This paper presents a conceptualized combined storm and wastewater treatment system for typical low income, high density drainage catchment similar to the case of Manila City. In many developing and tropical port cities in low-lying floodplains, local and regional floods and non sewered communities compound in a problem that manifests in surface water pollution. This results in particularly difficult situation to introduce major sewerage infrastructure and a centralized wastewater treatment system. Thus, to initiate and invite discussions on different and possible solutions is necessary to address the need to provide appropriate treatment systems in rapidly growing cities and urban populations. This paper aims to contribute to such discussions by suggesting possible treatment processes achievable in limited, under exploited spaces in urban drainage networks. Also the paper mentions systematic estimations relevant to assess and theoretically simulate suggested treatment system. Following Hvitved-Jacobsen etal (2002) recognition of the role of sewers as bioreactors, the proposed system takes reviewed physical and biological techniques to be installed parallel to the track of drainage channels that provides neither on-site nor off-site approaches, but “inline” treatment. Essentially, the system integrates sewers or drains in the entire treatment system. The estimated primary and secondary information on both physical and socio-economic parameters in the study site, and the adapted “alternative” principles of decentralized, small scale systems yielded a multiple site designs including the following treatment processes and units: 1) inline fine and micro screening; 2) vegetated submerged beds (VSB); 3) rotating biological contactors (RBC); and 4) “biotowers”. All were designed to capture and reduce typical wastewater constituents (TSS, BOD, TKN and Coliforms) continuously during “dry” and “average wet” conditions until year 2025. The system embodies an effective and alternative wastewater treatment system for Philippine effluent standards, replicable in other tropical urban drainage catchments.

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