Metals in greywater: Sources, presence and removal efficiencies

To date, risk assessment for greywater reuse systems has almost exclusively been focussed on pathogenic microorganisms and conventional water quality parameters. Little is known about the risks from micropollutants present in greywater, nor about the potential for greywater treatment systems to act as barriers for micropollutants. In this paper, the sources, presence and potential fate of cadmium, mercury, lead, and nickel in onsite greywater treatment systems are investigated. Annual metal loads from bathroom greywater are small in comparison with typical municipal wastewater loads but still do not always meet environmental quality standards for surface waters. This study shows that onsite treatment may remove a third to a half of lead, mercury and nickel from bathroom greywater but showed no significant removal of cadmium. Geochemical modelling indicates that metals should not pose a problem for reuse in toilet flushing, irrigation and clothes washing. Rather, the major potential problems associated with the presence of metals in greywater are related to the issue of sludge disposal.

Until recently, the vast majority of studies investigating greywater characteristics and reuse have concentrated on conventional wastewater monitoring parameters, and the knowledge relating to the sources and presence of metals in greywater remains very limited. There is also a lack of monitoring studies investigating the fate of metals in onsite greywater treatment plants and the metal removal efficiencies of characteristic greywater treatment trains. The work presented in this paper shows that a vast number of sources may contribute to greywater metal loads. The actual contributions from any single household will depend largely on the behaviour of the inhabitants, although piping and plumbing fixture materials are also recognised as Table 3 Annual inlet metal loads in grey- and domestic-wastewater per capita (g personÀ1 yÀ1) Metal Untreated greywater Untreated domestic and municipal wastewater [19] This study Cd Ni Pb Hg Sweden [16] Denmark Maximum 0.0015 0.17 0.12 0.21 0.0023–0.017 0.087–0.59 0.062–0.99 0.0004–0.0038 0.2–0.4 2–4 5–10 0.1–0.2 0.5–0.7 2–4 5–10 0.1–0.2 potential contributors to metal loads. In the Nordhavnsgaarden case study described here, metal concentrations were found to vary notably between sampling times reflecting the impact of residents behaviour on greywater metal loads, but in any case, the calculated annual metal loads from greywater were small in comparison with typical loads found in combined municipal wastewater. Fugacity modelling showed that the removal of metals in onsite treatment plants is probably primarily associated with sorption and settling, with the greywater metal contaminants investigated primarily partitioning to the sludge fraction during treatment. This indicates that the major potential problems associated with the presence of metals in greywater are related to the issue of sludge disposal. In many cases, this is discharged directly to sewer, in which case the risk to the environment should not differ from that associated with conventional wastewater treatment systems where the greywater treatment step is not included. This also indicates that greywater treatment probably does not act as a significant mitigation and treatment barrier for metals. Furthermore, in systems where the sludge disposal route is not to sewer (e.g. land disposal), the metal load of the greywater sludge fraction requires further attention, to ensure that soil guideline limits are not exceeded and that home grown food crops are not compromised by increasing metal contents. Soil texture, depth to groundwater, and effluent pH should also be carefully considered to ensure that the environment is adequately protected. Similar considerations are necessary when greywater effluent is to be reused for irrigation purposes. The results further indicate that the presence of these metals in greywater is unlikely to present any major obstacles for greywater reuse for toilet flushing. Moreover, although reuse for laundry washing may yield oversaturation of Pb minerals which may adhere to clothing, this is unlikely to lead to a significant increase in human exposure at the measured concentrations. Apart from Hg, the effluent metal concentrations all meet the WFD-EQS indicating that greywater metal concentrations do not present a risk to surface waters. This does not mean that greywater effluent is suitable for discharge to surface waters however, as factors such as sodicity and/or pathogen content (bacteria, virus, and protozoa [see e.g. 15,20]) may constitute a greater problem than that of metal content. Although greywater effluent Hg loads could be further decreased by air stripping, ongoing phasing out of ambient sources such as amalgam fillings may present a much more cost-effective option.