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Urban water supply and water treatment
Published in Sandy Cairncross, Richard Feachem, Environmental Health Engineering in the Tropics, 2018
Sandy Cairncross, Richard Feachem
The sand bed is 600 to 900 mm deep, but most of the filtration takes place in the top layer. At the very top of the sand bed a dense slimy layer of retained fine material develops, with an active flora and fauna. This biologically active zone, known as the ‘schmutzdecke’, is responsible for much of the water quality improvement provided by a slow sand filter. In particular, the schmutzdecke retains or kills the great majority of viruses, bacteria, protozoal cysts and helminth eggs and thus makes the slow sand filter a far more efficient pathogen-removing process than the rapid sand filter.
Removal of Particulate Matter by Filtration and Sedimentation
Published in Samuel D. Faust, Osman M. Aly, Chemistry of Water Treatment, 2018
The filter is operated with a water depth of 3–5 ft above the sand surface. A schmutzdecke forms, in which most of the solids are removed. The schmutzdecke is a layer of microorganisms that facilitate removal of particles. When the head lost through the filter becomes excessive (commonly 305 ft), the filter is removed from service, drained and cleaned. Filter runs can range from 1 to 6 months (see below).
Water treatment and distribution
Published in Nick F. Gray, Water Science and Technology: An Introduction, 2017
In contrast, slow sand filters employ a much finer sand of 0.15–0.3 mm in diameter. Slow sand filters are normally rectangular in plan with a layer of the fine sand up to 1 m in depth laid on a graded layer of coarse sand and gravel to prevent the fine sand blocking the system of underdrains beneath (Figure 17.10). On the surface of the sand, a gelatinous layer develops, which is rich in microorganisms (e.g. bacteria, protozoa, algae) called the schmutzdecke. It is this layer that is largely responsible for the treatment of the water by both physical removal of particles and the biological treatment of dissolved organic matter and nutrients. The top 2 mm is an autotrophic layer, a mixture of algae and nitrifying bacteria, removing N and P and releasing oxygen. Below this, heterotrophic bacteria dominate where residual organic matter is removed. The heterotrophs extend up to 300 mm within the sand layer. Raw water is allowed to flow onto the surface filter to a depth that will provide sufficient hydraulic head to drive it through the filter to give a hydraulic retention time of about 2 h. Design details are given in Figure 17.10. The quality of water is excellent (<1 NTU). However, as the rate of filtration is very slow (<0.1 m3 m2 h−1), large areas of filters are required (Example 17.1). The cost of slow sand filtration is much higher in capital terms compared to rapid gravity filters. They are also expensive to operate because solids retained at the surface of the filter increasingly impede drainage and so the top 10–15 mm of sand must be mechanically skimmed off every 2–4 months after the filter has been drained. This takes some time to complete, requiring extra standby filters. The fine sand layer must also be topped up every few years to maintain an adequate layer of fine sand. This layer should not be allowed to reduce to <0.5 m in depth. Slow sand filtration can be used on its own to treat high-quality surface water and groundwater, although if there are turbidity problems, then pre-filtration using either a rapid sand filter or a micro-strainer is necessary (Figure 17.2a). Slow sand filters do not work with coagulants, so there are significant reductions in the sludge produced compared with rapid sand filters, and they require high-quality raw waters with the low turbidities normally associated with reservoirs and lakes (<10 NTU). Rapid sand filters have been used as roughing filters to increase the loading rate of slow sand filters and to enable them to be used for treating poorer-quality raw waters. A layer of GAC can also be incorporated between the media layers to improve performance and to remove trace organics.
Pretreatment using Opuntia cochenillifera followed by household slow sand filters: technological alternatives for supplying isolated communities
Published in Environmental Technology, 2020
Bárbara Luíza Souza Freitas, Lyda Patricia Sabogal-Paz
HSSFs were developed to be operated intermittently [10]. This design gives freedom for the user to produce water according to their need. Intermittent flow operation is achieved by daily batch feeding; however, some changes were made at the filter outlet, which maintains a constant minimum water level at the top of the filter bed (stationary level). This operation condition generates the so-called pause period – the time needed for physical–chemical and microbiological processes acting on the schmutzdecke, which can treat the water. Huisam and Wood [11] describe the schmutzdecke region as the interface between top sand and water, which accumulates retained material and intense microorganism activity.