Sewerage Risk Management

Clearly the primary purpose of a sewer system is to carry flow and as far as the fee paying customers are concerned if it isn't doing that consistently well it is not providing a satisfactory service.  Flow includes all aspects of wastewater like household domestic sewage and storm run-off but in the case of industrial customers can include very special discharges with high pollution loads and these are subject to negotiated permits.

More than ever, sewerage utility customers expect drains and sewers to carry their flow away without negative impacts like:

1. Flooding inside or outside any property, and not just regarding the property that a customer pays sewerage charges for.
2. Pollution that is attributed to sewerage utilities, e.g. in receiving waters like streams, rivers, estuaries and beaches.
3. Pollution from smells/odours, noises (e.g. equipment) and visual pollution.

Generally speaking UK customers have an ‘out of sight, out of mind’ view of sewerage services whereby they are happy if they avoid any associated negative impacts.

Aspects of Performance

There are four primary aspects of sewerage performance: 

1. Structural – the aspect which if ignored in a gravity sewer eventually leads to sewer collapse and failure to achieve the primary purpose of the sewer (i.e. to carry flow).
2. Hydraulic – in particular the carrying of flow which in the event of failure results in surcharging and/or flooding.
3. Water Quality and any polluting impacts on the environment
4. Operations and Maintenance – the day to day running of sewer systems, including things like customer enquiries and sorting out local blockages, and activities to protect the integrity and longevity of the sewer system.

The Integrated Approach

The integrated approach considers all aspects of sewer system performance together since each aspect of performance very clearly interacts with each other and ignoring such interactions would nearly always have a negative impact on the value of any associated expenditure. 

Such sewerage interactions are very common relating to a multitude of components of each of the four main aspects of performance.

The integrated approach requires consideration of all interfaces between a sewer system and its surrounding environment, including of course the interface with a wastewater treatment works and any receiving water bodies, e.g. rivers.

The large number of sewer system problem interactions clearly make associated risk management processes more complex, however, they also facilitate delivering much better value (costs versus benefits) when considering incurring any intervention (expenditure) to improve service and address performance problems.  This is most readily achieved after acceptance and adoption of the key principles underpinning the SRM (integrated) Approach.

Principles of SRM (Integrated) Approach 

When considering any Intervention:

1. Consider first (second and third) retaining and reusing the existing ‘hole in the ground’ (the existing sewer) since it is generally an extremely valuable asset, e.g. by use of renovation and flow attenuation type options. 
2. Consider all aspects of performance together and seek trade-off solutions whereby more than one aspect of need can be addressed by an intervention option.
3. Look for phased interventions whereby expenditure is deferred to when it is really needed (whole life cost modelling and cost benefit analysis tools are available to assist).
4. Without compromising minimum acceptable performance criteria always seek best value solutions by adopting variable performance criteria that achieve the best balance (trade-off) between all aspects of performance and  cost.
5. Always avoid considering ‘blinkered’ approaches that just consider single aspects of need/performance.
6. Always seek to attenuate flows as high up the sewer system as feasible.
7. Generally and responsibly seek to minimise flows entering the sewer system.
8. Consider the needs and views of all stakeholders but clearly be willing to give due weighting to reflect agreed sewerage management priorities.

Example of performance interactions

As an illustration of the interaction of different aspects of sewer system performance consider a sewer with a flatter than desirable gradient (none self cleansing) and defective pipe joints and the following plausible problems which interact with each other:  Note that such defects may be the result of poor original workmanship and/or, say, mining subsidence over many years.

1. Ground water infiltration washes silts (fine particles) into the sewer that settle out in the sewer length, reducing cross sectional area and hydraulic capacity.  
2. The low sewer gradient results in particulate matter settling out of the sewage flow at low velocities and further reduces hydraulic capacity. 
3.   These two fairly common problems then cause regular surcharge that accelerates the washing in of fine particles, removes side support from the pipe that then fails (cracks) when a heavy load passes at ground level.
4. The cracked pipe deforms further reducing cross sectional area and accelerates the deterioration process.

The growing surcharge subsequently results in flooding and causes an upstream CSO to discharge prematurely, causing river pollution.

The first apparent problem in the example above is the accumulation of fine deposits and/or evidence of surcharge in manholes adjacent to the problem sewer and these may be first recognised by an operations/maintenance team who had reason to lift one or more relevant manhole covers.

Later identification of problems is when flooding and/or premature (earlier than acceptable) CSO discharges occurred.

Limited importance is often given to the earliest signs of sewerage service problems described in the example discussed above and the reasons are thought to be:

1. A lack of ‘joined-up’ logic/management that suitably reflects well-established and proven findings, possibly because the findings have not previously been communicated clearly enough.
2. A lack of high level conviction / commitment regarding the need for good quality sewerage data in integrated systems, e.g. as reflected by previous Drainage Area Plans being seen as quinquennial capital planning steps that did not link to operational decisions or day to day management matters.
3. A willingness to accept the risks that sewers will not fail in the short term and that expenditure is not justified, even though asset lives generally cannot be readily or specifically forecast.

Other Issues

It is noted that integrated approaches are clearly desirable beyond the sewerage utility’s boundary of responsibility (e.g. to pollution, urban development, sustainability, etc.) and across all aspects of major and minor systems (e.g. rivers, roads and sewers respectively).

This higher level need for integrated approaches generates complex interfaces between sewerage utilities, the EA, developers, planners and all concerned with effective River Basin Management (RBM) and a current example of the complexity is SUDS and who should take ownership for associated funding and long term management.

Defra is promoting associated progress with funding for case studies of integrated urban drainage management (IUDM) but possibly their predominantly hydraulic (only) focus will also run into problems associated with the Government’s commitment to public value for money (VFM) and sustainability agendas.

The Government’s commitment to Water Framework Directive (WFD) implementation by 2016 and growing EU pressures about a lack of Government commitment to the 1994 Habitats Directive further highlight the need for truly integrated approaches at all levels within a sustainable RBM philosophy.

In the integrated context Climate Change is clearly a further issue that concerns many aspects of water utility business.

Updated: 21/03/2013