Levels of trihalomethanes (THMs) in drinking water from water treatment plants (WTPs) in Nigeria were studied using a gas chromatograph (GC Agilent 7890A with autosampler Agilent 7683B) equipped with electron capture detector (ECD). The mean concentrations of the trihalomethanes ranged from zero in raw water samples to 950
In sub-Saharan Africa, uncontrolled microbiological contamination of drinking water sources is a commonplace and perennially presents a significant threat to public health. Disinfection is a critical step in drinking water treatment usually performed to safeguard the public health from pathogenic microbes and waterborne diseases [
The disinfection of water is an essential treatment process for safeguarding the quality of drinking water but could create undesirable chemical risk due to the formation of disinfection byproducts during chloramination, chlorination, and ozonation with natural organic matter. Since the early seventies, studies have revealed that chlorination produces potentially harmful DBPs with more than 600 DBPs detected and quantified in drinking waters [
According to documented report based on World Health Organization/UNICEF pilot study carried out in twelve states across eight hydrological areas in Nigeria, about 75% and 50% of water samples from protected dug wells and utility piped water (including water treatment plants) supplies are potentially contaminated with thermotolerant and faecal streptococci, respectively. Free chlorine was equally detected in piped water systems at levels ≥0.2 mg/l, (above maximum permissible level), which was attributed to poor dosing prior to distribution [
Four potable water treatment plants (WTPs) that utilize one of the two main treatment processes (chlorine-chlorine, chlorine-UV) used in Nigeria were selected for this study. Public and private WTPs in Lagos and Ogun States were selected and these include two public (OW and LW) and two private (HW and SW) water treatment plants. The codes used in the present study were adopted to protect the corporate identities of the companies involved. At the public water works, water is sourced from surface water (river) with the help of 33 kW low lift pump. Large debris and other particulate matters are prevented from entering the treatment process by coarse screens as the raw water flows into a tank. Coagulation, flocculation, and sedimentation processes are all done in a large rectangular reactor clarifier (RC). Preliming (using aluminium sulphate, Al2(SO4)3) and prechlorination (using 30 ppm chlorine powder) are performed simultaneously while the raw water is in the reactor clarifier. Water from the RC is pumped into several filter beds, which consist of layers of sand and gravels on top of the underdrain nozzles at the bottom of the filter beds. Any particulate matter that escapes the sedimentation process is trapped in the filter beds and thereafter directed through the underdrain nozzles designed to retain the filter media and allow the flow of water. From the filter beds, water is channeled into large reservoirs at the base of the treatment plant. Prior to the filtration process, backwashing is done to remove the debris that might have accumulated in the filter beds. This involves forcing water through the media in an opposite direction to isolate the filter from the treatment process and agitating the surface with the use of compressed air to loosen and flush the debris that has accumulated on it into the waste holding tank. A secondary disinfection involving chlorination is done to maintain a chlorine residual of 0.2 ppm as the water moves from the filter beds into reservoirs prior to distribution through an underground network of pipes for public consumption.
However, at the private water plants, raw groundwater (a borehole) is pumped into an aeration tank. It is subjected to prechlorination and preliming in the treatment tank where it is allowed to stand for six to eight hours to ensure enough contact time. Industrial filter tanks containing activated carbon and resins are employed to remove iron (Fe), odour, taste, and microsized particles from the water. The secondary disinfection is carried out by allowing the treated water to flow from the overhead tanks through the UV disinfection system, prior to packaging. The treatment goal of chlorination is to kill germs, thus inhibiting further biological activities, while improving the taste and odour of water. Raw water samples were taken directly from respective sources prior to primary disinfection process. More so, primary disinfection samples were collected from the large reservoirs after the primary disinfection stage, while the secondary water samples were obtained at the point of the distribution into pipes/packaging. Sampling of raw, primary, and secondary water samples from the public and private WTPs took place between January and May 2015. Water samples were taken in clean and well-marked 40 mL glass vials with screw caps lined with Teflon-faced septa. Each vial was filled to overflow ensuring that there are no air bubbles and headspace. After collection, 25 mg of ascorbic acid was added to each vial as a reducing agent to quench further production of disinfection byproducts (DBPs). The vials were then sealed and samples stored at 4°C prior to analyses.
All the reagents and chemicals used in this work are of HPLC grade and of highest purity. n-Pentane and n-hexane were purchased from Scharlau Chemie SA, Spain. An AccuStandard® Incorporated, USA, Commercial Stock Standard of Trihalomethanes Mix (1000
The internal standard was prepared by dissolving 5
Trihalomethanes (THMs) were isolated using a liquid-liquid extraction with HPLC grade pentane, and analyses were carried out using a gas chromatograph (GC) (7890A, Agilent, USA) with autosampler (7683B, Agilent, USA) equipped with an electron capture detector (ECD) based on USEPA method 551.1 [
The pH, temperature, chloride, total organic carbon (TOC), total dissolved solids (TDS), and residual chlorine measured for raw water samples from the private and public water treatment plants are presented in Table
Summary statistics for raw water samples collected from private and public water treatment plants.
pH | Temp. (°C) | Chloride (mg/L) | TDS (mg/L) | TOC (mg/L) | Res. Cl (mg/L) | |
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HWR |
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0.00 |
SWR |
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0.00 |
OWR |
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0.00 |
LWR |
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0.00 |
The raw water samples had pH values that were within the neutral pH range of 6.5 and 7.5, and the temperature ranged between
The concentrations of the trihalomethanes (Table
Average concentration (
Source | TCM | BDCM | DBCM | TBM | TTHMs |
---|---|---|---|---|---|
Jan. | |||||
OWR | BD | BD | BD | BD | 0 |
OWP | 997.43 | BD | BD | 0.40 | 997.83 |
OWS | 960.68 | BD | BD | BD | 960.68 |
LWR | BD | BD | BD | BD | 0 |
LWP | 716.04 | 0.42 | 0.38 | BD | 716.84 |
LWS | 825.04 | BD | BD | BD | 825.04 |
HWR | BD | BD | BD | BD | 0 |
HWP | 755.70 | BD | BD | BD | 755.70 |
HWS | 812.35 | BD | BD | BD | 812.35 |
SWR | BD | BD | BD | BD | 0 |
SWP | 999.64 | BD | BD | BD | 999.64 |
SWS | 950.97 | BD | BD | BD | 950.97 |
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Feb. | |||||
OWR | BD | BD | BD | BD | 0 |
OWP | 953.77 | BD | BD | BD | 953.77 |
OWS | 887.34 | BD | BD | BD | 887.34 |
LWR | BD | BD | BD | BD | 0 |
LWP | 900.70 | 0.42 | BD | BD | 901.12 |
LWS | 825.04 | BD | BD | BD | 825.04 |
HWR | BD | BD | BD | BD | 0 |
HWP | 916.94 | BD | BD | BD | 916.94 |
HWS | 872.00 | BD | BD | BD | 872.00 |
SWR | BD | BD | BD | BD | 0 |
SWP | 906.38 | BD | BD | BD | 906.38 |
SWS | 712.50 | BD | BD | BD | 712.50 |
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Mar. | |||||
OWR | BD | BD | BD | BD | 0 |
OWP | 561.86 | BD | BD | BD | 561.86 |
OWS | 582.32 | 0.41 | BD | BD | 582.73 |
LWR | BD | BD | BD | BD | 0 |
LWP | 620.55 | BD | BD | BD | 620.55 |
LWS | 846.79 | 0.41 | BD | BD | 847.20 |
HWR | BD | BD | BD | BD | 0 |
HWP | 777.70 | BD | BD | BD | 777.70 |
HWS | 803.42 | BD | BD | BD | 803.42 |
SWR | BD | BD | BD | BD | 0 |
SWP | 624.54 | BD | BD | BD | 624.54 |
SWS | 591.99 | BD | BD | BD | 591.99 |
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Apr. | |||||
OWR | BD | BD | BD | BD | 0 |
OWP | 31.21 | BD | BD | BD | 31.21 |
OWS | 28.34 | 0.41 | BD | BD | 28.75 |
LWR | BD | BD | BD | BD | 0 |
LWP | 21.92 | BD | BD | BD | 21.92 |
LWS | 28.44 | BD | BD | BD | 28.44 |
HWR | BD | BD | BD | BD | 0 |
HWP | 28.25 | BD | BD | BD | 28.25 |
HWS | 30.03 | BD | BD | BD | 30.03 |
SWR | BD | BD | BD | BD | 0 |
SWP | 28.87 | BD | BD | BD | 28.87 |
SWS | 32.11 | BD | BD | BD | 32.11 |
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May | |||||
OWR | BD | BD | BD | BD | 0 |
OWP | 30.45 | BD | BD | BD | 30.45 |
OWS | 34.15 | BD | BD | BD | 34.15 |
LWR | BD | BD | BD | BD | 0 |
LWP | 27.20 | BD | BD | BD | 27.20 |
LWS | 26.89 | BD | BD | BD | 26.89 |
HWR | BD | BD | BD | BD | 0 |
HWP | 32.90 | BD | BD | BD | 32.90 |
HWS | 29.20 | BD | BD | BD | 29.20 |
SWR | BD | BD | BD | BD | 0 |
SWP | 32.50 | BD | BD | BD | 32.50 |
SWS | 32.10 | BD | BD | BD | 32.10 |
R: raw, P: primary, S: secondary disinfection samples, and BD: below detection limit.
The average concentrations of trihalomethanes in primary and secondary water samples from the WTPs generally followed the sequence TCM > BDCM > TBM = DBCM, which was consistent with similar documented reports [
In surface and groundwater sources, the organic matter is predominantly derived from decayed or living plant materials. This natural organic matter is present in water sources in dissolved, particulate, and colloidal forms [
Risk assessment is a vital tool for regulation and prioritization of chemical contaminants in drinking water and could be expressed in terms of specific disease endpoints (e.g., cancer) [
The chronic daily intake of THMs through ingestion route of exposure is presented in Tables
Exposure assessment of THMs (mg/kg-day) through ingestion in adults
Sites | Jan. | Feb. | March | April | May | ||||||||||
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TCM × 10−2 | BDCM × 10−5 | DBCM × 10−5 | TCM × 10−2 | BDCM × 10−5 | DBCM | TCM × 10−2 | BDCM × 10−5 | DBCM | TCM × 10−3 | BDCM × 10−5 | DBCM | TCM × 10−3 | BDCM | DBCM | |
OWP | 4.51 | — | — | 4.31 | — | — | 2.54 | — | — | 1.41 | — | — | 1.37 | — | — |
OWS | 4.34 | — | — | 4.01 | — | — | 2.63 | 1.85 | — | 1.28 | 1.85 | — | 1.54 | — | — |
LWP | 3.23 | 1.89 | 1.72 | 4.07 | 1.89 | — | 2.81 | — | — | 9.90 | — | — | 1.23 | — | — |
LWS | 3.73 | — | — | 3.72 | — | — | 3.83 | — | — | 1.29 | — | — | 1.21 | — | — |
HWP | 3.41 | — | — | 4.15 | — | — | 3.52 | 1.85 | — | 1.27 | — | — | 1.49 | — | — |
HWS | 3.67 | — | — | 3.94 | — | — | 3.63 | — | — | 1.36 | — | — | 1.32 | — | — |
SWP | 4.52 | — | — | 4.09 | — | — | 2.63 | — | — | 1.30 | — | — | 1.47 | — | — |
SWS | 4.29 | — | — | 3.22 | — | — | 2.67 | — | — | 1.45 | — | — | 1.45 | — | — |
Exposure assessment of THMs (mg/kg-day) through ingestion in children (6–18 years)
Sampling sites | Jan. | Feb. | March | April | May | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
TCM × 10−2 | BDCM × 10−5 | DBCM × 10−5 | TCM × 10−2 | BDCM × 10−5 | DBCM | TCM × 10−2 | BDCM × 10−5 | DBCM | TCM × 10−3 | BDCM × 10−5 | DBCM | TCM × 10−3 | BDCM | DBCM | |
OWP | 3.98 | — | — | 3.81 | — | — | 2.24 | — | — | 1.25 | — | — | 1.22 | — | — |
OWS | 3.83 | — | — | 3.55 | — | — | 2.32 | 1.64 | — | 1.13 | 1.64 | — | 1.36 | — | — |
LWP | 2.86 | 1.68 | 1.51 | 3.59 | 1.67 | — | 2.48 | — | — | 0.87 | — | — | 1.09 | — | — |
LWS | 3.29 | — | — | 3.29 | — | — | 3.38 | — | — | 1.13 | — | — | 1.07 | — | — |
HWP | 3.01 | — | — | 3.66 | — | — | 3.10 | 1.64 | — | 1.12 | — | — | 1.31 | — | — |
HWS | 3.25 | — | — | 3.48 | — | — | 3.21 | — | — | 1.19 | — | — | 1.17 | — | — |
SWP | 3.99 | — | — | 3.62 | — | — | 2.49 | — | — | 1.15 | — | — | 1.29 | — | — |
SWS | 3.79 | — | — | 2.84 | — | — | 2.36 | — | — | 1.28 | — | — | 1.28 | — | — |
The lifetime incidence rates (LIR) of developing cancer from exposure to different DBPs through different pathways were calculated using (
Lifetime incidence rates of developing cancer by exposure to chloroform in adults
Sampling sites | Jan. | Feb. | Mar. | Apr. | May |
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OWP |
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OWS |
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LWP |
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LWS |
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HWP |
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HWS |
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SWP |
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SWS |
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Lifetime incidence rates of developing cancer by exposure to chloroform in children
Sampling sites | Jan. | Feb. | Mar. | Apr. | May |
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OWP |
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OWS |
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LWP |
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LWS |
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HWP |
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HWS |
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SWP |
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SWS |
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Total cancer incidence rate (×10−4) by exposure to chloroform in adults and children.
Jan. | Feb. | Mar. | Apr. | May | ||||||
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ADT | CHD | ADT | CHD | ADT | CHD | ADT | CHD | ADT | CHD | |
OW | 5.3996 | 4.7723 | 5.0769 | 4.4872 | 3.1551 | 2.7886 | 0.1642 | 0.1451 | 0.1781 | 0.1574 |
LW | 4.2496 | 3.7559 | 4.7588 | 4.2060 | 4.0662 | 3.5762 | 0.1386 | 0.1227 | 0.1491 | 0.1318 |
HW | 4.3240 | 3.8217 | 4.9331 | 4.3600 | 4.3600 | 3.8535 | 0.1607 | 0.1420 | 0.1420 | 0.1513 |
SW | 5.3789 | 4.7541 | 4.4641 | 3.9456 | 3.3546 | 2.9649 | 0.1682 | 0.1486 | 0.1486 | 0.1574 |
The median values and 5th and 95th percentiles of the cancer risk distributions from exposure to chloroform through ingestion pathway are summarized in Table
Excess cancer incidences from exposure to TCM through different ingestion (×10−4) in adults and children.
OW | LW | HW | SW | |
---|---|---|---|---|
Adults | 1.67 (0.17, 5.32) | 2.11 (0.14, 4.63) | 2.24 (0.15, 4.79) | 1.76 (0.15, 5.15) |
Children | 1.47 (0.15, 4.700) | 1.85 (0.13, 4.09) | 1.99 (0.14, 4.23) | 1.56 (0.15, 4.55) |
Data shown are median values of the risk distributions, and values in the parentheses are the 5th and 95th percentiles of the risk distributions.
In this study, the concentration of trihalomethanes was assessed in drinking water from four different water treatment plants (HW, OW, LW, and SW) in two heavily populated states in Nigeria. The levels of the total trihalomethanes were found to be higher at the initial stage of the research but gradually reduced probably due to more careful handling of the water treatment processes as the quality control officers were briefed on the results of previous analysis when the next samples were being collected. This is an indication that the treatment processes handling contributes a great deal to the formation of disinfection by-products in drinking water. This has to do partly with carefulness in the addition of chlorine, which is mostly used in all the water treatment plants in the sampled areas; the documented dosage is 20–30 mg/L but it does not seem this is strictly adhered to. In addition to this are the processes of sedimentation/flocculation and the maintenance of the right amount of residual chlorine in the distribution system.
For the water treatment plants to achieve the SON (0.001 mg/L) and USEPA (0.08 mg/L) recommended maximum permissible levels (MCL) for the TTHMs, a constant effort is required to reduce the concentrations of all the DBPs to the barest minimum. This can be achieved by protecting the source water from excessive pollution thereby reducing the DBP precursors. The use of chloramine as disinfectant has proven to produce very minimal amount of DBPs in countries like Europe, America, and Australia. Therefore, a switch from chlorine usage during primary disinfection by DWTPs in Nigeria to chloramine is strongly recommended with ultraviolet light as the secondary disinfectant. Operators of public and private water treatment plants should adopt viable alternative primary disinfection strategies such as using ClO2, ozone, and UV disinfection to chlorination. In addition, a thorough evaluation of the efficiency of WTPs and operational practices of the water treatment processes and the piped distribution network is proposed to examine the possible reasons for the exceedances of TTHMs.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
The authors declare no conflicts of interest.