Simple mid-depth transverse baffles to improve bacterial disinfection in a shallow maturation pond - performance evaluation and CFD simulation.

Sunlight-mediated disinfection in wastewater treatment ponds is widely recognised as the main role for disinfection, but pathogen removal can be limited by poor light penetration in the water column. The aim of this research is to evaluate a different and simple type of intervention for improving bacterial disinfection, by placing mid-depth transverse baffles perpendicular to the theoretical flow in an existing shallow maturation pond, aiming at increasing the opportunity for bacteria to receive radiation at the upper layers of the pond. In order to understand the hydrodynamic mechanisms and evaluate the effectiveness of the proposed intervention, a 3D Computational Fluid Dynamic (CFD) model was applied, including the transverse baffles and a particle-tracking model. This was complemented by monitoring E. coli in the pond, in the phase before and after inclusion of the transversal baffles. Overall, the transverse baffles increased disinfection efficiency by moulding/re-routing the hydrodynamic flow and increasing microorganism exposure to the sunlight.

Colmatação e desempenho de sistemas alagados construídos de escoamento horizontal subsuperficial ao longo de oito anos de operação / Clogging and performance of horizontal subsurface flow constructed wetlands over eight years of operation

RESUMO O presente trabalho objetivou avaliar as condições de funcionamento de dois sistemas alagados construídos de escoamento horizontal subsuperficial (SACs-EHSS), um plantado (SACP) com taboa (Typha latifolia) e outro mantido sem plantas (SACNP), ao longo de oito anos de operação, no tratamento de esgoto sanitário. Mediu-se a extensão do escoamento superficial, como indicativo da colmatação do meio poroso ao longo do tempo, além de diversas variáveis que possibilitem inferências sobre as possíveis perdas de eficiência do sistema na remoção de poluentes dessa água residuária. As avaliações indicaram que o meio poroso dos dois SACs se encontra colmatado (mais de 80% do leito apresentando escoamento superficial), estando o SACNP, atualmente, em pior situação, tendo havido prejuízo do seu potencial de remoção de sólidos e nitrogênio em relação ao SACP. Apesar disso, o desempenho em ambos os sistemas permanece satisfatório, não existindo tendência de decréscimo acentuado nas suas capacidades de remoção de poluentes, contrariando a expectativa e corroborando resultados obtidos em trabalhos de monitoramento desses sistemas por longos períodos de tempo. A carga aplicada e o tipo de pré-tratamento a que o esgoto foi submetido parecem ter sido mais decisivos na eficiência dos sistemas em remover nutrientes/poluentes do que o próprio período de operação das unidades, sobretudo no que se refere ao SACP.

ABSTRACT The objective of this work was to evaluate the conditions of operation of two horizontal subsurface flow constructed wetlands (HSSF-CW), one planted (PCW) with cattail (Typha latifolia), and another, maintained without plants (NPCW), along eight years of operation in sewage treatment. The extent of the overland surface flow was measured as an indicative of the clogging of the pore medium over time, besides several variables that allow to infer possible losses of treatment efficiency in the pollutants removal from wastewater. Evaluations indicated that the porous medium of the two CWs is clogged (more than 80% of the bed with overland surface flow), and that the NPCW is currently in a worse condition, with a reduction of its solids and nitrogen removal potential in relation to the PCW. Despite this, the performance in both systems remains satisfactory, and there is no tendency of decrease in the capacity to remove pollutants, contrary to expectations, and corroborating the results obtained in works that monitored these systems for long periods of time. The load applied and the type of pretreatment to which the influent was submitted seem to have been more decisive in the systems' efficiency in removing nutrients/pollutants than the operating period of the unit, especially for the PCW.