletion on relevant waterborne diseases in the Um Al Nasser network. SPSS and Excel programs were used to analyze the data collected about residual chlorine concentration in different locations of the water distribution network and the incidence of Pin Worms, Giardiasis, Amoebiasis and Diarrhea in the targeted locations. The results and analysis of winter data are shown in Figure 3 a, b, c and d and Table 2. The incidence of Pin worms decreases when the residual chlorine increase, the correlation is not significant R² = 0.13 (Figure 3a), while the ANOVA test is accounted for 0.267. Giardiasis incidence is not correlated with residual chlorine concentration as indicated by R² = 0.24 (Figure 3b) and ANOVA test 0.125 same as Amoebiasis where R² and ANOVA test accented for 0.22 and 0.141 (Figure 3c), respectively. Diarrhea incident cases are decrease when residual chlorine concentration increase significantly, the correlations are R² = 0.65 (Figure 3d) and ANOVA test 0.003 less than 0.05 indicating significant correlation between the decrease of the residual chlorine and the increase of incidence number. In general for correlated relations the ANOVA test should be less than 0.05.

Figure 3. Correlation factor between average residual chlorine and general water borne diseases in Winter

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Table 2. Statistical analysis and correlation of chlorine concentration in the distribution network and water borne diseases in Winter

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Figure 4. Correlation factor between average residual chlorine and general water borne diseases in Summer

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Table 3. Statistical analysis and correlation of chlorine concentration in the distribution network and water borne diseases in Summer

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As shown in Figure 4 a,b,c and d and Table 3 (results for summer months), the incidence of Pin worms is not correlated with residual chlorine concentration R² = 0.14 (Figure 4a) and ANOVA test 0.236. The increase of incidence cases of Giardiasis and Amoebiasis are not correlated with the decrease of residual chlorine R² = 0.26 and 0.14 (Figure 4b and Figure 4c) and ANOVA test 0.087 and 0.238, respectively, i.e. the increase of incident cases coincide with the decrease of residual chlorine, but this is not the case, the results show insignificant increase of residual chlorine and decrease of incident number of Pinworm, Giardiasis and Amoebiasis, indicating that the incident cases may have other cause such as contamination of food. The increase in Diarrhea is highly correlated with the decrease of residual chlorine concentration R² = 0.61 (Figure 4d) and significant ANOVA test 0.003.

4. Discussion

It has been shown in the literature that biofilms cells are more resistant to antimicrobial agents including disinfectants used in practice for water treatment ^{[19, 20]}.

Total coliform contamination percentages in wells fluctuated from 6% to 13% during the 5 years study conducted by Yassin et al., 2006 for the Gaza Strip. In water networks, the values fluctuated from 12% to 20%. Faecal coliform contamination in wells ranged from 2% to 8%, whereas, in networks, it ranged from 4% to 12%. It is obvious that total and fecal coliform contamination levels exceeded the World Health Organization (WHO) limit (< 5% for total coliform and free for faecal coliform), and these levels were generally higher in water networks than in wells. In the current study fecal coliforms were not tested, but the low residual chlorine in some locations of the network provides further evidence for facal coliform contamination.

Chlorination of water did not totally prevent reactivation of stressed bacterial cells, it however, helps to reduce the role of reactivation ^[20]. The correlation between the depletion of residual chlorine and the distance from the chlorination source in Um Al Nasser water distribution network as shown in Figure 2 for four years monitoring emphasizes the fact of microbial contamination along the distribution network, in this case chlorine is consumed in the network as an oxidant to the microorganism which is the mechanism of disinfection ^[21]. The chlorine is consumed exponentially with the distance from chlorination point at the well. Water distribution system in Um Al Nasser suffers from the infiltration of wastewater to the water distribution system especially during winter where the sewage system is overloaded in Gaza Strip ^{[22, 23, 24]}. Such a pollution is very common in the intermitted supply system due the difference in pressure inside and outside the pipe lead to the seepage of wastewater to the system. The exponential decay of chlorine in the distribution system indicates the existence of microorganisms as a consumptive matter to the chlorine. The chlorine itself causes many health problems such as asthma, cancer, fertility problems, heart diseases, eczema and birth defect ^{[25, 26, 27]} when outside the acceptable range given by WHO at the upper limits of 1.2 mg/l. The percentage of samples that are less than the minimum allowable was 68% and this prevents the hazardous materials to form in the water but did not provide adequate disinfection leading to growth of microorganisms within the water network. A study by ^[28] showed that some concentrations of residual chlorine in municipal water distribution network infringed recommended values according to WHO standards by 41.4%. Therefore the chlorination process should be monitored and fixed values of chlorine dosage should be added to the system according to WHO limitation and standards to prevent the buildup of biofilms in water distribution system. The problem of upper limits 1.2 mg/l of WHO are never reached in Um Al Nasser village, thus the previously mentioned health problems due to the upper limits are not detected in the village.

Nutrient availability, hydraulic conditions, water temperature, pH, the type and concentration of disinfectant residues will affect biofilm growth in water distribution networks ^{[29, 30, 31, 32]}.

Diarrheal diseases were the most frequently self-reported disease among residents in Gaza City. Such diseases were more prevalent among people using municipal water than people using desalinated water and water filtered at home for drinking ^{[33, 34, 35]}. Even though, there is traces of residual chlorine in all sampling points in the network, the Giardiasis and Diarrheal diseases still exist. The number of infected residents due to the lack of chlorine in the system and the significant correlation between the increase of Diarrhea incident and the decrease of chlorine concentration as shown in Figure 3d and Figure 4d agrees with the UN Humanitarian Country Team and AIDA, 2009 ^[36] study who mentioned that without access to safe water, adequate sanitation and proper hygiene, children are particularly vulnerable to sickness caused by water borne diseases. In Gaza, diarrhea, an easily preventable disease, is the cause of 12 cent of young deaths. Furthermore, lack of safe water is the cause of mal-nutrition for millions of children, which can have long term impacts on a child’s cognitive and physical development. The Department of Health of the UN Relief and Works Agency (UNRWA) reports that: “Watery diarrhea as well as acute bloody diarrhea and viral hepatitis remain the major causes of morbidity among reportable infectious diseases in the refugee population of the Gaza Strip” ^[15]. It is relevant to carefully assess whether chlorination of drinking-water in the public domain is a priority when a rural water-supply scheme is planned, especially if chlorination cannot be done reliably ^{[37, 38, 39]}. The continuous treatment with 0.3 mg/l free chlorine (limit value German Drinking Water Ordinance) leads to a reduction of the CFU/cm2 down to the detection limit after 70 days ^[40]. Chlorination and heath are not a problem in developing countries only, but it is a worry of high income countries, in Switzerland for example rural water supply and sanitation review showed that there is some decline in the incidence of water-related diseases, such as diarrheal diseases, due to improved water supply and sanitation coverage ^[41]. The results of the study conducted for three villages in a remote and rural area in the Vhembe District, situated in the north-eastern parts of Limpopo Province in South Africa show reductions in diarrheal morbidity of about 57% due to the improvement in water supply system ^[42].

5. Conclusion

This study shows the imbalance in the performance of the chlorination process in the Um Al Nasser water supply system. This can be clearly shown in the amount of residual chlorine in the intermitted water supply system. The number of samples which have a residual chlorine concentration less than 0.2 mg/l (the lowest value recommended by WHO) is 220 out of 324. A significant correlation between the distance and the decrease of chlorine concentration is shown, R² for 2010 is 0.75, while for 2011, 2012 and 2013 is 0.81, 0.72 and 0.58, respectively. Diarrhea incidence cases in winter and summer seasons are significantly correlated with chlorine concentration in the network. A strong correlation was found between the increase of chlorine concentration and the decrease of Diarrheal incidence R² = 0.65 and 0.61, winter and summer seasons, respectively. It is highly recommended to conduct further studies to identify the characteristics and types of micro-organisms to redefine of doses used or the use of multiple chlorine disinfection units in the network to substitute the depletion of chlorine dosage due to large water distribution systems. Moreover, to study the causes of other diseases such as Pinworm, Giardiasis, and Amoebias those have insignificant correlation with the current range of chlorine dosage.

References

[1]	Lenntech,1998-2009. Waterborne diseases by pathogenic microorganisms. https://www.lenntech.com/processes/disinfection/waterborne-diseasecontagion.htm.
	In article
[2]	Wingender, J and Hans-Curt Flemming. 2011. Biofilms in drinking water and their role as reservoir for pathogens. International Journal of Hygiene and Environmental Health 214: 417-423.
	In article		CrossRef PubMed
[3]	Coulliette, A., Kyle S. Enger, Mark H.Weir, Joan B. Rose. 2013. Risk reduction assessment of waterborne Salmonella and Vibrio by a chlorine contact disinfectant point-of-use device. International Journal of Hygiene and Environmental Health 216: 355-361.
	In article		CrossRef PubMed
[4]	Cohn, P. D, Cox, M. and Berger, P. S. 1999. Health and Aesthetic Aspects of Water Quality, In: Water Quality and Treatment. Ed. American Water Works Association, McGraw-Hill, Inc, 2. 1-2.86.
	In article
[5]	Khleifat, K., M. Abboud, W. Alshamayleh, A. Jiries and K.A. Tarawneh. 2006. Effect of chlorination treatment on gram negative bacterial composition of recycled wastewater. Pak. J, Biol. Sci, 9: 1660-1668.
	In article		CrossRef
[6]	World Chlorine Council (WCC). 2008. Drinking Water Chlorination. World Chlorine Council position paper. 2008. www.worldchlorine.org.
	In article
[7]	WHO, 1993b. Assessment of sources of air, water and land pollution. WHO/PEP/GETNET 93.1-A.P, WHO, Geneva.
	In article
[8]	Hamdan, S., Uwe Troeger and Abelmajid Nassar. 2007. Stormwater availability in the Gaza Strip, Palestine. Int. J. Environment and Health, Vol. 1, No. 4: 580-594.
	In article		CrossRef
[9]	Qahman, Abdelkader Larabi, Driss Ouazar, Ahmed Naji, Alexander H.-D. Cheng 2009. Optimal Extraction of Groundwater in Gaza Coastal Aquifer. J. Water Resource and Protection, 4, 249-259.
	In article		CrossRef
[10]	Ryde, K. AAFC-PFRA. 2006. Chlorine disinfection of private water supplies for household or agricultural uses.
	In article
[11]	Bartram, J., Gordon, B., 2008. The global challenge of water quality and health. Water Pract. Technol. 3 (4).
	In article
[12]	Abu Mourad T.A. 2004. Palestinian Refugee Conditions Associated with intestinal parasites and diarrhea Nuseirat refugee camp as case study. Public Health 118 (2): 131-142.
	In article		CrossRef PubMed
[13]	Coastal Municipal Water Utility (CMWU) 2012. Annual Report on Water Status in the Gaza Strip - Summary about Water and Wastewater Situation in Gaza Strip. CMWU library, Gaza.
	In article
[14]	Rosemarie S. Roedera, Johannes Lenzb, Peter Tarnea, Jürgen Gebel, Martin Exnerb, Ulrich Szewzyka. 2013. Long-term effects of disinfectants on the community composition of drinking water biofilms. International Journal of Hygiene and Environmental Health 213: 183-189.
	In article		CrossRef PubMed
[15]	UNRWA Epidemiological Bulletin, Vol. 1, Issue 13, Sunday 15 October 2009.
	In article
[16]	Palestinian Central Bureau of Statistics, PCBS. 2013. “Statistic Brief (Population, Housing and Establishment Census)”, Palestinian National Authority, Gaza, Palestine.
	In article
[17]	World Health Organization. Guidelines for drinking-water quality, 3rd ed, 2006.
	In article
[18]	Water Department, Um Al Nasser Municipality 2013. Data Base and Different Materials, Palestinian National Authority, Gaza strip, Palestine.
	In article
[19]	Okpara, C.G, N.F. oparaku and C.N. Ibeto. 2011. An overview of water disinfection in developing countries and potential of renewable energy. J. Environ. Sci. Technol., 4:18-30.
	In article		CrossRef
[20]	Sule I.O., Oyeyiola G.P, T.O. Agbabiaka. 2009. Comparative bacteriological analysis of chlorinated and dechlorinated pipeborne water. International Journal of biological science. 1: 93-98.
	In article
[21]	Parsons, S.A. and B. Jefferson, 2006. Introduction to Potable Water Treatment. Blackwell Publishing Ltd., Ames, Iowa.
	In article		CrossRef
[22]	Melad KA. 2002. Evaluation of groundwater pollution with wastewater microorganisms in Gaza Strip, Palestine. Egypt: MSc thesis, Ain Shams University.
	In article
[23]	Hamdan S. 2009. A literature based study of stormwater harvesting as a new water resource. Water Science & Technology-WST Vol 60 No 5 pp 1327-1339.
	In article		CrossRef PubMed
[24]	Shomar, B. 2010. Groundwater contaminations and health perspectives in developing world case study: Gaza Strip. Environ Geochem Health, 11 June 2010.
	In article
[25]	Sadiq, R. and Rodriguez, M.J. 2004. Disinfection by-products (DBPs) in drinking water and predictive models for their occurrence: a review, Science of the Total Environment, 321, 21-46.
	In article		CrossRef PubMed
[26]	Environmental Protection Agency (EPA). 2006 Initial Distribution System Evaluation Guidance Manual for the Final Stage 2 Disinfection Byproducts Rule, Appendix A, EPA 815-B-06-002.
	In article
[27]	Oparaku, N.F., B.O. Mgbenka and C.N. Ibeto. 2011. Waste water disinfection utilizing ultraviolet ligh. J. Environ. Sci. Technol., 4: 73-78.
	In article		CrossRef
[28]	Al-Safady, M. and H. Al-Najar 2011. Disinfection Process of Water Supply System in the Gaza Strip Between Real Practice and WHO Limitations. Research Journal of Environmental Sciences. 5 (5): 414-423
	In article		CrossRef
[29]	Norton, C.D., LeChevallier, M.W., 2000. A pilot study of bacteriological population changes through potable water treatment and distribution. Appl. Environ. Microbiol. 66, 268-276.
	In article		CrossRef PubMed
[30]	Clark, R.M., Thurnau, R.C., Sivaganesan, M. and Ringhand, P. 2001 Predicting the Formation of Chlorinated and Brominated By- Products. Journal of Environmental Engineering, 127, 493-501.
	In article		CrossRef
[31]	Brereton, J.A. and Mavinic, D.S. 2002 Field and material-specific simulated distribution system testing as aids to understanding trihalomethane formation in distribution systems. Can. J. Civ.Eng., 29/ 17-26.
	In article
[32]	Roberts Alley, P.E. 2007. Water Quality Control Handbook, Mc Graw Hill Companies, Second edition.
	In article
[33]	Yassin Maged, Salem S. Abu Amr, Husam M Al-Najar. 2006. Assessment of microbiological water quality and its relation to human health in Gaza Governorate, Gaza Strip . Public Health. 120, 1177-1187.
	In article		CrossRef PubMed
[34]	Abu Amr and M.M. Yassin. 2008. Microbial contamination of the drinking water distribution system and its impact on human health in Khan Yunis Governorate, Gaza Strip: Seven years of monitoring (2000-2006), Public Health 122: 1275-1283.
	In article		CrossRef PubMed
[35]	Yousef Salah. Abu Mayla and Salem Saleem. Abu Amr. 2010. Chemical and microbiological quality of drinking water in Gaza Strip, Palestine. Sci Vis 10 (3), 80-88.
	In article
[36]	UN Humanitarian Country Team and AIDA Association of International Development Agencies, The impact of the blockade on Gaza’s basic needs, 9 November 2009.
	In article
[37]	Peter K. Jensen, Jeroen H.J. Ensink, Gayathri Jayasinghe, Wim van der Hoek, Sandy Cairncross, and Anders Dalsgaard. 2003. Effect of Chlorination of Drinking-water on Water Quality and Childhood Diarrhoea in a Village in Pakistan. J HEALTH POPUL NUTR; 21(1): 26-31.
	In article		PubMed
[38]	Timothy C.N. 2003. Estimation of risk due to low doses of microorganisms: a comparison of alternative methodologies. American journal of epidemiology, 118: 573-582.
	In article
[39]	Alia, T. 2007. A study of free chlorine residual in the drinking water distribution system: Al-remal al-shimali quarter-lattakia city. Tishreen Univ. J. Stud. Res. Eng. Sci. Series.
	In article
[40]	Lenz J, S. Linke, S. Gemein, M. Exner, J. Gebel. 2010. Verification of the efficiency of chemical disinfection and sanitation measures in in-building distribution systems. International Journal of Hygiene and Environmental Health 213: 198-203.
	In article		CrossRef PubMed
[41]	Mwendera, E.J 2006. Rural water supply and sanitation (RWSS) coverage in Swaziland: Toward achieving millennium development goals. Physics and Chemistry of the Earth 31: 681-689.
	In article		CrossRef
[42]	Batsirai Majurua, M. Michael Mokoenaa, Paul Jagalsa, Paul R. Hunter. 2011. Health impact of small-community water supply reliability. International Journal of Hygiene and Environmental Health 214: 162-166.
	In article		CrossRef PubMed