I've been doing extensive ...

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.