CASE STUDY

INFLOW & INFILTRATION REDUCTION PROJECT

Village of Hastings

The Village of Hastings is located in the County of Northumberland north of Cobourg via Highway 45. The Village is located along the Trent River, which flows directly through the Village in a south-west to north-east direction. The Villages population is approximately 1,000 persons.

The existing sanitary sewer system in the Village of Hastings consists of 8.9 kilometres of gravity sanitary sewer, ranging in size from 200 mm (8 inches) to 600 mm (24 inches) in diameter, and 0.6 kilometres of sanitary force main, ranging in size from 100 mm (4 inches) to 200 mm (8 inches) in diameter. There are three sewage pumping stations in the system located on Homewood Avenue, on Front Street at Bridge Street, and at the Water Pollution Control Plant (WPCP) off Cedar Drive. The firm pumping capacity of the three pumping stations ranges from 7.6 L/s to 38 L/s. Collected sewage is pumped to the Villages Water Pollution Control Plant which has a design capacity of 1,060 m3/d. The plant is a single train oxidation ditch process with secondary clarification, effluent disinfection via chlorination, and aerobic sludge holding.

In 1990, the Council of the Village of Hastings authorized a sewer system needs study for the Village. The purpose of the study was to determine the present condition and performance of the sanitary sewer system. At that time, the capacity of the Village’s sewage treatment plant was being exceeded on a regular basis and a development freeze was in effect.

Staff reviewed water and sewage flow records in the Village for the period 1987 to 1990. This review indicated that the water flow was generally higher than the sewage flow during the summer months, an indication that a considerable amount of water, such as flushing hydrants and watering lawns and gardens, etc., is not being returned to the sanitary sewer system. However, in the fall and early spring average monthly sewage flows exceeded water usage and in some cases actually exceeded the plant design capacity. A comparison of monthly average sewage and water flows for the years 1987 to 1990 are presented in Figures 2 through 5.

Staff also reviewed historical records for raw sewage quality between 1987 and 1989. These data indicated that the average BOD5 concentration in the raw sewage ranged from 38 to 78 mg/L, which is significantly less than the normally expected level of 200 mg/l for municipal sanitary sewage. Both the high monthly average sewage flows during the fall and spring periods and low BOD5 concentrations indicated significant infiltration/inflow was reaching the sanitary sewer system.

Given that significant infiltration/inflow was suspected, an assessment and investigation program was conducted. The assessment consisted of drainage area identification, sewer inventory, sewer hydraulic capacity evaluation and a system operation and maintenance review. The inspection program consisted of manhole inspections, house-to-house survey’s, flow monitoring, smoke testing and a television camera inspection of both the mainlines and simultaneously in the lateral services.

Portable flow monitors were installed in five locations during the flow monitoring program. The results of the flow monitoring program allowed staff to isolate those areas of the sewer system with the highest extraneous flows (ie. greatest infiltration/inflow). The results of the flow monitoring were used to streamline the sewage collection system inspection.

As a result of the sewage collection system inspection, a considerable number of manholes were found to have deficiencies related to general inflow and infiltration. In addition, sump pumps were identified as being connected to the sanitary sewer system during the house-to-house survey, smoke emissions were identified through smoke testing and several sewer sections were found to be in need of repair as determined through in depth, isolated, television inspections.

A staged approach to sewer rehabilitation was proposed as a result of the investigations. Staff prepared specifications for the rehabilitation works in 1993. The sewer rehabilitation contract was awarded to D.M. Robichaud Associates, a sewer rehabilitation contractor specializing in trenchless technology, in 1994. The work was completed in 1994, 1995, and 1996.

D.M. Robichaud used a number of rehabilitation techniques on this project. The rehabilitation work included chemical sealing of manholes and sewer lines, structural spot repairs, and extensive manhole upgrades. Manholes in the system had benching replaced, precast joint/seams were sealed, new ladder rungs were installed, inflow protector bins were installed, and the chimney section was sealed using CretexTM seals. All joints within the mainline sewer were pressure tested and sealed, using a urethane chemical grout. All lateral service mouth connections were pressure tested and sealed. Several areas of the sewer system required inline reaming to remove root infestations, protruding lateral connections and calcium deposits prior to chemical grouting. Sixty service lateral repairs were completed. The work on the service laterals included: mini camera inspections and plotting, back to back clean out installations, chemical sealing and spot repairs, and full lateral lining. The total construction cost of the sewer rehabilitation program was approximately $420,000, sixteen percent below the budget of $500,000.

The impact of the rehabilitation works on influent flows to the water pollution control plant have been dramatic. A comparison of average monthly water and sewage flows for 1996 and 1997 indicated a significant drop in wastewater flows. Plant operations staff have reported a drop in influent wastewater flow of over 50%. Staff has conservatively estimated the flow reduction as 30% or 300 m3/d (66,000 igal/d). The unit cost of the capacity gained is therefore approximately $6.40/igal. The estimated cost to construct a 300 m3/d (66,000 igal/d) expansion at the Hastings WPCP, based upon a similar project being undertaken for the Village of Fenelon Falls, is $800,000. The unit cost of plant expansion is therefore approximately $12.12/igal almost twice the cost of the capacity gained via sewer rehabilitation. The Village of Hastings is once again open for business now that the development freeze has been lifted. Significant residential and commercial growth is in the planning stages.