Laserfiche WebLink
Feasibility Study <br />Water Treatment Plant <br />City of Lino Lakes, MN <br />WSB Project No. 015822-000 Page 18 <br />6.4.1 Conventional Gravity Filtration <br />For a Conventional Gravity Filtration system, raw water from the City’s wells would be <br />pumped to a gravity filtration treatment filtration where it is either aerated or injected with <br />chemicals such as chlorine and potassium permanganate to oxidize the manganese and <br />iron to solids that can be filtered out of the water. Following aeration or chemical <br />injection, the water flows by gravity into a detention tank to complete the oxidation of <br />manganese and iron. From the detention tank, the water continues to flow by gravity into <br />multiple concrete filter cells where the insoluble manganese and iron are removed by <br />anthracite and greensand or silica sand filter media. During the filtration process, <br />manganese and iron solids will accumulate on the filter media and restrict the flow of <br />water to create headloss. When the headloss has increased to a maximum level, the <br />filter media is backwashed and cleaned to remove the accumulated solids from the <br />media. After the solids have been removed from the media, the clean filter cells are put <br />back into service for another filtration cycle. The backwash wastewater is discharged into <br />an 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. The filtered water ultimately flows into an <br />underground clearwell tank before it is pumped into the distribution system with high <br />service pumps. Fluoride, chlorine and possibly ortho-phosphate would be added to the <br />finished water. In Minnesota, the vast majority of water treatment plants over 2,000 gpm <br />in capacity have been constructed with gravity filters and not pressure filters in the past <br />two decades. <br />The advantages of a Conventional Gravity Filtration system include: <br />1.) Odors in drinking water can easily be removed. <br />2.) Gravity filters are open to view and access. This is very advantageous in which it <br />enhances the observation, operation, and maintenance of the filter cells and <br />associated components. <br />3.) The walls of a concrete gravity system are integral and part of the exterior walls <br />of the building structure. This reduces the cost of the facility since the exterior <br />walls have a dual purpose. <br />4.) There are several filter equipment manufacturers having proven track records of <br />success ensuring quality equipment and competitive bid prices. <br />5.) Gravity filters are considered to be the standard of the industry for municipal <br />water filtrations. In Minnesota alone, there are many such facilities in existence <br />having capacities equal to or greater than identified in Lino Lakes. <br />6.) The existing well pumping capacities will possibly increase if the wells only need <br />to pump to the elevation of the aerator or detention tank of a gravity filtration <br />plant rather than to the water tower levels. <br />The disadvantages of a Conventional Gravity Filtration system include: <br />1.) Typically requires the facility to be constructed on two floor levels based on the <br />hydraulic profile of the aeration, detention tank, and filters. However, parts of the <br />plant can be constructed underground when the site has ten feet or more of <br />topographic relief.