In cities like Victoria, one or two percent of the rain that falls is impounded in the reservoirs that provide water for the inhabitants. The remainder runs off the surface, or soaks into the ground to contribute to the ground water. All this water in time runs into the sea. The only exception is the water that is taken up by plants and that subsequently evaporates off the leaves.
The impounded water is piped to houses where human, cooking and washing wastes are added, together with some fraction from such additives as soap and detergent and other household products.
Where there are roads, the water that runs off surfaces will be fed into a system of sewers. Many of the original ditches and streams will finish up as part of the piped system.
The wastes from households constitutes 'sanitary' sewage and wastes from run-off constitutes 'storm' sewage. Where the sanitary sewage is piped into the same sewers as the storm sewage the system is known as a 'combined' system. Where the sanitary sewage is piped separately it is known as a 'separate' system.
Many cities were developed using the combined system as being the more economic. However, where treatment was necessary, its adequate provision was difficult, noting that when it rained the storm sewage could be up to a hundred times the flow of the sanitary sewage. Commonly, some provision was made for the first flush of storm water, carrying much of the polluting load, by storing it for later treatment. By the 1950's, a consensus had been reached that the separate system was usually better. By then, most of the world's cities were already well developed so in the outcome a hotch-potch of systems came about as part of a gradual switch from the one system to the other.
Complicating this scenario was the added recognition by the 1970's of the possible dangers of storm water itself which have already been mentioned. It contains everything that is washed out of the air, the quality of which has been so much in question. To fully treat all the storm water is too horrendous a task to be practical for most communities except possibly over a very long time frame.
All sewers run downhill in order to carry the flow by gravity and inevitably it becomes necessary in some places to lift it by pumping. Most cities of any size will have many pumping stations. So long as the separate system is in operation all the sewage can be pumped. Where a combined system is in use, it is not practical to pump all the waste when it rains hard and some will be overflowed to nearby streams or rivers. The same may happen where a nominally separate system in fact contains storm water or where the pipes are old and allow infiltration of ground water.
By the mid 1960, like any other city of its size, Victoria had a mixture of sewers and pumping stations varying in age from new to over 50 years old and in adequacy from good to bad.