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Water treatment

Figure 3.9. Piezometer

Figure 3.9. Piezometer distributions along vertical flow CGF units of MSF pilot system. 81

Table 3.2 Maintenance criteria and cleaning procedures of MSF stages. 1991-1993. Units Criteria Cleaning Procedures DyGF Partial cleaning cycle. Frequent cleaning to recover filtration capacity of DyGF units. Every 8 days (until July 1991; Every 5 days (until March 1992), and Every 3 days (until April 1993). Higher frequency if it is necessary to recover filtration capacity. Total cleaning cycle (TCC). Occasionally, when flowfiltering capacity can not be recovered with the previous procedure. Clean the inlet and outlet boxes. Rake the top gravel layer: Close the filtered water outlet. Move or rake the surface layer of gravel in opposite direction to the flow, starting near the inlet and finishing at the overflow weir. Move the gravel until the wash water is visually similar to the raw water. Drain the filter bed: sequential opening and closures of drainage plug or valve (several times) and drainage of the filter bed until reaching effluent turbidities similar to those in raw water or until recovering initial flow filtering capacity. Remove and wash the gravel bed: Close the filtered water outlet and the water inlet. Remove the gravel layers taking care not to mix them. Wash them separately. Sieve the gravel fractions if they became mixed. Wash the pipe-work, floor and walls of the unit. Install the layers of gravel in the same order and thickness as they were initially packed. Open the water inlet and carry out a normal cleaning procedure. CGF Partial cleaning cycle (PCC). Frequent (every 30 days) cleaning to recover filtration capacity of CGF steps. Higher frequency if it is necessary to recover filtration capacity. Total cleaning cycle (TCC). Occasionally, when filtering capacity can not be recovered. Clean the inlet and outlet boxes. Clean the top gravel layer: Close the outlet valve. Move the surface gravel with a metal shovel, upflow filtration to waste until the wash water turbidity becomes similar to the influent water. Not applicable for HGF and MHGF. Drain the filter bed: sequential fast openings of drainage valve (5 to 8 times). Continuous drainage of the filter bed until reaching effluent turbidities similar to those in integrated water or until recovering initial flow filtering capacity. Open the water inlet to fill the filter and repeat the procedure if necessary. Remove and wash the gravel bed: Complete removal and washing of the gravel bed (similar to the procedure described for DyGF units). SSF Partial cleaning cycle (PCC). Maximum headloss (0.80 m) Note: Total-cleaning cycles (resanding) were not planned during test periods. Scrape the SSF skin: Drain the supernatant water with the overflow weir, also called “goose neck” device (Annex 1). Drain the top 5 to 10 cm of the sand filter bed by opening the drainage valve. Scrape the top 2 to 3 cm of the sand filter bed. Remove the scraped dirty sand. Level out the sand surface and fill the SSF unit from the bottom. Increase gradually the influent flow until reaching the normal operational velocity. High sampling frequencies were adopted for those parameters more relevant to the acute sanitary risks, such as thermotolerant (faecal) coliforms and turbidity or to the maintenance of the filtration units, such as suspended solids and head losses. A high frequency was also adopted in the case of colour, as a surrogate parameter of natural organic matter (NOM), due to its significance in establishing terminal chemical disinfection and the acceptance of the treated water by the consumers. Samples for colour were complemented with some samples for chemical oxygen demand (COD) and volatile solids. Low frequency was adopted for some parameters, such as iron and manganese, because of their stability. Samples were also collected for dissolved oxygen as an important factor for biological activity. Alkalinity and hardness were also monitored with a low frequency to verify the absence of precipitation in the filter media. Sampling was carried out using labelled containers. Plastic cylinders of 1,000 ml were used to collect samples for the majority of physicochemical parameters. Winkler bottles of 300 ml 82

Dec2015