In the present research, preliminary biological process was carried out using aerobic and anaerobic processes, with or without the provision of nutrients. The biological effects did not differ significantly among treatments. The physical-chemical treatment is typically applied as the first stage in research, but in the present work, the biological treatment successfully operated as the preliminary stage. After that, practically all turbidity was decreased using a coagulation-flocculation process without any pH adjustments, although colour was not totally removed. The results demonstrated that the 3000-ppm coagulant treatment almost entirely removed turbidity. As a result, this dosage was chosen as the one that would function best in the AOP to reduce colour. To low operational costs, the current research avoided employing chemicals to buffer the reaction media or to make any extra pH adjustments.
Leachates' level of pollution is often determined by the amount of rainfall and the types of waste present. The main characteristics of the leachate from Chimbo's landfill that was used both before and after the studies are summarized in Table 3. The humic compounds are present in large concentrations as seen by the leachate samples' alkaline pH and dark brown colour.
3.1. Effect of biological treatment
The average reactor temperature among all treatments was 28°C, which was higher than the 25°C laboratory temperature. Therefore, the removal of TS and VSS by biological means was positively influenced by time, nutrients, and aeration conditions. The mean reduction in TS and VSS across all treatments was 33% and 92%, respectively (Fig. 2).
Anaerobic treatment with N-P-K nutrients addition (T1) was more effective than just anaerobic condition for removing TS (T3). At the end of 42 days, the TS removal in T1 was 39% and in T3 it was 33%. In terms of suspended solids, 94% of the VSS was removed in T1, and 90% in T3. The most common biological treatment is anaerobic because it produces less sludge, is excellent for treating young leachates, and generates methane, which is both beneficial for the environment and the economy [13, 44]. The necessity of a well-balanced macro- and microelements environment for cell metabolism is one of the drawbacks of anaerobic treatment [45]. Co-digestion with different substrates, especially for intermediate or mature leachate, promotes an effective anaerobic digestion [33, 46].
The progression of how solids were reduced during a 42-day period is shown by the ratio between VSS and TS (Fig. 2). Environmental Protection Agency (EPA) for biosolids states that this ratio is necessary for the biostabilization of residual sludge. According to this source, the residual sludge is in the process of biostabilization or maturity if the change in the organic fraction indicated by VSS relative to total solids is less than 0.6; this is a quality indicator that allows controlling the quality of a biostable product. Despite the fact that our method employed a liquid phase, we would nevertheless use this limit value to assess the biodegradation criterion [46].
For all treatments, the initial VSS/TS ratio was 0.76 during the first week (Fig. 2). Later, this ratio was decreased in accordance with several biological treatments. With a VSS/TS ratio of 0.06 after 42 days for anaerobic treatment using N-P-K nutrients (T1) and 0.12 for anaerobic treatment using no nutrients (T3), 94% of volatile suspended solids were stabilized. Additionally, Fig. 2 illustrates the reduction of turbidity for all biological treatments as an indication of microbial activity for the degradation of organic compounds. In treatments T1 and T3, the turbidity was reduced by 72% and 60%, respectively.
At the aerobic treatment, the percentage of TS removed in T2 without nutrients was 32%, and in T4 with N-P-K nutrition, it was 29%. In T2, the VSS elimination was 89%, while in T4, it was 94%. The VSS/TS ratio in the aerobic treatment T2 was 0.12 while it was just 0.07 in T4. In T2 and T4, the turbidity was reduced by 79% and 60%, respectively. By oxidizing ammonia to nitrite and then converting nitrite to nitrate, nitrification takes place in the aerobic stage [47, 48]. Following aeration, heterotrophic bacteria convert nitrate to nitrogen during an anoxic phase [14, 17].
At the completion of the 42 days, the VSS/TS ratio varied between 0.06 and 0.12 for all treatments. Unpleasant odours disappeared, and the original dark brown colour turned light orange. No abnormal change was noticed because the pH level was maintained throughout all treatments. The fact that over 92% of volatile suspended particles were stabilized on average shows that the reactors were successfully removing organic components. Therefore, it is proven that microbial metabolism caused by aerobic and anaerobic treatments both successfully decreased the concentration of solids in young leachate.
Treatment (Factor A), and biodegradation time (Factor B) effects were analysed using analysis of variance (ANOVA) in Table 2 according to suspended solids, total solids, and turbidity. According to Table 2, there was no significant difference (p > 0.01) among all treatments; however, a significant response (p < 0.01) was obtained by biodegradation time.
Table 2
Summary of the analysis of variance (ANOVA) for treatment (Factor A), biodegradation time (Factor B) according to suspended solids, total solids, and turbidity.
Source of variation | Df | Suspended solids | Total solids | Turbidity |
Treatment | 3 | 0.978 (ns) | 0.511 (ns) | 0.558 (ns) |
Time | 1 | 0.000 (***) | 0.000 (***) | 0.000 (***) |
Treatment:Time | 3 | 0.931 (ns) | 0.897 (ns) | 0.891 (ns) |
Df = Degrees of freedom. |
Significance codes: 0 (***) 0.001 (**) 0.01 (*) 0.05 (.) 0.1 (-) no significance (ns)
3.2. Effect of coagulation
Figure 3 illustrates the effectiveness of various coagulant dosages while maintaining the flocculant dosage constant at 30 ppm. At 250 ppm, ferric chloride reduced 50% of the turbidity while PAC reduced approximately 38%. FeCl3 was still 66% more effective at 500 ppm than PAC, which was just 52% effective. To bring maximum particle removal, the coagulant dosage was doubled at this step. At a dosage of 750 ppm, both coagulants essentially equalled their efficiency (69%). Both PAC and FeCl3 were efficient in removing turbidity between 84 and 97% up to 3000 ppm. With increasing doses, contaminant removal grew and eventually reached its maximum asymptote at 3000 ppm. At 3000 ppm FeCl3, the efficiency for turbidity removal was at its maximum (97%). For FeCl3 dosages beyond this level caused a charge reversal and stopped colloid destabilization from proceeding. There was no requirement for pH modification because the initial pH was 7.0.
Turbidity may be effectively eliminated using both PAC and FeCl3. Ferric salts, as opposed to aluminium, were shown to be more effective. The particle surface charge, which is considered to have been initially negative, was reversed, however, in response to a high dose of the coagulant. According to experimental results, coagulation had the ability to neutralize the electrostatic charge that was present in leachate colloids, which lessened the attraction between negatively charged particles by generating protons.
According to reaction (I), an aluminium salt dissolves in water and losses three electrons from its final orbitals. Due to the aluminium atom's remaining 6 electrons in its electrical state, the hydration of the ion takes the form Al(H2O)6+3. In reaction (II), the bond between the positively charged aluminium cation and an oxygen atom of one of the six water molecules in the Al(H2O)6+3 results in a complex ion that behaves like a proton donor due to the increased polarity of the O-H bonds of the water molecule. As a result, hydrogen atoms are more likely to ionize. The solution becomes acidic because of the hydrolysis of the metal cation and proton generation. The same complex ionization occurs for ferric chloride as shown in (III) and (IV).
Al+ 3(s) + 6 H2O (l) → Al(H2O)6+3(aq) + 3e- (I)
Al(H2O)6+3(aq) + H2O (l) ⇋ Al(OH)(H2O)5+2(aq) + H3O+(aq) (II)
Fe+ 3(s) + 5 H2O (l) → Fe(H2O)5+3(aq) + 3e- (III)
Fe(H2O)5+3(aq) + H2O (l) ⇋ Fe(OH)(H2O)4+2(aq) + H3O+(aq) (IV)
Aluminium polychloride and ferric chloride removed colour, assisting in the removal both dissolved and fine suspended particles while maintaining the original pH. Ferric chloride is a viable coagulant for colour reduction, according to the literature, but only at pH 4, 6, and 12 [30]. In contrast to prior studies, which achieved an 84% removal of turbidity at 1900 ppm, in our experiments, PAC removed 89% of turbidity at 2000 ppm [16]. In our research, the addition of a 30-ppm anionic polyacrylamide flocculant enhanced turbidity reduction. Several studies have been conducted on leachate treatment, but little is known about the mechanisms that optimize and coagulate landfill leachate utilizing aluminium or even iron as coagulants [16].
The dosage of the coagulant depends on the hydrogen bonding system and colloidal electrostatic charge because the composition of leachate differs from city to city, as described in the literature; hence, applying the conclusions of other authors would not be appropriate. Along with mixing time and speed, the key factors to optimize are pH and various dosages of chemicals. Coagulation-flocculation was used in the current work to remove suspended solids in a substantial proportion without adjusting pH; this was appropriate to lower operational expenses and to speed up the AOP stage in the breakdown of organic matter and ammoniacal nitrogen.
3.3. Effect of an advanced oxidation process
Two supernatants from the coagulation-flocculation phases with PAC and FeCl3 were used in this advanced oxidation process. 4000, 8000, and 12000 ppm of a 50% hydrogen peroxide oxidant were used as doses. Commercial activated carbon was added at dosages of 4, 8, and 20 g/L as a catalyst (without any chemical modification). The duration of each treatment was limited to a maximum of 4 hours of sun exposure. Supernatant's original turbidity was 0 NTU, and its initial pH was 6.5. Effects of pH or UV light were not tested.
The AOP was ineffective for the supernatant produced by the PAC treatment. It is hypothesized that an environment containing Al+ 3 residues from the coagulation stage did not increase AOP at all; on the other hand, an increase in turbidity was observed in all treatments. Turbidity increased from 0 to a maximum of 389 NTU, as shown in Fig. 4. Since there was no colour removal in this instance, the objective was not accomplished when using PAC.
The 4 g/L dosage of activated carbon in all H2O2 concentrations had no effect on the turbidity and 89% of the colour was removed from the supernatant that resulted from the FeCl3 treatment. The removal of colour with 8 g/L activated carbon at 12000 ppm H2O2 was successful without causing any turbidity to rise. However, turbidity increased, and colour was not removed when activated carbon was used at 8 g/L, and H2O2 at 4000 ppm, and 8000 ppm. Besides, all H2O2 dosages resulted in an increase in turbidity at 20 g/L activated carbon.
Efficiency dropped as more H2O2, and AC were supplied to the system. As previously stated, a high concentration of H2O2 does not necessarily indicate an improvement in the removal of colour and turbidity [21]. The best treatment in the current study was activated carbon at 4 g/L and 4000 ppm H2O2.
The degree of treatment needed to remove dissolved solids, including coloured particles, was not provided by coagulation-flocculation operations. The results show that AOP significantly accelerated the degradation of emerging pollutants. When H2O2 and catalysts like Fe+ 3 and Fe+ 2 ions are combined, the peroxide decomposes to produce hydroxyl radicals (•OH) [17]. These radicals are highly reactive species that can aggressively degrade organic matter [49–52]. The use of AC as a catalyst in the current work was made possible by the material's high porosity, which would encourage the production of •OH. A reaction mechanism using activated carbon as a catalyst is not yet entirely researched. However, it is herein suggested in reactions (V) to (VIII) that H2O2 decomposes catalytically by the presence of AC [31].
AC + H2O2 → AC+ + OH- + •OH (V)
AC+ + H2O2 → AC + H+ + •O2H (VI)
Additionally, the residual Fe+ 3 ions from the coagulation step contribute to the Fenton-like reaction that produces •OH.
Fe+ 3 + H2O2 + hv → Fe+ 2 + OH+ + •O2H (VII)
Fe+ 2 + H2O2 + hv → Fe+ 3 + OH- + •OH (VIII)
3.4. Physico-chemical properties of the treated leachate
A high organic load and high biodegradability of the leachate are indicated by the ratio of BOD to COD, which has a mean value of 78% in Table 3. While a low BOD/COD ratio (between 20 and 40%) denotes the existence of low-biodegradable recalcitrant compounds, a high BOD/COD ratio (between 40 and 60%) defines a good biodegradability of wastewater [15, 53]. Heterotrophic microorganisms break down a significant percentage of biodegradable organic matter from young landfills into fatty acids. While BOD was reduced by 67% (2122 to 707 mg/L), COD was lowered by 60% (from 2720 to 1088 mg/L) during the anaerobic treatment employing nutrients (T1) (Table 3). According to some authors, anaerobic treatment may decrease COD by up to 74% and colour by up to 98% [15]. As a result, the anaerobic condition with nutrient addition (T1) was determined to be the optimal treatment in this work in terms of low energy consumption and operational expenses. A biological treatment alone is not a practical method to eliminate virus particles of SARS-CoV-2 in leachate. However, it is still uncertain if viruses may survive in the environment, including sewage and landfill leachate [19, 20]. Coagulation-flocculation is necessary to remove all turbidity, and accelerated oxidation to destroy pathogens and minimize virus particles is also important.
The coagulation treatment at 3000 ppm FeCl3 decreased COD by 80% from 1088 mg/L to 217 mg/L and BOD by 78% from 707 to 152 mg/L. Additionally, this treatment removed 53% of the colour. According to other investigations, a suspended particles reduction up to 95% was observed at 1509 ppm FeCl3 at a pH of 7.02 [54]. The AOP treatment reduced 95% COD, 96% BOD, and 89% colour using activated carbon at 4 g/L and 4000 ppm H2O2. Another study found that COD was removed by 82% applying 15 g/L of chemically modified granular AC and 4000 ppm H2O2 at pH 8.0 [27, 31].
Table 3
Landfill leachate analysis before and after biological, and physical-chemical treatments
Parameter | WW | Bio | C-F | AOP |
pH | 8.0 | 7.0 | 6.5 | 6.5 |
COD (mg/L) | 2720 | 1088 | 217 | 10 |
BOD5 (mg/L) | 2122 | 707 | 152 | 6 |
Turbidity (NTU) | > 4000 | 1110 | 0 | 0 |
Colour Pt-Co | > 600 | 350 | 283 | 30 |
ORP | 127 | 180 | 130 | 238 |
Conductivity (µS/cm) | 600 | 210 | 253.1 | 1.2 |
WW = Landfill leachate wastewater |
Bio = Anaerobic treatment using N-P-K nutrients
C-F = Coagulation-flocculation treatment using 3000 ppm FeCl3
AOP = Advanced oxidation process using 4 g/L AC and 4000 ppm H2O2