Operational efficiency of conventional iron removal plant in municipal water works of Bangladesh

Abstract

In Bangladesh about ninety percent of municipal water works are using ground water as a source of drinking water. The reason is this water requires no treatment for bacterial contamination and can be supplied directly. But ground water contains minerals like iron, manganese and arsenic etc. of which iron is most commonly present in ground water of this country. The presence of iron in ground is generating various complications in municipal water supply system. This complication is not limited to technical aspect but also to the management of municipal water works. The horizon of this complications due to this iron is increasing as the number of thana towns are upgraded to municipalities. In the past, inadequate measures have been taken to minimize the iron problem through installation of limited number of iron removal plants. Due to resource constraints installation of IRPs in large number could not be made possible. But with the number of municipalities the demand for removal of iron from water is growing. During eightees, the Netherlands govemment came forward to minimize the problem and installed a number of IRPs. But several operational difficulties showed up within short period of commissioning. In course of time the plant performances are reported to decline and their O&M cost stands so high that it becomes very difficult for the municipal authority to meet these cost out of their revenue income. In this study the declining performances of the IRPs were investigated. The study also examined the effectiveness of various steps of the treatment process and their findings were analyzed along with observational, experimental and theoretical findings. During the study it was found that these IRPs were not designed on the basis of any pilot plant performance or on the basis of any laboratory model test. Study also reveals that plant performances are declining due to some design faults. Design of Wash trough, filter underdrain and determination of backwash rate in accordance to grain size are worthwhile to mention. Poor backwash creates a couple of problems : the impurities are not completely removed at one hand and on the other hand premature filter breakthrough occurs which means water quality deteriorates much before filter head lodd reached to terminal head. It is found that no plants could meet neither the designed water quality nor WHO guideline value for drinking water. In this study, a laboratory model test was performed in order to determine the unit processes of treatment that would require for the study area on the basis of the investigation. A unit process of treatment is selected and design outline is given on the basis of the model test. Some operational procedures are suggested, as observed during the study. If these were implemented, plant performances would improve by 30 to 50% with respect to wash water requirement. Moreover some modifications are also suggested to improve the performances of the existing plants which can be made without disturbing whole process. These will reduce the operational cost of the plants. For IRPs to be constructed in future it is recommended to carryout detailed water quality investigation and pilot plant Ilaboratory model study before construction of trealment plant in large a scale. It is also recommended to strengthen close monitoring of plant performance with special focus on day to day O&M