Key factors governing alkaline pretreatment of waste activated sludge

Alkaline pretreatment is an effective technology to disintegrate sewage sludge, where alkali dosage and sludge concentration are two important factors. pH value or alkali concentration is usually adjusted in order to determine a proper dosage of alkali. Our work has found that this is not a good strategy. A new parameter, the ratio of alkali to sludge (Ra/s), is more sensitive in controlling the alkali dosage. The sludge concentration Cs and retention time t are two other important factors to consider. The validity of these arguments is confirmed with modeling and experiments. The individual effect of Ra/s, Cs and t was studied separately. Then the combined effect of these three factors was evaluated. The sludge disintegration degree of 44.7% was achieved with the optimized factors. Furthermore, an alkaline-microwave combined pretreatment process was carried out under these optimized conditions. A high disintegration degree of 62.3% was achieved while the energy consumption of microwave was much lower than previously reported.

Graphical abstractFor sludge alkaline-pretreatment, alkali dosage and sludge concentration (Cs) are two important influences, but alkali dosage is not suitably evaluated and the latter is rarely concerned with most previous experiments performing at a certain Cs. Therefore, a new parameter, the ratio of alkali to sludge (Ra/s), is defined in our work to determine alkali dosage. The individual effect of Ra/s, Cs and retention time t is examined separately, and the combined effect of these three factors is evaluated and optimized. The sludge disintegration degree (DD) of 44.7% by alkaline pretreatment alone was obtained with optimum Ra/s, Cs and t being 22%, 108 g·L− 1 and 11 h.Figure optionsDownload full-size imageDownload as PowerPoint slide