The Effects of Aluminium- and Ferric-Based Chemical Phosphorus Removal on Activated Sludge Digestibility and Dewaterability

Peter Ojo,Augustine Osamor Ifelebuegu

doi:10.3390/pr7040228

Abstract

The uses of Al3+ and Fe3+ salts in chemical phosphorus removal (CPR) in activated sludge plants have increased considerably in recent years and their full impacts on downstream processes such as dewaterability and digestibility are not fully understood. In this research, the effects of CPR on sludge digestibility and dewaterability were investigated in laboratory-scale experiments using sludge samples from a full-scale wastewater treatment plant. The results of the digestibility tests showed a 21% and 36% reduction in the biogas volume generated during anaerobic digestion of surplus activated sludge at 0.1 g/L doses of Al3+ and Fe3+ salts, respectively. This demonstrates that Al3+ dosing for CPR has less of a reduction effect compared with Fe3+ salts on biogas generation during anaerobic digestion of sludge. The dewaterability tests showed that primary sludge dewaterability was improved by up to 25% by Fe3+ and 16% by Al3+, while that of surplus activated sludge was reduced by 64% and 73%, respectively, at a metal salt dose of 50 mg/L. Consequently, a pre-precipitation process during CPR where phosphorus is removed in the primary tank would, therefore, enhance sludge dewaterability.

Highlights

The presence of nutrients, phosphates, present a huge challenge in the treatment of wastewater both in municipal and industrial wastewater treatment plants (WWTPs)
The results demonstrated that the use of Al3+ and Fe3+ salts reduced the digestibility of surplus activated sludge, with Al3+ salt having up to 15% less reduction effect compared with Fe3+
Salts improved the dewaterability of primary sludge by 16% and 25% but significantly reduced the dewaterability of surplus activated sludge (SAS) by 73% and 64%, respectively

Summary

Introduction

The presence of nutrients, phosphates, present a huge challenge in the treatment of wastewater both in municipal and industrial wastewater treatment plants (WWTPs). The activated sludge process (ASP) is the most widely used technique for municipal wastewater treatment and is able to achieve higher P removal to levels of less than 1 mg/L [1]. The two main processes for phosphorus removal are biological phosphorus removal (BPR) and chemical phosphorus removal (CPR). Both processes are used independently and, most times, in combination to achieve effluent consents for P. While BPR is considered the most environmentally friendly option, it has its limitations, the requirement for strong biological oxygen demand (BOD) in the influent wastewater. Most WWTPs in Europe use CPR to achieve their effluent P consents using mainly ferric (Fe3+ ) and aluminium (Al3+ ) salts for precipitation of phosphorus [2,3,4]

Objectives

Methods

Conclusion