What is more feasible; having separate sewerage & storm drainage or combined systems ?

In practice and due to blockage, illegal seweragelinks have been connected to the storm drainage networks, resulting in constraints to the capacity of the pipelines and pollution of the rivers where the collected waste water was discharged.

3 Answers

1. Thanks Ala! Yes, I think that public awareness campaigns can help reduce the number of illegal linkages if solid waste and sewage are sufficiently separated in the public's mind so that sewers are no longer considered a repository for everything that is undesirable. The actual acts of illegal linkage seem to be more reactionary than intentional: the head drainage engineers and government consultants that I've spoken with believe that it is actually government engineers themselves who are creating illegal linkages to assuage angry residents who have sewage backwashing into their homes. If we can lower the amount of blockages through reduced solid waste incursion, that may sufficiently reduce the pressure on ward engineers to stop them from creating such linkages. Another thing that may help is encouraging dialogue between drainage designers/central drainage department engineers and ward engineers. There is presently little communication between those who are designing and those who are maintaining, leading to numerous discrepancies in how the sewerage system is understood and treated.

2. Thanks Stephanie for your informative feedback on this issue. It would seem that implementing the aqueduct design instead of pipes could contribute to an improved capacity of the distribution network like in London and other parts of US and Europe. However the design of the STPs should also be upgraded to comply. Moreover, the treated sewage can be used for irrigation as in Abu Dhabi. I wonder if public awareness campaigns can assist in eliminating the illegal linkages or even law enforcement?

3. I've been doing extensive work on the sewerage system in Ahmedabad, India, which currently has two separate systems that are illegally connected all across the city. The city has recently sealed off all the stormwater outfalls that empty into the river, and installed two collector pipes to take everything into the STPs. While this solves the problem of wastewater pollution, it overloads STP capacity significantly during the monsoon. Although they have committed a large amount of money to new STPs over the years, reports tell of a large percentage of effluents going straight into the river downstream of the city due to insufficient STP capacity. From what I've seen of the STPs last year, the quality of the effluents they are releasing is far from adequate. There are pros and cons to having a separate system or a combined system. Boston, for example, is switching to a separate system after decades of having a combined system. But it has a particular situation where illegal linking isn't a practice, usage is fairly regulated, and maintenance is fairly effective. It's a delicate balance between preventative system design, informed usage of systems, and adequate maintenance practices. The state of sewerage design in general is antiquated, with oversized pipes that are prone to blockage, and no thought to the way that design shapes legal/illegal pipe usage as well as maintenance practices. The public in LDCs mistrusts government engineers and is used to taking problems into their own hands, privileging short term, localized solutions over long term, system-wide effects. Solid waste is dumped into pipes for convenience and pipes are corroded by industrial effluents, which are also conveniently emptied into pipes. Maintenance workers in LDCs do not have adequate equipment or training to deal with major blockages. The choice of a separate or combined system can be either feasible or a complete disaster depending on the context. Even if the design itself is poor, a public that complies with regulated use of pipes and effective maintenance practoces can make the system adequately functional. Even if it's a combined system with a new STP, decision-makers must ensure that STP capacity is capable of handling seasonal loads and routinely check to ensure that all effluents are actually being treated before release.