by Earl Brousseau, D.M. Robichaud Associates Ltd. Paper presented at First Nations Technical Services Corporation, Annual Sewer Conference (Orillia, Ontario, September, 1997). Table of Contents
1.0 Introduction
2.0 What is Sewer Rehabilitation?
3.0 Advantages of Sewer System Rehabilitation
4.0 Determining that a Infiltration/Inflow Problem Exists
5.0 Developing a Plan of Attack to Address Infiltration/Inflow Problems
6.0 Locating Infiltration/Inflow Sources
7.0 Repair Methods
8.0 Project Implementation and Funding
9.0 Conclusions
1.0 Introduction
This paper presents a review of applicable sewer rehabilitation techniques using trenchless technologies, many of which are applicable to small communities. The paper covers the advantages and disadvantages of sewer rehabilitation, defines infiltration/inflow, presents simple techniques that can be applied by municipal operators to determine if their system suffers from an infiltration/inflow problem, presents the methods to pin-point infiltration/inflow sources, identifies available methods for repairing I/I problems, and presents a case history of a successful sewer rehabilitation project. Finally, the steps that must be followed in order to secure funding in order to implement a sewer rehabilitation project are outlined.
2.0 What is Sewer Rehabilitation?
Most communities are serviced via a sanitary sewer system. The purpose of the system is to collect the liquid wastes (wastewater) from homes, institutional facilities, and industry in the community after they have been soiled by a number of uses and deliver these to a local treatment facility prior to discharge to the natural environment. Wastewater is essentially the water supply of the community after it has been fouled by a number of uses. For many communities the sanitary sewer collection and treatment system is a fundamental component of the communities public health facilities, since untreated wastewater contains disease causing organisms as well as a number of compounds which could harm the health of the community and natural environment.
The components of a typical sanitary sewer collection system include the connecting piping from homes and other buildings within the community to the sewer system (sewer laterals); and the collector and transport piping with the associated, manholes, and pumping stations. The collected wastewater is transported to the sewage treatment plant(s) which receive the collected sewage and provide the required treatment.
Sewer rehabilitation, which is the subject of this paper, is the repair of the sanitary sewer system and/or it’s components to maintain structural integrity and reduce infiltration/inflow into the sewer system. Infiltration is defined as the leakage of ground and storm waters into the sanitary sewer system by such means as defective pipes, pipe joints, defective lateral service saddle connections, manhole frame-chimney defects, manhole seam and wall cracking, and the inlet and outlet pipe locations. One of the most significant sources of infiltration, that is normally ignored, is riser connections and inspection tees in sanitary sewer laterals. In many cases where frost is in the ground the riser on a service lateral will separate at this connection point of the lateral resulting in significant infiltration. Infiltration is most often related to high groundwater level but can also be influenced by storm events, snow melt conditions, or leaking water mains. Inflow is defined as the entrance of storm water into the sanitary sewer system from sources such as roof leaders, basement footings, yard, and area drains, cooling water discharges, drains from springs and swamps, low manhole covers, cross connections from storm sewers and combined sewers, catch basins, storm water and surface runoff.
The rate and quality of infiltration depends on the length of the sewers, the material of construction, the population density (which affects the number and total length of house connections), the groundwater elevation, the number of defective joints, and cracking of the sewer system which may have occurred during installation or through later settlement. The rate of infiltration is typically greatest during the early spring when groundwater levels are highest or in areas of constantly high water tables.
The quantity of inflow depends on the number of improper connections to the sanitary sewer. Sump pumps, foundation drains, and cooling water discharge can provide steady flows when connected to the sanitary sewers. The steady flows are included in the measurement of infiltration but are not technically infiltration since they do not originate from leaking pipe joints. Roof leaders, yard drains, manhole covers, cross connections and catch basins connected to the sanitary system only provide flow during and immediately after a rain storm events.
3.0 Advantages of Sewer System Rehabilitation
Sewer system rehabilitation offers a number of community advantages including the following:
· Reduced pollution;
· Reduced operating costs; and,
· Reduced capital costs for collection system and treatment plant expansions.
As indicated previously, untreated wastewater contains disease causing organisms as well as a number of compounds which could harm the health of the community and natural environment. A leaky sewer system can increase pollution within a community in a number of ways. Cracks in collection system piping and defects in pipe joints will allow untreated wastewater to leak into the surrounding soil (exfiltration) and groundwater when groundwater levels are low, with resulting soil and groundwater contamination. In addition, excessive infiltration/inflow can result in wastewater flows which exceed the capacity of collection system piping, pumping stations, and treatment plants. The result will be sewer back-up into homes and other buildings in the community and by-passing and/or overflow conditions at sewage pumping stations and/or treatment plants. Sewage back-up causes obvious public health concerns and overflow and by-pass result in environmental contamination. Successful sewer rehabilitation helps to reduce the occurrence of these problems in a community.
Operating and maintenance costs at sewage pumping stations and treatment plants are directly related to the flow of wastewater to be pumped and treated. In general, higher wastewater flows result in higher operating costs for energy and treatment chemicals. Reducing infiltration/inflow and therefore wastewater flows will have a direct effect on operations costs.
In many communities excessive infiltration/inflow into the sanitary sewer system has increased wastewater flows to the sanitary sewer collection system and treatment facilities to such a degree that the existing facilities have reached their capacity far in advance of what was originally anticipated. For these communities, there are essentially three choices: limit growth within the community, expand existing facilities to accommodate the higher than anticipated flows, or reduce infiltration/inflow into the sewer system. Reducing infiltration/inflow into the sanitary sewer system is more cost effective than facility expansion in many cases.
4.0 Determining that a Infiltration/Inflow Problem Exists
How does an operator determine that her or she has an infiltration/inflow problem, given the significant benefits associated with the correction of this problem. Although there are a number of sophisticated techniques for determining the quantity of infiltration/inflow there are a number of very simply things the operators of the community wastewater system can do to determine whether or not they have a problem. These techniques involve review of historical records and field reconnaissance either during rainfall events or during low flow periods.
One of the easiest ways to determine whether your sanitary sewer system has a significant infiltration/inflow problem is to review historical flow and raw sewage quality records. This review would include a comparison of average wastewater flow and water usage records within the community, a comparison of wastewater quality during high rainfall periods and summer periods, and review of strip charts and other ‘real time’ flow records for known wet weather periods.
One of the easiest methods of determining whether or not your system is suffering from an infiltration/inflow problem is to compare average water usage with average wastewater flows at the community wastewater treatment plant. In many small communities, the people responsible for operating the water supply plant are the same people who are responsible for operating the wastewater collection and treatment system. In most communities where infiltration/inflow is not a problem, average wastewater flows will typically be 75% to 95% of average water usage. Therefore, an operator can quickly get an indication of whether or not they might have an infiltration/inflow problem within the sewer system by comparing average monthly water usage against average monthly wastewater flow. If the average monthly wastewater generation is significantly greater than water usage, especially during the early spring when groundwater levels tend to be higher, you may have an infiltration/inflow problem within the sanitary sewer collection system.
In some cases the operator will have historical wastewater flow data but no historical water usage records. In these cases the operator can calculate the average wastewater generation rate per person in the community, based upon the historical flow records collected at the wastewater treatment plant, and compare this value to known sewer design guidelines. In communities where infiltration/inflow is not excessive one would expect the average daily wastewater flow per person in the community to be in the range of 250 to 540 litres per person per day. If the average daily wastewater flow per person in your community is significantly higher than this value you most likely have an infiltration/inflow problem.
If for some reason the operator has no flow records at the wastewater treatment plant or those which are available are believed to be unreliable, one can use raw wastewater quality records in order to identify a potential infiltration/inflow problem. A comparison of raw wastewater quality between the summer and early spring can be used to indicate an infiltration/inflow problem may be present. For instance, if the five day biochemical oxygen demand (BOD5) concentration of the raw sewage entering your treatment plant is two to three times greater than that measured during the early spring, when groundwater levels tend to be high, there is most likely a problem with infiltration/inflow. The operator must be careful however that he or she is not making such determinations on the basis of only one or two samples, since the characteristics of raw wastewater tend to vary. A comparison of raw wastewater quality to typical values throughout the province can be used in those cases where seasonal wastewater quality records are unavailable. For instance, the five day biochemical oxygen demand (BOD5) concentration of raw sewage reaching the treatment plant in systems having minimal infiltration/inflow is typically in the range of 120 to 200 mg/L. If the BOD5 concentration of the raw sewage reaching your treatment plant is significantly lower than this range you likely have an infiltration/inflow problem.
In those cases where the operator does not have access to any flow or quality records, he or she may be able to determine they have a potential problem with infiltration/inflow in the sewer system by doing simple field reconnaissance during rainy weather. The field reconnaissance activities can include lifting manhole covers during a rainfall event to observe how full a sewer is flowing and the clarity of the wastewater. In those cases where infiltration/inflow is excessive the operator will notice a significant increase in sewage flow during a heavy rainfall with much of the flow appearing relatively clear (ie. most of the sewage flow is in fact groundwater and rain water). Additional clues to a potential infiltration/inflow problem include significantly longer pump run times at sewage pumping stations and incidents of by-passing and/or basement flooding during rain fall events.
5.0 Developing a Plan of Attack to Address Infiltration/Inflow Problems
The first step in any sewer rehabilitation program is to determine that a problem actually exists. This topic was covered in the previous section. The remaining steps in a sewer rehabilitation program are as follows:
(i) Infiltration
· Locating infiltration sources
· Eliminating infiltration by repair and/or replacement
· on-going maintenance
(ii) Inflow
· Discovering points of Inflow
· Determining if identified sources of inflow are legitimate
· Correcting inflow problems
· Establishing Policies.
Many of the tasks in a sewer rehabilitation program can be carried out using municipal works staff, consulting engineers and trenchless experienced contractors. Many of the above steps are discussed in the following sections.
6.0 Locating Infiltration/Inflow Sources
Click here for a pictoral guide to inflow/infiltration reduction techniques.
One of the primary goals of any sewer rehabilitation program is to isolate the source of the infiltration/inflow to as small an area as possible. There are a number of techniques available to assist the operator. These include wet weather manhole surveys, mainline and lateral service closed circuit television (CCTV), in sewer flow monitoring, house-to-house surveys, smoke testing, and comprehensive pipe joint test and seal programs. Each of these investigative techniques is discussed in more detail below.
Manhole Surveys are one of the simplest investigative techniques available to the operator for locating infiltration/inflow sources. Infiltration can enter a manhole structure through the frame-chimney joint adjustment ring and the brick and block area of manholes, the precast section seams, wall cracking, defective benching or at the connection point between the manhole and the inlet and outlet pipes. Inflow can enter a manhole through the vent holes in the manhole cover. Inflow will enter manholes located in low lying areas where rain water can pond over the manhole cover. Manhole surveys are conducted during wet weather, preferably during heavy rain fall periods. An operator can locate many infiltration problems by lifting manhole covers during a rainfall event and physically observing infiltration in through the manhole wall. Inflow sources are identified in a similar fashion. In many cases the operator can do a survey during a rainfall event noting manholes where rainwater is ponding over the cover or simply flowing into the manhole through the vent holes and around the cover seam. Those manholes requiring corrective action should be identified on a schematic of the sanitary collection system for immediate repair.
Mainline Closed Circuit Television Monitoring (Mainline CCTV) is accomplished by using closed-circuit systems specifically designed for sewer inspection. The system typically consists of a camera and monitoring equipment which allows the operator to control the speed and travel of the camera. CCTV monitoring allows the operator to identify actual leaks, areas of potential leaks such as pipe cracks or accumulations of mineral buildup, and service laterals which have a defective saddle connection at the main or a significant flow of clear water from the service line. CCTV monitoring should be carried out during high groundwater conditions when infiltration into the sewer system is at its peak. In addition, it is imperative to clean the sewer system prior to CCTV monitoring for excellent observations. The most accurate information can be obtained by inspecting sewer sections with the flow or from the upstream manhole and placing a plug up-line just prior to the inspection. This type of isolation technique will allow the operator to commence the inspection from a dry manhole base and thus be able to observe any flows or infiltration as it develops throughout the line being inspected. This technique eliminates the addition of upstream flows that will camouflage points of infiltration that may be active below the existing water flow levels. This type of inspection details the deficiencies and the results provide for accurate remedial costing.
Lateral Service Mini-Camera Closed Circuit Television Monitoring (Lateral Service CCTV) is accomplished by using closed-circuit systems specifically designed for lateral service sewer inspection. Lateral service lines observed to be flowing clear water steadily by the main CCTV camera unit should be inspected immediately with a mini-camera. The aim is to locate the exact source of the infiltration just observed on the main inspection system and detail that source on V.H.S. tape. Access can be obtained from either a clean out in the basement of the house or from a clean out on the front lawn. The lateral inspection will provide the detailed information as to the source of the infiltration and insight for rehabilitation costs and technique.
Smoke Testing is a relatively inexpensive and quick method of detecting inflow sources in a sanitary sewer system. Smoke testing is particularly effective in detecting inflow sources such as roof leaders, area drains, foundation drains, abandoned building sewers, cross-connected storm catch basins, and faulty service connections. Smoke generators are used to introduce the smoke required for smoke testing a section of sewer. Sewer plugs may be used to prevent smoke from escaping to adjacent sewer pipes and manholes and a blower is typically used to force the smoke into the sewer system. Smoke will escape from all inflow sources, such as roof leaders, and catch basins which are cross-connected to the sewer section being tested. It is important to notify community members that smoke testing is being conducted since a smoke discharge from a neighbours roof leader may lead people to believe the neighbours house is on fire. One very important point to remember with smoke testing is that a negative result does not necessarily mean that there are no inflow sources connected to the sewer section being tested since pipe sags or water traps may prevent smoke from passing through, resulting in an inaccurate determination.
House-to-House Surveys are typically conducted in order to identify sources of inflow originating within homes and other buildings. These field investigations are used to verify the discharge locations for sump pumps which are used to remove groundwater leakage from building basements. Most municipalities prohibit sump pump connections to the sanitary sewer system, since these pumps typically handle storm water. During a home inspection, the operator would identify those residents which have the sump pump connected to the sanitary sewer system. These connections would be re-routed at a later date in order to eliminate these flows from the sanitary sewer system.
Flow Monitoring Programs allow the operator to isolate areas or specific reaches of the sewer system which have excessive infiltration and/or inflow. There are a number of techniques available. Modern flow monitoring equipment typically has a data logging function which allows the operator to collect ‘real time’ flow records over an extended period of time. These data can then be correlated with rain fall events. Areas of the collection system which exhibit significant increases in peak flow over a dry weather period during a rainfall event most likely suffer from significant infiltration/inflow into the sewer system. Another technique that is commonly used is night flow isolation whereby an operator will measure flows in the sewer system in the early morning hours, between midnight and 6:00 A.M., when sewer flows should be virtually zero. Reaches of the sewer system with significant flows during the early morning hours most likely have significant infiltration.
Test and Seal programs are a cost effective way to identify defective lateral service mouth connecting fittings and sewer joints. Test and seal equipment includes a cylindrical packer with inflatable end elements. The packer is inserted into the sewer and then positioned across the pipe joint. The rubber end elements are inflated to isolate and the pipe joint which is then tested under an air/water pressure of 5 to 10 psi. If the air/water pressure holds constant the joint is passed. If the test pressure declines indicating the joint failed the test, a chemical grout is pumped through the joint into the surrounding ground until the chemicals solidify outside the pipe into a tight-fitting, waterproof seal, and an acceptable back pressure is achieved. The joint is then re-tested following the grouting procedure. The test and seal process is controlled and monitored by CCTV. The television monitor, pumps, air compressors, and chemical transfer equipment typically are housed in a van that acts as the operations and control centre. This test and seal method is very economical.
7.0 Repair Methods
There are a number of available methods for fixing infiltration/inflow problems once they have been discovered. In some cases, where a pipeline or manhole has deteriorated significantly replacement is the only option. In many cases, collection system components can be repaired using trenchless technologies which generally cost less than conventional replacement and involve little or no open trench excavation which results in reduced traffic disruption and public inconvenience. Available repair methods for manholes, sewers, and service laterals are presented here.
(i) Manholes Rehabilitation Options
The rehabilitation techniques that are normally used for manholes include coatings, ladder rung removal and replacement, structural lining, sealing of frame-chimney joint, precast chimney cone areas, installation of manhole bowls to collect inflow, and chemical grouting. Each of these techniques is briefly described below.
Coating systems can be applied to manholes to restore manhole structures for several years. The coating process includes cleaning and surface preparation followed by application of a cementitious material containing Portland cement, finely graded mineral fillers, and chemical additives. Coating systems can be used for the entire manhole, including reconstruction of the benching and invert.
Replacement of Ladder Rungs can be accomplished by drilling adequately sized holes in the side of the manhole wall, insertion of the new ladder rungs, and grouting of these new rungs into place.
Sealing of Frame-Chimney Adjustment Joint Section can be accomplished using a mechanical seal, such as a CretexTM seal. A CretexTM seal consists of a high quality rubber seal positioned around the entire chimney-frame area of the manhole. The seals are held in place by stainless steel bands with a positive locking mechanism. The seal prevents groundwater from infiltrating into the manhole in this area that is exposed to the frost/freeze and thaw zone. These seals are totally unaffected by freezing.
Manhole Bowls are used to prevent storm waters from entering the sanitary manhole through the holes and around the manhole cover. These devices are typically constructed of plastic, fibreglass, or stainless steel and are inserted under the manhole cover itself. The insert collects manhole inflow. Manhole inserts should include vacuum relief, gas relief valves, and watertight gaskets..
Manhole Chemical Grouting/CretexTM Seal Application is a cost-effective option for manhole rehabilitation. The success of chemical grouting in sealing existing leaks depends upon soil and groundwater conditions, injection patterns, gel time and component mixture, containment of excessive grout migration, and selection of the proper type of grout. There are several available urethane grouts for manhole rehabilitation. The chemical grouting process involves cleaning the manhole, drilling holes in the manhole wall for grout injection, and injecting a grout/water component mixture into these holes under pressure. The grout is pumped until the grout/water mixture is observed entering back into the manhole through existing cracks or precast seam faults and the sealant forms (gels) a new foam gasket material. The manholes are normally grouted prior to mainline sewer chemical grouting.
(ii) Pipeline Rehabilitation Options
Pipeline rehabilitation techniques include sliplining, cured-in-place pipe lining, pipe bursting, cleaning, root removal, internal chemical grouting, external grouting, mechanical sealing, and structural spot repairs.
Pipe Lining may be installed inside a sewer in either continuous or short lengths. Pipe lining techniques include cured-in-place pipe, deformed pipe, and spiral-wound pipe.
Sliplining involves inserting a new pipe into an existing sewer line as a continuous pipe or in short lengths. The space between the pipe lining and the existing sewer pipe is then grouted. In continuous sliplining lengths of pipe are butt fusion-welded together before installation in the sewer. The new pipe is either drawn or jacked through the existing pipe. As an alternative to sliplining with continuous pipes, short pipe lengths can be joined by gaskets or mechanical joints. They are inserted either by carrying individual pipes into the sewer and joining them in place or jacking pipes into place. This latter technique is typically suitable for large pipes only.
The Cured-In-Place-Pipe lining process involves inserting a thermal resin impregnated flexible felt woven liner into the sewer using either an inversion process or a pull-in process. The liner is inserted through existing manholes and depending on the system selected, installed using water inversion, steam, or air inflation. With water inversion, the lining is inverted under water pressure and cured by circulating hot water. With the pull-in version, the lining is winched into place and inflated into place against the sewer wall by an air bladder. The lining is then cured by circulating low pressure controlled hot water or steam. With the air version, the lining is inverted under air pressure and cured by introducing hot water or steam.
Pipe Bursting is a method of inserting a new pipe of equal or larger diameter into an existing pipeline by fragmenting the existing pipe work and forcing the material into the surrounding soil. The new pipe is then inserted into the enlarged hole.
Cleaning is required prior to CCTV inspection and rehabilitation. Pre-cleaning of sewers allows the entry of pipe liners and promotes a tight fit between the liner and the existing pipe material. Sewer cleaning is typically accomplished using a mobile high pressure sewer cleaning unit. Protruding lateral services, calcite buildups, roots, and broken pipe sections are normally removed using a power auger or reamer prior to cleaning and subsequent relining and/or structural spot repair.
Root Removal involves mechanical removal. Typically, roots are cut using a sewer flusher and reaming tools. Roots typically will grow back after each cutting operation therefore root cutting should be followed by immediate chemical treatment with a root retardent. The entry of roots into a sewer or service lateral will increase the rate of infiltration into the pipeline by expanding an existing joint. Root removal is compulsory prior to any remedial program.
Internal Grouting is the most common method of sealing leaking sewer joints in structurally sound sewer pipes. A cylindrical packer with inflatable end elements is used to seal small and medium sewer pipes up to 36 in. diameter. The packer is inserted into the pipe and positioned across the sewer joint. The end elements are inflated and the joint is tested using air/water pressure. If the joint fails the test, a chemical grout is pumped through the joint to the surrounding soil until the grout solidifies. The joint is then re-tested. This procedure is repeated if necessary until the joint is sealed. The test and seal process is controlled and monitored by closed circuit television. Special packers are available for testing and sealing the lateral service connection where the service mouth meets a mainline sewer. Large diameter sewers are sealed by man-entry mechanical injection devices that range in sizes up to 108 in. diameter.
Mechanical Seals consist of P.V.C. sectional pipe and/or rubber seal positioned across a defective joint and held in place by a stainless steel locking band. Mechanical seals maybe similar to the CretexTM seals used for sealing the chimney section of a leaking manhole.
(iii) Lateral Repair Techniques
Available techniques for service lateral repair include chemical grouting, spot repairs, and lining. These techniques are virtually scaled down versions of the same techniques used for mainline and collection sewers.
8.0 Project Implementation and Funding
The funding process for a sewer rehabilitation project is similar to any other funded capital works project. The first step is preparation of a sewer rehabilitation study. This study will identify the need for sewer rehabilitation and quantify some of the associated benefits. The preparation of a sewer rehabilitation study includes a review of historical water and sewage flow records, followed by many of the investigative techniques described in this paper. Much of this work including review of flow records, manhole surveys, and house-to-house surveys can be completed by the community’s operations staff with little or no additional equipment.
Once a rehabilitation study is completed the community would prepare an application for Preliminary Project Approval (PPA). The PPA would be used to provide the required funding for preparation of specifications and tendering of the rehabilitation project.
Once the rehabilitation works specifications are prepared, the community would prepare an Effective Project Approval (EPA) in support of the required capital dollars for the rehabilitation project. Construction dollars would be released once the EPA is approved and the community can proceed with award of the sewer rehabilitation project.
At the completion of the rehabilitation project, the community would prepare a Project Completion Report which outlines the history of the project and lessons learned. This history can then be used by other communities to assist them in conducting similar capital projects.
9.0 Conclusions
Sewer rehabilitation is a cost effective alternative to sewer replacement and facility expansion. This is illustrated in the case history presented here, where sewer rehabilitation techniques allowed the Village of Hastings to solve a treatment plant capacity problem very cost effectively. Much of the upfront work involved in a sewer rehabilitation project can be carried out by local staff or consulting engineers. Given the potential benefits, sewer rehabilitation should be critically evaluated by all communities considering a sewage treatment plant expansion.
For further technical information regarding this and other projects, please contact the writer, Earl M. Brousseau.