Laserfiche WebLink
<br />and mercury are critical contaminants that could affect the river, however, and a <br />requirement for some form of treatment is reasonable. <br /> <br />Expansion of the existing facility is necessary to meet higher flow rates with higher <br />organic loads. Pretreatment processes are in the process of being upgraded with new <br />bar screens and a new grit separator. As part of the pretreatment upgrade, a chemical <br />phosphorus removal system is being added to the facility. Primary treatment is to be <br />expanded to meet new flow demands with the addition of two new primary clarifier <br />settling tanks equal in size to the existing tanks. <br /> <br />Expansion of secondary treatment is critical to removing soluble organics from the <br />wastewater. The trickling filters are effective while operating in series mode, but need to <br />be switched to parallel mode to handle flow rates in excess of 2.9 mgd. In parallel <br />mode, the trickling filters can accommodate a maximum flow of 5.8 mgd, which is less <br />than the projected peak hourly flow rate of 7.0 mgd. Two models have been applied to <br />evaluate removal of organics from the wastewater. The results show that the trickling <br />filters would not meet the strict fixed loading requirements at flows as low as 2.0 mgd. <br />An activated sludge system would improve the systems performance considerably. The <br />facility has previously had upgrades in preparation for the addition of an activated sludge <br />system. An aeration tank and blower building have been constructed. Additional <br />aeration tanks would be needed to provide redundancy and to meet higher flow and <br />loading demands. <br /> <br />To complete the activated sludge system, a new secondary clarifier would be needed. <br />Solids loading to the secondary clarifiers would increase dramatically with an activated <br />sludge system, and an additional clarifier tank is needed for adequate settling. The new <br />settling tank would be equal in size to the three existing, operational tanks. <br /> <br />Two final steps to treatment are needed to meet permit requirements. As mentioned, <br />filtration is advised to polish the wastewater and reduce CBODs, TSS, phosphorus and <br />mercury concentrations to acceptable levels. Additionally, the existing UV disinfection <br />system is sized for an average flow of 1.1 mgd. Another channel is available for <br />installation of an additional train of UV disinfection bulbs. The existing train of UV bulbs <br />would need to be replaced with a higher intensity system; depending on the type and <br />efficacy of the previous filtration step. <br /> <br />Solids generated during wastewater treatment must be handled. Digestion facilities <br />would need to be added to meet new solids production requirements. An additional <br />primary digester, secondary digester, and digested sludge storage tank are needed to <br />adequately reduce, stabilize and store solids. <br /> <br />To meet space requirements for the expansion of the treatment facility, it is <br />recommended that the City investigate the possibility of land acquisition to the south of <br />the existing site currently owned by Great River Energy. A new main building with a full <br />Supervisory Control and Data Acquisition (SCADA) system and improved laboratory and <br />office space could then be built south of the existing parking lot. The existing main <br />building would be demolished to create space for treatment processes. Part of the <br />existing garage space could be preserved in the existing main building, however. <br />System hydraulics would be simplified by locating treatment processes in the same area <br />as existing processes, and costs for new treatment processes could be reduced <br />considerably. <br /> <br />1-2 <br />