Freezing as a combined wastewater sludge pretreatment and conditioning method

Laboratory experiments were carried out to investigate the potential of freezing as a combined sludge pretreatment and conditioning method, which could not only improve wastewater sludge dewaterability but also solubilize sludge solids. The effect of freezing temperature, freeze thaw cycles on solubilization of organic matter, release of nutrients, dewaterability and particle size distribution was examined. The treatment efficiency of the freezing technique was compared with thermal and combined thermal chemical (acidification or basification) pretreatment methods. The efficacy of the combined chemical and freezing treatment was also tested for the first time. Experimental results indicated that freezing was effective in the solublization of sludge solids. About 2- to 8-fold increase in soluble chemical oxygen demand (SCOD), 2–8 times in ammonia (NH3–N) and 1.5–2.5 times in orthophosphate (OPO4–P) concentrations were observed after freezing treatment. Sludge samples with combined basification and freezing treatments had the higher SCOD and NH3–N concentrations while acidifying prior to freezing did not enhance treatment efficiency. Freezing significantly improved sludge dewaterability. Freezing caused the aggregation of small particles and greatly increased the volume of large particles. Freezing has a potential as a combined sludge pretreatment and conditioning method that could provide more befits than as only a sludge conditioning technique.

Research Highlights
► Freezing is effective in solubilizition of organic matter and releasing nutrients.
► Acidification or basification prior to freezing affected treatment efficiency.
► Freezing agglomerated small particles and greatly enhanced dewaterability.
► Freezing could be used as a combined sludge pretreatment and condition method.