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Abstract
During the past decades, ultrasonic technology has gained wide interest as an effective mechanical pretreatment method for sludge, due to its excellent performance, good technical and operational stability, compactness, and environmentally friendly processing. This review is intended to provide an overview on the applications of ultrasonics in sludge solubilization and subsequent digestion, sludge sanitization, enhancement of enzymatic activity, resource recovery, extraction of chemicals, ultrasound assisted lysis–cryptic growth, and degradation of hazardous pollutants. Moreover, the working mechanisms behind the treatment and the influence of various process conditions, including power density/intensity, specific energy input, ultrasonic frequency, duration of ultrasonication (i.e., partial and complete ultrasonication, solids concentration, and type of sludge on the ultrasonic processing of sludge is specifically emphasized. Also, a discussion is included on the future research directions, an economical assessment of ultrasonic technique, and feasibility of full-scale implementation.
NOMENCLATURE
| A-A/O | = | anaerobic-anoxic-oxic |
| AD | = | anaerobic digestion |
| BAP | = | biochemical acid potential |
| BHC | = | Benzenehexachloride |
| BPA | = | bisphenol A |
| COD | = | chemical oxygen demand |
| CST | = | capillary suction time |
| DBP | = | di-butyl phthalate |
| DDT | = | dichlorodiphenyltrichloroethane |
| DEHP | = | bis (2-ethylhexyl) phthalate |
| DHA | = | dehydrogenase activity |
| DMS | = | dimethyle-sulphide |
| DNA | = | deoxyribonucleic acid |
| DPB | = | denitrifying phosphorus removal bacteria |
| DS | = | dry solids |
| EDC | = | endocrine disrupting compounds |
| EDTA | = | ethylenediaminetetraacetic acid |
| EPS | = | extracellular polymeric substances |
| F/M ratio | = | food to micro-organisms ratio |
| HRT | = | hydraulic retention time |
| LAS | = | linear alkylbenzene sulphonates |
| MBR | = | membrane bioreactor |
| MSPD | = | matrix solid-phase dispersion |
| NTU | = | nephelometric turbidity unit |
| OLR | = | organic loading rate |
| OPs | = | organophosphorus pesticides |
| OTC | = | organotin compounds |
| OUR | = | oxygen uptake rate |
| PAE | = | pthalic acid esters |
| PAH | = | polycyclic aromatic hydrocarbons |
| PAOs | = | phosphorus accumulating organisms |
| PCB | = | polychlorinated biphenyls |
| PCB | = | printed circuit board |
| PHA | = | polyhydroxy-alkanoates |
| PHB | = | Poly-β-hydroxybutyrate |
| PSP | = | primary sludge particulates |
| PVC | = | polyvinyl chloride |
| SBR | = | sequencing batch reactor |
| SCOD | = | soluble chemical oxygen demand |
| SP | = | proportion of sonicated sludge |
| SRT | = | solids retention time |
| SVI | = | sludge volume index |
| TN | = | total nitrogen |
| TP | = | total phosphorus |
| TS | = | total solids |
| TSS | = | total suspended solids |
| UAE | = | ultrasonic assisted extraction |
| USP | = | probe ultrasonication |
| VFA | = | volatile fatty acids |
| VS | = | volatile solids |
| VSS | = | volatile suspended solids |
| WAS | = | waste activated sludge |
| WWTP | = | wastewater treatment plant |
FUNDING
The authors would like to thank the National Taiwan University for financially supporting this research under Contract No. 99R40044.