Leachate treatment using sequential microwave and algal photo-bioreactor: Effect of pretreatment on reactor performance and biomass productivity.

The present study aims to design a lab-scale hybrid reactor, primarily focused on the removal of organics, nutrients, heavy metal and other toxic compounds, thereby, minimizing risk associated with the disposal of landfill leachate. The potential of a designed hybrid treatment system (i.e., sequential microwave (MW) with algal bioreactor) with and without pretreatment, i.e., coagulation-flocculation (CF), was evaluated based on several parameters. The CF pretreatment under optimized conditions has resulted in 90% turbidity and 76% COD removals from leachate; furthermore, the MW treatment achieved 91% ammonia removal from raw leachate. As a result, substantial algal growth was observed in the preliminary algal batch experiment conducted with MW and MW-CF treated samples. Subsequently, leachate treatment was carried out using sequencing batch reactor (SBR) systems, i.e., MW-algal SBR and CF-MW-algal SBR. Algal biomass growth and increment in DO level were observed in algal-SBR experiments. Under the optimized reactor conditions, TN and TP removal rates in the algal-SBR were found to be 1.67-20 mg/L/d and 0.6-9.6 mg/L/d, respectively. The majority of heavy metals present in the leachate were removed due to algal-uptake (mainly Zn2+) and bio-sorption (total-Fe, Cu2+ and Pb2+). Meanwhile, some amount of energy can be recovered from algal biomass as inferred from the cost benefit analysis. Overall, the hybrid treatment combining MW and algal-SBR has shown immense potential for sustainable leachate treatment.

Sequential coagulation/flocculation and microwave-persulfate processes for landfill leachate treatment: Assessment of bio-toxicity, effect of pretreatment and cost-analysis.

The possibility of landfill leachate treatment in a coupled microwave-persulfate (MW-PS) system with and without pretreatment, i.e. coagulation-flocculation (C-F) was investigated. The C-F pretreatment with alum and FeCl3 has reduced the turbidity from 90 NTU to 43 NTU and 10 NTU, respectively, at the optimized coagulant dosage. Moreover, 73% COD and 86% color removal was observed in C-F pretreatment with FeCl3. The application of MW-PS system (at 10 g/L of PS dosage) for pretreated leachate (FeCl3 dosage 1 g/L and pH 5.5) has produced a final COD removal of 89%. Similarly, alum pretreatment (dosage 1.6 g/L, pH 8.2) coupled with MW-PS system has achieved a total COD removal of 62%. In MW-PS system, the ratio of initial PS dosage to initial COD ratio has shown significant effect on leachate treatment. However, slightly lesser ammonia removal was observed in MW-PS (93%) compared to MW alone (97%) owing to reduction in pH of the system. The comparison of bio-toxicity (i.e. inhibition to aliivibrio fischeri) of treated samples from MW-PS and MW alone after pretreatment, i.e. 12.1 mg/L and 6.8 mg/L of equivalent ZnSO4 toxicity, indicated that MW-PS treated sample were found to be more toxic than MW alone treatment and raw leachate (7.6 mg/L and 7.2 mg/L of equivalent ZnSO4 toxicity, respectively) due to sulfate ion. This indicates that C-F followed by MW alone would be an ideal option for leachate treatment. The cost and energy estimation of MW and MW coupled systems well supported the above findings.

Mature landfill leachate treatment using sonolytic-persulfate/hydrogen peroxide oxidation: Optimization of process parameters.

The suitability of stand-alone ultrasound (US) system, coagulation pre-treatment followed by US, hydrogen peroxide added US system (US-H2O2) and persulfate added US system (US-PS) for the treatment of matured landfill leachate was investigated. With US system, around 67% COD removal and an increase in BOD/COD ratio were observed (from 0.033 to 0.142) after 15 min at 30% US amplitude. However, the energy input required for landfill leachate treatment in US system was found to be very high due to the presence of fixed solids. Coagulation pretreatment using alum was carried out to improve the overall COD removal and reduce the cost of treatment. As a result, the COD removal was increased to 78% (42% in pretreatment and 36% in US) in 15 min. On the other hand, US-H2O2 and US-PS hybrid systems have shown significant improvement in COD removals (93% and 86%, respectively) from raw leachate after 15 min. Subsequently, a three factor (i.e. PS dose (mg/L), H2O2 dose (mol/L), and US amplitude (%)) 5-level design of experiment was used to maximize the COD removal efficiency by response surface methodology (RSM). The RSM model generated a quadratic equation to accurately analyze the influence of input variables on COD removal efficiency (R2 of 0.92). A maximum COD removal of 98.3% was predicted using the model and the corresponding optimal experimental condition were identified as follows: PS dose ∼4700 mg/L, H2O2 dose ∼0.7 mol/L and US amplitude ∼49%. The overall observations reveals that PS and H2O2 coupled with US system has a great prospective to treat mature landfill leachate.