Abstract
The drinking water treatment plants (DWTPs) in the developing countries urgently need an efficient pre-treatment for nitrate (NO3−) removal to cope with the increasing NO3− pollution in raw water. An upflow sludge blanket (USB) reactor applied for NO3− removal from domestic wastewater may be adopted by the DWTPs. However, studies on the optimal carbon-to-nitrogen ratio (C/N) and operation of USB reactor at short hydraulic retention times (HRT) for high-rate polluted raw water pre-treatment are lacking. In this study, we first investigated the optimal C/N for biological NO3− removal in a sequencing batch reactor (SBR). An USB reactor was then operated with the optimal C/N for pre-treating synthetic raw water contaminated with NO3− (40 mg N L− 1) to monitor the NO3− removal performance and to examine opportunities for reducing the HRT. After operating the SBR with designed C/N of 4, 3 and 2 g C g− 1 N, we selected C/N of 3 g C g− 1 N as the optimal ratio due to the lower carbon breakthrough and nitrite (NO2−) accumulation in the SBR. The USB reactor achieved complete NO3− and NO2− removal with a lower designed C/N of 2 g C g− 1 N due to the longer sludge retention time when compared with that of SBR (10 d). The high specific denitrification rate (18.7 ± 3.6 mg N g− 1 mixed liquor volatile suspended solids h− 1) suggested a possible HRT reduction to 36 min. We successfully demonstrated an USB reactor for high-rate NO3− removal, which could be a promising technology for DWTPs to pre-treat raw water sources polluted with NO3−.
Highlights
Nitrate (NO3−) pollution is increasingly threatening the water quality in many developing countries undergoing rapid urbanization, mainly due to agricultural runoffs, livestock wastewater and discharge of inadequately treated domestic wastewater
This study aimed to investigate the effect of carbon-to-nitrogen ratio (C/N) on the NO3− removal performance of an upflow sludge blanket (USB) reactor pretreating synthetic raw water contaminated with NO3−
We found that the dissolved organic carbon (DOC)/NOx−-N of the USB reactor (1.4 ± 0.4 g DOC g− 1 NOx−-N) was slightly lower than the theoretical C/N for denitrification (1.5 g C g− 1 N) [16, 17]
Summary
Nitrate (NO3−) pollution is increasingly threatening the water quality in many developing countries undergoing rapid urbanization, mainly due to agricultural runoffs, livestock wastewater and discharge of inadequately treated domestic wastewater. Malaysia is one these developing countries experiencing temporal elevated nitrate nitrogen (NO3−-N) up to 35 mg L− 1 in rivers [1, 2]. Most of the drinking water treatment plants (DWTPs) in the developing countries lack a treatment technology for NO3− removal from raw water because most of these plants are still using conventional process [4]. Research study on a robust raw water pre-treatment technology for NO3− removal is urgently needed to protect the public health. Biological denitrification was commonly reported for NO3− removal from domestic wastewater in activated sludge process [7,8,9]
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