Harvested Rainwater Quality of Dhaka and Its Importance as the Alternative to Surface and Groundwater

Abstract

Water is the most essential prerequisite for life on earth. Fortunately, among the total surface of the earth, 75% is covered with a water body. But unfortunately, most of the portion of this water is saline (about 35 parts per thousand). This saline water contains a high concentration of salts and inorganic metals (Total Dissolved Solids, TDS >35,000 mgL^-1), and hence it is unacceptable for human consumption [1]. Only, 3% of this total water is safe for drinking and other applications including irrigation and industrial purposes. But, the major portion (68.9%) of the freshwater is capped in Glaciers and Icecaps. The rest of the 29.9% is groundwater and 1.2% is surface water (atmosphere, lakes, river) respectively [2]. As a result, the dependency on groundwater has become the most reliable solution for modern civilization. But, the ever-increasing population and the heavy consumption of groundwater, causing continuous groundwater depilation around the globe [3]. This depletion of the aquifer layer is seriously affecting the geological and environmental orientation and hence increasing the possibility of earthquakes, and landslides especially in megacities like Dhaka (Capital of Bangladesh) [4]. Many alternatives like surface water treatment, boiling, and filtration of surface water, desalination of seawater, etc. are being adopted around the developed and undeveloped countries. But, none of this process seems ultimately efficient enough in the sense of cost-effectiveness. Towards this, the naturally harvested rainwater could be the most potential source as an alternative to surface and groundwater especially for Bangladesh (around 2000 mm rainfall yearly) [5]. This much rainwater could be a natural solution towards the alternative of the costly surface water treatment plants of municipalities. It is noteworthy that many countries like Germany, India, Australia, etc. have also imposed strict regulations on rainwater use and few of these countries have large rainwater harvesting systems for their cities' water supply [6].

Although the quality of the rainwater varies largely on the total environment of a particular region, it is ideally safe for different applications without any further treatment viz. gardening, car washing, constructions, irrigations, and so forth. But, all the rainwaters collected from different regions are not readily drinkable and safe for other sophisticated purposes too. For instance, Husam et al. have studied the water quality of Hebron, Palestine, and found a significant number of heavy metals (Cr, Mn, Co, Ni, Cu, Zn, Mo, Ag, Cd, Bi, and Pb) which is greater than the prescribed value of world health organization (WHO) [7]. The high presence of heavy metals especially lead is very threatening to human health. They have studied other water parameters like (pH, salinity, TDS,) and found that the rainwater of the Hebron is not safe for drinking. So, the prior quality assessment of rainwater including the presence of heavy metal contents is highly required to announce it as safe to drink. This study covered the total quality assessment of harvested rainwater including the determination of pH, conductivity, Dissolved oxygen, Hardness, DO salinity, TDS, Alkalinity, CO₃^2- in Dhaka city, and especially the number of heavy metal contents by performing Inductively coupled plasma mass spectroscopy (ICP-MS). Also, the anions of the samples were investigated by ion chromatography. The results of the heavy metals are presented in (Table:1) and other parameters reveal that the rainwater quality of Dhaka city is quite excellent according to the WHO standard guideline for safe drinking water [8]. Although this rainwater of Dhaka city is completely feasible with the WHO standards, it is suggestive to drink it after charcoal filtration and if necessary ultrafiltration to remove any microbiological contamination. It is also suggestive to collect the rainwater in a proper scientific way to get rid of the initial dust contamination from the collection system. Thus, it can be concluded that the rainwater of Dhaka city corporation could be the most prospective as the alternative solution towards the very cost-effective and high-quality water supply as well as achieving the SDG-6 goals for this country.