In recent years, new methodologies and accelerated programs have been developed to take advantage of the information obtained from sewer line maintenance operations. Such programs incorporate information gathered from various maintenance activities with basic sewer evaluations to create a system that can remedy and prevent future malfunctions and failures more effectively and efficiently.
A study performed by the American Society of Civil Engineers reports that the most important maintenance activities are cleaning and CCTV inspections. Below shows the most important maintenance activities (cleaning and CCTV inspections) and the frequency of various maintenance activities:
Cleaning – 29.9 % of system per year
Root Removal – 2.9 % of system per year
Manhole Inspection – 19.8 % of system per year
CCTV Inspection – 6.8 % of system per year
Smoke Testing – 7.8 % of system per year
A maintenance plan attempts to develop a strategy and priority for maintaining pipes based on several of the following factors:
Problems- frequency and location; 80 percent of problems occur in 25 percent of the system (Hardin and Messer, 1997)
Age- older systems have a greater risk of deterioration than newly constructed sewers
Construction material- pipes constructed of materials that are susceptible to corrosion have a greater potential of deterioration and potential collapse
Non-reinforced concrete pipes, brick pipes, and asbestos cement pipes are examples of pipes susceptible to corrosion. Pipe diameter/volume conveyed- pipes that carry larger volumes take precedence over pipes that carry a smaller volume
Location- pipes located on shallow slopes or in flood prone areas have a higher priority
Force main vs. gravity-force mains have a higher priority than gravity, size for size, due to the complexity of the cleaning and repairs
Subsurface conditions- depth to groundwater, depth to bedrock, soil properties (classification, strength, porosity, compressibility, frost susceptibility, erodibility, and pH
Corrosion potential- Hydrogen Sulfide (H2S) is responsible for corroding sewers, structures, and equipment used in wastewater collection systems. The interior conditions of the pipes need to be monitored and treatment needs to be implemented to prevent the growth of slime bacteria and the production of H2S gases
There are many advantages and disadvantages to inspection techniques in the collection system.
Below some of the limitations are listed:
Visual Inspection – In smaller sewers, the scope of problems detected is minimal because the only portion of the sewer that can be seen in detail in near the manhole. Therefore, any definitive information on cracks or other structural problems is unlikely. However, this method does provide information needed to make decisions on rehabilitation.
Camera Inspection – When performing a camera inspection in a large diameter sewer, the inspection crew is essentially taking photographs haphazardly, and as a result, the photographs tend to be less comprehensive.
Closed Circuit Television (CCTV) – This method requires late night inspection and as a result the TV operators are vulnerable to lapses in concentration. CCTV inspections are also quite expensive and time consuming.
Lamping Inspection – The video camera does not fit into the pipe and during the inspection it remains only in the maintenance hole. As a result, only the first 10 feet of the pipe can be viewed or inspected using this method.
There are also advantages and disadvantages to different cleaning methods as shown below:
* Balling – cannot be used effectively in pipes with bad offset joints or protruding service connections because the ball can become distorted
* Jetting – The main limitation of this technique is that cautions need to be used in areas with basement fixtures and in steep-grade hill areas
* Scooter – When cleaning larger lines, the manholes need to be designed to a larger size in order to receive and retrieve the equipment. Otherwise, the scooter needs to be assembled in the manhole. Caution also needs to be used in areas with basement fixtures and in steep-grade hill areas.
In general, these methods are only successful when necessary water pressure or head is maintained without flooding basements or houses at low elevations.
* Bucket Machine – This device has been known to damage sewers. The bucket machine cannot be used when the line is completely plugged because this prevents the cable from being threaded from one manhole to the next. Set-up of this equipment is time-consuming.
* Flushing – This method is not very effective in removing heavy solids. Flushing does not remedy this problem because it only achieves temporary movement of debris from one section to another in the system.
* High Velocity Cleaner – The efficiency and effectiveness of removing debris by this method decreases as the cross-sectional areas of the pipe increase. Backups into residences have been known to occur when this method has been used by inexperienced operators. Even experienced operators require extra time to clear pipes of roots and grease.
* Kite or Bag – When using this method, use caution in locations with basement fixtures and steep-grade hill areas.
* Rodding – Continuous rods are harder to retrieve and repair if broken and they are not useful in lines with a diameter of greater than 300 mm (0.984 feet) because the rods have a tendency to coil and bend. This device also does not effectively remove sand or grit, but may only loosen the material to be flushed out at a later time.