Wastewater and Sewage Sludge Effects on the Absorption of Some Heavy Metals in Soil and Mint

Introduction
Due to its especial geographic condition, Iran, as a semi-arid country, suffers from water shortage for domestic, industrial and agricultural usages. As limited water supplies, the use of non-conventional water resources such as industrial or municipal wastewater, is common practice in many parts of the world including Iran. It is estimated that at least 20 million hectares in 50 countries are irrigated with raw or partially treated wastewater. Also it is estimated that potential volume of wastewater in urban and rural areas in Iran will be 4369 and 823 million m3, respectively.
On the other, the application of sewage sludge in agricultural soils has been widespread in many countries around the world and also in Iran. The application of sewage sludge in agricultural has been shown to improve soils such as organic matter, nutrients, porosity, aggregate stability, bulk density and plant productivity. Despite the growing interest in wastewater and sewage sludge usage, excessive application of them may have some harmful effects such as human health problems, runoff and leaching of nutrients to surface and groundwater, undesirable chemical constituents, pathogens, accumulations of heavy metals in plants and soils, negative environmental and health impacts. So, wastewater and sewage sludge application should be under controlled conditions to minimize health risks of pollution to agricultural products, soil, ground and surface water.
The use of medicinal plants and herbal medicines is increasing rapidly all over the world, which shows the importance of cultivation and production of these plants. One of the important needs of medicinal plants, in order to achieve high performance and quality, is plant nutritional needs. Therefore increasing in soil fertility, could be increased yield of medicinal plants.
Medicinal plants today are cultivated commercially in polluted environments where soil, water and air contain rather high levels of pollutants. These plants appear to be a good choice for phytoremediation since these species are mainly grown for secondary products (essential oil) thus the contamination of the food chain with heavy metals is eliminated. Aromatic and medicinal plants also have a demonstrated ability to accumulate heavy metals.
The mint (Mentha spicata) is a medicinal plants and it has received considerable economic importance due to the large demand for its essential oil in food, pharmaceutical, cosmetic and hygiene industries.
The objective of this study was to determine the use of wastewater, sewage sludge on growth and nutrient concentrations of mint (Mentha spicata).
Matherials and Methods
This study was conducted at the greenhouse of Bu-Ali Sina University located in Hamedan, western Iran. The experiment was run in May 2015- February 2016), including 5 months for soil preparation (in lysimeters) and 5 months for crop cultivation and harvesting. To evaluate the use of wastewater and its sewage sludge on growth and nutrient concentration of mint (Mentha spicata), a factorial experiment based on completely randomized was designed with three replications in 27 lysimeters. The factors included three types of watering: tap water (W1), raw wastewater (W2) and treated wastewater (W3), three sewage sludge levels: 0 (S1), 50 (S2) and 100 tha-1 (S3) and three time of harvesting (T1, T2 and T3). Therefore, 9 treatments (W1S1 to W3S3) were considered for investigation. It is noted that the time of harvesting factor was not considered to be an independent factor.
27 volumetric lysimeters were applied as Cultivation beds (26 × 30 × 30 cm). The soil had two layers: the upper layer (0 to 50 cm) with sandy loam texture, and the bottom layer (50 to 110 cm) with sandy clay loam texture. After beds preparation, mint (Mentha spicata) was planted in them. The raw and treated wastewaters as well as sewage sludge were prepared from wastewater treatment plant of Hamedan. Also, municipal compost was prepared from Kermanshah Compost Company. Overall, 13 irrigation programs with 10-day intervals were applied. The plants shoot were harvested at the full flowering stage in 3 times. At the end of each harvesting stage, the fresh and dry weights, as well as the amount of N, P, K, Ni, Cd and Pb of mint were measured. Also after the last harvesting, soil samples were taken at two depth of lysimeter: D1: 10, D2: 40 and D3: 85 cm. Soil samples were analyzed for Ni, Pb, Cd, total N, total p and K.


Discussion of Results
The ANOVA test for effects of water quality and compost levels on fresh and dry weight as well as nutrient in mint showed that the amount of N, K, P and Cd were influenced by interaction between water quality and sewage sludge levels. But amount of Ni and Pb in shoot of mint was not influenced by interaction between water quality and sewage sludge levels.
The results showed that using of wastewater and sewage sludge heavy metals in soil, so, compared to control, W2S3 were increases Pb, Ni and Cd in soil 304, 375 and 208 percent respectively.
The heavy metals accumulated in soil surface layers (at depths of 10) and heavy metal concentrations and their ranges decreased with deepening of soil layer. had low vertical movement. Preferential flow and metal complexation with soluble organic apparently allow leaching of heavy metals.
Also, the result showed that application of wastewater and sewage sludge increased fresh and dry weights of mint. As, in compartion control, W3S3 incresed fresh weight and dry weight of plant to 257 and 239 %, respectively. Also, performance of plant were increased during next harvesting. Many of researchers reported that providing balance nutrients and gradual nutrient release from organic sources during period of growth can be positive role in enhancing the growth of mint. The reasons for increasing the yield of plants include: Having the organic matter, increasing soil water holding capacity, strengthening the plant hormone-like activities, increasing nutrient uptake by plants and to generally improve the chemical and physical soil structure, noted.
The amount of nitrogen in plant was varied among the treatments. The maximum (4.65 %) and minimum (1.11 % dry weight) amounts of nitrogen were observed in W2S3 and W1S1, respectively. Total N concentration in W2S3 was 3.12 times greater than that in W1C1. Also the highest amount of P (0.67 % dry weight) and K (3.85 mg in dry weight) was observed in W2S3.
Also, the results indicate that using the wastewater (raw wastewater andtreated wastewater) and sewage sludge (50 and 100 tha-1), compared to the control (fresh water and soil without any sewaage), increas heavy metals (Ni, Pb and Cd) of mint. As, maximum and minimum of heavy metals concentration in the shoot of mint were obtained in W2 and W3 as well as in C3 and C1, respectively.The concentrations of heavy metals (mg kg-1 dry wt.) in mint ranged from 0.01 to 0.57 for Pb, 0.02 to 0.71 for Ni and 0.01 to 0.3 for Cd. The results of this study showed that the amount of heavy metals (Ni, Pb and CD), were lower than the international Cd standard level of FAO. The reasons for the increase in the amount of nitrogen, phosphorus and potassium in plant can be noted rich wastewater and sludge in elements.
Conclusions

In thudy, the effects of wastewater sewage sludge on nutrient concentrations and growth of mint (Mentha spicata) were studied.The results showed that the use of wastewater (raw and treated) and sewage sludge different levels of sludge, increased performance and uptake N, P, K, zinc, Pb, Ni and Cd in plant significantly. Although the concentration levels of Pb, Ni and Cd is lower than the limit of FAO standard, but long-term use of these wastes should be done cautiously.