Environmental Engineering Reference
In-Depth Information
an inevitable consequence of the fact that the Irwell flows through the cities of
Manchester and Salford, from which large quantities of anthropogenic waste
are generated. Indeed, in dry weather over 90% of the Irwell's flow comprises
sewage effluent from the various WwTWs and industrial effluents along the
river. Much of the organic pollution found in the Rivers Irk and Medlock can
also be attributed to inadequately treated sewage and, until recently, farm
waste. 1 The construction of the sewer network is such that localised storm events
also led to frequent Combined Sewer Overflow (CSO) spill events (Rees & White
1993 ), particularly in the Rivers Irk and Medlock. Large volumes of organic
pollutants would therefore enter the Irwell adding to the dry weather load.
The deep (up to 9m), slow flowing water and steep vertical sides resulting
from the canalisation of the River Irwell to form the MSC exacerbate the water
quality problems (Hendry et al. 1997 ). Canalisation fundamentally changes the
hydraulic regime of tributaries by slowing the flow, which has a profound
effect upon water quality (Hellawell 1989 ). The consequential high residence
time renders pollutants difficult to flush out and particulate contaminants
settle readily onto the sediment layer, building up gradually to eventually
create a thick layer of severely contaminated sediment. A non-uniform annual
deposition of 0.5 m of fresh sediments occurs within the Turning Basin (APEM
1996 ). Sediment depth varies from 1.2m to 4.2m between Woden Street
Footbridge and Mode Wheel Locks, and the total quantity of accumulated
sediment is estimated at 460 000m 3 (APEM 1996 ).
These sediments exert a demand for oxygen (Sediment Oxygen Demand SOD)
on the overlying water that, combined with the high retention time, facilitate
stratification and bottom water anoxia during the warmer months. A confined
anoxic layer in the lower depths of stratified water does not present a signifi-
cant problem to pelagic fish populations. However under certain conditions, for
example, following a CSO event coincident with warm dry weather, the deoxygen-
ated layer can extend toward the surface and create total water column anoxia.
Anoxia can occur in the absence of a CSO release during prolonged spells of hot,
dry weather, particularly when there is little flow through the locks and minimal
aeration from wind-driven mixing. In addition to the inadequate mixing and
SOD, the high water column BOD and ammonia contribute to the oxygen defi-
ciency (Hendry et al. 1997 ). Stratification prevents reaeration of the lower layer
(hypolimnion) and when combined with a high BOD, also leads to anoxic sedi-
ments (Moss et al. 1986 ). Although the stratification eventually breaks down, the
rate of atmospheric oxygen diffusionmay still be exceeded by the BOD and/or SOD.
Sediment cores taken at various sites in the upper MSC and analysed for gas
release and oxygen demand exerted on the overlying water highlighted the
importance of SOD to the overall oxygen budget of the MSC (APEM 1990a , b ).
1
For a time, intensive pig farming in the Medlock Valley formed a significant contribution
to the organic pollution load.
Search WWH ::




Custom Search