In my humble opinion, I think Storm water should flow artificially to the water courses, being collected for small rainfalls to the wastewater treatment plants. The latter could reduce flooding and other side effects such as soil erosions, which would benefit both inhabitants and environment.

Many thanks,

Debra

Thanks Dear Debora for your good contribution. In fact it seems that some experts advocate the treatment of pluvial water corresponding to the first moments of the atmospheric precipitation. In my perspective, the problem in practice is how to perform this efficiently, due to the enormous variability and disproportion of flows.

Historically, the first sewer systems were built to collect only pluvial flows (storm water) with necessarily large sections. With the construction of the water supply systems for supplying the biggest cities with and after the industrial revolution, the rejected wastewaters from the domestic uses were connected to the existing pluvial systems and thus the first combined systems were created. This explains somehow the initial rationality in the XIX century of such systems. I made a study with a survey in the beginning of decade of 90 in the littoral center region of my country about municipal water supply, municipal sewerage systems, and municipal wastes that included many municipalities. Rarely wastewater treatment plants worked properly on that time. A great development has been observed since then, due to also to great investments in this sector. In my opinion, treatment of sanitary wastewater requires separate systems.

Surely that I agree with you, Dear Mario. In my opinion, it does not make any sense the treatment of urban storm water (in excess) in conventional separated storm water systems, at least for the wastewater treatment plants conventional developed for sanitary sewage. It seems even a little absurd. But that is merely my opinion. I recognize that the human knowledge is still limited and we must be always open to new ideas and innovation.

You mentioned a very important aspect in current conception of the systems, Dear C. Lewis. The separated storm water networks must be reduced to a minimum and suitably integrated in the urban landscape with many permeable areas, particularly green spaces, in order to minimize floods, great flows of pluvial water in the sewers and superficial runoff uncontrolled. The problem increases greatly with the massive and dense construction everywhere of concrete buildings and impermeable surfaces, in the urban agglomerates (and likely also with the climate changes that have been observed?). The separated sanitary sewer systems, with much smaller diameters and much larger extension serving the urban agglomerates, can transport thus without unwanted overflows the much smaller flows collected to the treatment facilities.

Urbanization and higher population concentration leads to reduction in permeable surfaces as you have noted,  thus the need to collect and dispose of the stormwater through underground sewers. When the population was small, the dilution effect of the receiving waters was considered sufficient treatment that combined sewer overflow outfalls were designed during high rainfall - the portion that went to the WWTP was simply primary clarified and disinfected before discharge. As urban populations grew the receiving waters dilution effect was no longer there creating  water pollution along with bioaccumulation of pollutants in sediments from decades of disposal. With the formation of the EPA in the US in the 1970s, additional treatment and stormwater treatment have led to decreased pollution and recovery of once contaminated area. With slatwater intrusion in freshwater aquifers and increased water demand causing in areas river flows and reservoirs to drop, it make sense now to collect the stormwater and treat it for reuse or recharging aquifers.

It is interesting to see that homes in Caribbean Islands collect rainwater through their lime washed roofs into cisterns and this is their primary source for potable water use. Remember a place  I stayed in Bermuda had that system as all the neighbouring houses.

Here are some links on stormwater regulation developments by the EPA in the US:

http://www.pollutiononline.com/doc/epa-agrees-revisit-polluted-runoff-next-year-0001?sectionCode=News&templateCode=Single&user=3132979&source=nl:43919&utm_source=et_6214185&utm_medium=email&utm_campaign=POL_2015-09-23&utm_term=87e38578-c2c2-477c-ba3f-234f00553775&utm_content=EPA%2bAgrees%2bTo%2bRevisit%2bRules%2bFor%2bPolluted%2bRunoff%2bBy%2bNext%2bYear

http://www.pollutiononline.com/doc/epas-stormwater-program-needs-a-significant-0001http://www.pollutiononline.com/doc/facilities-across-the-southeast-ordered-to-0001

Thanks Dear Mohan for your good answer raising other important aspects in sewerage systems and pollution control. In my opinion, the old combined systems are based on a wrong idea of development and urbanization, with low environmental concerns. Currently they are obsolete.

One of the problems in big cities is the water supply, for example in Mexico City this resource is becoming increasingly scarce due to urban population growth. However, the infrastructure of a system for collecting storm water in an old-growth anarchic-economic city it would be, as is the case of Mexico City, extremely expensive. In this case the proposal is the collection of storm water in each building/house and use that water for domestic cleaning/toilet because it contains different types of bacteria and surely heavy metals, and other contaminants; or it is possible to use a filtering system to remove such pollution and it might be used for domestic consumption but finally it would end in the sanitary sewerage.

Here is a another article which shows how an urban pollution river problem is successfully implemented but the increasing number of residents and tourists who like to partake of nature have created new river contamination problem:

http://www.sgvtribune.com/environment-and-nature/20131008/bacteria-levels-up-in-forest-areas-but-los-angeles-river-cleaner-health-report-says

By the way anyone here  taking an interest in step wells used for water harvesting which were used in India until 19th century and now are being restudied and some being put back into operation.  Stormwater did get harvested at least 5,000 years ago. See links below:

http://www.thisiscolossal.com/2015/08/stepwell-architecture/

http://www.archdaily.com/395363/india-s-forgotten-stepwells

http://petapixel.com/2015/09/05/photos-the-beautiful-architecture-of-indias-ancient-stepwells/

http://www.ted.com/talks/anupam_mishra_the_ancient_ingenuity_of_water_harvesting?language=en

Came across an article showing how an architect has incorporated  rainwater harvesting and cooling aspects of stepwells, wastewater recycling along with screen cladding into a green building design:

http://www.treehugger.com/green-architecture/architect-uses-ancient-techniques-cool-modern-building-india.html

Some great photos of more stepwells below:

http://twistedsifter.com/2012/12/chand-baori-stepwell-india/

http://www.ancient-origins.net/history/beautiful-utilitarian-stepwells-india-are-danger-becoming-extinct-003770

Thanks Dear Victor for another good contribution. In fact Mexico City is internationally known for being an enormous urban center that faces problems, particularly related with water resources that are enormous engineering challenges for the Civil, Hydraulic, and Environmental Engineers. The water cycle needs to be considered in an integrated mode at a basin level to guarantee the correct exploration and equilibrium of the natural resources. Complementarily, local solutions for harvesting of rain water, water supply, and sanitary sewers, reducing superficial runoff and storm water flows may be in fact particularly suitable in this case.

Thanks Dear Mohan for your contributions, with the amazing step wells for harvesting rain water that I did not know and that seem to make part of Indian tradition?

In urban agglomerates, for the reduction (at least) of instantaneous flows of storm water, can be used retention ponds, natural or artificial. There are also separate systems no conventional with permeable chambers that connect the sewer pipes and that allow the infiltration of storm water in the soil and the recharge of underground aquifers.

Dear António, 

You ask me to answer, but i just have learned about your question from those answers. Then i thank you for that.

Have a nice time

Helena

Sure, Dear Helena that was a pleasure to share also this question in the sequence of my submarine outfalls question that I also shared with you, given that we have one near Lisbon. However, we can naturally participate in any RG question, independently if it is shared or not by its proponent. When I participate in a RG discussion, naturally that I am also there for learn.

Dear @Antonio, this is case study from Germany!

"Separate sewerage consists in the separate collection of municipal wastewaters (blackwater from toilets, greywater and industrial wastewater) and surface run-off (rainwater and stormwater). The separate collection prevent the overflow of sewer systems and treatment stations during rainy periods and the mixing of the relatively little polluted surface run-off with chemical and microbial pollutants from the municipal wastewater. The design of the sewers and the (semi-)centralised treatment stations thus needs to consider the volume of the wastewater only and the surface run-off and rainwater can be reused (e.g. for landscaping or agriculture) after a simplified treatment...

The construction costs might be higher than for the combined sewer system because two separated networks are necessary. These must also be maintained and operated separately. Also to replace the combined system by a separated system is very costly..."

http://www.sswm.info/content/separate-sewers

Antonio -Many of the very elaborate stepwells are found in the north-western parts of India which is semi-desert thus the development of these stepwells to collect and store water during the non-monsoon months. They were important centers around which the towns and communities lived. As indicated the elaborate ones were usually built by ruling kings and queens and religious temples were also incorporated. Hindus venerate water and have religious purification ceremonies with water like bathing in sacred holy waters like the Ganges, Kumbh Mela festivals, etc. Christians have the baptism in holy water once - did this ritual come by or be influenced by the Hindus? but that is another different discussion!!

Here are a couple of links to the Rani ki Vav stepwell which was desilted giving its history and functions:

http://www.amusingplanet.com/2014/11/rani-ki-vav-queen-stepwell-in-india.html

http://whc.unesco.org/en/list/922

And in Los Angeles, California, the severe shortage of raw water supplies has the agencies recently moving forward to reclaim more treated wastewater. See link below.

http://www.latimes.com/local/lanow/la-me-ln-mwd-recycled-water-20150922-story.html

Here is a link below to article on how the centuries old lakes/reservoir/stepwells/tanks system were dismantled  and filled up  for housing  the growing population in Bangalore, India. Throw in corrupt politicians pandering to industrial polluters, the situation gets worse, not solved.

http://thewire.in/2015/05/23/beneath-the-foam-and-fire-2342/

Good case study fro Ohio:  Lima Sewer Separation Project to Begin in Next Few Weeks! I was thinking that in US it was separated everywhere long time ago!

"Lima City Council approved an ordinance for construction inspection of a sewer separation project. The project costs just under $3 million and will separate sewer lines from sanitary lines in Lima..."

http://www.hometownstations.com/story/30144345/lima-sewer-separation-project-to-begin-in-next-few-weeks

In fact strictly maintenance costs may be larger because of two public networks of pipes in separate systems, but operation costs (even without considering environmental concerns) can be normally smaller. As observed by Ljubomir, the replacement of the old combined systems by separate networks, certainly that is not a problem only of the third world.