Assessing environmental impacts of large centralized wastewater treatment plants with combined or separate sewer systems in dry/wet seasons by using LCA

Siti Safirah Rashid,Yong-Qiang Liu

doi:10.1007/s11356-020-08038-2

Siti Safirah Rashid, Yong-Qiang Liu

Open Access

https://doi.org/10.1007/s11356-020-08038-2

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Abstract

Rainfall can affect influent flow rate and compositions of wastewater, and thus further affect wastewater treatment performance and the effluent quality. This study aims to study the influence of rainfall on the environmental impacts of centralized wastewater treatment plants. The correlations between rainfall, and influent flow rate and compositions of wastewater in wet and dry seasons with two sewer systems, i.e. combined and separate sewer systems, were primarily established. Environmental impacts were assessed with life cycle assessment (LCA) to understand the temporal environmental burdens in wet and dry seasons. Functional units as per m3 treated wastewater (FU1) and as per kg PO43-eq. removed (FU2), respectively, were used to evaluate impacts of wastewater treatment to the environment. Strong correlation between rainfall and the influent flow rate was found in the wastewater treatment plants with either a combined sewer system (with Pearson correlation coefficient r at 0.66) or a separate sewer system (with r at 0.84), where r represents the strength of the association between two variables. The rainfall effect is more obvious on the eutrophication potential and global warming potential than on other environmental indicators while sewer system, i.e. combined or separate, seems not important in the two cases studied. Both wastewater treatment plants (WWTPs) show a lower environmental burden in the wet season than in the dry season partially due to the dilution of wastewater by using FU1. The WWTP receiving high strength wastewater, however, demonstrates higher environmental impacts in the wet season by using FU2 than FU1, due to the less efficient treatment caused by heavy rainfall. Meanwhile, it is found that environmental impacts from the WWTP receiving low strength wastewater have no difference when using either FU1 or FU2. The results indicate that the environmental burdens particularly eutrophication and global warming caused by WWTPs are dependent on the correlations of rainfall intensity with wastewater quantity and quality instead of combined or separate sewer system. This could be used to guide a stricter control of eutrophication in a more sensitive season in more vulnerable receiving waters.

Highlights

Municipal wastewater treatment plants mainly deal with domestic wastewater, but it is a very common practice worldwide thatResponsible editor: Philippe Loubet Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.storm runoff, through a combined sewer system, is combined with domestic wastewater for treatment
& When per m3 treated wastewater was used as the functional unit, all environmental impact categories in Malaysian Sewage Treatment Plant (MSTP) except eutrophication potential are almost similar in dry and wet seasons while Millbrook Wastewater Treatment Work (MWTW) shows higher environmental burdens in the dry season than a wet season for all seven environmental impact categories
& When per kgPO43-eq removed was used as the functional unit, all seven environmental impacts in MWTW showed higher values in the wet season than the dry season, while the selection of either of functional units has no influence on the environmental impact categories in MSTP

Summary

Introduction

Wastewater characteristics (e.g. concentrations of pollutants) in influent are one of the most important parameters to affect wastewater treatment efficiency, and effluent quality, leading to different environmental impacts from WWTPs. So far, the vast majority of LCA studies of WWTPs, were based on the dry weather conditions without considering rainfall effects, which does not enable a holistic view at the scale of the year with the temporal variability of environmental burdens. The vast majority of LCA studies of WWTPs, were based on the dry weather conditions without considering rainfall effects, which does not enable a holistic view at the scale of the year with the temporal variability of environmental burdens This is important to the vulnerable receiving waters as dry weather–based environmental impact assessment might overestimate or underestimate the environmental burdens such as eutrophication and ecotoxicity

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