Action: Set or improve minimum sewage treatment standards
Key messagesRead our guidance on Key messages before continuing
- One study examined the effects of improving minimum sewage treatment standards on subtidal benthic invertebrates. The study was in the Bay of Biscay (Spain).
COMMUNITY RESPONSE (1 STUDY)
- Overall community composition (1 study): One before-and-after, site comparison study in the Bay of Biscay found that after introducing a secondary treatment of sewage wastewaters, invertebrate community composition at an impacted site did not significantly change compared to unimpacted sites.
- Overall richness/diversity (1 study): One before-and-after, site comparison study in the Bay of Biscay found that after introducing a secondary treatment of sewage wastewaters, invertebrate richness and diversity at an impacted site did not significantly change compared to unimpacted sites.
POPULATION RESPONSE (1 STUDY)
- Overall abundance (1 study): One before-and-after, site comparison study in the Bay of Biscay found that after introducing a secondary treatment of sewage wastewaters, total cover of invertebrates significantly increased at an impacted site at 8 m but not 3 m depth, compared to unimpacted sites.
Untreated sewage reaching the marine environment can impact subtidal benthic invertebrates through the introduction of bacteria, excess nutrients, toxic substances and solid particles, and through changes in salinity (McGann et al. 2003). Setting minimum sewage treatment standards, or improving the standards already in place, could potentially ensure that pollution level and associated risks to subtidal benthic invertebrates are minimized. For instance, improving standards can be achieved by installing a secondary treatment involving the mechanical and biological removal of settleable solids and dissolved organic compounds (Bustamante et al. 2012).
Evidence for related interventions is summarised under “Threat: Pollution – Limit, cease or prohibit the dumping of untreated sewage”, “Limit, cease or prohibit the dumping of sewage sludge” and “Limit the amount of storm wastewater overflow”.
Bustamante M., Bevilacqua S., Tajadura J., Terlizzi A. & Saiz-Salinas J.I. (2012) Detecting human mitigation intervention: Effects of sewage treatment upgrade on rocky macrofaunal assemblages. Marine Environmental Research, 80, 27-37
McGann M., Alexander C.R. & Bay S.M. (2003) Response of benthic foraminifers to sewage discharge and remediation in Santa Monica Bay, California. Marine Environmental Research, 56, 299–342.
Supporting evidence from individual studies
A before-and-after, site comparison in 2001–2009 of four rocky seabed sites in Plentzia Bay, southern Bay of Biscay, northern Spain (Bustamante et al. 2012) found that improving the treatment of sewage wastewaters before discharge at one impacted site did not result in changes in invertebrate community composition or diversity after three years. Community composition did not change over time at the impacted site nor at three adjacent unimpacted sites, and communities appeared to be similar at all sites both before and after sewage treatment improvement (data reported as statistical model results and graphical analyses). In addition, diversity did not change at the impacted site or unimpacted sites over time (data reported as five diversity indices). Total species cover significantly increased at 8 m depth at the impacted site (before: 14–20%; after: 42–46%) compared to the unimpacted site (before: 3–42%; after: 4–42%), but not at 3 m depth where cover changed similarly at the impacted site (before: 11–20%; after: 31–63%) and the unimpacted sites (before: 5–50%; after: 23–98%). Raw sewage had been released into the intertidal area at the study area for 40 years until physical-chemical treatment was introduced in 1998. In 2006, a secondary biological treatment was introduced. Every two years between 2001 and 2009, one impacted site and three adjacent unimpacted sites were surveyed. Six locations/site were surveyed in summer (three at 3 m depth, three at 8 m). Invertebrate species were counted, and their cover visually estimated in three 40 x 40 cm quadrats.