The study aimed to investigate the biochemical up gradation of duckweed (DW)-Spirodela polyrhiza biomass cultivated in wastewater and then its further utility in anaerobic digestion (AD). For chemical scaling of DW biomass, a batch-scale duckweed reactor was designed using urban wastewater (WW) and changes in WW characteristics were recorded at the end. The WW showed the significant reduction (p < 0.05) in pH- 16.9; electrical conductivity (EC)-67.6%; biochemical oxygen demand (BOD)-62.6%; nitrate nitrogen (NO3--N)-76.5%; orthophosphate (PO4-3-P)-76%; sulphate (SO4-2)-86.9%; sodium (Na+)-12.0%; calcium (Ca+2)-75.9% and, potassium (K+)-53.6% after duckweed treatment. After treatment, the DW biomass was harvested and analysed for biochemical properties. Results showed an increase in carbohydrate (45.5%), starch (40.8%), lipid (46.4%) and, protein (56.4%) contents. In the second stage of experiments, the harvested DW biomass was mixed (v/v) with waste activated sludge (WAS) and inoculum acclimatized anaerobic granular sludge (AAGS) to produce four anaerobic batch setups: T-1-DW/WAS/AAGS (50:10:40), T-2-DW/WAS/AAGS (40:20:40), T-3-DW/WAS/AAGS (30:30:40) and, T-4-AAGS (100%) and production of methane was recorded for 35 d. The methane production was recorded in the ranges of 3001 (T-3)-5491 (T-1) mL. The rate of methane generation in all batch reactors was in the order: T-1 (24.01) > T-2 (15.13) > T-3 (9.55). Results thus, revealed that the high content of DW in reactor caused positive effect on methane generation. During the process, soluble chemical oxygen demand (SCOD) and volatile solids (VS) also reduced 36.8-79.7% and, 42.9-70.9%, respectively. Gompertz model validates the experimental methane yield in all setups. Our study indicates that DW can be sustainable tool to solve two major problems: wastewater treatment and renewable energy production under clean development approach. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.