That strategy will already include reactive root cutting and jet cleaning, some CCTV survey work to identify the cause and extent of structural failures and spot repairs. This strategy allows crews to keep up with reactive works, but problems are defined as important or urgent, with only the urgent problems addressed as there is no time for the important jobs.
The key to success is to therefore develop a pro-active strategy that reduces blockages and overflows, freeing up crews to work on the important jobs rather than waiting until the important becomes urgent and crews have to start running – which can result in out of hours work with clean up, compensation costs and bad publicity.
A comprehensive study investigating the economic analysis of root control and back water flow control relating to infiltration and inflow by the US EPA (600/2-77-017a) found that "Mechanical cleaning offers a positive initial result but appears to make root growth more of a problem over the long run. Agencies judged to have the most comprehensive root control programs also appeared to have excellent preventative maintenance program."
So how is a comprehensive root control program developed? The program has to be proactive, targeted, comprehensive and ongoing to achieve the desired results. A chemical control program can deliver such results. San Diego City claims to have reduced overflows by 57 per cent as a result of chemical root control programs. The first step is to identify areas and sections of pipe that have or are likely to experience sewer blockages. Typically this information is gathered from previous blockage reports. The reports will identify areas where blockages have occurred and chemical control will be of significant benefit.
Best results will be achieved if longer sections of pipe are treated, rather than just one man hole length that has had a blockage, particularly if the landscape is consistent over the street length. If there are tree roots in one section of main and there are trees all the way up the street, it is fair to assume that tree roots will already be in the pipe or have begun to attack the pipe. Where blockages have occurred and a root cutter has been used to clear the pipe, chemical treatment should be undertaken within six months but not less than 6 weeks after cutting. Chemical root treatments should be scheduled two times each year. This allows cleared lines to be treated before reblocking occurs, if cleared correctly. It also allows the roots to repair and begin to regrow to better absorb the herbicide.
The application process should include a herbicide killing agent, a growth inhibiting agent and have a controlled expanding foam method of delivery.
A database should be maintained of all manhole sections treated with a chemical treatment. This will allow for the monitoring of the performance of the treatment process. It will also ensure that re treatment dates are scheduled as part of ongoing works.
Property branch lines are a common entry point into sewers for roots of trees planted in nature strips and houses. After root cutting, a CCTV inspection will often reveal roots entering from the branch were the cause of a blockage. These roots are pruned back by the cutter and immediately start regrowing thicker and faster. Chemical treatment of reticulation lines allows the herbicide to penetrate a short section of branch line as part of the process. Many Authorities are now also treating branch lines with in house staff or contractors to ensure blockages do not originate form the branch lines into the main, causing a costly and inconvenient blockage.
Once the chemical treatment process is completed the treated lines should be left undisturbed for a period of six months. This allows the dead roots to decay and disintegrate. The roots will drop off gradually into the flow and be captured for disposal at the treatment plant. Dead roots in the pipe are weak and flimsy and rarely cause a blockage either after dieing, before falling off into the flow or as they flow down stream. The smaller roots drop off quicker than larger roots which disintegrate over time. Any large dead roots left after six months will not restrict CCTV inspection equipment and ultimately a more complete survey will be achieved as a result of the foaming program. The inspections allow better viewing of the pipe condition with the roots dead and the pipe clear. This improved information will assist decision makers deciding if pipe sections require relining, replacement or just to be maintained on the three year foaming program.
The advantage of killing the roots over cutting the roots out is three fold. Firstly, the roots in the pipe are killed along with the root in the pipe wall and a few centimetres outside the pipe. As the dead roots rot the cracks in the pipe made by the growing root can close up under ground pressure. This action assists in reducing inflow and infiltration. It also preserves the quality of the ground water by reducing outflow of sewerage into the environment.
Secondly, unlike root cutting that stimulates more aggressive regrowth and only removes up to 80 per cent of the root mass leaving root vials at the top where they enter, the herbicide foam fills the pipe and kills all roots it comes into contact with. Destroying roots with herbicides eliminates the ongoing use of a root cutter. The root cutter is an abrasive action on the pipe and can damage or dislodge the rubber joiner seals. Over time this significantly reduces the life expectancy of the asset.
Thirdly, as part of the herbicide application process a growth inhibiting agent, Dichlobenil, a surface active chemical combined with Bentonite clay, is applied to the sewer and adheres to cracks, defects and organic matter in the pipe. Dichlobenil does not kill anything - it is a growth inhibiting agent, a soil sterilant that stops plant cells dividing and hence halts growth.
The effectiveness of chemical treatment will be reduced if the correct methods are not followed. Some Authorities use chemical treatment as a pro-active tool. If you cut or have to clear heavy blockages to allow for chemical treatment the effectiveness of the killing agent is reduced. The bleeding roots reduce the amount of chemical absorbed. Very heavy root masses restrict the passage of the foam and reduce the contact between the roots and the herbicide. Roots not coming into contact with the herbicide will not die. Dirty pipes with heavy build up of debris, particularly grease, protect the roots from contact with the herbicide. If the pipes cannot be flushed clean to expose the roots with a low pressure wash (1500 psi) then the pipe may need high pressure jetting or a degreasing agent prior to chemical treatment.
To achieve positive results from chemical root treatments the need for such treatments should first be evaluated based on blockage reports. Also older areas with well established gardens and mature trees are areas that will benefit from a chemical treatment program. Once a pipe is treated with chemicals it should be left undisturbed for at least six months Jetting of residual lines will remove the growth inhibiting agents and reduce the effectiveness of the program. Once a program starts lines should be retreated prior to the expiry of the guarantee of three years. The second and subsequent treatments will offer accumulative benefits and improved performance. The cost of chemical treatment when amortised over the period of the guarantee is considerably cheaper than scheduled root cutting. It is also cheaper than reactive cleaning based on blockages and over flows.
For a section of pipe on a scheduled cutting program, the cost per metre is $2.50 and it is cut each 12 months. The cost over the three year period is therefore $7.50 per metre. If it is cut every six months then the cost over three years is $15.00. Chemical treatment of a 150 mm pipe guaranteed for three years will cost between $3.60 and $5.00 per metre.
The US EPA report (600/2-77-017a) stated that "Chemical control with Vaporooter is not 100 per cent effective, but repeated applications over several years approaches this limit. No other method of control approaches the effectiveness found using Vaporooter."
"Mechanical cleaning offers a positive initial result but appears to make root growth more of a problem over the long run. Agencies judged to have the most comprehensive root control programs also appeared to have excellent preventative maintenance programs."