Wednesday, 20 May 2009

Sewage Spillage in Jersey

I've just been listening to BBC Radio Jersey where a spokesman for Transport and Technical Services was explaining to the presenter that in very wet conditions, they have to let raw sewage seep into the bay, because even the cavern (a large storage area rather like a capacity) cannot cope with the overload.

There were several things not said, however....

I clearly remember an earlier explanation being given a few years ago, which I suspect is the right one, that the capacity of the pumping system from the West of the Island is not fast enough to take the increase in volume in raining weather. It functions in that respect rather like the road network, which has several feeder roads from Western and North-Western parishes all coming together into one road at Beaumont. I suspect the lack of capacity in flow is the real problem with the sewage system, and why it has either to be sent out to sea in such cases, or blow covers and go into Goose Green marsh.

The other matter, which is obvious, is that rain water runoff and the sewage network are combined. Extreme wet weather does not cause humans (as far as I am aware) to suddenly increase their use of the toilet, shower, bath etc - the culprit is the rainwater, and the fact that the system does not split the two. The more rainwater is set off on its own system, and discharged to sea, the less of a problem there would be. In the 1970s, there were inspections of properties in St Brelade to ascertain that this was the case, and presumably to try and rectify it. I don't know what happened to that survey and its aftermath, but nothing much seems to have been done. Perhaps we need a code, like that which is in force in Ohio (1)

1101.2 Where required. All roofs, paved areas, yards, courts and courtyards in buildings shall drain into a separate storm sewer system, or a combined sewer system, or to an approved place of disposal..

1101.3 Prohibited drainage. Storm water shall not be drained into sewers intended for sewage only.

But perhaps there are concerns that storm runoff may contain unacceptable levels of pollutants (from roads etc)? There are, and have been, modern ways of dealing with this that reduce the impact of storm rainwater's load on the system. There is a very good paper from 2003, called "EUROPEAN APPROACHES AGAINST DIFFUSE WATER POLLUTION CAUSED BY URBAN DRAINAGE" (2)  which deals with this, among other matters:

Storm water management

Storm water management concepts are combining unsealing of paved areas, infiltration of runoff from disconnected areas, storm water re-use, distributed retention, delayed transport and treatment (the latter to be described in the following subsection). The pressure to rethink conventional drainage systems and realise such modern concepts is due to mainly water quantity problems (insufficient hydraulic capacity of sewer systems as well as of streams and rivers) but also quality requirements. In the sense of sustainable development, ecological criteria are taken into account in these drainage concepts which are potentially much closer to nature than the traditional approach has been. The decentralised solutions (e.g. infiltration structures as main element) are used as best management practices (BMP) and are recently named 'sustainable
urban drainage system' (SUDS). Practical planning experience shows the necessity to involve drainage planners into town and traffic planning at an early stage because boundary conditions are fixed then which are very important for feasibility and efficiency of the local storm water management concept.

SUDS are a very popular topic in urban drainage in Germany. Beginning with first exemplary projects in the late 1980s which already include investigations of impacts on groundwater quality SUDS are now widely used in drainage planning. The approach is also beginning to be used more extensively in other European countries, e.g. in the UK, France and Switzerland as well as in the US and in Australia.

The first mentioned two elements of SUDS (unsealing and infiltration) as source control measures have a reduction effect on the runoff volume, the others an attenuation effect on the peak flows. Both effects are reducing the hydraulic stress for the receiving waters (disturbance of benthic fauna). Infiltration closes the natural water cycle by increasing the ground water feeding. Böhm et al. (1999) stated an efficient decrease of emissions of hazardous substances into receiving waters (especially heavy metals, nutrients only to a minor extent), but partly these loads are transferred to soils and wastes. Therefore measures at source are necessary in parallel (see sub-section 'Replacement of hazardous substances'). Additionally it is expected that the elimination efficiency of the existing treatment structures improves by 10-15 % in terms of pollutant load emissions due to the reduced inflow rates.


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