The implementation of wet weather treatment lines as an adaptation measure for climate change events and resilente of the drainage systems

One of the main evidence of climate change effects is the increase in the frequency of extreme precipitation events and sea-level rise. Portugal already has registered an increase of the total precipitation attributed to heavy and very heavy precipitation and, consequently, a tendency to have more intense daily rainfall events, leading to increasing problems of flooding and combined sewer overflows (CSO) (de Lima et al., 2013).

Wastewater Treatment Plants, also mentioned as Water Resource Recovery Facilities (WRRF), could be designed to treat part of the CSO. However, with the increase of extreme precipitation events, not only the treatment process efficiency decreases but also the hydraulic capacity of the treatment installation will most likely exceed the design capacity, giving no other choice than to bypass the treatment process during the precipitation event with all the eventual consequences to the discharges means.

There are several solutions to wet weather flow and CSO control in the drainage and treatment system. US EPA (1995), classifies four categories of control measures:

1. source controls, that reduce the volumes, peak flows or pollutant loads entering the collection system; 
2. collection system controls, that applies measures and technologies to maximize the storage capacity in the drainage system; 
3. storage technologies to store wet weather flows; and
4. treatment technologies, to reduce the pollutant load in the CSO to receiving waters.

Nevertheless, all these solutions can also lead to several problems on the treatment process, namely, the increase in first flush loads; longer periods of the installation operation in full hydraulic load after the event; a decrease of the treatment process efficiency, and, reduced biomass performance in the biological treatment processes (Capodaglio, 2004), giving sometimes no order choice than increase the treatment capacity for the wet weather.

Águas do Tejo Atlântico  adopted wet weather line in WRRF treatment processes as an efficient climate change adaptation measure and for CSO control. Alcântara WRRF, one of biggest WRRF in Portugal, was already designed with a Wet Weather Line using a physical-chemical process named Actiflo® as a side-stream treatment, which increases the treatment capacity of the installation from 3.3 m3/s, in Dry Weather, to 6.6 m3/s, when heavy rainfall periods occur.

More recently, Beirolas WRRF, near Parque das Nações in Lisbon, has started the construction of a Wet Weather Line (Desadeg® technology) to increase the capacity of treatment during heavy precipitation events, improving the resilience of the drainage system and adapting Beirolas WRRF to the most probably increase of extreme precipitation events in Lisbon.

– Capodaglio A.G. (2004) Improving Sewage Treatment Plant Performance in Wet Weather. In: Marsalek J., Sztruhar D., Giulianelli M., Urbonas B. (eds) Enhancing Urban Environment by Environmental Upgrading and Restoration. Nato Science Series: IV: Earth and Environmental Sciences, vol 43. Springer, Dordrecht, DOI: 10.1007/1-4020-2694-3_15.

– de Lima, M. I. P., Santo, F. E., Ramos, A. M., and Trigo, R. M. (2015) Trends and correlations in annual extreme precipitation indices for mainland Portugal, 1941–2007. Theor Appl Climatol, 119: 55, DOI:10.1007/s00704-013-1079-6.

– U.S. Environmental Protection Agency (1995). Combined Sewer Overflows Guidance for Long-Term Control Plan. EPA 832-B-95-002.