Microbial decomposition of dissolved organic matter and its control during a sorption experiment

This paper describes a detailed investigation on the effect of microbial activity on apparent dissolved organic matter (DOM) sorption in batch experiments and means to control it. The adsorption and decomposition dynamics of DOM derived from three organic wastes (green manure, pig manure, and sewage sludge) were performed at 4 or 22°C in the presence and absence of 5 mM NaN₃. No DOM decomposition occurred in sorption experiments performed at 4 or 22°C receiving NaN₃, while significant reduction was noted at 22°C without NaN₃ addition after 24 h shaking. The sequence of DOM susceptibility to microbial decomposition in a 24-h period was: green manure > pig manure > sewage sludge. A shaking period of 2 h was found too short for notable microbial activity as compared with that of 24 h shaking but long enough to achieve equilibrium adsorption at 22°C. Therefore, sorption experiments performed at ambient temperatures with a long equilibrium period will overestimate the amount of DOM adsorbed and lead to an incorrect interpretation of the DOM adsorption behavior because of the involvement of microbial decay of DOM. It is recommended that batch equilibrium DOM adsorption studies be performed at either 4°C for 24 h or at ambient temperatures for 2 h, while addition of 2.5 to 5 mM of NaN₃ is also a possible alternative for an equilibrium period of > 2 h.