Effect of storage time on in-situ parameters and total nitrogen analysis results of tofu liquid waste samples

Abstract

Field parameter measurement is usually constrained by time management, causing measurement delays and, therefore, different results between field parameters and the actual environmental conditions. Tofu liquid waste contains a low pH (3-5) and suspended, colloidal, and dissolved organics matter at high concentrations; the latter might change rapidly and also cause changes to other parameters. Tofu liquid waste from a tofu factory outlet was used in this study. The field parameters in this study included pH, total dissolved solids (TDS), and electrical conductivity (EC) by storage time variation of 15, 20, 25, and 30 minutes. The total nitrogen was analyzed by the Kjeldahl method with a storage time variation of 2, 4, 6, and 8 days. The measurement of pH found no significant difference (sig > 0.05) up to 30 minutes of storage time, while EC and TDS significantly differed within 20 minutes. The total nitrogen was stable up to the second day of storage time at a storage temperature of 4-8 °C without preservatives. In water quality monitoring, the results of water samples analysis should be as close as possible to their values in the environment. This research shows that field parameters' results can change quickly if the sample handling and analysis time are not by procedures, especially for EC and TDS. In addition, this study also shows that without giving H2SO4, the value of total nitrogen in tofu liquid waste samples decreased significantly after the second day of storage at 4-8 °C.