Development of water lettuce (Pistia spp.) based biochar filter for the treatment of industrial wastewater: a green approach

Biochar (BC) is a viable wastewater treatment agent. Water lettuce (Pistia spp.) an invasive aquatic plant in Sri Lanka has been used in this study to produce BC by pyrolysis. Industrial water was filtered through activated and non-activated BC samples having 65–125 mm and 125–250 mm particle sizes. Filter with sand and gravel served as standard for comparison. The filter unit containing activated BC showed higher treatment efficiency. pH was reduced from 8.72 ± 0.01 to 6.87 ± 0.01; biological oxygen demand (BOD₅) from 57 ± 1.73 to 15.67 ± 1.15 ppm; chemical oxygen demand (COD) from 512 to 128 ppm, total Kjeldahl nitrogen (TKN) from 158.67 ± 2.89 ppm to 33 ppm, ammoniacal nitrogen from 22.63 to 3.13 ± 0.01 ppm; chromium from 0.0625 to 0.0245 ppm, and zinc from 0.9886 to 0.2105 ppm. Optimization study revealed that the BC integrated filter units could be used to treat wastewater in three cycles. Thereafter their efficiency is reduced. The BCs with small particle size (65–125 mm) recorded the highest treatment efficiency. This particle size showed 99.72% turbidity reduction. Reduction of total phosphate, ammoniacal nitrogen, total suspended solids, COD, nitrate, BOD5 and TKN was recorded as 97.82%, 91.49%, 88.38%, 88.18%, 87.44%, 84.38% and 84.38% respectively. BC derived from Pistia spp. has proven to be promising adsorbent for wastewater treatment. Findings of the study show a long-term green waste­water treatment method that makes effective use of an invasive aquatic plant.