Fertigation Efficacy of Municipal Wastewater for Leafy Vegetables

Abstract

Fresh water scarcity has attracted farming communities to utilize wastewater for irrigation. Municipal wastewater (MW) enriched with diverse contaminants including heavy metals is still preferred due to higher concentration of plant nutrients, continuous availability and low cost. Vegetables irrigated with such water usually contain high contents of heavy metals, which are considered harmful for consumers. Treated wastewater application for irrigation of vegetables can be an effective strategy to minimize health risks associated with the consumption of contaminated vegetables. Present study was conducted at Pir Mehr Ali Shah, Arid Agriculture University, Rawalpindi Pakistan with two cultivars of spinach and lettuce. Municipal wastewater was collected following standard procedures from Nullah Lei, Gawalmandi, Rawalpindi, Pakistan and it was treated with two types of trickling filters (TFs) i.e. (1) BT using biological (rice husk, corn cob and wheat straw) and (2) PT using physical (sand, medium, fine and very fine gravel) substrates. Physico-chemical characteristics of all irrigation treatments and soils were determined with (MF, PTF, BTF, TWF and GF) and without NPK supplementation (MW, PT, BT, TW and GW). Leafy vegetables i.e. lettuce [leaf (L1), ice-berg (L2)] and spinach [desi palak (S1), lahori palak (S2)] were analyzed for metal accumulation in roots and leaves, plant growth/productivity and biochemical parameters in all treatments. Physico-chemical parameters (pH, NO31-N, COD, PO43-) and five heavy metals (Ni, Mn, Cu, Co and Cd) exceeded irrigation standards in MW whereas the rest of the six parameters and seven metals were within allowable limits. Physico-chemical parameters of correspondingly irrigated soils were within safer limits of soil standards except two metals (Cu and xxi Cd) which were above safer limits due to wastewater irrigation. Among studied vegetables higher metal accumulation was recorded in S1 (Pb, Co, Zn) and S2 roots (Ni, Cu, Cr) in MW and MF treatments respectively compared to control treatments. Higher accumulation in leaves was found in S1 (Pb, Mn, Cr) and S2 (Ni, Zn, Fe, Cu, Co, Cd) in MW and MF respectively. Bioconcentration factor (BCF) for all metals was recorded higher in all vegetables roots in MW whereas S1 (MW) and S2 (MF) leaves in respective treatments. Significant human health risk is speculated on ingesting heavy metal hyperaccumulating S1 and S2 in MW and MF treatments respectively. The BT treated water showed remarkable reduction in all physico-chemical parameters, particularly heavy metals compared to PT and values of studied parameters closely matched with TW. All studied soil parameters in PT, PTF, BT, BTF, TW and TWF were found within safe limits due to irrigation with treated water. Root and leaf metal accumulation was again recorded higher for S1 and S2 in PT and PTF treatments compared to BT, BTF, TWF and TW revealing better wastewater treatment efficiency in trickling filter containing biological substrates. Similarly BCF for metals and health risks in adults and children were only found in spinach in PT and PTF treatments while no risk was found in BT and TW treatments. Lettuce was found less metal hyperaccumulating compared to spinach. In this study, fertigative efficacy of municipal wastewater (MW) was recorded but at the cost of human health risk and risk pattern was; S2>S1>L1>L2.