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  • There is an opportunity here to redirect investment towards investigation of the actual risk profile of private sewers and the risks they present to achieving customer service standards.

    Mike Sanderson, Strategy Director, 1Spatial

Want holistic wastewater network knowledge within hours? Mike Sanderson from 1Spatial explains how to quickly and cheaply get a handle on private sewer assets and how they fit with the wider network.

Current estimates of the length of the private sewers transferred to water company ownership a year ago have a confidence interval of +/- 40 per cent. Since there is uncertainty and an absence of hard facts, Ofwat has implemented a cost pass-through arrangement to the transferred assets during the current asset management plan. This cost pass-through arrangement is expected to cease at the next price review in 2014. By then, Ofwat will expect additional information to be available, based on experience, following transfer.

So how should water companies go about amassing information on their new assets and assessing baseline costs? UK Water Industry Research (UKWIR) and WRc have built on work; popularly known as the English House Condition Survey. This provides detailed estimates of pipe lengths and manhole numbers for different house types and ages in matrix form, as shown in the table overleaf. This information can be used as a baseline-check during the private sewers inferencing process.

Since 2001, Ordnance Survey has produced products under the OS MasterMap® brand name. These products are databases of information in their own right. The road centre lines, the buildings and topographic identifier references are valuable master data tools and fundamental in building asset knowledge bases. These products have now been enhanced by a common gazetteer known as AddressBase®, enabling information flow from local authority building control departments to the pre build layer in OS MasterMap to be speeded up. AddressBase also contains other valuable information, such as floor elevations over Ordnance Survey data.

As a consequence, it is now possible to take the existing sewer layout and create business rules for each cell in the English House Condition Survey matrix (shown in the table) to create private sewer and drain networks (see before and after pictures). This uses information about the boundaries of a property from the Ordnance Survey products (rear fence, centreline, and so on) and existing information on the public sewer network (for example, separate or combined), as well as the house type and age. The age-bands enable network layouts to be determined, based on the probability that bathrooms and toilets were inside or outside and the probability of multiple toilets inside.

The rules are developed in common language (not a programming language), so they can be readily understood and altered rapidly by engineers. All of the datasets described above can be interrogated simultaneously, meaning running and rerunning the rules takes only a few hours for an entire sewerage company area on a reasonably-sized hardware configuration.

The approach can guide and be validated through field survey. 1Spatial has been working with Tobin Consulting Engineers to determine a sampling basis that can quickly improve the 40 per cent confidence interval around the quantity of transferred assets.

Typical house type/age classification:

Property typeAge
DetachedUnknown
Semi-detachedPre-1900
Terrace1900-1944
Flats1945-1979
Non-domestic1980-present

The rules that created the layout can be written as metadata into asset records, together with the date that the rules were run, so that it is clear from the asset record that these are inferred, not surveyed, assets. Over time, as the real layout becomes clear through ongoing field survey and operational work, the real network can be recorded accurately.

This means that the sewerage company will be able to start to benefit from managing its network as an integrated whole. There are several benefits:

  • creation of an asset record, against which to record operational and survey information. By following the process outlined, companies can derive a fully connected wastewater network within hours, at a fraction of the cost previously estimated (by UKWIR in 2002). Sewerage firms released from the manual creation of records can move forward rapidly to undertake risk studies, particularly in relation to the ingress of groundwater into the private network;
  • highlighting hotspots for survey work. This approach does not lend itself to identifying pipework in all areas, nor does it include septic tank arrangements. The gaps in the inferred network could be regarded as potential priorities for field survey work. However, these need to be looked at in conjunction with the development of risk models. 1Spatial has been working to determine what additional information can be mobilised or determined from additional existing datasets to infer the network layout with increasing confidence. The further development of inference rules, working on floor elevations of properties to develop likely private sewer gradients and even to infer the presence of hitherto unknown pumping stations (without the need for hydraulic models) has also been undertaken by 1Spatial and Tobin;
  • provide a visualisation of the network layout. Operationally, it is valuable to know whether there is access to private sewers in the back gardens of properties; and
  • improve information on customers by linking assets to properties. The creation of a fully connected sewerage network means that it will be easier to connect customer records to both the clean and wastewater asset records in a water and sewerage company.

There is an opportunity to redirect investment, initially allocated to expensive and expansive survey and mapping approaches, towards investigation of the actual risk profile of private sewers and the risks they present to achieving customer service standards. Inadvertently, the transfer process and legislation may prove the catalyst for creating water and wastewater smart grids.

The products used

Contact us

For more information:

Contact: Mike Sanderson, Strategy Director

Email: info@1spatial.com

Phone: (+44) 012­23 420414

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