Utilization of artificial recharged effluent for irrigation: pollutants' removal and risk assessment

Water shortage is a serious environmental issue in arid and semi-arid regions that requires the exploration of water reuse options (Bakopoulou et al. 2011; Wei et al. 2012). To solve the water crisis throughout the world, rainfall, deep groundwater, seawater, etc., have been recognized as new alternative water resources (Elimelech & Phillip 2011; Erban et al. 2013; Jung et al. 2015). However, exploitation of the above water resources is always restricted by the local, economic and technological conditions, which limits their further application (Busch & Mickols 2004; Henriques et al. 2015). Considering that municipal wastewater treatment plants (WWTP) produce stable and abundant flows during different seasons, development of secondary effluent as an attractive alternative for water reuse is practically urgent (Kalkan et al. 2011; Zucker et al. 2015). Thus, how to efficiently recharge and then reuse the secondary effluent is rapidly becoming a necessity for many municipalities throughout the world (Wei et al. 2009). Generally, the traditional reuse approaches of secondary effluent include mainly agricultural irrigation, industrial processing, cooling water, toilet flushing, wetland habitat creation, restoration and maintenance, groundwater recharging, landscape water replenishment, among others (Bunani et al. 2015).

China is one of the largest agricultural countries in the world and the amount of water used for agriculture irrigation accounts for about 70% of the total water consumption (Li et al. 2010; Zhao et al. 2014); However, the lack of water sources constrains the development of agriculture (especially for northwest China), and the increase in population and shrinking of water supplies has strengthened this water shortage (Huang et al. 2012). Recently, the search for a renewable water source, such as secondary effluent, for agricultural irrigation is considered to be practically applicable. However, a relatively high level of heavy metals, persistent organic pollutants, salt ions, and other elements, as well as a certain amount of pathogenic bacteria have provided limits to its direct agricultural reuse (Rizzo et al. 2014). Earlier work by Lado et al. (2012) demonstrated that a significant accumulation of total organic matter, Cu, Ni, Zn and B in the top 1.0–1.5 m soil layer during long-term irrigation (>7 yr) with secondary treated wastewater, might be potentially toxic to some sensitive crops. Friedman et al. (2007) observed that higher levels of nitrogen and Mn were accumulated in celosia and phosphorus in sunflower during irrigation with secondary-treated municipal effluents. Recent work by Bakopoulou et al. (2011) pointed out that secondary effluents produced in the Thessaly region are suitable for irrigation reuse, especially for crops which are not used raw by humans; however, the seasonal and site-specific inhibition of growth on the plants were noted. Oron et al. (2006) stated that a UF/RO system would significantly improve the water quality of the secondary effluent, and minimize the health risk of agricultural products which had been irrigated with reclaimed wastewater. Generally, the existence of heavy metals, salinity, sodium, residual chlorine and other containments could partially affect plant growth, and consequently negatively affect the crop characteristics and soil properties. To effectively remove these hazardous pollutants in the secondary effluent, pre-treatments such as coagulation, ozone oxidation, granular activated carbon adsorption and membrane filtration have been applied as tertiary treatments (Bixio & Wintgens 2006; Kalkan et al. 2011; Pramanik et al. 2015). Generally, the above-mentioned pre-treatments are expensive and need chemical additions or regeneration steps. Therefore, less expensive and environmentally friendly methods for secondary effluent pre-treatment are desired.

Soil aquifer treatment (SAT) is an artificial water recharge technique frequently used to renovate domestic effluents for potable and non-potable purposes (Xue et al. 2013; Schaffer et al. 2015). The water quality is substantially improved via the integrated functions of biological, chemical, and physical processes in the aquifer (Hübner et al. 2014). To this end, the SAT system may be an alternative approach to the treatment of secondary effluent, and it has the advantages of lower cost, easier operation and greater efficiency. The SAT system can store the reclaimed water in the aquifer layer, with no evaporation losses and also can achieve inter-seasonal and inter-year storage (Dillon et al. 2006). Although several studies in the past years have attempted to explore secondary effluent as a water resource for agriculture irrigation, to the authors' knowledge seldom do researchers mention the application of SAT as a pre-treatment for secondary effluent irrigation.

The goal of this work was to: (1) study the effect of SAT pre-treatment on water quality for irrigation; (2) evaluate the effect of SAT pre-treatment on production of typical agricultural crops; and (3) assess the possible health risk of reusing the reclaimed water for irrigation.

Secondary effluent was obtained from the Taiping WWTP, Harbin (China). Collection of secondary effluent was performed at least once a month between February 2012 and August 2014. The secondary effluent had a pH value of 7.2 ± 0.1, chemical oxygen demand (COD) of 45.1 ± 5.2 mg/L, dissolved organic carbon (DOC) of 10.1 ± 1.3 mg/L, ultraviolet light adsorption at 254 nm (UV-254) of 14.3 ± 2.1 m⁻¹, with suspended solids (SS) of 13.9 ± 6.2 mg/L, ammonia of 10.2 ± 2.1 mg N/L, nitrate of 21.7 ± 2.1 mg N/L, chlorine of 121.7 ± 7.1 mg/L, of 59.0 ± 8.1 mg/L, total hardness of 157.2 ± 15.1 mg CaCO₃/L, and TP of 1.2 ± 0.3 mg/L. The tertiary effluent was obtained from the pre-treatment of the secondary effluent by ferric chloride coagulation/sand filtration.

Laboratory-scale soil column system which simulated aquifer conditions in a series of three 55 cm columns (diameter 10 cm) was constructed and operated for a period of more than two years, which was operated with a cycle of 16 h wetting/8 h drying. The influent was pumped upwards through the column at a desired flow rate of 15 mL/h controlled by a peristaltic pump. A complete description of the operation parameters of the SAT system is given in Xue et al. (2007). Experimental soil samples were collected from the dry bed of Songhua River in Harbin (potential recharge sites). After air drying, soil samples with particles greater than 2 mm were sieved out, then packed into the columns and further compacted to field density. The soil samples had an average pH of 8.2, organic carbon content (OC) of 2.9%, cation exchange capacity of 7.4 cmol/kg, and soil composition of 49.3% sand, 44.5% silt, and 6.2% clay, respectively.

Secondary, tertiary (coagulated-filtrated) and the SAT effluents were used for irrigation purposes, with tap water as a control. Five different types of crops, namely (Lachca sativa – lettuce, Brasica rapa var chinensis – pak choi, Cucumis sativus – cucumber, Brassica oleracea – cabbage, and Zea mays – maize) were selected as experimental objects, in which the lettuce and pak choi (common names) were used for studying the effect of different irrigation regimes on the inhibition/promotion of the growth and yields of vegetables, while cucumber, cabbage, and maize (common names) were used for evaluating accumulation of heavy metals in the crops.

After irrigation, the soil samples were air-dried, crushed and passed through a 2-mm sieve for further analysis. Soil pH and electrical conductivity were determined from suspension with a 1:1 soil:water ratio according to US Environmental Protection Agency (EPA) methods 9050 and 9045C, respectively. Calcium carbonate (CaCO₃) was measured with a calcimeter. Soil elements (K, Ca, Mg, Na, Cu, Fe, Mn, Zn, Ni, Cr, and Pb) were extracted after digestion with 3:1 (V/V) concentrated HCl-HNO₃ and measured by atomic absorption spectrophotometer (ICP-AES, Optima 5300DV, Perkin Elmer, USA).

The phytotoxicity of the irrigated water samples was evaluated according to Bakopoulou et al. (2011), where the micro-biotest measures the root growth rate of lettuce and pak choi periodically. Specifically, 10 seeds of each plant were placed at equal distances near the middle ridge of the test plates on black filter paper placed on reference soil which was hydrated with secondary, tertiary, and SAT effluents. Another series of plates (containing reference soil) was hydrated with tap water as a control. Three replicates were used for each plant species. The plates were placed vertically in a holder and incubated for 3 days at 25 °C in the dark. Pictures of the test plates at the end of the exposure period were taken by a digital camera and the length of the root of each plant was measured using the Image Tool 3.0 software (UTHSCSA, USA). The percent inhibition of root growth was calculated according to Bakopoulou et al. (2011).

Fresh weight (wet weight) of the vegetables and leaves was recorded immediately after 40 days' growth. Specifically, the collected vegetables and leaves were first washed with tap water, then rinsed with distilled water and dried in an oven at 80 °C for 72 h, finally ground in a stainless steel mill and retained for further analysis (Singh & Bhati 2005).

The crops cucumber, cabbage, and maize used for the irrigation study were planted in spring and harvested in autumn, and irrigated in the same condition (irrigation was performed once 3–5 d, 2 L water/strain). For comparison, the secondary effluent, tertiary effluent, and SAT effluent (0.5, 1.0, and 1.5 m) were selected as the irrigation water samples. Once the crops were harvested, each of the samples was immediately collected and homogeneously mixed and then stored in plastic bags for further analysis.

All the collected water samples were filtrated using 0.45 μm cellulose nitrate membrane filter and stored at 4 °C prior to analysis. Parameters, namely, pH, water content, organic matter, COD, soluble COD (SCOD), total nitrogen (TN), ⁠, NO₃^—N, and total phosphorus (TP) of the dewatered sludge were analyzed according to Standard Methods (APHA 2005). Samples for the analyses of heavy metals within the vegetables were pre-treated by wet-clear up-method (HNO₃:HClO₄ = 4:1), the concentration of heavy metals in the extracts was then determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 5300DV, Perkin Elmer, USA). Before the trihalomethane formation potential analysis, all the water samples were diluted to produce a DOC concentration of 1 mg/L. The concentration of DOC and UV-254 was analyzed using Shimadzu TOC-5000 total organic carbon analyzer and Shimadzu UV-2550 ultraviolet-visible spectrophotometer (Shimadzu, Japan).

As shown in Table 1, more than 70% of ⁠, TP, biochemical oxygen demand (BOD₅), Cr, Cd, As, Zn, Ni, and Mn in the secondary effluent was efficiently removed during the laboratory-scale 1.5 m depth SAT operation, ascribing to the efficient filtration, adsorption, and biodegradation within the vadose ozone and aquifer zone of the SAT system (Dillon et al. 2006; Alidina et al. 2014). The artificial SAT system also exhibited an efficient removal of the organic matter in secondary effluent, with a removal efficiency of 51.3% for COD and 58.7% for TOC. The majority of those removed organics were non-aromatic components, as evident by a relatively lower reduction of 35.7% for UV-254. For comparison, a relatively lower removal efficiency of 19.4% for ⁠, 30.0% for ⁠, and 32.1% for Cl⁻ was observed when the SAT operation progressed. The hardness of the SAT effluent increased 55.3% as compared to the secondary effluent, ascribing to the desorption and cation exchange of the sediments within the packed soil (Candela et al. 2007; Long et al. 2011; Qiu et al. 2016). As the bulk removal efficiency of pollutants for the artificial 1.5 m SAT column is much lower than the actual groundwater recharging systems, the reusing of SAT effluent for agricultural irrigation water is meaningful in practice. In addition, the top layer of the SAT system played a key role in pollutants' reduction during the secondary effluent recharging process, evidenced by the predominant removal of most pollutants within the top 0.5 m depth soil, noted in Table 1.

Table 1

Water quality of secondary, tertiary and 0.5, 1.0, and 1.5 m depth SAT effluents

Parameters .	Secondary effluent .	Tertiary effluent .	SAT effluent .			Parameters .	Secondary effluent .	Tertiary effluent .	SAT effluent .
			0.5 m .	1.0 m .	1.5 m .				0.5 m .	1.0 m .	1.5 m .
COD (mg/L)	45.1	37.4	29.4	23.8	21.9	Cl− (mg/L)	121.7	109.2	101.3	90.6	82.7
BOD5 (mg/L)	24.6	11.4	7.3	5.4	4.9		59.0	48.2	49.4	44.3	41.3
TOC (mg/L)	14.5	9.3	8.2	6.6	6.0	Cd (μg/L)	10.10	1.00	3.25	1.14	0.72
SS (mg/L)	13.9	5.4	5.1	3.1	1.9	Na (mg/L)	94.70	63.2	77.4	63.7	57.6
UV254 (mg/L)	0.14	0.08	0.11	0.10	0.09	Cr (μg/L)	20.30	5.05	2.81	1.22	0.22
Turbidity (NTU)	6.2	4.1	3.4	3.3	3.2	Pb (μg/L)	13.09	1.02	8.31	6.12	4.23
Hardness (mg CaCO3/L)	157.2	197.5	189.4	221.3	244.2	Mn (μg/L)	220.3	79.6	112.4	72.1	60.9
pH	7.20	7.42	7.31	7.66	7.94	Mg (μg/L)	11.37	11.15	5.24	3.97	3.24
TP (mg/L)	1.20	0.58	0.11	0.09	0.08	Cu (μg/L)	92.31	47.22	40.25	21.33	14.25
TN (mg/L)	31.4	24.2	23.7	22.1	21.8	Ni (μg/L)	0.005	0.004	0.001	0.00	0.00
	10.2	7.6	4.1	2.0	0.2	Hg (μg/L)	2.19	1.40	1.02	0.91	0.83
	21.7	15.4	19.6	19.9	17.5	Zn (μg/L)	60.00	14.00	36.2	25.7	23.50
Fecal coliforms (L−1)	17,000	<2.2	110	32.0	11.0	As (μg/L)	0.50	ND	0.11	0.04	0.00

Parameters .	Secondary effluent .	Tertiary effluent .	SAT effluent .			Parameters .	Secondary effluent .	Tertiary effluent .	SAT effluent .
			0.5 m .	1.0 m .	1.5 m .				0.5 m .	1.0 m .	1.5 m .
COD (mg/L)	45.1	37.4	29.4	23.8	21.9	Cl− (mg/L)	121.7	109.2	101.3	90.6	82.7
BOD5 (mg/L)	24.6	11.4	7.3	5.4	4.9		59.0	48.2	49.4	44.3	41.3
TOC (mg/L)	14.5	9.3	8.2	6.6	6.0	Cd (μg/L)	10.10	1.00	3.25	1.14	0.72
SS (mg/L)	13.9	5.4	5.1	3.1	1.9	Na (mg/L)	94.70	63.2	77.4	63.7	57.6
UV254 (mg/L)	0.14	0.08	0.11	0.10	0.09	Cr (μg/L)	20.30	5.05	2.81	1.22	0.22
Turbidity (NTU)	6.2	4.1	3.4	3.3	3.2	Pb (μg/L)	13.09	1.02	8.31	6.12	4.23
Hardness (mg CaCO3/L)	157.2	197.5	189.4	221.3	244.2	Mn (μg/L)	220.3	79.6	112.4	72.1	60.9
pH	7.20	7.42	7.31	7.66	7.94	Mg (μg/L)	11.37	11.15	5.24	3.97	3.24
TP (mg/L)	1.20	0.58	0.11	0.09	0.08	Cu (μg/L)	92.31	47.22	40.25	21.33	14.25
TN (mg/L)	31.4	24.2	23.7	22.1	21.8	Ni (μg/L)	0.005	0.004	0.001	0.00	0.00
	10.2	7.6	4.1	2.0	0.2	Hg (μg/L)	2.19	1.40	1.02	0.91	0.83
	21.7	15.4	19.6	19.9	17.5	Zn (μg/L)	60.00	14.00	36.2	25.7	23.50
Fecal coliforms (L−1)	17,000	<2.2	110	32.0	11.0	As (μg/L)	0.50	ND	0.11	0.04	0.00

ND: not detected.

Generally, the tertiary treatment of coagulation/filtration showed a relatively lower removal rate of the organic contaminants, ⁠, Cl⁻ and majority heavy metal ions, leading to an average removal efficiency of 17.1% for COD, 53.8% for BOD₅, 35.7% for TOC, 32.1% for ⁠, and 10.3% for Cl⁻, respectively. For comparison, a relatively higher removal rate of 29.0% for ⁠, 42.9% for UV-254, and 92.3% for Pb was also observed. Previous study demonstrated that the existence of non-biodegradable organic, phytotoxic metals like Cr, Pb, Cu, Mn, and Zn in secondary effluent markedly enhanced the phytotoxicity of the target crops (Ellouze et al. 2009; Ma et al. 2015), thus, the efficient removal of these pollutants during SAT operation might guarantee a safe agricultural reusing of secondary effluent.

As shown in Figure 3, the concentration of Cr in the SAT effluent irrigated maize accounted for 34.9% and 40.5% of the bulk accumulation of Cr in the secondary and tertiary effluents irrigated samples, respectively, and was 50.8% and 76.2% for cucumber and 58.1% and 46.4% for cabbage for comparison. Similarly, the concentration of Pb in the SAT effluent irrigated cucumber, maize, and cabbage decreased 61.1%, 43.8%, and 27.3% in comparison with the secondary effluent irrigated crops, respectively. Concentration distribution of element Cd exhibited a similar trend after the irrigation with SAT effluent and secondary effluent. Thus, a general conclusion that can be drawn from the above is that the reuse of SAT effluent would decrease the accumulation of heavy metals of Cr, Pb, and Cd in vegetables as compared with that of secondary and tertiary effluents. Conversely, the concentration of Cu, Zn, and Ni in the SAT effluent irrigated crops only decreased 9.3 ∼ −2.9% for cucumber, 11.4 ∼ −9.1% for cabbage, and 11.3 ∼ 3.2% for maize in comparison with the corresponding crops irrigated with tap water and tertiary/secondary effluents. Generally, the accumulated Cr, Pb, Cd, Ni in cucumber was significantly decreased for SAT effluent irrigation, and Cr, Pb, Cu for cabbage, and Cr, Pb, Cd and Cu for maize. This demonstrates that SAT pre-treatment was efficient and useful for heavy metal pollution control during secondary effluent irrigation.

The health risk of the different effluent irrigations via the food chain can be calculated based on the hypothesis that the exposure frequency is once per day, with an exposure dosage of 10 mL. As shown in Table 2, the calculated annual risk of the chemical non-carcinogens during the secondary effluent irrigation was 1.18 × 10⁻¹¹, and decreased to 7.62 × 10⁻¹² for tertiary effluent, and further declined to 3.64 × 10⁻¹² for the 1.5 m depth SAT effluent. It is obvious that irrigation using the SAT effluent would significantly decrease the health risk of chemical non-carcinogens to farmers, and is meaningful for agricultural irrigation purposes.

As shown in Table 3, the carcinogenic risk caused by inhalation during the secondary, tertiary, 0.5 m depth SAT and 1.5 m depth SAT effluents' irrigations were 5.69 × 10⁻⁷, 4.31 × 10⁻⁷, 3.50 × 10⁻⁷, and 4.58 × 10⁻⁸, respectively, and are much lower than the threshold value (5.0 × 10⁻⁵/yr) recommended by ICRP. Overall, the carcinogenic risk of the chemical carcinogens during the SAT effluent irrigation was approximately an order of magnitude lower than the irrigation with secondary effluent. Moreover, element Cr contributed as much as 80% of the bulk carcinogenic risk during the SAT effluent irrigation, and was the predominant precursor of the chemical carcinogens within the SAT effluent.

Assuming that the annual exposure frequency of the irrigated water to a farmer was 40 times per year, with an exposure dosage of 1 mL every time, the corresponding infection probability of the secondary, tertiary, 0.5 m depth SAT and 1.5 m depth SAT effluents were calculated and the results are listed in Table 4.

As shown in Table 4, the annual risk of rotavirus to the farmer was 6.72 × 10⁻⁴ for the secondary effluent irrigation, which exceeded the maximum acceptable value of EPA (10⁻⁴/yr) for individual risks. For comparison, the calculated annual risk of the tertiary effluent irrigation declined to 5.28 × 10⁻⁶, and further to 1.83 × 10⁻⁷ for the 1.5 m SAT effluent. Similarly, the annual risk of the hepatitis A virus to farmers during the secondary effluent irrigation also exceeded the EPA benchmark, indicating that the typical virus in the secondary effluent should be of a high concern for its potential human health risk. As expected, the health risk of hepatitis A virus during the tertiary and SAT effluents' irrigation was much lower, and both can meet the requirement of the criteria of EPA. Generally, the annual risk of a typical virus during the SAT effluent irrigation was three to four orders of magnitude lower than that of the secondary effluent irrigation, clearly demonstrating that the control of health risks during the secondary effluent irrigation would benefit from the pre-treatment of SAT.

The following conclusions are drawn based on the presented experimental results:

1. SAT system exhibited a relative higher removal efficiency of most pollutants within the secondary effluent as compared to the tertiary treatment, especially for the reduction of NH₄⁺-N, TP, BOD₅, Cr, Cd, As, Zn, Ni, and Mn.

2. Irrigation with secondary, tertiary, and SAT effluents enhanced the growth of pak choi, with a root growth inhibition rate of −5.9% for secondary, −21.6% for tertiary, and −23.2% for SAT effluents, respectively. The noticeable toxicity of secondary effluent was observed for the growth of lettuce with a positive root growth inhibition rate of 21.3%, while the irrigation with the tertiary and SAT effluents enhanced the growth of lettuce. Moreover, irrigation with secondary, tertiary, and SAT effluents led to slight increases in the productivity both for lettuce and pak choi.

3. SAT pre-treatment was efficient for heavy metal pollution control during secondary effluent irrigation. Specifically, accumulation of Cr, Pb, Cd, Ni in cucumber during the SAT effluent irrigation significantly declined as compared to the irrigation with tap water, tertiary/secondary effluents, and Cr, Pb, Cu for cabbage, and Cr, Pb, Cd, and Cu for maize.

4. The carcinogenic risk of the chemical carcinogens during the 1.5 m SAT effluent irrigation was approximately an order of magnitude lower than that of secondary effluent irrigation, and the corresponding annual risk of Coxsackie, rotavirus and hepatitis A virus declined three to four orders of magnitude.

The authors gratefully acknowledge funding from Project 51408159 supported by National Nature Science Foundation of China, the support by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2013TS03), and the China Postdoctoral Science Foundation funded projects (2013T60375 and 2012M520744).