quality control of potable water supplies from swcc's

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13) Total Organic Carbon (TOC): TOC was analysed using a Beckman Industrial Model 915B TOC analyzer after removing the inorganic carbon compounds from the samples by acidification and helium sparging (25). 14) Chloroform and Total trihalomethanes (THMs): These species were determined by purge and trap technique as per USEPA method #624 (26). 5 ml water samples were purged with helium gas for 10 minutes using Tekmar model LSC-2 liquid sample concentrator. The trapped organics from the tenax column of the concentrator were desorbed and analysed gas chromatographically using Hewlett Packard GC-5880 installed with 63 Ni EC detector and DB-5 capillary column. 15) Total coliform and faecal coliform bacteria: Multiple tube method was used with McConkey broth to determine the most probable number (MPN) of coliform bacteria (27) EC broth was used for the confirmatory tests for coliform bacteria. Since none of the samples tested positive for faecal bacteria, the reliability of the method was occasionally tested with water samples collected from sewage treatment plant. RESULTS AND DISCUSSION In 1984 WHO has published the guideline values for several water quality parameters (1). These guidelines were intended to supersede both the “European standards for drinking water”(28) and “The International standards for drinking water (29) which had been in existence for more than a decade. The new guideline values are not to be considered as recommended standards but were intended for use by countries as a basis for the development of standards, which, if properly implemented, will ensure the safety of drinking water supplies (1). By doing so WHO appreciates the fact that it may not be possible for all countries to provide potable water that attains a recommended or standard set of water quality parameters. Nevertheless each country is expected to try to develop water quality standards as close as possible to the guideline values in an endeavour to protect public health. The results of the present work on the monitoring of water quality parameters of the desalination - derived drinking water from the three MSF desalination plant in the Eastern Coast of Saudi Arabia are given in Tables 1 and 2. Parameters which are considered to have significant effect on the health of the population are grouped in Table 1, while the constituents of aesthetic importance are given in Table 2. This categorization has been done according to that of WHO guide lines. The last column of each table contains the guideline values set by WHO for the purpose of easy reference and comparison with the results of the present study. Before reviewing the results a brief description of the quality of the product distillate generally obtained from MSF plants, and the post - treatment methods employed at each plant before distribution of the water for public consumption may be helpful. All three plants under this study are based on multistage flash evaporation process. The distillates obtained from these plants were specified not to have more than 25 ppm of total dissolved solids (TDS). During the past several years of the operational experience TDS of the distillates 64
from Al-Jubail and Al-Khafji plants seldom exceeded this limit though lately, there have been several instances of high TDS in the distillates of Al-Khobar plant. In Al-Jubail plant the combined distillates are disinfected with chlorine gas and then treated with CO2/lime dosing. Both at Al-Khafji and Al-khobar plants chlorination is done by the addition of sodium hypochlorite from sea water electrolysers. pH is adjusted by CO2/lime dosing in Al-Khobar. The pretreated distillates from Al-Jubail and Al-Khobar plants are blended with good quality brackish well water to upgrade the mineral contents in several blending station facilities located near the respective distribution centers. Al- Khafji does not have blending facility due to the absence of good quality ground water nearby. The potable water available from all these facilities are constantly monitored for residual chlorine, which is controlled between 0.2 - 0.5 ppm, and pH in order to make it safe for human consumption and non-corrosive to equipments and pipings. Various constituents present in desalination derived potable water thus may originate from several sources such as carry over in the product dististillates from the plant, treatment chemicals such as CO2, lime, chlorine/hypochlorite constituents from the brackish water used for blending, contamination from or reaction products of water carrying equipments or treatment chemicals etc. A periodic assessment of the quality of the potable water thus becomes essential to ensure its safety for human consumption. The parameters which are considered to be toxic or have some adverse effects on human health are listed in Table 1. This table contains nine health related inorganic constituents besides organic and microbiological parameters monitored during this study. In general it can be seen that none of the samples tested during the period of this study exceeded the guideline values recommended for any of the parameters set by WHO. Cyanide, mercury and selenium were totally absent in all samples tested. Lead, cadmium and arsenic appeared in few samples, occasionally. However, their concentrations were found to be negligible than the WHO guideline values by an order of magnitude or more. Presence of chromium was more significant than other constituents. This is not surprising since traces of chromium may leach into water from the stainless steel parts extensively used in the plant. The concentrations detected are much lower than the guideline values. Though chromium is well known for its toxicity, traces of chromium intake is considered necessary according to some researches due to its beneficial effects on glucose metabolism. Nitrate varied from 0 - 2.04 and is considerably less than the guide line value of 10 ppm. Among the other inorganic constituents the concentrations of fluoride (F-) in drinking water samples require special mentioning. There is no conclusive evidence to indicate that fluoride is an essential ion for human nutrition. Extensive studies conducted in U.S., Europe and several other countries indicate that there is some correlation between dental health and fluoride content in drinking water. In general the data indicate that water containing fluoride upto an optimal concentration of 1.5 ppm reduces dental caries especially among children while higher concentrations result in mottling of teeth and dental fluorosis (30). The concentration of fluoride found in various samples analysed during the present study varied from Nil in Al-Khafji to 0.13 mg/L in Qatif 65
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