While state and federal governments continue to expect water authorities to do more with less in order to maintain ageing sewer reticulation assets, developments in technology are offering new ways to meet solve these challenges. It is now more important than ever to develop an effective overall strategy that will deal with the immediate reactive challenges faced by many authorities, as well as the development of planning and maintenance strategies that will shift the focus from reactive to proactive solutions.

The strategy will include reactive root cutting, jet cleaning, and CCTV survey work to identify the cause and extent of structural failures, the level of root intrusion, and when that will result in a system failure. Asset replacement or upgrading will also form part of the strategy. This design will allow crews to keep up with reactive works, while also enabling them to plan and undertake preventative measures in order to extend the asset life or increase the assets capacity.

Chemical power

The professionals that wage a trenchless war on sewer blockages and structural damage have used a variety of methods to rid their systems of roots.

“By far, the most common method has been mechanical root cutting. Unfortunately, today’s most common solution is tomorrow’s blockage problem, and a future candidate for replacement,” says Mr Horne.

Surprisingly, the uptake of herbicide control of tree root intrusion in sewer pipes has been slower than expected, and has resulted in an enormous drain on water authority budgets and equipment, he says.

“Although cutting is a necessary tool in the arsenal for clearing blocked lines, it has its drawbacks. In addition to potentially damaging pipes and manholes, cutting roots actually causes them to grow back thicker and healthier.”

The persistent problem of root blockages led Sacramento County in California to conduct research in the 1960s to find a long term solution. In 1967, the County called on botanist Leonard Oliver at the University of California, Davis to conduct a study to determine why their cutting efforts were not successful.

Attempting to understand the nature of various types of roots and the effects they were having on sewer systems, the study quickly took on all the characteristics of an archaeological dig. Working sewer lines were exposed through careful excavation, root growth patterns were examined, and the results were documented and analysed.

The researchers found that the physical environment within a sewer is an ideal growth environment for roots. Since mechanical root cutting only intensified the problem, they began to search for alternative solutions, primarily using chemical treatments. From an exhaustive list of potential chemistry, they discovered a unique chemical that was very effective at killing all types of roots: metham sodium. Metham’s unique ability to kill all roots on contact without harming the plant itself, combined with the fact that it dissipated quickly without leaving any harmful residue behind, made it the first choice. The discovery of the right herbicide was only the beginning of the development of the product that pioneered chemical root control, which remains the most effective means of eliminating roots to this day, says Mr Horne.

The research team went on to combine the attributes of metham with a powerful growth inhibitor, dichlobenil. Final commercialisation required testing compatibility with various surfactants to identify a way to deliver the herbicides in dense foam, thereby ensuring the chemical made contact with all roots and pipe surfaces. Effectively, this was the foundation of root foaming of sewers, and for the past 40 years, “it has proven to be a most effective formula for eliminating roots and preventing re-growth, and will continue to be for years to come,” says Mr Horne. The treatment process

An effective root control program starts by identifying areas and sections of pipe that have, or are likely to experience, sewer blockages. A report will identify areas where blockages have occurred and chemical control will have significant benefit. The best results will be achieved if longer sections of pipe are treated rather than just one manhole length that has had a blockage, particularly if the landscape is consistent over the street length.

To ensure the best possible result, water authorities will often cut and clean manhole lengths prior to treatment with a chemical herbicide. Chemical treatment should be undertaken not less than six weeks after the cleaning process, in order for the roots to regrow into the pipe. This delay is critical, as the root foaming procedure requires contact between the roots and the chemical to allow the roots to absorb the herbicide.

Once the chemical treatment process is complete, the treated lines should be left undisturbed for a period of six months, says Mr Horne. This will allow 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 after dying, before falling off into the flow or as they flow downstream.”

Treatment advantages

Mr Horne says the advantage of killing the roots over cutting is threefold. 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 may close up under ground pressure. This action will assist reducing inflow and infiltration, while also preserving the quality of the groundwater by reducing outflow of sewage into the environment.

Secondly, unlike root cutting that stimulates more aggressive re-growth and only removes up to 80 per cent of the root mass, the herbicide foam will fill the pipe and kill all roots it touches. Destroying to roots with herbicides eliminates the need to continually use a root cutter.

Thirdly, as part of the herbicide application process, a growth inhibiting agent should be applied to the sewer that adheres to cracks, defects and organic matter in the pipe. This critical aspect of a foaming program controls new root growth into the sewer and allows for a three year respite from tree root infiltration.

“Effective root foaming programs are being undertaken right across Australia. Some authorities are now undertaking branch line applications from the boundary to the reticulation network using in-house crews and mini foamer systems with great results,” says Mr Horne.