Enhanced Coagulation as a Pretreatment for Low Temperature Wastewater

Abstract

The effects of low temperature on enhanced coagulation were studied. A new composite coagulant called SynthA was synthesized. The effects of enhanced coagulation on the removals of dissolved organic matter, dissolved organic nitrogen, and so on under room temperature or low temperature (2-5℃) were determined, and their influences on biological treatments were investigated by using membrane fractionation distribution, three-dimensional fluorescence spectrum (3DEEM), and differential ultraviolent absorbance. The results showed that, under room temperature, the removals of particulate COD, particulate nitrogen, colloidal COD, and colloidal nitrogen were highly correlated with turbidity reduction by coagulation using aluminum chloride (AlCl3), poly aluminum chloride (PACl), and SynthA as coagulants separately, while the relationship was not clear between the dissolved parameters and turbidity reduction. The reduction of fluorescence value of dissolved organic matter after coagulation was much higher than that of dissolved COD. Dissolved organic nitrogen (DON) is removed to the greatest extent by preset coagulation along with particulate nitrogen (PN) and colloidal nitrogen (CN). Low temperature affected enhanced coagulation in many aspects. It inhibited turbidity reduction and COD removal by the three coagulants with the order being AlCl3 > PACl > SynthA. It exhibited differential influences on the removals of particulate, colloidal and dissolved COD, and nitrogen, and it showed greater adverse effects on particulate and colloidal COD and nitrogen. The fluorescence value of dissolved organic matter in low temperature water showed a significant increase, and its reduction by coagulation was high, compared with that in room temperature water. Low temperature coagulation exerted greater impacts on ultraviolet differential absorbance than did room temperature. Under low temperatures, slight increases of total nitrogen (TN) removal, DN, and DON removals were achieved by using SynthA as coagulant, and removals of PN and CN were maintained, compared with room temperature. As an example, when SynthA dosage was above 30 mg ·L-1, DON removal reached 28.5%-41.7% at low temperature, while the removal was only 17%-31.4% at room temperature. A large portion of the COD and some TN were removed by coagulation as a pretreatment, indicating that a large amount of the time in an aeration pond could be reduced, and the removal efficiency of TN would be stabilized. Therefore, in winter, the decrease of biological treatment efficiency could be alleviated to some extent by using enhanced coagulation with an adaptable coagulant, such as SynthA as a pretreatment, which would relieve the stress of denitrogen and stabilize treatment efficiency.