Behaviour and fate of metals in urban wastewater treatment plants: a review

Abstract

The concerns on metals in urban wastewater treatment plants (WWTPs) are mainly related to its contents in discharges to environment, namely in the final effluent and in the sludge produced. In the near future, more restrictive limits will be imposed to final effluents, due to the recent guidelines of the European Water Framework Directive (EUWFD). Concerning the sludge, at least seven metals (Cd, Cr, Cu, Hg, Ni, Pb and Zn) have been regulated in different countries, four of which were classified by EUWFD as priority substances and two of which were also classified as hazardous substances. Although WWTPs are not designed to remove metals, the study of metals behaviour in these systems is a crucial issue to develop predictive models that can help more effectively the regulation of pre-treatment requirements and contribute to optimize the systems to get more acceptable metal concentrations in its discharges. Relevant data have been published in the literature in recent decades concerning the occurrence/fate/behaviour of metals in WWTPs. However, the information is dispersed and not standardized in terms of parameters for comparing results. This work provides a critical review on this issue through a careful systematization, in tables and graphs, of the results reported in the literature, which allows its comparison and so its analysis, in order to conclude about the state of the art in this field. A summary of the main consensus, divergences and constraints found, as well as some recommendations, is presented as conclusions, aiming to contribute to a more concerted action of future research.

Identification of the Chemical Forms of Heavy Metals in Municipal Sewage Sludge as a Critical Element of Ecological Risk Assessment in Terms of Its Agricultural or Natural Use

“…257) [ 8 ], whereas in the European Union (EU), to which Poland belongs, in the Council Directive of 12 June 1986 (86/278/EEC) [ 20 ]. However, it must be emphasized that processes commonly used in municipal WWTPs do not guarantee definite heavy metal removal [ 21 ]. So, application of sewage sludge to land solves the problem of its final disposal, but on the other hand may pose the potential risk associated with the secondary contamination of soil and ground water by heavy metals present in the sludge [ 12 ].…”

Assessment of the Bulgarian Wastewater Treatment Plants’ Impact on the Receiving Water Bodies

“…This increases freshwater scarcity worldwide, since the contaminated water may cause human diseases due to the wide variety of viruses, bacteria, and protozoa, these waters may contain. Apart from these biological contaminants, the wastewater effluents are also polluted with chemicals, e.g., nitrogen, phosphorus, heavy metals [2], and organic compounds [3,4] among which detergents, pesticides, hydrocarbons, and metabolites. Wastewater effluents rich in decomposable organic matter are the primary cause of organic pollution.…”

“…Wastewater effluents rich in decomposable organic matter are the primary cause of organic pollution. Most heavy metals present in the water are associated with industrial discharges but are also found in the wastewater treatment plants’ (WWTP) effluents [2]. Therefore, the management and utilization of natural resources need to be further improved, and human pollution activities to be reduced.…”

Occurrence and Fate of Heavy Metals in Municipal Wastewater in Heilongjiang Province, China: A Monthly Reconnaissance from 2015 to 2017

“…For instance, the average removal ratios range from 20% to 97% for As, 10% to 79% for Cd, 33% to 98% for Hg, 50% to 77% for Cr, and 30% to 96% for Pb. This is attributed to the fact that MWTPs are not designed for removing heavy metals, and the apparent removal in most of the cases is the result of the metals partitioning to the solid phase of the treatment systems [52]. Thus, the release of heavy metals from MWTPs into the environment should not be ignored.…”