Background

One of the most recruited methods to treat waste materials and their disposal is sanitary landfills. This method is used in many countries around the world. Researchers have shown that between 40 to 80% of municipal solid waste (henceforth MSW) is disposed of in developed countries whereas this rate reaches 60 to 90% in developing countries [1].

Leachate micro-organisms can be either broken down or absorbed via the process of biological uptake [2]. This process significantly depends on the establishment of microbial populations as a response to the contaminants’ loading. It is indispensable to investigate the actual pollution plumes to gain efficient insights into the complicated framework to foresee the fate of the contaminants [3].

Due to Tehran’s waste analysis about 68.8% of the solid waste is biodegradable materials which lead to produce huge amounts of leachate with the high organic loads [4, 5]. This issue was studied in previous studies which show leachate also contains high volume of inorganic and organic compounds in two phases of suspended or dissolved [6, 7]. Hence, leachate is known as hazardous liquid can easily infect the surface water and groundwater [8]. Moreover, as an efficient disposal alternative, leachates are sometimes used for irrigation. This method is appropriate for the polishing of the pre-treated leachate [9].

Appropriate treatment of leachate consequently is regarded as a daunting challenge [10]. The occasional existence of highly concentrated heavy metals makes the biological treatment very difficult due to its toxic effects on microbes [11].

It is possible that leachates contain colossal amounts of organic matter in the forms of biodegradable and biorefractory carbon, nitrogen (bio-nitrogen, ammonia, and nitrate nitrogen), heavy metals and etc. [12].

Two systems namely soil and water are known as the natural treatment systems. When the leachate is on the land surface, under controlled conditions, to achieve a certain level of filtration and treatment through physical, chemical and bio processes in the matrix of water-soil-plant, the soil systems are suggested. Moreover, less energy consumption is needed in the land treatment methods rather than the filtration methods which are mostly due to moving and spreading the leachate [13].

In addition, to achieve the best efficiency among the various irrigation methods, regards to the characteristics of the land treatment with leachate, slow irrigation system is presented. Besides, it leads to high removal of contaminants, BOD5 and depositions.

In the land treatment method the pre-treatment process such as screening and primary sedimentation leads to remove suspended solid which prevents damage to irrigation facilities and machinery [14].

Mainly, leachate contains heavy metals. Consequently, a crucial characteristics of plants in phytoremediation needs to be hyper-accumulation. Such type of vegetation is selected due to the physical potential to tolerate and assimilate toxic substances, growing rate, depth of its roots, and capability to degrade or bio accumulate the contaminants in its roots, branches and leaves [15, 16].

Using plants in two environments of soil (purification, refinement, preservation, and sustaining soil, sediments) and contaminated waters in previous studies is well accepted [17]. In phyto-remediation methods, one of the significant issues is choosing the kind of the plants. However, plants can lead to water and soil contamination removal and also degrade the bio contaminants. Moreover, they can filter, trap, absorb and stabilize the heavy metals.

Vetiver (Chrysopogonzizanioides) is known as a successfully permanent grass to decline the contaminations due to the mentioned details. It originates from Indian peninsula and for the first time it was used by the World Bank in India [17]. Using Vetiver as a treatment for sewage and leachate was a start point of a great novelty and show the extraordinary capability of plants. In additions, it leads to a green, natural, simple and practical treatment method with low expenses.

The purpose of this study is to evaluate the efficiency of leachate land treatment with the help of Vetiver plant to reduce the COD, BOD5, TSS, TDS and the existing TOC in the leachate in municipal landfills leachate after the pre-treatment stage. Moreover, the above mentioned factors against the current standards are compared and discussed.

Methods

This research presents the results of a laboratory study in a pilot scale which soil profile as a biological filter to attenuation of organic and inorganic contaminants from landfill’s leachate was examined after pre-treatment stage. However, land treatment of leachate in Aradkouh’s landfills has been chosen as the study area.

Landfill’s characteristics

Aradkouh’s landfill is on the 25th km of Tehran-Qom road and is used as a disposal facility for more than 40 years. More than 8000 ton solid waste is transported to Aradkouh landfill every day which leads to generate leachate about 637500 liters per day. The underground waters level is very low and the wells have a low rate of discharge between 0.5 to 3 liters per second of salty water.

Raw leachate’s characteristics

The location of sampling effects on the type of leachate. So, fresh leachate is sampled which is needs to sample in the waste discharge place. Usually, these places are dangerous and very hard to take the samples. Otherwise, leachate passes a long way every day and absorb huge amount of contaminants and finally get mixed with the old leachate which leads to lose the initial characteristics. Moreover, the characteristics of generated leachate in Kahrizak’s landfill are given in Table 1.

Table 1 Raw leachate’s characteristics in Aradkouh’s landfill
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The pilot’s characteristics

In general, three pilots were used and their charecteristics are shown in Table 2. Three various scenarios are conducted to treat the leachate in three pilots. In the first pilot (pilot A), an artificial arrangement soil was used where (from down to top) the first layer is the region soil and the second is sand and rock-stone. On the other hand, the second pilot (pilot B) the natural form of the local soil is used whereas in the third one (pilot C) the local’s natural soil planted by Vetiver is used (Figure 1).

Table 2 The pilot’s characteristics
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Figure 1
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Stages leachate movements from storage to pilots.

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Soil analysis

Physical characteristics of soil such as bulk density, moisture, etc. which are determined according to the standard method of ASTM are presented in Table 3.

Table 3 The results of soil studied in Aradkouh’s landfill
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As can be indicated, the analysis of the sample soil including the soil texture and composition are given in Table 4.

Table 4 Composition of soil studied
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Initial preparation and irrigation

Irrigation technique for forming the structure of soil was conducted for six weeks. The soil underwent wet and dry periods. After that, the output leachate from anaerobic lagoons with a retention time of one month is replaced for irrigation. To achieve the slow rate irrigation leachate sprayed in three rates (0.2, 0.6 and 1 ml/min). However, the process is done at a certain filtration rate and at specific time intervals according to the of slow rate irrigation’s form.

Laboratory analysis

For running the laboratory tests, the sampling was conducted twice a week. Moreover, to prevent anaerobic conditions the pilots are irrigated with three days interval between irrigations. The samples from the pilots were gathered three times at each rate of 0.2, 0.6 and 1 ml/min, respectively. After that, the output sewage liquids were tested to estimate the COD, BOD5, TDS, TSS, TOC and EC. To determine the level of reduction done by soil and plant the results were compared to the pre-treatment stage. It should be mentioned that, the experiments were conducted according to the APHA standard [18]. In addition, to avoid any change in pH values while pH-meter is used due to the CO2 evolution, pH was determined immediately after the sampling (pH Tutor, Mfg. by: Eutech Instruments). All the experiments were carried out three times and the mean values are used in this report.

Statistical analysis

To analysis the results the Duncan test was used. Duncan test can be applied when the number of samples is more than three. In this test, the significant level of (α = 0.05) is assumed.

Results and discussion

pH, conductivity and TDS

In this section, the results of the experiments which were conducted in various rates and heights or the three considered pilots are presented and discussed in Tables 5, 6 and 7. In each 30 cm from the top to down of each pilots an output sample was tested which we name them O1, O2 and O3. It should be mentioned that the input of the pilots was the output of pre-treatment section.

Table 5 The results of the experiment at the rate of 0.2 ml/min
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Table 6 The results of the experiment at the rate of 0.6 ml/min
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Table 7 The results of the experiment at the rate of 1 ml/min
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Due to the Tables 5, 6 and 7 the rate of EC decline in various rates for all the pilots was determined. The best efficiency was obtained at the medium rate (0.2 ml/min) at pilot C for the sample O3. Moreover, the amount of TDS was decreased along the soil profile. However, the maximum reduction rate was obtained at the medium rate of 0.2 ml/min at pilot C for the sample O3. This can be defined by passing the leachate throw the various layer of the soil, biomasses are being formed which lead to the adsorption of some ions by the soil and decrease the TDS.

With regards to the achieved P-value, the amount of EC and TDS are 0.015 and 0.044, respectively. According to the assumed α = 0.05, the p-value is less than α which implies that with varying the flow rate and the pilots cause negligible changes to amounts of EC and TDS.

COD & BOD5

Tables 8, 9, 10 present the COD and BOD5 values which were determined due to various rates in the three studied pilots.

Table 8 The results of the experiment at the rate of 0.2 ml/min
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Table 9 The results of the experiment at the rate of 0.6 ml/min
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Table 10 The results of the experiment at the rate of 1 ml/min
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The output COD and BOD5 values due to various rates in the three studied pilots are shown in Figures 2 and 3 which were compared with standards discharging to surface waters, agricultural purposes and irrigation.

Figure 2
figure 2

COD values due to various rates.

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Figure 3
figure 3

BOD5 values due to various rates.

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With respect to the Tables 8, 9 and 10, the variation of COD and BOD5 values due to different rates for each samples are illustrated. Moreover, the special trends are shown and discussed in following. The best efficiency was reached at the medium rate (0.2 ml/min) at the pilot C for the sample O3. Moreover, the output results at the speed of 0.2 ml/min at the pilot C for the sample O3 meets the both standards discharging to surface waters and agricultural purposes and irrigation.

Along the soil profile the amounts of COD and BOD5 were declined. Some parts of particulate COD were decreased by the mechanism of sedimentation and colloidal material adsorbed and then deposited. Some parts of organic materials were reduced by anaerobic decomposition mechanism. BOD5/COD ratio was decreased, because microorganisms that were present in the biofilm layer consumed organic matter, also biodegradable organic matter was decomposed thus this ratio is reduced.

In addition, the most elimination rate is founded in the pilot A at the first 30 cm (O1), all the three flow rates. However, as much as the soil aggregation gets finer the retention time and the elimination efficiency increased more.

As can be seen, if the organic loading gets more intense (increase the input flow rate), the percentage of removal efficiency would decreases due to the declination of the hydraulic retention time which leads to descend the rate of reduction.

In pilot C in addition to the natural soil zone, the plant growth with the spray roots is effective in the elimination of leachate pollutants. The plant growth regards to the Rhizosphere which is the gathering place of microorganisms and the level of the biological activities are high, with the coexistence between the microorganisms and the absorption of plant roots, lead to more elimination of the leachate pollutants.

The p-values for the COD and the BOD5 are 0.28 and 0.411, respectively. However, it can be concluded that due to varying the flow rates and the pilots the amounts of COD and BOD5 are fluctuated.

TSS & TOC

Tables 11, 12, 13 show the results of TSS and TOC which were measured for the outputs of the three pilots due to the various speed ratios.

Table 11 The results of the experiment at the rate of 0.2 ml/min
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Table 12 The results of the experiment at the rate of 0.6 ml/min
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Table 13 The results of the experiment at the rate of 1 ml/min
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The measured values of TSS for the samples due to different speed ratios are illustrated in Due to the Tables 11, 12, 13, the best efficiency was achieved at the medium rate of 0.2 ml/min in the sample of O3 at pilot C. Moreover, TOC along the profile of soil was decreased and the maximum reduction caused at the medium speed ratio of 0.2 ml/min at sample O3 at pilot C. To sum up, the best efficiency was obtained at the pilot C, sample O3 whilst the average amounts of TSS and TOC removal were %98.2 and %94.9.

The solid suspended materials are separated before the other materials caused by the flowing throw the soil. A proportion of TSS is eliminated by the screening and sedimentation mechanism and another proportion of fine, soluble and colloidal materials are adsorbed. However, due to the low flow rate the sedimentation is increased. It should be mentioned that, in the systems with rapid penetration most of the solid materials are separated in the surface of the soil. Therefore, it would be possible that the wastewater materials blocking the surface. Hence, drainage systems are suggested.

As can be seen in the above tables, growth in the load of organic materials leads to decrease the elimination efficiency which is caused by increasing the retention time.

As can be seen, in Table 14 a brief literature review of attenuation of landfill leachate with compared to the results of this study is given.

Table 14 Literature data of land treatment techniques for reduction of organic material
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Conclusions

In this research the capability of land filtration and treatment in natural reduction of organic and inorganic contaminants existing in the leachate after the pre-treatment stage was studied. Based on comparison of the results a few points can be concluded which are presented as follows.

  1. 1.

    Land treatment of leachate has a significant positive effect on COD removal and pH stabilization. Due to the achieved results of pilot C (Sample O3) at the rate of 0.2 ml/min the maximum reduction efficiency including COD, BOD5, TSS & TOC at 99.1%, 99.7%, 98.2% & 94.9% were obtained, respectively.

  2. 2.

    The higher the organic rate load is, the lower the efficiency of the removal would be. In lower hydraulic loads, the reduction rate of organic and inorganic contaminant due to the longer hydraulic retention time is more.

  3. 3.

    Using Vetiver plant in land treatment and filtration of leachate increases the efficiency. It also plays a key role in the contaminant reduction at an appropriate rate (to remove organic and inorganic contaminant, pre-treatment is essential which leads to increase the expenses).

  4. 4.

    This filtration technique compared to the advanced filtration methods, only needs to transmit the leachate from output of the initial sedimentation system, spreading and spraying it on the surface requires energy. Moreover, it needs less mechanical facilities. Also, it requires lower and easier levels of maintenance.