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Feasibility Study <br />Water Treatment Plant <br />City of Lino Lakes, MN <br />WSB Project No. 015822-000 Page 19 <br />6.4.2 Biological Gravity Filtration <br />The manganese and iron that exist in the City’s well water could be treated without <br />chlorine and potassium permanganate chemicals with Biological Gravity Filtration. In <br />addition, the disinfection byproducts (Total Trihalomethanes and Total Haloacetic Acids) <br />that are formed in the City’s water when chlorine reacts with the natural organic matter <br />could be totally eliminated with biological filtration. Biological filtration is commonly <br />practiced in the United States and Europe and is endorsed by the Minnesota Department <br />of Health (MDH) as an efficient and effective method for treating manganese, iron, <br />ammonia, and other contaminants. A Biological Gravity Filtration plant is basically a <br />Conventional Gravity Filtration plant but with a modified aeration process and without <br />chemicals added prior to the filtration process. Fluoride, chlorine and possibly ortho- <br />phosphate would be added to the finished water just like a conventional gravity filtration <br />plant. <br />In addition to the advantages described above for a Conventional Gravity Filtration <br />system, the advantages of a Biological Gravity Filtration system include: <br />1.) Higher filtration rates. <br />2.) Longer filter runs (reduces backwash supply and wastewater volumes). <br />3.) Significant savings in chemical costs (chlorine is needed for disinfection only and <br />no potassium permanganate is needed). <br />4.) Reduction in disinfection byproducts. <br />5.) The design of a conventional gravity filtration process with detention and aeration <br />can easily be modified to accommodate biological filtration after the successful <br />completion of a biological filtration study as required by MDH. <br />The disadvantages of Biological Gravity Filtration System include: <br />1.) Additional monitoring required with inhouse laboratory equipment for dissolved <br />oxygen levels, etc. These tests are relatively easy to do, however, and automatic <br />dissolved oxygen sensors are available to provide continuous monitoring. <br />6.4.3 Pressure Filtration <br />For a pressure filtration system, raw water from the City’s wells would be pumped directly <br />through multiple horizontal steel pressure filters with anthracite and greensand or silica <br />sand filter media. The raw water would be pumped under pressure from the well pumps, <br />through the filters, and into the water distribution system with no intermediate pumping <br />steps. The oxidation of manganese and iron would occur inside the pipelines located <br />downstream of the plant chemical feed systems and above the filter media. Similar to <br />gravity filtration, as the water passes through the filter media, the insoluble solids of <br />manganese and iron are removed from the water and create filter headloss. When the <br />headloss has increased to a maximum level, the filter media is backwashed and cleaned <br />with distribution system water to remove the accumulated solids from the filter media. <br />After the solids have been removed from the media, the clean filter cells are placed back <br />into service for another filtration cycle. The backwash wastewater is discharged into an <br />underground holding tank where the clean backwash wastewater (supernatant) is <br />recycled back to the head of the plant for additional treatment and the settled solids are <br />discharged into the sanitary sewer system. Fluoride, chlorine and possibly ortho- <br />phosphate would be added to the finished water.