Investigation of landfill leachate treatability for reuse in agricultural purposes

One of the main solid waste management methods in Iran as a developing country is the landfill, into which about 84% of generated municipal solid waste (10 million tons per year) was disposed, with approximately, 0.5 m³/ton leachate production rate. The fundamental problem of solid waste landfilling is leachate production. Generally, land-fill leachates (LFL) are characterized by dark color, bad smell, high organic and nitrogen loads, heavy metals, humic substances and recalcitrant compounds which make leachate treatment challenging and complicated (Kamenev et al. 2002; Zouboulis et al. 2004).

The age of landfill and thus the degree of solid waste stabilization has a significant effect on LFL characteristics (Renou et al. 2008). Among the different LFL properties, the ratio BOD₅/chemical oxygen demand (COD) is explanatory of LFL age due to its direct relationship to the biodegradability of LFL. Table 1, shows a classification of LFL characteristics expressed by Ahmad & Lan (2012). Consistent with this, biological treatments such as UASB, stabilization ponds, activated sludge, trickling filters, biodiscs and sequencing batch reactor (SBR), have shown to be very useful for young leachates treatment (Kang & Hwang 2000; Ding et al. 2001; Spagni & Marsili 2009; Yan & Hu 2009). For old or partially stabilized leachates, the most efficient way to treat them is physicochemical processes including coagulation-flocculation-sedimentation, membrane processes (reverse osmosis, ultra, and Nanofiltration), ammonia stripping and advanced oxidation processes (Rivas et al. 2004; Ntampou et al. 2005). High concentrations of COD and N-NH₃ are the principal pollutants of LFL. In the past decade, many investigations were conducted to decrease COD and N-NH₃ from LFL, by applying combined chemical, biological and physical processes to achieve desirable results (Wu et al. 2009; Yan & Hu 2009).

In various studies (Modenes et al. 2012; Shrawan et al. 2013), the BOD₅/COD ratio has been increased by Fenton's process followed by biological treatment in order to decreasing COD loading. Air stripping has been used as a conventional method for reducing ammonia concentration from LFL (Kilic et al. 2007; Hasar et al. 2009; Guo et al. 2010), whereas the existence of N-NH₃, can be useful for agricultural purposes. Accordingly, there is no limitation in Iran's Agricultural Water Standard (IAWS) for ammonia. On the other hand, due to many economic barriers, implementation of advanced LFL treatment plant in developing countries is not executable. According to the UN Development Program (United Nations 1992), the level of Iran's per capita water resources has been predicted to fall to as little as 816 m³ in 2025, down from 2,030 m³ in 1990. It is clear Iran is facing an impending water crisis of staggering proportion (MOE 2010).

Therefore, the objective of current study is an investigation of LFL treatment feasibility in order to reuse for agricultural purposes as a new nutrient source. To achieve this target, all characteristics of Sari, Iran's landfill leachate were measured, and then combinations of biological (SBR), Chemical (Fenton's process) and physical (Ultrafiltration) processes have been evaluated for leachate treatment according to IAWS (MOE 2010).

Leachate was collected from Sari's municipal landfill located in the north of Iran and the characteristics were evaluated in triplicate and compared with the agricultural water standard of Iran (MOE 2010). According to Table 2, BOD₅, raw COD, filtered COD, turbidity, Zn, Pb, TSS, N-NH₃, and color have a higher value more than the standard limit, so in this study, the performance of treatment methods has been evaluated according to these parameters.

For adaptation of bacteria to leachate, activated sludge from municipal wastewater treatment plant was fed with synthetic wastewater containing KH₂PO₄, CO(NH₂)₂, C₆H₁₂O₆ + H₂O, and required trace elements for 7 days. After this period, 5% (v/v) of synthetic wastewater was diminished and replaced with leachate for 20 days to prepare activated sludge for using in SBR and membrane SBR (MSBR).

A reactor with a total volume of 3 L and a working volume of 2 L was used. Air was supplied at the bottom of the reactor to maintain dissolved oxygen concentration between 2 and 4 mg/l. The system was operated at approximately 8-hour cycles including 7-hour aeration, and 1 hour sedimentation. The volume exchange ratio was 50%. The reactor operated at room temperature (20–27°C) without pH control.

A membrane separation process was coupled to a SBR for using simultaneously biological oxidation and physical separation processes in a reactor. The separation module was made with propylene hollow fiber membranes (100-nm pore size, 6 m² surface area) vertically submerged in the 218 L (useful volume) bioreactor with SBR operation by hydraulic retention time of 24 hr. A blower with a diffuser was placed under the membrane to avoid fouling of the membrane through promoting shear over its surface and produced stable dissolved oxygen concentration of 2–5 mg/l in the bioreactor. Required suction was provided by a vacuum pump and the membrane module was also backwashed with effluent for 10 min every day to remove deposits on the membrane surface.

A bench-scale jar-testing apparatus equipped with 1,000-mL beakers was used to conduct optimization Fenton experiments. Before starting the experiments, leachate was brought to room temperature at approximately 23°C, and leachate was diluted to 50% (v/v) for decreasing COD strength. First, the mixing speed and pH was adjusted to 200 rpm and 3 respectively, then the desired amount of H₂O₂ was added to each sample jar-test beaker. Approximately 1 min later, the desired amount of FeSO₄-7H₂O was added in a single step. A 30 min oxidation period was provided, followed by coagulation by increasing the pH to 8.3 using 10 M NaOH for 5 min. Afterward, 15 min of flocculation time was given by reducing the mixing speed to 30 rpm. Finally, a 30 min sedimentation period was provided to settle iron sludge, the supernatant was collected and stored for analysis and using in downstream treatment.

In order to assess the performance of the Fenton process for leachate treatment in combination with other systems (SBR & MSBR), it was necessary to study the optimal operational conditions of the Fenton process to achieve maximum COD, color, and turbidity removal. It was conducted one variable at a time by modifying the value of one particular variable while keeping all other variables close to the favourable conditions (Zhang et al. 2005; Di Laconi et al. 2006).

According to optimum operational parameters, pH = 3, reaction time = 75 min, Fe²⁺ = 1,750 mg/l and H₂O₂ = 3,000 mg/l, the Fenton process was performed, and the effluent was stored for using in the SBR reactor. Characteristics of the Fenton process effluent are presented in Table 3. As it has been shown, except for COD and BOD, other parameters are the sub-standard limits. The BOD₅/COD ratio of the effluent leachate from the Fenton reactor was improved from 0.36 to 0.78, which could be sufficient to have an efficient biological treatment. Since this effluent was connected to SBR reactor for further treatment.

The SBR was operated in two different phases, the operation with diluted leachate (phase 1) and the operation with Fenton-treated effluent (phase 2). The cycle period for both phases was 24 hr and divided into five steps: filling (0.25 hr), aeration-reaction (22 hr), settling (0.75 hr), decant (0.25 hr) and idle (0.75 hr). Table 4 shows the characteristics of the effluent SBR reactor. Although these results indicated SBR reactor reduced the values of COD and BOD₅ of leachate, it still could not meet the reuse standard for agriculture. Therefore, better biological treatment is needed to combine with the Fenton process. Guo et al. (2010), has investigated leachate treatment using a combined stripping, Fenton, SBR, and coagulation process. COD and N-NH₃ of the SBR effluent which was operated after Fenton process was 700 mg/l and 25 mg/l with %57 and %16 removal efficiency, respectively (Guo et al. 2010).

As mentioned, the BOD₅/COD ratio of raw leachate was 0.36 (Table 2) which means the biodegradability of leachate is low and therefore, the maximum removal efficiency of MSBR is regarding the biodegradable portion and remaining pollutants are non-biodegradable. Accordingly, final BOD₅ and COD removal efficiency have been %92 and %76, respectively and therefore, BOD₅/COD ratio of MSBR effluent decreased to 0.11, which agreed with other studies (Neczaj et al. 2005; Laitinen et al. 2006) In the Fenton process, effluent COD (54 mg/l) reached the standard limit by the destruction and oxidation of non-biodegradable compounds and the BOD₅/COD ratio increased to 0.12 which is a little more than the MSBR system. The maximum removal efficiency of ammonia in MSBR was %64 which is due to nitrification reactions. Existence of denitrification process in biological flocs due to simultaneous nitrification and denitrification event is predictable (Pochana & Keller 1999) and membrane separation can affect nitrogen compounds rejection from the LFL stream (Trebouet et al. 2001). As expected, MSBR does not have suitable removal efficiency for turbidity (%45) and color (%32), these results are due to the ultrafiltration module. But the Fenton process increased dramatically the removal efficiency of turbidity and color to %98 and %97, respectively. These results have a perfect match with other investigations (Wang et al. 2012). Other parameters have received standard limits through the biological treatment and the membrane separation.

In this study, treatability of leachate by different processes was evaluated. The results indicated that the MSBR + Fenton process has a sufficient performance for treatment of leachate pollutants. The total removal efficiency of the main pollutants through this process, such as COD, BOD₅, color, and turbidity was 89%, 96%, 98% and 99%, respectively. Total ammonia removal was 88% which means there is a remaining part of ammonia in the effluent, which can be useful for agricultural lands. However, evaluation of high ammonia-leachates through the MSBR + Fenton process is recommended. The results of the present investigations suggest the possibility of leachate-treated for using in agricultural purposes that are a promising strategy to tackle important environmental challenges and water scarcity in Iran.