Screens:
Screens are used on many surface water intakes to remove particulate material and debris from raw water. Weeds and debris can be removed using coarse screens, whereas smaller particles including fish can be removed using coarse screens, whereas smaller particles including fish can be removed using band screens and microstrainers. Ahead of coagulation and subsequent filtration, microstrainers are used as a pretreatment to reduce solids loading.
Gravel Filters:
Turbidity and algae can be removed using gravel filters, which consist of a rectangular channel or a tank divided into several sections filled with graded gravel (size range four to 30mm). An inlet distribution chamber allows the raw water to enter through and flow horizontally through the tank, encountering first the coarse and then the finer gravel. An outlet chamber collects the filtered water with solids being removed from the raw water accumulated on the floor of the filter.
Slow Sand Filters:
Turbidity, algae, and microorganisms can also be removed using slow sand filters. A simple and reliable process, slow sand filtration is often suitable for the treatment of small supplies provided that sufficient land is available. Slow sand filters usually consist of tanks containing sharp sand (size 0.15-0.30mm) to a depth of between 0.5 to 1.5m.
Activated Carbon:
Using physical adsorption, contaminants can be removed using activated carbon. This will be affected by the amount and type of the carbon, the nature and concentration of the contaminant, retention time of water in the unit and general water quality (temperature, pH, etc.). One of the most common mediums is granular activated carbon (GAC) although powdered activated carbon (PAC) and block carbon are also sometimes used. Filter media is contained in replaceable cartridges and a particulate filter at the outlet of the cartridge is used to remove carbon fines from the treated water.
Aeration:
Aeration is designed to transfer oxygen into water and remove gases and volatile compounds by air stripping. A common method is packed tower aerators as a result of their compact design and high energy efficiency. To achieve air stripping various techniques can be used including countercurrent cascade aeration in packed towers, diffused aeration in basins and spray aeration.
Membrane Processes:
Reverse Osmosis (RO), Ultrafiltration (UF), Microfiltration (MF), and nanofiltration (NF) are the most commonly used membranes for water treatment processes. Previously applied to the production of water for industrial or pharmaceutical applications, membranes are being applied to the treatment of drinking water. Membrane processes can provide adequate removals of pathogenic bacteria, Cryptosporidium, Giardia, and potentially, human viruses and bacteriophages. In a notable case study, companies from the Netherlands and Denmark are working on integrating enzymes into membrane technology for the removal of pesticides and pharmaceutical residues from drinking water.
