Since digging up countless miles of the 6,500 mile system to clean out the pipes is not a cost-effective solution, the program hinges on the combination of a strict cleaning schedule and chemical applications.
A new three-year contract begins with city crews cleaning the pipes using mechanical rodding machines and high-velocity sewer cleaners. Then, the pipes are chemically treated with the active ingredients metam-sodium and dichlobenil, which have been used in sewer maintenance for more than 30 years.
Once the chemical has been applied, the pipes are cleaned again six months later to prevent the collection of dead roots and other debris that could possibly cause blockages.
“Common blockages are caused by either fats, oils or greases, which can be fought by developing ordinances and scheduling routine cleanings; or root intrusion – and this requires a different solution, as regular cleaning does not prevent root regrowth,” said Barry Berggren, division manager for the City of Los Angeles wastewater collection system. “It’s a lot like pruning a tree, you cut roots from a sewer, but they will just grow back thicker and stronger.”
The chemical effectiveness is twofold. First, the metam-sodium breaks down into a gas that kills roots on contact, while the dichlobenil inhibits root growth and prevents the root from growing back. However, when the metam-sodium breaks down, it targets only the roots – it does not move through the root system to kill the plant. Thus far, the city’s root control program is said to have a 99.6 per cent success rate – or, in other terms, about 0.4 per cent of the pipes treated will experience an overflow within two years.
While the metam-sodium is a somewhat restricted chemical, it has been used for years in the agriculture industry and has no effect on the downstream 80 to 400 million gallon treatment plants.
Although the program has been successful and pipe maintenance remains on a 20 to 24 month cycle, it seems that the root problem is not easily remedied.
“More than 50 per cent of the root problem stems from private house laterals – the pipes between the house and the connection to the city’s mainline sewer,” said Mr Berggren. “Most of the time, roots grow from the private laterals and move into the city system.”
Often private homeowners do not realise that the roots have infiltrated their systems and although there are private sewer cleaners that will come out and clean your pipes, the roots that are cleared from the homeowner’s pipes are often pushed into the city’s pipes, causing
Throwing another wrench into the program, each pipe segment must be analysed to ensure that it is given the correct treatment. The dose of the chemicals and cleaning depends on the type of tree, the size of the root, the severity of intrusion, and the slope and movement of water through the pipe.
Generally two types of roots are found in sewer lines – veils and tails. Veil roots penetrate the top or sides and hang from the pipe’s upper surface and skim the steady flow. Tail roots, however, grow into the pipe from any direction and continue to grow downstream. Eventually, the tail root fills the pipe and causes major damage. Tail roots upward of 40 feet long have been removed from sewers.
Root growth is usually less prevalent in late spring and summer when the tree’s canopy is growing. Once fall and winter blow in, the lack of rainfall and colder temperatures cause the roots to go in search of a suitable source of nutrients. A microscopic opening would allow roots to infiltrate the pipe’s integrity.
In addition to overflows and blockage, roots also cause a reduction in hydraulic capacity and loss of self-cleaning velocities as it solidifies its home in the pipe.
The program in LA
With its humble beginnings in a pilot program covering only 150 miles of sewer pipe, root control in Los Angeles has long since been a priority. In the early 1990s, a chemical treatment program began but the end of the decade brought a change in which pesticides were restricted and the program was dissolved.
The Bureau of Sanitation spent approximately $US750,000 by the end of the fiscal year in 2002 on the pilot program. That price tag has since increased to $US2 million per year.
Nevertheless, the benefit of the program clearly outweighs any monetary sum, saves money in the long run and is a step toward addressing the Environmental Protection Agency’s (EPA) sanitary sewer overflows concern. The EPA estimates that 43 per cent of overflows are caused by pipe blockages and approximately half of those are caused by roots.
The program in Los Angeles seeks to take on this problem and is so large that it takes two separate contractors to do the job. Both Pacific Sewer Maintenance and Dukes Root Control will receive $US1 million a year for three years, beginning in May 2006, and the root control program is just one of many efforts in sewer line maintenance. Crews work seven days a week to ensure that the system remains free flowing.
“The root control program is reaching maturity and will, most likely, continue for the foreseeable future,” says Mr Berggren. “There is little that can be done to prevent roots from entering an established sewer, especially in an arid region such as Los Angeles.”