11-08-1999 Council Packet - Laserfiche WebLink

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2-inch forcemain cannot be cleaned with the City’s conventional sewer cleaning equipment due to its small diameter and due to bends in the forcemain. Lift Station 5 pumps through a short 2-inch forcemain to a Wayzata gravity line. The 2-inch forcemain is heavy wall high density polyethylene and is believed to be in generally sound condition, but in need of cleaning. There is approximately 2,000 feet of 2-i»“ch forcemain in this area. This area has poor soils and high groundwater, which complicates construction involving deep excavations. The gravity sewers in this area were televised in 1999 and are in generally sound condition. Lift Stations 2,3, and 4 are in need of complete replacement. Since Lift Station 5 is of the same age and construction, replacement of Lift Station 5 should be considered. Inflow and Infiltration (I & I) - Ferndale Area The February 1999 sewer televising in this area shows two residential ser\ ices on Ferndale Road West that appear to be contributing significant I & I. Field observations at Lift Station No. 3 show a third residential service that appears to be contributing significant I & I. Observations in July 1999 at Lift Station No. 2 confirmed that there is a significant, steady stream of clear water from the gravity sewer entering the lift station. During the spring and early summer, the lift stations in this area run near continuously due to increased I & I, and high level alarms in Lift Station No. 2 are almost constant. This problem is aggravated by deposits in the 2-inch forcemain which lower the lift station pumping rates. There is also minor infiltration directly into Lift Station No. 1. Flow Conditions and Proposed Hydraulic Improvements - Ferndale Area With minor forcemain improvements, the existing 2-inch forcemain is adequate to serve this area in conjunction with the proposed lift station improvements. The proposed improvements will be able to accommodate some amount of Inflow and Infiltration. It should be noted that despite the alarm history in this area, sewer back-ups have not been a problem. This is attributable to basement elevations being significantly above lift station elevations in most cases. There are approximately ten residential service connections tributary to Lift Station No. 2. Typically, peak sanitary flow rates correspond to 1 gpm per residential service connection, for larger service areas. In the Ferndale area, this would result in an estimated peak flow of 10 gpm. Due to the small system size, the peaking factor will be higher than for a large system. Items such as private water treatment facilities and draining of hot tubs may also contribute to higher peak flows. The most conservative approach would be to assume that all 10 service connections are contributing significant flow at the same time. Based on a “fixture unit curve”, this would result in a flow of 45 K:\/i3999l07\wortf\repcrt3\!etier nportdoc 3