A sludge purging procedure that increases the robustness of upflow anaerobic sludge blanket reactors in sewage treatment

This work proposes a sludge purging procedure from upflow anaerobic sludge blanket reactors in small wastewater treatment plants. The following characteristics of the anaerobic sludge supported its development: specific methanogenic activity, biodegradability and dewaterability. The sludge samples from the reactor blanket zone presented low specific methanogenic activity, low biodegradability and good dewaterability. On the other hand, sludge samples from its bed zone showed higher biodegradability and specific methanogenic activity, as well as equivalent dewaterability and excellent sedimentability. These characteristics indicate that the common practice of sludge purging from the bed zone is a mistake and must be avoided. Nevertheless, the sludge blanket usually presents high water contents (total solids < 3%), which can overload hydraulically the drying beds. To prevent this problem, the sludge purge from the blanket zone should be fractionated into three discharges of lower volume during the first 3 days of the dewatering cycle.