Reusing existing infrastructure to create a wastewater pumping station for nine homes

By Doug Pease

In 2011, R.V. Anderson Associates Ltd. was retained by the City of St. Catharines to undertake a review of the existing Nadine Crescent wastewater collection system, prepare recommendations to improve servicing and reduce maintenance.

Nadine Crescent is a residential street located in the City of St. Catharines’ Erion Heights neighborhood. It is separated from the rest of the Erion Heights neighborhood by Francis Creek and the Francis Creek Natural Area, which are approximately eight metres below the surrounding grade.

The existing sanitary sewer gravity collection system servicing Nadine Crescent is a separated system, which was constructed in 1998. It consists of a single 200 mm gravity sewer that conveys flows from the homes on Nadine Crescent to a 1,200 mm maintenance hole (MHB), located at the southeast end of the street. A pair of 75 mm diameter inverted siphons, acting as duty and standby, then convey flows under Francis Creek and discharge to the gravity sewer on Erion Road The inverted siphons are each approximately 260 m in length, and extend from MHB to maintenance hole A (MHA) located on Erion Road, just west of Francis Creek Road.

Under normal conditions, a single siphon conveyed all flow from Nadine Crescent. As the level in the sewer rose, the standby siphon automatically activated. If the level continued to rise, MHB overflowed through a 200 mm gravity pipe into an adjacent precast concrete chamber.

This three-metre diameter chamber was equipped with a single ½ horsepower submersible pump, configured to discharge into one of the inverted siphons. Under this condition, one of the inverted siphons was essentially converted into a force main, that operated in parallel with the other one.

The existing system had been experiencing ongoing operational difficulties, primarily relating to blockages in the inverted siphons. In order to ensure continued reliable wastewater collection, the City undertook a maintenance program to flush the inverted siphons on a regular basis in order to clear out any blockages. However, despite the City’s efforts, the inverted siphons continued to plug and cause problems in the upstream system.

Review of existing system

A thorough review of the existing system was conducted, including evaluation of as-built information for the existing system, interviews with City operations and maintenance staff, and visits to the site. Ultimately, two primary factors were identified as likely contributing to the reoccurring blockages.

The first factor related to the scouring velocity, or cleansing velocity, within the inverted siphons. Scouring velocity is defined as the minimum velocity within a pipe required to keep solids in suspension within wastewater. The Ontario Ministry of the Environment (MOE) Design Guidelines for Sewage Works (2008) recommends a minimum scouring velocity of 0.6 m/s within force mains. Based on peak flows within the Nadine Crescent system, the maximum flow velocity through one of the inverted siphons was only 0.3 m/s. During average flow periods, the velocity could drop to nearly nothing. As a result, it was likely that solids had been settling out of suspension from the wastewater and had been depositing in the inverted siphons.

Secondly, it was found that the actual internal diameter of the inverted siphons was only 71 mm. This is much less than the minimum diameter of 100 mm recommended by the MOE Design Guidelines in order to convey typical solids found in municipal wastewater. The inverted siphons did not have any sort of upstream bar screens to prevent larger solids from entering. As such, it was possible that larger solids in the wastewater might have been contributing to plugging.