Modified field method for testing activated sludge condition in chambers with variable wastewater levels

The paper presents a proposal to modify a field method of testing the condition of activated sludge using a 30-min volume of sludge (settling test). To verify the validity of the modified method of testing the condition of activated sludge, field tests were performed in two onsite wastewater treatment plants. In these plants, the reaction chambers were fed by gravity from the primary sedimentation tank throughout the day. For such treatment plants, the activated sludge condition test varies during a 24-h period. Testing the condition of activated sludge using the modified volume of sludge gives stable results over a 24-h period. To analyse the results, a statistical test was performed to compare the results obtained in the traditional volume of sludge and the modified one. The analysis showed that the results of the sludge volume test are incorrect if the wastewater sample is taken at a time other than the final filling of the bioreactor (just before the sedimentation phase) due to sludge thickening. The proposed modified sludge volume test allows the operator to perform the test at any time of day with a correct result.