Pros and Cons of separating rainwater from sewers to prevent sewer overflow in urban areas

Making the case for a separated sewer

A 2013 report by the World Meteorological Organization already shows a >10% increase in precipitation, and predictions are even worse for the upcoming decades. In a combined (conventional) sewer system, stormwater runoff is combined in a single pipe with wastewater from homes, businesses, and industry. During drier weather, the stormwater and wastewater are carried to the sewage treatment plant together. But ever more frequent torrential rainfalls and high volumes of stormwater can exceed the capacity of a combined sewer system.

The excess, untreated  amounts of water overflow then empty directly into our waterways, or worse, onto our streets. Apart from causing flooding and consequently hazardous situations for both pedestrian and road traffic, there's also a significant health threat with the unhygienic situation that occurs when untreated sewer water flows into the streets.

Three-tier sewer replacement approach

To improve water management and protect the sewage system from damage, cities are beginning to conduct sewer renovation of their underground pipe and drainage systems – by separating rainwater from the sewage system. This is done first in two tiers: separating them overall, then separating the sewer mains that serves private property. The third tier involves the process of encouraging/stimulating property owners to follow suit.

Tier One – Separating sewer systems in the municipality overall

In an urban municipality’s two-pipe separated sewer system, stormwater is collected through storm drains and travels through a separate pipe from the one that manages household sewage and wastewater. In this type of system, the overall sewer mains are separated, with each of the two sewer programs operating at the same time – and the storm drains carry the stormwater runoff separately from the wastewater.

Tier Two – Separating the sewer mains that serves private property

A scaled-down version of the same process (described above) applies to separating the sewer mains that serve private properties. In the city of Vancouver (British Columbia, Canada), for example, this smaller project was conducted on the basis of three conditions: that sewer line blockages had definitively originated fully or partially from the city mains, that the city’s sewer line maintenance programme was no longer effective, and that the property owner has fully replaced his/her private property sewer line but has found the city’s line to be in poor condition.

Tier Three – Replacing the private property portion of the sewer line

A completely symbiotic operation of sewer system overhaul happens when the private property owner makes the investment to replace his/her own sewer line. Thus, from tier to tier, the sewer lines are managing stormwater and sewage/wastewater separately, efficently and sustainably.

It may be argued that there is also a fourth tier: at the level of the waste water treatment plant itself. There are so many questions, considerations and concerns that arise when discussing the feasibility of separating sewer systems, like – for instance – if the stormwater is separated from the sewage/wastewater, will there be enough flow left to move the sewage along? Here are some of the pros and cons:

PROS of a separate sewer system

• Eliminates combined sewer overflow which, in turn, helps to prevents pollution
• Mitigates the problem of flooding by increasing capacity
• Allows stormwater to be used as a resource Optimizes performance of the waste water treatment plant (WWTP)
• In the long term, the efficiency and longevity of a separated system will pay for itself (ROI)

CONS of a separate sewer system

• Too costly – at every tier, it is a costly endeavor
• Too disruptive – the reconstruction process will disrupt the urban areas (i.e. businesses, traffic)
• May result in an increase in pollutants loading to receiving waters, as a result of the increased discharge of untreated surface run-off
• Incomplete or improperly separated sewer system efforts can result in poor sanitary sewer system performance
• Inadequate follow-up by operations and maintenance (O&M) will defeat the purpose and cost of overhauling the sewer system

There have been many debates on the subject of combined versus separate sewer systems – throughout Europe rel="noopener noreferrer" and very much so in the Netherlands. As an industry leader in rainwater and wastewater management solutions, Wavin is very much engaged in this discussion.

Wavin’s understands water management

Wavin’s attenuation and infiltration systems like Q-Bic, Q-BB, AquaCell and our latest product, Q-Bic Plus, enables underground tanks to be created quickly and easily. Designed for use in locations where there are heavy traffic loads and where local groundwater levels are high, the Q-Bic Plus is based on a modular concept that only uses side panels where they are really needed in an infiltration/attenuation tank. The side plates are only placed on the outside surfaces of the tank. Every part of the system can be clicked together, without the use of connector pins or tools, which greatly increases installation speed. Made from virgin polypropylene, it is very robust and can withstand extreme loads.

Keeping the sewers clean

Sewer systems can clog up with waste, debris, sediment, tree roots and leaves – especially during heavy rainstorms. The more traditional concrete sewer pipes have less flow and are prone to breakage, also contributing to the clogging. Wavin’s plastic sewage pipe sewage systems and Tegra manholes will simply never corrode, and they’re really easy to clean and inspect.

Let’s keep the conversation going!

Climate change is forcing municipalities around the world to come up with sustainable solutions which poperly address the new challenges of rainwater management. The notion of overhauling and separating the sewage system is a major discussion point in the overall conversation of how climate change is affecting the way municipalties and business/home owners handle the challenges of excessive rainfall and flooding. So let’s keep the dialogue open, shall we?

We’d love to hear your thoughts on the pros and cons of separating rainwater from sewers, so please post them on our Facebook page. And if you want to overhaul your sewer system, but need some guidance on how to do it, just contact your local Wavin office and we will be happy rel="noopener noreferrer" to help. A big thanks to Martijn Tilma, Urban Water Advisor of Royal Haskoning DHV, for his help and proofreading. His knowledge and smart insights made this a more thoughtful article.