Evaluation of Organic Matter Release and Economy for Various Pretreatments of Sewage Sludge

Abstract

Mechanical, chemical, and biological methods are always used to pretreat sewage sludge. To determine which pretreatment can release more organic matter from sewage sludge, and therefore, make it more economical, mechanical pretreatment (ultrasonic treatment at 20 kHz), chemical pretreatment (pH 10), and biological pretreatment (anaerobic conditions at 70℃) were compared. Results showed that all three pretreatments increased the organic matter release of sewage sludge; the initial total soluble protein and carbohydrate concentration of which was only 418.9 mg·L-1 (cal. as COD) and the DNA content was 18.1 mg·L-1. However, the pH 10 and anaerobic (70℃) pretreatments resulted in a greater organic release than the ultrasonic (20 kHz) pretreatment, with total soluble protein and carbohydrate concentrations of 7516.0 mg·L-1 and 7892.5 mg·L-1, and DNA contents of 1343.3 mg·L-1 and 1766.1 mg·L-1, respectively. Flow cytometry was adopted to assess cell morphology. The cell mortality rates of sludge after pretreatment accounted for 61.6%, 59.9%, and 34.5% respectively, which was improved by 45.6% (at pH 10), 43.9% (under anaerobic conditions at 70℃), and 18.5% (with ultrasonic pretreatment at 20 kHz) compared with raw sewage sludge. At the same time, organic matter removal ratios of sludge after pretreatment were 19.1% (at pH 10), 13.8% (under anaerobic conditions at 70℃), and 7.6% (with ultrasonic pretreatment at 20 kHz). Moreover, the pretreatment of per liter sludge at pH 10 saved 28.5% and 124.1% more than ultrasonic pretreatment (20 kHz) and anaerobic (70℃) pretreatment. Taking both organic matter release and economic analysis into consideration, the chemical method of pretreatment (at pH 10) was identified as the best option.