Applicability of drinking water treatment residue for lake restoration in relation to metal/metalloid risk assessment

Drinking water treatment residue (DWTR), a byproduct generated during potable water production, exhibits a high potential for recycling to control eutrophication. However, this beneficial recycling is hampered by unclear metal/metalloid pollution risks related to DWTR. In this study, the pollution risks of Al, As, Ba, Be, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, and Zn due to DWTR application were first evaluated for lake water based on human health risk assessment models and comparison of regulatory standards. The risks of DWTR were also evaluated for sediments on the basis of toxicity characteristics leaching procedure and fractionation in relation to risk assessment code. Variations in the biological behaviors of metal/metalloid in sediments caused by DWTR were assessed using Chironomus plumosus larvae and Hydrilla verticillata. Kinetic luminescent bacteria test (using Aliivibrio fischeri) was conducted to analyze the possibility of acute and chronic detrimental effects of sediment with DWTR application. According to the obtained results, we identify a potential undesirable effect of DWTR related to Fe and Mn (typically under anaerobic conditions); roughly present a dosage threshold calculation model; and recommend a procedure for DWTR prescreening to ensure safe application. Overall, managed DWTR application is necessary for successful eutrophication control.

3 coupled plasma-atomic emission spectrometry (ICP-AES, ULTIMA, JY, France). Therefore, except the four undetectable elements, the risks of other metals and As were assessed herein.

Incubation test
To examine the effect of lake water pH, 100 g of wet sediments was placed into eight beakers (1 L). Four beakers contained 7 g of air-dried DWTR, while the other four beakers were used as controls. The DWTR represented approximately 10% of the sediments in dry weight 1 . Lake water of 500 mL was added to the beakers slowly to avoid solids resuspension.
Next, lake water pH was kept at 5.5-6.0 and 8.5-9.0, respectively, using either HCl or NaOH.
Each group has two parallel samples and pH was daily adjusted. The beakers were covered with a gas-permeable film and incubated at 15 °C in dark.
To examine the effect of lake water redox conditions, similar to the test of the pH effect, each group had two parallel samples. One group was put into a culture tank. Then, the gas extraction and replacement processes were performed three times by a Unijar Suction System (Unitech BioScience Co., Ltd, China) for the tank to create anaerobic condition. The replacement gas contained N 2 (80%), CO 2 (10%), and H 2 (10%). The other was covered by a gas-permeable film to maintain aerobic condition.
In the above two tests, 10 mL lake water was collected every 10 d (the tests lasted 30 d), and the metals and As concentrations were determined using ICP-AES. After incubation, the sediments with and without DWTR were freeze-dried, ground, and sieved to a diameter less than 0.15 mm for further analysis. The properties of lake water during the tests can be seen in Table S10.
till usage at -20 °C. Briefly, a pure culture of A. fischeri was prepared in supplemented seawater complete media (SSWC media) and incubated over night (90 rpm, 20 °C). After the turbidity of the bacteria suspension reached 500-700 formazin turbidity units (FTU), the culture was diluted with SSWC media to an initial turbidity of 20 FTU approximately. The bacteria suspension and SSWC media (blank) was transferred to a 96-well plate, and an initial measurement of luminescence and optical density (λ=578 nm) was performed after pre-tempered for 30 min. Subsequently, the sediment extracts and controls were added, and a kinetic measurement of luminescence and optical density was conducted for 24 h by the plate reader (infinite M200, Tecan, Switzerland) and positioned in a cooling incubator (Thermo Fisher Scientific, USA) at 15 °C. Each sample was tested in triplicate.
Test solution preparation: Aqueous extracts of samples were prepared according to Ocampo-Duque et al. 9 with some modification. Sample of 3 g was mixed with 30 mL of 3% (w/v) aq. NaCl solution, shaking for 12 h at 20 °C, and then filtering on 0.45 um pore diameter membrance filters. Sediment preparation: The DWTR were mixed with sediment at dosages accounting for 0 (control) and 10% of sediment in dry weight. The mixtures were incubated for 10 d 1 . After incubation, the mixtures were freeze-dried, ground, and sieved to a diameter less than 1 mm.
In the untreated cultures, the transition between the exponential and the stationary growth phase was reached after approximately 10 h, and the maximum luminescence was reached in the late stationary growth phase after approximately 16 h. Accordingly, 10 and 15 h could be the suitable exposure time for the analysis of the growth inhibition and chronic luminescence inhibition compared to the controls, respectively. The acute luminescence 7 inhibition was measured after the bacteria were exposed 30 min.

Statistical analysis
Data analysis was performed using SPSS version 18.0. For fractionation and TCLP analysis, the relative standard deviation of three parallel sub-samples for each sample was less than 10%. Kolmogorov-Smirnov tests indicated that the data from the replicate samples followed a normal distribution. Therefore, one-way analysis of variance (ANOVA), based on α = 0.05, was used to determine the differences of data obtained accordingly.  The extractability of the metals and As in raw sediments.

Human health risk assessment for metal/metalloid in lake water
A human health risk assessment model was used to determine the potential the metal and As risk of DWTR addition to lake water. The assessments can be divided into non-carcinogenic and carcinogenic risks, which are based on the oral and dermal exposure routes. The detailed calculation methods are presented below. Normally, the potential non-carcinogenic risk is of concern when the hazard quotient (HQ) or hazard index (HI) exceeds 1 11 . Carcinogenic risk is the probability of an individual developing any type of cancer from the lifetime exposure to a carcinogen. The acceptable or tolerable risk for regulatory purposes is in the range of 10 −6 to 10 −411 .
For the non-carcinogenic effects, it can be calculated using the following equations 12,13 .
The HQ for the metal and As in dermal exposure (HQ dermal ) under different pH and redox conditions was below 1 (Table S8), indicating that dermal non-carcinogenic risk for each metal and As in lake water with DWTR addition was not of concern. For the HQ in oral exposure (HQ oral ), with the exceptions of As and Mn, the values for the other metals were also below 1. In comparison, HQ oral exceeded 1 for Mn in the overlying lake water with DWTR addition under anaerobic condition and pH 5.5-6.0, and that for Mn and As in lake water without DWTR addition under anaerobic condition. These results indicated that DWTR addition did not cause concerns for the oral non-carcinogenic risk for most metals in lake water and can even eliminate the concerns for As risk under anaerobic condition. However, at low pH, the addition may cause concerns for Mn oral non-carcinogenic risk.
The HI for the metal and As by dermal and oral exposure (HI dermal and HI oral ) is the sum of HQ dermal and HQ oral for each metal and As in lake water (Table S8). The HI dermal values for the metal and As in lake water under different conditions were below 1, indicating that dermal non-carcinogenic risk for the metal and As in lake water with DWTR addition was not of concern. However, the HI oral for the metal and As exceeded 1 in the overlying lake water with DWTR addition under anaerobic condition and pH 5.5-6.0, and without DWTR addition under anaerobic condition, while for other conditions, the HI oral was below 1. These results demonstrated that the oral non-carcinogenic risk for the metal and As in lake water with and without DWTR addition may be of concern. Further calculation showed that the HQ oral for Mn accounted for 95 and 79% of the HI oral for lake water with DWTR addition under anaerobic condition and pH 5.5-6.0, while for lake water without DWTR addition under anaerobic condition, the HQ oral for As and Mn accounted for 37 and 58% of the HI oral , respectively. Therefore, DWTR addition, on the one hand, increased HI oral by increasing Mn HQ oral , heightening the concerns for oral non-carcinogenic risk of metal/metalloid in lake water under anaerobic condition and low pH. On the other hand, the addition could reduce HI oral by decreasing As HQ oral , alleviating the concerns for oral non-carcinogenic risk under anaerobic condition.
In this study, among detectable metal/metalloid in lake water, only As had a carcinogenic risk (Table S9). The As dermal carcinogenic risks in lake water with and without DWTR addition were in the acceptable range (< 3.4×10 −6 ). The As oral carcinogenic risks in lake water were mostly not acceptable, and the risks for As with and without DWTR addition were in the same order of magnitude (1.4×10 −4 to 6.2×10 −4 ). The only exception was in the case of a pH range of 5.5 to 6.0, in which the As oral risk was < 9.9×10 −5 . These results suggested that DWTR addition could not reduce the As oral carcinogenic risks to an acceptable range for lake water.   c Data cannot be calculated because As concentration in lake water was below detection limit.

Water quality assessment based on comparison of metal/metalloid concentrations in lake water with water quality criteria
Metal/metalloid concentrations in lake water were compared with National Recommended Water Quality Criteria for fresh water in USA 16 and Environmental Quality Standard for Surface Water in China (GB3838-2002) 17 . The National Recommended Water Quality Criteria for fresh water in USA divided the metals/metalloids into two categories, i.e.
priority pollutants, including Ag, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Se, and Zn, and non priority pollutants, including Al and Fe. The standards for the pollutants commonly had two thresholds, which were criteria maximum concentration (CMC) and criterion continuous concentration (CCC). The CMC is an estimate of the highest concentration of a material in surface water to which an aquatic community can be exposed briefly without resulting in an unacceptable effect. The CCC is an estimate of the highest concentration of a material in surface water to which an aquatic community can be exposed indefinitely without resulting in an unacceptable effect. Among the priority pollutants, Ag, Be, Cd, Co, Cr, Hg, Sb, Se, and Pb were not detectable in lake water, sediments or DWTR. The concentrations of As, Cu, Ni, and Zn in lake water were clearly below the CMC and CCC. For the non priority pollutants, the concentrations of Al were clearly below the CMC and CCC. However, Fe concentrations in lake water exceeded the CCC (Fe 1000 μg L -1 ) under anaerobic condition, and DWTR addition increased the exceeding.
The Environmental Quality Standard for Surface Water in China (GB3838-2002) 17 divided surface water into five classes according to its purpose for use and protection target, and the standard of Class Ⅲ was adopted herein. The Class Ⅲ is mainly for class two protection areas for centralized potable water source, and protection areas for general fishing and swimming in China, and is referred to As, Cd, Cr, Cu, Hg, Se, Pb, and Zn. Besides the undetectable Cd, Cr, Hg, Se, and Pb, the concentrations of As, Cu, and Zn in lake water were clearly below the standards (Figure 2). There also have supplementary standards for the The risk assessment code of metal/metalloid in sediments Table S8 The calculated risk assessment code of the metals and As in sediments with and with DWTR addition.  The selected properties of overlying lake water in the incubation test The DO levels of lake water during the test of the pH effect were 4.8-7.0 mg L -1 , while during the test of the redox conditions effect, the DO levels of lake water under aerobic condition were 4.3-6.4 mg L -1 , and the oxidation-reduction potential of lake water under anaerobic condition was -87--308 mV. In addition, the pH of lake water under anaerobic condition was 6.6-6.7, and under aerobic condition, it was 7.8-8.3.  Figure S1. The results of the metals and As fractionation in sediments with and without DWTR addition after incubation tests. SWR and SNR represent sediments with and without DWTR, respectively; Low pH and High pH represent pH 5.5-6.0 and 8.5-9.0, respectively.