RESUMO
Slow sand filtration is a common technology providing potable water in rural households across Latin America, Asia and Africa. Two PVC household slow sand filters (HSSF) were operated in continuous (C-HSSF) and intermittent (I-HSSF) flow modes for eight consecutive months. A non-woven blanket was installed on the fine sand top to facilitate cleaning with scheduled maintenance undertaken every 30 days. The efficiency of each HSSF was evaluated via physico-chemical indicators (reduction of turbidity and colour) with biological performance assessed via total coliform and E. coli enumeration post treatment. There were no statistically significant differences between the continuous flow and intermittent flow models for physical-chemical and total coliform reduction parameters. However, when evaluating E. coli, C-HSSF performed better (p = 0.02). The non-woven blanket was subjected to weekly analysis using a Clark-type amperometric microsensor (diameter < 20â µm), which measured dissolved oxygen (DO) concentration in the adherent biofilm. DO microprofiles illustrated a variation in biofilm growth, which were associated with a progressive increase in the HSSF efficiency. The maximum DO depletion value measured during several months of operation showed no significant difference between I-HSSF and C-HSSF (p = 0.98). The microsensor measurements provided unprecedented results in real time. These results can help to understand the efficiency of the filter in relation to the biofilm growth, the dissolved oxygen depletion and turbidity removal.
Assuntos
Água Potável , Purificação da Água , Purificação da Água/métodos , Escherichia coli , Filtração/métodos , Oxigênio , Dióxido de SilícioRESUMO
This study aimed to evaluate the efficiency of four household slow sand filter (HSSF) models for the removal of microorganisms from river water throughout the development of their biological layers (schmutzdecke). Two models were designed to be operated continuously (HSSF-CC and HSSF-CT) and two intermittently (HSSF-ID and HSSF-IF). Filters were fed daily with 48 L pre-treated river water (24â h sedimentation followed by filtration through a non-woven synthetic blanket). Water samples were quantified by coliform group bacteria and analysed by bright field microscopy to visualize the microorganisms. Total coliform reduction was between 1.42 ± 0.59 log and 2.96 ± 0.58 log, with continuous models showing a better performance (p-values < 0.004). Escherichia coli reduction varied from 1.49 ± 0.58 log to 2.09 ± 0.66 log and HSSF-IF, HSSF-CC and HSSF-CT presented a similar performance (p-values > 0.06), slightly better than the one presented by HSSF-ID (p-value=0.04). Microorganisms, such as algae, protozoa and helminths were detected by microscopy in raw water and pre-treated water. Algae were the most significant group in these samples, although they were not visualized by bright field microscopy in the filtered water. Results showed the potential of HSSF in microbiological risk reduction from river water, which increases the range of point-of-use water treatments in rural communities. However, additional studies of the HSSF biological layer must be performed.