2. Potentially Toxic Elements 2. POTENTIALLY TOXIC ELEMENTS: SOURCES, PATHWAYS, AND FATE THROUGH URBAN WASTEWATER TREATMENT SYSTEMS The aim is to reduce inputs of pollutants entering the wastewater system to background levels because this represents the minimum potential extent of contamination that can be achieved. Potentially toxic elements are of concern because of their potential for long-term accumulation in soils and sediments. The majority of metals transfer to sewage sludge (see Fig 2.1). However, 20% may be lost in the treated effluent, depending on the solubility and this may be as high as 40% - 60% for the most soluble metal, Ni. Although the use of sludge on agricultural land is largely dictated by nutrient content (nitrogen and phosphorus), the accumulation of potentially toxic elements in sewage sludge is an important aspect of sludge quality, which should be considered in terms of the long-term sustainable use sludge on land. Application of sludge to agricultural land is the largest outlet for its beneficial use and this is consistent with EC policy of waste recycling, recovery and use. This is a critical issue due to the increasing amount of sludge produced, the increasingly stringent controls on landfilling, the public opposition to incineration (a potential source of further atmospheric pollution), and the ban on disposal at sea. Consequently sludge quality must be protected and improved in order to secure the agricultural outlet as the most cost effective and sustainable option. Figure 2.1: Origin and fate of metals during treatment of wastewater [from ADEME, 1995] Pollutants in Urban Waste Water and Sewage Sludge 12 2. Potentially Toxic Elements 2.1. Sources and pathways of potentially toxic elements in UWW The average concentrations of potentially toxic elements in domestic and commercial wastewater are given in Table 2.1. The maximum concentrations of potentially toxic elements found in commercial wastewater are generally greater than those in domestic wastewater. This is supported by Scandinavian studies [SFT-1997a, 1997b, 1999] considering all urban sectors together, which judged that commercial and light industrial sectors contributed larger loads of potentially toxic elements to urban wastewater than household sources. Table 2.1 Concentrations of metals in domestic and commercial wastewater [Wilderer and Kolb, 1997 in Munich, Germany] Element Domestic Commercial Wastewater [mg.l-1] Wastewater [mg.l-1] Pb 0.1 £ 13 Cu 0.2 0.04-26 Zn 0.1-1.0 0.03-133 Cd <0.03 0.003-1.3 Cr 0.03 £20 Ni 0.04 £7.3 Table 2.2 Potentially toxic elements in UWW from various sources (% of the total measured in the UWW) Pollutant Country Domestic Commercial Urban Not Reference Wastewater Wastewater Runoff Identified Cd France 20 61 3 16 ADEME, 1995 Norway 40 SFT report 97/28 UK 30 29 41 WRc, 1994 Cu France 62 3 6 29 ADEME, 1995 Norway 30 SFT report 97/28 UK 75 21 4 WRc, 1994 Cr France 2 35 2 61 ADEME, 1995 Norway 20 SFT report 97/28 UK 18 60 22 WRc, 1994 Hg France 4 58 1 37 ADEME, 1995 Pb France 26 2 29 43 ADEME, 1995 Norway 80 SFT report 97/28 UK 43 24 33 WRc, 1994 Ni France 17 27 9 47 ADEME, 1995 Norway 10 SFT report 97/28 UK 50 34 16 WRc, 1994 Zn France 28 5 10 57 ADEME, 1995 Norway 50 SFT report 97/28 UK 49 35 16 WRc, 1994 Pollutants in Urban Waste Water and Sewage Sludge 13 2. Potentially Toxic Elements Cd distribution Cu distribution Domestic Domestic Storm events Storm events Commercial Commercial Non Identified Non Identified Cr distribution Hg distribution Domestic Domestic Storm events Storm events Commercial Commercial Non Identified Non Identified Pb distribution Ni distribution Domestic Domestic Storm events Storm events Commercial Commercial Non Identified Non Identified Zn distribution Domestic Storm events Commercial Non Identified Figure 2.2 Pie charts showing the breakdown of potentially toxic elements entering UWW from different sources in France (ADEME 1995) This uses the French data in Table 2.2 but is included to give a clearer visual representation of the source breakdown for the different metals. Pollutants in Urban Waste Water and Sewage Sludge 14 2. Potentially Toxic Elements The data in Table 2.2 and Figure 2.2 show that for some elements over 50% of the potentially toxic elements in wastewater are unaccounted for. This is in line with findings by Critchley & Agy [1994] Better source inventory data is essential in order to effectively target reductions in emissions from all the different sources. It may be that identification of some of the industrial sources will require increased trade effluent discharge controls if concentrations of pollutants are to be reduced. Domestic and urban run-off sources may require different types of action, such as changes in products used. Emissions of potentially toxic elements from industrial point sources were the major sources of pollution to urban wastewater. However, stringent and more widespread limits applied to industrial users has reduced the levels of potentially toxic elements emitted by industry into urban wastewater considerably. This continues a general decline of potentially toxic elements from industrial sources since the 1960s, due to factors such as cleaner industrial processes, trade effluent controls and heavy industry recession. For example, the liquids used in metal finishing typically contain 3-5 mg.l-1 of copper, 5-10 mg.l-1 of chromium, 3-5 mg.l-1 of zinc, 5-10 mg.l-1 of zinc, 1-5 mg.l-1 of cyanide, and 10-50 mg.l-1 of suspended solids [Barnes, 1987]. However, metal finishing industries are now required to pre-treat these liquids before disposal, reducing toxic discharges by 80-90%. In the Netherlands, a survey of potentially toxic element load in UWW influent [SPEED, 1993], also made estimations for 1995 and forecasts up to 2010. The overall prevalence of potentially toxic elements in the UWW system is expected to decrease, mainly due to a decrease in runoff and industrial sources, while the potentially toxic elements share in WWTS loads from households was expected to increase. As industrial sources of potentially toxic elements in UWW decline, the relevant importance of diffuse sources will increase. Wiart and Reveillere [1995] carried out studies at the Achères WWTS in France. Their studies showed a significant decrease (50-90%) in the potentially toxic element content of sewage sludge since 1978, following the application of the "at-source discharge reduction" policy [Bebin, 1997]. However, the main concern is now with organic pollutants, and current regulations require monitoring of the influent, in order to set up a baseline database from which limits may then be devised. Pollutants in Urban Waste Water and Sewage Sludge 15 2. Potentially Toxic Elements 2.1.1 Domestic sources Domestic sources of potentially toxic elements in wastewater are rarely quantified due to the difficulty in isolating them. Domestic sources include the potentially toxic elements discharged from the household to UWW collecting systems and, in addition, corrosion from materials used in distribution and plumbing networks, tap water and detergents. A study by RIVM (Dutch Institute of Public Health and the Environment) in the Netherlands [SPEED, 1993], quantified the waterborne emissions of potentially toxic elements from household sources, dentistry and utility buildings in the urban environment. Table 2.3 shows the data of waterborne potentially toxic elements emissions in tonnes per annum. Table 2.3 Emissions by Dutch households of potentially toxic elements [adapted from SPEED, 1993]. Potentially Gross waterborne emissions* tonnes.y-1 to surface toxic element water (1993) Household Dentistry Utility buildings sources Copper 94 0.6 27 Zinc 118 - 26 Lead 13 - 3.1 Cadmium 0.7 - 0.2 Nickel 7.3 - 0.9 Chromium 2.9 - 0.3 Mercury 0.3 2.3 0.01 * 96 % of the waterborne emissions are expected to go to the UWW collecting systems, with 4% going directly to surface waters. Domestic products containing potentially toxic elements used on a regular basis at home and/or at work, are also reviewed by Lewis [1999]. The following lists the principal PTEs and products containing them that may enter urban wastewater; Cadmium: is predominantly found in rechargeable batteries for domestic use (Ni-Cd batteries), in paints and photography. The main sources in urban wastewater are from diffuse sources such as food products, detergents and bodycare products, storm water [Ulmgren, 2000a and Ulmgren, 2000b]. Copper: comes mainly from corrosion and leaching of plumbing, fungicides (cuprous chloride), pigments, wood preservatives, larvicides (copper acetoarsenite) and antifouling paints. Mercury: most mercury compounds and uses are now banned or about to be banned, however, mercury is still used in thermometers (in some EU countries) and dental amalgams. Also, mercury can still be found as an additive in old paints for water proofing and marine antifouling (mercuric arsenate), in old pesticides (mercuric chloride in fungicides, insecticides), in wood preservatives (mercuric chloride), in embalming fluids (mercuric chloride), in germicidal soaps and antibacterial products (mercuric chloride and mercuric cyanide), as mercury-silver-tin alloys and for "silver mirrors". Nickel: can be found in alloys used in food processing and sanitary installations; in rechargeable batteries (Ni-Cd), and protective coatings. Lead: The main source of lead is from old lead piping in the water distribution system. It can be found in old paint pigments (as oxides, carbonates), solder, pool cue chalk (as carbonate), in certain cosmetics, glazes on ceramic dishes and porcelain (it is banned now Pollutants in Urban Waste Water and Sewage Sludge 16 2. Potentially Toxic Elements for uses in glazes), also in "crystal glass". Lead has also been found in wines, possibly from the lead-tin capsules used on bottles and from old wine processing installations.