Modelo de avaliação de alternativas de tratamento descentralizado de esgoto doméstico para pequenas comunidades.

Abstract

The objectives of this research were the investigation of conventional and compact wastewater biological treatment systems concerning the sanitary quality of the effluent and costs estimation for construction, operation and maintenance prior to designing a Model of Assessment of Decentralize Wastewater Treatment Systems based on integrated economical and multi criteria and multi objective analysis, which may be suitable for studies prior the implementation of rural and suburban treatment systems. The research was performed between March, 2011 and November, 2013. Four anaerobic systems, one aerobic, and one compact anaerobic-aerobic system were operated and monitored under household low flow settings (270L.day-1) in the course of nine months. The physical, chemical and microbiological analysis has taken place weekly. Given the importance of the final disposal of effluent in the soil in decentralized systems, infiltrations tests both in laboratory and pilot scales were achieved. Henceforth, the monitored systems were designed accordingly to Brazilian wastewater treatment systems guidelines, having their total costs estimated and valuated to the Net Present Value for 20 years, under a rate of 12% per year. Posteriorly, an Assessment Model based on linked computer worksheets was built, embedding an economic appraisal by Cost & Benefit Method, as well as a multi objective evaluation, by the Compromise Programming Method. The treatment and final disposal alternatives combination included in the Model, as well as the effluent infiltration behavior in experimental soakaways was mathematically modeled. The modeling of the effluent concentration has performed a close-fitting with the experimental data both of the physical, chemical and microbiological parameters, with absolute differences between 0,05% and 5,45%. The modeling of soakaway behavior ahead of the septic tank has lean towards clogging 50% to 58% faster than the soakaway ahead of UASB reactor. Comparisons among community on-site and single household on-site treatment systems were made, with cost results for pipeline been the most substantial part of total cost, on behalf of 50% to 67% of those costs. The equilibrium point for the maximum numbers of homes to be added in a cluster was 15 homes. Regarding household treatment systems, the configuration Conventional UASB + Anaerobic Filter + Chlorination has shown to be the best alternative relating to total costs. The Compact Treatment System has shown as the best alternative concerning suitability for the standards of household agricultural reuse practice, with its configuration with UV tertiary treatment to be chosen as a promising feasible alternative for household wastewater treatment prior to reuse in household agriculture, although the alternative UASBY + FaFint + UV + Reuse was chosen as the Compromise Alternative, becoming yet, under economical emphasis ranking, the most Strong Alternative of the analysis. Innovative Compact low cost Systems based on anaerobic-aerobic concept of treatment, has shown to be promising for household wastewater treatment prior to reuse in agriculture in background of decentralized wastewater management for rural and suburban areas.