An integrated approach for landfill leachate treatment using Plasma/UV pre-treatment and indigenous green microalgae

An increase in municipal solid waste (MSW) production is usually the inevitable result of a continuously growing population. Landfilling is the most applied method worldwide to deal with the produced MSW. However, one major drawback of landfilling is the production of a complex, toxic and hazardous type of wastewater which is landfill leachate (LL). Various methods for LL treatment exist which can mainly be divided into biological and physical-chemical methods, however integrating both approaches result in more effective treatment. In this study, soil samples (with leachate runoff) from a landfill leachate treatment site in Chesterfield, UK are collected. Indigenous microalgal strains from the samples are isolated, purified and genetically identified using five different primers which allowed the sequencing of most of the rDNA (18S, 5.8S, ITS1 & ITS2 regions). The identified sequences are submitted to the NCBI GenBank and given accession numbers, identification revealed a community of four green microalgae namely two strains of Chlorella vulgaris, one strain of Chlorococcum species and one strain of Scotiellopsis reticulata. The threshold concentration of LL tolerated by the four green microalgae after dilution with distilled water (v/v) is tested by growing them separately in different LL concentrations (5%, 10%, 15%, 20% and 25%). The two strains of Chlorella vulgaris were the only strains capable of showing a significant growth increase (p<0.05) in the challenging concentration of 20% LL (v/v) which is usually inhibitory to other algal strains. Hence, concentration 20% LL is chosen as a platform for the following screening experiment to test the most potent strain/s in growth and biotreatment of LL. One replicate of one of the strains of Chlorella vulgaris (C.V.M*) (CCAP 211/141) started showing an outstanding growth at the end of a 30-day experiment with a significant ammonia removal (p<0.05). To confirm these findings, the two replicates of Chlorella vulgaris (C.V.M* and C.V.N) are further tested in 20% LL (supplemented with phosphate) in a 30-day experiment. A dramatic increase in the growth of C.V.M* by 19-fold over its peer C.V.N with a 75% ammonia removal (starting from concentration 290.73 mg/L) are observed. This percentage is further improved when implementing plasma technology (with fluidic oscillator incorporated) as a pre-treatment step. The plasma pre-treatment induced LL colour change after three hours (from dark-brown to yellow) and reduced ammonia-N concentration by 1.9-fold. Strain C.V.M* showed the highest growth in 20% LL pre-treated by plasma compared to the untreated LL, with a final biomass yield of 0.38 g/L, a total increase in the total ammonia-N removal (79%) and a significant decrease (p<0.05) in the pH value from 8.6 to 6.67 at the end of the experiment. Whole genome sequencing for both strains revealed differences in genotypes of 15169 single nucleotide polymorphism (SNP) loci and 2046 insertion-deletion (indel) loci. This in turn might indicate the possibility of a developed mutation or sexual reproduction that might have increased the ability of strain C.V.M* (CCAP 211/141 ) to tolerate harsher conditions and higher ammonia-N concentrations in 20% diluted LL. This might pave the way for a possible powerful candidate in LL treatment using a highly tolerant algal strain which when coupled with plasma pre-treatment might provide a possibly effective, eco-friendly, and sustainable LL treatment approach.