Conference Agenda

Base Infiltration (BI) can substantially hinder a collection system’s ability to convey wastewater. It is a capacity thief hiding in plain sight that operates 24 hours a day. Wastewater flows from some basins evaluated by the author have been found to be comprised of more than 60% BI. The expanding interest in modeling the performance of collection systems over extended periods has enhanced the need for more accurate estimates of BI contributions. Also, with the expanding popularity of trenchless rehabilitation methods, there is an increasing need to verify post-construction BI reductions. However, there is no clear-cut universally accepted method by which to determine or otherwise verify degree of BI from collection system basins.

This presentation addresses four empirical methods used to determine degree of BI based on sewer flow data, including % Minimum Method, the Wastewater Production Method, the Stevens-Schutzbach Method, and the Min Factor (Mitchell) Method. Each method is evaluated using 45 case study system basins. The basis and assumptions of each method will be shown. These empirical methods were tested against a chemical parameter verification method that involves regressing hourly parameter concentrations (TOC, BOD, TSS, and COD) with sewage flow rates; regression graphs of which will be presented.

The simple-to-use empirical Stevens-Schutzbach equation is recommended as a universal, yet conservative BI quantification Method and the Min Factor or Mitchell Method is based on WEF guidance and considered to be an accurate and defensible method.

The City of Bellingham’s Roeder Lift Station is located near the industrial area along Bellingham Bay and serves much of the northern part of the City. The existing lift station was suspected to be operating under reduced reliability conditions during peak storm events. The City contracted with Carollo Engineers to complete an alternatives analysis and design to increase the existing lift station’s pumping capacity from approximately 8 million gallons per day (mgd) to 18 mgd.

The existing dry-pit/wet-pit station is located on a small city-owned parcel of land surrounded by Port of Bellingham (Port) and Burlington Northern Santa Fe (BNSF) railroad properties. The station’s existing 18-inch diameter force main is routed through a City utility easement on Port and BNSF properties including crossing under a BNSF spur track to reach the discharge manhole location. Two lift station retrofit alternatives and one new submersible lift station alternative were evaluated, including installation of a parallel force main alternative. The selected alternative included a new submersible lift station 1,100 feet northwest of its current location. The lift station design included a combination of variable speed driven non-clog submersible pumps and screw centrifugal pumps in self-cleaning pre-rotation basins with a pumping range of 1 to 18 mgd. Other improvements included 2,500 lineal feet of parallel 14-inch and 28-inch diameter force mains with trenchless installation beneath the railroad, installation of a 36-inch diameter gravity sewer main and manholes, and miscellaneous 24-inch diameter water main replacement.

One of the challenges of designing the new lift station included impacts from the 100-year flood plain, sea level rise, and the potential for tsunami flooding. Based on available mapping, the location of the new lift station is within the FEMA 100-year flood plain and the Tsunami Design Zone. The FEMA flood maps do not take into consideration future long-term change due to climate change and the rising sea level. In addition, the project needed to consider impacts related to potential tsunami flooding. As such the new station was elevated 9 feet above the existing grade. This presentation will summarize the analysis and design related to the proposed improvements.

Equipment can vibrate when not properly installed. Many times, the root cause of the problem is incorrect installation of the equipment base to the concrete foundation – which was the situation for the case study that this presentation will cover.

Initially, the team removed the old steel base, put in a new stout base, and installed it properly with jack screws and high-strength flowable grout. While this seemed, at first, to solve the problem, over the course of 8 hours, the vibration on the pump increased and the pump had to be shut down.

The engineering team and client maintenance staff then decided to bring in the “big guns” and performed a vibration and modal analysis of the newly rebuilt pump, as well as three other pumps (700 HP and two 1000 HP pumps).

The three major issues identified were: 1.) Grout holes in the steel base but no grout visible in the grout holes or smaller vent holes; 2.) The base was not level and exceeded the Hydraulic Institute/ American National Standards Institute (HI) and American Petroleum Institute (API) criteria of 0.004 inches; 3.) Leveling nuts were used to level the steel base frame. This held the steel frame in the air with no solid support.

This presentation will focus on the vibration equipment used, the data collection methods employed, and the analysis performed to determine the cause of the excessive vibration of the pump bearings.