MULTI-MEDIA WATER FILTERS (Depth Filters)
Advanced Multi-Media Filtration Systems (Depth Filters)
Advanced multi-media filtration systems represent a significant improvement over their single-media predecessors, chiefly due to the complex filter bed dynamics achieved through the innovative selection and arrangement of diverse filter media. These systems enable the provision of high-quality filtered water at substantially elevated flow rates, in contrast to the performance of conventional sand filters. In traditional sand filters, finer and lighter sand particles tend to gather at the top of the bed, while heavier and coarser particles settle at the bottom after backwashing. The filtration process is mainly restricted to the upper layers of the bed. In contrast, the multi-media filter’s configuration is reversed, with heavier but lighter-density particles migrating to the top during backwashing, while finer and heavier-density particles remain at the bottom. This is made possible by the meticulous selection of suitable media, which confers multiple benefits. The entire bed acts as a filter, not just the surface layers, capturing turbidity throughout its depth and retaining more solids before requiring backwashing.
Multi-Media Filter Composition
The filter bed typically consists of three layers of different media, with a cumulative depth of 26 to 40 inches. In a three-layer configuration, the top layer is made up of large, lightweight anthracite coal particles 15 to 18 inches deep, with a particle size of 1.0 to 1.5 mm and a density of 1.35 to 1.75. The middle layer comprises heavier, smaller calcined aluminum silicate or sand particles 8 to 15 inches thick, with a size of 0.5 to 0.6 mm and a density of 2.65. The bottom layer is composed of garnet particles 3 to 6 inches deep, with a size of 0.2 to 0.3 mm and a density of 4.0 to 4.2, making it 50 to 60% heavier than sand.
Backwashing and Maintenance
Multi-media filters undergo backwashing akin to sand filters, using reverse or upward water flow. Each layer retains its stratification due to the varying densities. In a four-media filter, an additional top layer of 3 to 6 inches of larger, lighter plastic pillows is added, with a particle size of 2.0 to 4.0 mm and a density slightly greater than water’s 1.0.
Benefits
The multi-media filter can operate considerably longer before requiring backwashing due to its enhanced capacity for retaining turbidity. Unlike traditional filters that only capture turbidity in the upper layers, the multi-media filter does so across the bed’s full depth. This system is well-suited for closed pressure tanks, reducing bed cracking and turbidity breakthrough, and eliminating the need for visual inspection. Utilizing pressure tanks instead of open basins or filters is especially beneficial for point-of-use filtration and small community water supplies. The system supports higher filtration flow rates, allowing for the use of smaller diameter tanks without compromising performance. The filtered water achieves exceptional clarity thanks to the garnet particles capturing fine turbidity. Moreover, the multi-media filter can treat water at a much higher flow rate than single-media sand filters, producing a significantly greater volume of filtered water.
Central Filtration Systems
In small community water supply systems, the conventional sedimentation tank is replaced by a centrifugal separator, occupying only 1% of the space. These separators, which have been used extensively in mining and mineral recovery, subject solid particles to high centrifugal forces, driving them to the separator walls and into a collection device. The clarified water is then directed to the center and exits through the top. These separators can remove up to 98% of suspended particles as small as 74 micrometers, which is less than the diameter of a human hair.
The multi-media filter diminishes the traditional reliance on alum as a coagulant, complementing it with a polymer that creates a more robust floc effective over a wider turbidity range.
Contact Clarification
A contact clarifier tank enables hydraulic contact flocculation and surface storage clarification, replacing traditional paddle flocculation and protracted quiescent clarification. The subsequent sand filter is replaced with a more efficient multi-media filter, reducing the process time from over 4.5 hours to just 8 minutes. Multi-media systems can achieve dramatic reductions in NTU levels, from 200 NTU to 0.42 NTU in high turbidity waters and from 25 NTU to 0.15 NTU in low turbidity waters.
Given recent reports highlighting unsafe contamination levels in U.S. tap and well water due to industrial and environmental pollution, a robust water filtration system is indispensable for monitoring and ensuring the quality and safety of drinking water. Reverse osmosis systems can remove 90-99% of contaminants, providing households with healthy water. Our featured drinking water filter system offers both health benefits and convenience.
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