Bleaching powder dosing for disinfection in waste water treatment plant

Bleaching powder dosing for disinfection in waste water treatment plant

Sources of Informations given below
Manual  http://cpheeo.gov.in/upload/uploadfiles/files/Chapter%206-9.pdf

1. https://www.epa.ie/pubs/advice/drinkingwater/Disinfection2_web.pdf
2.http://jeb.co.in/journal_issues/200710_oct07/paper_04.pdf

 The pilot scale disinfection units located at two different UASBR based STPs have been selected for this study. One of these pilot plants has been constructed after 27 MLD UASBR based STP, Noida (Uttar Pradesh, India) and is based on chlorination using calcium hypochlorite (either slurry form or liquid high test hypochlorite, HTH) with a capacity of 2 million liter per day (MLD). The other 1 MLD DHS-biotower pilot plant, developed by Prof. H. Harada (Machdar et al., 1997), is based on fixed film aerobic process and has been constructed after 40 MLD UASBR based STP, Karnal (Haryana, India). These STPs, based on UASBR technology, consists of preliminary treatment (Screen and Grit Chamber), UASBR and final polishing pond (Fig. 1). A schematic diagram of the pilot plants at Noida and Karnal is presented in Fig. 2 and 3 respectively. The chlorination unit of 2 MLD consists of a sump, mixing unit and baffled contact chamber (3m x 7m) and a specific dose of chlorine was mixed with a separate pipe. The 1 MLD DHS-biotower based on fixed film aerobic process consists of a sump and reactor having plastic rectangular sheets onto which triangular prism polyurethane foams were tiled. Raw sewage after UASBR treatment was supplied to the chlorination unit and DHS-biotower
In case of chlorination unit, initially the bleaching powder was used as a disinfectant from day 1 to day 48 and later on it was found that there was no residual chlorine in the bleaching powder, so the bleaching powder was replaced with calcium hypochlorite as disinfectant after 48th day. The dose of disinfectant was 5 mgl-1 from day 1 to day 16 and was increased to 20 mgl-1 from day 20. The contact time for the reaction between wastewater and disinfectant was 30 min throughout the study.
 Results and Discussion Reduction of fecal coliform: To achieve good disinfection, the contact time and dose are extremely important. A longer contact time is required for complete disinfection to occur. As per Lindsay (2004), a contact time of 30 minutes is a minimum, and if the dose remains constant, the contact time may necessitated to be increased at low temperatures or higher pH to obtain the same level of disinfection. Typical chlorine doses for municipal wastewater disinfection are about 5-20 mgl-1 with a contact time of 30 to 60 min (Lazarova et al., 1999). During this study, 5 and 20 mgl-1 of disinfectant doses for a fixed contact time of 30 min was applied. It was observed that a dose of 20 mgl-1 was very effective for the reduction in indicator microorganism (fecal coliforms). Initially when a dose of 5 mgl-1 was used, the reduction in fecal coliforms count was less than 90%. It may be attributed to the fact that the minimum dose required for the killing of microorganism was not sufficient as there was high chlorine demand due to the presence of high concentration of organic matter and suspended solids in the UASBR effluent. The average fecal coliforms count was reduced from 3.58 x 105 (± 1.02 x 105 ) MPN/ 100ml to nil at a dose of 20 mgl-1 and 30 min contact time (Fig. 4a). Thus the results shows that almost 100% removal of fecal coliforms has been observed after chlorination; however at certain days i.e., from day 7 to day 14 and from day 44 to day 53 (Fig. 4a), the number of fecal coliforms present in the wastewater effluent after disinfection was increased. It could be explained on the basis of type of chemical used for the chlorination. During this period, bleaching powder was used. Later on it was found that the chemical used at the plant does not have any residual chlorine. Chlorination was switched to the fortified brand of the calcium hypochlorite, high test hypochlorite (HTH). It indicated that chlorination was very effective in the removal of fecal coliforms if the regular operation and maintenance (O and M) is ensured.