Revisiting wastewater pH standards: A policy lever for mitigating coastal acidification and enhancing blue carbon

Global ocean acidification driven by atmospheric CO2 uptake is well recognized; however, coastal zones are subject to additional, localized acidification pressures. Among these, the chronic discharge of low-pH-treated wastewater (often pH 6.0), permitted under many current regulations, represents a significant but often overlooked stressor. This practice introduces highly acidic loads into sensitive nearshore ecosystems that are chemically incompatible with ambient seawater (pH ∼8.1). This perspective argues for reframing effluent pH not only as a pollutant parameter to be bounded but also as a modifiable policy lever. Revising discharge standards to require a minimum effluent pH > 8.0 for marine outfalls offers a novel pathway to mitigate localized coastal acidification. Furthermore, this approach aligns with emerging ocean alkalinity enhancement strategies, potentially enhancing coastal carbon sequestration and offering cobenefits such as reduced metal toxicity. Such a policy shift necessitates technological adaptation but promises significant benefits for coastal resilience and broader ocean sustainability goals.

Introduction: An overlooked driver of coastal acidification

Ocean acidification, which is generally attributed to rising atmospheric CO2 concentrations, now requires crucial reframing for coastal waters. Over the past century, the average pH of surface ocean waters has decreased from approximately 8.2 to 8.0. This seemingly small change corresponds to a ∼60% increase in hydrogen ion concentration that significantly alters the chemical dynamics of seawater.1 Although this shift has occurred gradually on a global scale, it has already disrupted marine carbonate chemistry and impaired biological functions across a range of ecosystems.

In contrast, treated effluents discharged from wastewater treatment plants often introduce far more concentrated acidic loads into receiving waters. Current regulations in many jurisdictions permit effluent pH values as low as 6.0. Indeed, a review of standards across diverse global regions, including Asia, the Americas, Europe, and the Global South (Figure 1), reveals a commonality: wastewater effluent pH discharge limits are often set with a lower boundary at pH 6.0 or even lower in some instances. This pH is up to two orders of magnitude greater than that of natural seawater (pH ∼8.1). In absolute terms, wastewater at pH 6.0 contains approximately 158 times more hydrogen ions than seawater does at pH 8.2, on the basis of the logarithmic nature of the pH scale.2 The sheer volume of global wastewater, exceeding 420 billion tons annually, ensures that even a fraction discharged at the lower pH limit contributes a substantial cumulative acid load, particularly impacting poorly buffered nearshore systems such as estuaries and semi-enclosed bays. This “silent accumulation” highlights a critical regulatory gap where legally compliant discharges are ecologically incompatible with marine resilience.