Transferring private sewers: The long and the short
The quantity of private sewerage being transferred to sewerage companies is largely unknown. Claire Brown, a consultant scientist at WRc, reports on a new model to gauge the scale of works
Once a water service company decides that certain sewers are not on the sewer map, they become like a dependent child - dependent on the homeowners for repair. Often insurance companies or local authorities have to pick up the pieces and pay for the repair. But sometimes the homeowner has to pay the cost.
This might be acceptable if the pipe was on your land and you were the only person to use it. Presently, though, many of these private sewers are shared by several property owners, and stretch beyond property boundaries into highways and other people's gardens.
Many people thought this system was unfair. So, on February 22, the government decided to transfer these difficult sewers to the sewerage companies in England and Wales. But what does this really mean?
Private sewers and the lateral drains that go beyond the property's boundary will be transferred. Generally, it looks like sewers and lateral drains that drain by gravity will be transferred overnight, followed by a phased approach to sewers served by pumping stations, and the pumping stations themselves.
Nearly every property in the country has a lateral drain - whether it is a metre or a mile long. Many sewers were built in gardens and then connected to the main sewer in the street. These main sewers tend to be operated by the sewerage companies.
So what's the problem?
There are lots of problems. To begin with, the total amount of pipes to be transferred is not known. Secondly, no-one knows where the pipes are. Very few people actually have plans of their drainage. And the condition of the pipes needs to be considered.
In order to quantify some of these problems, WRc undertook a project for UKWIR. This project modelled the length to be transferred and the cost nationally to the sewerage companies.
The asset model considered how many properties were built with foul and surface water draining separately on site. Occasionally, the overall length of foul and surface water sewers were not double that of combined sewers. Unit lengths for foul and surface water sewers and lateral drains were calculated from plans. These were separated into seven different types of property in six different age bands. These ages bands ranged from 1850 to the present. A matrix was created for each type of drain and sewer, within each age band and property type.
Consideration was also given to the location of these properties. An additional matrix of unit lengths was created for properties located in an area with 50 or more houses that were all privately sewered. Often the drainage layouts of these areas varied, giving different average lengths for foul and surface water sewers and drains in each property type, and in each age band.
The date the property was built was also considered, as in 1937 sewers were transferred to the sewerage companies (these became known as Section 24 sewers).
These sewers were not included in the overall length, but the lateral drains from these properties were.
The number of inspection chambers on drains and in gardens were considered alongside the number of manhole chambers. An average number per metre was established.
Previously, drainage systems were built with interceptors (sometimes called blind siphons, Windsor traps or bauchan traps). These were installed to prevent odours entering the property. But these were phased out around the time of World War II as more appliances came with traps built in. Many interceptors were removed if they became problematic, and so the survival rate of interceptors also had to be considered.
Once the asset model had been developed, the project began to investigate the frequency of tasks on those pipes. Finally, costs were associated with those tasks.
The tasks identified included urgent repairs, blockage clearance, proactive cleaning, rodent baiting, flooding costs, and mapping.
Urgent repairs are for those sewers that are not functioning as sewers and have to have some repair or need to be rehabilitated in order to work properly. Many sewers were installed in back gardens and not offered for adoption.
This meant that they did not need to meet water industry standards, so they were never inspected by a company sewerage inspector. This means the initial condition of private sewers is unknown.
It follows that without a starting point the current condition cannot be predicted. Therefore, this aspect of the model is based on engineering judgement.
Little information about the amount of work undertaken on private drains is kept. There are sources of information from insurers and local authorities. But there is an unknown element.
Some homeowners do not have the option of going through the council as the property is not a council house. Others are unaware that their insurance covers it. Others still have no insurance. It is that proportion of work that is unknown. For this reason, the model predicts blockage rates based on data from the insurance sector and knowledge of Section 24 sewers.
Proactive cleaning and rodent baiting will continue but on a greater scale as there will be a larger network to maintain. The cost of these activities was taken as an average of current prices used by sewerage companies.
The project assumes little increase in the number of properties flooded by hydraulic overload, but the number of properties flooded by other causes may increase.
When it comes to mapping these sewers, it will be a massive task. The project proposes that sewers and drains are mapped reactively, for example when the drain or sewer is investigated for an incident such as a blockage or flooding. This follows the principle set out in the New Roads and Street Works Act 1991. Proactive mapping could be undertaken from existing plans attached to legal agreements.
The workload for each activity was established per metre and multiplied by the total length of pipe to be transferred. The model looks at lateral drains and private sewers separately to minimise the error.
Overall, the model calculates the total length of pipe for transfer but this can be separated out. There are specific lengths for lateral drains this is separated in the amount of foul drain in the highway or in private land. The same is possible for surface water. These categories are repeated for private sewers that will be transferred.
So what did WRc find?
The total estimate of pipes for transfer is about 250,000km. This includes lateral drains and sewers for both foul and surface water drains in all ages and property types. This also includes the category for estates that are privately sewered.
There are about 700,000 inspection chambers on lateral drains, and a further 3.7 million manhole chambers with less than 400,000 chambers in the highway, all expected to be transferred. The area of greatest uncertainty is the number of interceptors, as the number built in the first place is not recorded. Some local authorities (back when they were rural and urban district councils) insisted they were installed.
In 1911, the model building regulations (that local authorities used to draft their own regulations) contained build standards on interceptors. But, by the time these were made into a national building regulation in 1965, the standard for interceptors had disappeared.
The date when interceptors stopped being installed is different in each local authority area, and often varies across the modern boundaries. But the total number of interceptors is about 2.1 million interceptors.
The question then becomes one of location. If the interceptor is on the inside of the property boundary, it will not be transferred. If it is outside the boundary, it will be. If it is on the boundary, then only time will tell where responsibility will lie.
The model estimated that up to 240,000 blockages a year will also be transferred to sewerage company responsibility. This is estimated by Defra to be less than half the work that private drainage contractors deal with. Most of these blockages are caused by sewer abuse rather than from broken drains.
What happens next?
Currently WRc is reviewing this national model for individual sewerage companies. Part of this process is to identify areas where there are sewers in poor condition. This will enable the sewerage companies to address the work required earlier than they would otherwise.
For the future the water industry and government needs to prevent the creation of more private sewers, by creating a national standard that sewers and drains should be built to. For those drains and sewers that are transferred, a code of practice needs to be developed so that insurance companies drain repairers and sewerage undertakers can work together.