Predicting and Improving the Dewaterability of Waste Activated Sludge Through Moisture Distribution Analysis

Abstract

The dewatering of waste activated sludge is one of the most costly and least understood operations in wastewater treatment. While much effort has been put into streamlining operation of sludge dewatering equipment to maximize moisture removal, until recently, little attention was paid to the water within sludge. Sludge water molecules can be subdivided into several categories according to various properties with efforts made to convert sludge water from one category to another with the goal of making it easier to remove. This thesis tests two different ideas. First, that a dewaterability prediction test can be developed that is based on the distribution of moisture within activated sludge. Second, that sludge water can be converted from one form to another thereby improving sludge dewatering.

A centrifugal dewatering test was developed based on the idea that as centrifugal forces approach infinity, all of the free, or easily removable, water is withdrawn leaving only bound water. The centrifugal dewatering test consisted of centrifuging a 24-ml sample of waste activated sludge at ˜150,000 x g for 30 minutes. Following centrifugation, the samples were inverted and supernatant was removed. The water remaining in the pellet was considered to be the bound water and represented the portion of sludge water that cannot be removed by mechanical dewatering equipment. Comparisons were made between the centrifugal dewatering test and full-scale wastewater treatment plant data. Significant positive correlations were made between test results and plant results for six of the eight plants involved in the study. The tests value is that it could be used to predict trends in sludge dewaterability in advance of solids handling equipment. The purpose of developing the test was to demonstrate that investigations into sludge dewatering operations should include the entire wastewater treatment process, as well as, solids handling equipment.

The second part examines the idea that sludge moisture present as bound water can be converted to free water thereby improving the dewatering process. It was hypothesized that an input of disruptive energy could change the distribution of moisture within activated sludge. Heat treatment (40, 60, and 80°C), sonication, alkaline treatment, and cation addition were each evaluated alone, and in combination, for their ability to alter moisture distribution. Results indicated that only sonication, both alone and combined with cation addition, led to increased dewaterability and compactibility, as well as, decreased bound water. Cation addition alone led to increased dewaterability.